DOI: 10.1148/rg.243035126
RadioGraphics 2004;24:637-655
© RSNA, 2004
CT Angiography of Intracranial Aneurysms: A Focus on Postprocessing1
Bernd F. Tomandl, MD,
Niels C. Köstner,
Miriam Schempershofe,
Walter J. Huk, MD,
Christian Strauss, MD,
Lars Anker, MD and
Peter Hastreiter, PhD
1 From the Department of Neurosurgery, University of Erlangen-Nuremberg, Schwabachanlage 6, D-91054 Erlangen, Germany. Presented as an education exhibit at the 2002 RSNA scientific assembly. Received May 5, 2003; revision requested June 13 and received July 30; accepted July 31. All authors have no financial relationships to disclose. Address correspondence to B.F.T. (e-mail: tomandl@neuroradiologie-erlangen.de).
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Abstract
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Computed tomographic (CT) angiography is a well-known tool for detection of intracranial aneurysms and the planning of therapeutic intervention. Despite a wealth of existing studies and an increase in image quality due to use of multisection CT and increasingly sophisticated postprocessing tools such as direct volume rendering, CT angiography has still not replaced digital subtraction angiography as the standard of reference for detection of intracranial aneurysms. One reason may be that CT angiography is still not a uniformly standardized method, particularly with regard to image postprocessing. Several methods for two- and three-dimensional visualization can be used: multiplanar reformation, maximum intensity projection, shaded surface display, and direct volume rendering. Pitfalls of CT angiography include lack of visibility of small arteries, difficulty differentiating the infundibular dilatation at the origin of an artery from an aneurysm, the kissing vessel artifact, demonstration of venous structures that can simulate aneurysms, inability to identify thrombosis and calcification on three-dimensional images, and beam hardening artifacts produced by aneurysm clips. Finally, an algorithm for the safe and useful application of CT angiography in patients with subarachnoid hemorrhage has been developed, which takes into account the varying quality of equipment and software at different imaging centers.
© RSNA, 2004
Index Terms: Aneurysm, CT, 17.12116, 17.73 • Aneurysm, intracranial, 17.73 • Computed tomography (CT), angiography, 17.12116 • Computed tomography (CT), image processing, 17.12117
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LEARNING OBJECTIVES FOR TEST 1
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After reading this article and taking the test, the reader will be able to:
- Describe the technique of CT angiography performed for detection of intracranial aneurysms.
- List the technical considerations in and limitations of the most common methods of image postprocessing.
- Discuss when to use CT angiography as the sole diagnostic test in a patient with subarachnoid hemorrhage before therapy.
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Introduction
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Sudden onset of vigorous headache typically is the leading symptom in patients with subarachnoid hemorrhage (SAH) caused by the rupture of an intracranial aneurysm. Computed tomography (CT) is the first step in the examination of these patients. Once SAH is confirmed, it is paramount to detect the source of bleeding in order to initiate therapy. Digital subtraction angiography (DSA) is still the most sensitive tool for the detection of intracranial aneurysms. The selective intraarterial injection of contrast medium ensures optimal enhancement of the intracranial arteries with superior resolution compared with that of CT or magnetic resonance (MR) angiography. However, DSA has the disadvantage of being an invasive study. The risk of acquiring a permanent neurologic deficit with cerebral angiography in patients with SAH is below 0.1% (1). Despite this relatively low risk, a noninvasive method yielding three-dimensional (3D) information for the planning of therapeutic intervention is desirable.
The reported sensitivity of CT angiography lies in the range of 80%–97% (2–8) depending on the size and location of an aneurysm (3). In all of these studies, some kind of 3D visualization was used to analyze the CT angiography data. Little is known about the influence of postprocessing methods like maximum intensity projection (MIP), shaded surface display (SSD), and direct volume rendering (dVR) on the detection rate of intracranial aneurysms. However, it can be assumed that the same CT angiography data may lead to varying detection rates when different visualization strategies, computer platforms, and graphics hardware are used (9–12).
In the first section of this article, technical aspects of CT angiography with a focus on data acquisition are discussed. In the second section, different methods for the postprocessing of CT data are presented, including the analysis of source images and the methods currently available for two-dimensional (2D) and 3D postprocessing, such as high-resolution dVR. Then, typical pitfalls encountered while working with CT angiography data are demonstrated. Finally, we propose a reasonable paradigm for the use of CT angiography in patients with SAH, taking into account that this method is still not a standardized procedure.
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Technique of Intracranial CT Angiography
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CT angiography can be defined as a fast thin-section volumetric spiral (helical) CT examination performed with a time-optimized bolus of contrast medium in order to enhance the cerebral arteries (13). In order to visualize the intracranial arteries, the examination includes the region from the first vertebral body up to the vertex. It is important to include the atlas in the study to ensure incorporation of the posterior inferior cerebellar artery (PICA), which has an extracranial origin from the vertebral arteries in about 18% of cases (14).
