CT angiography of the lower extremities The peripheral vascular system presents unique imaging...

July 2004 ONLINE SUPPLEMENT TO APPLIED RADIOLOGY � 45

Contrast Use in CTA Applications

Computed tomography (CT) is asimple and robust method forevaluating the peripheral arterial

system and for diagnosing peripheral arte-rial disease. Advances in technology andvariability in patient physiology makecontrast administration challenging, how-ever. This article will review patientpreparation for CT angiography (CTA)of the lower extremities, as well as opti-mal scanner settings, contrast administra-tion, and visualization techniques.

The essentialsA multirow scanner is essential for

CTA of the lower extremities; however,the number of detector rows the scanneris equipped with (4, 8, or 16) is of lessimportance. In fact, all of the clinicalstudies of lower-extremity CTA reportedin the scientific literature to date havebeen conducted on 4-row scanners, withimpressive results.1-5

Several very practical details are im-portant in ensuring the success of lower-extremity CTA. It is recommended thattape be used to keep the patient’s kneesand feet together. (Usually, at our insti-tution, we wrap a pillowcase or towelaround the knees and feet first.) Withouttape, the patient’s natural inclination is tolet the knees fall away from one another. Ifthat happens, the field-of-view in the re-construction must be opened substantiallyin order to include the proximal anterior

tibial arteries, and in-plane resolution islost. The patient must not have a pillowunder the knees, however, as the arterialsystem will move up and down relative tothe table, which creates a similar problemfor the reconstruction field-of-view.

The patient should be positioned nearthe isocenter of the scanner. Centeringthe patient provides the best in-plane andthrough-plane resolution for visualizingwhat are frequently very tiny vessels.

We use a 20-gauge antecubital intra-venous line for contrast delivery, andnonionic iodinated contrast material. Thescanning range for most studies is fromthe celiac artery through the toes, a totalof 105 to 130 cm, depending on theheight of the patient. The entire studytakes about 20 minutes to perform.Three-dimensional (3D) visualization isessential. Volume renderings, maximumintensity projections (MIPs) and curvedplanar reformation (CPRs) are all usefulvisualization methods.

Scanner settingsThere are a range of approaches to

image acquisition, depending on thedetector configuration and the desiredcoverage. Full anatomic coverage, fromthe celiac arteries through the toes (105to 130 cm), is generally indicated for theevaluation of atherosclerotic occlusivedisease. More limited distal coverage (40to 60 cm) is typically indicated prior to

reconstructive surgery, including fibulartransfer grafts and pectoral flap mobiliza-tion for revascularization of an area ofosteomyelitis or tissue injury secondaryto trauma.

Figure 1 describes available detectorconfigurations for full anatomic coverage,using a 4-, 8-, or 16-row scanner. In eachcase, only 1 configuration emerges as anacceptable choice. For example, with a 4-channel scanner, a 2.5- to 3.0-mm colli-mation will accomplish the study in 30 to40 seconds, whereas a 1.0- to 1.5-mm col-limation will result in an unacceptablyslow image acquisition. With an 8-chan-nel scanner, by comparison, a detectorcollimation of 2.5 to 3 mm is seldom used,because a higher-resolution alternative is available (8 × 1.0 to 1.5 mm). With the16-row scanner, a 0.5- to 0.75-mm colli-mation results in excessive image noise,particularly in the abdomen and pelvis. A1.0- to 1.5-mm collimation accomplishesthe study in 15 to 20 seconds, with anacceptable noise level.

Figure 3 shows a full-coverage acquisi-tion from above the celiac arteries throughthe feet, using a 4 × 2.5-mm detector colli-mation. The patient had claudication, andfemoral pulses were absent. The scanreveals occlusion of the distal abdominalaorta, proximal common iliac, and leftsuperficial femoral arteries, with exten-sive collateralization reconstituting thelower extremity arterial system.

CT angiography of the lower extremitiesThe peripheral vascular system presents unique imaging challenges.

Geoffrey D. Rubin, MD and Dominik Fleischmann, MD

Dr. Rubin is an Associate Professor of Radiology and Section Chief of Cardiovascular Imaging, and Dr. Fleishmann isan Assistant Professor of Radiology at Stanford University School of Medicine, Palo Alto, CA.

