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1 Department of Diagnostic Radiology, University of Konkuk, Chung-Ju Hospital,
620-5 Kyohyun-Dong, Chung-Ju City, Chung-Buk 380-704, South Korea.
2 Department of Diagnostic Radiology, University of Korea, Kuro Hospital, 80
Guro-Dong, Guro-Ku, Seoul 151-050, South Korea.
Received September 17, 2003;
accepted after revision November 17, 2003.
Supported by the academic research fund of Konkuk University in 2003.
Abstract
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SUBJECTS AND METHODS. The study group was composed of 210 patients who underwent MRI of the lumbosacral spine. The position of aortic bifurcation was evaluated using MRI. Midline and sagittal MRI were also assessed to identify the proximal portion of the right renal artery and the tip of the conus medullaris. These locations were recorded in relation to the upper or lower half of the adjacent vertebral body or the adjacent intervertebral disk space.
RESULTS. The most common site of aortic bifurcation was at the L4 vertebra (83%). In most patients two separated iliac arteries were apparent at the level of the L4–L5 disk space (93%), and the right renal artery was located between the lower half of the L1 vertebra and the upper half of the L2 vertebra (92%). The position of the conus medullaris was variably located, and the most frequent site was at the L1 vertebra (56%).
CONCLUSION. The aortic bifurcation, the right renal artery, and the combination of these structures can be reliable landmarks for determining the lumbar vertebral segments on MRI or CT. However, conus medullaris cannot be considered a good landmark because of its variable locations.
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Usually, lumbosacral spine radiographs are used for identifying lumbar vertebral segments. In several methods for numbering the lumbar vertebrae on lumbosacral radiographs [4, 5], the transverse diameter of the pedicle and the interpedicular distance are frequently largest at the L5 vertebra. Likewise, the L3 vertebra usually exhibits the longest transverse process of all lumbar segments, and the size of the neural foramen gradually increases to the last lumbar vertebra. The iliac wings are also generally found between the L4 and L5 vertebrae.
If the radiograph is not available, the paraspinal structures are necessary landmarks for the exact localization of lumbar vertebral segments on MRI. For instance, various structures such as the aortic bifurcation [6–8], inferior vena cava confluence [7], renal artery [9, 10], conus medullaris [11, 12], superior mesenteric artery, psoas muscles, pedicles, neural foramen, and iliolumbar ligament [13] can be observed on lumbar MRI, but only the aortic bifurcation, proximal right renal artery, and conus medullaris were consistently observed in our study. Although various articles on the anatomic locations of each of these three structures have been published [6–12], finding an article on selecting the location of lumbar vertebrae through anatomic location and traits is difficult.
Therefore, this study aimed to describe the anatomic location and significance of the aortic bifurcation, right renal artery, and conus medullaris when locating lumbar vertebral segments on MRI. In addition, this study examined changes in the location of paraspinal structures and their differences with what is normal in the case of lumbosacral transitional vertebra.
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MR images were obtained with a 1.0-T scanner (Magnum, Medison). Imaging was performed with all the patients lying in a supine position using a Quad thoracolumbar spinal coil for the lumbar spine and a body coil for the cervicothoracic spine.
The MRI sequences performed on all the patients were sagittal T1-weighted spin-echo (TR/TE, 400/17; acquired signals, 2; slice thickness, 4 mm; field of view, 280 x 280 mm; matrix, 184 x 512 mm; acquisition time, 4 min 54 sec), sagittal T2-weighted fast spin-echo (2,900/108; acquired signals, 3; slice thickness, 4 mm; field of view, 280 x 280 mm; matrix, 192 x 256 mm; acquisition time, 4 min 38 sec), and axial T2-weighted fast spin-echo (4,300/108; acquired signals, 3; slice thickness, 4 mm; field of view, 130 x 210 mm; matrix, 120 x 256 mm; acquisition time, 4 min 18 sec).
