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Vascular and Interventional Radiology |
1 Department of Radiology, Nagasaki University School of Medicine, 1-7-1 Sakamoto, Nagasaki 852-8501, Japan.
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Abstract |
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MATERIALS AND METHODS: Twenty-three patients suspected of having lower extremity ischemia underwent three-dimensional dynamic contrast-enhanced subtraction MR angiography of the aortoiliac arteries and arteries of the lower extremity. As the reference standard, conventional angiography was also performed. For data analysis, the arterial system was divided into 10 segments. Each segment was classified as normal, mildly stenosed, moderately stenosed, severely stenosed, or occluded.
RESULTS: At conventional angiography, 83 stenosed segments (14 mildly stenosed, 16 moderately stenosed, 14 severely stenosed, and 39 occluded) were identified in a total of 423 segments. For the segments with more than mild stenosis, MR angiography was 97.1% sensitive and 99.2% specific.
CONCLUSION: Three-dimensional dynamic contrast-enhanced subtraction MR angiography has high sensitivity and specificity. This technique is a noninvasive alternative to conventional angiography for screening patients suspected of having lower extremity ischemia.
Index terms: Arteries, extremities • Arteries, MR, 92.12917, 92.129412, 92.12942, 92.12943, 98.12917, 98.129412, 98.12942, 98.12943 • Arteries, stenosis or obstruction, 92.72, 98.72 • Magnetic resonance (MR), comparative studies, 92.1211, 92.12942, 98.1211, 98.12942
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Introduction |
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MR angiography of aortoiliac arteries and arteries of the lower extremity has been hampered by the need to depict long segments of the arteries in short imaging times. On the basis of the T1-shortening effect associated with intravenously administered contrast material, a three-dimensional (3D) sequence with very short repetition times has been developed that enables the assessment of vascular disease (2,6). Because contrast-enhanced techniques are combined with a 3D sequence with very short repetition times, imaging with a 3D contrast-enhanced technique can be performed along the length of a vessel in a shorter examination time.
In recent reports (1,2,4), 3D contrast-enhanced subtraction MR angiography has been used successfully for the evaluation of patients with atherosclerotic occlusive peripheral vascular disease. In the present study, a 3D fast spoiled gradient-echo (SPGR) sequence with image subtraction was evaluated in patients after the injection of contrast material. A coronal acquisition was completed in 32 seconds for a limited number of sections that covered the area of interest. The purpose of this study was to determine the clinical feasibility of 3D dynamic contrast-enhanced subtraction MR angiography of the aortoiliac arteries and arteries of the limb (above the ankle joint) in patients with clinically symptomatic lower extremity ischemia. To our knowledge, this study represents the first blinded evaluation of long arterial segments in which 3D contrast-enhanced subtraction MR angiography was compared with conventional angiography.
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MATERIALS AND METHODS |
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The indications for this study were intermittent claudication in 19 patients (83%) and rest pain in four patients (17%). Patients with intermittent claudication had a Fontaine grade of IIa or IIb. The Fontaine classification describes peripheral vascular occlusive disease in terms of four categories: grade I, asymptomatic; grade IIa/b, mild to moderate claudication; grade III, rest pain; and grade IV, tissue loss. Nonhealing ulceration was not seen in any patient, and none of the patients had undergone amputation.
MR Angiography
MR angiography was performed with a 1.5-T Signa Horizon (n = 19) or Advantage (n = 4) imager (GE Medical Systems, Milwaukee, Wis); the torso multicoil array was used for the entire study. Three acquisitions usually were needed (Fig 1). The patient's feet were taped together to prevent motion artifact. Venous access was achieved with a 21-gauge needle inserted in the antecubital fossa or forearm. For contrast enhancement, gadopentetate dimeglumine (Magnevist; Nihon-Schering, Osaka, Japan) was administered by means of hand injection at about 10 seconds before the start of imaging. Normal saline solution (10 mL) was injected after the contrast material to flush the intravenous tubing. The contrast material was administered at a dose of approximately 5–6 mL for each injection, with a maximum total dose of 15–20 mL (maximum of 0.2 mmol/kg) for multiple injections.
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Images of three axial localizing sections were obtained with a two-dimensional (2D) fast SPGR sequence as a guide to select the appropriate coronal plane. A 3D fast SPGR sequence (9.8/1.4 [repetition time msec/echo time msec], 256 x 128 matrix, one signal acquired, 20°–30° flip angle, 2–3-mm section thickness, 28 sections, 32–45-cm field of view) was used after the injection of contrast material. A coronal acquisition was completed for a limited number of sections that covered the area of interest. The imaging time per measurement was 32 seconds.
