Technical considerations for lower limb multidetector computed tomographic angiography

Transcription

1 Technical considerations for lower limb multidetector computed tomographic angiography Vascular Medicine 16(2) The Author(s) 2010 Reprints and permission: sagepub. co.uk/journalspermissions.nav DOI: / X vmj.sagepub.com Aoife N Keeling 1,2, Cormac Farrelly 1, James C Carr 1,2 and Vahid Yaghmai 1 Abstract Multidetector computed tomography (MDCT) enables imaging of the entire arterial tree non-invasively. Optimal technical considerations for performing MDCT angiography (MDCTA) are essential for accurate diagnosis and atherosclerotic disease stratification. This review article focuses on the various technical aspects necessary for peripheral computed tomographic angiography (CTA) acquisition. Common clinical indications for peripheral MDCTA and the latest scan protocols are described. The essential issue of radiation dose reduction is discussed, along with methods of optimal contrast bolus detection and delivery. Post-processing techniques are also presented. Previously, digital subtraction angiography was the only established reliable imaging technique to quantify atherosclerotic disease load; however, MDCTA may now challenge this old gold standard, along with other non-invasive techniques such as magnetic resonance angiography (MRA). Keywords CT angiography; multidetector computed tomography; peripheral artery disease; techniques Introduction Digital subtraction angiography (DSA) remains the gold standard method for evaluation of atherosclerosis within the peripheral vasculature. However, this is an invasive technique with the potential for iatrogenic complications. Therefore, non-invasive imaging methods are desirable among patients and physicians alike. With the advent of multidetector computed tomography (MDCT), non-invasive angiography has become a viable option as a result of increased speed, spatial resolution and volume coverage. 1 Isotropic data sets can now be acquired with MDCT angiography (MDCTA), allowing for 3D reconstruction and many post-processing methods to enable both diagnostic interpretation and eloquent anatomical and pathological arterial display. The alternative non-invasive imaging techniques to MDCTA are magnetic resonance angiography (MRA) and duplex ultrasound (US), both of which are widely used clinically. MRA has been shown to have similar diagnostic accuracy to that of MDCTA; 2,3 however, direct comparison studies in the literature are lacking, likely due to the fact that each modality is usually compared to DSA for the purposes of a reference standard. While MRA lacks the need for ionizing radiation, its major advantage over MDCTA, MRA is limited due to cost, scanner availability, local expertise and need for gadolinium contrast agent which has recently been shown to cause nephrogenic systemic fibrosis in patients with impaired renal function. 4 Duplex US, while readily available, cheap and safe, has a reduced sensitivity compared with both MRA and MDCTA, 5 and also has a lower physician confidence. 6 The aim of this review article is to identify various technical aspects essential to high-quality peripheral MDCT angiography acquisition. Current common clinical indications for peripheral MDCTA are outlined. Scan protocols can be variable due to different detector and tube numbers as well as tube rotation speeds. Therefore, scan parameters based on various CT scanners are presented. The essential issue of radiation dose reduction, optimal contrast bolus 1 Department of Radiology, Division of Cardiovascular Imaging, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL, USA 2 Department of Radiology, Division of Interventional Radiology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL, USA Corresponding author: Aoife Keeling Department of Radiology, Division of Cardiovascular Imaging Northwestern Memorial Hospital Northwestern University Feinberg School of Medicine 737 N. Michigan Ave, Suite 1600 Chicago, IL USA aoifekeeling@hotmail.com

2 132 Vascular Medicine 16(2) Table 1. Summary scan protocols for 4- and 64-detector scanners for both Siemens and GE Detector number 4 (Siemens) 4 (GE) 64 (Siemens) 64 (GE) Gantry rotation time 0.5 seconds 0.8 seconds 0.33 seconds 0.6 seconds Pitch Table increment 15 mm/ mm/ mm/ mm/360 Table speed 30 mm/second 19 mm/second 63 mm/second 63 mm/second Scan time 43 seconds 69 seconds 20 seconds 20 seconds Adapted from Fleischmann et al. 8 timing and delivery, and various post-processing methods are presented. Scan protocols Scan protocols are highly variable and depend on the type of MDCT scanner available, 4-, 16-, 64-, 128- or 320-detector, or dual source/energy and also depend on the vendor. A suitable protocol should be chosen and programmed into the individual scanner for routine use. To obtain the most optimal MDCTA, a 16-slice scanner or higher is desirable owing to the ability for faster scanning, thinner collimation with near isotropic data and reduced contrast dose. Peripheral MDCTA protocols include an initial topogram, from the diaphragm to the feet, and an arterial phase-timed acquisition from the celiac axis to the feet. It has been previously reported that there is a wide range of variability of transit times from the aorta to the popliteal artery across different patients with different peripheral artery disease (PAD) stages. 