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Vascular and Interventional Radiology |
1 Department of Radiology, University of Utah Health Sciences Center, 50 N Medical Dr, Salt Lake City, UT 84132 (M.G., H.C.Y.)
2 Department of Radiology, Hospital of the University of Pennsylvania, Philadelphia (E.S.S., L.A., A.H.S., R.D.S.G., R.A.B., M.C.S., M.D.S.).
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Abstract |
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MATERIALS AND METHODS: Fifty-four patients underwent conventional angiography and gadolinium-enhanced three-dimensional gradient-echo MR angiography. Three angiographers blinded to each other's interpretations and the MR angiographic findings assessed the conventional angiograms for renal artery stenosis. Similarly, three blinded MR imagers evaluated the MR angiograms.
RESULTS: Interobserver variability for the degree of renal artery stenosis in the 107 kidneys evaluated was not significantly different between the two modalities. The mean SD of the degree of stenosis was 6.9% at MR angiography versus 7.5% at conventional angiography (
.05, P > .05). In 70 kidneys (65%), the average degree of stenosis reported by the readers for the two modalities differed by 10% or less. In 22 cases (21%), the degree of stenosis was overestimated with MR angiography by more than 10% relative to the results of conventional angiography. In 15 cases (14%), the degree of stenosis was underestimated with MR angiography by more than 10%.
CONCLUSION: Gadolinium-enhanced MR angiography permits evaluation of renal artery stenosis with an interobserver variability comparable with that of conventional angiography.
Index terms: Angiography, comparative studies, 961.122 • Magnetic resonance (MR), comparative studies, 961.12942, 961.12943 • Magnetic resonance (MR), vascular studies, 961.12942, 961.12943 • Renal arteries, stenosis or obstruction, 961.721
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Introduction |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Once these patients are identified as being at higher risk of renal artery stenosis, the choice of the best test for diagnosis of renal artery stenosis is controversial. Accurate identification of patients with correctable renovascular hypertension can be difficult with use of standard noninvasive techniques because Doppler ultrasonography and radionuclide renography provide only indirect evidence of the presence of renal artery lesions. Conventional angiography has been the standard method of evaluating patients suspected to have renal artery stenosis. Many of these patients have concurrent renal insufficiency and may be at risk for renal toxic effects from iodinated contrast material (4,5). In addition, the risk of catheter-induced atheroembolism is increased in patients with concurrent atherosclerosis of the abdominal aorta (5).
Magnetic resonance (MR) angiography has several advantages over conventional angiography. MR angiography is noninvasive and allows direct visualization of the renal arteries without use of iodinated contrast material or ionizing radiation. Several recent articles have described techniques of gadolinium-enhanced MR angiography for evaluation of the renal arteries (6–11). One important limitation of MR angiography is the difficulty of evaluating intraparenchymal branches of the renal artery. However, distal lesions may be problematic from a therapeutic standpoint because conventional radiologic or surgical repair is usually difficult if not impossible. When conventional angiography is used as the standard of reference, the sensitivity for detection of clinically important renal artery stenosis with gadolinium-enhanced MR angiography is 70%–100% (6–10). If conventional angiography is considered the standard of reference, then comparing MR angiography with conventional angiography can at best show MR angiography to be as accurate in evaluation of vessels. Therefore, we compared the performance of gadolinium-enhanced MR angiography and conventional angiography in diagnosis of renal artery stenosis without using one modality as the standard of reference by assessing intermodality and interreader variability.
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MATERIALS AND METHODS |
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MR angiography was performed with a 1.5-T unit with an enhanced gradient system (Signa; GE Medical Systems, Milwaukee, Wis) during rapid intravenous bolus injection of 0.2–0.3 mmol/kg gadopentetate dimeglumine (Magnevist; Berlex Laboratories, Wayne, NJ). A torso phased-array coil was used in 50 of the 54 patients. The MR angiography sequence was performed in a 22–32-second breath hold with the following parameters: repetition time msec/echo time msec = 4.8–7.0/1.1, 256 x 128 matrix, 64-kHz receiver bandwidth, 26–36-cm field of view, 45°–60° flip angle, 2.0–2.8-mm section thickness, and one signal acquired. The 54 cases were reformatted at an independent workstation (GE Medical Systems) by a single radiologist (M.G.), who was blinded to the clinical history and the conventional angiographic and MR angiographic readings. Standard frontal and oblique coronal images of the renal arteries were produced.
