Predictive Value of Conventional Computed Tomography

Abstract

The present study aimed to evaluate the diagnostic reliability of computed tomography in determining the proximal extent of abdominal aortic aneurysms and the possibility of infrarenal clamping.

Preoperative computed tomographic findings, together with the operative data for 95 patients, were retrospectively analyzed in light of the operative findings. Eighty-nine (93.68%) of the patients were men and 6 (6.32%) were women, with a mean age of 66.27 ± 18.14 years.

Diagnosis of infrarenal aneurysm by computed tomography was confirmed at the time of surgery in 91 (95.79%) of 95 patients. The negative-predictive value of computed tomography in detecting supra-aneurysmal renal arteries was found to be 95.79%. The specificity was 98.91%. Infrarenal cross-clamping was performed in 59 (62.11%) of 95 patients, whose aortic segments between the renal artery orifices and the proximal borders of the aneurysms had a mean length of 26.4 ± 7.11 mm by computed tomography. Suprarenal clamping was required in 36 (37.89%) of the 95 patients, whose aortic segments had a mean length of 12.7 ± 3.48 mm.

We conclude that conventional computed tomography is reasonably accurate in determining the proximal extent of abdominal aortic aneurysms. Although there is a high rate of error in determining the possibility of infrarenal clamping when no specific measurements are taken, infrarenal clamping can be planned when measurement by computed tomography shows a length of ≥26 mm between the renal arteries and the proximal extent of the aneurysm. In patients with shorter aortic segments, suprarenal aortic clamping should be considered. (Tex Heart Inst J 2002;29:172–5)

Among people older than 60 years, about 4% to 11% have abdominal aortic aneurysms (AAA), either diagnosed or undiagnosed. ¹ Without surgery (the usual treatment), the mortality rate is quite high, but it varies in accordance with the size and growth characteristics of the aneurysm. ^1,2 About half of all AAA patients can be expected to die of aneurysmal rupture if their aneurysms are not surgically repaired. ¹ Multiple studies have shown that larger aneurysms are most likely to rupture. Nevitt and coworkers ³ reported that the risk of rupture is zero over 5 years for patients with an aneurysm less than 5 cm in diameter and 25% for patients with an aneurysm 5 cm or more in diameter at the time of the rupture. Most AAAs occur below the origin of the renal arteries and extend to the iliac bifurcation. ⁴ However, approximately 10% of aneurysms involve the origin of the renal arteries, extend more proximally, and affect the other visceral artery orifices. ^1,5

Preoperative determination of an aneurysm's origin as infrarenal, juxtarenal (within 1–2 cm of the origin of the renal arteries), or suprarenal is important in choosing both the surgical approach (transperitoneal or retroperitoneal) and the proximal surgical clamp site. ⁶ Distinctions between juxta- and suprarenal aneurysms can be very subtle even during surgery; determining an aneurysm's origin with preoperative diagnostic tools is understandably more difficult. ⁶ One of the most important techniques in the preoperative evaluation of AAA is computed tomography (CT), which may provide important information concerning the diameter of the aneurysm, proximal and distal extensions, anatomic variations that can complicate the surgical approach, perianeurysmal inflammation, and aneurysmal rupture. By eliminating degradations caused by respiratory, pulsatile, or intestinal peristaltic motion, the more rapid spiral CT technique has provided high-resolution data sets with near-anatomic representation in multiple dimensions. ^7,8 Although spiral CT has been available since the 2nd half of the 1980s, most AAA patients still undergo surgery in accordance with conventional CT findings.

The aims of this study were to evaluate the usefulness of conventional CT imaging in demonstrating an aneurysm's origin and in predicting the possibility of placing an infrarenal clamp. This last we hoped to achieve by determining what length of aorta, upon CT measurement, was needed to enable placement of a clamp between a renal artery orifice and the proximal border of an infrarenal aortic aneurysm.

Patients and Methods

From the beginning of January 1993 through May 2001, 116 patients underwent elective surgical repair of AAA at our institution. Twenty-one of these patients were excluded from the study because their preoperative CT findings indicated suprarenal aneurysms. We retrospectively analyzed the preoperative CT findings and the operative data on the remaining 95 patients, who all had diagnoses of infrarenal AAA by conventional CT. Eighty-nine (93.68%) of these patients were men and 6 (6.32%) were women. The mean age at the time of surgery was 66.27 ± 18.14 years. All patients had undergone contrast-enhanced CT after intravenous administration of 100 to 150 mL of iodinated contrast material as a dynamic bolus. Computed tomographic scans had been obtained with a commercially available scanner (General Electric Sytec 3000; G.E. Medical Systems; Milwaukee, Wis). Images of contiguous 5- to 10-mm-thick sections had been obtained from the diaphragmatic hiatus through the iliac bifurcation in a cephalocaudad direction. There was no interslice spacing.

A midline transperitoneal incision had been used in all patients. If it was of normal diameter, the aortic segment just below the renal arteries and proximal to the aneurysm's border was explored and freed from the adjacent tissues to evaluate the possibility of infrarenal clamp placement. Every effort was made to evaluate this segment of aorta for the placement of a clamp, in order to perform a secure anastomosis. If such a segment was not available, or if it was not long enough to enable safe clamp placement, supraceliac clamping was performed.

In the 95 patients with a preoperative CT diagnosis of infrarenal aneurysm, we used the CT images retrospectively to measure the length of the aorta between the renal arteries and the proximal border of the aneurysm.