Cervicothoracic sagittal scout images were used as the gold standard for determining the numbering of the lumbar vertebral segments (Fig. 1A). These cervicothoracic sagittal scout images were made up of six sagittal images of T2-weighted fast spin-echo imaging (2,700/108; acquired signals, 2; section thickness, 5 mm; matrix, 192 x 256 mm; intersection gap, 0.5 mm; field of view, 450 x 450 mm). These scout images included the C1–L1 vertebrae. An oil capsule was used as a skin marker at the level of the lower thoracic spine by palpating the xiphoid process located between the T10 and T11 vertebrae.
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The vertebral level showed the oil capsule on the cervicothoracic sagittal scout image and on the sagittal T2-weighted fast spin-echo image of the lumbar spine to be in the same position (Figs. 1A and 1B), which enabled comparing the cervicothoracic sagittal scout image with the lumbar sagittal image and consequently determining the level. Specifically, the cervical, thoracic, and lumbosacral vertebrae were counted caudad from the C2 to the S1 vertebra, assuming that there were seven cervical and 12 thoracic vertebrae.
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Axial T2-weighted fast spin-echo imaging was used to evaluate the aortic bifurcation. The routine axial scan was focused on the disk space of the lumbar spine. Toward this end, an additional axial study was performed on the vertebral body on top of the disk space in which both common iliac arteries were seen for the first time. The longest horizontal position before the aorta was bifurcated into both common iliac arteries was selected as the point of aortic bifurcation (Fig. 1C).
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Sagittal T1-weighted spin-echo and sagittal T2-weighted fast spin-echo images were used in the study of the right renal artery. The proximal portion of the right renal artery was defined as a small round signal void at the right paracentral portion of the sagittal images (Fig. 2A). A line at the center of the proximal right renal artery was subtended to the adjacent vertebral body or intervertebral disk space, with the position defined in relation to the adjacent vertebra.
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For conus medullaris, the tip of the conus was defined as the most distal point of the cord on the T1- and T2-weighted sagittal sequences (Fig. 2B). As in the case of the right renal artery, the location was selected by comparing the vertebral body or disk space when linking the vertical line with the long axis of the cord in the distal point of the spinal cord. The axial sequences were used to prevent the thickening of the filum terminale and its effects.
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The level of their locations was recorded in relation to the upper or lower half of the adjacent vertebral body or the adjacent intervertebral disk space, assuming that a disk was equal in height to half of a vertebra (Fig. 1B).
For purposes of statistical evaluation, each level was assigned a number; the T12–L1 intervertebral disk space was 1, and the upper half of the L5 vertebral body was 14. The bivariate correlation analysis with Pearson's correlation coefficient was performed to investigate the relationship of the aortic bifurcation with the right renal artery and with the conus medullaris. The Student's t test was also used to assess differences in parameters for age and sex. All p values less than 0.05 were considered a statistically significant difference.
Furthermore, the incidence of the lumbosacral transitional vertebra and the location distribution of paraspinal structures were examined by dividing the transitional vertebra into the lumbarization of S1 and sacralization of L5. Lumbarization refers to the condition in which the first sacral segment has the radiographic appearance of a lumbar vertebra. On the other hand, in sacralization, the lowest lumbar vertebra has some or all the anatomic characteristics of the first sacral segment [14]. Lumbosacral spine radiographs were also used for the evaluation on the transitional vertebra.
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The proximal portions of the right renal artery were situated at the L1–L2 intervertebral disk space in 109 patients (52%), at the lower half of the L1 vertebral body in 68 patients (32%), at the upper half of L1 in 15 patients (7%), at the upper half of L2 in 17 patients (8%), and at the T12–L1 intervertebral disk space in one patient. The mean position of the proximal right renal artery was at the lower half of L1 (mean, 3.6; SD, 0.78). In most patients (92%), the proximal portion of the right renal artery was located between the lower half of the L1 vertebra and the upper half of the L2 vertebra.
