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Original Research |
1 Department of Diagnostic Radiology, Tohoku University Graduate School of
Medicine, 1-1 Seiryo, Aoba, Sendai, Miyagi, 980-8574, Japan.
2 Department of Nephrology, Endocrinology and Vascular Medicine, Tohoku
University Graduate School of Medicine, Miyagi, Japan.
Received October 23, 2007;
accepted after revision February 13, 2008.
Address correspondence to T. Matsuura
(t.matsuura{at}rad.med.tohoku.ac.jp).
Abstract
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MATERIALS AND METHODS. Contrast-enhanced MDCT was performed in 104 patients with thoracoabdominal vascular disease using an 8-MDCT scanner. Both axial and multiplanar images were reviewed by two radiologists. The following points regarding the right adrenal vein were evaluated: degree of visualization; relationship to accessory hepatic or other veins; anatomy, including location of the orifice in relation to the surrounding structures; direction from the inferior vena cava; and length and diameter.
RESULTS. The right adrenal vein was detected in 79 (76%) of 104 patients. The right adrenal vein formed a common trunk with the accessory hepatic vein in six (8%) of the 79 patients. The orifice was craniocaudally located between the level of vertebrae T11 and L1. Among the 73 patients, the right adrenal vein joined the inferior vena cava in the right posterior quadrant in 71 patients (97%) and in the left posterior quadrant in two (3%). The transverse direction from the inferior vena cava was posterior and rightward in 56 patients (77%) and posterior and leftward in 17 (23%); the vertical direction from the inferior vena cava was caudal in 65 (89%) and cranial in eight (11%) patients. The length and diameter averaged 3.8 and 1.7 mm, respectively.
CONCLUSION. MDCT enabled the identification of the right adrenal vein and delineation of its anatomy, including its position and relationship to surrounding structures.
Keywords: adrenal gland • adrenal venous sampling • aldosteronism • anatomy • CT • right adrenal vein
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Selective adrenal venous sampling can be difficult to perform because catheterization of the right adrenal vein generally remains difficult, whereas catheterization of the left adrenal vein is a relatively simple procedure [6–11]. The difficulty associated with catheterization of the right adrenal vein appears to result from its small size and variable anatomy: It is a vein that usually drains directly into the inferior vena cava (IVC) at a variable angle [8, 12].
MDCT could possibly guide adrenal venous sampling if it were capable of delineating the anatomy of the right adrenal vein. Although Daunt [12] referred to the feasibility of identifying the right adrenal vein with MDCT, to our knowledge a detailed analysis regarding visualizing the anatomy of the right adrenal vein with MDCT has never been made. We undertook this study to determine how frequently the right adrenal vein can be identified on MDCT and what spectrum of anatomic variations is seen.
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Patients
We performed a retrospective analysis of CT images from 104 consecutive
patients (69 men, 35 women; mean age, 67 years; range, 26–89 years) with
a presumptive diagnosis of thoracoabdominal vascular disease who underwent
contrast-enhanced MDCT of the thoracoabdominal aorta and iliac arteries
between June 2002 and April 2003. Frequent experience of identifying the right
adrenal vein in everyday practice with MDCT for thoracoabdominal vascular
disease led us to choose these patients as subjects for our study. Forty of
these patients had aortic dissection; 26, thoracic aortic aneurysm; 25,
abdominal aortic aneurysm; one, aortitis; and 12, atherosclerosis and
suspected aortic aneurysm. Informed consent for contrast-enhanced CT had been
obtained from all patients before their CT examination.
CT Examinations
The scanner was an Aquilon 8-MDCT scanner (Toshiba). Scans were obtained
with the following parameters: 0.5 second per rotation, 1-mm collimation, 14
mm/s table increment (pitch, 7.0), tube voltage of 120 kV, and the tube
current was 350 mA. The patients' spines were positioned near the isocenter of
the CT gantry. Patients were asked to hold their breath for approximately 40
seconds during scanning immediately after the inhalation of oxygen. When
patients could not hold their breath for the entire scanning time, they were
instructed to breathe shallowly after holding their breath for as long as
possible.
Before scanning was started, 100 mL of contrast material containing 300 mg I/mL ([iopamidol] Iopamiron, Schering) was injected into an antecubital vein at a rate of 3.5 mL/s. The scanning delay was set by means of an automatic triggering system (SureStart, Toshiba). When the attenuation value at the level of the ascending aorta reached a preset threshold (an absolute attenuation value of 85 H) in three consecutive sampling points, scanning started automatically.
