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1
Department of Radiology, University René
Descartes-Paris V, Hôpital Ambroise
Paré, 9 ave. Charles de Gaulle, 92104
Boulogne, France.
2
Department of Radiology, Gustave-Roussy Institute, 94805 Villejuif,
France.
3
Department of Vascular Surgery, University
René Descartes-Paris V,
Hôpital Ambroise
Paré, 92104 Boulogne, France.
Received March 27, 1998;
accepted after revision June 22, 1999.
Address correspondence to S. D. Qanadli.
Abstract
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SUBJECTS AND METHODS. Dual-slice helical CT angiography was performed in 47 consecutive patients (mean age, 59 years) with abdominal aortic aneurysm to determine whether we could then evaluate the extent of aneurysm and see associated renal, celiac, mesenteric, and iliofemoral artery disease. Results were compared with those of digital subtraction angiography (n = 47) and surgery (n = 37).
RESULTS. The proximal and distal extents of abdominal aortic aneurysm correlated well with surgical findings. Dual-slice helical CT showed all main (n = 102) and accessory (n = 13) renal arteries with a sensitivity of 91% and a specificity of 100% for revealing associated renal artery stenosis exceeding 50%. Sensitivity and specificity of dual-slice helical CT for revealing stenosis exceeding 75% in celiac and superior mesenteric arteries were both 100%. Three of four iliofemoral artery stenoses and two occlusions of the common iliac artery were revealed by dual-slice helical CT.
CONCLUSION. Helical CT angiography with dual-slice scanning is a useful and minimally invasive technique that can provide with high accuracy all the necessary information for treatment of abdominal aortic aneurysm.
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Conventional angiography is considered the gold standard for the detection and quantification of vascular occlusive disease. However, the size and extent of the aneurysm can be underestimated or sometimes misinterpreted [1, 2]. MR angiography and helical CT have been recently introduced as minimally invasive techniques for the pretherapy assessment of abdominal aortic aneurysms [3, 4, 5, 6, 7]. In helical CT angiography, a trade-off exists between longitudinal resolution, z-axis coverage, and scanning time. The dual-slice concept is an improvement of helical CT technology designed to achieve faster scanning [8]. Dual-slice helical CT provides wider z-axis coverage than single-slice helical CT and thus doubles the volume scanned within a given time and resolution.
The purpose of this study was to evaluate the usefulness and efficacy of dual-slice helical CT for pretherapy assessment of abdominal aortic aneurysm, and the sensitivity and specificity of this technique for detecting and grading associated renal, celiac, mesenteric, and iliofemoral diseases.
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Dual-Slice Helical CT Protocol
CT scans were performed with a commercially available CT Twin Flash scanner
(Elscint, Haifa, Israel). This scanner is equipped with a double array of
detectors, so that with 10-mm collimation, two contiguous 5-mm sections can be
obtained, each with a 1-sec rotation when the dual-slice scanning mode is
used. Craniocaudal helical CT was performed in a single breath-hold with the
following parameters: 120 kVp, 199 mA, 1-sec gantry rotation, 10-mm
collimation, and table speed of 15 mm/sec (pitch of 1.5). Contrast material
using the monophasic bolus injection technique was administered IV with an
automated injector (MCT FLS; Medrad, Rungis, France) through an 18- to
21-gauge catheter. A total of 120-150 ml of 30% iodinated contrast material
(iobitridol [Xenetix; Guerbet, Aulnay-sous-Bois, France]) was administered at
a rate of 4 ml/sec with a 20- to 25-sec interval between the injection and the
acquisition. No previous timing bolus was used. Z-axis coverage
ranged from 300 to 470 mm (from the level of the 12th thoracic vertebra to the
bifurcation of the femoral arteries). Scanning time ranged from 20 to 32 sec
(average, 23 sec). Images were reconstructed using a 180° linear
interpolation algorithm and a standard kernel. Effective slice thickness was
5.5 mm reconstructed at 2.7-mm intervals (overlap, 50%).
Multiplanar two-dimensional reformations and three-dimensional maximum-intensity-projection (MIP) reconstructions were performed for each case on an independent workstation (Omni View or OmniPro, Elscint). MIP reconstructions were used in two ways: bone structures were first eliminated from axial images and the edited images were subsequently used to produce MIP reconstruction of the entire data volume (Fig. 1D); and three to five overlapped images were used to obtain MIP reconstruction in the caudocranial view without the removal of bone structures (Fig. 1E). The last reconstruction is rapid and is adapted to analyze renal and celiomesenteric arteries because these vessels are rarely completely imaged in one axial transverse section. Linear cut lines for multiplanar image reformation were interactively defined on axial images or MIP reconstructions.
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DSA
DSA (Integris, Philips Medical Systems, Best, the Netherlands; ADAC, ADAC
Laboratories, Milpitas, CA) examinations were performed via a transfemoral
(n = 45) or transbrachial (n = 2) artery approach using 5-
or 4-French catheter with 32% ioxaglate sodium:ioxaglate meglumine (Hexabrix;
Guerbet) or 30% iobitridol (Xenetix; Guerbet) injected at a rate of 10- to
20-ml/sec for a total of 20-40 ml per series. Posteroanterior and lateral
projections of the abdominal aorta and posteroanterior or oblique projections
of the iliofemoral arteries were obtained in each patient. Supplementary
angiography was performed as needed at various injection rates. Selective
renal artery catheterization was not performed in this series.