The conus medullaris was variably located at the lower half of the L1 vertebral body in 62 patients (30%), at the upper half of L1 in 57 patients (27%), at the L1–L2 intervertebral disk space in 55 patients (26%), at the upper half of L2 in 30 patients (14%), and at the intervertebral disk space of T12–L1 in six patients (3%). The mean position of conus medullaris was at the lower half of L1 (mean, 3.2; SD, 1.1).
The Pearson's correlation (r = 0.535, p = 0.00001) between the aortic bifurcation and the right renal artery revealed a more significant positive correlation than the relationship (r = 0.138, p = 0.1337) between the aortic bifurcation and conus medullaris. Thus, the right renal artery was located at a more caudal level as the point of aortic bifurcation descended. No significant difference in the paraspinal structures was observed between male and female patients or with increasing age (p > 0.05).
Of the 210 patients, transitional vertebrae were seen in 20 patients (9.5%), lumbarization of S1 in 11 patients (5.2%; male-to-female ratio, 4:7), and sacralization of L5 in nine patients (4.3%; male-to-female ratio, 7:2).
Of the 11 cases of S1 lumbarization, nine cases were aortic bifurcations at the lower half of L4; the upper half of L4 and the L4–L5 intervertebral disk space accounted for one case each. The right renal artery was at the L1–L2 intervertebral disk space in six patients, at the upper half of L2 in four patients, and at the lower half of L1 in one patient. In the case of conus medullaris, seven cases of it were found at the upper half of L2, three cases at the L1–L2 intervertebral disk space, and one case at the lower half of L1.
Of the nine cases of L5 sacralization, aortic bifurcation was at the L3–L4 intervertebral disk space in seven cases and at the lower half of L3 and the upper half of L4 in one case each. On the other hand, in six patients the right renal artery was at the upper half of L1, and one patient each had it at the T12–L1 disk space, the lower half of L1, and the L1–L2 disk space. In the case of conus medullaris, there were three cases each at the T12–L1 disk space and the upper half of L1, two cases at the lower half of L1, and one case at the L1–L2 disk space.
In short, paraspinal structures in the lumbarization of S1 were located more toward the caudal direction compared with cases without lumbosacral anomalies (Fig. 3A, 3B, 3C, 3D). The paraspinal structures in the sacralization of L5 were located more toward the cephalad direction (Fig. 4A, 4B, 4C, 4D).
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Peh et al. [15] used cervicothoracic sagittal imaging for numbering the vertebral segments and reported that the transitional vertebra was in 13.2% of the patients. We also used the cervicothoracic sagittal image for the objective determination of lumbar vertebral segments. On the basis of this gold standard, we examined the location and distribution of the three paraspinal structures: aortic bifurcation, right renal artery, and conus medullaris. Confluences of the inferior vena cava, the superior mesenteric artery, and the psoas muscle were also examined on MRI, but the confluences were excluded from this study because it was difficult to determine starting and splitting points accurately. In addition, the pedicle, neural foramen, and iliolumbar ligament were not included in our study.
According to Gray's Anatomy [16], the point of aortic bifurcation is located at the front of the body of L4. On the other hand, Chithriki et al. [6] found that 67% of all cases in MRI research indicated that aortic bifurcation occurred at the L4 vertebral body. The lower level of L4 was prominent in the CT research of Kornreich et al. [7]. In our study, the point of aortic bifurcation was at the anterior part of the L4 vertebral body in 83% of our patients. In particular, 45% were at the upper half of L4 and 38% at the lower half of L4. On the axial images, the first bifurcated common iliac arteries appeared in the L4–L5 disk space in approximately 93%, in the L5–S1 disk space in 5%, and in the L3–L4 disk space in 2%. In the absence of specific information on the location of the lesion on abdominal sonography, abdominal or pelvic CT, and lumbar spine CT, the point of aortic bifurcation can contribute to forecasting the location of the lesion.