46 � ONLINE SUPPLEMENT TO APPLIED RADIOLOGY


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The study took approximately 70 sec-onds to complete. With 2.5-mm-thicksections, the vessels are a little less dis-tinct than they are with thinner sections,but image quality is more than adequatefor making important observations onvessel patency and stenosis, and for guid-ing routine therapy.

Figure 4 demonstrates a study of dis-tal anatomy, acquired with a 4-row scan-ner and a 1.5-mm detector collimation.This patient had osteomyelitis and waspreparing to undergo surgery to revascu-larize the distal calf. The MIPs depict thearterial system with the intricate detailnecessary for surgical planning.

The images in Figure 5 were acquiredwith 16-row scanner and a 0.625-mmdetector collimation. In this patient with a tibial plateau fracture, CTA was per-formed to determine whether the pop-liteal artery had been injured. There is noocclusion and no extravasation of contrastmaterial. Such studies have become a com-mon application of CTA in our practice.

Contrast deliveryFor all their advantages, fast scan

acquisitions can complicate contrastdelivery. Table 1 outlines three protocolswe have used for peripheral runoff stud-

ies performed on 4-, 8-, and 16-rowscanners. A total of 89 patients are repre-sented. (Unpublished data.)

There is an almost 4-fold reduction inimage acquisition time when comparinga 4-row scanner with a 16-row scanner.Enhancement is not necessarily betterwith the 16-row scanner, however.Defined as the average arterial attenua-tion from the aorta to the feet, enhance-ment increases substantially when goingfrom a 4-row to an 8-row scanner,because contrast delivery is much faster.Similarly, contrast efficiency, defined asaverage enhancement divided by theiodine load and normalized to the cover-age distance, goes up by about 50%.

Contrast efficiency improves furtherwith the 16-row scanner. Average attenua-tion drops, however, because there is notas much time for the contrast bolus to fullydevelop and opacify the vessels. Whethersuch differences in attenuation affect theaccuracy of diagnosis or clinical manage-ment decisions is yet to be determined.

Certain aspects of contrast utilizationare unique to imaging the peripheral vas-culature. A separate study, as yet unpub-lished, by Dominik Fleischmann, MD, atour institution, examined peripheral arte-rial enhancement in 20 patients with

peripheral arterial occlusive disease.After injecting a small test-bolus, Fleis-

chmann observed an aortic contrast transittime of 14 to 28 seconds, with a mean of20 seconds—standard findings for aorticCTA. To measure the contrast transit timebetween the aorta and the popliteal artery,he injected a second contrast bolus anddocumented its arrival in the poplitealartery. The aortopopliteal transit timeaveraged 10 seconds, representing anaverage contrast flow rate of 65 mm/sec.There was substantial variability amongpatients, however, with a minimum of 4 seconds (177 mm/sec) and a maximumof 24 seconds (30 mm/sec)—almost a 4-fold difference. What is intriguing is thatit confirms angiographic experience, asthere was no correlation between contrastflow rate and clinical stage of disease.Therefore, contrast transit times are not predictable by any means other than direct measurement.

Table 2 presents the effect of contrastflow rates on scanning parameters.With a slower scan (detector collima-tion: 4 × 2.5 mm, 8 × 1.25 mm, or 16 ×0.625 mm), table speed is 30 mm/sec.The time it takes to scan from aorta toankle, or a distance of about 1200 mm,is 40 seconds. In a patient with a low

FIGURE 1. Detector configurations for a full anatomic scan, from theceliac arteries to the feet.

FIGURE 2. Detector configurations for distal scan, from knees to feet.



FIGURE 3. (A and B) Full-coverage acquisition from above theceliac arteries through the feet, using a 4- × 2.5-mm detector colli-mation (frontal and lateral maximum intensity projections). Thepatient had claudication, and femoral pulses were absent. The scanreveals occlusion of the distal abdominal aorta, proximal commoniliac, and left superficial femoral arteries, with extensive collateral-ization reconstituting the vascular system. The study took approxi-mately 70 seconds to complete. (Reprinted with permission fromRubin GD, Schmidt AJ, Logan LJ, Sofilos MC. Multidetector row CTangiography of lower extremity arterial inflow and runoff: Initial experi-ence. Radiology. 2001;221:146-168.1)