The imaging protocol was completed with three contiguous data acquisitions. The first measurement was used as the mask. The measurement of the aortoiliac arteries was begun 30–35 seconds after the start of the injection of contrast material, during a breath hold. The measurement of the arteries of the lower extremity was begun 35-40 seconds after the start of the injection without the use of a breath-hold technique. The magnitude data of the first measurement usually were subtracted on a section-by-section basis from the magnitude data of the second measurement. The data from the third measurement usually were discarded. The purpose of the third data acquisition was to measure delayed administration of contrast material in elderly patients and in patients with poor cardiac output. The data of the aortoiliac arteries and the arteries of the entire limb were acquired in less than 40 minutes, which included the time needed for patient positioning. A maximum intensity projection algorithm was used for image reconstruction, typically at 18° increments around the coronal orientation (Fig 2).
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Imaging Analysis
All MR angiograms were separately evaluated by two experienced radiologists (E.S., I.S.). Conventional angiograms were also interpreted in a blinded fashion by the same radiologists at a different time. After the lesions were graded on both MR and conventional angiograms, each lesion was cross checked for correspondence. Vascular segments that were graded differently by the two observers were reevaluated for a final consensus interpretation.
For the comparative analysis of the performance achieved with the techniques, the arterial system was divided into 10 segments: the common iliac artery, the external iliac artery, the common femoral artery, the superficial femoral artery, the popliteal artery, the tibioperoneal trunk, three trifurcation vessels (above the ankle joint), and bypass grafts. Each segment was evaluated for patency during MR angiography and conventional angiography by using a five-point scale, on the basis of the most severely altered portion of the segment. Each segment was classified as normal, mildly diseased (<50% luminal narrowing), moderately diseased (50%–74% luminal narrowing), severely diseased (75%–99% luminal narrowing), or occluded. Otherwise, the lesion was interpreted as severely diseased with reconstitution.
Statistical Analysis
Calculations of sensitivity, specificity, and accuracy for detection of stenotic lesions were made in each patient by using conventional angiographic findings as the standard of reference.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Conventional catheter angiography has remained the reference standard for the evaluation of peripheral ischemia. The method is, however, invasive and expensive. Complication rates of 2%–5% have been reported, of which up to 0.14% represented to major complications, including hemorrhage, arterial injury, thrombosis, infection, and adverse reactions to iodinated contrast material (4, 10–12). As a result, a noninvasive imaging technique that demonstrates atherosclerotic disease, arterial stenosis, or vascular occlusions in patients with peripheral occlusive disease would be of clinical value.
The usefulness of MR imaging for demonstrating flowing blood has been well documented, and many techniques have been reported. The 2D time-of-flight technique has developed into the standard MR angiographic method used for the examination of patients with atherosclerotic occlusive disease of the aortoiliac arteries and the arteries of the lower extremity. However, the 2D time-of-flight technique has major limitations. Imaging with a nonenhanced 2D time-of-flight technique in the lower extremities is difficult, because it requires a long imaging time and is often limited in regions of turbulence or slow flow, which are commonly present in diseased lower extremity arteries (2,13).
In recent years, 2D and 3D contrast-enhanced techniques have been successfully used to study vascular disease. In general, thinner sections can be obtained with 3D techniques than with 2D techniques. Thinner sections increase resolution, and the signal-to-noise ratio available with the 3D technique is higher than that available with the 2D technique. Because contrast-enhanced MR angiography is not dependent on the inflow of unsaturated spins, signal intensity loss due to saturation effects in the distal portion of vessels is not a problem. In addition, ghost artifacts are virtually eliminated. Imaging with a 3D contrast-enhanced technique can be performed along the length of a vessel with a shorter examination time. Moreover, image subtraction can be used to eliminate the background signal intensity (especially high-signal-intensity bowel gas and fat components) and to improve vessel conspicuity (1–7).
In this study, the ability to depict the aortoiliac arteries and arteries of the lower extremity by using a 3D fast SPGR technique with image subtraction and the injection of contrast material was evaluated. With this technique, a coronal acquisition was completed in 32 seconds for a limited number of sections that covered the area of interest, and all data of the aortoiliac arteries and arteries of the lower extremity were acquired in less than 40 minutes, which included the time for patient positioning.
Although the number of patients in this study was small, 3D dynamic contrast-enhanced subtraction MR angiography had a sensitivity of 97.1% and a specificity of 99.2% for the differentiation of vessels with a stenosis of greater than 50%. Our results suggest that this technique is useful for screening patients suspected of having lower extremity ischemia.
All patients with symptomatic arteriosclerotic peripheral vessel disease need an extensive diagnostic work-up, including some noninvasive tests, to establish an accurate diagnosis. Precise anatomic knowledge of the vascular system is mandatory for treatment planning and decisions regarding the most appropriate type of intervention (percutaneous transluminal angioplasty, bypass surgery, amputation). The vascular surgeon and the interventional radiologist need an overview of the entire vascular system, including the status of the arterial inflow tract and runoff vessels, the location of stenoses and occlusions, the presence of collateral vessels, and so on (13). The MR angiographic technique we used provides almost all the necessary information in a noninvasive manner. In comparison with conventional angiography, however, MR angiography yields limited information about the character of vessel walls and flow dynamics. Our results do not suggest that MR angiography can replace all the information obtained at conventional angiography.