On our four-row multisection scanner (Somatom 4 Volume Zoom; Siemens Medical Solutions, Erlangen, Germany), we used the following parameters: 120 kVp, 200 mAs, collimation of 4 x 1 mm, table feed of 2.7 mm per rotation, and rotation time of 0.5 seconds. Image reconstruction parameters were as follows: section thickness of 1.25 mm, overlapping steps of 0.5 mm, and field of view (FOV) of 120 mm2. The applied narrow FOV of 120 mm2 leads to an excellent in-plane resolution (0.23 x 0.23 mm2) and reproduces all relevant information (Fig 1). In addition, lateral parts of the skull are already eliminated, which simplifies the postprocessing of source data. It is possible to perform reconstructions in steps of 0.23 mm to produce isotropic data (15), thus yielding voxels of equal extent in all three dimensions. In our experience, this does not noticeably increase image quality while doubling the number of source images, thus leading to an extension of time spent on postprocessing source data.
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Figure 1a. Effect of FOV on image quality of 3D imaging with SSD in a patient with two aneurysms (arrows in b-d) at the bifurcation of the left middle cerebral artery (MCA). (a) CT image shows the areas covered by three different values for FOV: 200, 120, and 60 mm2. (b) SSD image obtained by reconstructing the data with an FOV of 200 mm2. Arteries appear blurred. (c) SSD image obtained by reconstructing the data with an FOV of 120 mm2. Vascular anatomy is shown more clearly than in b. This FOV contains all relevant intracranial arteries from which aneurysms usually originate while providing good in-plane resolution. Thus, we always use this FOV for detection of intracranial aneurysms. (d) SSD image obtained by reconstructing the data with an FOV of 60 mm2. There is even better demonstration of vascular anatomy than in c. It is sometimes useful to perform a second reconstruction with a narrow FOV such as this when CT angiography is used for therapy planning and very detailed information is required.
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Figure 1b. Effect of FOV on image quality of 3D imaging with SSD in a patient with two aneurysms (arrows in b-d) at the bifurcation of the left middle cerebral artery (MCA). (a) CT image shows the areas covered by three different values for FOV: 200, 120, and 60 mm2. (b) SSD image obtained by reconstructing the data with an FOV of 200 mm2. Arteries appear blurred. (c) SSD image obtained by reconstructing the data with an FOV of 120 mm2. Vascular anatomy is shown more clearly than in b. This FOV contains all relevant intracranial arteries from which aneurysms usually originate while providing good in-plane resolution. Thus, we always use this FOV for detection of intracranial aneurysms. (d) SSD image obtained by reconstructing the data with an FOV of 60 mm2. There is even better demonstration of vascular anatomy than in c. It is sometimes useful to perform a second reconstruction with a narrow FOV such as this when CT angiography is used for therapy planning and very detailed information is required.
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Figure 1c. Effect of FOV on image quality of 3D imaging with SSD in a patient with two aneurysms (arrows in b-d) at the bifurcation of the left middle cerebral artery (MCA). (a) CT image shows the areas covered by three different values for FOV: 200, 120, and 60 mm2. (b) SSD image obtained by reconstructing the data with an FOV of 200 mm2. Arteries appear blurred. (c) SSD image obtained by reconstructing the data with an FOV of 120 mm2. Vascular anatomy is shown more clearly than in b. This FOV contains all relevant intracranial arteries from which aneurysms usually originate while providing good in-plane resolution. Thus, we always use this FOV for detection of intracranial aneurysms. (d) SSD image obtained by reconstructing the data with an FOV of 60 mm2. There is even better demonstration of vascular anatomy than in c. It is sometimes useful to perform a second reconstruction with a narrow FOV such as this when CT angiography is used for therapy planning and very detailed information is required.
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Figure 1d. Effect of FOV on image quality of 3D imaging with SSD in a patient with two aneurysms (arrows in b-d) at the bifurcation of the left middle cerebral artery (MCA). (a) CT image shows the areas covered by three different values for FOV: 200, 120, and 60 mm2. (b) SSD image obtained by reconstructing the data with an FOV of 200 mm2. Arteries appear blurred. (c) SSD image obtained by reconstructing the data with an FOV of 120 mm2. Vascular anatomy is shown more clearly than in b. This FOV contains all relevant intracranial arteries from which aneurysms usually originate while providing good in-plane resolution. Thus, we always use this FOV for detection of intracranial aneurysms. (d) SSD image obtained by reconstructing the data with an FOV of 60 mm2. There is even better demonstration of vascular anatomy than in c. It is sometimes useful to perform a second reconstruction with a narrow FOV such as this when CT angiography is used for therapy planning and very detailed information is required.
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For enhancement of intracranial arteries, 100 mL of contrast medium (Ultravist 300; Schering, Berlin, Germany) was injected intravenously at a flow rate of 4 mL/sec by using a power injector (EnVision CT injector; Medrad, Indianola, Pa). A bolus tracking method (16) was used routinely to achieve optimal synchronization of contrast medium flow and scanning. Once the injection is started, the bolus tracking software measures attenuation values within one internal carotid artery (ICA), and the spiral scan is automatically started as soon as a threshold of 100 HU is exceeded.
