Price of Isotropy in Multidetector CT

doi:10.1148/rg.271065037

HOME

HELP

FEEDBACK

SUBSCRIPTIONS

Price of Isotropy in Multidetector CT¹

Neal C. Dalrymple, MD, Srinivasa R. Prasad, MD, Fadi M. El-Merhi, MD and Kedar N. Chintapalli, MD

¹ From the Department of Radiology, University of Texas Health Science Center at San Antonio, Mail Code 7800, 7703 Floyd Curl Dr, San Antonio, TX 78229-3900. Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received March 21, 2006; revision requested June 5 and received July 10; accepted July 11. All authors have no financial relationships to disclose.

View larger version (36K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 1a. Geometry of isotropic and anisotropic acquisitions. Anisotropic data consist of voxels that have a section thickness greater than the x- and y-axis dimensions of the facing pixels. Section thickness along the z-axis is four times the size of each pixel in a but only twice the size of each pixel in b. Although both data sets are anisotropic, there is a significant difference in image quality for three-dimensional applications, with improved longitudinal spatial resolution in b compared with that in a. When the section thickness is equal to the pixel size, as in c, the data are isotropic.

View larger version (38K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 1b. Geometry of isotropic and anisotropic acquisitions. Anisotropic data consist of voxels that have a section thickness greater than the x- and y-axis dimensions of the facing pixels. Section thickness along the z-axis is four times the size of each pixel in a but only twice the size of each pixel in b. Although both data sets are anisotropic, there is a significant difference in image quality for three-dimensional applications, with improved longitudinal spatial resolution in b compared with that in a. When the section thickness is equal to the pixel size, as in c, the data are isotropic.

View larger version (41K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 1c. Geometry of isotropic and anisotropic acquisitions. Anisotropic data consist of voxels that have a section thickness greater than the x- and y-axis dimensions of the facing pixels. Section thickness along the z-axis is four times the size of each pixel in a but only twice the size of each pixel in b. Although both data sets are anisotropic, there is a significant difference in image quality for three-dimensional applications, with improved longitudinal spatial resolution in b compared with that in a. When the section thickness is equal to the pixel size, as in c, the data are isotropic.

View larger version (29K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 2a. General principles of detector configuration at multidetector CT with a 16-channel scanner. (a) Diagram of beam geometry with the 16 x 0.625 mm detector configuration shows narrow collimation of the x-ray beam, with exposure of only the central detector elements (DE). The data acquisition system (DAS), a type of switchable circuit system, is set to sample each of the central elements individually. This setting permits reconstruction of 0.625-mm-thick sections, the thinnest possible with this scanner platform. Note that a portion of the beam (rose bands) extends beyond the active detector elements. This area of overextension, called the penumbra, is necessary to ensure exposure of the most peripheral of the active detector elements, but beam overextension results in some radiation exposure that does not directly contribute to the image. (b) Diagram of beam geometry with the 16 x 1.25 mm detector configuration shows wider collimation of the x-ray beam to expose all the detector elements. The data acquisition system samples combined data from the small central elements while collecting data separately from each of the larger peripheral elements. In this setting, a larger volume of tissue is exposed per gantry rotation, but axial sections cannot be reconstructed to a thickness of less than 1.25 mm. Because the penumbra is a smaller percentage of the overall beam, the efficiency of the acquisition is increased and the patient is exposed to less additional radiation.

View larger version (31K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 2b. General principles of detector configuration at multidetector CT with a 16-channel scanner. (a) Diagram of beam geometry with the 16 x 0.625 mm detector configuration shows narrow collimation of the x-ray beam, with exposure of only the central detector elements (DE). The data acquisition system (DAS), a type of switchable circuit system, is set to sample each of the central elements individually. This setting permits reconstruction of 0.625-mm-thick sections, the thinnest possible with this scanner platform. Note that a portion of the beam (rose bands) extends beyond the active detector elements. This area of overextension, called the penumbra, is necessary to ensure exposure of the most peripheral of the active detector elements, but beam overextension results in some radiation exposure that does not directly contribute to the image. (b) Diagram of beam geometry with the 16 x 1.25 mm detector configuration shows wider collimation of the x-ray beam to expose all the detector elements. The data acquisition system samples combined data from the small central elements while collecting data separately from each of the larger peripheral elements. In this setting, a larger volume of tissue is exposed per gantry rotation, but axial sections cannot be reconstructed to a thickness of less than 1.25 mm. Because the penumbra is a smaller percentage of the overall beam, the efficiency of the acquisition is increased and the patient is exposed to less additional radiation.

