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Vascular and Interventional Radiology |
1 From the Institute of Diagnostic Radiology (R.J., C.F., E.W., W.B., F.A.F.) and Vascular Surgery, Department of Surgery (W.L.), Friedrich-Alexander-University Erlangen-Nürnberg, Maximiliansplatz 1, D-91054 Erlangen, Germany; and Institute of Radiology, Landesnervenklinik Wagner-Jauregg, Linz, Austria (C.F., F.A.F.). Received December 8, 2003; revision requested February 13, 2004; final revision received May 12; accepted June 23. Address correspondence to R.J. (e-mail: rolf.janka@idr.imed.uni-erlangen.de).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Twenty-seven patients with peripheral arteriosclerotic occlusive disease underwent contrast-enhanced MR angiography with an automatic moving-table technique. In addition, lower-leg and pedal arteries were examined without table movement (hybrid technique). Two radiologists independently reviewed MR angiograms to assess image quality and grade stenosis in 13 segments per leg. Each was blinded to patients’ clinical data. Twenty-five of the patients also underwent conventional angiography. Stenosis grade at conventional angiography was assessed by two radiologists in consensus. Interobserver variability for stenosis grade at MR angiography was calculated with Cohen 
test. Specificity and sensitivity of MR angiography in detection of stenosis of more than 50% and occlusion were calculated for both observers. The study was approved by the local ethics committee.
RESULTS: In 14 of the 27 patients, hybrid technique was superior to moving-table technique because there was less venous overlap (11 patients), fewer motion artifacts (one patient), or both (two patients). In nine patients, there was no difference between techniques; in four patients, moving-table technique was superior. Stenosis grade was analyzed in 698 segments with MR angiography and in 638 segments with both conventional and MR angiography. Analysis of interobserver agreement with MR angiography yielded a score of 0.84. For the 638 segments evaluated with both conventional and MR angiography, observers 1 and 2 assigned same grade of stenosis with both modalities in 558 and 555 segments, respectively. Sensitivity for stenoses greater than 50% and occlusion was 94.4% and 91.1% for observers 1 and 2, respectively, and specificity was 90.6% and 91.3%. More distal runoff vessels were shown with MR angiography in seven cases and with conventional angiography in two cases.
CONCLUSION: Contrast-enhanced MR angiography of the peripheral vessels with a 1.0-T system and dedicated peripheral angiography coil is feasible, and in some cases, it provides additional information compared with conventional angiography.
© RSNA, 2005
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Up to now, most studies of peripheral contrast-enhanced MR angiography have used a 1.5-T unit. In a prior study performed at 1.0 T (15), contrast-enhanced MR angiography did not allow reliable evaluation of lower-limb arteries. That study, however, was performed with a body resonator rather than a dedicated phased-array coil. The purpose of our study was to prospectively determine whether it was feasible to perform contrast-enhanced MR angiography of the peripheral arteries from the distal aorta to the pedal arteries with a 1.0-T system and a dedicated phased-array coil.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Peripheral arteriosclerotic occlusive disease was graded according to Rutherford classification (7). Moderate claudication (grade I, category 2) was found in two patients, severe claudication (grade I, category 3) was found in 20, ischemic rest pain (grade II, category 4) was found in three, and minor tissue loss (grade III, category 5) was found in two. Risk factors for peripheral arteriosclerotic occlusive disease were cigarette smoking in 18 patients, hypercholesterolemia in eight, hypertension in 12, and diabetes in 10.
Our study was approved by the ethics commission of the University of Erlangen-Nürnberg. Informed consent was obtained from all patients.
Imaging Techniques
Contrast-enhanced MR angiography was performed with a 1.0-T MR unit (Magnetom Harmony; Siemens, Erlangen, Germany). The gradient field strength was 20 mT/m, and the minimal gradient rise time was 400 µsec. A dedicated peripheral angiographic coil was used (Peripheral CP Angio Array Coil; Siemens). This dedicated coil, which is for use at 1.0 T, was developed analogously to the 1.5-T peripheral MR angiography coil (11,14). The total length of the coil in the z direction is 950 mm. There are eight circularly polarized channels; four are on each side. The peripheral angiographic coil is compatible with other surface coils. In this study, we combined the body phased-array coil and the spine coil that is used to measure the pelvic region.
Contrast-enhanced MR angiography was performed with a three-station approach. Imaging parameters and automatic shimming for the three examination stations were selected independently. Three-dimensional data sets were collected with a three-dimensional fast low-angle shot sequence in coronal orientation, with a field of view of 450 mm. Sequence parameters and voxel sizes are shown in Table 1.
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Unenhanced images were acquired before the administration of contrast material. Butylscopolaminiumbromide (20 mg) was given intravenously to prevent bowel motion. A total of 30 mL of gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany) was injected into the cubital vein with an automatic power injector (Spectris; Medrad, Pittsburgh, Pa). For moving table angiography, 20 mL of gadopentetate dimeglumine was administered by using a biphasic protocol with a flow rate of 1.0 mL/sec (10 mL) and 0.5 mL/sec (10 mL) followed by a 20-mL saline flush (0.5 mL/sec). Single-step angiography of the lower-leg and pedal arteries was performed with 10 mL of gadopentetate dimeglumine (1.0 mL/sec) followed by a 20-mL saline flush (1.0 mL/sec). A delay of 5 minutes between moving table and single-step angiography was chosen to reduce contrast material artifacts from the first measurement. The postcontrast data set from single-step angiography was measured twice without delay. The timing of data acquisition after the administration of gadopentetate dimeglumine was calculated on the basis of bolus arrival time, as measured with the two-dimensional fluoroscopic sequence, plus 2 seconds. Postprocessing included subtraction of the unenhanced MR angiography data sets from the contrast-enhanced data sets and maximum intensity projection of the subtracted data. Maximum intensity projections were obtained in 11 projections over 180°, beginning from the left lateral projection and proceeding to the right lateral projection.
Conventional angiography was performed within 10 days (mean, 3.3 days) of MR angiography. All examinations were performed by a radiology resident and supervised by an interventional radiologist with 9 years of experience in angiography. The investigators performing conventional angiography were blinded; that is, they did not know which patients in their daily routine were patients from this study. The examinations were performed with a Polytron (Siemens) conventional angiography unit in six patients, an Iconos R220 (Siemens) unit in 11 patients, and an Axiom Artis TA unit (Siemens) in eight patients.
Pelvic angiography was performed after femoral arterial puncture and insertion of a 4-F Universal Flush catheter (Cordis, Miami Lakes, Fla) in the distal aorta. This catheter is shaped like a small sidewinder catheter with side holes. This shape facilitates a cross-over maneuver in the aortic bifurcation for selective angiography (Fig 1a, 1b). The region from the pelvis to the pedal arteries was examined on a step-by-step basis. In each step, 15–30 mL of iomeprol (Imeron 300; Altana Pharma, Konstanz, Germany) was injected with a flow rate of 4–12 mL/sec by means of a power injector (Mark V; Medrad). Selective catheterization of the femoral arteries was performed, and special projections were obtained if considered necessary.
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Image Evaluation
Each MR angiogram was evaluated independently by two radiologists (F.A.F., R.J.) with 5 years experience in contrast-enhanced MR angiography of the peripheral arteries. Images were evaluated with regard to image quality and stenosis grade. The image quality of the MR angiograms was evaluated for each of the four regions (pelvis, upper leg, lower leg, and pedal) according to three criteria: vascular anatomy (1 = very good, 2 = good, 3 = sufficient, and 4 = poor), motion artifacts (1 = no motion artifact, 2 = slight motion artifact, 3 = moderate motion artifact, and 4 = substantial motion artifact), and venous overlap (1 = no venous overlap, 2 = slight venous overlap of the superficial veins, 3 = strong venous overlap of the superficial veins, and 4 = venous overlap of the deep veins). Overall image quality was defined as the worst value achieved with any of the three criteria. If there was a difference in the quality of images obtained in the lower-leg and pedal arteries with the moving-table technique and the single-step approach, the observers had to determine the reason for the difference.
The signal-to-noise ratio of the lower-leg arteries with the moving table and single-step techniques was evaluated on the basis of the anteroposterior views of the maximum intensity projections. Thus, regions of interest were placed within the best-visualized lower-leg artery in the middle third of the lower leg on each leg on the same site (mean size, 0.07 cm2). Noise was measured as the standard deviation within a large region of interest (mean size, 80.30 cm2) between both legs.
