Published online Mar 31, 2010.
https://doi.org/10.3348/jksr.2010.62.3.263
Radiologic Evaluation of the Renal Axis in Patients with an Accessory Renal Artery
Abstract
Purpose
The purpose of this study is to evaluate the association between an accessory renal artery (ARA) and the renal axis seen on radiographs.
Materials and Methods
The MDCT axial images of 428 patients were used to detect the presence of an ARA and its location. The plain radiographs were used to measure the renal axis angle, which was the angle between the longitudinal spinal axis and the renal axis. We correlated these results to determine the association between an ARA and the renal axis.
Results
Of the 856 kidneys (428 patients), 19 kidneys had an ARA in the upper pole, 63 kidneys had an ARA in the hilum and 20 kidneys had an ARA in the lower pole. The mean renal axis angles of these three groups were 16.7 degrees, 15.9 degrees and 11.2 degrees respectively. The mean renal axis angle without an ARA was 16.8 degrees. The renal axis angles with an ARA in the upper pole or hilum showed no significant differences compared to those without an ARA. However, the renal axis angle with an ARA in the lower pole was significantly smaller than those without an ARA.
Conclusion
On plain radiographs, the axis of kidneys with an ARA in the lower pole maybe more vertical than those without an ARA.
Renal arteries are usually is the largest paired visceral branches of the abdominal aorta. A single renal artery supplies each kidney in most individuals. The renal arteries typically arise just below the origin site of the superior mesenteric artery and they travel laterally behind the renal veins (1, 2). Among the various types of renal artery anomalies, the most clinically important variation is the accessory renal artery. Accessory renal arteries are especially important in situations when accurate depiction of the renal circulatory system is required. These situations include surgical reconstruction of the abdominal aorta, renal transplantation and renovascular hypertension (3, 4, 5).
When studying the radiographs of the abdomen and pelvis, abnormally vertical renal axes were seen in some patients. Some of these patients had underlying diseases such as scoliosis of the lumbar spine, horseshoe kidneys, renal masses, renal cysts or other retroperitoneal lesions that might cause a mass effect (6, 7). In clinical practice, we have experienced several cases of vertical renal axes where an accessory renal artery was found in the lower pole of the kidney. This led us to suspect the association between the renal axis and the accessory renal artery not only in the lower pole, but also in the upper pole and the hilum. To the best of our knowledge, there have been no previous studies on the relationship between these two factors. The purpose of this study is to evaluate the association between an accessory renal artery and the renal axis seen on radiographs.
MATERIALS AND METHODS
Study Population
From March 1, 2008 to April 1, 2008, 500 consecutive patients who underwent abdominal multidetector row computerized tomography (MDCT) were chosen for possible entrance into this study. The patients with no arterial phase images or 3-D angiography images were chosen because the patients who underwent these studies usually had an underlying disease such as a renal mass or chronic renal disease. Therefore, we used the axial images that were taken during the portal phase as the reference standard. Fifty patients were excluded from this study because they'd had no radiographs taken in the past. Additional, another 22 patients were excluded from this study because of various reasons that might have an effect on the renal axis; 12 cases of lumbar spine scoliosis, two cases of renal mass, two cases of renal cysts, two cases of polycystic kidneys, one case of a post-kidney transplantation state, one case of nephrectomy, one case of hematoma in the psoas muscle and one case of autosomal dominant polycystic kidney disease. The final study population was 428 patients (244 males and 184 females; age range: 4-85 years, mean age: 55.7 years). Since there were two kidneys in each patient, a total of 856 kidneys were examined. The radiographs used in this study were the KUB (n = 127), the IVP (n = 102), the simple abdominal radiograph in the supine position (n = 136) and the lumbar spine AP view (n = 63). CT topograms were not used because clear visualization of the renal poles was difficult with using this modality.
CT Examination
MDCT examinations were performed on a 16-detector row CT scanner (LightSpeed Pro; General Electric Medical System, Milwaukee, WI, USA) or on a 64-detector row CT scanner (Brilliance 64; Philips Medical Systems, Cleveland, OH, USA). The parameters for the 16-detector row CT were a gantry rotation speed of 0.5 seconds, a detector configuration of 1.25 mm × 16 mm and a table feed of 20 mm per gantry rotation. The parameters for the 64-detector row CT scanner were a gantry rotation speed of 0.75 seconds, a detector configuration of 0.625 mm × 64 mm and a table feed speed of 40 mm per gantry rotation. The MDCT procedure and breathing techniques were explained to each patient before scanning. A 20-gauge peripheral or central line was inserted into a median cubital vein for administering intravenous contrast material. Any oral contrast medium was not given. A power injector was used to inject 100 mL of iopromide (Ultravist 370; Schering, Berlin, Germany) through the veins at a flow rate of 3 mL/s. The portal phase was defined as 70 seconds after the injection of the contrast media. Continuous 5-mm-thick sections and 5-mm intervals were used for both scanners for the axial images.