Transverse sections were reconstructed with a 1-mm section thickness at 0.5-mm intervals. Originally, the reconstruction field of view was set to approximately 40 cm to evaluate the entire aorta and iliac arteries. Reconstruction with a small field of view of 15 cm around the aorta and spine was also performed to evaluate the artery of Adamkiewicz. We used the image data of a 15-cm field of view for retrospective evaluation of the right adrenal vein in this study.
CT Interpretation
Images were interpreted using a stand-alone workstation (ZioM900, Amin). We
used a cinemode display of transverse and multiplanar reformation (MPR) images
to evaluate the right adrenal vein. Two radiologists, one with 6 and the other
with 18 years of experience, independently analyzed the CT images. In cases of
disagreement, a final consensus was reached through interobserver
discussion.
Definition of the Right Adrenal Vein on CT Images
The extraglandular part of the right adrenal vein was identified according
to the following criteria: an enhanced tubular or linear structure that arose
from the right adrenal gland and eventually entered the IVC either directly or
indirectly; it may be in contrast to the background of the intervening adipose
tissue. In the absence of substantial intervening adipose tissue, it may be in
contrast to the surrounding organs such as the right adrenal gland and liver
because of its denser enhancement than that of those organs.
Points of Evaluation
We evaluated the following points regarding the right adrenal vein: the
degree of visualization; whether the common trunk was formed with accessory
hepatic or other veins before entering the IVC; the anatomy of the right
adrenal vein, including the location of the orifice; and its direction,
length, and diameter where it directly joined the IVC. For describing the
direction of the right adrenal vein, we used a 3D coordinate system and
aligned the rectangular coordinate axes with the body axes (i.e.,
anteroposterior = x-axis, transverse = y-axis, and vertical
= z-axis). The positive x-, y-, and z-axes
pointed posteriorly, rightward, and caudally, respectively.
Degree of visualization of the right adrenal vein—The rate of visualization and number of the right adrenal veins were checked. The degree of visualization was arbitrarily graded on a 5-point scale as follows: excellent if the right adrenal gland was distant from the IVC by more than 2 mm, which brought about a sufficiently long right adrenal vein through the surrounding adipose tissue; good if the right adrenal gland was distant from the IVC by less than 2 mm but the right adrenal vein was in good contrast to the surrounding background; fair if the right adrenal gland was distant from the IVC by less than 2 mm and it was in relatively good contrast; poor if the right adrenal gland was distant from the IVC by less than 2 mm and the right adrenal vein was barely in contrast; and none if the right adrenal vein was not visualized.
We defined the former three groups (excellent, good, and fair) as those with an unequivocally identified right adrenal vein in which the following anatomic analyses were made; the latter two groups (poor and none) were categorized as those with an unidentified right adrenal vein and were excluded from the following analyses.
Relationship of the right adrenal vein to accessory hepatic or other veins—We examined whether a common trunk for the right adrenal vein and the accessory hepatic or other veins was formed before entering the IVC. When the right adrenal vein directly entered the IVC but almost shared a common orifice with an accessory hepatic or other vein, that was also recorded. In patients with a common trunk, its length—that is, the distance of the insertion of the right adrenal vein from the insertion of the accessory hepatic vein to the IVC—was measured.
Location of the right adrenal vein orifice in relation to surrounding
structures—The craniocaudal level of the right adrenal vein orifice
was specified relative to vertebral bodies and disks. Vertebral bodies were
divided into three segments: superior, middle, and inferior. The transverse
distance from the right margin of the vertebral body was measured. The
vertical distance from the lower end of the ipsilateral renal vein orifice was
measured because the large-caliber right renal vein sometimes played a role as
a landmark during catheterization. The position of the orifice was also
evaluated along the circumference of the IVC as an
angle—
—in the xy plane
(Fig. 1A).
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1—between the right adrenal
vein and the x-axis when the right adrenal vein was projected onto
the xy plane (Fig.
1B), and the other was the craniocaudal
angle—2—between the right adrenal vein and the
z-axis when the right adrenal vein was projected onto the vertical
plane that paralleled the right adrenal vein
(Fig. 1C).