Data Analysis
Two experienced radiologists who were unaware of the angiographic and
surgical findings reviewed in consensus all CT examinations. Combined analysis
of axial transverse images, multiplanar reformatted images, and MIP
reconstructions was used in all patients. Length and diameter measurements
were obtained from the adequate projection and at the discretion of the
investigators. Quantification of the degree of stenosis was obtained on axial
transverse or reformatted images or MIP reconstructions. An effort was made to
perform the measurements on two-dimensional reformatted images from MIP
reconstruction to generate the vessel section perpendicular to the vessel
axis. When measurements differed, the mean value was used. No comparison
between data from axial transverse images and those provided by different
reconstruction methods was made.
The image analysis included evaluation of aneurysm anatomy (location and extent) and assessment of associated vascular lesions (celiac trunk, superior mesenteric artery, inferior mesenteric artery, renal arteries, and iliac and common femoral arteries). An aneurysm was considered type A if it originated more than 1.5 cm below the origin of the most caudal renal artery. It was considered type B if it originated cranially to within 1.5 cm of the most caudal renal artery, and type C if it extended up to the level of the renal arteries. The distal neck of the abdominal aortic aneurysm was classified regarding the distance between aneurysm and aortic bifurcation as subtype "a" if it was greater than or equal to 1 cm and as subtype "b" if it was less than 1 cm. The superior mesenteric artery was analyzed in the supramesenteric segment. Superior mesenteric artery and celiac trunk stenoses were depicted and graded. Inferior mesenteric artery patency at its origin was assessed. Visualization of main and accessory renal arteries and detection and grading of renal artery stenoses were assessed, as was the evaluation of iliac and common femoral arteries (aneurysm, stenosis, and patency of internal iliac arteries).
The presence of thrombotic deposits or calcifications in the adjacent aorta or iliac arteries was specified, as were anatomic variants or abdominal diseases.
Vascular stenoses (celiac trunk, superior mesenteric artery, renal arteries, iliofemoral arteries) were graded using a four-point scale: grade 0, 0-49% reduction in arterial diameter; grade 1, 50-74% reduction; grade 2, 75-99% reduction; and grade 3, occlusion. For grades 1 and 2, objective measurement of the luminal diameter at the maximum reduction of diameter and at the normal vessel were made to obtain the degree of stenosis. The choice of the normal vessel site was at the discretion of the investigators.
DSA examinations were independently analyzed by two radiologists who were unaware of the CT findings. Criteria described for the extent of abdominal aortic aneurysm and grading of stenosis were also applied. Quantification of the degree of stenosis was performed using a commercially available algorithm (ADAC Laboratories).
The radiologists' evaluations of the aneurysms were compared with the surgical findings in 37 patients. Surgical evaluation was obtained by two of three surgeons. The extent of the aneurysm was determined by visual inspection and palpation or according to the need for suprarenal cross clamping. The renal arteries were evaluated after careful mobilization of the left renal vein. Measurements were not obtained before surgery. Evaluation of the celiac trunk and the superior mesenteric artery was not available in five patients.
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Abdominal Aortic Aneurysm Anatomy
Dual-slice helical CT correctly indicated the type of aneurysm, and CT
results correlated with surgical findings in all patients who underwent
surgery (Table 1). Four
patients (11%) had aneurysm type A, subtype a; 16 (43%), type A, subtype b;
three (8%), type B, subtype a; eight (22%), type B, subtype b; and six (16%),
type C, subtype b. On DSA, six aneurysms (13%) were incorrectly classified:
four type B, subtype a, according to both CT and surgical findings, were
classified type A, subtype a on DSA; one type B, subtype b was misclassified
as type A, subtype b; and one type A, subtype b was misclassified as type A,
subtype a.
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The mean outer transverse diameter of the aneurysm was 5.8 ± 1.5 cm (range, 3.6-11 cm). The mean lumen diameter was 2.9 cm. A mural thrombus in the aneurysmal segment of the aorta was seen in 43 patients (91%). A thrombus of the adjacent aorta was seen in three patients (6%). One patient required preoperative conversion of infrarenal cross-clamping to suprarenal cross-clamping because of mural thrombus of the adjacent aorta. Aneurysm of the aorta at the level of the celiac artery was seen in one patient (2%).
Evaluation of the Renal Arteries
DSA revealed 115 renal arteries: 102 main arteries, and 13 accessory renal
arteries. All main and accessory renal arteries were depicted on CT. In three
patients (6%), CT depicted an accessory renal artery that had been considered
the bifurcation of the main renal artery on DSA in one patient because it was
superimposed on the contrast material-filled aortic lumen. In the two
remaining patients, an accessory renal artery of the left kidney was not
identified (Fig. 1A,
Fig. 1B,
Fig. 1C,
Fig. 1D,
Fig. 1E,
Fig. 1F) on DSA. Axial
transverse images and localized MIP reconstruction, from to three to five
images in the caudocranial view, helped to identify an accessory renal artery.