With 2D time-of-flight MR angiography, the aortoiliac arterial segments are difficult to image because of abdominal wall and bowel motion artifacts and the more oblique course of the arterial segments, which can cause a substantial amount of in-plane flow. An additional limitation is the loss of flow signal intensity in reconstituted popliteal arteries due to saturation of the retrograde flow signal by means of the venous presaturation pulse. This in-plane flow can cause partial saturation of the flowing spins and diminished signal intensity from flowing blood. Additional problems occur with horizontal or cephalad-directed vascular orientations such as a cross-femoral bypass graft (1,12). In this study, we used image subtraction during breath holding to reduce abdominal wall and bowel motion artifacts in the aortoiliac arterial segments. In the evaluation of the iliac artery, popliteal segments, and bypass grafts, this technique had high sensitivity and specificity. These results show that 3D dynamic contrast-enhanced subtraction MR angiography was superior to 2D time-of-flight MR angiography for the iliac artery, popliteal segments, and bypass grafts (Figs 4, 5).
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For each patient, the peak circulation time was estimated on the basis of age and clinical status. In previous reports (7,14–18), the time needed for intravenously administered contrast material to reach the abdominal aorta varied substantially, from 10 to 50 seconds for injections into an antecubital vein. The circulation time is generally longer in older patients and in patients with poor cardiac output, and the time to reach the aortoiliac arteries and arteries of the lower extremity is also longer.
Because the 3D fast SPGR sequences fill in k space linearly, from bottom to top, the central portion of k space is acquired during the middle portion of the acquisition. It is the central portion of k space with the low-spatial-frequency information that dominated image contrast (7,19). In our study, measurement of the aortoiliac arteries and the arteries of the lower extremity began 30–40 seconds after the start of injection of contrast material. The bolus administration of contrast material was timed so that the arterial phase would occur during acquisition of most of the central portion of k space.
In our study, stenosis was overlooked or overestimated on MR angiograms of seven segments; this was mainly due to suboptimal image subtraction owing to patient motion, limited spatial resolution of the image, and dephasing from complex and high-velocity flow (2,20). Although we used three acquisitions for long segments of a vessel with the torso multicoil array to increase the arterial signal-to-noise ratio, the limited spatial resolution of the images was a major problem in the grading of the degree of stenosis. If contrast-enhanced 3D imaging is performed for the examination of the separate long segment of a vessel in a shorter examination time, the spatial resolution of the image and the total volume of contrast material are limited.
Previous authors (2) addressed the accuracy of MR angiography in the evaluation of the tibial arteries and showed that 2D time-of-flight MR angiography is at least as accurate as conventional angiography and that contrast-enhanced 3D techniques do not necessitate image subtraction for the examination of the blood vessels in the calf. However, we believe that contrast-enhanced 3D techniques necessitate image subtraction for the examination of a long segment of a vessel, to eliminate background signal intensity and to improve vessel conspicuity because of limited spatial resolution, the total volume of contrast material, and the total imaging time. Further optimization of this technique is needed to improve its accuracy for the purpose of grading the degree of stenosis. In the future, the use of 3D sequences in shorter times will reduce motion artifacts and improve spatial resolution.
In summary, 3D dynamic contrast-enhanced subtraction MR angiography has high sensitivity and specificity. The technique of 3D dynamic contrast-enhanced subtraction MR angiography is a noninvasive alternative to conventional angiography for screening patients suspected of having lower extremity ischemia. Because contrast-enhanced methods are fast, they can be used for the rapid coverage of portions of the aortoiliac arteries and arteries of the lower extremity with 3D fast SPGR MR imaging. However, MR angiography remains an investigational technique that may potentially replace conventional angiography in certain clinical settings.
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Footnotes |
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Abbreviations: SPGR = spoiled gradient echo 2D = two-dimensional 3D = three-dimensional
Author contributions: Guarantor of integrity of entire study, E.S.; study concepts and design, E.S., I.S.; definition of intellectual content, E.S., I.S.; literature research, E.S., Y.M.; clinical studies, E.S., I.S., Y.M., H.H.; data acquisition, E.S., I.S., Y.M., H.H.; data analysis, E.S., I.S.; statistical analysis, E.S., Y.O.; manuscript preparation, E.S.; manuscript editing and review, E.S., I.S., R.H., K.H.
Received October 31, 1997;
revision requested December 23, 1997; revision received July 6, 1998;
accepted September 11, 1998.
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References |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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