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Statistical Analysis
To determine the interobserver agreement for detecting the right adrenal
vein obtained before consensus, kappa statistics were used. The kappa values
were evaluated as follows: < 0.20, poor agreement; 0.21–0.40, fair
agreement; 0.41–0.60, moderate agreement; 0.61–0.80, good
agreement; and 0.81–1.00, excellent agreement.
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Relationship of the Right Adrenal Vein to Accessory Hepatic or Other Veins
Among the 79 patients who had an identifiable right adrenal vein (excellent
to fair), the right adrenal vein and accessory hepatic vein formed a common
trunk before entering the IVC in six (8%;
Fig. 3) patients, the right
adrenal vein entered the IVC directly but almost shared a common orifice with
an accessory hepatic vein in seven (9%), and it entered the IVC independently
of other veins in the remaining 66 (84%) patients. In the six patients with a
common trunk, the mean length of common trunk was 4.7 ± 2.0 mm. No case
occurred in which the right adrenal vein formed a common trunk with veins
other than an accessory hepatic vein.
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Angle , representing the position of the orifice along the
circumference of the IVC, is shown in
Figure 4B. Angle ranged
from –7° to 71° (mean, 39° ± 16°). The right
adrenal vein joined the IVC in the right posterior quadrant in 71 (97%) of the
73 patients, most frequently in a range of angulation between 20° and
60°. In two (3%) of the 73 patients, the right adrenal vein occurred in
the left posterior quadrant.
Direction of the Right Adrenal Vein from the IVC
The angles 1 in the transverse plane and 2 in the vertical plane
are shown in Figures 4C and
4D. The angle 1 ranged
from –36° to 47° (mean, 8° ± 18°), ranging
between 0° and 20° in almost half of the patients. The direction from
the IVC was posterior and rightward in 56 (77%) and posterior and leftward in
17 (23%) of the 73 patients.
The angle 2 ranged from 30° to 136° (mean, 73° ±
23°); most patients showed a value between 50° and 90°. The
direction from the IVC was caudal in 65 (89%) and cranial in eight (11%) of
the 73 patients.
When the angles in the transverse plane (1) and in the vertical plane
(2) were combined in individual patients, the course of the right adrenal
vein from the IVC took a posterior, rightward, and caudal direction in 52
(71%) patients; a posterior, leftward, and caudal direction in 13 (18%); a
posterior, rightward, and cranial direction in four (5%); and a posterior,
leftward, and cranial direction in four (5%) of the 73 patients.
Length and Diameter of the Right Adrenal Vein
The mean length was 3.8 ± 1.7 mm, and the mean diameter at the
junction with the IVC was 1.7 ± 0.6 mm.
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The direction of the right adrenal vein from the IVC is known to usually take a posterior, rightward, and caudal course [18], which was also confirmed in our study. In a minority of the cases, however, the vein proceeded posterior and leftward in the transverse plane, and cranial in the cranial–caudal direction. This information should also be of much help for catheterization.
A common trunk of the right adrenal vein with an accessory hepatic vein was found in six (8%) of the 79 patients in our series. In addition to those cases, the right adrenal vein took a separate course but almost shared a common orifice with an accessory hepatic vein in another seven (9%) of the 79 patients. In the literature [14–16], a common trunk of the right adrenal vein with an accessory hepatic vein has been reported in approximately 10–21% of individuals; a common trunk might have included both types of variation, although no detailed distinction between them has been provided. In these types of variation, a common trunk may work as a landmark for catheterization of the right adrenal vein. Furthermore, recognition of these variations should be important preoperative information because the tip of the catheter might better be advanced selectively into the right adrenal vein: Keeping the catheter in the common trunk may result in a contaminated sample of blood, and erroneous advancement selectively into the accessory hepatic vein would provide spurious results when sampling.
In contrast to the relatively simple catheterization of the left adrenal vein, with a success rate of more than 90%, catheterization of the right adrenal vein remains difficult. Its success rate generally remains at approximately 60–70% [6, 8–11], although an exceptionally high success rate of 96% was reported by Young et al. [7], who stated that their rate might have been attributable to the efforts of an angiographer who specialized in the procedure. The difficulty may arise because of the small size of the right adrenal vein, its anatomic variations in the junction with the IVC, or confusion with accessory hepatic veins that may form a common stem with the right adrenal vein [8, 12].