In the types A and B groups, two accessory renal arteries had their origins in
the aneurysm (Fig. 2A,
Fig. 2B,
Fig. 2C).
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DSA revealed 11 renal artery stenoses and one renal artery occlusion in 10 patients (21%) (Table 2). Four stenoses were grade 1 and seven were grade 2 (Fig. 1A, Fig. 1B, Fig. 1C, Fig. 1D, Fig. 1E, Fig. 1F). All stenoses and occlusions were depicted on CT except one grade 1 stenosis. Five grade 2 stenoses were correctly quantified on CT, but three grade 1 stenoses were incorrectly quantified as grade 2. Sensitivity and specificity of dual-slice helical CT were 91% and 100%, respectively, for the detection of renal artery stenosis. Sensitivity and specificity were both 100% when only grade 2 stenoses were considered.
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A retroaortic left renal vein was depicted in one patient, and one patient had an unknown right renal carcinoma revealed on CT.
Despite preoperative knowledge by 10 patients of renal artery stenosis, only one underwent an aortorenal bypass graft at surgery.
Evaluation of the Celiac Trunk and the Superior and Inferior
Mesenteric Arteries
On DSA, five grade 1 and two grade 3 stenoses were visualized in the celiac
trunk (Table 3). Dual-slice
helical CT depicted all lesions, but two were incorrectly quantified (one
grade 1 stenosis was mistaken for grade 2, and one grade 3 stenosis was
mistaken for grade 2). Sensitivity and specificity of dual-slice helical CT in
the detection of grades 2 and 3 stenoses of the celiac trunk were both
100%.
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Two grade 1, one grade 2, and one grade 3 stenoses of the superior mesenteric artery were depicted on both CT and DSA (Table 3). Dual-slice helical CT misclassified one grade 0 stenosis as grade 1. However, surgery indicated that stenosis related to an eccentric plaque was probably underestimated by DSA on lateral projection (Fig. 3A, Fig. 3B, Fig. 3C).
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One patient had one grade 2 stenosis of the celiac trunk and one grade 1 stenosis of the superior mesenteric artery. One other patient had occlusion of both the celiac trunk and the superior mesenteric artery.
At surgery, the inferior mesenteric artery was patent in 33 (89%) of the 37 surgical patients. Dual-slice helical CT identified the inferior mesenteric artery at its origin in 31 patients (84%) (Fig. 2A, Fig. 2B, Fig. 2C) when it was opacified in only 18 patients (49%) of the 37 surgical patients on DSA. Two patients underwent reimplantation of the inferior mesenteric artery, and one patient had an aortomesenteric bypass graft.
Evaluation of the Iliofemoral Arteries
Aneurysmal dilatation extending into the iliac arteries was seen in six
patients (13%) on both DSA and CT. In two patients, aneurysm of the right
common iliac artery was missed on DSA (Fig.
1A, Fig. 1B,
Fig. 1C,
Fig. 1D,
Fig. 1E,
Fig. 1F).
Four grade 1 stenoses were depicted on DSA in four patients (9%): one in the right common iliac artery, two in the external iliac arteries, and one in the right common femoral artery. A grade 3 stenosis was depicted in the right common iliac artery on DSA in two patients (4%) (Fig. 4A, Fig. 4B). All stenoses but one (of the femoral artery) were depicted on CT. However, two grade 1 stenoses were incorrectly quantified as grade 2 stenoses. All these patients underwent an aortobifemoral bypass graft.
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In the internal iliac arteries, two stenoses (one grade 2 and one grade 3) were seen on DSA. Only the grade 3 stenosis was depicted on CT.
In other respects, dual-slice helical CT accurately showed spatial configuration of the aorta, the femoroiliac arteries, and calcium deposits (Figs. 4A, Figs. 4B and 5).
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Simoni et al. [11] studied 33 patients with abdominal aortic aneurysm using 5-mm collimation and a pitch of 1. These authors reported that seven patients (21%) with ectatic common iliac arteries required immediate additional conventional scans to complete the evaluation. Technically, z-axis coverage can be enlarged using the same collimation by increasing the table speed (a pitch of 2 to obtain 30-cm z-axis coverage), which results in reduction of the signal-to-noise ratio and increases the broadening of the section profile, or by increasing the acquisition time.
Dual-slice helical CT with overlapping reconstruction improves the longitudinal resolution with acceptable axial resolution and acquisition time. Our results show that this technique is useful for the evaluation of both the proximal and distal extents of abdominal aortic aneurysm and iliac artery involvement. The type of aneurysm was correctly evaluated by dual-slice helical CT in all patients in whom surgical correlation was available.
Abdominal aortic aneurysms were readily located through the combined analysis of axial and multiplanar reformatted images and MIP reconstructions. We used combined analysis of all images because evaluation of axial images alone has been reported to over-estimate the proximal extent of the aneurysm [2, 10, 12]. