Published online before print May 20, 2003, 10.1148/radiol.2281020375
(Radiology 2003;228:284-289.)
© RSNA, 2003
Peripheral Vessels: MR Angiography with Dedicated Phased-Array Coil with Large–Field-of-View Adapter—Feasibility Study1
Franz A. Fellner, MD,
Martin Requardt, PhD,
Werner Lang, MD,
Claudia Fellner, PhD,
Werner Bautz, MD and
Alexander Cavallaro, MD
1 From the Institute of Diagnostic Radiology (F.A.F., C.F., W.B., A.C.) and Department of Surgery (W.L.), Friedrich-Alexander-University Erlangen-Nürnberg, Maximiliansplatz 1, D-91054 Erlangen, Germany; Institute of Neuroradiology, Oberösterreichische Landesnervenklinik Wagner Jauregg, Linz, Austria (F.A.F.); and Department of Magnetic Resonance, Siemens Medical Solutions, Erlangen, Germany (M.R.). Received April 8, 2002; revision requested July 3; final revision received October 8; accepted October 14. Address correspondence to F.A.F. (e-mail: franz.fellner@gespag.at).
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ABSTRACT
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At magnetic resonance (MR) angiography with conventional phased-array coils, visualization of the vascular tree from the infrarenal aorta to the pedal arch is not possible in most patients. For this purpose, the authors developed a dedicated adapter with a large field of view that allows coverage of a body length of approximately 1,380 mm. Among five patients with peripheral arteriosclerotic disease, four underwent both conventional angiography and MR angiography. One hundred fourteen vascular segments from the infrarenal aorta to the feet were evaluated. Agreement between findings at conventional angiography and those at MR angiography was 94.7% (108 of 114) for all segments (96.1% [25 of 26] in the abdomen or pelvis, 97.5% [39 of 40] in the thigh, and 91.7% [44 of 48] in the calf or foot).
© RSNA, 2003
Index terms: Arteries, MR, 92.12942, 95.12942, 96.12942, 98.12942, 92.72, 95.72, 96.72, 98.72 • Arteries, peripheral, 92.12942, 95.12942, 96.12942, 98.12942, 92.72, 95.72, 96.72, 98.72 • Magnetic resonance (MR), vascular studies, 92.12942, 95.12942, 96.12942, 98.12942
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INTRODUCTION
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Owing to rapid advances in computer hardware and software, magnetic resonance (MR) angiography has developed increasingly into a noninvasive alternative to invasive conventional angiography. In a growing number of centers, it is replacing conventional angiography in the diagnosis of carotid stenoses (1–4). Results in preliminary clinical studies to evaluate peripheral vessels are promising (5–9). Further improvements are achieved with dedicated phased-array coils (10–12).
According to the guidelines of the TransAtlantic Inter-Society Consensus (TASC) working group for the use of imaging procedures in the management of peripheral artery disease (PAD), conventional angiography remains the method of choice (13). The TASC working group regards duplex sonography and MR angiography as alternative methods whose relative value in terms of possible use with conventional angiography still has to be established with further studies.
With regard to conventional angiography in the investigation of PAD, the TASC requires visualization of the vascular tree from the infrarenal aorta to the pedal arteries. Visualization of distal vessels is required for pedal bypasses, especially in patients with diabetes. Thus, MR angiography must satisfy this requirement if it is to replace conventional angiography. However, visualization of the entire vascular tree is only possible with very short patients by using dedicated phased-array coils. Therefore, we developed a dedicated adapter with a large field of view (FOV) to enlarge coverage of the dedicated phased-array coil system. The purpose of our study was to evaluate the feasibility of this system in patients with PAD.
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Materials and Methods
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Coil System
To visualize the vascular tree from the infrarenal aorta to the pedal arch, we used a 16-channel receiver system that consisted of the body and spine arrays and the peripheral angiographic coil. This 12-channel combination was expanded to include the large-FOV adapter and the body extender coil in a 16-channel system. The large-FOV adapter displaced the spine array coil at 370 mm along the table. At the same time, it deactivated the first two receptor elements in the spinal array, which freed a socket for the body extender coil. The two receiver channels of the body extender coil were then mapped onto the leads of the two deactivated spine array elements. In this way, about 1,500 mm of body length can be covered by array coils where each coil element images its own part of the patient’s anatomy. In the current study, however, we used a total coverage of 1,380 mm, with a 60-mm overlap between the different levels. Figure 1 shows the coil coverage of the 12- and 16-channel systems.
