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contrast-enhanced magnetic resonance angiography, duplex ultrasound, peripheral arterial disease. Introduction. Management strategies of peripheral arterial ...
Vascular Diagnostics

Diagnostic Accuracy of Contrast-Enhanced Magnetic Resonance Angiography and Duplex Ultrasound in Patients With Peripheral Vascular Disease

Vascular and Endovascular Surgery 44(7) 576-585 ª The Author(s) 2010 Reprints and permission: sagepub.com/journalsPermissions.nav DOI: 10.1177/1538574410377018 http://ves.sagepub.com

Dra Alicia Bueno, MD1, Francisco Acı´n, MD1, Dra Cristina Can˜ibano, MD1, Jose L. Fernandez-Casado, MD1, and E. Castillo, MD2

Abstract Introduction: Noninvasive techniques such as duplex ultrasound (DU) and contrast-enhanced magnetic resonance angiography (CE-MRA) are valid alternatives in the preoperative evaluation of such patients. Our aim is to assess the diagnostic accuracy of CE-MRA and DU in patients with peripheral arterial disease (PAD). Methods: Forty consecutive patients underwent DU, hybrid CE-MRA, and digital subtraction angiography (DSA). Magnetic resonance angiography and DSA images were evaluated independently and in a blinded fashion. Every segment was graded as normal, stenosed less than 50%, stenosed more than 50%, or occluded. Results: There were 1720 segments for analysis. Duplex ultrasound depicting stenosis >50% demonstrated a sensitivity (S) 81.4%, specificity (E) 99%, positive predictive value (PPV) 96.2%, and negative predictive value (NPV) 94.8%. Occlusions showed S 90%, E 97%, PPV 98.1%, and NPV 88.4%. Magnetic resonance angiography depicting stenosis >50% demonstrated a S 91%, E 99%, PPV 96.7%, and NPV 97.6%. Occlusions showed S 95.4%, E 98%, PPV 98.4%, and NPV 94.7%. Conclusion: Combined CE-MRA and DU is the first diagnostic approach in the preoperative assessment of PAD, leading to the use of DSA for selected cases Keywords contrast-enhanced magnetic resonance angiography, duplex ultrasound, peripheral arterial disease

Introduction Management strategies of peripheral arterial disease (PAD) depend on the patient’s clinical status and the precise location, length, and severity of the atherosclerotic lesion. Treatment planning differs for patients with intermittent claudication and patients with limb-threatening ischemia. Treatment options for lower extremity peripheral vascular disease have expanded, therefore the need for obtaining high quality diagnostic arterial images. Digital subtraction angiography (DSA) was considered gold standard, but arterial puncture, ionizing radiation, and potential nephrotoxicity of iodinated contrast agents are drawbacks. In addition, DSA has been questioned as gold standard because it may fail to reveal patent distal vessels, especially in patients with multilevel occlusive lesions and low inflow pressure.1 Duplex ultrasonography (DU) was developed in the 1980s as an alternative to DSA to acquire direct physiologic and morphologic information about affected arteries.2,3 It is a well-established noninvasive technique with good sensitivity and specificity; however, it is operator-dependent and does not provide and easy-to-interpret map of the vascular system that is useful for treatment planning.4 In the last decade, contrast material-enhanced magnetic resonance angiography (CE-MRA) has evolved into one of the 576

safest, fastest, and most accurate noninvasive diagnostic imaging methods for evaluating peripheral vascular disease.5-8 However, MRA may have some technical limitations. Patients’ demand for less-invasive methods of treatment and technologic advances are the key points in the evolution toward less-invasive diagnostic tests in the treatment of PAD.9 The aim of this study is to prospectively assess the diagnostic accuracy of CE-MRA using a hybrid technique and DU of the entire lower limb vascular tree, in patients with PAD.