FIGURE 4. Study of distal anatomy, acquired with a 4-row scanner anda 1.5-mm detector collimation. This patient had osteomyelitis and waspreparing to undergo surgery to revascularize the distal calf. The maxi-mum intensity projection images depict the arterial system in theexquisite detail necessary for surgical planning. (Reprinted with permis-sion from Rubin GD, Schmidt AJ, Logan LJ, Sofilos MC. Multidetectorrow CT angiography of lower extremity arterial inflow and runoff: Initialexperience. Radiology. 2001;221:146-168.1)

A B


blood flow rate (30 mm/sec), contrastflows in perfect synchrony with thetable speed. In a patient with an aver-age blood flow rate, however, contrastflow from aorta to ankle outpaces thetable speed. The contrast bolus must beat least 22 seconds long to ensure thatopacification is still adequate as thefoot is being scanned. In a patient with

a fast blood flow rate (177 mm/sec), thecontrast bolus must be at least 33 sec-onds in length.

With faster scans (detector collima-tion: 8 × 2.5 mm or 16 × 1.25 mm), thetime it takes to scan from aorta to ankleis only 20 seconds. In a patient with aslow blood flow rate, the scanner canoutrun the contrast bolus, rather than

lagging behind it. In some patients, thishas created a challenge when attempt-ing to examine vessels in the feet using a16 × 1.25-mm detector collimation. Insuch cases, it is necessary to wait 20 sec-onds after contrast arrival in the aorta tobegin the scan, so that the vessels of thefeet are opacified at the time of dataacquisition. (The goal is to image the tailof the bolus in the abdomen and the headof the bolus in the feet).

Because there is substantial andunpredictable variability among patientsin lower-extremity flow rates, the bestapproach appears to be the use of a longcontrast bolus—30 to 35 seconds—in allcases. In short, bolus duration is drivennot by the speed of the scan but by theneed to ensure that the entire arterial sys-tem, from aorta to feet, is opacified at thetime of image acquisition.


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FIGURE 5. (A) Frontal volume-rendered image. (B) Posterior oblique volume-rendered image. (C) Cranial oblique volume-rendered imagefollowing digital disarticulation of the femur and patella. Images acquired with a 16-row scanner and a 0.625-mm detector collimation. In thispatient with a tibial plateau fracture, CTA was performed to determine whether the popliteal artery had been injured. There is no occlusionand no extravasation of contrast material.

A B C

Table 1. Peripheral runoff studies (n = 89)

Scanner 4-row 8-row 16-rowRotation time (sec) 0.8 0.5 0.5Detector configuration 4 × 2.5 8 × 1.25 16 × 1.25Scan time (sec) 70 44 22Contrast concentration (mgI/mL) 300 350 350Iodine dose (g) 54 58.8 42Attenuation (HU) 282 418 365Contrast efficiency (HU/gI•mm) 6.4 9.1 11.5Data collected by Alessandro Napoli, MD. (Unpublished data.)


Determining the optimal time to beginimage acquisition is more complex thanmerely fitting the contrast bolus into thescan range, however. It may not be desirableto image just as the bolus arrives. The in-tensity of enhancement builds up over time,so the distal end of the bolus is far moreenhancing than it is at the proximal end.6

How the scan will be triggered is anotherimportant consideration in determininghow much contrast to deliver. There aretwo fundamental ways to determine thedelay from venous injection of contrastmaterial to enhancement of the abdominalaorta. The first is to use a preliminary testbolus. With this approach, the time to peakcontrast enhancement directly measurescontrast medium transit time. Direct bolustriggering is the other approach. Auto-mated or visual detection of contrast arrival,defined by a predetermined attenuationthreshold, triggers scanning.

The length of the contrast bolus willdepend on whether scan timing is deter-mined by a test bolus or bolus triggering.Assume that a 30- to 35-second injectionis needed to ensure adequate opacifica-tion of the entire periphery in the major-ity of patients. That bolus size will besufficient if scan timing is determined bya preliminary test injection. A longerbolus will be necessary, however, if scantiming is determined by bolus triggering,because of the delay between the time ofcontrast arrival at the abdominal aortaand the actual triggering of the scan. Thisdelay varies from one scanner to another,

but it can be up to 8 seconds. If the delayis 8 seconds, then a 38- to 43-second con-trast bolus will be needed.

With fast scanning, scanning should bedelayed by 20 seconds, even if a test bolusor bolus triggering is used. (It is not neces-sary to add 8 seconds to the bolus length to account for the delays associated withbolus triggering, however.) The benefit ofthe 20-second delay is that imaging takesplace later in the rising contrast enhance-ment curve, which potentially results inmore homogeneous and intense opacifi-cation throughout the acquisition.