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Figure 1a. Functional description of the large-FOV adapter. (a) The adapter reroutes the circularly polarized spine coil signal (1, 2, 3) and, at the same time, frees a socket (4) for alternative use. This socket maps the signals from the body extender coil to the circularly polarized spine elements 1 and 2. (b) The large-FOV adapter (2) displaces the spine array coil at 370 mm along the table. (c) Sagittal scout images and (d) matching MR angiograms (coronal maximum intensity projections) obtained in different patients show the configuration of the coil elements (peripheral array and circularly polarized spine array coil) (c) without and (d) with the adapter. In c, visualization of the infrarenal aorta is incomplete. In d, the vascular tree can be seen from the thoracic aorta to the arteries in the feet.
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Figure 1b. Functional description of the large-FOV adapter. (a) The adapter reroutes the circularly polarized spine coil signal (1, 2, 3) and, at the same time, frees a socket (4) for alternative use. This socket maps the signals from the body extender coil to the circularly polarized spine elements 1 and 2. (b) The large-FOV adapter (2) displaces the spine array coil at 370 mm along the table. (c) Sagittal scout images and (d) matching MR angiograms (coronal maximum intensity projections) obtained in different patients show the configuration of the coil elements (peripheral array and circularly polarized spine array coil) (c) without and (d) with the adapter. In c, visualization of the infrarenal aorta is incomplete. In d, the vascular tree can be seen from the thoracic aorta to the arteries in the feet.
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Figure 1c. Functional description of the large-FOV adapter. (a) The adapter reroutes the circularly polarized spine coil signal (1, 2, 3) and, at the same time, frees a socket (4) for alternative use. This socket maps the signals from the body extender coil to the circularly polarized spine elements 1 and 2. (b) The large-FOV adapter (2) displaces the spine array coil at 370 mm along the table. (c) Sagittal scout images and (d) matching MR angiograms (coronal maximum intensity projections) obtained in different patients show the configuration of the coil elements (peripheral array and circularly polarized spine array coil) (c) without and (d) with the adapter. In c, visualization of the infrarenal aorta is incomplete. In d, the vascular tree can be seen from the thoracic aorta to the arteries in the feet.
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Figure 1d. Functional description of the large-FOV adapter. (a) The adapter reroutes the circularly polarized spine coil signal (1, 2, 3) and, at the same time, frees a socket (4) for alternative use. This socket maps the signals from the body extender coil to the circularly polarized spine elements 1 and 2. (b) The large-FOV adapter (2) displaces the spine array coil at 370 mm along the table. (c) Sagittal scout images and (d) matching MR angiograms (coronal maximum intensity projections) obtained in different patients show the configuration of the coil elements (peripheral array and circularly polarized spine array coil) (c) without and (d) with the adapter. In c, visualization of the infrarenal aorta is incomplete. In d, the vascular tree can be seen from the thoracic aorta to the arteries in the feet.
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To examine the abdominal and pelvic levels, four elements of the spine array were used together with two elements each of the body array and the body extender coil. For the thigh and lower leg levels, four of the peripheral angiographic coil elements were used.
The coil system, including the large-FOV adapter, is commercially available but is specific to the 1.0- and 1.5-T MR imagers used in the current study.
MR Imaging Techniques
All examinations were performed with a 1.5-T whole-body system (Magnetom Symphony; Siemens Medical Systems, Erlangen, Germany) with the coil system. The maximum magnetic field gradient was 30 mT/m, and the minimum gradient rise time was 240 µsec.
MR angiography was performed in three stages by means of automatic table movement. Images were acquired with a three-dimensional fast low-angle shot, or FLASH, sequence in the coronal plane. The three imaging levels were the following: (a) the abdominal and pelvic levels; (b) after automatic table movement, the crural and pedal vessels; and (c) after automatic table movement, the femoral vessels. At each level, three-dimensional MR angiographic data sets were acquired before and after intravenous administration of contrast material. Gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ) was administered with an automatic injector (Spectris; Medrad, Pittsburgh, Pa). Subsequently, the images were subtracted. The delay for acquisition of the three-dimensional data sets was calculated by measuring the arrival time of a test bolus (2 mL of gadopentetate dimeglumine at a flow rate of 1 mL/sec, followed by a 20-mL saline flush at a flow rate of 1 mL/sec). Temporal resolution was one image per second in the abdominal aorta.
For the pelvic and crural levels, a 10-mL bolus of gadopentetate dimeglumine was injected at a flow rate of 1 mL/sec followed by a 20-mL saline flush at a flow rate of 1 mL/sec. Imaging at the pelvic level was performed with a breath-hold technique. For the femoral level, a 15-mL bolus of gadopentetate dimeglumine was injected at a flow rate of 1 mL/sec followed by a 20-mL saline flush at a flow rate of 1 mL/sec.
The delay between the nonenhanced and contrast material–enhanced measurements at each level was calculated on the basis of the test bolus measurement in the abdominal aorta. For imaging of pelvic arteries, the delay was calculated as the bolus arrival time in the abdominal aorta minus 2 seconds. For imaging of arteries in the thighs, the delay was calculated as the bolus arrival time in the abdominal aorta plus 5 seconds. For imaging of arteries in the lower legs and feet, the delay was calculated as the bolus arrival time in the abdominal aorta plus 3 seconds. These delay times for the different levels proved to be valuable with regard to the different time intervals between the sequence start and the acquisition of k-space center (time to k-space center).
Measurement Parameters per Level
In the abdomen and pelvis, MR angiography was performed with the following parameters: repetition time msec/echo time msec of 3.85/1.39, flip angle of 25°, spectral fat saturation, 64 partitions, FOV of 500 mm, rectangular FOV of 68.8%, one signal acquired, acquisition time of 24 seconds, voxel size of 1.6 x 1.0 x 1.5 mm, and time to k-space center of 7.8 seconds.
In the thigh, MR angiography was performed with the following parameters: 3.78/1.38, flip angle of 35°, fat saturation, 64 partitions, FOV of 500 mm, rectangular FOV of 68.8%, one signal acquired, acquisition time of 24 seconds, voxel size of 1.6 x 1.0 x 1.4 mm, and time to k-space center of 7.7 seconds.
In the lower legs and feet, MR angiography was performed with the following parameters: 3.88/1.45, flip angle of 25°, fat saturation, 96 partitions, FOV of 500 mm, rectangular FOV of 68.8%, one signal acquired, acquisition time of 44 seconds, voxel size of 1.2 x 1.0 x 1.2 mm, and time to k-space center of 14.6 seconds.
The different levels were imaged with an overlap of 60 mm, which amounts to an overall image length of 1,380 mm. Geometric distortion in the craniocaudal direction at both ends of the FOV, which was 500 mm, was compensated completely by means of automatic distortion correction and the 60-mm overlap between levels.
Before the abdominal and pelvic levels were imaged, 1 mL of an antiperistalsis drug (scopalamine butylbromide, Buscopan; Boehringer, Ingelheim, Germany) was administered intravenously to reduce intestinal peristalsis.
Total acquisition time, including data processing, was approximately 40 minutes. Examinations in all patients were performed without problems.
Patients
In this preliminary study, five consecutive male patients (age range, 54–72 years; mean age, 62 years ± 8 [SD]) with a history of PAD were examined. One of these patients had diabetes and aneurysmal or cardiac disease. All patients gave written informed consent. The study had institutional review board approval. Four of the five patients successfully underwent conventional angiography 2–5 days before MR angiography (Polytron CA; Siemens Medical Systems). Pelvic conventional angiography was performed after femoral or brachial arterial puncture and insertion of a 4-F pigtail catheter in the distal aorta. The region from the pelvis to the feet was examined step by step. At each step, 15–30 mL of iodinated contrast medium (Imeron 300; Altana Pharm, Konstanz, Germany) was injected automatically (Mark V; Medrad) at a flow rate of 5–8 mL/sec.
In one of the five patients, MR angiography was performed first. In that patient, MR angiography and duplex sonography revealed identical results concerning the iliac and femoral arteries; therefore, the vascular surgeon did not see an indication for conventional angiography and decided to perform an iliac-femoral artery crossover bypass.
Image Evaluation
Stenoses in the four patients were assessed with both conventional angiography and MR angiography of the following vessel segments: infrarenal aorta, common iliac artery, external iliac artery, internal iliac artery, common femoral artery, deep femoral artery, proximal femoral artery, distal femoral artery, popliteal artery, tibioperoneal trunk, anterior tibial artery, peroneal artery, posterior tibial artery, dorsalis pedis artery, and plantaris pedis artery.
The conventional angiograms were evaluated initially by one radiologist (A.C.) to classify the degree of stenosis and then by a vascular surgeon (W.L.) to make treatment decisions. One week later, the MR angiograms were evaluated by another radiologist (F.A.F.), and again the vascular surgeon made treatment decisions, this time on the basis of the MR angiograms. Therapy was started after evaluation of the imaging findings; therefore, the interval between the two reading sessions was limited to 1 week.
The stenoses were classified into the following grades: 0%–49% stenosis, grade 1; 50%–69%, grade 2; 70%–99%, grade 3; and occlusion, grade 4. Agreement between conventional angiography and MR angiography was defined as identical grading of stenoses with both imaging modalities.
Venous overlap at the different levels was assessed with the following scale: 0 = no venous overlap, 1 = venous overlap without impairment of evaluation of arteries, and 2 = venous overlap with impairment of evaluation of arteries. The veins were defined according to the clinical anatomic classification of clinical signs, etiologic classification, anatomic distribution, and pathophysiologic dysfunction, or CEAP (14).
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Results
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The bolus arrival times in the abdominal aorta ranged between 26 and 38 seconds (mean, 31 seconds ± 5). With the regimes for calculating the delay times (as described in Materials and Methods), MR angiograms depicted the entire vascular tree from the infrarenal aorta to the pedal vessels in all patients. Owing to the increased coverage with the large-FOV adapter, the thoracic aorta was also depicted in each patient. The quality of the conventional and MR angiograms was considered adequate for determining a therapeutic regimen (Fig 2). The virtually isotropic voxel size also provided adequate quality for the sagittal and oblique sagittal-coronal maximum intensity projections (Fig 2b).
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Figure 2a. MR angiograms show peripheral vessels from the lower descending thoracic aorta to the pedal vessels in a 54-year-old patient with PAD. (a) Oblique maximum intensity projection shows occlusion of the right common and external iliac artery and depicts the superficial femoral artery (arrowheads). Arteriosclerotic mural irregularities of the common iliac artery are seen, as well as a moderate degree of stenosis of the external iliac artery (small arrow) and occlusion of the superficial femoral artery (large arrow). (b) Oblique and (c) sagittal maximum intensity projections depict occlusion of both superficial femoral arteries (arrows in b) and occlusion of the distal arteries to the pedal arch (arrow in c). Image quality is sufficient on both projections as a result of virtually isotropic voxel size.
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Figure 2b. MR angiograms show peripheral vessels from the lower descending thoracic aorta to the pedal vessels in a 54-year-old patient with PAD. (a) Oblique maximum intensity projection shows occlusion of the right common and external iliac artery and depicts the superficial femoral artery (arrowheads). Arteriosclerotic mural irregularities of the common iliac artery are seen, as well as a moderate degree of stenosis of the external iliac artery (small arrow) and occlusion of the superficial femoral artery (large arrow). (b) Oblique and (c) sagittal maximum intensity projections depict occlusion of both superficial femoral arteries (arrows in b) and occlusion of the distal arteries to the pedal arch (arrow in c). Image quality is sufficient on both projections as a result of virtually isotropic voxel size.
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Figure 2c. MR angiograms show peripheral vessels from the lower descending thoracic aorta to the pedal vessels in a 54-year-old patient with PAD. (a) Oblique maximum intensity projection shows occlusion of the right common and external iliac artery and depicts the superficial femoral artery (arrowheads). Arteriosclerotic mural irregularities of the common iliac artery are seen, as well as a moderate degree of stenosis of the external iliac artery (small arrow) and occlusion of the superficial femoral artery (large arrow). (b) Oblique and (c) sagittal maximum intensity projections depict occlusion of both superficial femoral arteries (arrows in b) and occlusion of the distal arteries to the pedal arch (arrow in c). Image quality is sufficient on both projections as a result of virtually isotropic voxel size.
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The stenosis classifications, which were determined visually, are shown in the Table. There was 94.7% (108 of 114) agreement between MR angiography and conventional angiography for the 114 vascular segments evaluated (Fig 3). Stenoses were overestimated with MR angiography compared with conventional angiography in five segments and were underestimated in one segment. In all patients, the difference was one grade. Agreement was closest at the abdominal or pelvic levels (96.1% [25 of 26] and the level of the thighs (97.5% [39 of 40]). A somewhat lower agreement (91.7% [44 of 48]) was found at the level of the lower legs or feet. In all four patients, treatment decisions were the same with MR angiography and conventional angiography.
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Figure 3a. MR angiograms obtained with the large-FOV adapter in a 56-year-old patient with PAD. (a) Coronal maximum intensity projection and (b) anteroposterior conventional angiogram show occlusion of the left superficial femoral artery (large arrows) and multiple low-grade stenoses (small arrows).
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Figure 3b. MR angiograms obtained with the large-FOV adapter in a 56-year-old patient with PAD. (a) Coronal maximum intensity projection and (b) anteroposterior conventional angiogram show occlusion of the left superficial femoral artery (large arrows) and multiple low-grade stenoses (small arrows).
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There was no relevant superimposition of veins in the crural sections that would have impaired vascular evaluation. In five patients (15 levels), assessment of venous overlap revealed no overlap in eight levels, overlap without impairment of evaluation of arteries in seven levels (three in the thigh and four in the lower leg or foot), and overlap with impairment of diagnostic evaluation in none of the levels. Classifications of overlapping veins, on the basis of clinical signs, etiologic classification, anatomic distribution, and pathophysiologic dysfunction, were A2, A3, A5, A13, and A18 (greater saphenous vein above knee, greater saphenous vein below knee, nonsaphenous veins, superficial femoral vein, and muscular and perforating veins of the calf, respectively).
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Discussion
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The use of MR angiography of the peripheral vessels to evaluate PAD is relatively new. Initial clinical results are promising with MR angiography compared with conventional angiography, particularly when dedicated phased-array coils are used (10–12). The clinical potential of this technique is acknowledged by the TASC working group: "In some cases MR angiography (especially with contrast enhancement) may be as good as conventional angiography. Further studies are needed to determine to what extent MR angiography may replace angiography" (13). In some cases, MR angiography is not only as good as conventional angiography but is superior, especially in cases of poor peripheral contrast at conventional angiography as a result of proximal occlusions (15–17).
To replace conventional angiography in imaging of PAD, however, MR angiography must satisfy the requirements for conventional angiography. In their guidelines for conventional angiography, the TASC working group requires "visualization from the infrarenal aorta to the pedal arteries." This is often not possible with conventional phased-array coil systems, particularly in tall patients. The phased-array coil system with the large-FOV adapter used in the current study allows complete visualization of the vascular tree from the descending thoracic aorta to the pedal arch. Thus, the current system not only meets the requirements of the TASC working group, it actually depicts the vascular system for a longer distance than is required clinically. The large-FOV adapter can be used without constraining the various MR angiographic techniques. MR angiographic techniques such as one-step acquisition, acquisition of all levels after one bolus injection of contrast medium, and hybrid techniques can be used.
Results of this feasibility study demonstrate the ability of MR angiography with this coil system to fulfill the clinical requirements for conventional angiography of peripheral vessels in cases of PAD given by the TASC working group (visualization from the infrarenal aorta to the pedal arch). This study, which was conducted to prove the clinical equivalence between conventional and MR angiography, was performed in a small group of patients. In the future, large clinical studies are needed.
In summary, development of this phased-array coil system with the large-FOV adapter is a step toward the potential replacement of conventional angiography with MR angiography in the diagnosis of PAD.
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FOOTNOTES
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Abbreviations: FOV = field of view,
PAD = peripheral artery disease,
TASC = TransAtlantic Inter-Society Consensus
Author contributions: Guarantors of integrity of entire study, F.A.F., W.B.; study concepts, F.A.F., C.F., M.R.; study design, F.A.F., A.C.; literature research, F.A.F., C.F., W.L., W.B.; clinical studies, F.A.F., C.F., A.C.; experimental studies, F.A.F., M.R., C.F., W.L.; data acquisition, C.F., F.A.F., A.C.; data analysis/interpretation, F.A.F., A.C., W.B.; statistical analysis, F.A.F., A.C.; manuscript preparation, F.A.F., C.F., W.L.; manuscript definition of intellectual content, F.A.F., A.C., W.B.; manuscript editing, F.A.F., M.R., W.L., C.F.; manuscript revision/review, W.L., W.B., C.F.; manuscript final version approval, F.A.F., C.F.
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R. Janka, C. Fellner, E. Wenkel, W. Lang, W. Bautz, and F. A. Fellner
Contrast-enhanced MR Angiography of Peripheral Arteries including Pedal Vessels at 1.0 T: Feasibility Study with Dedicated Peripheral Angiography Coil
Radiology,
April 1, 2005;
235(1):
319 - 326.
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ABSTRACT
INTRODUCTION
