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DOI: 10.1148/rg.255055014
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RadioGraphics 2005;25:1141-1158
© RSNA, 2005

EDUCATION EXHIBIT

Quantitative Vascular Measurements in Arterial Occlusive Disease1

Hideki Ota, MD, Kei Takase, MD, Hiroya Rikimaru, MD, Masahiro Tsuboi, MD, Takayuki Yamada, MD, Akihiro Sato, MD, Shuichi Higano, MD, Tadashi Ishibashi, MD and Shoki Takahashi, MD

1 From the Department of Diagnostic Radiology, Tokohu University Graduate School of Medicine, 1–1 Seiryo, Aoba, Sendai, Japan (H.O., K.T., T.Y., A.S., S.H., T.I., S.T.); the Department of Radiology, Ishinomaki Red Cross Hospital, Ishinomaki, Miyagi, Japan (H.R.); and the Department of Radiology, Furukawa City Hospital, Furukawa, Miyagi, Japan (M.T.). Presented as an education exhibit at the 2004 RSNA Annual Meeting. Received January 28, 2005; revision requested March 10 and received April 21; accepted April 22. All authors have no financial relationships to disclose. Address correspondence to H.O. (e-mail: h-ota{at}rad.med.tohoku.ac.jp).


   Abstract
Top
 Abstract
Introduction
 Technical Considerations
 Imaging in the Quantification...
 Quantification of Arterial...
 Conclusions
 References
 
Accuracy in quantifying arterial occlusive disease requires an understanding of the relevant technical considerations and familiarity with the strengths and weaknesses of various imaging modalities in this setting. The degree of stenosis is evaluated in terms of diameter stenosis, which can be measured on either projection images or cross-sectional images, or area stenosis, which can be measured only on cross-sectional images. With projection images, the minimum luminal diameter should be sought on multiple images obtained at different angles. The reference site used for measurement should be noted and may be located at the level of the lesion or in a normal-looking portion of the stenotic vessel near the lesion. Multi–detector row computed tomographic (CT) angiography and magnetic resonance (MR) angiography are starting to replace digital subtraction angiography in quantifying arterial occlusive disease. CT angiography allows accurate evaluation without reducing in-plane resolution, although beam-hardening artifacts from high-attenuation structures can degrade image quality. MR angiography is useful even in cases of severe calcification but has a lower spatial resolution. Ultrasonography (US) may also be helpful in quantifying arterial occlusive disease; US analysis is almost always based on blood flow velocity measurement. Precise measurements of stenotic occlusion will help determine optimal therapy for affected patients.

© RSNA, 2005


   Introduction
Top
 Abstract
 Introduction
Technical Considerations
 Imaging in the Quantification...
 Quantification of Arterial...
 Conclusions
 References
 
In assessing patients with arterial occlusive disease, quantitative analysis of the degree of arterial steno-occlusive change is essential for determining optimal therapy. Digital subtraction angiography (DSA) has long been the standard of reference for quantitative measurement. However, other imaging modalities such as multi–detector row computed tomographic (CT) angiography, magnetic resonance (MR) angiography, ultrasonography (US), and intravascular US are also used for vascular analysis, and under some conditions DSA may not be the standard of reference.

In this article, we review technical considerations in the quantitative measurement of arterial occlusive disease. In addition, we discuss and illustrate the use of the aforementioned imaging modalities as well as various postprocessing techniques—multiplanar reformation (MPR), volume rendering (VR), maximum intensity projection (MIP), and curved planar reformation (CPR)—in this setting. We also discuss specific issues relating to the quantification of arterial occlusive disease in selected arteries, including the internal carotid artery (ICA), renal arteries, and lower extremity arteries.


   Technical Considerations
Top
 Abstract
 Introduction
 Technical Considerations
Imaging in the Quantification...
 Quantification of Arterial...
 Conclusions
 References
 
For quantitative measurement of stenotic occlusion, the residual lumen at the lesion site is compared with the lumen at a reference site. The degree of occlusion is measured in terms of the diameter or area of stenosis. The percentage of stenosis is calculated as (1 – L/R) x 100, where L and R are the area or diameter of the lesion and of the reference site, respectively.

Reference Site
There are two methods of assigning the reference site (Fig 1). In the first method, a normal-looking portion of the stenotic vessel either proximal or distal to the lesion serves as the reference site. In the second method, the reference site is located at the level of the lesion. The percentage of stenosis may differ according to the location of the reference site (1,2).



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  		Figure 1.  Drawing illustrates measurements used to determine the degree of vascular stenosis. Rp and Rd indicate the luminal diameter or area in the normal-looking portion of the vessel proximal (Rp) and distal (Rd) to the stenotic lesion (L). Re is the estimated luminal diameter or area at the level of the lesion. Any of these values—Rp, Rd, or Re—can be used as the reference value, and it should be noted which of the three is used. The percentage of stenosis is calculated as (1 – L/R) x 100, where L = lesion diameter or area and R = diameter or area at the reference site.

 
Diameter Stenosis
Traditionally, only diametric measurements have been used for the evaluation of stenosis because the images provided by catheter-based angiography, which for many years has been the standard of reference in this setting, are projection images of the affected vessel, making diameter stenosis measurable but not area stenosis. For evaluating diameter stenosis, the minimum luminal diameter at a target site is determined. The projection image should be generated at an angle that allows measurement of the minimum luminal diameter; this dimension may be unmeasurable in cases of eccentric stenosis with images generated at suboptimal angles (Fig 2). In contrast, on cross-sectional images, the minimum luminal diameter can be measured accurately without difficulty (Fig 2).



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  		Figure 2.  Drawings illustrate how projection images (a, b) and a cross-sectional image (c) are used to measure the diameter of an eccentric arterial stenosis. In a, the minimum luminal diameter (Da) is depicted at the optimal projection angle. In b, the degree of stenosis is underestimated because the minimum luminal diameter (Db) is depicted at a suboptimal projection angle, making it larger than Da. The cross-sectional image is oriented perpendicular to the vessel and accurately depicts lumen morphology, making the minimum luminal diameter easy to measure. Da and Db in c correspond to the diameters measured in a and b.

 
Area Stenosis
Some reports state that a reduction in cross-sectional area correlates better with the hemodynamic effect of stenosis than does a reduction in diameter (3). Therefore, change in cross-sectional area has been proposed as a more accurate way of measuring arterial stenosis (4). Evaluation with true cross-sectional images that are oriented perpendicular to the affected vessel is essential for quantification of area stenosis. Such evaluation is relatively time consuming if the border of the lumen is traced manually. Software programs (eg, Advanced Vessel Analysis; GE Medical Systems, Milwaukee, Wis) are now commercially available that can measure area stenosis automatically and help reduce the amount of time required for analysis.

Relationship between Diameter and Area Stenosis
It is important to note whether an arterial stenosis is measured in terms of diameter or area because the two percentages do not correspond (3). Figure 3 illustrates the relationship between area reduction and diameter reduction in cases of completely concentric stenosis: The degree of area reduction is greater than the degree of diameter reduction, unless there is either no stenosis (0% reduction) or total occlusion (100% reduction). In cases of eccentric stenosis, the relationship between area and diameter reduction is not constant.



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  		Figure 3.  Graph (top) illustrates the relationship between area reduction and diameter reduction in a completely concentric stenosis. The relationship is described by the equation A = D x (2 – [D/100]), where A = percentage of area reduction and D = percentage of diameter reduction. Drawings at bottom illustrate cross-sectional views of lumina at various percentages of area and diameter stenosis.

 

   Imaging in the Quantification of Stenosis
Top
 Abstract
 Introduction
 Technical Considerations
 Imaging in the Quantification...
Quantification of Arterial...
 Conclusions
 References
 
In the evaluation of arterial stenosis, the inherent strengths and limitations (eg, with respect to spatial resolution [Table 1] or artifacts) of the imaging modality being used should be considered to ensure accurate quantification.


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  		Table 1. Spatial Resolution of Various Imaging Modalities

 
Digital Subtraction Angiography
As mentioned earlier, for many years DSA has been the standard of reference for quantifying arterial stenosis. Despite the emergence of other less invasive modalities that provide high-quality images, DSA is still used as a diagnostic tool for several reasons. DSA yields findings that are easy to interpret and can depict the whole target artery with a higher spatial resolution than multi–detector row CT angiography or MR angiography (Table 1). In addition, DSA allows easier, more accurate lumen evaluation in calcified vessels than does multi–detector row CT angiography and in vessels containing stents than do multi–detector row CT angiography and MR angiography owing to fewer artifacts from calcification or metal structures, although an extremely dense structure may still cause some problems.

However, DSA also has some shortcomings. This modality is a kind of "luminography" that provides little information about vessel wall morphology or the course of the vessel at the occluded segment. Because DSA yields two-dimensional (2D) images, multiple views are obtained at different angles for the evaluation of stenosis. Even with multiple views, however, eccentric stenosis or stenosis of a tortuous vessel may be underestimated (5). Overlapping vessels may also interfere with the assessment of stenosis (Fig 4). Rotational DSA better depicts stenosis than does DSA in two or three projections (6).



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  		Figure 4a.  Stenoses of the left external and right common iliac arteries in a 70-year-old woman. (a) Anteroposterior DSA image fails to depict stenosis of the left external iliac artery because of the enhancement of the overlying left internal iliac artery (straight arrow). However, stenosis of the right common iliac artery can be seen (curved arrow). (b) DSA image (30° right anterior oblique angle) allows differentiation of the enhanced left external iliac artery from the left internal iliac artery, demonstrating 60% diameter stenosis of the former artery (arrow) relative to the distal reference site (arrowhead).

 


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  		Figure 4b.  Stenoses of the left external and right common iliac arteries in a 70-year-old woman. (a) Anteroposterior DSA image fails to depict stenosis of the left external iliac artery because of the enhancement of the overlying left internal iliac artery (straight arrow). However, stenosis of the right common iliac artery can be seen (curved arrow). (b) DSA image (30° right anterior oblique angle) allows differentiation of the enhanced left external iliac artery from the left internal iliac artery, demonstrating 60% diameter stenosis of the former artery (arrow) relative to the distal reference site (arrowhead).

 
Because projection images but not cross-sectional images can be used for quantification, only diameter reduction can be measured with DSA. The minimum luminal diameter should be sought on multiple views obtained at different angles. Treatment planning requires both (a) the ratio between the diameters of the stenosis and reference site and (b) the absolute value of the luminal diameter. In DSA, the shorter the distance between the x-ray source and the target vessel, the greater the magnification of the projected image. The obliquity of the angle between the x-ray source and the target vessel also influences luminal diameter measurement. Therefore, calibration to the correct geometric magnification is essential. This calibration is performed with use of a known distance (eg, catheter diameter in the "catheter" method, four radiopaque dots arranged in a square in the "dot" method, or a radiopaque grid in the "grid" method) (7).

Multi–Detector Row CT Angiography
Current multi–detector row CT provides high-resolution isotropic volume data that have almost equal submillimeter resolution along the x-y plane and the z axis (Table 1). A major advantage of multi–detector row CT angiography is that it can provide information about extraluminal structures, wall status (eg, calcifications, plaques), and the course of an occluded vessel, in addition to information made available with intraluminal contrast material–enhanced imaging. All of this information is essential for the success of an interventional procedure. Furthermore, optimal window width and level settings allow differentiation between luminal enhancement and adjacent high-attenuation structures such as calcifications and indwelling stents. A shortcoming of multi–detector row CT angiography is that dense calcification may degrade the accuracy of the evaluation of stenosis due to beam-hardening artifacts.

With current workstations, both original axial CT scans and reformatted images can be assessed (8). Processed images used for vascular evaluation usually include MPR images, VR images, MIP images, and CPR images. Each display technique has strengths and weaknesses with regard to quantification (Fig 5, Table 2). It is recommended that all of these techniques be used for comprehensive evaluation of arterial stenosis.



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  		Figure 5a.  (a) Drawing illustrates a tortuous vessel with upper concentric stenosis with surrounding mural calcification and lower eccentric stenosis with partial calcification. The two planes oriented perpendicular to the vessel indicate the levels at which cross-sectional images were obtained. The curved plane oriented longitudinally along the vessel indicates the cutting plane of the CPR image (cf e). (b) Drawings illustrate cross-sectional images (cf a), which accurately depict luminal configurations and mural calcifications, thereby allowing measurement of both diameter stenosis and area stenosis. (c) Drawing illustrates a VR image that provides a comprehensive overview of the vessel. The upper stenosis is partially hidden by mural calcification. (d) Drawing illustrates an MIP image; overlying mural calcification makes evaluation of the upper stenosis impossible. (e) Drawing illustrates a CPR image that depicts the upper stenosis with part of the surrounding mural calcification as well as the lower stenosis.

 


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  		Figure 5b.  (a) Drawing illustrates a tortuous vessel with upper concentric stenosis with surrounding mural calcification and lower eccentric stenosis with partial calcification. The two planes oriented perpendicular to the vessel indicate the levels at which cross-sectional images were obtained. The curved plane oriented longitudinally along the vessel indicates the cutting plane of the CPR image (cf e). (b) Drawings illustrate cross-sectional images (cf a), which accurately depict luminal configurations and mural calcifications, thereby allowing measurement of both diameter stenosis and area stenosis. (c) Drawing illustrates a VR image that provides a comprehensive overview of the vessel. The upper stenosis is partially hidden by mural calcification. (d) Drawing illustrates an MIP image; overlying mural calcification makes evaluation of the upper stenosis impossible. (e) Drawing illustrates a CPR image that depicts the upper stenosis with part of the surrounding mural calcification as well as the lower stenosis.

 


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  		Figure 5c.  (a) Drawing illustrates a tortuous vessel with upper concentric stenosis with surrounding mural calcification and lower eccentric stenosis with partial calcification. The two planes oriented perpendicular to the vessel indicate the levels at which cross-sectional images were obtained. The curved plane oriented longitudinally along the vessel indicates the cutting plane of the CPR image (cf e). (b) Drawings illustrate cross-sectional images (cf a), which accurately depict luminal configurations and mural calcifications, thereby allowing measurement of both diameter stenosis and area stenosis. (c) Drawing illustrates a VR image that provides a comprehensive overview of the vessel. The upper stenosis is partially hidden by mural calcification. (d) Drawing illustrates an MIP image; overlying mural calcification makes evaluation of the upper stenosis impossible. (e) Drawing illustrates a CPR image that depicts the upper stenosis with part of the surrounding mural calcification as well as the lower stenosis.

 


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  		Figure 5d.  (a) Drawing illustrates a tortuous vessel with upper concentric stenosis with surrounding mural calcification and lower eccentric stenosis with partial calcification. The two planes oriented perpendicular to the vessel indicate the levels at which cross-sectional images were obtained. The curved plane oriented longitudinally along the vessel indicates the cutting plane of the CPR image (cf e). (b) Drawings illustrate cross-sectional images (cf a), which accurately depict luminal configurations and mural calcifications, thereby allowing measurement of both diameter stenosis and area stenosis. (c) Drawing illustrates a VR image that provides a comprehensive overview of the vessel. The upper stenosis is partially hidden by mural calcification. (d) Drawing illustrates an MIP image; overlying mural calcification makes evaluation of the upper stenosis impossible. (e) Drawing illustrates a CPR image that depicts the upper stenosis with part of the surrounding mural calcification as well as the lower stenosis.

 


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  		Figure 5e.  (a) Drawing illustrates a tortuous vessel with upper concentric stenosis with surrounding mural calcification and lower eccentric stenosis with partial calcification. The two planes oriented perpendicular to the vessel indicate the levels at which cross-sectional images were obtained. The curved plane oriented longitudinally along the vessel indicates the cutting plane of the CPR image (cf e). (b) Drawings illustrate cross-sectional images (cf a), which accurately depict luminal configurations and mural calcifications, thereby allowing measurement of both diameter stenosis and area stenosis. (c) Drawing illustrates a VR image that provides a comprehensive overview of the vessel. The upper stenosis is partially hidden by mural calcification. (d) Drawing illustrates an MIP image; overlying mural calcification makes evaluation of the upper stenosis impossible. (e) Drawing illustrates a CPR image that depicts the upper stenosis with part of the surrounding mural calcification as well as the lower stenosis.

 

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  		Table 2. Characteristics of Image Display Techniques Used with Multi–Detector Row CT Angiography for Vascular Evaluation

 
Postprocessing Techniques
Multiplanar Reformation.— With MPR, any desired 2D section, whether oriented longitudinal or perpendicular to the target vessel, can be generated from a stack of CT scans. Isotropic data sets obtained with current multi–detector row CT allow generation of any sectional MPR images without reducing in-plane resolution.

Cross-sectional MPR images are very useful for quantitative analysis because they accurately depict lumen shape (9,10). One drawback of using cross-sectional MPR is that it is more difficult to get an overview of the arteries and to determine lesion length than with VR, MIP, or CPR. Another drawback of cross-sectional MPR is that image generation is time consuming, especially in tortuous vessels. Currently available software programs allow prompt generation of cross-sectional MPR images, thereby facilitating cross-sectional analysis (Fig 6), although manually corrected image generation may be required in cases in which automated generation is unsuitable (eg, in calcified or bifurcating vessels) (11). Both diameter reduction and area reduction can be measured with use of cross-sectional MPR images.



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  		Figure 6a.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 


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  		Figure 6b.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 


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  		Figure 6c.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 


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  		Figure 6d.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 


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  		Figure 6e.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 


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  		Figure 6f.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 


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  		Figure 6g.  Stenosis of the right external iliac artery in a 70-year-old man. The reformatted images in a–f were generated from multi–detector row CT data obtained with semiautomated software (Advanced Vessel Analysis, GE Medical Systems) for vascular analysis. (a) VR image provides an overview of the stenotic artery. The black line connects the midpoints of the cross-sectional vascular lumen and is drawn automatically after the starting point and endpoint of the target vessel have been plotted. Lines b–d indicate the levels at which cross-sectional MPR images were obtained. (b–d) Cross-sectional MPR images obtained at different levels (cf lines b–d in a) demonstrate varying degrees of luminal enhancement (arrowhead). Both luminal diameter and area (shown as percentages) were calculated automatically after level d had been chosen as the reference site. (e) Stretched CPR image (left) shows the target vessel. Diagram (right) shows the calculated luminal area throughout the vessel (gray area). Lines b–d indicate the levels at which the cross-sectional images were obtained (cf b–d), with the corresponding area measurements given in square millimeters. (f ) CPR image shows the distribution of plaques and calcifications in the vessel (arrows), together with stenoses. (g) DSA image demonstrates 60% diameter stenosis in the proximal external iliac artery (b) and 70% diameter stenosis in the distal external iliac artery (c). Line d indicates the reference site.

 
Volume Rendering.— In generating a VR image, the CT values used to construct the image are assigned as either "visible" or "invisible," with the "visible" values displayed in various colors and at various enhancement levels (8). The role of VR in multi–detector row CT angiography is to provide an overview of the target vessel (Fig 6a), since VR yields three-dimensional images that can be rotated on a workstation. However, VR has two drawbacks: (a) The appearance of the lumen can vary according to the threshold value, and (b) mural calcification or an indwelling stent may make evaluation of the vessel difficult.

Maximum Intensity Projection.— In MIP, the highest CT value along any viewing ray from the desired direction is extracted and projected onto the image. Although depth information about the spatial relationship to anatomic structures perpendicular to the projection ray is lost, the image can be rotated and viewed from any angle on a workstation to avoid the interposition of structures other than the target vessel. The bone elimination technique or partial MIP excluding bone attenuation is essential for processing vascular MIP images.

The role of MIP in multi–detector row CT angiography is to generate a DSA-like image and provide an overview of the target vessel. MIP is not suitable for the evaluation of stenosis in cases of dense calcification or indwelling stents, which may obscure contrast material in the lumen (Fig 7a).



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  		Figure 7a.  Bilateral renal artery stenoses in a 58-year-old man. Multi–detector row CT was performed following stent placement. (a) MIP image depicts bilateral indwelling stents in the renal arteries. However, it is impossible to evaluate the patency of the lumen within the stents. (b) Curved MPR image generated along the central line of the bilateral renal arteries clearly depicts luminal patency within the stents.

 


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  		Figure 7b.  Bilateral renal artery stenoses in a 58-year-old man. Multi–detector row CT was performed following stent placement. (a) MIP image depicts bilateral indwelling stents in the renal arteries. However, it is impossible to evaluate the patency of the lumen within the stents. (b) Curved MPR image generated along the central line of the bilateral renal arteries clearly depicts luminal patency within the stents.

 
Curved Planar Reformation.— A CPR image is a 2D image that is created by sampling CT volume data along an expected curved plane. A plane oriented along the longitudinal axis of the target vessel is usually used for vascular analysis (Fig 7b).

The role of CPR in multi–detector row CT angiography is to depict the distribution of stenosis, along with mural plaques and calcifications (Fig 6f). A drawback of CPR is that the value of the vascular images that are created depends on the course of the curved plane that is selected, with the possibility of generating an erroneous appearance of stenosis.

MR Angiography
Time-of-flight and contrast-enhanced MR angiography are widely used for arterial analysis, although contrast-enhanced MR angiograms are usually of better quality and take much less time to acquire. An advantage of MR angiography for quantitative measurement is that it has little artifact from calcification, although dense calcification may contribute to local susceptibility-related artifacts (12). Even when an artery is severely calcified, MR angiography can depict the luminal configuration, whereas CT may not (Fig 8). Furthermore, there is no need for bone elimination when generating three-dimensional images from MR angiographic data, which represents an advantage of MR angiography over CT.



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  		Figure 8a.  Stenosis of the left common iliac artery in a 74-year-old man. (a) MIP image from multi–detector row CT data depicts severe bilateral calcifications in the iliac arteries. The degree of luminal stenosis is not assessable at any calcified site. (b) Cross-sectional image of the left external iliac artery from multi–detector row CT data is suspicious for luminal stenosis (arrow). However, evaluation of the degree of stenosis is difficult even after setting the window width and level due to beam-hardening artifact from the severe calcification. (c) MIP image from contrast-enhanced MR angiographic data obtained at 30° right anterior and caudal oblique angles demonstrates 60% diameter stenosis of the left common iliac artery (arrow).

 


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  		Figure 8b.  Stenosis of the left common iliac artery in a 74-year-old man. (a) MIP image from multi–detector row CT data depicts severe bilateral calcifications in the iliac arteries. The degree of luminal stenosis is not assessable at any calcified site. (b) Cross-sectional image of the left external iliac artery from multi–detector row CT data is suspicious for luminal stenosis (arrow). However, evaluation of the degree of stenosis is difficult even after setting the window width and level due to beam-hardening artifact from the severe calcification. (c) MIP image from contrast-enhanced MR angiographic data obtained at 30° right anterior and caudal oblique angles demonstrates 60% diameter stenosis of the left common iliac artery (arrow).

 


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  		Figure 8c.  Stenosis of the left common iliac artery in a 74-year-old man. (a) MIP image from multi–detector row CT data depicts severe bilateral calcifications in the iliac arteries. The degree of luminal stenosis is not assessable at any calcified site. (b) Cross-sectional image of the left external iliac artery from multi–detector row CT data is suspicious for luminal stenosis (arrow). However, evaluation of the degree of stenosis is difficult even after setting the window width and level due to beam-hardening artifact from the severe calcification. (c) MIP image from contrast-enhanced MR angiographic data obtained at 30° right anterior and caudal oblique angles demonstrates 60% diameter stenosis of the left common iliac artery (arrow).

 
Like DSA, contrast-enhanced MR angiography yields a luminal image that provides little information about the vessel wall or extravascular structures. Although additional conventional MR imaging does provide such information, it reveals little about the status of the lumen within a stent or about the stent itself due to susceptibility artifact. MR angiography tends to lead to overestimation of vascular narrowing, probably owing to spin dephasing caused by turbulent flow at a stenosis site and partial volume averaging effects (Fig 9) (13).



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  		Figure 9a.  Mild stenosis of the right renal artery in a 62-year-old woman. (a) Contrast-enhanced MR angiogram shows 70% diameter stenosis of the right renal artery. (b) DSA image obtained during transcatheter measurement of arterial pressure shows only mild (45%) diameter stenosis. In addition, there was no significant pressure gradient in the artery. These findings prevented unwarranted intervention.

 


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  		Figure 9b.  Mild stenosis of the right renal artery in a 62-year-old woman. (a) Contrast-enhanced MR angiogram shows 70% diameter stenosis of the right renal artery. (b) DSA image obtained during transcatheter measurement of arterial pressure shows only mild (45%) diameter stenosis. In addition, there was no significant pressure gradient in the artery. These findings prevented unwarranted intervention.

 
For quantitative analysis of stenosis, MIP images are widely used for displaying MR angiographic findings (12,14–16). However, stenosis can be overestimated on MIP images because the subtle signal intensity at the stenotic region is obscured by background signal intensity (13). Review of the individual source images, image reformation in the transverse plane, or cross-sectional imaging is required for reliable analysis (4,17,18). Mallouhi et al (19) reported that VR images were more accurate in detecting and quantifying renal artery stenosis than were MIP images because of improved vascular delineation.

Ultrasonography
US is a widely available, relatively inexpensive, and completely noninvasive technique for vascular imaging compared with other modalities. The advantage of US is that it provides information about blood flow as well as vessel morphology.

The inherent limitations of US are that (a) sound waves cannot penetrate thick bone structures, dense calcification, or air; and (b) fat tissue can degrade vascular evaluation because of significant attenuation. Furthermore, US is operator dependent, so quality control is necessary for reliable examination.

The length of the sound wave emitted from the transducer determines spatial resolution and attenuation. As the frequency of the sound wave increases, the wavelength decreases and the spatial resolution increases (Table 1). However, the more the frequency increases, the more the ultrasound beam is attenuated. Therefore, a lower-frequency (2.5–3.5-MHz) transducer with lower spatial resolution should be used for adequate penetration of deeper structures such as the renal and iliac arteries. For superficial structures such as a carotid artery, a higher-frequency (7–10-MHz) transducer can be used, thereby allowing higher spatial resolution (20).

For quantifying vascular stenosis, both B-mode and Doppler US are used. B-mode US is used for anatomic evaluations such as vessel caliber, mural configuration, and relationship to extravascular structures. Cross-sectional images obtained perpendicular or longitudinal to the vessel are desirable.

Doppler US is used to measure blood flow velocity. The basic principle of Doppler US is as follows: When sound waves are reflected from moving blood, the frequency and wavelength of the returning waves are shifted, a phenomenon known as Doppler shift (21,22). Accordingly, blood flow velocity can be calculated using a known Doppler shift value and incident angle. A small incident angle ({theta}) of 60° or less (ie, cos {theta} ≥ 0.5) is essential to minimize measurement error (Fig 10) (21).



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  		Figure 10a.  (a) Diagram shows incident angles ({theta}) between the ultrasound beam and the direction of blood flow, either relatively narrow ({theta}1 ≤ 60°) or wider ({theta}2 > 60°). The relationship between Doppler shift ({Delta}f ) and blood flow velocity is described by the equation {Delta}f = (2f x v x cos {theta})/c, where f = frequency of the sound wave emitted by the transducer, v = velocity of blood flow, and c = velocity of the ultrasound beam in tissue ({approx}1540 m/sec). (b) Graph illustrates the relationship between blood flow velocity and incident angle when the measured value of Doppler shift ({Delta}f ) is constant. This relationship is described by the equation v = {Delta}f x (c/[2f x cos {theta}]). Because flow velocity is directly proportional to the value of 1/cos {theta}, it varies according to the incident angle. The incident angle has a potential measurement error that represents the difference ({Delta}{theta}) between its real value and its value as estimated visually by the operator. When the incident angle is less than or equal to 60° ({theta}1), the measurement error in calculated flow velocity ({Delta}V1) is relatively small. When the angle is greater than 60° ({theta}2), the error ({Delta}V2) is exaggerated. In other words, the closer the incident angle is to 90°, the greater the error in calculated velocity ({Delta}v).

 


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  		Figure 10b.  (a) Diagram shows incident angles ({theta}) between the ultrasound beam and the direction of blood flow, either relatively narrow ({theta}1 ≤ 60°) or wider ({theta}2 > 60°). The relationship between Doppler shift ({Delta}f ) and blood flow velocity is described by the equation {Delta}f = (2f x v x cos {theta})/c, where f = frequency of the sound wave emitted by the transducer, v = velocity of blood flow, and c = velocity of the ultrasound beam in tissue ({approx}1540 m/sec). (b) Graph illustrates the relationship between blood flow velocity and incident angle when the measured value of Doppler shift ({Delta}f ) is constant. This relationship is described by the equation v = {Delta}f x (c/[2f x cos {theta}]). Because flow velocity is directly proportional to the value of 1/cos {theta}, it varies according to the incident angle. The incident angle has a potential measurement error that represents the difference ({Delta}{theta}) between its real value and its value as estimated visually by the operator. When the incident angle is less than or equal to 60° ({theta}1), the measurement error in calculated flow velocity ({Delta}V1) is relatively small. When the angle is greater than 60° ({theta}2), the error ({Delta}V2) is exaggerated. In other words, the closer the incident angle is to 90°, the greater the error in calculated velocity ({Delta}v).

 
Blood flow velocity varies according to lumen status. The calculated velocity is used for characterizing arterial hemodynamics, since flow velocity usually increases in a stenotic lumen (Fig 11). Thus, various velocity-related indices have been developed for grading stenosis, such as PSV and PSV ratio, the latter being calculated by dividing the PSV at or immediately downstream from the stenosis by the PSV upstream from the stenosis. An increase in either PSV or PSV ratio indicates stenosis.



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  		Figure 11.  Drawing illustrates the changes in blood flow caused by a stenosis. The peak systolic velocity (PSV) in the proximal nonstenotic lumen (Vp) is normal. At the stenosis, flow velocity increases as the blood passes through the restricted area. The ratio between the PSV at the stenosis (Vs) and Vp is used to evaluate the degree of stenosis. A jet is observed in the poststenotic lumen, with subsequent turbulent flow near the vessel wall in cases of severe stenosis.

 
The pitfalls of blood flow velocity measurement are as follows: (a) the velocity is also influenced by clinical factors such as cardiac function; and (b) in cases of tandem stenoses, false-negative findings may occur at the distal stenosis due to decreased proximal flow. Both pitfalls may be partially eliminated by calculating PSV ratio instead of PSV.

Intravascular US
Because intravascular US is an invasive and relatively costly imaging modality, its major role in the clinical setting is limited to guiding various catheter-based interventions in patients with vascular disease (23).

Intravascular US can provide high-resolution images from inside the vessel (Table 1). For a currently used (20–40-MHz) transducer, the resolution is about .05–.08 mm in an axial direction (ie, parallel to the ultrasound beam). Intravascular US can depict mural structures including the intima, media and adventitia, and atherosclerotic plaques, as well as help measure the luminal area (Fig 12) (24–26).



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  		Figure 12a.  Intravascular US images of a peripheral artery show the residual lumen as an anechoic area (dotted circle in b). The outer echolucent layer (dashed circle in b) represents the media, the relatively echogenic area between the circles represents thickened intima containing plaque, and the echogenic area outside the dashed circle represents the adventitia and periadventitial tissues.

 


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  		Figure 12b.  Intravascular US images of a peripheral artery show the residual lumen as an anechoic area (dotted circle in b). The outer echolucent layer (dashed circle in b) represents the media, the relatively echogenic area between the circles represents thickened intima containing plaque, and the echogenic area outside the dashed circle represents the adventitia and periadventitial tissues.

 
Transducer obliquity or a tortuous vascular course can result in suboptimal quality at oblique cross-sectional imaging, resulting in overestimation of dimensions and a false impression that the vessel is elliptic (26). An intravascular US catheter that is larger than practicable for the lumen may fail to cross the stenotic lesion or cause mechanical disruption of plaques (27).

Stenosis is quantified with appropriately obtained cross-sectional images. Lumen measurements are made on the basis of the interface between the lumen (echolucent region) and the leading edge of the intima (echogenic region). Both the cross-sectional area and the diameter of the lesion can be measured to calculate the degree of stenosis (26).


   Quantification of Arterial Occlusive Disease in Selected Arteries
Top
 Abstract
 Introduction
 Technical Considerations
 Imaging in the Quantification...
 Quantification of Arterial...
Conclusions
 References
 
Internal Carotid Artery
In major clinical studies from the North American Symptomatic Carotid Endarterectomy Trial (NASCET) and the European Symptomatic Carotid Surgery Trial (ECST), it has been established that carotid endarterectomy is more effective than medical therapy in reducing stroke risk in symptomatic patients with severe (70%–99%) carotid artery stenosis (28,29). Carotid endarterectomy may also be beneficial for asymptomatic patients with carotid artery stenosis of less than 60% (30). Thus, quantification of stenosis of the ICA is important for patient treatment. Because the reference sites used to calculate the degree of diametric stenosis differ between the NASCET and the ECST (Fig 13), it should be noted which reference site was used for quantification.



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  		Figure 13.  Drawing illustrates how the percentage of diameter stenosis in an ICA is measured in the NASCET and the ESCT. The NASCET uses the formula 1 – (a/c), where a is the residual luminal diameter at the stenosis and c is the luminal diameter at a visible, disease-free point above the stenosis. The ECST uses the formula 1 – (a/b), where b is the estimated luminal diameter at the level of the lesion based on a visual impression of where the normal arterial wall was before development of the stenosis. CCA = common carotid artery, ECA = external carotid artery.

 
US plays an important role in the evaluation of carotid artery stenosis because it is by far the most common imaging modality in this setting and carotid endarterectomy is performed solely on the basis of US findings at many institutions (31,32). A consensus statement by the Society of Radiologists in Ultrasound regarding US criteria for the diagnosis of ICA stenosis was recently published (Fig 14, Table 3) (31). US measurement of flow velocity in the ICA has a specific pitfall: If an ICA has significant stenosis, a compensatory increase in blood flow frequently occurs in the contralateral ICA to maintain intracranial blood flow. Thus, overestimation of stenosis may occur in ICAs with less than 70% stenosis that are contralateral to ICAs with greater than 70% stenosis or an occlusion (33).



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  		Figure 14a.  Stenosis of the left ICA in a 59-year-old man. (a) Color Doppler flow US image shows more than 50% stenosis caused by plaques at the origin of the ICA. (b) Duplex US image shows a PSV of 296 cm/sec, a finding that indicates a stenosis of at least 70%. (c) MIP image from multi–detector row CT data shows the levels at which cross-sectional images were obtained (d, e). (d, e) Cross-sectional images obtained at the stenotic site (d) and the reference site (e) demonstrate 70% diameter stenosis and 90% area stenosis. (f ) DSA image shows 70% diameter stenosis (as measured with NASCET criteria).

 


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  		Figure 14b.  Stenosis of the left ICA in a 59-year-old man. (a) Color Doppler flow US image shows more than 50% stenosis caused by plaques at the origin of the ICA. (b) Duplex US image shows a PSV of 296 cm/sec, a finding that indicates a stenosis of at least 70%. (c) MIP image from multi–detector row CT data shows the levels at which cross-sectional images were obtained (d, e). (d, e) Cross-sectional images obtained at the stenotic site (d) and the reference site (e) demonstrate 70% diameter stenosis and 90% area stenosis. (f ) DSA image shows 70% diameter stenosis (as measured with NASCET criteria).

 


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  		Figure 14c.  Stenosis of the left ICA in a 59-year-old man. (a) Color Doppler flow US image shows more than 50% stenosis caused by plaques at the origin of the ICA. (b) Duplex US image shows a PSV of 296 cm/sec, a finding that indicates a stenosis of at least 70%. (c) MIP image from multi–detector row CT data shows the levels at which cross-sectional images were obtained (d, e). (d, e) Cross-sectional images obtained at the stenotic site (d) and the reference site (e) demonstrate 70% diameter stenosis and 90% area stenosis. (f ) DSA image shows 70% diameter stenosis (as measured with NASCET criteria).