7 A second, later, limited acquisition from the lower thighs to the feet, in the event that the contrast has a slower transit time on the symptomatic side or bilaterally, or if the scanner outruns the contrast bolus, is an optional addition. As mentioned previously, different detector numbers and different manufacturers will require different scan protocols. Fleischmann et al. eloquently summarized their experience 8 (Table 1). With increasing numbers of detectors there is the advantage of increased temporal and spatial resolution allowing acquisition of sub-millimetre isotropic data sets, which in turn improve diagnostic sensitivity and specificity. However, the disadvantage of increased detector numbers is that the scan can be too fast and thus outrun the contrast bolus (as mentioned in Contrast concentration, injection rate and volume). Therefore, the fastest scanning mode is not necessarily the optimal mode for extremity MDCTA. The rotation time of new scanners (64-slice and beyond) should be slowed. 8 For instance, although the Siemens 64-slice scanner has a rotation speed of 0.33 r/s, for extremity MDCTA a rotation speed of 0.5 r/s may be more optimal, particularly in the setting of low cardiac output or severe PAD. Dose Currently, CT accounts for approximately 75% of the total radiation dose delivered by medical imaging. 9 With the increasing widespread use of CT and individual patients having multiple scans over their lifetimes, low-dose techniques are increasingly desired. However, standard dose settings for peripheral MDCTA with a tube voltage of 120 kv and a maximum tube amperage of 300 mas result in a radiation exposure and dose of mgy and 9.3 msv, respectively. 1 Automated tube current modulation that is available on all modern scanners allows for a more patient-specific dose delivery with significant dose savings. Fraioli et al. performed peripheral MDCTA at 50 mas, 100 mas and 130 mas in three groups of patients with PAD and compared the findings to the gold standard, DSA. 10 No difference in qualitative analysis of arterial segments were determined between the three MDCTA groups; similar sensitivity and specificity for diagnosing PAD was achieved with MDCTA performed at 50 mas, 100 mas and 300 mas. 10 Optimal image quality with diagnostic accuracy for PAD was achieved with the low-dose technique, allowing a 74% reduction in effective dose to the patient. 10 Willmann et al. directly compared radiation dose exposure to patients undergoing peripheral MDCTA with a 16-detector CT versus that of DSA and determined a mean effective radiation dose of 3.0 msv (range, msv) in men and 2.3 msv (range, msv) in women for MDCTA versus 11 msv (range, msv) for both sexes for DSA. 11 The authors utilized an online modulation of tube current to reduce radiation exposure. 11 Similarly, Greess 12,13 demonstrated reduced effective doses with an attenuation-based online modulation of tube current. Furthermore, lowering the kv will have a greater impact than ma on reducing radiation dose and will also improve the contrast signal to noise ratio (Figure 1). 14,15 For larger patients, these tube factors should be increased to reduce image noise. Another dose reduction strategy is to optimize the scan parameters by limiting the z-axis coverage to the area that is clinically relevant. Obtaining the smallest field of view will improve the quality of reformatted images. A new feature available on the Siemens 128-slice scanner consists of an adaptive dose shield which can block irrelevant pre-spiral and post-spiral radiation with dynamic diaphragms, thus ensuring that only clinically essential radiation exposure occurs. Spatial resolution MDCTA requires high spatial resolution to enable accurate detection and interrogation of small arteries. Both in-plane and through-plane spatial resolution are important indices that require optimization to provide isotropic source data. Isotropic means that the spatial resolution is approximately

3 Keeling AN et al. 133 A B Figure 1. (A) A 69-year-old male with bilateral short distance claudication. Dual-energy MDCTA was performed. This axial image was performed at 80 kv and demonstrates large eccentric foci of heavy calcification within the right popliteal artery (arrow). Note the almost concentric calcification within the left popliteal artery. (B) Dual-energy image of the same patient and at the same table position as Figure 1A, acquired at 140 kv. Note that the blooming from the eccentric foci of calcification within the right popliteal artery (arrow) is reduced compared with the 80 kv image. B A Figure 2. (A) A 48-year-old male presented to the emergency room following a fall from a step ladder. Five years previously he had a posterior left knee dislocation after a road traffic accident. Axial MDCTA demonstrates a large hematoma medially within the left thigh. There are high attenuation areas within the hematoma consistent with recent hemorrhage. There is also a large pseudoaneurysm within the hematoma. (B) This coronal maximum intensity projection (MIP) MDCTA image demonstrates the pseudoaneurysm donor artery, which is the left superficial femoral artery (SFA). Note the large pseudoaneurysm/hematoma combination displacing the superficial femoral artery (SFA) laterally (arrow). No active contrast extravasation was identified. equal in all planes, which is necessary to enable multiplanar image reconstruction following acquisition. Isotropic data avoids loss of spatial resolution in one plane and reduces partial volume effects following source data reconstruction. 16 Increased scanner detector numbers have allowed for increased through-plane spatial resolution as the detector width is reduced with increased numbers ( mm with four-detector row CT; mm with 64-detector row

4 134 Vascular Medicine 16(2) CT). 17 In-plane spatial resolution has not been improved by the new multidetector scanners as it is determined by detector geometry and the convolution kernel (reconstruction algorithm). 16 However, the in-plane spatial resolution also depends on the field of view and the reconstruction matrix, allowing for improved resolution with smaller reconstruction slice thickness and smaller field of view (Figure 2B versus Figure 3A). A reconstructed slice thickness of 1 mm or less improves detection of disease. Very thin slices will result in increased noise. Although reducing the slice collimation during image acquisition will result in improved spatial resolution, this may result in an increased radiation dose, as the automated dose modulation may increase mas to compensate for increased image noise. 18 Contrast detection Perhaps the most challenging aspect of extremity MDCTA is obtaining optimal enhancement of the arterial tree without venous contamination. The run-off vessels in the symptomatic limb can be problematic with MDCTA as a result of proximal/in-flow stenoses (slower flow) or alternatively distal hyperemia at the site of arterial ulceration (faster flow), as these can cause arterial flow discrepancy between the two lower limbs. Thus, one limb will have a faster flow rate than the other, resulting in good contrast opacification of the arteries ipsilaterally but failed opacification contralaterally. Various methods for optimizing peripheral arterial contrast opacification exist, including alteration of contrast concentration and volume, timing of contrast injection with respect to scan acquisition, the use of power injectors and extra scan acquisitions. Contrast concentration, injection rate and volume As a result of faster scan times with MDCTA, optimizing and maximizing arterial enhancement has become more of a challenge. 19,20 In order to separate arteries from veins and achieve a diagnostic quality study, an arterial density value of greater than 200 Hounsfield units (HU) is desirable during the scan acquisition. 21,22 It is, therefore, crucial to time the contrast bolus correctly and to be cognizant of the factors that affect the time to contrast peak, namely the iodine concentration, injection volume and rate, and patient s cardiac status. 19,20,23,24 Some authors have reported that contrast material with a higher iodine concentration improves vascular enhancement, if all other parameters are held constant, in a porcine model and within human cirrhotic livers. 19,25 However, a recent review attempted to determine the difference that a contrast media iodine concentration would make to image quality of MDCTA in the peripheral vasculature. 22 All of the studies reviewed used concentrations of 300 mg I/ml and higher; while demonstrating improved arterial enhancement and visualization with higher iodine concentrations, there was no clear evidence of a significant difference in diagnostic efficacy for the different iodine concentrations. 22 Appropriate A B Figure 3. (A) A 40-year-old male with peripheral artery disease; status post left common iliac artery and left external iliac artery stenting via the right common femoral artery approach. This sagittal maximum intensity projection (MIP) MDCTA image demonstrates a right common femoral arterial pseudoaneurysm (arrow). (B) A volume-rendered view demonstrating the right common femoral arterial pseudoaneurysm; note the left common and external iliac arterial stents in situ.

5 Keeling AN et al. 135 clinical decisions can still be made with lower iodine concentrations. Iezzi et al. also determined no significant differences in diagnostic ability of MDCTA for PAD, comparing 300 mg I/ml with 400 mg I/ml on a four-detector scanner. 26 This could perhaps be explained by applying findings from the coronary MDCTA literature, where Becker et al. determined that coronary artery attenuation levels of HU were optimal for the evaluation of coronary artery disease, as higher attenuation values may underestimate the amount of atherosclerosis due to obscuration of vessel wall calcification. 27 A large metaanalysis including 957 patients, performed to determine the diagnostic accuracy of peripheral MDCTA compared to DSA, reported that the contrast media iodine concentration varied between 300 and 400 mg I/ml among studies, with a median volume of 130 ml (range ml). 28 With respect to injection rate, a faster rate achieves a higher density within the target artery; however, rapid administration of contrast medium shortens the plateau phase of contrast enhancement, reducing the window for optimal arterial enhancement. 19,20,29 Fleischmann et al. recommend tailoring the injection rate to the patient s weight, with an average rate of 4 6 ml/second in a biphasic injection. 8 An important consideration with the fast 16- and 64-slice scanners is the possibility of outrunning the contrast bolus in patients with low cardiac output or in cases that require long z-axis coverage (abdominal and extremity MDCTA). To overcome this issue, one can slow down the scanner by increasing the gantry rotation time and slowing the table speed. 7,8 Contrast timing: fixed scan delay A standard timed scan delay following intra-venous (IV) contrast administration in order to acquire images in the arterial phase is reported by several authors. 30,31 Catalano et al., for example, used a fixed scan delay of 28 seconds in 50 patients for peripheral MDCTA and determined that this method provided a diagnostic exam in all patients with no cases of poor arterial enhancement. 31 By setting the same scan delay for every patient, one is assuming that exactly the same hemodynamic conditions exist in all patients. A fixed scan delay method does not allow for variation from the normal physiology (i.e. low blood pressure, low cardiac outputs, hypovolemia, high outputs). Fixed scan delays are simple to use but are more suited to slower scanners. Today s fast scanners require more precise timing of contrast bolus as acquisition times are extremely short and the slightest variations in contrast enhancement will adversely affect image quality. Contrast timing: test bolus Another method to optimize arterial contrast opacification is that of a test bolus or timing bolus acquisition. Described by a number of investigators, 1,32 this technique involves administering a small bolus (10 30 ml) of IV contrast material followed by serial CT data acquisitions at one table position, usually at the level of the celiac axis. Images are acquired, following a scan delay of 8 10 seconds, every 1 2 seconds for a predetermined number of images (20 40 images) or until the CT technologist chooses to manually stop the acquisition following the contrast peak within the aorta. A time versus Hounsfield unit (attenuation) curve is then generated by placing a region of interest over the contrast opacified aorta (Figure 4A). The time taken to reach peak opacification is then used as the scan delay for the actual peripheral MDCTA and thus corresponds to the time taken for the contrast to pass from the IV injection site to the aorta. This method is useful as it detects variable transit A B Figure 4. (A) During a test bolus a time versus Hounsfield unit (attenuation) curve is generated by placing a region of interest over the contrast opacified aorta. This graph plots Hounsfield units (HU) on the y-axis versus time (seconds) on the x-axis. The time taken to reach peak opacification (11 seconds in this case) is then used as the scan delay for the actual peripheral CTA and thus corresponds to the time taken for the contrast to pass from the IV injection site to the aorta. The peak HU is an indirect measure of how tight the contrast bolus is. (B) Similarly, for bolus tracking, a region of interest is placed inside the middle of the aortic lumen which subsequently measures the Hounsfield units of the aortic lumen on subsequent scanning. A time (seconds) versus HU (attenuation) curve is generated with HU on the y-axis versus time on the x-axis. When the aortic region of interest registers a HU of a pre-determined value, in this case 100 HU, this then triggers the scan to start.

6 136 Vascular Medicine 16(2) times between patients with different hemodynamic states and allows individualization of scan delays. It does, however, increase the total volume of iodinated contrast used and also increases the radiation dose required. The problem of flow discrepancy between the two lower limbs may be somewhat reduced by using an adaptive method of contrast detection, a variation of the test bolus technique, described by Qanadli et al. 33 Laswed et al. applied this adaptive method clinically in patients with PAD, with DSA correlation, and determined that MDCTA has a sensitivity and specificity of 100% for arterial lesion detection on a per patient basis. 34 When analysed on a per segment basis, MDCTA for lesion detection in the below-knee arteries had a sensitivity and specificity of 91% and 96%, respectively, and for the distal pedal arteries 100% and 90%, respectively. 34 This method was found to be reproducible, had high image quality, avoided the problem of venous overlay and resolved the issue of differential peripheral arterial opacification. 34 Limitations of this method include the complexity of choice of scan timings used (since peak enhancement of the popliteals can be quite variable), the additional contrast volume required for the test bolus and the complexity of the scan protocol. bolus is necessary. Thus, patients will need to have a wellpositioned, large-bore (16 18 G), intravenous cannula within the antecubital fossa. A dual-head power injector allows for both contrast and saline to be administered separately, concurrently and sequentially. This means that two separate injection phases are possible, the first with 100% iodinated contrast and the second with a 100% saline flush. The advantages of this method are that arterial contrast enhancement is both improved and also prolonged, and the contrast dose is reduced because the saline flush at the end clears contrast from the upper limb veins. 23,38,39 Additional scan acquisition Owing to inter-individual and inter-limb variability in arterial flow rates, the scan acquisition can sometimes be too fast and thus outrun the contrast bolus, resulting in failed contrast opacification of the run-off vessels. To compensate for this and avoid obtaining a non-diagnostic study, many authors advocate the option of a built-in second delayed acquisition from the lower thighs to the feet if poor opacification is observed during the first run. 8 Contrast timing: bolus tracking Another method for determining optimal scan enhancement is contrast bolus tracking. Used by many authors, 32,35,36 this is an efficient way of optimizing the peripheral arterial opacification. Initially, a single low-dose CT image is obtained, without contrast administration, at the level of the celiac axis. A mm 2 circular region of interest is placed inside the middle of the aortic lumen and this will subsequently measure the Hounsfield units of the aortic lumen on subsequent scanning. At 10 seconds following IV contrast administration, serial low-dose monitoring CT scans are obtained at the same table position (celiac axis level) at 2-second intervals. When the region of interest detects a preset contrast enhancement level (usually a HU value), there is automatic triggering of the scanner to acquire images in the desired scan range, usually from the level of the celiac axis to the feet (Figure 4B). This time-efficient method ensures optimal arterial enhancement within the region of interest. Its advantage over the test bolus method is its lower contrast usage. Contrast timing: summary Of the various techniques described above, each has advantages and disadvantages. When compared directly, bolus tracking was found to yield a higher attenuation value and more homogenous arterial enhancement than a timing bolus technique. 37 Also, the timing bolus method requires an extra ml of contrast medium. Thus, if contrast volume is a concern, then the bolus tracking method would be more appropriate. Dual-head power injectors In order to achieve the desired arterial Hounsfield unit, a fast iodinated contrast injection with a tight arterial contrast Post processing Owing to the large volume of data or data explosion 40 obtained with peripheral MDCTA, accurate, time-efficient and reproducible image post processing to enable disease interpretation is now a requirement. 8,41 43 The axial image plane, formerly the plane with the highest image quality in a single detector CT scanner acquisition, is no longer the only plane available for image interpretation. 44 For the majority of patients with PAD, axial image viewing is timeconsuming, inefficient and often less accurate than viewing reformatted images. 45 With MDCTA acquisition, near isotropic data sets can be obtained and thus manipulated in all imaging planes and projections without significant loss of image quality to enable an eloquent display of the arterial anatomy and pathology. 46 Therefore, a dedicated 3D workstation to enable a real-time interactive approach to image manipulation and interpretation has now become a necessity. Many authors advocate the use of 3D image display, both for diagnosis and procedure planning. 43,47 49 Available image post-processing techniques on the clinical 3D workstations include multiplanar reconstruction (MPR), maximum intensity projection (MIP), volume rendering (VR) and shaded surface display (SSD). Multiplanar reconstruction (MPR) The MPR algorithm enables a re-ordering of specific acquired image voxels along a predefined vascular center line to provide a 2D image of the vessel of interest. As all arteries are curved at some point in their distribution, curved MPR (CMPR) is an extension of the MPR process that enables display of a curved plane prescribed along an individual vessel contour or center line, thus displaying the entire vessel midline on a single 2D image 46 (Figures 5C, 5D and 6). CMPRs provide a comprehensive cross-sectional display of arterial luminal sizes over long segments and can

7 Keeling AN et al. 137 Figure 5A. A 72-year-old male presented with bilateral short distance intermittent claudication, worse on the left side. This axial image from the MDCTA at the level of the groin demonstrates a severe stenosis within the proximal left superficial artery (arrowhead) with marked arterial wall thickening consistent with atherosclerotic plaque. The right superficial artery also demonstrates a severe stenosis (curved arrow). Axial source data can be used to problem solve any issues identified within the reconstructed images. Figure 5C. The left-hand image demonstrates a volumerendered scout view with the curved arrow at the site of interest, the bifurcation of the left common femoral artery, which is represented in the curved multiplanar reformatted (CMPR) view. CMPR on the right demonstrates a severe stenosis in both the proximal profunda femoris artery and the superficial femoral artery (arrowhead). Note the CMPR view on the right (arrow) masks the profunda femoris stenosis. Figure 5B. A volume-rendered view identifies the left (arrowhead) and right (arrow) proximal superficial femoral arterial severe stenoses. be especially useful in reviewing large vascular territories such as the peripheral arterial tree. 50 CMPRs are the most equipped post-processing method to reduce artifact from vessel calcification and arterial stents. However, CMPR is user-dependent as it requires manual or semi-automated tracing of each vessel center line. 51 Also, it is imperative that at least two orthogonal planes for each arterial segment are created to ensure accurate quantification of eccentric atherosclerotic plaque. 8 The orthogonal planes are in fact Figure 5D. The left-hand image demonstrates a volumerendered scout view with the curved arrow at the site of interest, the bifurcation of the right common femoral artery, which is represented in the curved multiplanar reformatted (CMPR) view. The CMPR view on the right side demonstrates a severe stenosis in the proximal superficial femoral artery (arrowhead and arrow). Both CMPR views are at 90 to each other.

8 138 Vascular Medicine 16(2) a threshold attenuation value and selecting out the highest attenuation voxels along lines projected through the given volume data set. 54 These selected high-attenuation voxels are then incorporated into a 2D angiogram-like image, useful for demonstrating vessel opacification and residual vessel lumen. The limitations of MIP include vessel obscuration by other high-attenuation voxels, such as calcification, stents or bone, and the inabi lity to display 3D relationships of vessels and adjacent anatomical structures. 55 These limitations become a major problem with heavy arterial wall calcification or arterial stents, as the vessel lumen can become obscured. 32 Also, when vessel relationships need to be determined prior to surgical intervention, this 2D MIP method is limited. 56 However, MIP weaknesses can be reduced by editing adjacent high-density structures (bones, vessel wall calcification and stents), using alternate planes of projection and setting variable attenuation threshold values. 46 Figure 6. A 59-year-old female with 1-mile left-sided intermittent claudication underwent a peripheral MDCTA. The left-hand image demonstrates a volume-rendered scout view with the curved arrow at the site of interest, the left midsuperficial femoral artery, which is represented in the curved multiplanar reformatted (CMPR) view. The CMPR view on the right demonstrates a moderate stenosis (arrow), which is only identified on one of the CMPR views, confirming the eccentric nature of the plaque. This stresses the importance of two 90 CMPR views. essential for determining the true cross-sectional diameter of a vessel. Other limitations of CPRs are that only one arterial segment can be displayed at a time and the limited spatial perception due to the absence of anatomical landmarks such as vessel bifurcations. 51 A new potential solution for these limitations has been described: that of multipath curved planar reformations (MPCPRs). 52 This MPCPR method allows multiple longitudinal vessel cross-sections to be displayed simultaneously and therefore allows branching patterns to be seen without obscuring vessel wall calcifications and stents and enables restoration of spatial perception. Roos et al. report that MPCPR is currently the most comprehensive technique to visualize the peripheral arteries in patients with PAD; however, it cannot completely replace MIP and VR. 53 Maximum intensity projection (MIP) The technique of MIP provides images that most closely resemble those obtained with conventional DSA and therefore are often desired by interventionalists to enable a quick overall review of the peripheral vasculature for anatomical and significant lesion determination, prior to endovascular or surgical treatment (Figures 2B, 3A, 7A and 7B). To obtain a MIP image, a specific algorithm is applied to the source data within the 3D workstation. This algorithm involves applying Volume rendering (VR) Again, dedicated computer software on the 3D workstation allows for a VR algorithm to be applied to the source data. The principle of VR involves taking the entire volume of source data, adding the contributions of each voxel along a line from the viewer s eye through the data set, and displaying the resulting composite for each pixel of the display 55 (Figures 3B, 5B, 8 and 9). VR preserves 3D anatomical relationships, unlike MIP; however, VR, like MIP, still has limitations with vessel calcification and arterial stents (Figure 10). Therefore, this fundamental limitation precludes the exclusive use of VR and MIP techniques in a large proportion (approximately 60%) of patients with PAD. 57 Shaded surface display (SSD) SSD is a process in which apparent surfaces are determined within the volume of data and an image representing the derived surfaces is displayed. 55 SSD provides an anatomical overview but like MIP and VR, has difficulty discriminating calcification, fails to display lumen detail and can over-estimate stenosis; therefore, SSD is generally not recommended for vessel caliber measurements. 46 Combination of methods In clinical practice, a combination of the available 3D reformatting methods are employed to accurately diagnose PAD. In the event that complete evaluation of all arterial segments is hindered, then review of the original axial source images can be performed. Anatomical overview and quick significant disease localization can be achieved with the MIPs or the VR images. If arterial calcification or a stent obscure the arterial lumen, then a combination of MPR/CMPR is used. Dual-energy CT acquisition is a potential option for severe arterial calcification, as this tool enables subtraction of the wall calcification to unmask the adjacent arterial lumen. 58,59 The principal of dual energy lies in exploiting the energy dependence of the CT attenuation

9 Keeling AN et al. 139 A B Figure 7. (A) A 69-year-old male with a swelling behind the right knee. This sagittal maximum intensity projection (MIP) MDCTA image demonstrates a fusiform aneurysm of the proximal popliteal artery with minimal mural hematoma. (B) A coronal MIP MDCTA image shows a popliteal tight stenosis inferior to the fusiform aneurysm. of calcium and iodine, thus the Hounsfield unit will differ at different energy levels due to a difference in electron density of calcium and iodine. With a dual-source CT scanner, one tube can be set to 80 kv (Figure 1A) and the other to 140 kv (Figure 1B), enabling two different energy data sets to be acquired simultaneously with one acquisition. A dual-energy post-processing software program is available to enable calcified structures (atherosclerotic plaque and bone) to be identified separately from iodinated contrast, and subsequently calcification can be removed. Post-processing limitations Technical limitations to post processing may exist, with the main ones being secondary to heavy arterial wall calcification or to the presence of an arterial stent, as discussed above. Reviewing the axial source data can be a simple measure to overcome these problems. 50 Post-processing artifacts can also occur, secondary to inadvertent vessel subtraction, particularly with MIP images. Other limitations of MIPs include anatomic overlap of adjacent structures with increasing slab thickness, along with loss of structural detail with increased slab thickness. MIP images can be degraded by high-density structures within the imaged field of view, including bone, calcium, stents and coils. Also, with MIP, there can be a limited grading of stent lumens and thus difficulty with evaluating in-stent re-stenosis. With MPRs (especially curved MPRs), artifacts, such as production of pseudostenoses and/or occlusions as a result of inaccurate center-line definition, can occur. There can be limited spatial perception with MPR data sets. Operator dependence can be a limitation of CMPRs. Eccentric atherosclerotic plaque may be underestimated with the CMPR technique as a result of tracing only the center line of the artery. Volume-rendered images may not demonstrate aneurysmal dilation of a vessel due to partial luminal thrombosis. Again, the axial source data can act to solve these problems. Also, anatomic overlap of adjacent structures and loss of structural detail with increased slab thickness can occur with VR post-processing techniques similar to MIP. To date, there are no fully automated algorithms to detect vessel center-lines, to segment out bony structures, or to detect/subtract vessel wall calcification. However,

10 140 Vascular Medicine 16(2) Figure 9. A 61-year-old female with right-sided rest pain underwent a MDCTA. This volume-rendered posterior view demonstrates a long occlusion of the right distal superficial femoral artery (SFA) and popliteal artery with a short segment of popliteal reconstitution. Note a large collateral vessel from the SFA within the adductor canal. Figure year old female, status post uterine artery embolization for a fibroid uterus. A volume-rendered view showing a right external iliac artery (REIA) dissection (arrow to arrowhead) and the fibroid uterus (star). This dissection was surgically repaired with a vein bypass graft. with constant software updates, these tools are likely to become available in the near future. 8 A new investigational tool is dual-energy MDCTA that allows automatic subtraction of bone and potentially calcified atherosclerotic plaque, based on differential attenuation of bone, calcium and iodine on a dual-energy scan as discussed above. Figure 10. A 58-year-old female complaining of right-sided intermittent claudication at 200 m. This posterior volumerendered view of a MDCTA demonstrates bilateral superficial femoral arterial stents in situ, with evidence of a tight stenosis within the right superficial femoral artery distal to the stent (arrow). Clinical applications of peripheral MDCTA The most common indication for peripheral MDCTA in many centers is for further evaluation of PAD. The diagnosis has already been established clinically, with ankle brachial indices and/or duplex ultrasound, but the extent and severity of arterial involvement are of interest to vascular surgeons, interventional radiologists and vascular medicine physicians (Figure 5). The TransAtlantic Inter-Society Consensus (TASC) guidelines recommend appropriate endovascular or surgical treatment of PAD based on lesion location, lesion number, lesion severity, lesion length and lesion morphology. 60 Peripheral MDCTA is well positioned to evaluate all of these criteria to enable appropriate treatment planning. A recent meta-analysis reported a significant increase in both sensitivity and specificity for peripheral arterial lesion detection compared with DSA, when studies using 16- or 64-detector CT scanners were compared to those using two- or four-detector scanners. 28 Accurate depiction of the peripheral arteries was not possible with the use of single-slice CT scanners due to the lack of spatial resolution. 61,62 Four-detector scanners, while achieving high sensitivity and specificity within large arteries such as the aorta

11 Keeling AN et al. 141 and iliacs, % and % respectively, 32,36,63 still underperformed compared to DSA within smaller vessels such as the renals, internal iliacs and calf arteries. 32,36,63 When the number of detectors is increased to 16, not only do overall sensitivity and specificity for arterial stenosis detection increase, but so do the sensitivity and specificity for determining atherosclerotic lesions within the small popliteo-crural arteries, rising to 96.5% and 95.5%, respectively, in a study by Willmann et al. secondary to improved spatial resolution and increased interobserver agreement. 11 MRA is also a useful tool for TASC determination; however, cost, scanner availability, patient contraindications, local expertise and the risk of nephrogenic systemic fibrosis can limit this tool. 28 MDCTA can also be employed to perform bypass or vein graft surveillance 35 or follow-up of lesions treated with percutaneous transluminal angioplasty (PTA), 8 stenting, cryoplasty, atherectomy and endarterectomy. Availability and rapid access places MDCTA at the forefront for fast and complete arterial assessment in lower limb injury, both in trauma settings and iatrogenic events (Figure 8). 64,65 Lower limb aneurysmal (Figure 7) and pseudoaneurysmal (Figures 2 and 3) disease can be eloquently evaluated with MDCTA as sac size, relation to parent vessel, thrombus load and distal run-off can all be demonstrated prior to covered stent placement or surgical repair. 8 Rarer indications encountered include vasculitis, 65 arteriovenous malformations 35 and popliteal entrapment syndrome. 66 Limitations of MDCTA MDCTA, similar to conventional DSA, utilizes ionizing radiation to acquire image data with an average effective dose reported at 12.2 mgy by Catalano et al. 31 and a CT dose index of mgy, with an effective dose index of 9.3 msv reported by Rubin et al. 1 Interestingly, the effective dose index for MDCTA of the peripheral arteries was 3.9 times lower than that for DSA: 9.3 msv versus 36.2 msv, respectively. 1 Low-dose techniques, involving image acquisition at a reduced mas (i.e. 50 mas) have been reported to be feasible and accurate when compared to DSA. 10 MDCTA also necessitates the IV administration of contrast medium, which is a potentially nephrotoxic agent, and thus care needs to be employed in contrast administration to patients with renal impairment. There is a lack of dynamic information with MDCTA in comparison to timeresolved MRA or DSA; this can even render the scan nondiagnostic if there are marked differential flow rates in the run-off arteries below the knee. An adaptive method of contrast administration described previously may prove to alleviate this issue. 34 Similar to most imaging techniques, patient motion will degrade MDCTA image quality and result in blurred images. A strategy to reduce this problem is to wrap a towel around the patient s lower limbs and fix them together with adhesive tape in order to reduce any inadvertent limb motion. 1 Image display and data storage are also limitations, as vast amounts of data are acquired with each MDCTA examination. Remote access to large datasets is becoming more available with the use of thin clients and central servers for image processing. Conclusion Since the introduction of multidetector computed tomography (MDCT), one now has the ability to image the entire arterial tree in a non-invasive fashion in well under a minute. Attention to optimal technical considerations is essential for performing MDCT angiography in order to achieve accurate diagnosis. Care must be taken to maintain the radiation dose as low as reasonably achievable. Reproducible methods of image post-processing are now available. Previously, digital subtraction angiography was the only established reliable imaging technique to quantify atherosclerotic disease load; however, in this new millennium, MDCTA may now challenge this old gold standard. Acknowledgement There are no conflicts of interest. Funding There was no financial support for this study. References 1. Rubin GD, Schmidt AJ, Logan LJ, Sofilos MC. Multi-detector row CT angiography of lower extremity arterial inflow and runoff: initial experience. 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