View larger version (31K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 3a. Detector configurations and minimal voxel dimensions at multidetector CT with a matrix-array four-channel scanner. (a) Left: Diagram of beam geometry with a detector configuration of 4 x 1.25 mm. Four central elements are exposed, and four data components with a section thickness of 1.25 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed near-isotropic voxels, which on axial images have a z-axis depth of 1.25 mm, approximately twice the x- and y-axis dimension (0.7 mm). (b) Left: Diagram of beam geometry with a detector configuration of 4 x 2.5 mm. Eight central elements are exposed in pairs, and four data components with a section thickness of 2.5 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are four times as long in the z-axis as in the x- and y-axes. (c) Left: Diagram of beam geometry with a detector configuration of 4 x 3.75 mm. Twelve central elements are exposed in triplets, and four data components with a section thickness of 3.75 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are approximately six times as long in the z-axis as in the x- and y-axes. (d) Left: Diagram of beam geometry with a detector configuration of 4 x 5.0 mm. All 16 detector elements are exposed, and four data components with a section thickness of 5.0 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are eight times as long in the z-axis as in the x- and y-axes.

View larger version (27K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 3b. Detector configurations and minimal voxel dimensions at multidetector CT with a matrix-array four-channel scanner. (a) Left: Diagram of beam geometry with a detector configuration of 4 x 1.25 mm. Four central elements are exposed, and four data components with a section thickness of 1.25 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed near-isotropic voxels, which on axial images have a z-axis depth of 1.25 mm, approximately twice the x- and y-axis dimension (0.7 mm). (b) Left: Diagram of beam geometry with a detector configuration of 4 x 2.5 mm. Eight central elements are exposed in pairs, and four data components with a section thickness of 2.5 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are four times as long in the z-axis as in the x- and y-axes. (c) Left: Diagram of beam geometry with a detector configuration of 4 x 3.75 mm. Twelve central elements are exposed in triplets, and four data components with a section thickness of 3.75 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are approximately six times as long in the z-axis as in the x- and y-axes. (d) Left: Diagram of beam geometry with a detector configuration of 4 x 5.0 mm. All 16 detector elements are exposed, and four data components with a section thickness of 5.0 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are eight times as long in the z-axis as in the x- and y-axes.

View larger version (30K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 3c. Detector configurations and minimal voxel dimensions at multidetector CT with a matrix-array four-channel scanner. (a) Left: Diagram of beam geometry with a detector configuration of 4 x 1.25 mm. Four central elements are exposed, and four data components with a section thickness of 1.25 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed near-isotropic voxels, which on axial images have a z-axis depth of 1.25 mm, approximately twice the x- and y-axis dimension (0.7 mm). (b) Left: Diagram of beam geometry with a detector configuration of 4 x 2.5 mm. Eight central elements are exposed in pairs, and four data components with a section thickness of 2.5 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are four times as long in the z-axis as in the x- and y-axes. (c) Left: Diagram of beam geometry with a detector configuration of 4 x 3.75 mm. Twelve central elements are exposed in triplets, and four data components with a section thickness of 3.75 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are approximately six times as long in the z-axis as in the x- and y-axes. (d) Left: Diagram of beam geometry with a detector configuration of 4 x 5.0 mm. All 16 detector elements are exposed, and four data components with a section thickness of 5.0 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are eight times as long in the z-axis as in the x- and y-axes.