If bolus tracking is not available, the test bolus method should be applied to calibrate timing of the data acquisition (17): Ten seconds after bolus injection of 20 mL of contrast medium, a dynamic single-axial-section study (one scan every 2 seconds) at the level of the first cervical vertebral body is started until the contrast material appears as hyperattenuating spots in the ICAs. By using this technique, the time interval between bolus administration and the beginning of data acquisition can also be determined individually.
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Analysis of CT Angiograms
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The examples shown in this article were created with the regular software of the workstation supplied with a Somatom Volume Zoom CT scanner (Syngo Wizard version VA 40C; Siemens Medical Solutions). The dVR images were created on a separate workstation (Syngo Leonardo 2002B; Siemens Medical Solutions).
Prior to any kind of postprocessing, such as 3D visualization, a detailed review of the source images that are the basis of CT angiography is mandatory (18). These source images contain the entire information that is available from the data. Even the most sophisticated methods for 3D imaging will lead to a distinct loss of data and, thus, potentially important information. Partial thrombosis or calcification of an aneurysm will be missed if the source images are not reviewed in a meticulous way. The interactive analysis of the source images should be done on a workstation rather than by looking at hard copies in order to develop a better perception of the course and the relationships of the intracranial arteries of interest. A wide window setting is necessary to enable differentiation between arteries filled with contrast medium, bone, and calcifications (Fig 2).
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Figure 2. Importance of an adequate window setting to demonstrate the intracranial arteries within the skull base. CT image obtained with a window width of 500 HU and a center of 150 HU. Both ICAs can be clearly differentiated within the carotid canals (arrows).
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Many aneurysms can already be detected by analyzing the source images. Smaller aneurysms below 5 mm in diameter are often difficult to detect on the basis of source images alone. Therefore, several methods for 2D and 3D postprocessing have been developed that allow more detailed analysis and in addition an "angiographic" representation of CT angiography data.
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Postprocessing of CT Angiography Data
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The basic principle of 2D and 3D postprocessing is to input cross-sectional images into a computer and thereby to create a so-called volume. One can imagine that this procedure is like putting back together the pieces of a sliced potato. Once a volume is created, several methods for 2D and 3D visualization exist (19). The easiest way to analyze a volumetric data set is multiplanar reformation (MPR), in which from a given angle of view a plane is reconstructed in a defined depth of the volume. This way it is possible to create coronal, axial, sagittal, as well as any kind of oblique sections. The quality of the reconstructions depends on the voxel size. With the use of isometric data (ie, voxels have the same depth, length, and height), all images are of the same quality as the basic source images (20). In contrast to MIP and the 3D methods discussed later, the reconstructed planes contain all information that is contained in the source images. Therefore, MPR should always be the method of first choice for the further examination of CT angiography data (21).
To create useful "angiographic" representations from CT angiography data, it is always necessary to eliminate disturbing structures from the volume in order to ensure an unobstructed view of the circle of Willis and its related arteries. For this purpose, several graphical tools exist that vary depending on the software of the workstation used. To eliminate the straight sinus and other veins that always prohibit an unobstructed view of the basilar artery, a so-called clip plane can be applied parallel to the clivus (Fig 3). This kind of data manipulation is always necessary when MIP or 3D visualization of CT angiography data is performed by using one of the following methods.
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Figure 3a. Preparation of the volume for analysis. (a) Posterosuperior image shows large veins (arrowheads), which are typically also visible in CT angiography of intracranial vessels and preclude an unobstructed view of the circle of Willis and the basilar artery (arrow). (b) Left lateroposterior image shows easy elimination of the most obscuring venous structures by using a clip plane (dotted white line) parallel to the clivus. (c) Posterosuperior image obtained after application of the clip plane (dotted white line) shows that the basilar artery is demonstrated completely (arrowheads).
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Figure 3b. Preparation of the volume for analysis. (a) Posterosuperior image shows large veins (arrowheads), which are typically also visible in CT angiography of intracranial vessels and preclude an unobstructed view of the circle of Willis and the basilar artery (arrow). (b) Left lateroposterior image shows easy elimination of the most obscuring venous structures by using a clip plane (dotted white line) parallel to the clivus. (c) Posterosuperior image obtained after application of the clip plane (dotted white line) shows that the basilar artery is demonstrated completely (arrowheads).
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Figure 3c. Preparation of the volume for analysis. (a) Posterosuperior image shows large veins (arrowheads), which are typically also visible in CT angiography of intracranial vessels and preclude an unobstructed view of the circle of Willis and the basilar artery (arrow). (b) Left lateroposterior image shows easy elimination of the most obscuring venous structures by using a clip plane (dotted white line) parallel to the clivus. (c) Posterosuperior image obtained after application of the clip plane (dotted white line) shows that the basilar artery is demonstrated completely (arrowheads).