 


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  		Figure 14d.  Stenosis of the left ICA in a 59-year-old man. (a) Color Doppler flow US image shows more than 50% stenosis caused by plaques at the origin of the ICA. (b) Duplex US image shows a PSV of 296 cm/sec, a finding that indicates a stenosis of at least 70%. (c) MIP image from multi–detector row CT data shows the levels at which cross-sectional images were obtained (d, e). (d, e) Cross-sectional images obtained at the stenotic site (d) and the reference site (e) demonstrate 70% diameter stenosis and 90% area stenosis. (f ) DSA image shows 70% diameter stenosis (as measured with NASCET criteria).

 


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  		Figure 14e.  Stenosis of the left ICA in a 59-year-old man. (a) Color Doppler flow US image shows more than 50% stenosis caused by plaques at the origin of the ICA. (b) Duplex US image shows a PSV of 296 cm/sec, a finding that indicates a stenosis of at least 70%. (c) MIP image from multi–detector row CT data shows the levels at which cross-sectional images were obtained (d, e). (d, e) Cross-sectional images obtained at the stenotic site (d) and the reference site (e) demonstrate 70% diameter stenosis and 90% area stenosis. (f ) DSA image shows 70% diameter stenosis (as measured with NASCET criteria).

 


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  		Figure 14f.  Stenosis of the left ICA in a 59-year-old man. (a) Color Doppler flow US image shows more than 50% stenosis caused by plaques at the origin of the ICA. (b) Duplex US image shows a PSV of 296 cm/sec, a finding that indicates a stenosis of at least 70%. (c) MIP image from multi–detector row CT data shows the levels at which cross-sectional images were obtained (d, e). (d, e) Cross-sectional images obtained at the stenotic site (d) and the reference site (e) demonstrate 70% diameter stenosis and 90% area stenosis. (f ) DSA image shows 70% diameter stenosis (as measured with NASCET criteria).

 

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  		Table 3. Gray-Scale and Doppler US Criteria for Diagnosis of ICA Stenosis

 
CT angiography and MR angiography are also reliable tools for detecting carotid artery stenosis (34). These modalities are superior to US in that they are relatively objective, less operator dependent, and allow arterial evaluation from the aortic arch to the intracranial arteries.

The sensitivities and specificities of US, CT angiography, and MR angiography for detecting 70%–99% stenosis of the ICA are compared with those of DSA in Table 4 (35,36).


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  		Table 4. US, CT Angiography, and MR Angiography versus DSA in the Detection of 70%–99% Stenosis of the ICA

 
Although treatment strategy has been based on the results of large trials in which DSA was the standard of reference, the aforementioned minimally invasive modalities are now starting to replace DSA. Currently, stent placement is emerging as a treatment for ICA stenosis, and some suggest that it may be as effective as surgery (37). Thus, DSA will remain as a diagnostic tool during endovascular therapy for ICA stenosis.

Renal Arteries
Because significant renal artery stenosis causing renovascular hypertension is potentially curable with vascular intervention (38–40), quantification of the degree of stenosis is essential for patient treatment.

Because it is difficult to visualize the entire main renal artery at US due to its deeper location, flow velocity measurements made with Doppler analysis are used for grading renal artery stenosis (Fig 15, Table 5) (40,41). The reported sensitivities and specificities of Doppler US for detecting significant renal artery stenosis compared with DSA are 84%–98% and 90%–98%, respectively (41–45). Specific drawbacks of US in evaluating the renal artery are (a) degrading factors such as interposition of bowel gas and obesity and (b) difficulty in visualizing accessory renal arteries that may play a role in the development of renovascular hypertension.



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  		Figure 15a.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 


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  		Figure 15b.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 


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  		Figure 15c.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 


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  		Figure 15d.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 


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  		Figure 15e.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 


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  		Figure 15f.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 


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  		Figure 15g.  Left renal artery stenosis in a 65-year-old woman. (a) Duplex US image shows a PSV of 328 cm/sec in the left renal artery, a finding that suggests significant stenosis. (b) VR image (20° left anterior oblique angle) from multi–detector row CT data shows the levels at which cross-sectional images were obtained (c, d). (c, d) Cross-sectional images demonstrate a stenosis (c) causing 62% diameter reduction and 85% area reduction relative to the reference site (d). (e) DSA image (20° left anterior oblique angle) obtained prior to angioplasty shows 65% diameter stenosis in the left main renal artery. (f, g) Intravascular US images obtained at the stenotic site (f ) and the distal reference site (g) show 60% diameter stenosis and 80% area stenosis with thick plaque (red-orange area).

 

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  		Table 5. Duplex US Criteria for Diagnosis of Renal Artery Stenosis

 
CT angiography and contrast-enhanced MR angiography are reliable modalities for evaluating renal artery stenosis. In a study by Willmann et al (46), there was no significant difference between the two modalities in the detection of significant stenoses of the aortoiliac and renal arteries, each having a sensitivity and a specificity of approximately 92% and 99%, respectively. Furthermore, a meta-analysis indicated that both modalities were superior to US in detecting renal artery stenosis (47). Noninvasive US can be used for screening and follow-up, whereas contrast-enhanced multi–detector row CT angiography or MR angiography may become a first-line modality for scrutiny. The role of DSA will be limited to treatment-related procedures performed during percutaneous revascularization.

Atherosclerotic stenosis most often involves the proximal one-third of the main renal arteries (48), and the right renal artery tends to originate from the aorta anterolaterally whereas the left one does so posterolaterally or laterally (49). Consequently, evaluation of renal artery stenosis with projection images requires an appropriate viewing angle to avoid overlap of the origins of the renal arteries with their parent artery, the aorta. In measuring the degree of renal artery stenosis with DSA, CT angiography, or MR angiography, the reference site should be a normal-looking segment proximal to the stenosis or distal to the poststenotic dilatation (40).

Lower Extremity Arteries
Planning of a therapeutic procedure for patients with advanced peripheral occlusive disease requires information about the degree and length of the lesion and the configuration of the inflow and runoff vessels.