View larger version (31K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 3d. Detector configurations and minimal voxel dimensions at multidetector CT with a matrix-array four-channel scanner. (a) Left: Diagram of beam geometry with a detector configuration of 4 x 1.25 mm. Four central elements are exposed, and four data components with a section thickness of 1.25 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed near-isotropic voxels, which on axial images have a z-axis depth of 1.25 mm, approximately twice the x- and y-axis dimension (0.7 mm). (b) Left: Diagram of beam geometry with a detector configuration of 4 x 2.5 mm. Eight central elements are exposed in pairs, and four data components with a section thickness of 2.5 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are four times as long in the z-axis as in the x- and y-axes. (c) Left: Diagram of beam geometry with a detector configuration of 4 x 3.75 mm. Twelve central elements are exposed in triplets, and four data components with a section thickness of 3.75 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are approximately six times as long in the z-axis as in the x- and y-axes. (d) Left: Diagram of beam geometry with a detector configuration of 4 x 5.0 mm. All 16 detector elements are exposed, and four data components with a section thickness of 5.0 mm each are acquired per gantry rotation. Right: Diagram shows the reconstructed voxels, which are eight times as long in the z-axis as in the x- and y-axes.

View larger version (95K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 4a. Coronal multidetector CT images of the right hip, obtained with various detector configurations on a four-channel scanner. In each case, a fixed tube current of 240 mA was used, and gantry rotation time and table speed were constant. Data were reconstructed with an overlap and with each section thickness available with the particular detector configuration used. The reconstructed section thickness and increment were 5.0 mm and 2.5 mm (a), 3.75 mm and 1.8 mm (b), 2.5 mm and 1.25 mm (c), and 1.25 mm and 0.625 mm (d). Note that reconstruction with the thinnest section available is required for adequate evaluation of the articular cortex and joint space of the hip. Config. = detector configuration, CTDI_w = weighted CT dose index, Time = scanning time.

View larger version (98K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 4b. Coronal multidetector CT images of the right hip, obtained with various detector configurations on a four-channel scanner. In each case, a fixed tube current of 240 mA was used, and gantry rotation time and table speed were constant. Data were reconstructed with an overlap and with each section thickness available with the particular detector configuration used. The reconstructed section thickness and increment were 5.0 mm and 2.5 mm (a), 3.75 mm and 1.8 mm (b), 2.5 mm and 1.25 mm (c), and 1.25 mm and 0.625 mm (d). Note that reconstruction with the thinnest section available is required for adequate evaluation of the articular cortex and joint space of the hip. Config. = detector configuration, CTDI_w = weighted CT dose index, Time = scanning time.

View larger version (99K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 4c. Coronal multidetector CT images of the right hip, obtained with various detector configurations on a four-channel scanner. In each case, a fixed tube current of 240 mA was used, and gantry rotation time and table speed were constant. Data were reconstructed with an overlap and with each section thickness available with the particular detector configuration used. The reconstructed section thickness and increment were 5.0 mm and 2.5 mm (a), 3.75 mm and 1.8 mm (b), 2.5 mm and 1.25 mm (c), and 1.25 mm and 0.625 mm (d). Note that reconstruction with the thinnest section available is required for adequate evaluation of the articular cortex and joint space of the hip. Config. = detector configuration, CTDI_w = weighted CT dose index, Time = scanning time.

View larger version (101K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 4d. Coronal multidetector CT images of the right hip, obtained with various detector configurations on a four-channel scanner. In each case, a fixed tube current of 240 mA was used, and gantry rotation time and table speed were constant. Data were reconstructed with an overlap and with each section thickness available with the particular detector configuration used. The reconstructed section thickness and increment were 5.0 mm and 2.5 mm (a), 3.75 mm and 1.8 mm (b), 2.5 mm and 1.25 mm (c), and 1.25 mm and 0.625 mm (d). Note that reconstruction with the thinnest section available is required for adequate evaluation of the articular cortex and joint space of the hip. Config. = detector configuration, CTDI_w = weighted CT dose index, Time = scanning time.

View larger version (147K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 5a. Volume-rendered images obtained with different section thicknesses at four-channel CT. The first row of data on the images is the detector configuration, the middle row is the weighted CT dose index, and the bottom row is the scanning time. (a) Image obtained with a reconstructed section thickness of 5.0 mm and an increment of 2.5 mm shows the right hepatic artery (arrow) where it arises from the celiac artery. The origin of the left hepatic artery (arrowhead) at the left gastric artery also is visible but is not well depicted. (b) Image in a similar orientation, obtained with a reconstructed section thickness of 2.5 mm and an increment of 1.25 mm, shows both hepatic arteries more clearly and with a smoother appearance than in a.

View larger version (160K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 5b. Volume-rendered images obtained with different section thicknesses at four-channel CT. The first row of data on the images is the detector configuration, the middle row is the weighted CT dose index, and the bottom row is the scanning time. (a) Image obtained with a reconstructed section thickness of 5.0 mm and an increment of 2.5 mm shows the right hepatic artery (arrow) where it arises from the celiac artery. The origin of the left hepatic artery (arrowhead) at the left gastric artery also is visible but is not well depicted. (b) Image in a similar orientation, obtained with a reconstructed section thickness of 2.5 mm and an increment of 1.25 mm, shows both hepatic arteries more clearly and with a smoother appearance than in a.

View larger version (34K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 6a. Detector configurations and voxel dimensions at 16-channel multidetector CT. (a) Left: Diagram shows narrow collimation, with exposure of only the central 16 detector elements. Each element functions as a separate unit, and 16 sections with a thickness of 0.625 mm each are acquired per gantry rotation. Right: Diagram shows that the reconstructed voxels are isotropic, with about equal length in each dimension. (b) Left: Diagram shows wide collimation, with exposure not only of the central small elements but also of larger elements at the periphery. Central elements function in pairs, and peripheral elements are used individually. As a result, 16 sections with a thickness of 1.25 mm each are acquired per rotation. Right: Diagram shows that reconstructed voxels are anisotropic, about twice as long in the longitudinal plane as in the transverse plane.

View larger version (34K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 6b. Detector configurations and voxel dimensions at 16-channel multidetector CT. (a) Left: Diagram shows narrow collimation, with exposure of only the central 16 detector elements. Each element functions as a separate unit, and 16 sections with a thickness of 0.625 mm each are acquired per gantry rotation. Right: Diagram shows that the reconstructed voxels are isotropic, with about equal length in each dimension. (b) Left: Diagram shows wide collimation, with exposure not only of the central small elements but also of larger elements at the periphery. Central elements function in pairs, and peripheral elements are used individually. As a result, 16 sections with a thickness of 1.25 mm each are acquired per rotation. Right: Diagram shows that reconstructed voxels are anisotropic, about twice as long in the longitudinal plane as in the transverse plane.

View larger version (131K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 7a. Depiction of a lung nodule with different detector configurations on a 16-channel CT scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Coronal reformatted image obtained with the wide detector configuration (reconstructed section thickness, 1.25 mm; increment, 0.625 mm) provides excellent depiction of a nodule and adjacent vessels in the upper lobe of the left lung. (b) Coronal reformatted image obtained with the narrow detector configuration (reconstructed section thickness, 0.625 mm; increment, 0.312 mm) provides slightly improved depiction of vessels and interstitial lines, but there is no alteration in the quality of depiction of the nodule.

View larger version (134K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 7b. Depiction of a lung nodule with different detector configurations on a 16-channel CT scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Coronal reformatted image obtained with the wide detector configuration (reconstructed section thickness, 1.25 mm; increment, 0.625 mm) provides excellent depiction of a nodule and adjacent vessels in the upper lobe of the left lung. (b) Coronal reformatted image obtained with the narrow detector configuration (reconstructed section thickness, 0.625 mm; increment, 0.312 mm) provides slightly improved depiction of vessels and interstitial lines, but there is no alteration in the quality of depiction of the nodule.