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Maximum Intensity Projection
The term maximum intensity projection (MIP) means that from any given angle of view only the brightest voxels of a volume are collected and used to create an image (22). Therefore, MIP is not a 3D method, as it creates 2D images in which voxels from different locations within the volume are collapsed into one plane. Thus, depth information is lost and it is not possible to tell whether a structure is located in the front or back on the basis of a single MIP image. Because calcifications and bone are brighter than contrast material–filled arteries, it is possible to differentiate levels of attenuation (eg, to recognize a calcified artery).
The use of MIP as a method to create CT angiograms is limited due to the fact that the skull base has a much higher attenuation than the intracranial arteries and therefore has to be eliminated when MIP is used for image reconstruction (23). When dealing with intracranial aneurysms, it is often not possible to clearly depict the relations of the aneurysm to its adjacent arteries. Furthermore, with the use of MIP, small aneurysms will often be missed as they are eclipsed by the signal of their parent vessels averaged into the same 2D plane (Fig 4).
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Figure 4a. Influence of 3D visualization techniques on the detection of intracranial aneurysms. (a) MIP image (superior view) shows the bifurcation of the left MCA (arrow). Owing to the lack of depth information, the image does not allow visualization of two aneurysms at this site. (b, c) SSD (b) and dVR (c) images (superior views) of the bifurcation of the left MCA show the two aneurysms (arrows).
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Figure 4b. Influence of 3D visualization techniques on the detection of intracranial aneurysms. (a) MIP image (superior view) shows the bifurcation of the left MCA (arrow). Owing to the lack of depth information, the image does not allow visualization of two aneurysms at this site. (b, c) SSD (b) and dVR (c) images (superior views) of the bifurcation of the left MCA show the two aneurysms (arrows).
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Figure 4c. Influence of 3D visualization techniques on the detection of intracranial aneurysms. (a) MIP image (superior view) shows the bifurcation of the left MCA (arrow). Owing to the lack of depth information, the image does not allow visualization of two aneurysms at this site. (b, c) SSD (b) and dVR (c) images (superior views) of the bifurcation of the left MCA show the two aneurysms (arrows).
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In contrast to the two methods for 3D visualization described later, MIP is not threshold dependent and therefore is relatively easy to use. It has to be kept in mind that MIP uses only about 10% of the information contained in a given volume. In our experience, MIP is of minor use for the creation of CT angiograms in order to search for and analyze aneurysms but is often very helpful when used interactively on the workstation in thin sections of about 10–20 mm in addition to MPR (24) (Fig 5). In contrast to threshold-dependent methods, smaller arteries are displayed without user interaction (22).
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Figure 5a. Analysis of CT angiography data with MPR and thin-section MIP. (a-c) Sagittal (a), coronal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on threshold-based 3D images. (d-f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sections of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d).
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Figure 5b. Analysis of CT angiography data with MPR and thin-section MIP. (a-c) Sagittal (a), coronal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on threshold-based 3D images. (d-f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sections of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d).
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Figure 5c. Analysis of CT angiography data with MPR and thin-section MIP. (a-c) Sagittal (a), coronal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on threshold-based 3D images. (d-f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sections of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d).
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Figure 5d. Analysis of CT angiography data with MPR and thin-section MIP. (a-c) Sagittal (a), coronal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on threshold-based 3D images. (d-f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sections of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d).
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Figure 5e. Analysis of CT angiography data with MPR and thin-section MIP. (a-c) Sagittal (a), coronal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on threshold-based 3D images. (d-f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sections of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d).
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Figure 5f. Analysis of CT angiography data with MPR and thin-section MIP. (a-c) Sagittal (a), coronal (b), and axial (c) MPR images show a small aneurysm at the bifurcation of the right MCA (arrow). Note the large intracerebral hematoma (arrowheads in a), which is usually not demonstrated on threshold-based 3D images. (d-f) Sagittal (d), coronal (e), and axial (f) MIP images obtained with thin sections of 20 mm show the aneurysm more clearly (arrow) and show the intracerebral hematoma as well (arrowheads in d).
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Threshold-dependent Methods for 3D Visualization
Shaded surface display (SSD) and direct volume rendering (dVR) are more difficult to use than MIP. They require the user to define thresholds for the selection of voxels on the basis of their attenuation (measured in Hounsfield units). For SSD, typically upper and lower thresholds are defined and from a chosen angle of view the first layer of voxels with an attenuation within the defined parameters is displayed. Therefore, the images show the surface of these structures and provide valuable information about the 3D shape of an object (25). On the other hand, all structures are shown in the same color and information about the attenuation of a structure is lost completely. For example, it is not possible to see calcifications within an artery on SSD images. Since MIP retains information about the attenuation of objects yet does not allow the depth perception provided by SSD, both methods may be used to complement one another.
The definition of the thresholds is performed interactively by the user and significantly influences the appearance of the vascular structures (7) (Fig 6). Setting the lower threshold to a low value (eg, 100 HU) will result in an image showing many vascular structures, including the veins and small arteries. When the lower threshold is increased (eg, to 200 HU), structures of low attenuation such as intracranial veins and small arteries will disappear completely and the major arteries will appear smaller. The "ideal" threshold to depict intracranial arteries has to be found interactively and depends on several parameters, including the injection rate of the contrast medium and cardiac output, both of which influence the attenuation of the contrast material–filled vasculature (26).
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Figure 6a. Threshold-dependent 3D visualization with SSD. (a) Superoposterior view obtained with a lower threshold of 100 HU shows smaller arteries like the left PICA (arrow) and venous structures (arrowheads). (b) Superoposterior view obtained by increasing the lower threshold to 200 HU shows arteries that appear thinner compared with those in a and even demonstrate discontinuities (arrow). The venous structures are nearly eliminated (arrowheads), resulting in a less complex image.
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Figure 6b. Threshold-dependent 3D visualization with SSD. (a) Superoposterior view obtained with a lower threshold of 100 HU shows smaller arteries like the left PICA (arrow) and venous structures (arrowheads). (b) Superoposterior view obtained by increasing the lower threshold to 200 HU shows arteries that appear thinner compared with those in a and even demonstrate discontinuities (arrow). The venous structures are nearly eliminated (arrowheads), resulting in a less complex image.
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From the point of view of a computer scientist, MIP and SSD are types of "volume rendering" in which only one layer of voxels is used for visualization. In the medical literature, the term volume rendering is reserved for the technique described next, in which all voxels of a volume are considered for visualization (27).
Direct volume rendering (dVR) is the most sophisticated method for 3D visualization. The basic principle is to select several groups of voxels according to their attenuation in Hounsfield units and to assign them a color and a so-called opacity (28,29) (Fig 7). When dVR is used to create CT angiograms, the voxels of high attenuation containing information about bony structures are selected separately from those voxels with an attenuation between 100 and 300 HU containing information about contrast-enhanced vascular structures. This selection allows the creation of 3D images showing red arteries and white bone. A high opacity will lead to images that look similar to those produced by SSD. Use of a low opacity can result in the creation of transparent objects (eg, it is possible to make intracranial arteries visible beneath a layer of skull bone) (Fig 8). Selecting only a small group of voxels with a high opacity allows creation of a "virtual endoscopic" view in which the thin layer of voxels resembles the vessel wall (Fig 8e) (30).
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Figure 7a. Basic principles of volume rendering. Groups of voxels are selected according to their Hounsfield unit values. Every group has its own color and opacity. A low opacity makes the objects transparent. dVR image (superior view) of a patient with two aneurysm clips (a) and photograph of the workstation screen (b). The voxels representing the metal clips (arrows in a) are colored blue with a high opacity. Bone (voxels between 200 and 2,000 HU) is colored white with an opacity of 49%. Finally, the voxels between 90 and 300 HU that contain the vascular information are colored red with a 50% opacity.
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Figure 7b. Basic principles of volume rendering. Groups of voxels are selected according to their Hounsfield unit values. Every group has its own color and opacity. A low opacity makes the objects transparent. dVR image (superior view) of a patient with two aneurysm clips (a) and photograph of the workstation screen (b). The voxels representing the metal clips (arrows in a) are colored blue with a high opacity. Bone (voxels between 200 and 2,000 HU) is colored white with an opacity of 49%. Finally, the voxels between 90 and 300 HU that contain the vascular information are colored red with a 50% opacity.
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Figure 8a. Different possibilities for examining an aneurysm of the left MCA with dVR. (a) Frontal image obtained without shading. (b) Frontal image obtained with shading (addition of an artificial light source), which gives the objects more depth. (c) Frontal image obtained by selecting only a small group of voxels with a low opacity. The vessels appear transparent, thus allowing visualization of a branch of the MCA running behind the aneurysm (arrow). (d) Left frontolateral transparent image allows the orifice of the feeding artery to be seen through the aneurysm (arrow). (e) Frontal "virtual endoscopic" image, obtained by selecting only a small group of voxels with a high opacity, shows a pseudoconnection between the aneurysm and an adjacent artery (arrow). This "kissing vessel" artifact is a partial volume problem that is often seen on CT angiograms of intracranial aneurysms. (f) Anterocaudal image obtained with high opacity shows the close relationship of the aneurysm to the lower branch of the MCA (arrow).
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Figure 8b. Different possibilities for examining an aneurysm of the left MCA with dVR. (a) Frontal image obtained without shading. (b) Frontal image obtained with shading (addition of an artificial light source), which gives the objects more depth. (c) Frontal image obtained by selecting only a small group of voxels with a low opacity. The vessels appear transparent, thus allowing visualization of a branch of the MCA running behind the aneurysm (arrow). (d) Left frontolateral transparent image allows the orifice of the feeding artery to be seen through the aneurysm (arrow). (e) Frontal "virtual endoscopic" image, obtained by selecting only a small group of voxels with a high opacity, shows a pseudoconnection between the aneurysm and an adjacent artery (arrow). This "kissing vessel" artifact is a partial volume problem that is often seen on CT angiograms of intracranial aneurysms. (f) Anterocaudal image obtained with high opacity shows the close relationship of the aneurysm to the lower branch of the MCA (arrow).
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Figure 8c. Different possibilities for examining an aneurysm of the left MCA with dVR. (a) Frontal image obtained without shading. (b) Frontal image obtained with shading (addition of an artificial light source), which gives the objects more depth. (c) Frontal image obtained by selecting only a small group of voxels with a low opacity. The vessels appear transparent, thus allowing visualization of a branch of the MCA running behind the aneurysm (arrow). (d) Left frontolateral transparent image allows the orifice of the feeding artery to be seen through the aneurysm (arrow). (e) Frontal "virtual endoscopic" image, obtained by selecting only a small group of voxels with a high opacity, shows a pseudoconnection between the aneurysm and an adjacent artery (arrow). This "kissing vessel" artifact is a partial volume problem that is often seen on CT angiograms of intracranial aneurysms. (f) Anterocaudal image obtained with high opacity shows the close relationship of the aneurysm to the lower branch of the MCA (arrow).
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Figure 8d. Different possibilities for examining an aneurysm of the left MCA with dVR. (a) Frontal image obtained without shading. (b) Frontal image obtained with shading (addition of an artificial light source), which gives the objects more depth. (c) Frontal image obtained by selecting only a small group of voxels with a low opacity. The vessels appear transparent, thus allowing visualization of a branch of the MCA running behind the aneurysm (arrow). (d) Left frontolateral transparent image allows the orifice of the feeding artery to be seen through the aneurysm (arrow). (e) Frontal "virtual endoscopic" image, obtained by selecting only a small group of voxels with a high opacity, shows a pseudoconnection between the aneurysm and an adjacent artery (arrow). This "kissing vessel" artifact is a partial volume problem that is often seen on CT angiograms of intracranial aneurysms. (f) Anterocaudal image obtained with high opacity shows the close relationship of the aneurysm to the lower branch of the MCA (arrow).
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Figure 8e. Different possibilities for examining an aneurysm of the left MCA with dVR. (a) Frontal image obtained without shading. (b) Frontal image obtained with shading (addition of an artificial light source), which gives the objects more depth. (c) Frontal image obtained by selecting only a small group of voxels with a low opacity. The vessels appear transparent, thus allowing visualization of a branch of the MCA running behind the aneurysm (arrow). (d) Left frontolateral transparent image allows the orifice of the feeding artery to be seen through the aneurysm (arrow). (e) Frontal "virtual endoscopic" image, obtained by selecting only a small group of voxels with a high opacity, shows a pseudoconnection between the aneurysm and an adjacent artery (arrow). This "kissing vessel" artifact is a partial volume problem that is often seen on CT angiograms of intracranial aneurysms. (f) Anterocaudal image obtained with high opacity shows the close relationship of the aneurysm to the lower branch of the MCA (arrow).
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Figure 8f. Different possibilities for examining an aneurysm of the left MCA with dVR. (a) Frontal image obtained without shading. (b) Frontal image obtained with shading (addition of an artificial light source), which gives the objects more depth. (c) Frontal image obtained by selecting only a small group of voxels with a low opacity. The vessels appear transparent, thus allowing visualization of a branch of the MCA running behind the aneurysm (arrow). (d) Left frontolateral transparent image allows the orifice of the feeding artery to be seen through the aneurysm (arrow). (e) Frontal "virtual endoscopic" image, obtained by selecting only a small group of voxels with a high opacity, shows a pseudoconnection between the aneurysm and an adjacent artery (arrow). This "kissing vessel" artifact is a partial volume problem that is often seen on CT angiograms of intracranial aneurysms. (f) Anterocaudal image obtained with high opacity shows the close relationship of the aneurysm to the lower branch of the MCA (arrow).
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There are so many possibilities to create images that it is impossible to compare studies of CT angiography with volume rendering from different institutions. As with SSD, the appearance of the intracranial arteries strongly depends on the selected thresholds. Lower values will show more vessels including venous structures and thus lead to a more complex image. It very much depends on the experience of an individual user to select these thresholds in an efficient way. To ensure a good detection rate for aneurysms, it is mandatory to evaluate the 3D models in a standardized way (Fig 9). The areas where aneurysms are most likely to occur have to be inspected in particular detail. This is extremely important because there is more than one aneurysm in about 20% of patients with SAH (31). Although it is customary to use red for the group of voxels containing the vascular structures, it is not clear whether the colors have an influence on aneurysm detection (Fig 10).
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Figure 9a. Standard 3D projections obtained by using dVR interactively on the workstation (left) and diagrams of the corresponding arterial anatomy (right). This systematic analysis is of extreme importance, especially when an aneurysm is detected at first glance, to ensure that additional aneurysms are not missed. ACA = anterior cerebral artery, PCA = posterior cerebral artery, PCom = posterior communicating artery, VA = vertebral artery. (a) Superior view of all of the intracranial arteries. In many cases, larger aneurysms are immediately visible on this overview. (b) Posterior view of the basilar and vertebral arteries. Aneurysms of the PICA and the basilar artery tip can be detected on this view. AICA = anterior inferior cerebellar artery. (c) Lateral view of the intracranial part of the ICA. Note that the ICA is partially obscured by osseous structures, making detection of aneurysms in this area difficult with 3D images alone. (d) Unobstructed view of the MCA bifurcation obtained from a superior angle. (e) Unobstructed view of the anterior communicating artery (ACom) obtained from a superior angle. (f) Unobstructed view of the anterior communicating artery (ACom) and the bifurcation of the left MCA obtained from an inferior angle after elimination of the skull base by using a clip plane (Fig 3).