Methods Patient Selection From July 2006 to January 2007, 40 consecutive patients scheduled for DSA because of lower extremity arterial disease 1 Vascular Surgery Department, Hospital Universitario de Getafe, Madrid, Spain 2 Radiology Department, Hospital Universitario de Getafe, Madrid, Spain

Corresponding Author: Dra. Alicia Bueno, Medical Staff Vascular Surgery Department, Hospital Universitario de Getafe, Crta Toledo Km 12 Getafe, 28905 Madrid, Spain Email: [email protected]

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Table 1. Risk Factors and Clinical Situation Number of Patients Hypertension DM Hyperlipidemia Ischemic cardiopathy Renal insufficiency Smokers Men Clinical situation (III/IV)

40

1720 segments

62.5% 57.5% 15% 25% 15% 60% 90% 15%/50%

Abbreviation: DM, diabetes mellitus.

were included in this prospective study. Patients with metal implants were excluded. The risk factors for cardiovascular disease in the patient population as well as their clinical situation are detailed in Table 1. All patients underwent duplex ultrasound, hybrid MR angiography, and DSA at our institution. Both, the symptomatic and the contralateral limbs, were included in the analysis.

Duplex Ultrasonography Ultrasound examinations were performed by one vascular surgeon credited for noninvasive laboratory explorations, using an Esaote scanner colour duplex US equipment (Esaote Technos, Ge´nova, Italy). The abdominal aorta and pelvic iliac arteries were scanned using a 2.5 to 5 MHz convex transducer. Lower limbs were examined with a 7.5 MHz linear transducer. In every arterial segment, the peak systolic velocity (PSV) ratios and the waveforms were recorded and studied. The common femoral artery waveforms were classified by shape (trifasic, bifasic, or monofasic) and by acceleration time (more or less than 120 msec). No contrast agent was used in any patient. The patients lay in supine position. Previous fasting time was not always possible for scanning the abdominal aorta. The entire peripheral arterial tree was studied from the abdominal aorta to pedal vessels in both limbs. The information obtained was depicted in an anatomical diagram of the entire vascular tree, which was presented to the rest of the members of the department. The hemodynamic significance of lesions in the aortoiliac and femoropopliteal tract was graded by peak systolic velocity (PSV) ratios, calculated as the PSV in the stenosis divided by the PSV in the prestenotic region. Significant lesions were identified by a PSV ratio of 2.5 or more, for a stenosis >50%, or by the absence of Doppler signals in case of occlusion. A segment with both stenosis and occlusion was graded as occluded. In addition to assessment of hemodynamic parameters, our study protocol also focused on the vascular anatomy and vascular arterial calcifications.

Magnetic Resonance Angiography Contrast-enhanced magnetic resonance angiography was performed using a 1.5-T MR unit system (Magnetom Avanto;

Siemens Medical Systems, Erlangen, Germany) equipped with high-performance gradients 30 mT/m and maximal slew rate of 125 mT/m per second. Our hybrid protocol consisted of 2 distinct acquisitions using a dedicated phased-array peripheral angio coil, assessing first the infragenicular arteries and followed by the remaining peripheral arterial tree, from abdominal aorta to popliteal arteries, for both limbs. All studies were performed without knowledge of the patient’s clinical situation. Infragenicular MRA. Lower limbs were positioned in external rotation to obtain maximal contact between the surface coils and the calf and ankle and to optimize the study of pedal vessels. The infrapopliteal arteries were studied in the coronal plane with a 3D gradient-echo T1-weighted fast low-angle shot spoiled sequence (FLASH; TR: 3.55 ms /TE:1.3 ms/ FA:25! ) with a field of view of 500 cm and an image matrix of 512 " 308 obtaining 88 partitions of 1 mm of thickness. In addition, a parallel imaging acquisition technique (iPAT) with an acceleration factor of 2 was used, resulting in a total acquisition time of 22 seconds. A series obtained before contrast enhancement was used as a mask for subtraction from subsequent series. Gadolinium-chelate (Multihance; Bracco, Milano, Italy) was administered with a dose of 0.1 mmol/kg body weight at 3 mL/s, followed by a 30-mL saline flush administered at the same rate. To optimize bolus tracking, we used a dedicated software (CARE bolus, Siemens Medical Solutions). Axial 2D single-slice multiphase gradient-echo images were acquired at the level of the popliteal artery to determine the arrival of the contrast bolus. The acquisition rate was approximately 1 image per second. The 3D contrast-enhenced sequence was launched when the bolus arrived in the popliteal artery (or the collaterals in cases of popliteal artery occlusion) and measured 2 times, followed by an automatically performed subtraction of the unenhanced 3D mask sequence. Aortic and lower limb MRA. After finishing the infragenicular study, a 2-station (aortoiliac and femoral) bolus chase MRA was performed with an identical 3D FLASH sequence using an additional phased-array body coil to the previously mentioned and a moving table technique. Only the first station was studied with the patient in breath-hold. The parameters (field of view, matrix size, partition number, and slice thickness) at each station were optimized to permit rapid scanning while maximizing resolution. Again, an unenhanced acquisition was first obtained for each station to serve as a mask for further examination. Real time bolus tracking was performed using the CARE bolus system mentioned previously. An axial 2D single-slice image was placed at the level of the yuxtarenal aorta to determine the arrival of the contrast bolus. The slice was repeated each second after injection of 0.01 mmol/kg of gadolinium-chelate. Two-station stepping-table MRA was performed when enhancement 3D mask sequence was automatically subtracted from the 3D contrast-enhanced sequence for each station. Angiograms of the subtracted images were created using the maximum-intensity projection (MIP) algorithm. 577

578 For each station, a series of 3 MIP images of each leg was generated using 3 projections, left anterior oblique (30! ), posteroanterior, and right anterior oblique (30! ). All MRA studies were printed on hard-copy films. Window level settings for all MIP images were adjusted to maximize arterial contrast and minimize the background signal. Postprocessing was standardized as follows: angiograms of the subtracted images were created using the MIP algorithm, and a series of 6 MIP images was generated for each 30! rotational increment from right lateral to left lateral. The images were presented in the anteroposterior projection, right lateral, and left lateral projections.

Digital Subtraction Angiography All DSA examinations were performed by experienced angiographers. They used a digital angiographic unit (Siemens Angiostar. Germany). They were unaware of patients’ identity and clinical situation and included from abdominal aorta to pedal vessels in both limbs. All the studies were done through a retrograde common femoral artery approach. Multiple DSA images were obtained by injecting iodixanol 320 (Visipaque, General Electric Healthcare) with a power injector (Medrad Mark V Plus). The DSA examinations consisted of anteroposterior overlapping evaluations of the lower extremities, except for the pedal and ankle stations, where a 15- to 30! -external rotation of the foot was used. For each station, the frame rate was 4 images per second, and images were acquired until complete lack of visibility of the most distal artery of the station. We did not use drug vasodilatation in any patient. Images were stored on a hard disk before filming. For each station, the first image of a series was selected as a mask. The unenhanced mask image was automatically subtracted from the contrastenhanced images for each station. The final angiogram was obtained by adding a selection of 2 to 3 subtracted images. Subtracted images were selected to obtain homogeneous opacification of the entire arterial tree along the feet-head axis. Images were printed on hard copy. Maximun time between DSA, MRA, and DU was 15 days, and they were performed without knowledge of either the patients’ clinical situation or other diagnostic tests. Some patients did not undergo the 3 diagnostic tests because of medical or organizational reasons, and these patients were excluded from the analysis.

Image Analysis For each DSA study and MRA, patient names were omitted and the clinical history number was assigned to each examination. Their lectures were done in a random way, in different order and on separate days. Magnetic resonance angiography and DSA images were evaluated independently and in a blinded fashion by 2 vascular surgeons who completed the same form. All disagreements were reviewed by a third reader who decided between the 2 prior categories. Analysis of the vascular tree was accomplished by 578

Vascular and Endovascular Surgery 44(7) dividing the arterial system into 43 segments per patient: abdominal aorta, common iliac artery, external iliac artery, common femoral artery, profunda femoral artery, superficial femoral artery (upper, middle, and lower third), popliteal artery (upper and lower half), tibioperoneal trunk, anterior tibial artery (upper, middle, and lower third), peroneal artery (upper, middle, and lower third), posterior tibial artery (upper, middle, lower third, and retromaleolar segment), and pedal artery. Every segment were graded as normal, stenosed less than 50%, stenosed more than 50%, or occluded. For both techniques, the degree of stenosis was measured by dividing the minimal luminal diameter in the segment by the maximal observed. When there was more than 1 stenosis in a segment, the stenosis with the highest grade was chosen for classification. The double and blind lecture of the duplex scanning was technically impossible. It was interpreted in the same way as the DSA and MRA in terms of number of segments and distribution and grading of stenosis. We have tried to adhere to the STARD guidelines for reporting of diagnostic accuracy studies.10

Statistical Analysis Statistics were calculated for the entire vascular tree and for each region separately (pelvis, upper-leg, and lower-leg). Separate interpretations were used to calculate the sensitivity and specificity, with 95% confidence intervals (CIs) of our CE-MRA and DU studies, for detecting stenoses greater than 50% and occlusions, using DSA as the gold standard. Interobserver agreement for CE-MRA and DSA was determined by calculating Cohen’s Kappa coefficient for each observer. Intertechnique agreement was determined for the overall analysis, infrapopliteal, femoropopliteal, and pelvic analysis, by calculating Cohen’s Kappa coefficient. Kappa values superior to .80 were considered an almost perfect agreement. Statistical analysis was performed by the SPSS statistical package (version 12.0).

Results A total of 1720 arterial segments were assessed using DSA as the gold standard.

Duplex Ultrasound Compared to DSA Compared with DSA, DU depicting stenosis >50% demonstrated a sensitivity of 81.4% (CI 0.76-0.85), specificity of 99% (CI 0.98-0.995), positive predictive value of 96.2% (CI 0.92-0.98), and negative predictive value of 94.8% (0.93-0.96). The values for diagnosing occlusion are sensitivity of 90% (CI 0.85-0.93), specificity 97% (CI 0.94-0.99), positive predictive value 98.1% (CI 0.95-0.99), and negative predictive value 88.4% (CI 0.83-0.91). The percentage of missing segments for analysis was 8.5%. The values by territories are shown in Table 2. Total examination time was 60 to 90 minutes.

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Table 2. Duplex Ultrasound (DU) Compared to Digital Subtraction Angiography (DSA)

Sensitivity (CI)

Especificity (CI)

PPV (CI)

NPV (CI)

Percentage Missing Segments for Analysis of the Territory

Aortoiliac

>50% Occlusion

100% (0.678-1) 100% (0.56-1)

99.5% (0.97-0.99) 100% (0.67-1)

91.6% (0.59-0.99) 100% (0.56-1)

100% (0.97-1) 100% (0.67-1)

10.7%

Femoropopliteal

>50% Occlusion

87% (0.78-0.92) 93.6% (0.83-0.98)

98% (0.96-0.99) 100% (0.94-1)

95.2% (0.87-0.98) 100% (0.92-1)

96% (0.93-0.97) 95.2% (0.87-0.98)

4.58%.

Infrapopliteal

>50% Occlusion

77% (0.7-0.83) 88% (0.83-0.92)

99% (0.97-0.99) 96.4% (0.91-0.98)

97% (0.92-0.99) 97.6% (0.94-0.99)

91.5% (0.88-0.93) 84.04% (0.77-0.89)

9.79%.

Abbreviations: NPV, negative predictive value; PPV, positive predictive value.

Table 3. Contrast-Enhanced Magnetic Resonance Angiography (CE-MRA) Compared to Digital Subtraction Angiography (DSA)

Sensitivity (CI)

Especificity (CI)

PPV (CI)

NPV (CI)

Percentage Missing Segments for Analysis of the Territory

Aortoiliac

>50% Occlusion

100% (0.69-1) 100% (0.56-1)

99% (0.96-0.99) 100% (0.69-1)

85% (0.56-0.97) 100% (0,56-1)

100% (0.98-1) 100% (0.69-1)

5.7%

Femoropopliteal

>50% Occlusion

97.8% (0.91-0.99) 97% (0.88-0.99)

98.3% (0.96-0.99) 100% (0.95-1)

95% (0.87-0.98) 100% (0.93-1)

99% (0.97-0.99) 97.8% (0.91-0.99)

1.45%

Infrapopliteal

>50% Occlusion

88% (0.82-0.92) 94.8% (0.91-0.97)

99% (0.98-0.99) 97% (0.93-0.98)

98.8% (0.95-0.99) 97.9% (0.95-0.99)

95.3% (0.92-0.97) 92.7% (0.87-0.95)

4.8%

Abbreviations: NPV, negative predictive value; PPV, positive predictive value.