View larger version (62K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 8a. Three-dimensional volume-rendered images from renal CT angiography with a 16-channel scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Image reconstructed from anisotropic data provides satisfactory depiction of the aorta and central vessels. (b) Image reconstructed from isotropic data provides slightly improved definition of smaller vessels. The automated "seed and grow" software program used to create these images provided better depiction of peripheral branches of the renal vessels with the use of isotropic data.

View larger version (101K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 8b. Three-dimensional volume-rendered images from renal CT angiography with a 16-channel scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Image reconstructed from anisotropic data provides satisfactory depiction of the aorta and central vessels. (b) Image reconstructed from isotropic data provides slightly improved definition of smaller vessels. The automated "seed and grow" software program used to create these images provided better depiction of peripheral branches of the renal vessels with the use of isotropic data.

View larger version (29K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 9a. Detector configurations and voxel dimensions at 40-channel multidetector CT. (a) Left: Diagram shows narrow beam collimation, with exposure of only the central 40 detector elements. In this setting, each element functions as a separate unit, and 40 sections with a thickness of 0.625 mm each are acquired per gantry rotation, yielding long-axis coverage of 25 mm. Right: Diagram shows that reconstructed voxels in this mode are isotropic, with approximately equal length in each dimension. (b) Left: Diagram shows wide beam collimation, with exposure not only of the central small elements but also of the additional larger elements at the periphery of the array. The central elements function in pairs, and the larger peripheral elements are each used individually. As a result, 32 sections with a thickness of 1.25 mm each are acquired per gantry rotation, yielding long-axis coverage of 40 mm. Right: Diagram shows that reconstructed voxels in this mode are anisotropic, approximately twice as large in the longitudinal plane as in the transverse plane.

View larger version (28K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 9b. Detector configurations and voxel dimensions at 40-channel multidetector CT. (a) Left: Diagram shows narrow beam collimation, with exposure of only the central 40 detector elements. In this setting, each element functions as a separate unit, and 40 sections with a thickness of 0.625 mm each are acquired per gantry rotation, yielding long-axis coverage of 25 mm. Right: Diagram shows that reconstructed voxels in this mode are isotropic, with approximately equal length in each dimension. (b) Left: Diagram shows wide beam collimation, with exposure not only of the central small elements but also of the additional larger elements at the periphery of the array. The central elements function in pairs, and the larger peripheral elements are each used individually. As a result, 32 sections with a thickness of 1.25 mm each are acquired per gantry rotation, yielding long-axis coverage of 40 mm. Right: Diagram shows that reconstructed voxels in this mode are anisotropic, approximately twice as large in the longitudinal plane as in the transverse plane.

View larger version (176K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 10a. Coronal reformatted images from renal CT angiography performed with a 40-channel scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Image reconstructed from anisotropic data provides excellent definition of the origins of both renal arteries and depicts both calcified and noncalcified components of plaque. (b) Image reconstructed from isotropic data provides minimally improved definition of the margins of noncalcified plaque in the left renal artery, with an increase of 20% in the estimated radiation dose to the patient. Although the scanning time required for isotropic data acquisition is 60% greater than that for anisotropic data acquisition, the difference (3 sec) is unlikely to have a significant effect on breath-hold or contrast-enhanced multiphase imaging.

View larger version (173K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 10b. Coronal reformatted images from renal CT angiography performed with a 40-channel scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Image reconstructed from anisotropic data provides excellent definition of the origins of both renal arteries and depicts both calcified and noncalcified components of plaque. (b) Image reconstructed from isotropic data provides minimally improved definition of the margins of noncalcified plaque in the left renal artery, with an increase of 20% in the estimated radiation dose to the patient. Although the scanning time required for isotropic data acquisition is 60% greater than that for anisotropic data acquisition, the difference (3 sec) is unlikely to have a significant effect on breath-hold or contrast-enhanced multiphase imaging.