A further complication of contrastdelivery is that blood flow in the legsmay be asymmetric if, for example, thereis a tight stenosis or aneurysm on onlyone side. In such cases, use of an evenlonger contrast bolus may be warranted.

Venous opacification may make itmore difficult to evaluate the arterial sys-tem in certain cases. Early venous opaci-fication almost always occurs in cases ofipsilateral inflammation, resulting fromcellulitis or ischemic ulceration. Both arecommon in patients with peripheral vas-cular disease. Occasionally, venousopacification is attributable to sponta-neous arteriovenous shunting, which inpatients with atherosclerotic occlusivedisease is likely due to plaque rupture insmall vessels.

Visualization techniquesMaximum-intensity projections pro-

vide a useful overview of the peripheral

vessels. To effectively use MIPs in theperiphery, however, bone must be editedout. This is a time-intensive endeavor,although tools are becoming available tomake it more efficient. In addition, calci-fication can obscure underlying vessels.

Curved planar reformations are longi-tudinal sections along the length of thevessel. A CPR enables visualization ofboth soft and hard plaque and is espe-cially useful in cases of circumferentialcalcium, where it permits examination ofthe lumen adjacent to the calcium,including regions of stenosis. In addition,no bone editing is necessary.

A CPR does not provide as good anoverview of the peripheral vasculatureas does the MIP, however. In addition,this technique is insufficient unless win-dow and level settings are optimized.When evaluating calcification in periph-eral arteries, for example, it is importantto use a bone window to avoiding“blooming” and overestimation of calci-fication size. In very small vessels, suchas the peroneal artery, even a CPR maynot be able to discriminate arterial lumenfrom calcification.

ConclusionCT has become a simple and robust

method for assessing the peripheral arte-rial system and diagnosing peripheralarterial disease. Optimization of contrastadministration is increasingly challeng-ing as scanner speeds increase. Newways to evaluate each patient’s circula-


Table 2. Influence of contrast flow rates on image acquisition

Slow scan––Detector collimation: Fast scan––Detector collimation: 4 × 2.5, 8 × 1.25, 16 × 0.625 8 × 2.5, 16 × 1.25Table speed = 30 mm/sec Table speed = 60 mm/sec

Blood Contrast travel CT table travel Min. CT table travel Min.flow time to ankle time to ankle bolus length time to ankle bolus length

(mm/sec) (sec) (sec) (mm) (sec) (sec)30 40 40 0 20 (20)65 18 40 22 20 2177 7 40 33 20 13

Aorta to ankle distance = 1200 mm. Data collected by Dominik Fleischmann, MD. (Unpublished data.)


tory physiology and compensate for itwhen administering contrast materialare needed.

Advances in scanner speed are unlikelyto reduce the volume of contrast used dur-ing examination of the peripheral vascularsystem but will improve image resolution.Finally, additional detector rows createadditional data to analyze. We must con-tinue to find more effective and automatedways to manipulate and process such largedata sets.

REFERENCES1. Rubin GD, Schmidt AJ, Logan LJ, Sofilos MC. Multi-detector row CT angiography of lower extremity arterialinflow and runoff: Initial experience. Radiology.2001;221:146-158.2. Martin ML, Tay KH, Flak B, et al. Multidetector CTangiography of the aortoiliac system and lower extrem-ities: A prospective comparison with digital sub-traction angiography. AJR Am J Roentgenol. 2003;180:1085-1091.3. Ofer A, Nitecki SS, Linn S, et al. Multidetector CTangiography of peripheral vascular disease: Aprospective comparison with intraarterial digital sub-traction angiography. AJR Am J Roentgenol. 2003;180:719-724.4. Catalano C, Fraioli F, Laghi A, et al. Infrarenal aorticand lower-extremity arterial disease: Diagnostic perfor-mance of multi-detector row CT angiography. Radiol-ogy.2004;231:555-563.5. Ota H, Takase K, Igarashi K, et al. MDCT comparedwith digital subtraction angiography for assessment oflower extremity arterial occlusive disease: Importanceof reviewing cross-sectional images. AJR Am JRoentgenol. 2004;182:201-209.6. Fleischmann D. Present and future trends in multipledetector-row CT applications: CT angiography. EurRadiol.2002;12 (suppl 2):S11-S15.

DiscussionLEO P. LAWLER, MD, FRCR:

Geoff, thank you; it was a very goodtalk. I agree that nothing exposes ourdeficiency in terms of our understandingof contrast dynamics as much as multi-detector CT. It seems to me, however,that as we go forward with peripheralvascular imaging, it will require someway of knowing what the Hounsfieldunits are doing both in the arteries andthe veins as you scan, knowing what ishappening both proximally and distally.Then, somehow, you could feed thatback to the table pitch. That seems to methe only way forward to stop venous

contamination and to make sure asym-metric inflow disease is imminent. Whatare your thoughts?

GEOFFREY D. RUBIN, MD:With respect to venous contamination, Ithink we are in much better shape withCT than with MR. It seems to me thatvenous contamination typically onlyoccurs in a pathologic setting in whichwe have abnormal arteriovenous shunt-ing. In many respects, I like to be able toknow that it is there and to see it. So I amnot sure that it is a big problem movingforward.

But what you have said about beingable to somehow detect and compensatefor the variations in flow is very, veryintriguing. The question is, can we reallydo that on the fly? We know that what ishappening at the injector will influencelocal arterial opacity sometime, perhaps 20 seconds, in the future at least; in somecases, even more. I am not absolutely cer-tain that there is a way that we can moni-tor the progression of the injection andfeedback to have any effect on the scan.But, I do wonder if there are routines thatwe might develop to preliminarily test aperson’s circulation, to spot-check theaorta or the popliteal artery. Then wecould use that to come out with the appro-priate strategy. But, at this stage, that is arather cumbersome thing to do.

Nevertheless, I think it should be ourgoal to develop an algorithmic approach to at least understand these contrastdynamics. If we do know what these aortopopliteal transit times are, then we should understand the optimum wayto deliver contrast for that individual.

LAWLER: It is a very small area and itis highly selective. So, perhaps, it is not amajor issue for a lot of people. But I dothink the question really arises in cases such as in the posttraumatic leg that ishyperemic or perhaps if they are going tofix an ulcer with a free flap. This is reflectedby the lack of literature below the calf: thequestion is: Can I sacrifice this vessel tomake a free flap? I think you would be verycourageous to make that decision when

you have venous contamination, and youcannot confidently say that all three ves-sels running into the foot are clearly open.

RUBIN: It has been our experiencethat, even in the setting of venous con-tamination, we always look at the trans-verse sections and we believe we canvisualize arterial anatomy in spite ofvenous contamination. We have a papersubmitted specifically addressing thoseissues. The surgical plan was always inagreement with what was demonstratedon imaging, and the outcomes of theoperations were viable at least 3 monthsdown the line, which suggests that CTdoes adequately provide the information.

However, in particular, in acutely trau-matized patients in whom there is massivesoft-tissue injury, you can see remarkablevenous opacification, particularly in theupper extremities. I think there we seemore venous contamination than in thelegs, and it really just requires some dili-gence to examine the transverse sectionsand mentally filter out the veins. As long asyou are paging through, it works prettywell. But you are right, the volume render-ings can be very confusing.

U. JOSEPH SCHOEPF, MD: Ibelieve the images that you show aremuch more impressive than anythingthat we have available with MR thesedays, for example. But where exactly doyou see CT as opposed to MR in its indi-cation for doing that kind of study?

RUBIN: Right now it is pretty clearthat the rate of voxel acquisition for anequally sized voxel on CT is far outstrip-ping MR. As I have indicated, we are get-ting to the point at which CT may be get-ting too fast and we have to wait for thebolus anyway. I think MR’s great hoperight now is the successful developmentof parallel imaging, which is going to re-quire more widespread use of higherfield-strength magnets, like 3T. A highfield-strength magnet will compensatefor the signal-to-noise limitations thatcome about from parallel imaging whileallowing for the improved spatial resolu-tion that is possible with parallel imaging.


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So, who knows? If there is widespreaddeployment of 3T MR scanners or maybehigher field-strength scanners and suc-cessful implementation of parallel imag-ing, then maybe MR will give images likethis. But at this stage, you are right. We tryto do peripheral MRA as frequently aspossible, but at this stage, CTA is almostalways a better quality study.