Contrast-Enhanced Magnetic Resonance Angiography Compared to DSA Compared with DSA, CE-MRA depicting stenosis >50% demonstrated a sensitivity of 91% (CI 0.87-0.94), specificity of 99% (CI 0.98-0.99), positive predictive value of 96.7% (CI 0.93-0.98), and negative predictive value of 97.6% (0.960.98). The values for diagnosing occlusion are sensitivity of 95.4% (CI 0.92-0.97), specificity 98% (CI 0.95-0.99), positive predictive value 98.4% (CI 0.96-0.99), and negative predictive value 94.7% (CI 0.91-0.96). The percentage of missing segments for analysis was 4.01%. The values by territories are shown in Table 3.

Interobserver Agreement for CE-MRA and DSA For both reviewers and all locations, kappa values for interobserver agreement were .93 for CE-MRA and .72 for DSA. This difference was constant through all the territories. The results by territories are shown in Table 4. Complete agreement was found between CE-MRA, DSA, and DU in 83.6% of the segments (1438/1720), while 7.8% (135/1720) of the segments were classified in different categories in 1 of the 3 tests, and another 8.5% (147/1720) of the lectures were not available because of the missing data (Table 5).

Of the 135 discrepant results, 100 (74%) involved infrapopliteal vessels. Most of them involved segments that were occluded on DSA but patent or stenosed on CE-MRA and DU. By territories, the aortoiliac area had an 11.1% (31/280) of missing segments mainly for difficulties at performing the DU, 4.6% (22/480) for femoropopliteal area, and 9.8% (94/960) for infrapopliteal area. We did not have any problem in relation to venous contamination or motion artefacts as a cause for inadequate images, which could have rendered discrepant or missing results in CE-MRA.

Discussion Peripheral arterial disease is a part of systemic atherosclerosis. Continuous improvement in vascular and interventional techniques has opened a wide variety of options for the treatment of these patients. Unless intervention is being considered, imaging studies are not generally needed, and their results will influence the choice and nature of intervention. Imaging techniques have assumed a greater role in the decision-making process of whether a patient with PAD is a suitable candidate for endovascular or open-surgical revascularization. Conventional DSA is still the standard imaging technique used in the assessment of PAD, although it has a complication rate of approximately 1%. Contrast-enhanced magnetic resonance angiography and 579

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Table 4. Interobserver Agreement for CE-MRA and DSA

Aortoiliac Femoropopliteal Infrapopliteal General Concordance

CE-MRA Kappa (CI)

DSA Kappa (CI)

0.95 (0.93-0.97) 0.97 (0.95-0.99) 0.91 (0.88-0.93) 0.93 (0.91-0.95)

0.78 (0.66-0.9) 0.69 (0.62-0.75) 0.66 (0.62-0.70) 0.72 (0.68-0.74)

Abbreviations: CE-MRA, contrast-enhanced magnetic resonance angiography; DSA, digital subtraction angiography.

Table 5. Discrepant Results in the Three Tests, Analysed by Territory Aortoiliac Femoropopliteal Infrapopliteal Total Same results Different results Missing Total

247 2 31 280

425 33 22 480

766 100 94 960

1438 135 147 1720

DU are used as noninvasive or minimally invasive alternatives to DSA for the detection, grading, and treatment of PAD patients. Since the advent of contrast-enhanced ultrafast studies in 1995,11 MRA technology has remarkably improved. The Working Group on Cardiovascular Magnetic Resonance of the European Society of Cardiology reported that assessment of lower extremity arteries in patients with suspected PAD is a class I indication for MRA, based on the fact that it provides rapid and comprehensive information that allows quick and accurate decisions in clinical practice.12 There are various techniques for MRA evaluation, depending on gradient-echo sequence, spin-echo sequences, preparatory pulses, etc.13 Multiple studies comparing CE-MRA with DSA have shown a sensitivity and specificity greater than 95% in the detection of significant peripheral stenosis,14-23 with some authors declaring that MRA is as good as or superior to DSA in detecting distal run-off vessels, especially distal to highgrade stenoses or occlusions.1,8,24 One of the initial problems with the MRA was dependent on the timing of the image acquisition in accordance with the maximal contrast enhancement to optimize the signalto-noise ratio. Technological improvements prevent venous overlap.6,7 Previously to the application of different protocols like TRICKS,5 moving-bed infusion tracking technique, and the hybrid technique, the quality of images from distal vessels was poor. The problem in at least 5% to 10% of the studies23,25 was venous contamination. Kalle et al26 introduced the hybrid technique. This technique, used in our study, permitted a highquality and low-risk examination of the patients. As a result, we did not have any nonassesed segments because of venous overlap. These and other technical pitfalls of CE-MRA can be overcome with some strategies/techniques.27 One of these strategies is the use of tourniquets or blood pressure cuffs in a suprapopliteal location, and although effective, it is not a well-tolerated measure, especially in patients with critical limb ischemia. 580