Although the US diagnostic criteria for detecting significant stenosis of lower extremity arteries are based on Doppler US findings in many studies (Table 6), consensus has not been reached on the validated cutoff value. Some studies apply a PSV ratio of 2.0 as a means of differentiating between stenoses of less than 50% diameter reduction and those of greater than 50% reduction, whereas others use 2.5 or 3.0 as a cutoff value (50,51). A meta-analysis by Visser and Hunink (50) compared the diagnostic performance of duplex US with that of conventional angiography in detecting significant stenosis from peripheral arterial occlusive disease. Duplex US demonstrated a sensitivity of 87.6% and a specificity of 94.7% and less discriminatory power than MR angiography. Furthermore, duplex US evaluation of the arteries of an entire lower extremity is operator dependent and labor intensive and does not provide a "road map" equivalent to that provided by angiography. Thus, duplex US would be expected to play only a supplementary role in evaluating restricted segments of lower extremity arteries.


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  		Table 6. Duplex US Criteria for Diagnosis of Stenosis of Lower Extremity Arteries

 
Both multi–detector row CT angiography and contrast-enhanced moving-bed MR angiography allow evaluation of the entire aortoiliac and lower extremity arteries in a single series. The recently reported sensitivities and specificities of four–detector row CT angiography compared with DSA in detecting significant stenoses of the aortoiliac and lower extremity arteries are 91%–99% and 83%–99%, respectively (10,52–55), whereas those of contrast-enhanced MR angiography are 93%–96% and 90%–95%, respectively (56,57). The disadvantage of the two modalities is that the evaluation of infrapopliteal arteries might be suboptimal due to limited spatial resolution or venous contamination (53,55,57). This problem may be overcome by using (a) CT scanners with 16 or more detector rows with submillimeter collimation, or (b) MR angiographic techniques such as additional dedicated imaging of calf arteries with a reduced field of view and sensitivity encoding (58,59).

The feasibility of visualizing the course of the occluded segment is a great advantage of multi–detector row CT angiography in planning vascular intervention or bypass surgery, especially for lower peripheral arteries (Fig 16). A specific pitfall of multi–detector row CT angiography can occur when the timing of scanning does not coincide with the arrival of contrast material in the targeted lower extremity arteries. This discrepancy may cause misdiagnosis or overestimation of stenotic occlusion in calf arteries due to insufficient luminal enhancement in conditions such as low cardiac output or aortic aneurysm. This problem may be overcome with additional scanning of calf arteries immediately following normal scanning (Fig 17).



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  		Figure 16a.  Occlusion of the right iliac artery in a 72-year-old man. (a) Coronal CPR image from multi–detector row CT angiographic data shows the right iliac artery with a 7-cm-long occluded segment (arrows), a finding that allowed planning of vascular intervention. (b) DSA image shows occlusion of the right iliac artery but provides no information about the occluded segment. The segment distal to the occlusion is faintly enhanced (arrow). (c) DSA image obtained just after stent placement shows revascularization of the artery.

 


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  		Figure 16b.  Occlusion of the right iliac artery in a 72-year-old man. (a) Coronal CPR image from multi–detector row CT angiographic data shows the right iliac artery with a 7-cm-long occluded segment (arrows), a finding that allowed planning of vascular intervention. (b) DSA image shows occlusion of the right iliac artery but provides no information about the occluded segment. The segment distal to the occlusion is faintly enhanced (arrow). (c) DSA image obtained just after stent placement shows revascularization of the artery.

 


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  		Figure 16c.  Occlusion of the right iliac artery in a 72-year-old man. (a) Coronal CPR image from multi–detector row CT angiographic data shows the right iliac artery with a 7-cm-long occluded segment (arrows), a finding that allowed planning of vascular intervention. (b) DSA image shows occlusion of the right iliac artery but provides no information about the occluded segment. The segment distal to the occlusion is faintly enhanced (arrow). (c) DSA image obtained just after stent placement shows revascularization of the artery.

 


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  		Figure 17a.  Peripheral arterial occlusive disease in a 70-year-old man. (a) MIP image from multi–detector row CT angiographic data obtained with the usual protocol shows insufficient bilateral enhancement of the calf arteries because of excessive table speed relative to blood flow velocity. Because this insufficient enhancement was noticed at real-time monitoring, additional scanning was performed to evaluate the infrapopliteal arteries. (b) MIP image from data obtained during scanning of the infrapopliteal arteries shows sufficient luminal enhancement of the calf arteries. Note the bilateral occlusion of the peroneal arteries.

 


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  		Figure 17b.  Peripheral arterial occlusive disease in a 70-year-old man. (a) MIP image from multi–detector row CT angiographic data obtained with the usual protocol shows insufficient bilateral enhancement of the calf arteries because of excessive table speed relative to blood flow velocity. Because this insufficient enhancement was noticed at real-time monitoring, additional scanning was performed to evaluate the infrapopliteal arteries. (b) MIP image from data obtained during scanning of the infrapopliteal arteries shows sufficient luminal enhancement of the calf arteries. Note the bilateral occlusion of the peroneal arteries.

 
DSA has largely been replaced by other less invasive modalities for evaluating lower extremity arteries, although it may allow more accurate evaluation of small calf arteries with a higher spatial resolution. The role of DSA will also be limited to treatment-related procedures for vascular intervention in lower extremity arteries.


   Conclusions
Top
 Abstract
 Introduction
 Technical Considerations
 Imaging in the Quantification...
 Quantification of Arterial...
 Conclusions
References
 
To obtain accurate measurements in arterial occlusive disease, it is essential to understand the technical considerations in quantitative measurement and to take advantage of the strengths and recognize the weaknesses of individual modalities. Precise quantification will allow optimization of therapy for patients with arterial occlusive disease.


   Footnotes
 

Abbreviations: CPR = curved planar reformation, DSA = digital subtraction angiography, ECST = European Symptomatic Carotid Surgery Trial, ICA = internal carotid artery, MIP = maximum intensity projection, MPR = multiplanar reformation, NASCET = North American Symptomatic Carotid Endarterectomy Trial, PSV = peak systolic velocity, VR = volume rendering, 2D = two-dimensional


   References
Top
 Abstract
 Introduction
 Technical Considerations
 Imaging in the Quantification...
 Quantification of Arterial...
 Conclusions
 References
 

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