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Figure 9b. Standard 3D projections obtained by using dVR interactively on the workstation (left) and diagrams of the corresponding arterial anatomy (right). This systematic analysis is of extreme importance, especially when an aneurysm is detected at first glance, to ensure that additional aneurysms are not missed. ACA = anterior cerebral artery, PCA = posterior cerebral artery, PCom = posterior communicating artery, VA = vertebral artery. (a) Superior view of all of the intracranial arteries. In many cases, larger aneurysms are immediately visible on this overview. (b) Posterior view of the basilar and vertebral arteries. Aneurysms of the PICA and the basilar artery tip can be detected on this view. AICA = anterior inferior cerebellar artery. (c) Lateral view of the intracranial part of the ICA. Note that the ICA is partially obscured by osseous structures, making detection of aneurysms in this area difficult with 3D images alone. (d) Unobstructed view of the MCA bifurcation obtained from a superior angle. (e) Unobstructed view of the anterior communicating artery (ACom) obtained from a superior angle. (f) Unobstructed view of the anterior communicating artery (ACom) and the bifurcation of the left MCA obtained from an inferior angle after elimination of the skull base by using a clip plane (Fig 3).
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Figure 9c. Standard 3D projections obtained by using dVR interactively on the workstation (left) and diagrams of the corresponding arterial anatomy (right). This systematic analysis is of extreme importance, especially when an aneurysm is detected at first glance, to ensure that additional aneurysms are not missed. ACA = anterior cerebral artery, PCA = posterior cerebral artery, PCom = posterior communicating artery, VA = vertebral artery. (a) Superior view of all of the intracranial arteries. In many cases, larger aneurysms are immediately visible on this overview. (b) Posterior view of the basilar and vertebral arteries. Aneurysms of the PICA and the basilar artery tip can be detected on this view. AICA = anterior inferior cerebellar artery. (c) Lateral view of the intracranial part of the ICA. Note that the ICA is partially obscured by osseous structures, making detection of aneurysms in this area difficult with 3D images alone. (d) Unobstructed view of the MCA bifurcation obtained from a superior angle. (e) Unobstructed view of the anterior communicating artery (ACom) obtained from a superior angle. (f) Unobstructed view of the anterior communicating artery (ACom) and the bifurcation of the left MCA obtained from an inferior angle after elimination of the skull base by using a clip plane (Fig 3).
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Figure 9d. Standard 3D projections obtained by using dVR interactively on the workstation (left) and diagrams of the corresponding arterial anatomy (right). This systematic analysis is of extreme importance, especially when an aneurysm is detected at first glance, to ensure that additional aneurysms are not missed. ACA = anterior cerebral artery, PCA = posterior cerebral artery, PCom = posterior communicating artery, VA = vertebral artery. (a) Superior view of all of the intracranial arteries. In many cases, larger aneurysms are immediately visible on this overview. (b) Posterior view of the basilar and vertebral arteries. Aneurysms of the PICA and the basilar artery tip can be detected on this view. AICA = anterior inferior cerebellar artery. (c) Lateral view of the intracranial part of the ICA. Note that the ICA is partially obscured by osseous structures, making detection of aneurysms in this area difficult with 3D images alone. (d) Unobstructed view of the MCA bifurcation obtained from a superior angle. (e) Unobstructed view of the anterior communicating artery (ACom) obtained from a superior angle. (f) Unobstructed view of the anterior communicating artery (ACom) and the bifurcation of the left MCA obtained from an inferior angle after elimination of the skull base by using a clip plane (Fig 3).
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Figure 9e. Standard 3D projections obtained by using dVR interactively on the workstation (left) and diagrams of the corresponding arterial anatomy (right). This systematic analysis is of extreme importance, especially when an aneurysm is detected at first glance, to ensure that additional aneurysms are not missed. ACA = anterior cerebral artery, PCA = posterior cerebral artery, PCom = posterior communicating artery, VA = vertebral artery. (a) Superior view of all of the intracranial arteries. In many cases, larger aneurysms are immediately visible on this overview. (b) Posterior view of the basilar and vertebral arteries. Aneurysms of the PICA and the basilar artery tip can be detected on this view. AICA = anterior inferior cerebellar artery. (c) Lateral view of the intracranial part of the ICA. Note that the ICA is partially obscured by osseous structures, making detection of aneurysms in this area difficult with 3D images alone. (d) Unobstructed view of the MCA bifurcation obtained from a superior angle. (e) Unobstructed view of the anterior communicating artery (ACom) obtained from a superior angle. (f) Unobstructed view of the anterior communicating artery (ACom) and the bifurcation of the left MCA obtained from an inferior angle after elimination of the skull base by using a clip plane (Fig 3).