A reliable way to delineate the right adrenal vein anatomy before adrenal venous sampling has not been previously available, but our study suggests the potential for MDCT to provide detailed information on the right adrenal vein anatomy similar to other small arteries [12, 19–21]. Such information would be useful in planning adrenal venous sampling. For patients with suspected primary aldosteronism, at our institution we actually perform MDCT with 1-mm collimation not only for imaging of the adrenal gland but also for mapping the right adrenal vein before venous sampling, which allows us to catheterize easily and efficiently. Indeed, angiographers can perform catheterization of the right adrenal vein in a reasonable examination time if such information is available before venous sampling is undertaken (Figs. 5A, 5B, 5C, and 5D). We believe that preoperative recognition of the craniocaudal level of the orifice, the location of the orifice along the circumference of the IVC, the direction from the IVC, and knowing whether a common trunk with an accessory hepatic vein is formed would be especially valuable for right adrenal vein catheterization and subsequent venous sampling. Although undergoing MDCT for mapping the right adrenal vein may provide some degree of radiation exposure, fluoroscopy time might be diminished to some degree at venous sampling with detailed information on the right adrenal vein anatomy, which remains to be further studied.
Unequivocal identification of the right adrenal vein has not always been obtained. Several factors may contribute to these failures: inappropriate timing of the scan, artifacts due to insufficient breath-holding, or dense contrast material reflux from the right atrium to the IVC, attributable to deteriorated heart function. Concerning the timing of the scan, the CT protocol used in this study was that designed for showing the arterial structures. Despite such a CT protocol, the rate of successful identification of the right adrenal vein may well be regarded as relatively high, although a slightly later phase may have shown the right adrenal vein better. The rate of identification would likely be further improved if more optimal scan timing for the right adrenal vein is established, which remains to be determined. In addition to the degree of contrast enhancement of the right adrenal vein, abundance of the surrounding adipose tissue appears to affect the detection of the right adrenal vein.
Although insufficient breath-holding might be caused by the long duration of the scan in this study, it was found in only two patients. Because a relatively old 8-MDCT scanner was used, some patients could not breath-hold during scanning. However, 64-MDCT and a targeted scanning protocol for adrenal veins would solve this problem. Further study using a 64-MDCT scanner for patients with primary aldosteronism is necessary to confirm the accuracy and the usefulness of adrenal venous visualization by MDCT.
Our study has several limitations. First, the right adrenal veins were not confirmed by venography or autopsy, which are the gold standards of right adrenal vein assessment. However, we believe that an enhanced tubular or linear structure arising from the right adrenal gland, running through the intervening adipose tissue, and finally entering the IVC is the right adrenal vein; it is easily differentiated from the right adrenal arteries because the latter vessels originate from the right renal arteries, aorta, or inferior phrenic arteries and never join the IVC. Although the adrenal gland, an oblong structure in close proximity to the IVC, may be confused with the right adrenal vein in axial images alone, evaluation of multiplanar reformatted or volume-rendered images would help differentiate them.
Second, as mentioned previously, our study was not designed specifically to assess the right adrenal vein. We chose as subjects those patients who underwent contrast-enhanced MDCT of the thoracoabdominal aorta and iliac arteries with a field of view of 15 cm or less because we recognized that the right adrenal vein was often visualized in the late arterial to early venous phases in our experience, and because the portal phase of routine abdominal CT images with a field of view of 30 cm or more was considered inappropriate in terms of spatial resolution. With the CT protocol being more targeted to imaging the right adrenal vein, the scanning time could be shorter and so reduce movement artifacts.
Third, patients with confirmed hyperaldosteronism were not included as subjects in our study, and these patients would be the most likely reason for adrenal venous sampling. However, ease of identification of the right adrenal vein seemed unaffected by the incidental presence of a moderate-sized mass in the right adrenal gland in this study (Fig. 5B).
Finally, how this information affects the work of angiographers was not examined in this study. MDCT provides a static image, whereas angiography is a dynamic study. Breathing and Valsalva maneuvers will affect the anatomy of the IVC and consequently that of the right adrenal vein. Therefore, further study should be conducted on whether preoperative MDCT really decreases the time of the adrenal venous sampling procedure, increases the rate of successful sampling, and contributes to few er complications if MDCT images are reviewed before venous sampling.
In conclusion, MDCT enabled the identification of the right adrenal vein and delineation of its anatomy, including its position and relationship to the surrounding structures such as the IVC, in a high percentage of patients. This preoperative information may be helpful in the catheterization of the right adrenal vein for adrenal venous sampling.
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