Regarding the different protocols, the no-significant difference in diagnostic accuracy has already been described at the pelvic and thigh stations between bolus-chase and hybrid techniques, but the quality of the images are superior with the hybrid technique at the calf-distal station.23 The hybrid MRA enables an initial acquisition focused on distal vessels, thus avoiding the presence of early venous opacification and improving the spatial resolution. Our study confirms the good performance of a hybrid MRA protocol in PAD. Our results to detect the existence of stenosis superior to 50% in terms of sensitivity and specificity of 91% and 99%, respectively, and for detection of occlusion of 95.4% and 98%, respectively, compare favorably with other results.13-17,21-23,28-33 The superiority of the CE-MRA is also clear in the infrapopliteal vessels.13,16,20,28,30,31 In our study, 74% of the discrepant results among the 3 diagnostic tests involved the infrapopliteal vessels. Most of them were segments that were occluded on DSA but patent or stenosed on MRA or DU. Our interobserver agreement for MRA was 93%, which compares favorably with other concordances.5,14,20,31 This high interobserver agreement indicates that dedicated readers are interchangeable, and this fact makes the technique acceptable in clinical practice. Contrary to what some authors have reported,20 lack of experience with the MRA has not lead us to a bad interobserver agreement. This could be interpreted as a pitfall, due to the special interest employed in the reading of the images. In comparison with DSA, CE-MRA has some important advantages such as absence of risks of ionizing radiation, arterial catheterization, absence of calcium-induced artefacts, dynamic imaging, and 3D imaging data set. It is safe and has good repeatability. However, it has some disadvantages: it is not feasible in patients with metallic implants or severe claustrophobia nor is it accurate in case of vascular stents.34 In our study, we excluded from the analysis 1 patient who suffered from claustrophobia. Patients with vascular stents were also excluded. The often-cited lack of nephrotoxicity of CE-MRA is overcome and relies on the smaller amounts of gadolinium-base contrast agents (Gd-CA) used for MRA compared to computed tomography angiography (CTA) and DSA. Gadolinium-base contrast agents carry the potential to induce contrast-induced nephropathy (CIN) if applied at similar high doses as iodine contrast agents. Nephrogenic systemic fibrosis (NSF) is a disease related to the administration of Gd-CA in patients with chronic kidney disease and diabetic patients with nephropathy. The main etiologic factors contributing to NSF are renal impairment and gadolinium exposition. The Gd-CA dose and also the overall acumulative dose appear to contribute to the disease. It is very important to asses the creatinine level and the creatinine clearance in all patients. With the standard dose of 0.1 mmol-/kg body weight at 3 mL/s, no single case of CIN or NSF occurred in our patient population, and we did not administer higher doses. Other authors have described an incidence of 0.22% of NSF in cases of high-dose administration and no cases with the standard

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Table 6. Diagnostic Accuracy for Detection of Stenosis 50% or More or Occlusion With Different Assessment Methods Study

Number of Patients/Segments

Fontaine stage III/IV (%)

Sensitivity (CI)

Especificity (CI)

3/89 NR 13/0 15/50

91.9 (0.87-0.95) 99.5 (0.97-1) 96.3 (0.93-0.97) 91.0 (0.87-0.94)

63.7 (0.56-0.70) 98.9 (0.97-0.99) 96.9 (0.95-0.97) 99.0 (0.98-0.99)

NR 0/12 15/50

85.9 (0.76-0.92) 88.4 (0.83-0.92) 81.4 (0.76-0.85)

96.2 (0.91-0.98) 88.5 (0.83-0.92) 99.0 (0.98-0.99)

Fontaine stage III/IV (%)

Sensitivity (CI)

Especificity (CI)

NR 13/0 15/50

95.5 (0.88-0.98) 95.1 (0.91-0.97) 95.4 (0.92-0.97)

99.1 (0.98-0.99) 99.3 (0.98-0.99) 98.0 (0.95-0.99)

NR 0/12 NR 15/50

89.5 88.8 94.3 90.0

98.3 96.1 99.3 97.0

Contrast enhanced magnetic resonance angiography Cronberg16 35/418 Steffens17 50/900 Winterer24 76/1780 Present 40/1720 Duplex ultrasonography Hatsukami18 29/243 Sensier19 76/469 Present 40/1720 Abbreviation: NR, not reported.

Table 7. Diagnostic Accuracy for Detection of Occlusion With Different Assessment Methods Study

Number of Patients/Segments

Contrast enhanced magnetic resonance angiography Steffens17 50/900 Winterer24 76/1780 Present 40/1720 Duplex ultrasonography Hatsukami18 29/243 Sensier19 76/469 Zeuchner44 54/322 Present 40/1720

(0.78-0.96) (0.83-0.92) (0.84-0.98) (0.85-0.93)

(0.95-0.99) (0.93-0.98) (0.98-0.99) (0.94-0.99)

Abbreviation: NR, not reported.

dose. We carefully balance the potential risks and benefits of CE-MRA in patients with a creatinine clearance of less than 30 mL/1.73 m2 per minute. The increasing awareness of Gd-related complications in renally impaired patients forces us to stricter rules in application of CE-MRA in these patients. All of this is resulting in decreasing rates since 2007. The first report of peripheral arterial duplex ultrasound scanning dates from 1980s.3 Since then, the time-consuming requirement to evaluate both lower extremities and the operator dependence of the technique14,15 has not become widespread as a definitive preoperative diagnostic test, with the exception of some centers.35 Our results for detection of stenosis superior to 50% or more in terms of sensitivity and specificity of 81.4% and 99%, respectively, and for detection of occlusion of 90% and 97%, respectively, compare favorably with other results.14,21,22,36 In our laboratory, the aortoiliac level carries an 11.1% of missing segments. This compares favorably with other percentages.37 Direct visualization of aortoiliac segment is not possible in 5% to 25% of patients because of intestinal gas, vessels tortuosity, obesity, vessel depth, calcification, and a difficult angle of insonation. The assessment of the aortoiliac segments, waveform, and acceleration time are of special value when direct aortoiliac DU is not possible. This allows a quick, accurate, reliable, and noninvasive diagnosis that reduces the time needed for a complete lower limb DU.38,39 At the diffusely affected infrapopliteal level, where a clear anatomical increase in PSV is difficult to find, it is important to differentiate patent arteries and their suitability for surgery

or endovascular treatment, from occluded arteries. So, morphologic characteristics acquire an important role together with hemodynamic parameters in the distal territory with proximal occlusive disease. Tables 6 and 7 show data for detection of stenosis superior to 50% or occlusion. The clinically important question is whether CE-MRA and/or DU are useful for predicting which patients are candidates for revascularization, surgical or endovascular, or no treatment at all. In our study, CE-MRA and DU are equally effective in the decision-making process, with the exception of the infrapopliteal level, where CE-MRA was found to be more reliable. Nevertheless, the lower sensibility and specificity of DU at the infrapopliteal level would make us think that DU cannot detect any significant stenoses or occlusions. In practice, it is unlikely to misclassify a whole limb as normal, thus avoiding further imaging studies. All these results have to be interpreted in the context of DSA being gold standard. It is known that the stenoses percentages measured on single plane DSA images do not always correlate with either the true 3-dimensional morphologic features or the hemodynamic relevance of a given stenosis. So, single plane arteriography as gold standard can be questioned. In that way, this information can be obtained from DU and the 3D reconstruction of the MRA images. Grading vessel calcification has a high success rate in endovascular procedures and in choosing the target for distal bypasses.40 This can be achieved with DU but not with MRA. One criticism of our study is the design of the analysis of the image. Our analysis is segment-based and does not agree with 581

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Figure 1. The results of the contrast-enhanced magnetic resonance angiography (CE-MRA), digital subtraction angiography (DSA) and Duplex Ultrasound (DU) were presented in a map that showed the arterial tree from the aorta to the pedal vessels, so the information presented was the overall patient’s imaging information: 1.A to 1.C, Contrast-enhanced magnetic resonance angiography; 2.A to 2.E, Digital subtraction angiography; 3, DU.

this criticism. We accept that the exact limits of the different segments can overlap from one reading to another and from one reader to another, but contrary to what some authors say,33,20 we defend the validity of this kind of analysis per segment in terms of diagnostic accuracy. 582

However, the results of the CE-MRA and DU were presented in a map that showed the arterial tree from the aorta to the pedal vessels. The information presented was the overall patient’s imaging information (Figure 1). We defend the necessity of studies comparing the therapeutic decisions based on

Bueno et al patient’s history and preoperative imaging modality because the results after surgery or endovascular treatment are influenced not only by the preoperative studies but also by factors like quality of vein and operative skills. The choice between DU and CE-MRA for initial imaging work-up in patients with PAD will depend on the experience of the physician that performs the test and the quality of the equipment. De Vries et al,4 in a multicenter randomized trial, reduced the number of DSA after DU or CE-MRA in patients with PAD who needed to undergo imaging work-up by 42%. An explanation is that CE-MRA was presented as a road map with the whole lower limb vascular information, showing more clearly the location, severity, extent and grouping of stenoses, quality of outflow, and vascular wall appearance. Most vascular surgeons will not plan their interventions based solely on the results of DU mapping, with the exception of some centers.25,35 Contrast-enhanced magnetic resonance angiography is a minimally invasive alternative imaging modality for preoperative study of patients with PAD, and together with DU will show the presence of calcium on the vascular wall and the hemodynamic characteristics of the lesion. The extensive use of DU in endovascular therapies as the sole imaging test has been the reason for some authors to advocate the use of the term Interventional Ultrasonography.41 Other authors42,43 reduced DSA tests by 80% when performing DU as the first step in patients with critical limb ischemia. The suboptimal identification of distal targets ranged between 17%43 and 20%,42 in which case DSA should have been performed. We cannot offer this data because our analysis was per segment instead of by patient or whole artery, but we found no differences in interpretation between CE-MRA, DSA, and DU in 83.6% of the segments. These numbers could be translated to unnecessary DSA tests in an important number of patients. We cannot forget the patients’ preferences in the work-up of PAD. Visser et al9 reported that the majority of them had no preference between MRA and DU but most of them rejected the idea of DSA. In favor of both techniques is the noninvasive or minimally invasive nature and the outpatient characteristics. In up to 25% of patients, direct visualization of the aortoiliac level is not possible by DU. Also in the infrapopiteal sector, we find some difficulties due to vessel calcification. In both levels, disease characteristics are pivotal in the decission-making process. Contrast-enhanced magnetic resonance angiography adds some important advantages, especially in these 2 sectors: absence of calcium-induced artefacts and 3D imaging data set. These factors render CE-MRA as the imaging technique that complements the information acquired by DU. This precise information is used to make a decision. Duplex ultrasound is still operator-dependent, very timeconsuming in complete studies, and calcium can pose some problems, especially in diabetic patients. Although there is the possibility of selective studies and it is a good way to show the components of the atherosclerotic plaque like calcium. Magnetic resonance angiography has some drawbacks such as presence of metal implants, stents and cardiological devices,

583 which are still a problem for imaging, claustrophobia (less than 5% of the population), and overestimation of stenoses between 30% and 40% of the cases. However, MRA gives us multiplanar views, which have always been a disadvantage to DSA, especially in the aortoiliac segment. We are convinced that the combination of CE-MRA and DU should be the first diagnostic approach in the preoperative assessment of patients with PAD, whatever their category may be. We also recommend that DSA should be used in patients only when additional information is required. Declaration of Conflicting Interests The author(s) declared no conflicts of interest with respect to the authorship and/or publication of this article.

Funding The author(s) received no financial support for the research and/or authorship of this article.

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