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Figure 9f. Standard 3D projections obtained by using dVR interactively on the workstation (left) and diagrams of the corresponding arterial anatomy (right). This systematic analysis is of extreme importance, especially when an aneurysm is detected at first glance, to ensure that additional aneurysms are not missed. ACA = anterior cerebral artery, PCA = posterior cerebral artery, PCom = posterior communicating artery, VA = vertebral artery. (a) Superior view of all of the intracranial arteries. In many cases, larger aneurysms are immediately visible on this overview. (b) Posterior view of the basilar and vertebral arteries. Aneurysms of the PICA and the basilar artery tip can be detected on this view. AICA = anterior inferior cerebellar artery. (c) Lateral view of the intracranial part of the ICA. Note that the ICA is partially obscured by osseous structures, making detection of aneurysms in this area difficult with 3D images alone. (d) Unobstructed view of the MCA bifurcation obtained from a superior angle. (e) Unobstructed view of the anterior communicating artery (ACom) obtained from a superior angle. (f) Unobstructed view of the anterior communicating artery (ACom) and the bifurcation of the left MCA obtained from an inferior angle after elimination of the skull base by using a clip plane (Fig 3).
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Figure 10a. Visualization of the intracranial arteries with 3D dVR performed by using different colors. Superior views show the arteries colored red (a) and blue (b). It is not known whether the colors affect the detection rate of intracranial aneurysms with CT angiography.
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Figure 10b. Visualization of the intracranial arteries with 3D dVR performed by using different colors. Superior views show the arteries colored red (a) and blue (b). It is not known whether the colors affect the detection rate of intracranial aneurysms with CT angiography.
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Unfortunately, the quality of dVR highly depends on many factors like the quality of the workstation and the applied rendering algorithm (29). Therefore, dVR is not a unique method and available studies are still not comparable.
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Detection of Intracranial Aneurysms with CT Angiography
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In a systematic review of studies published between 1988 and 1998, the average sensitivity of CT angiography for the detection of intracranial aneurysms was about 90%. Aneurysms with a size of 3 mm or below were detected in 61% of cases, whereas the sensitivity for aneurysms with a larger diameter was 96% (32). More recent studies found even higher overall detection rates of up to 97%, and some authors already solely rely on the findings of CT angiography in patients with SAH (21,33,34). Because the detection rate depends on technical requisites and user-dependent postprocessing techniques, this is not yet accepted practice in most radiology departments.
For the increasing number of aneurysms treated by using an endovascular approach, CT angiography may be used as a tool for therapeutic decision making and therapy planning: A major advantage of CT angiography compared with DSA is the generation of 3D information on the exact anatomy of the intracranial arteries. In addition to the mere detection of aneurysms, these 3D models can be most helpful for therapy planning as well (21). When intravascular coiling is the therapy chosen on the basis of the CT angiographic findings, additional pretherapeutic DSA is not necessary, as it is part of the coiling procedure. In addition, CT angiography can help predict the ideal angle for the endovascular approach. Information on the exact dimensions of an aneurysm provided by CT angiography can be used to determine the diameter of the first coil (Fig 11). For the neurosurgeon, information about adjacent vascular structures and the possibility of simulating the intraoperative view prior to surgery are often helpful (Fig 12) (35).
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Figure 11a. Use of CT angiography for planning endovascular therapy. (a) Anterior 3D dVR image obtained with a high opacity shows an irregular aneurysm of the left intracranial carotid bifurcation (arrow). (b) Anterior transparent dVR image obtained with a low opacity shows the measured diameters of the dome and neck of the aneurysm (arrow). The maximal diameter of the dome was almost 7 mm. (c) Posteroanterior DSA image of the left ICA obtained after placement of the first coil. Because the exact measurements of the aneurysm (arrow) were determined with CT angiography, a 7-mm-diameter platinum coil was used first.
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Figure 11b. Use of CT angiography for planning endovascular therapy. (a) Anterior 3D dVR image obtained with a high opacity shows an irregular aneurysm of the left intracranial carotid bifurcation (arrow). (b) Anterior transparent dVR image obtained with a low opacity shows the measured diameters of the dome and neck of the aneurysm (arrow). The maximal diameter of the dome was almost 7 mm. (c) Posteroanterior DSA image of the left ICA obtained after placement of the first coil. Because the exact measurements of the aneurysm (arrow) were determined with CT angiography, a 7-mm-diameter platinum coil was used first.
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Figure 11c. Use of CT angiography for planning endovascular therapy. (a) Anterior 3D dVR image obtained with a high opacity shows an irregular aneurysm of the left intracranial carotid bifurcation (arrow). (b) Anterior transparent dVR image obtained with a low opacity shows the measured diameters of the dome and neck of the aneurysm (arrow). The maximal diameter of the dome was almost 7 mm. (c) Posteroanterior DSA image of the left ICA obtained after placement of the first coil. Because the exact measurements of the aneurysm (arrow) were determined with CT angiography, a 7-mm-diameter platinum coil was used first.
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Abstract
LEARNING OBJECTIVES FOR TEST...