Generally, the ostia of the right renal artery lies between the levels of the lower third of the L1 vertebra and the upper third of the L2 vertebral bodies and slightly below the level of the superior mesenteric artery [10, 17, 18]. It can be determined by cadaveric, angiographic, and helical CT studies. However, finding studies that have examined the origin of the renal artery using MRI is difficult. This study examined changes in the location of the proximal portion of the right renal artery that appears as a small round signal void on MRI, especially on sagittal images. The right renal artery was used because observing it in the sagittal area was much easier because the right renal artery is longer than the left renal artery. Likewise, the left renal artery arises from the posterolateral part of the aorta, whereas the right renal artery arises from the anterolateral part of the aorta [10]. In this study, we found that the proximal portions of the right renal artery were distributed as follows: approximately 52% at the L1–L2 intervertebral disk space, 32% at the lower half of the L1 vertebral body, and 8% at the upper half of the L2 vertebral body. In short, 92% of the origin of the right renal artery was located between the lower half of L1 and the upper half of L2 focusing on the L1–L2 disk space, thus helping determine the location of the lumbar vertebral segment.
To our knowledge, the first cadaveric study that described the position of the adult conus medullaris was the work of Thomson [19]. In other cadaveric studies, conus medullaris was reportedly distributed from the T12 to L3 vertebrae; on average, it was located between the L1 and L2 vertebrae [20, 21]. Saifuddin et al. [11] studied the location of the conus medullaris using MRI. These researchers found that the mean position of the conus was the lower third of L1, the range of which was from the middle third of T12 to the upper third of L3, and that the variation in the positions of conus followed normal distribution. In this study, the mean position of the conus medullaris was the lower half of L1. Similar to the results of other studies, its range was from the T12–L1 intervertebral disk space to the upper half of L2 vertebra. In comparison with the aortic bifurcation and the right renal artery, however, the conus medullaris showed a wide distribution. Thus, it was inadequate as a paraspinal structure for determining the location of the lumbar vertebral segment. This inadequacy is caused by the aortic bifurcation and the right renal artery being distributed mostly at two levels, whereas the conus medullaris is evenly distributed across four levels.
The results of our study show that the aortic bifurcation and the right renal artery have a more significant positive correlation than the relationship between the aortic bifurcation and the conus medullaris. Thus, the origin of the right renal artery (i.e., a vascular structure) tends to move in the same direction as the point of aortic bifurcation.
Generally, the lumbosacral junction is the most common site for transitional vertebrae to occur. Here, the lowest lumbar vertebra may have some or all the anatomic characteristics of the first sacral segment (sacralization). Conversely, the first sacral segment may have the radiographic appearance of a lumbar vertebra (lumbarization) [2, 3, 14]. Despite the small number of cases, in the 20 cases of transitional vertebra, 11 cases showed that paraspinal structures of S1 lumbarization were positioned more toward the caudal location, whereas nine cases showed that the paraspinal structures of L5 sacralization were positioned more toward the cephalad location. These findings are similar to the results of Chithriki et al. [6] regarding aortic bifurcation. When the cervicothoracic sagittal scout image is not used and the vertebral segment is counted using a morphology standard, the lumbarization of S1 can be questioned if the paraspinal structures are located at a higher place than the normal range; such will be the case for the sacralization of L5 if they are located at a lower place. However, because not enough cases of transitional vertebrae are available, further studies need to be carried out on the location of paraspinal structures in the transitional vertebrae.
In conclusion, the aortic bifurcation, the right renal artery, and the combination of these structures can be a reliable marker when locating lumbar vertebral segments on MRI. However, the conus medullaris cannot be considered a good landmark because of its variable location. In most patients, the bifurcated common iliac arteries are seen at the L4–L5 disk space, and the proximal right renal artery around the L1–L2 disk space. In our preliminary study, when these paraspinal structures were in outlying positions from the main locations, transitional vertebrae were possible. Their clinical applications include the accurate localization of the lumbosacral disk disease before surgery, the evaluation of transitional vertebra, and the localization of lumbar vertebral segments in an axial CT study.
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