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1 All authors: Department of Diagnostic Radiology, University Hospital of Regensburg, Franz-Josef-Strauss-Allee 11, D-93042 Regensburg, Germany.
Received September 14, 1999;
accepted after revision November 10, 1999.
Address correspondence to M. Völk.
Abstract
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SUBJECTS AND METHODS. Forty consecutive patients (age range, 25-81 years; mean, 62.9 ± 11.9 years) with suspected renal artery stenosis underwent intraarterial digital subtraction angiography and gadolinium-enhanced MR angiography, performed on a 1.5-T system with fast low-angle shot three-dimensional imaging (3.8/1.49 [TR/TE], 25° flip angle, 10-sec acquisition time, and 1.5-mm partition thickness). Three time-resolved phases were obtained in a single breath-hold. Digital subtraction angiography and gadolinium-enhanced MR angiography were evaluated by four observers who studied 80 main renal arteries and 19 accessory vessels to evaluate the degree of stenosis. A stenosis reducing the intraarterial diameter by more than 50% was regarded as hemodynamically significant. Interobserver variability was calculated.
RESULTS. Only one gadolinium-enhanced MR angiography study was not of diagnostic quality, as a result of failure of the power injector. All main branches were of diagnostic quality in 38 (97.4%) of the remaining 39 gadolinium-enhanced MR angiography studies. Seventeen (89.5%) of 19 accessory renal arteries were depicted with gadolinium-enhanced MR angiography. The overall sensitivity for significant stenoses was 92.9%. The overall specificity was 83.4%, and the overall accuracy was 85.9%. Interobserver variability of gadolinium-enhanced MR angiography exceeded that of digital subtraction angiography.
CONCLUSION. Time-resolved three-dimensional gadolinium-enhanced MR angiography is a useful noninvasive method of screening suspected renal artery stenosis because of its easy application, short examination time, and high sensitivity despite of its higher interobserver variability.
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The purpose of this prospective study was to evaluate the diagnostic accuracy and interobserver variability of time-resolved three-dimensional gadolinium-enhanced MR angiography in the detection of renal artery stenosis and accessory renal arteries in comparison with that of digital subtraction angiography.
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Imaging Technique
Contrast-enhanced MR angiography was performed on a 1.5-T superconducting
scanner unit (Magnetom Symphony; Siemens Medical Systems, Iselin, NJ). A
four-element phased array body coil was used in all patients. A
three-dimensional gradient-echo sequence and a fast low-angle shot
three-dimensional sequence with a repetition time of 3.8 msec, an echo time of
1.49 msec, a flip angle of 25°, and an acquisition time of 10 sec was
applied. An unenhanced study was acquired as a reference scan before
administration of contrast material. Bolus timing was not performed. Fifteen
seconds after IV bolus injection of 25 ml of gadopentetate dimeglumine
(Magnevist; Berlex Laboratories, Wayne, NJ) followed by 25 ml of saline
solution with an injection rate of 2.5 ml/sec, contrast-enhanced MR
angiography was performed in a 30-sec breath-hold period (three consecutive
measurements). The IV gadolinium bolus was injected with a power injector
(Spectris; Medrad, Pittsburgh, PA) through a peripheral arm vein, such as in
the antecubital fossa. A central line was not used in any of the patients.
A true fast imaging with steady precession sequence (TR/TE, 5.98/3; flip angle, 70°; number of slices, 19; slice thickness, 8 mm; field of view, 320 x 240 mm; matrix, 256 x 153) in coronal and sagittal orientations was used as a localizer. The coronal contrast-enhanced MR angiography slab was placed parallel to the abdominal aorta and covered the main aortic branches and the aortic bifurcation. In the fast low-angle shot three-dimensional sequence the field of view was 300 x 362.5 mm and the matrix size was 157 x 256. Readout band-width was 488 pixels per hertz. K-space asymmetry was 48% in read, 37% in the direction of phase encoding (line), and 50% in the direction of phase encoding (partition). Partition thickness was 1.5 mm after K-space interpolation using zero filling in the direction of partition encoding. Slab thickness was 69 mm. The in-plane spatial resolution of 1.67 x 1.17 mm lead to a voxel volume of 2.9 mm3.
A maximum-intensity-projection algorithm was applied to all contrast-enhanced MR angiography studies after subtraction of the unenhanced measurement by using the commercially available software on the MR imaging system (Magnetom Symphony, Numaris 3.5 software, version VA11A; Siemens). Maximum intensity projections were reconstructed in steps of 9° (range, 1-180°).
Digital subtraction angiography was the standard of reference. Digital subtraction angiography was performed using a pigtail catheter and the digital subtraction technique in all 40 patients. Additional selective angiography was necessary in four patients to better visualize the stenosis in the proximal two thirds of the main renal artery. Nonionic contrast material with 300 mg I/ml was injected intraarterially (volume, 30-40 ml; injection rate, 15-20 ml/sec; frame rate, 2/sec). Imaging was initially performed in the posteroanterior projection. Oblique views were obtained once stenotic areas were identified or when the renal artery ostium was not visible.
In 33 patients, digital subtraction angiography and contrast-enhanced MR angiography were performed within 24 hr of each other; in seven patients, both studies were performed within a range of 1 day to 4 months (mean, 46.7 ± 39.0 days) of each other. The average time interval in all 40 patients was 8.3 ± 23.6 days (range, 0-120 days). No radiologic or surgical intervention was performed during the period between the two studies.
Image Analysis
Four radiologists experienced in cardiovascular imaging assessed the
digital subtraction and MR angiograms independently in a randomized order for
maximum renal artery stenosis, visibility of the main segmental branches of
the renal arteries, and number and abnormalities of accessory renal arteries.
Right and left renal arteries were analyzed separately. Observers were not
aware of digital subtraction angiographic findings when analyzing
contrast-enhanced MR angiographic findings and vice versa.
Image analysis was based on original contrast-enhanced MR angiography data sets, maximum-intensity-projection reconstructions, and digital subtraction angiograms. Only the main renal arteries proximal to the first segmental branch were studied for stenoses. All observers were not aware of the clinical history and the interpretations of the other observers. The degree of stenosis was defined as the ratio between the narrowest diameter within the stenosis (A) and the diameter of the nearest downstream uninvolved segment of the main renal artery (B): 100 x (1 - A/B) [6]. The images were printed. For the measurement of the diameters of the vessels, a jeweler's eyepiece marked to the tenth of a millimeter was used. Based on this ratio (percentage of diameter reduction) the results were classified as no stenosis (0%), mild stenosis (1-29%), moderate stenosis (30-49%), significant stenosis (50-99%), and occlusion (100% stenosis). We used an approach similar to that of Hany et al. [7], who regarded stenoses of more than 50% as hemodynamically significant. The mean value of all four observers derived from digital subtraction angiography was the standard of reference.
The length and visibility of main and segmental branches of the renal arteries were classified as 0 when only the level of the ostium was visualized, 1 when only the proximal third was visualized, 2 when only the proximal and middle thirds were visualized, 3 when the total main vessel was visualized, and 4 when the main artery with segmental branches was visualized.
Assessment of renal arteries also included the determination of number,
degree of stenosis, and other abnormalities of accessory arteries. Stenoses
were classified as significant (
50% of diameter reduction) or
nonsignificant (<50% of diameter reduction).
Statistical Analysis
Statistical analysis was performed with Excel software (Microsoft, Redmond,
WA) and SPSS for Windows version 7.0 (SPSS, Chicago, IL). Sensitivity,
specificity, and accuracy were calculated on the basis of the classifications
of each observer and the pooled data for 78 renal arteries. For the
calculation of results of sensitivity, specificity, and accuracy, the ordinal
scale was used (no stenosis, mild stenosis, etc.).
Cohen's kappa value (
), linear regression coefficients (r),
and the mean values of the absolute differences of the digital subtraction
angiography and contrast-enhanced MR angiography results of each observer
versus those of the others were calculated as indicators of interobserver
agreement and variability. For this analysis, the interval scale was used
(calculated percent values). Using kappa statistics, interobserver agreement
was considered slight at a value equal or less than 0.2; fair, 0.21-0.40;
moderate, 0.41-0.60; substantial, 0.61-0.80; or almost perfect, 0.81-1.00
[8].
The two-tailed Wilcoxon's rank sum test was applied for each comparison of digital subtraction angiography versus contrast-enhanced MR angiography. The level for a statistically significant difference was set at a p value of less than 0.05.
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Vessel visibility was graded independently by all observers. In one of the 39 patients, only the proximal and middle thirds of the renal artery were visualized (category 2). In the remaining 38 (97.4%) of 39 contrast-enhanced MR angiography studies, all main renal branches were of diagnostic quality up to the distal third or the distal third with segmental branches (categories 3 and 4). No venous overlay on maximum-intensity-projection reconstructions was observed in any arterial phase contrast-enhanced MR angiography study (Fig. 2A,2B,2C). In 31 patients, the first measurement after contrast-material administration showed arterial contrast, whereas, in eight patients, the second measurement showed arterial contrast.
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The mean value was 92.9% for sensitivity, 83.4% for specificity, and 85.9% for accuracy. Sensitivity, specificity, and accuracy of each observer and mean values including their 95% confidence intervals are shown in Table 1. The kappa values of each pair of observers ranged from moderate to substantial agreement for digital subtraction angiography and from fair to substantial agreement for contrast-enhanced MR angiography (Table 2). The linear regression coefficients of each observer and the absolute differences in percentage of stenosis between the two estimates for each possible pair of observers in digital subtraction angiography and contrast-enhanced MR angiography are shown in Table 2. The results of the linear regression coefficient (r) were not significantly different when assessed with Wilcoxon's rank sum test (p = 0.062). The mean value of the absolute differences between contrast-enhanced MR angiography and digital subtraction angiography mean values was 12.6%. The absolute differences in percentage of stenosis between the two estimates and kappa values of digital subtraction angiography and contrast-enhanced MR angiography were statistically significant (p < 0.032).
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The mean value of the standard deviation of the degree of maximum stenosis for all 78 vessels determined with digital subtraction angiography among all four observers was 4.7 versus 6.8 for contrast-enhanced MR angiography. The stenoses measured 26.3% ± 4.7% for digital subtraction angiography versus 32.4% ± 6.8% for contrast-enhanced MR angiography. Regarding hemodynamically significant stenoses (>50%), digital subtraction angiography yielded a mean value of 64.3% versus 65.2% in contrast-enhanced MR angiography. Figure 3A,3B shows a case of disagreement regarding the grading of stenosis with MR angiography (significant stenosis) and digital subtraction angiography (no significant stenosis). However, the linear regression coefficient (r) for contrast-enhanced MR angiography and digital subtraction angiography results was 0.869.
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Accessory Renal Arteries
Seventeen (89.5%) of 19 accessory renal arteries were visualized using
contrast-enhanced MR angiography. Intraarterial angiography revealed three
stenoses of accessory renal arteries, which were also detected with
contrast-enhanced MR angiography (Fig.
4). All three stenoses were graded as significant (>50%) by all
observers using both digital subtraction angiography and contrast-enhanced MR
angiography. The remaining accessory vessels were normal at intraarterial
angiography (n = 16) and contrast-enhanced MR angiography (n
= 14).
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Because of the very short acquisition time for a three-dimensional, single fast low-angle shot data set with K-space interpolation (10 sec), no bolus timing is necessary and multiple data sets can be reconstructed for different phases of contrast media passage within one breath-hold period [9, 10]. Other contrast-enhanced MR angiography sequences were performed with an average acquisition time of 22 sec [11]. These contrast-enhanced MR angiography techniques require bolus timing because of the relatively long acquisition time [11, 12]. According to Schoenberg et al. [13], new multiphase sequences with an acquisition time of 6.4 sec are a reliable technique for arterial phase MR angiography of the renal arteries and are not susceptible to artifacts caused by the overlay of enhanced renal veins. In our study, the acquisition time was 10 sec and venous overlay was not a limitation. Venous overlay can occur because of either bolus mistiming or acquisition times of 20-30 sec. Substantial parenchymal enhancement limits the delineation of distal and segmental arteries even if optimal bolus timing is achieved [13]. In our study, only one examination was not of diagnostic quality because of the failure of the power injector. The other 39 examinations yielded excellent arterial contrast in the maximum-intensity-projection reconstruction with the time-resolved technique. A general limitation of the time-resolved technique appears in patients with severe heart failure, aortic dissection, or large aortic aneurysm caused by the prolonged blood circulation time. In these cases, bolus timing might be necessary.
Gilfeather et al. [12] showed that the interobserver variability of digital subtraction angiography and contrast-enhanced MR angiography does not differ significantly when digital subtraction angiography and contrast-enhanced MR angiography are evaluated by two different groups of expert observers. Unliked the study by Gilfeather et al., in our study all digital subtraction angiography and contrast-enhanced MR angiography examinations were interpreted by the same four observers. The resulting total numbers of observations were almost identical in both studies (Gilfeather et al., n = 642; our data, n = 640), leading to three and six interobserver comparisons, respectively.
Like that of Gilfeather et al. [12], our interobserver agreement for both techniques (linear regression coefficient) regarding the degree of renal artery stenosis was not significantly different. According to Bland and Altman [14], comparison of a new measurement technique with an established one using correlation coefficients is inappropriate. Even Landis and Koch [8] agreed that the classification of the strength of agreement with kappa statistics is clearly arbitrary. Thus, we additionally used the absolute differences in percentage of stenosis between the two estimates for each pair of observers as statistical methodology for the description of interobserver variability [14, 15]. As we have mentioned, the linear regression coefficient showed no statistically significant difference (Wilcoxon's rank sum test). However, looking at mean kappa values, the interobserver agreement was considered to be moderate for contrast-enhanced MR angiography and substantial for digital subtraction angiography (p < 0.032, Wilcoxon's rank sum test). Looking at the absolute differences in percentage of stenosis, contrast-enhanced MR angiography was significantly inferior compared with digital subtraction angiography (Wilcoxon's rank sum test). The confidence intervals also showed great variability between the observers.
Like former studies, our data revealed a slight overestimation of the degree of stenosis by contrast-enhanced MR angiography (26.3%) compared with the standard of reference (32.4%) [16, 17].
The overall sensitivity for hemodynamically significant stenoses in contrast-enhanced MR angiography was 92.9% (confidence interval, 76.2-99.9%) when digital subtraction angiography was used as the standard of reference. The sensitivity of contrast-enhanced MR angiography reported by other investigators ranged from 70% to 100% [7, 13, 18,19,20,21]. In our study, two (2.5%) of 80 vessels were classified as not of diagnostic quality. In contrast, another study reported dropout rates varying from 5.4% to 8.7%, depending on the applied MR imaging sequence [22]. Contrast-enhanced MR angiography studies seem to be more reliable in terms of dropouts. This is a result of the T1 shortening effect of gadolinium and the fact that contrast-enhanced MR angiographies are nearly independent of flow and pulsatility effects; thus, they have fewer artifacts compared with phase-contrast and time-of-flight techniques [18].
One important limitation of contrast-enhanced MR angiography is the difficulty of evaluating segmental branches of renal arteries [12]. Our results show that segmental branches were visible in 61.3% of the patients. However, our study did not include patients with stenoses in the distal part of the vessel. This limitation must be recognized in any discussion of high values of sensitivity and specificity both in this study and in other studies; however, stenoses of the distal third of the renal arteries are relatively uncommon. Segmental stenoses may be a problem in terms of diagnostic accuracy and therapeutic consequences.
In conclusion, time-resolved contrast-enhanced MR angiography is a reliable technique for arterial phase renal angiography and is potentially an appropriate screening technique for patients with suspected renal artery stenosis. Because of the time-resolved character of this sequence, segmental arteries were revealed before parenchymal enhancement obscured them from view. However, interobserver variability was higher in contrast-enhanced MR angiography than in digital subtraction angiography.
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-space-centered, breath-hold MR angiography of renal arteries and the
abdominal aorta. J Magn Reson Imaging
1997;7:617
-622[Medline]
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