Image Interpretation
All image interpretation was performed on a PACS monitor. The senior resident measured the angle of the renal axis. On the radiographs, the angle between the longitudinal axis of the spine and the renal axis was measured and this was rounded off to the first decimal point. The longitudinal axis of the spine was decided by drawing a line from the spinous process of the first lumbar vertebra to the spinous process of the fifth lumbar vertebra. The renal axis was decided by drawing a line from the upper most pole to the lower most pole (Fig. 1). Four weeks later, two radiologists reviewed all the patients' MDCT images for the presence of accessory renal arteries and they reached a consensus. One radiologist had 9 years experience and the other radiologist was a resident with 3 years of training. The MDCT images were reviewed for the presence of accessory renal arteries. The accessory renal artery was defined as an extrarenal artery originating from the aorta and entering directly into the renal capsule or the renal hilum (8). The distributions of the accessory renal arteries were divided into three groups; the upper pole of the right/left kidney, the hilum of the right/left kidney and the lower pole of the right/left kidney.
Fig. 1
A 44-year-old male. The longitudinal axis of the spine was determined by drawing a line (a) from the spinous process of the first lumbar vertebra to the spinous process of the fifth lumbar vertebra. A perpendicular line (b) was drawn to this longitudinal axis (a) and the angle (c) between this line and the renal axis (d) was measured and rounded off to the first decimal. The angle (e) between the longitudinal axis of the spine and the renal axis was obtained by subtracting angle (c) from 90 degrees.
Statistical Analysis
In each group, the angles of the right and left kidneys were compared with each other. The Wilcoxon signed-rank test was used, and a p value of less than 0.05 indicated that the results had a statistically significant difference. We then we used ANOVA to compare the angles of the four groups with one another. If the results showed that some groups had significant differences with each other, then a post-hoc test was done to determine which two groups were involved. The SPSS program for Windows (Version 16.0, SPSS Inc., Chicago, Illinois, USA) was used for all the statistical analysis.
RESULTS
The mean angles of the right and left kidneys, according to the location of the accessory renal artery, are shown in Table 1. There were no significant differences between the right and left kidneys in every group.
Table 1
Association of an Accessory Renal Artery (ARA) with the Renal Axis Angle Seen on Radiographs
The mean angle of the kidneys without an accessory renal artery (n = 754; right: 391, left: 363) was 16.8 degrees (range: 6.8 - 28.1 degrees) (Fig. 2). The mean angle of the kidneys with an accessory renal artery in the upper pole (n = 19; right: 5, left: 14) was 16.7 degrees (range: 9.9 - 23.1 degrees) (Fig. 3). The mean angle of the kidneys with an accessory renal artery in the hilum (n = 63; right: 36, left: 27) was 15.9 degrees (range: 11.3 - 22.9 degrees) (Fig. 4). The mean angle of the kidneys with an accessory renal artery in the lower pole (n = 20; right: 11, left: 9) was 11.2 degrees (range: 2.0 - 19.7 degrees) (Fig. 5). These results showed that there were no significant differences among the first three groups; those without an accessory renal artery, those with an accessory renal artery in the upper pole and those with an accessory renal vein in the hilum. However, the group with an accessory renal artery in the lower pole had significant differences compared with these three groups (p < 0.05) (Table 2).
Fig. 2
A 32-year-old female. The intravenous pyelography film of the left kidney with no accessory renal arteries. The angle between the renal axis and the longitudinal axis of the spine was 19.6 degrees.
Fig. 3
B, C. Axial images of the CT scan taken during the portal phase show an accessory renal artery originating from the aorta (B, arrow) and inserting directly into the upper pole (C, arrow).
A 57-year-old female.
A. The intravenous pyelography film of the left kidney. The angle between the renal axis and the longitudinal axis of the spine was 20.6 degrees.
Fig. 4
B, C. The axial images of the CT scan taken during the portal phase show an accessory renal artery originating from the aorta (B, arrow) and inserting directly into the middle pole (C, arrow).
A 31-year-old female.
A. Intravenous pyelography film of the left kidney. The angle between the renal axis and the longitudinal axis of the spine was 20.1 degrees.
Fig. 5
B, C. The axial images of the CT scan taken during the portal phase show an accessory renal artery originating from the aorta (B, arrow) and inserting directly into the lower pole (C, arrow).
A 65-year-old male.
A. The intravenous pyelography film of the left kidney. The angle between the renal axis and the longitudinal axis of the spine was 5.6 degrees.
Table 2
Association of an Accessory Renal Artery (ARA) with the Renal Axis Angle seen on Radiographs
DISCUSSION
There are several conditions that may cause an abnormal renal axis. These include horseshoe kidneys, spina bifida or retroperitoneal tumors such as renal tumors (6, 7). But as we studied the radiographs of the patients without these underlying diseases, we noticed some kidneys that had unusually vertical axes. Further studies showed that some of these patients had an accessory renal artery in the lower pole. This led us to believe that an accessory renal artery was another condition that may result in an abnormal renal axis.
Different variations of the renal arteries are caused by the development of the mesonephric arteries during the process of embryogenesis. A vascular network that originates from the mesonephric arteries supplies the kidneys, the suprarenal glands and the gonads located on both sides of the aorta. This network is known as the rete arteriosum urogenitale and it is located between the sixth cervical vertebra and the third lumbar vertebra. During the sixth through the ninth week of gestation, the kidneys ascend from their original location in the pelvic cavity to the first lumbar vertebra level, while maintaining their arterial supply from these mesonephric arteries. Over time, most of the arteries regress in a sequential fashion and leave only one mesonephric artery. This becomes the single main renal artery (9, 10, 11). During this ascent, the psoas muscle, which travels inferolaterally in an oblique fashion, has an effect on the renal axis. The upper pole comes to lie more closely to the lumbar spine than the lower pole. But when one of the mesonephric arteries fails to regress, an accessory renal artery is formed and this additional vascular structure is thought to have an influence on the renal axis. When this accessory renal artery is formed in the upper pole, there is no significant change in the renal axis because the upper pole is naturally closer to the spinal axis. The accessory renal artery that enters the hilum also has no effect on the renal axis because it courses along the main renal artery. However, an accessory renal artery in the lower pole is suspected of giving resistance to the lower pole and so this kidney moves farther away from the spinal axis.
Accessory renal arteries were previously defined as aberrant arterial branches that originated directly from the aorta and they usually served a small portion of the kidney (11, 12, 13). Kadir et al. (8) divided these accessory renal arteries into two types. The common type is when the accessory artery enters into the renal hilum along with the main renal artery. The other type is when the accessory artery enters directly into the capsule of the polar regions. Ozkan et al. (14) named the second group of accessory renal arteries polar or aberrant arteries.
There are a few other renal vascular variations that must be differentiated from an accessory renal artery (15, 16). The extrahilar branching variant is when the main renal artery branches before it reaches the renal hilum. When this occurs within 2 cm of the renal artery origin, it is referred to as an early branching artery (17). Detection of this variant is important before renal transplantations because a proper anastomosis is needed. Another variant is capsular arteries where tiny arteries perfuse the renal capsule. They may arise from the main renal arteries, the renal artery branches or other retroperitoneal vessels (15, 16).
Although conventional angiography has traditionally been used for evaluating renal artery variations, it is a very invasive and time-consuming procedure that uses large amounts of ionizing radiation and nephrotoxic contrast media (18). Therefore, the minimally invasive MD-CT angiography has replaced conventional angiography as the gold standard for evaluating kidneys via vascular imaging (19). Even though a plain radiograph can never replace MDCT angiography for detecting an ARA, a more vertical renal axis seen on a plain radiograph might warn the radiologist of a possible ARA in the lower pole, which will lead to a more careful observation of the MDCT angiograms.
Our study has several limitations. First, our study showed an ARA occurrence rate of 13.5% which was slightly lower compared to that of a previous study by Kawamoto et al. (20), which reported that accessory renal arteries were found in 17.6% of 74 kidneys. This difference may be due to fact that we used the MDCT axial scan images that were taken during the portal phase with a 5 mm thickness as the reference standard, unlike the previous study that used MDCT angiography images during the arterial phase to detect the presence of an accessory renal artery. We chose the portal phase images rather than the arterial phase images because the patients who underwent arterial phase scans usually had underlying renal diseases. By using the arterial phase images as the reference standard, more patients would have been excluded, and so this would have led to a much smaller pool of subjects. Since the branching of every accessory renal artery from the aorta was thoroughly examined, the possibility of misinterpreting an accessory renal artery with an accessory renal vein was minimized. Second, the renal poles were unclear on some of the radiographs, which might have led to some error in determining the renal axis. Since the angle of the renal axis was measured manually by a single observer, there might have been some error in measurement. To minimize these errors, we studied a larger pool of patients than that of the previous studies, and since the presence of the ARA was unknown when the renal axis was determined and the angle was measured, there was no bias toward a certain group.
In conclusion, the angle of the axis of the kidneys with an ARA in the hilum and the upper pole showed no significant differences compared to those kidneys without an ARA. However, the angle of the axis in the kidneys with an ARA in the lower pole was significantly smaller, and this resulted in a more vertical renal axis being seen on simple radiographs.
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