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Original Research |
1 All authors: Division of Interventional Radiology, Mount Sinai Medical Center, One Gustave L. Levy Pl., Box 1234, New York, NY, 10029.
Received September 22, 2006;
accepted after revision January 26, 2007.
Address correspondence to R. A. Lookstein
(robert.lookstein{at}msnyuhealth.org).
Abstract
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MATERIALS AND METHODS. A total of 218 abdominal MDCT angiograms from a single tertiary care referral center were reviewed. The subjects were 108 patients who had 127 arteries with more than 50% ostial atherosclerotic renal artery stenosis. Vessel analysis software was used to measure renal artery length, cross-sectional area, and maximum diameter. Differences between mean values for women and men and for left and right renal arteries were measured with a two-tailed Student's t test.
RESULTS. Significant differences for men and women were found in average maximum cross-sectional area distal to the point of stenosis (0.3 ± 0.19 vs 0.23 ± 0.09 cm2, p = 0.006) and the corresponding maximum diameter (6.9 ± 1.7 vs 6.1 ± 1.1 cm2, p = 0.003). Average lengths of the main renal artery did not differ significantly for men and women. Differences for the left and right main renal arteries were found in minimum area (i.e., area of maximum stenosis, 0.08 ± 0.04 vs 0.06 ± 0.03 cm2, p = 0.03), area proximal to the bifurcation (0.26 ± 0.11 cm2 vs 0.23 ± 0.07 cm2, p = 0.02), and length (38.5 ± 12.6 vs 48.7 ± 16.2 mm, p = 0.0002).
CONCLUSION. Significant anatomic differences exist between the left and right renal arteries, between the renal arteries in men and those in women, and from one person to the next. Many of these differences are relevant to the design and use of distal protection devices in stenting of the renal arteries.
Keywords: anatomy • CT angiography • renal disease • vascular stent
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Although studies evaluating distal protection in the renal circulation are underway, to our knowledge no dedicated renal protection device exists. Off-label use of coronary and carotid devices has been frustrated by significant anatomic differences between the coronary, carotid, and renal artery systems (Figs. 1, 2, 3, 4). The renal arteries differ from the carotid arteries in their direct origin from the aorta, from the coronary arteries in their larger size, and from both in their shorter length between their origin from the aorta and their bifurcation [7, 10]. The purpose of this study was to use 3D evaluation of CT angiographic data sets to elucidate the anatomy of the renal artery in the presence of ostial atherosclerotic plaque. With more detailed anatomic data, renal artery–specific embolic protection may improve outcome after PTRA.
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We used dedicated 3D vascular reconstruction software (Vitrea 2, Vital Images) to conduct a retrospective review of MDCT renal angiographic data in an effort to quantify anatomic limitations in deploying distal protection devices in the management of ostial renal artery stenosis. All abdominal MDCT angiographic studies performed at a single tertiary care referral center between June 2002 and September 2005 were screened (406 renal arteries in 298 patients).
Subject Selection
Included for postprocessing with 3D vessel reconstruction were 108 patients
with ostial atherosclerotic renal artery stenosis. Thirty-two of the patients
were women (age range, 57–92 years; mean age, 79 years), and 76 were men
(age range, 58–97 years; mean age, 77 years). A total of 127 native
renal arteries (72 left, 55 right) with at least 50% ostial stenosis were
examined. Accessory renal arteries and renal arteries with stents already in
place were excluded.
Image Acquisition and Postprocessing
All images were acquired with a 16- or 64-MDCT scanner. Acquisition
parameters for 16-MDCT were as follows: gantry rotation time, 0.5 second;
detector configuration, 16 x 0.75 channels/mm; pitch, 1.2; table
advancement per 360° gantry rotation, 16 mm; table speed, 29 mm/s.
Acquisition parameters for 64-MDCT were as follows: gantry rotation time, 0.5
second; detector configuration, 64 x 0.6 channels/mm; pitch, 0.85; table
advancement per 360° gantry rotation, 17 mm; table speed, 32 mm/s.
Cross-referenced axial, coronal, and sagittal projections were used to select
the renal arteries from a volumetric data set. The center-line tool from a 3D
workstation (Vitrea 2, Vital Images) was used to reconstruct renal artery data
sets into curved multiplanar images (Fig.
5).
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The schema for measurement of the renal arteries is shown in Figure 6A, 6B, 6C. The length of each renal artery was measured in two segments: L1, defined as the distance from the ostium to the point of maximum stenosis, and L2, defined as the length from the point of maximum stenosis to the bifurcation of the main renal artery. The sum of these two values was defined as the effective length of the main renal artery. Three cross-sectional luminal areas were measured along the vessel path. The area at the point of maximum stenosis (minimal vessel area) was defined as A1. The area of maximum dilatation distal to the point of stenosis was defined as A2. The area of the main renal artery immediately proximal the bifurcation was defined as A3. Maximum diameters D2 and D3 were measured at points in each vessel corresponding to A2 and A3. A single estimation of the percentage relative stenosis of each vessel was calculated as percentage stenosis = [(A3 – A1) / A3] x 100.
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Statistical Analysis
Data were segregated into those for men, for women, for the left main renal
artery, and for right main renal artery. The mean value ± SD within
each group was calculated for each variable. Groups and subgroups (e.g., left
renal arteries in men) were compared with one another by use of a two-tailed
Student's t test, a significant difference being p 
0.05. Multivariate regression analysis was used to discern how strongly the
sex of the subjects and the side of the artery related to the measured
anatomic dimensions. The relation between renal artery anatomy and subject age
also was analyzed. No corrections were made for multiple testing. All data
were analyzed with SAS software (SAS Institute).
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Length
Selected length results are shown in Figure
7A,
7B. For all vessels studied,
mean L1 was 11.3 ± 5.3 mm, mean L2 was 31.6 ± 14.3 mm, and mean
L1 + L2 was 42.9 ± 15.0 mm. Regardless of sex, mean L1 of the left main
renal artery was 10.4 ± 4.7 mm and of the right artery was 12.4
± 5.9 mm, and these values were significantly different (p =
0.05). Significant differences were not found in the subgroups men and women.
In all patients, mean left L2 was 28 ± 12.4 mm, mean right L2 was 36.3
± 15.6 mm, and the difference was significant (p = 0.002).
Significant differences in L2 also were found in the subgroups men and women.
In men, mean left L2 was 28.0 ± 12.9 mm, and mean right L2 was 35.8
± 15.7 mm (p = 0.02). In women, mean left L2 was 28.1 ±
11.5 mm, and mean right L2 was 37.0 ± 15.8 mm (p = 0.05).
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Total lengths of the main renal artery (L1 + L2) were significantly different between the left and right sides within both sex subgroups and within the study population as a whole. Mean left L1 + L2 was 38.5 ± 12.6 mm, and mean right L1 + L2 was 48.7 ± 16.2 mm (p = 0.0002). In men only, mean left L1 + L2 was 38.7 ± 12.9 mm, and mean right L1 + L2 was 49.0 ± 15.7 mm (p = 0.002). In women, mean left L1 + L2 was 37.8 ± 11.8 mm, and mean right L1 + L2 was 48.1 ± 17.3 mm (p = 0.04). No statistically significant differences were found between men and women in any of the length measurements in the study group as a whole or as segregated left and right renal artery groups (Tables 3 and 4).
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Area
Selected area results are shown in Figure
8A,
8B. For all vessels studied,
the mean cross-sectional luminal area of the renal artery at the point of
maximum stenosis (A1) was 0.07 ± 0.04 cm2. For all vessels,
the mean cross-sectional area at the point of maximum dilatation distal to the
point of stenosis (A2) was 0.27 ± 0.17 cm2. The mean area of
the left and right main renal arteries proximal to the bifurcation (A3) for
all vessels together was 0.25 ± 0.10 cm2.
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Mean areas proximal to the bifurcation (A3) of the left and right main renal arteries were significantly different in the entire study population and among the men. Regardless of sex, mean left A3 was 0.26 ± 0.11 cm2, and mean right A3 was 0.23 ± 0.07 cm2 (p = 0.02). Among men, mean left A3 was 0.28 ± 0.11 cm2, and mean right A3 was 0.24 ± 0.07 cm2 (p = 0.01). Mean left and right A3 values were not statistically different among the women.
All mean areas were significantly different between men and women without regard to left or right (Tables 3 and 4). In addition, all mean areas were significantly different for men versus women in the left renal artery subgroup. Mean A1 in men, regardless of side, was 0.08 ± 0.04 cm2; mean A1 in women was 0.06 ± 0.03 cm2 (p = 0.004). For only the left renal artery, mean A1 in men was 0.08 ± 0.04 cm2, and mean A1 in women was 0.06 ± 0.04 cm2 (p = 0.02). Mean A1 was not statistically different between men and women within the right renal artery subgroup.
Mean A2 was significantly different between men and women in general and within the right renal artery subgroup. Regardless of side, mean A2 in men was 0.3 ± 0.19 cm2, and mean A2 in women was 0.23 ± 0.09 cm2 (p = 0.006). In the left renal artery subgroup, mean A2 in men was 0.29 ± 0.13 cm2, and mean A2 in women was 0.23 ± 0.09 cm2 (p = 0.02). There was no significant difference in mean A2 between men and women within the right renal artery subgroup.
For differences in mean A3 between men and women, significant differences were found within the entire study population and within the left renal artery subgroup. In the group as a whole, mean A3 in men was 0.26 ± 0.10 cm2 and in women was 0.21 ± 0.09 cm2 (p = 0.002). Considering only left renal arteries, mean A3 was 0.28 ± 0.11 cm2 in men and 0.21 ± 0.11 cm2 in women (p = 0.01). No significant differences in mean A3 were found between men and women within the right renal artery subgroup.
Diameter
Selected diameter results are shown in Figure
8A,
8B. The mean maximum diameter
at the point of maximum dilatation distal to the point of stenosis (D2) was
6.7 ± 1.6 mm. The mean maximum diameter of the left and right renal
arteries proximal to the bifurcation (D3) was 6.4 ± 1.3 mm. The mean
maximum diameters at the point of maximum dilatation distal to the stenosis
(D2) were not significantly different between left and right in the study
population as a whole. Among the men, mean left D2 was 7.2 ± 1.6 mm,
and mean right D2 was 6.4 ± 1.8 mm (p = 0.03). No significant
difference between left and right mean D2 values was found among the women.
Likewise, mean left and right maximum diameters of the renal artery proximal
to the bifurcation (D3) were not significantly different within the group as a
whole or among the women. Among the men, however, mean left D3 was 6.9
± 1.3 mm, and mean right D3 was 6.3 ± 1.0 mm (p
=0.02).
For all arteries, regardless of side, mean D2 among the men was 6.9 ± 1.7 mm, and mean D2 among the women was 6.1 ± 1.1 mm (p = 0.003). In the left renal artery subgroup, mean D2 was 7.2 ± 1.6 mm among the men and 5.9 ± 0.9 mm among the women (p = 0.00005). There was no significant difference in mean D2 values between men and women in the right renal artery subgroup. Likewise, mean D3 values varied significantly between men and women for all arteries and within the left renal artery subgroup (Tables 3 and 4). Mean D3 among the men regardless of side was 6.7 ± 1.3 mm, and mean D3 among the women was 5.9 ± 1.2 mm (p = 0.001). In the left renal artery subgroup, mean D3 among the men was 6.9 ± 1.3 mm and among the women was 5.7 ± 1.3 mm (p = 0.02). There was no significant difference in mean D3 between men and women within the right renal artery subgroup.
Percentage Stenosis
For every group and subgroup, average percentage stenosis varied from 69.9%
± 11.8% to 72.6% ± 11.6%. There were no significant differences
in percentage stenosis between left and right renal arteries or between men
and women within the study population as a whole or within any subgroup
(Tables 3 and
4).
Regression Analysis
All lengths were significantly related to the side of the renal artery as
follows: L1, p = 0.0395; L2, p = 0.0011; and L1 + L2,
p = 0.0001. None of the areas or diameters varied significantly with
respect to side, although A1 (p = 0.066) and A3 (p = 0.0576)
approached significance. None of the lengths was significantly related to sex.
All areas and diameters did relate significantly to sex as follows: A1,
p = 0.0111; A2, p = 0.0280; A3, p = 0.0036; D2,
p = 0.0086; and D3, p = 0.0010. Percentage stenosis related
significantly to neither sex nor side of the renal artery. Likewise, no
relation was found between patient age and renal artery anatomic
measurements.
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Numerous interventional radiologists have begun using carotid or coronary distal protection devices for the off-label indication of PTRA. Early enthusiasm for this practice has been tempered, however, by concerns about the inadequate design of these devices for use in the renal circulation. To our knowledge, our study was the first performed with 3D imaging of the renal artery anatomy as pertains to PTRA with distal protection.
Left renal arteries were found to be an average of 3.9 ± 1.3 cm long, and right renal arteries were 1 cm longer, at 4.9 ± 1.6 cm. Concordant lengths and differences were found in the L2 component, or distal protection device landing zone, that is, the length of the main renal artery beyond the point of maximum stenosis. No significant length differences were found between men and women. The mean maximum diameter at the point of maximum dilatation distal to the point of stenosis (D2) was close to 7 mm (6.9 ± 1.5 mm) in the left renal artery and 6.5 mm (6.4 ± 1.6 mm) in the right renal artery. In men, who made up 70% of our study population, mean left D2 was almost 1 mm larger than mean right D2 (7.2 ± 1.6 vs 6.4 ± 1.8 mm). Regardless of side, the mean D2 in men (6.9 ± 1.7 mm) was approximately 1 mm greater than that in women (6.1 ± 1.1 mm).
One limitation of this study was the exclusion of accessory renal arteries. There was a wide range of areas and diameters in the studied vessels, and that range would have been wider if all renal arteries had been included. Most intervention, however, is directed at lesions in the dominant renal arteries. The intention of this study was to delineate the anatomy of those vessels.
Another limitation was the lack of data regarding the average lengths of renal segmental and lobar arteries. The distal wire of an embolic protection device often is placed in these segmental vessels during PTRA to accommodate the bulkier portion of the device within the distal L2 landing zone of the main renal artery (Fig. 1). These wires are typically 3 cm long, and the distal tips may enter the terminal portion of a segmental vessel, causing spasm, perforation, and dissection. The ability to accurately characterize these small vessels with MDCT has not been proven and was considered beyond the scope of our analysis.
A third limitation was reliance on data analysis with multiple Student's t tests, which increased the probability of alpha error. To correct for this limitation, multivariate linear regression analysis was also conducted. The results of that analysis supported the conclusions drawn from the Student's t test results.
Our data reflect the anatomic features of patients undergoing revascularization procedures at a busy tertiary care referral center. Our findings suggest that most commercially available distal protection devices do not accommodate renal artery anatomy for PTRA. For example, the GuardWire (Medtronic) balloon occlusion and aspiration system is short enough in the main renal artery landing zone for that device to appear ideal to protect most renal arteries, even those with a short distance between the aorta and the renal artery bifurcation. However, the balloons of currently available GuardWire models inflate only to a diameter of 3–6 mm, in many cases leaving emboli considerable room to pass. Similarly, most filter protection devices approved for use in coronary and carotid interventions filter only up to 6 mm of the vessel diameter. These devices typically occupy a substantial proportion of the length of the main renal artery. In our study, the average distal protection device landing zone (D2) was 3 cm in most of the vessels, a length that would not accommodate most devices on the market. Our results did show that the crossing profiles of most currently available distal protection devices (3.2–3.5 French, or 0.0089–0.0107 cm2) should accommodate most atherosclerotic lesions (mean area at maximum stenosis for all subjects, 0.07 ± 0.04 cm2).
Finally, clear differences in renal artery dimensions exist between men and women, left and right, and one subject and the next within any subgroup. This variability strongly suggests the utility of 3D vessel analysis in assessing both the appropriateness of percutaneous revascularization and the feasibility of distal protection in each case.
PTRA is a less expensive, minimally invasive alternative to open revascularization that may prove more effective than medical management alone in certain cases of refractory hypertension and chronic renal insufficiency. Our data suggest that relatively straightforward design modifications can further improve the safety and efficacy of distal protection devices in the renal circulation and improve outcome after PTRA. Because of the variability of anatomic features among individual patients, our results also suggest the utility of 3D imaging analysis of all patients before they undergo PTRA.
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