In the analysis of stenosis grade, the vascular tree from the pelvic arteries down to the pedal vessels was divided into 13 segments for each leg (common and external iliac arteries, common and deep femoral arteries, proximal and distal superficial femoral arteries, popliteal artery, tibioperoneal trunk, anterior and posterior tibial arteries, peroneal artery, and dorsalis and plantaris pedis arteries). Thus, we evaluated 702 segments in 27 patients with contrast-enhanced MR angiography and 650 segments in 25 patients with conventional angiography. Stenoses were classified according to the following grades: Grade 1 indicated stenosis of less than 50%; grade 2 indicated stenosis of 50%–69%; grade 3 indicated stenosis of 70%–99%, and grade 4 indicated occlusion. The grade of stenosis for all segments was independently evaluated by two radiologists (F.A.F., R.J.).
The grade of stenosis for all segments was independently evaluated by two radiologists (F.A.F., R.J.). The grade of stenosis at conventional angiography was determined by two blinded radiologists (R.J., E.W.) in consensus. If the grade of stenosis determined with conventional and MR angiography differed by more than one point with the four-point scale, the difference was considered to be important.
Two radiologists (E.W., R.J.) and one vascular surgeon with 18 years experience in peripheral arteriosclerotic occlusive disease (W.L.) evaluated in consensus whether contrast-enhanced MR angiography image quality alone was sufficient for further therapeutic decisions. In doing so, they used all information from MR angiography, including images of the lower-leg and foot arteries. Thus, a reduction in image quality due to venous enhancement or motion artifacts with one of these methods did not have an influence on their decision. In the case of disagreement between the observers, a majority decision was made.
If there was a substantial difference between grade of stenosis at conventional angiography and that at MR angiography, an attempt was made to find out which method showed the correct anatomic situation. For this purpose, two radiologists and the vascular surgeon (E.W., R.J., W.L.) reanalyzed conventional and MR angiograms in consensus. If conventional angiograms revealed fewer distal runoff vessels or did not seem to show the correct anatomic situation, it was determined whether the examination had been performed properly, that is, with selective catheterization and a sufficient number of projections.
Statistical Analysis
Statistics were calculated for the entire vascular tree and for each region separately (pelvic, upper-leg, lower-leg, and pedal vessels). A P value of less than .05 was interpreted as indicating a statistically significant difference, and a P value of less than .01 was interpreted as indicating a highly significant difference. The entire statistical analysis was performed with SPSS for Windows (version 11.0; SPSS, Chicago, Ill).
The mean image quality of all MR angiograms was calculated for each observer for the pelvic, upper-leg, lower-leg, and foot regions.
The average signal-to-noise ratio of the lower-leg arteries, as determined with the moving-table technique and the single-step technique, was compared with the t test.
Interobserver variability of contrast-enhanced MR angiography with regard to grade of stenosis was calculated with the Cohen test. The specificity and sensitivity of contrast-enhanced MR angiography in the detection of stenosis of more than 50% and occlusion were calculated for both observers, with conventional angiography used as the standard of reference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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In two patients, eight segments of the lower leg and foot were not visualized sufficiently with conventional angiography. In one patient, this was due to an occlusion of the superficial femoral artery and a high-grade stenosis of the deep femoral artery. In the other patient, the segments could not be visualized sufficiently because selective catheterization was not performed. Thus, 642 of 650 segments evaluated with conventional angiography and 638 segments evaluated with both conventional angiography and contrast-enhanced MR angiography were analyzed.
Image Quality
A comparison of the two diagnostic strategies used with contrast-enhanced MR angiography of lower-leg and pedal arteries showed that the automatic moving-table technique was better than the single-step technique in four patients because of a better signal in the arterial vessels. The single-step technique was superior to the moving-table technique in 14 patients. Patient motion between the unenhanced and contrast-enhanced data sets was responsible for the lower image quality in three patients. Venous overlap impaired visualization of the arterial anatomy in 13 patients with the moving-table technique. Two patients had motion artifacts and venous overlap. There were no differences between the techniques in nine patients.
The average signal-to-noise ratio of the lower-leg arteries was 86.0 with the moving-table technique and 45.8 with the single-step technique. The difference was highly significant.
The mean image quality of the MR angiograms as determined by observers 1 and 2 was 2.0 and 1.8, respectively, in the pelvic region, 1.7 and 2.1 in the upper-leg region, 1.9 and 1.7 in the lower-leg region, and 1.9 and 2.0 in the foot region.
The radiologist and vascular surgeon determined that all MR angiograms were sufficient for planning therapy.
Grade of Stenosis
The degree of interobserver agreement, as calculated with the test for all segments, was 0.84. It was best in the lower-leg and foot region (0.86 and 0.87, respectively), followed by the upper-leg region (0.83) and the pelvic region (0.77).