Conventional angiography was performed with pigtail catheters and a film or digital subtraction technique. Nonionic contrast material was injected intraarterially at 15–40 mL/sec for up to 5 seconds. Imaging was initially performed in the anteroposterior projection; oblique views were obtained once stenotic areas were identified. Selective catheterization of the renal artery was left to the discretion of the attending vascular radiologist and was performed when considered necessary for adequate diagnosis. The two imaging studies were performed 0 days to 4 months apart (average, 2.3 weeks). No surgical or radiologic intervention was performed between the two studies.
Three experienced angiographers (R.D.S.G., R.A.B., M.C.S.) assessed the conventional angiograms for maximal renal artery stenosis; three experienced body MR imagers (E.S.S., L.A., A.H.S.) assessed the MR angiograms for maximal renal artery stenosis. Only the main renal arteries were assessed. The readers were blinded to the clinical data and each other's interpretations. The degree of renal artery stenosis was graded as a percentage of the luminal diameter. Maximal stenosis was defined as the ratio between the narrowest diameter within the stenosis and the diameter of the nearest downstream uninvolved segment of the main renal artery. When vessel branching or poststenotic dilatation precluded measurement of a normal distal renal artery segment, an estimate was made on the basis of the nearest normal upstream renal artery segment. When two or more stenoses were identified in a single renal artery, the most severe stenosis was used for grading and analysis. The mean and SD of the degree of maximal renal artery stenosis were separately determined for each patient from the values determined by the three readers for each imaging study.
On a more clinically applicable level, the degree of maximal renal artery stenosis determined by each reader was transformed with a five-point ordinal scale: 0 = no stenosis, 1 = less than 50% stenosis, 2 = 50%–75% stenosis, 3 = greater than 75% stenosis, 4 = occlusion. The Cohen 
statistic was then determined for each pairwise comparison between the three readers involved in each imaging study. All statistical analyses were performed with SPSS for Windows version 7.0 (SPSS, Chicago, Ill). The level for a statistically significant difference was set at P .05.
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RESULTS |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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When the statistic was used to measure interobserver variability (after the data had been transformed into more clinically relevant categories of disease), there was slightly better agreement between the conventional angiographic measurements of maximal stenosis among the three readers than between the MR angiographic measurements. This difference is shown in the Table.
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The interpretations of the two imaging studies were then compared with one another. In 70 arteries (65%), the average degree of stenosis estimated by the readers of the two imaging studies differed by 10% or less. In 22 arteries (21%), the degree of stenosis was overestimated with MR angiography by more than 10% relative to the results of conventional angiography. In 15 arteries (14%), however, the degree of stenosis was underestimated with MR angiography by more than 10% relative to the results of conventional angiography.
After the data were transformed into predefined degrees of stenosis with the five-point ordinal scale, the transformed data were used to analyze the degree of consensus among readers. There were 62 vessels (58%) for which all three readers of MR angiograms agreed with each other (ie, gave the degree of stenosis the same rating on the ordinal scale). There were 72 vessels (67%) for which all three readers of conventional angiograms agreed with each other.
Patients undergoing evaluation for renal artery stenosis who have normal vessels or vessels with less than 50% maximal stenosis generally do not need radiologic or surgical intervention, whereas those with 50% or greater stenosis may be appropriate candidates for intervention. Therefore, a more clinically relevant decision for the triage of patients suspected to have renovascular lesions should be based on the agreement among readers about clinically important disease (
50% maximal stenosis) versus clinically unimportant disease (<50% maximal stenosis). Under these more lenient conditions, there were 91 vessels (85%) for which all three readers of MR angiograms agreed on the presence or absence of clinically important stenosis. There were 98 vessels (92%) for which all three readers of conventional angiograms agreed on the presence or absence of clinically important stenosis. There was unanimous agreement among the readers of conventional angiograms and the readers of MR angiograms as to the degree of clinically important or unimportant stenosis in 84 vessels. In 81 of these 84 vessels, the conventional angiographic and MR angiographic interpretations agreed with each other (Fig 4). For one vessel, all three readers of MR angiograms agreed that the vessel had 50% or greater stenosis, whereas all three readers of conventional angiograms agreed that there was no clinically important stenosis (Fig 5). For two vessels, all three readers of conventional angiograms agreed that there was 50% or greater stenosis, whereas all three readers of MR angiograms agreed that there was no clinically important stenosis (Fig 6).
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Among the 83 vessels that at least one reader of MR angiograms interpreted as normal or as having less than 50% stenosis, all three readers agreed on the absence of clinically important stenosis in 67 (81%). Among the 84 vessels that at least one reader of conventional angiograms interpreted as normal or as having less than 50% stenosis, all three readers agreed on the absence of clinically important stenosis in 74 (88%).