SCHOEPF: Also, I was really in-trigued by the data that you showed onhemodynamics in the lower extremityrunoff studies. One of the arguments thatis brought against the use of CT, overdigital subtraction angiography, forexample, is exactly that point, that youare not able to visualize or assess a phe-nomenon, such as arterial inflow, todetermine whether the lesion within thelower runoff is really hemodynamicallysignificant. Do you believe that some ofthose developments might come inhandy to overcome those limitations?

RUBIN: In general, what our vascularsurgeons like to repeatedly remind us isthat they treat patients, not scans; or theytreat patients, not angiograms. Therefore,they are going to perform bypass on apatient based on that patient’s symptoms.They know that if there appears to be ahigh-grade stenosis downstream andthere is a high-grade stenosis upstream,they need to fix them both. Otherwise,their graft is ultimately going to be lessviable and potentially thrombose.

So these issues of using the contrastdynamics in the setting of occlusive dis-ease to lead to treatment have been lessrelevant for us. There are situations, suchas in arterial venous fistulae, which obvi-ously are very specialized. In those cases,the contrast dynamics may be more use-ful. As we move ahead to 64-row CTscanners, chances are, we will be able topark ourselves at the nidus of an arteri-ovenous malformation and really ob-

serve these sorts of phenomena. But I donot think that comes up very frequently atthis stage.

ELLIOT K. FISHMAN, MD: Atleast in my experience with peripheralimaging of both upper and lower extrem-ities, saline chasers have worked reallywell. I think it really helps in terms of get-ting good homogeneous opacification,especially of the lower extremities. Doyou have any experience doing that?

RUBIN: Yes. We are using salinechasers on the dual-chamber injector.Although, I must say that I do not have areally good intuition that our opacifica-tion is any more homogeneous. Typically,what it does is prolong the bolus, whichgives us a little more usable bolus.

I am not sure that homogeneous opaci-fication is necessarily what we are afterhere in the periphery. In a sense, the proxi-mal vessels are big and the distal vesselsare small and are more susceptible to vol-ume averaging. So, in some respects, Ithink it is a good thing that the vessels getbrighter as you go along, because youhave a harder time detecting small vesselsthan you do big ones. I am sure that usinga saline chaser is an advantage. It is defi-nitely an advantage to return pooled contrast in the veins into the central circu-lation. I think we need more investigationas to what ultimate impact it has on thequality of the CTA data set.

FISHMAN: Right, but I think it hasmade a very big difference in terms of get-ting smaller vessels in, say, the legs. Wehave been using a 50 mL saline chaser.That seems to have made a major differ-ence, at least for us.

W. DENNIS FOLEY, MD: Geoff, Iwas interested in your different transittimes. You said that it was unrelated tothe patient’s clinical symptomatology.

RUBIN: Yes, unrelated even to theseverity of their disease.

BRIAN R. HERTS, MD: Was it cor-related with heart rate?

RUBIN: It was not correlated withanything that we measured.

LAWLER: You are filling the tube ofthe aorta with so much contrast, and it is abig elastic artery that is so highly vari-able. So, before the contrast even gets tothe lower extremities, there is going to bea massive variation. It will vary even ifyou use a constant because of the rate thatends up feeding into the inflow vessel ofthe iliac. Again, it is going around thewhole lungs, as well, before it comes tothe left side. There are so many variablesbefore you even get there.

RUBIN:There are a tremendous num-ber of variables in terms of the relativevascular resistance of different majorbeds and what percentage of the cardiacoutput is being directed to the legs or not.That is difficult.

FOLEY: You probably have to de-termine the transit time from the aorta tothe ankle.

RUBIN: But, how are we going to dothat? You notice that I made a big as-sumption here, which is that it is a con-stant rate. Dominik measured from theaorta to the popliteal and I just assumedthat it is the same rate down to the ankle.I think it is going to be very difficultbecause if you just give a 16-mL bolus,you are not going to be able to see that inthose tiny vessels down there.

So I think it is probably not unreason-able to consider that the aortopoplitealtransit time is very closely linked to theaortoanterior tibial or dorsalis pedis tran-sit time. I think we are going to have tobegin operating on the assumption thatmeasuring aortopopliteal and not aorto-ankle arterial transit times probablywould get us pretty close and just seewhat limitations are associated withthose measurements.

CT angiography of the lower extremities The peripheral vascular system presents unique imaging...

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