For the 638 segments analyzed with both conventional and MR angiography, observers 1 and 2 determined that the grade of stenosis matched in 558 (87.5%) and 555 (87.0%) segments, respectively. The best result was achieved in the pelvis, with 133 of 146 segments having the same grade at conventional and MR angiography (91.1%, both observers) (Table 2). With use of conventional angiography as the standard of reference, the overall sensitivity of MR angiography for stenoses of more than 50% and occlusion was 94.4% for observer 1 and 91.1% for observer 2; specificity was 90.6% and 91.3%, respectively (Table 3, Figs 1, 2).
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Results of repeat analysis of conventional and MR angiograms showed that conventional angiography failed to show patent runoff vessels in seven segments. One of these segments was located in the upper leg (deep femoral artery), three were located in the lower leg (one in the tibioperoneal trunk and two in the anterior tibial artery), and three were located in the foot (two in the dorsalis pedis artery and one in the plantaris pedis artery). This was caused by occlusion proximal to that segment in five cases and failure to perform selective catheterization in two cases. In one case, a stenosis in the proximal superficial femoral artery was overestimated with conventional angiography because of an occlusion of the pelvic arteries. In another case, a high-grade stenosis in the distal superficial femoral artery was underestimated at conventional angiography because only a single-plane view was obtained.
In nine cases, MR angiography revealed a high-grade stenosis and conventional angiography showed no stenosis larger than 50%. Six of these stenoses were located in the upper-leg segments (three in the superficial femoral artery, two in the deep femoral artery, and one in the popliteal artery). All of these patients had multiple irregularities at conventional angiography. The remaining three segments were in the tibioperoneal trunk (1) and the foot (2). Two segments in the foot were determined to be occluded at MR angiography and patent at conventional angiography. In one of 21 segments, the correct anatomic situation could not be determined.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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It is interesting that interobserver agreement was better in the lower legs and feet than in the pelvic and upper-leg regions. In the pelvis and thighs, some arterial segments have stenoses and dilatations or postoperative changes (Fig 1), which makes it difficult to define the normal vessel diameter. This could have influenced interobserver agreement and may be an explanation for the better value in the lower-leg and foot regions.
Unlike most published studies performed at 1.0 or 1.5 T, in our series, pedal arteries were imaged and evaluated as well. In patients with critical leg ischemia, diabetic ulcers, or gangrene, the vascular surgeon needs information about the patency of the distal lower-leg arteries and the runoff situation, especially with regard to the integrity of the pedal arch (6,7,16). Kreitner et al (10) showed that contrast-enhanced MR angiography has the potential to depict these vessels and can show even more distal patent arteries than does conventional angiography (10).
The angiograms that did not depict two patent vessels were not obtained with a selective technique. The remaining five segments were located distal to occluded vessels. This phenomenon has already been described in the literature (17–19); authors noted that in patients with proximal severe obstructions or occlusions, more crural arterial segments were detected with contrast-enhanced MR angiography than with conventional angiography. In a review article based on 28 published studies of peripheral arteries with unenhanced MR angiography (13), contrast-enhanced MR angiography (14), or both (1), Eiberg et al (20) found inherent deficits of conventional angiography. They concluded that treatment of lower limb occlusive disease on the basis of only conventional angiography may be problematic.
In our study, none of the runoff vessels detected with contrast-enhanced MR angiography but not with conventional angiography was used for a distal bypass graft; therefore, it was not possible to verify the findings with surgery. Carpenter et al (21) showed that graft patency rates for bypasses of vessels depicted at contrast-enhanced MR angiography that were occult with conventional angiography are similar to those of bypasses performed in vessels detected with conventional angiography. In the study by Kreitner et al (10), treatment plans were changed in seven of 24 patients with diabetes and peripheral arterial occlusive disease after repeat evaluation of conventional angiographic findings on the basis of contrast-enhanced MR angiography.