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DISCUSSION |
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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There is an important difference in the method of evaluating conventional angiographic and MR angiographic data sets. With conventional angiography, there is a set number of two-dimensional images from which the diagnosis of renal artery stenosis must be made. In our study, the angiographer performing the examination decided on the number of projection images that were to be recorded in an analog or digital format during rapid injection of intraarterial contrast material. All subsequent conventional angiographic interpretations were limited by these images. This situation is not reflective of the usual clinical situation, in which each angiographer can perform many imaging runs in different projections until a satisfactory study has been performed. The former situation is also different from MR angiography, in which only a single three-dimensional data set is obtained but can be evaluated in an infinite number of projections. In routine clinical practice, the MR angiographic reader often uses a computer workstation to evaluate the three-dimensional data set in any number of projections needed. Therefore, to a large degree, the accuracy of interpretation of MR angiograms depends on the sophistication of the image reconstruction software and the facility with which a radiologist can manipulate images using that software.
In the current study, we attempted to minimize the dependence of the MR angiographic interpretations on these two factors by having a single blinded individual proficient with both the software and the workstation process all of the reconstructions. Thus, the readers of MR angiograms were essentially in a situation comparable with that of the readers of conventional angiograms. Unfortunately, this situation also limited the readers of MR angiograms to interpreting only the reconstructed images rather than allowing them to perform their own reconstructions.
In this study, both the SD and the range of maximal stenosis values were marginally larger for the conventional angiographic interpretations than for the MR angiographic interpretations, although neither difference was statistically significant. However, for every other measure of interobserver variability, the MR angiographic interpretations demonstrated marginally greater variation than the corresponding conventional angiographic interpretations. The reasons for these differences are not clear.
An important finding of this study is that the interpretation of both imaging modalities is far from uniform. When 50% maximal stenosis was used as the cutoff for clinically important disease, there was agreement among all three readers of MR angiograms in 24 of 40 vessels (60%) for which at least one reader reported clinically important stenosis; the corresponding result for the readers of conventional angiograms was slightly better at 23 of 32 vessels (72%). Both modalities appeared to perform better in the exclusion of clinically important stenosis, but conventional angiography again performed slightly better than MR angiography. Among vessels for which at least one reader reported absence of clinically important stenosis, the other two readers agreed in 88% of cases with conventional angiography versus 81% of cases with MR angiography.
The obvious advantages of conventional angiography are the ability to determine the clinical importance of suspect lesions by performing hemodynamic measurements in addition to intraarterial angiography and the ability to perform endovascular therapy concurrently. However, these advantages have to be weighed against the higher cost and greater morbidity of conventional angiography. Our finding that the interobserver variability of MR angiography in diagnostic interpretation is only marginally inferior to that of conventional angiography further supports the use of MR angiography as potentially the most appropriate tool for screening patients in whom there is a high clinical suspicion of atherosclerotic renovascular disease. A comprehensive MR imaging examination, including MR angiography, can also provide important information regarding kidney size, qualitative renal function, accessory renal arteries, and the renal veins, as recently shown by Low et al (11) in patients being evaluated for transplantation. This information can be used in making patient treatment decisions.
However, MR angiography is not without limitations. Our study concentrated on use of MR angiography for evaluation of the main renal artery without addressing the importance of intraparenchymal renal artery disease, which occurs in patients with fibromuscular dysplasia and a number of arteritides. MR angiography still lacks the spatial resolution for adequate visualization of small accessory and branch vessels. Fortunately, in the setting of hypertension, nonatherosclerotic causes represent the minority of cases (12). Clinical evaluation remains critical, and younger patients or patients in whom an arteritis is suspected should be considered for conventional angiography rather than MR angiography.
In conclusion, MR angiography may be the most appropriate modality for screening patients suspected to have renovascular disease because the interobserver variability in diagnostic interpretations is only marginally inferior to that of conventional angiography.
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Footnotes |
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From the 1997 RSNA scientific assembly.
Author contributions: Guarantors of integrity of entire study, M.G., H.C.Y., M.D.S.; study concepts, M.G., H.C.Y., M.D.S.; study design, M.G., M.D.S.; definition of intellectual content, M.G., M.D.S.; literature research, M.G.; clinical studies, A.H.S., L.A., E.S.S., M.C.S., R.D.S.G., R.A.B.; data acquisition, M.G.; data analysis, M.G., H.C.Y., M.D.S.; statistical analysis, M.G., M.D.S., H.C.Y.; manuscript preparation, editing, and review, M.G., H.C.Y.
Received February 18, 1998;
revision requested May 4, 1998; revision received June 25, 1998;
accepted August 24, 1998.
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TOPAbstractIntroductionMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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