Statistical analyses were performed by the SPSS/PC+ (version 7.5) computer software (SPSS; Tokyo, Japan). A probability (P) of less than 0.05 was considered significant. The mean, median, and standard deviation values of all data were calculated and indicated. The surgical finding was used as the standard of reference in determining the proximal extent of the aneurysm. The sensitivity, specificity, accuracy, and positive and negative predictive values of CT in helping to reveal the position of the aneurysm in relation to the renal arteries were calculated. The ability of CT to predict both an infrarenal aneurysm and the possibility of infrarenal clamping were compared, by means of Student's t-test, with operative findings in those regards.

Results

Proximal Extent of the Aneurysm

The predictive value of conventional CT in determining the proximal extent of AAAs was investigated in all 95 patients who had been preoperatively diagnosed with infrarenal lesions. At surgery, 91 of these 95 patients were confirmed to have infrarenal AAAs, for a statistically negative predictive value (indicating unaffected renal arteries) of 95.79%. Moreover, 1 patient who had been excluded from the study group by a preoperative CT diagnosis of suprarenal AAA was found at surgery to have an infrarenal aneurysm, which brought the infrarenal total to 92 (79.31%) of 116 patients. The specificity was therefore 98.91% (the number of infrarenal aneurysms predicted by CT and confirmed by surgery divided by the total number of infrarenal aneurysms confirmed by surgery). There was not a significant difference between CT results and operative findings in regard to establishing the presence of infrarenal aneurysm (P = 0.386).

Possibility of Infrarenal Aortic Clamping

Preoperatively, no attempt had been made to predict the possibility of infrarenal cross-clamping on the basis of CT measurements in the 95 patients who had been diagnosed with infrarenal lesions. Upon surgery, infrarenal clamping could be performed in only 59 (62.11%) of the 95 patients, and the clamp had to be placed suprarenally in the remaining 36 (4 of whom had suprarenal lesions). The statistically negative predictive value was therefore 62.11% (the infrarenal clamping number at surgery divided by the infrarenal aneurysm number at CT). There was a significant difference between CT results and operative findings in regard to establishing whether an infrarenal clamp could be used (P ≤ 0.001).

On the basis of our retrospective CT measurements, the 59 patients in whom infrarenal clamping was possible had a mean aortic segment length of 26.4 ± 7.11 mm, between the renal artery orifice and the proximal border of the aneurysm. In contrast, the 36 patients in whom infrarenal clamping was not possible had a mean aortic segment length of 12.7 ± 3.48 mm upon CT measurement.

Discussion

Generally, surgeons can plan their operative strategies in accordance with the findings of conventional CT. Because the proximal extent of the AAA usually determines the site of the proximal aortic clamp, it is important to predict the clamping level before the operation. Suprarenal clamping has greater potential for damage to the kidneys or other visceral organs, and for temporary coagulation defects. ^9–12

To the best of our knowledge, there is only 1 other study that investigated CT measurement of infrarenal aortic segments as a means of determining the possibility of clamping. ⁵ In their study of 30 patients with ruptured AAAs, Cohan and coworkers ⁵ reported that CT predicted the possibility of infrarenal clamping with 100% certainty when an aneurysm appeared to originate at least 3 cm distal to the origin of the main renal artery. They also found that suprarenal clamping was required in 3 of 13 patients whose infrarenal aneurysms began less than 30 mm distal to the renal arteries, and that infrarenal clamping was possible in all 12 patients whose infrarenal aneurysms began at least 30 mm distal to the renal artery orifices at CT. ⁵

In contrast to their study, we excluded patients with ruptured aneurysms, because retroperitoneal hematoma makes it difficult to place an infrarenal clamp, even if there is a neck of infrarenal aorta. Moreover, the borders of an aneurysm are not easily delineated in cases of rupture, which makes a study based on subtle CT measurements more difficult. ¹ It has been reported that infrarenal clamping in patients with ruptured AAAs prolongs the operative time and is impeded by poor exposure of the neck of the aneurysm. ^13,14

Conventional CT is useful for aneurysm detection and overall measurement of the lesion, but cannot be used in detecting all accessory renal arteries or in grading stenoses of the aorta and of the renal, mesenteric, and celiac arteries. Magnetic resonance angiography, spiral CT, spiral CT angiography, and digital subtraction angiography have been introduced more recently and are more sensitive techniques in the evaluation of AAAs. ^6,15,16 Spiral CT can detect a greater number of renal artery orifices and accessory renal arteries and in many patients can better define the relationships of these vessels to aortic aneurysms. ⁵ There is also clear evidence that spiral CT's use of contiguous images at thicknesses of less than 5 mm is extremely effective in detecting renal artery origins. ⁵ The 5- to 10-mm-thick sections used for the evaluation of our patients by conventional CT lessened the overall resolution and doubtless caused some errors in displaying the length of the aneurysm's neck and in showing the relationship of the aneurysm to the renal arteries. This could be responsible for the discrepancies between the CT results and the surgical findings. We believe that the use of contrast-enhanced spiral CT in thin (5-mm or less) contiguous slices more accurately displays the length between the renal artery orifices and the proximal level of the aneurysm, thus providing more information that is useful in operative planning.

Conclusion

Conventional CT is reasonably accurate in determining the proximal extent of abdominal aortic aneurysms. There is a high rate of error in determining the possibility of infrarenal clamping if no specific CT measurements are taken. However, infrarenal clamps can be placed when CT measurement shows a length of ≥26 mm between the renal arteries and the proximal extent of the aneurysm. In patients with shorter aortic segments, suprarenal aortic clamping should be considered.