Visualization of the lower-leg and pedal arteries requires good spatial and temporal resolution. In their study, Kreitner et al (10) examined just one lower leg and foot in the sagittal orientation by using a head coil. They achieved a voxel size of 0.8 x 1.5 x 1.1 mm. In our study, we used a dedicated vascular angiography coil that covered both legs and feet. The voxel size of lower-leg and pedal arteries was 0.9 x 1.3 x 1.2 mm with the moving-table technique and 0.9 x 1.3 x 1.3 mm with the single-step technique. These voxel sizes are comparable to those achieved by using the best results in previous studies with 1.5-T units (12–14,22) and are substantially better than those achieved in another study performed at 1.0 T by Laissy and co-workers (23), who achieved a voxel size of 1.6 x 1.8 x 4 mm. The acquisition time for the pelvic and upper-leg arteries was 42 seconds plus 14 seconds for two table movements. With use of the automatic moving table technique, this resulted in venous overlap in 13 of 27 patients, despite elliptical k-space filling. In the automatic moving-table technique, the time between unenhanced and contrast-enhanced imaging is longer than that with the single-step technique, which makes patient movement between images more likely. In three of 27 patients (two of whom also had venous overlap), image quality at the third station was dramatically reduced due to motion artifacts with the automatic moving-table technique. The signal-to-noise ratio of the lower-leg arteries with the automatic moving-table technique, however, was significantly better than that with the single-step technique. This is most likely due to residual contrast material from the first measurement, which resulted in residual signal in the arteries on the native image and signal loss after subtraction. In four of 27 patients, this led to better visualization of the lower-leg and foot arteries with the automatic moving-table technique. No difference between the techniques was seen in nine patients.
Measurement of the entire vascular tree from the pelvis to the pedal arch with one bolus of contrast material and two table moves seemed to provide good results in nearly half of our patients. With an additional image of the lower-leg arteries, the drawbacks of the automatic moving-table technique could be overcome in the other half of the patients independently of the severity of peripheral arteriosclerotic occlusive disease. New protocols like parallel imaging and faster gradient systems may reduce the acquisition time and make the automatic moving-table technique the method of choice.
There are some probable limitations in our study. One investigator (R.J.) analyzed both MR angiograms and conventional angiograms, and this could have biased the results of the study. In all cases, however, the interpretation of the conventional angiograms occurred at least 5 days after MR angiography. In addition, the conventional angiograms were evaluated in consensus with another radiologist.
Conventional and MR angiography are both indirect methods of depicting an anatomic situation. Both methods have their limitations. Conventional angiography is limited because it allows just a restricted number of projections. Furthermore, visualization of a vessel segment is dependent on the amount of contrast material reaching that segment, which can be reduced in distal to high-grade stenoses and occlusions. Contrast-enhanced MR angiography has a limited spatial and temporal resolution compared with that of conventional angiography. Khilnani et al (24) tried to overcome the latter problem by performing time-resolved two-dimensional MR angiography from the adductor canal to the feet.
Grading of stenosis in adjacent segments was assumed to be mutually independent. If the stenosis grades determined at conventional and MR angiography were significantly different, images were reevaluated with regard to any influence by an adjacent segment. Nevertheless, we cannot rule out the possibility that the grade of stenosis of one segment had an influence on that of an adjacent segment.
Calculation of sensitivity and specificity requires simplifying assumptions. One is implicit in the reduction of data to a two-by-two table, whereas another is that there exists a method, referred to as the reference standard, that yields an accurate picture with regard to stenoses of more than 50% (25). Because of its good temporal and spatial resolution, conventional angiography is accepted as the standard of reference. In our study, conventional angiography failed to depict seven patent runoff vessels. Maybe this rate could have been improved by using a contrast material with a higher iodine concentration or with intraarterial injection of a vasodilative substance.
In conclusion, these first clinical results suggest that contrast-enhanced MR angiography may be a sufficient imaging tool, even at 1.0 T, and it is comparable with and in some cases probably even superior to conventional angiography when it comes to the evaluation of vessel patency distal to statistically significant stenoses or occlusions. Peripheral contrast-enhanced MR angiography from the pelvis down to the pedal arteries, including the pedal arch, is feasible with good image quality with a 1.0-T system and a dedicated phased-array coil combination.
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, R.J., F.A.F.; study concepts, F.A.F., R.J., C.F., W.B.; study design, R.J., F.A.F., C.F., W.L., E.W.; literature research, R.J., W.L., C.F., F.A.F., E.W.; clinical studies, R.J., C.F., W.L., F.A.F., E.W.; data acquisition, R.J., C.F.; data analysis/interpretation, R.J., F.A.F., W.L., W.B., E.W.; statistical analysis, R.J., C.F.; manuscript preparation, F.A.F., R.J., E.W.; manuscript definition of intellectual content, R.J., F.A.F.; manuscript editing, R.J., F.A.F., C.F., W.L., E.W.; manuscript revision/review and final version approval, all authors
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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