View larger version (97K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 11a. Magnified views of the abdominal aorta at the level of the hepatic artery, from CT angiography performed with a 40-channel scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Volume-rendered image reconstructed from anisotropic data provides adequate depiction of a stenosis of the replaced right hepatic artery at its origin from the superior mesenteric artery (arrow). The use of automated segmentation software consistently resulted in the removal of a calcified plaque (arrowhead) at the origin of the superior mesenteric artery. (b) Volume-rendered image reconstructed from isotropic data clearly shows the plaque (arrowhead).

View larger version (122K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 11b. Magnified views of the abdominal aorta at the level of the hepatic artery, from CT angiography performed with a 40-channel scanner. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Volume-rendered image reconstructed from anisotropic data provides adequate depiction of a stenosis of the replaced right hepatic artery at its origin from the superior mesenteric artery (arrow). The use of automated segmentation software consistently resulted in the removal of a calcified plaque (arrowhead) at the origin of the superior mesenteric artery. (b) Volume-rendered image reconstructed from isotropic data clearly shows the plaque (arrowhead).

View larger version (28K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 12a. Detector configurations and voxel dimensions at 64-channel multidetector CT. Because the width of the incident beam does not change between detector configurations, the concepts of narrow and wide collimation do not apply. (a) Left: Diagram shows the detector configuration for thin-section acquisitions. With each detector element used individually, 64 sections with a thickness of 0.625 mm each are acquired per gantry rotation, resulting in long-axis coverage of 40 mm. Right: Diagram shows that reconstructed voxels in this mode are isotropic, with approximately equal length in each dimension. (b) Left: Diagram shows the detector configuration for acquisition of thicker sections. Although the beam collimation does not change, the data acquisition system pairs the elements for the receipt of data. As a result, 32 sections with a thickness of 1.25 mm each are acquired per gantry rotation, while long-axis coverage remains constant. Right: Diagram shows that reconstructed voxels in this mode are anisotropic, approximately twice as large in the longitudinal plane as in the transverse plane.

View larger version (27K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 12b. Detector configurations and voxel dimensions at 64-channel multidetector CT. Because the width of the incident beam does not change between detector configurations, the concepts of narrow and wide collimation do not apply. (a) Left: Diagram shows the detector configuration for thin-section acquisitions. With each detector element used individually, 64 sections with a thickness of 0.625 mm each are acquired per gantry rotation, resulting in long-axis coverage of 40 mm. Right: Diagram shows that reconstructed voxels in this mode are isotropic, with approximately equal length in each dimension. (b) Left: Diagram shows the detector configuration for acquisition of thicker sections. Although the beam collimation does not change, the data acquisition system pairs the elements for the receipt of data. As a result, 32 sections with a thickness of 1.25 mm each are acquired per gantry rotation, while long-axis coverage remains constant. Right: Diagram shows that reconstructed voxels in this mode are anisotropic, approximately twice as large in the longitudinal plane as in the transverse plane.

View larger version (139K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 13a. Volume-rendered images from 64-channel multidetector CT. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Image reconstructed from anisotropic data (section thickness, 1.25 mm; increment, 0.625 mm) clearly depicts a peripheral aneurysm of the left renal artery. (b) Image reconstructed from isotropic data (section thickness, 0.625 mm; increment, 0.3 mm) provides sharper definition of vessel margins and allows visualization of small lumbar and mesenteric vessel branches.

View larger version (148K):
[in this window]
[in a new window]
[Download PPT slide]

Figure 13b. Volume-rendered images from 64-channel multidetector CT. The first row of data on the images is the detector configuration, the middle row is the volume CT dose index, and the bottom row is the scanning time. (a) Image reconstructed from anisotropic data (section thickness, 1.25 mm; increment, 0.625 mm) clearly depicts a peripheral aneurysm of the left renal artery. (b) Image reconstructed from isotropic data (section thickness, 0.625 mm; increment, 0.3 mm) provides sharper definition of vessel margins and allows visualization of small lumbar and mesenteric vessel branches.

Price of Isotropy in Multidetector CT1

Price of Isotropy in Multidetector CT¹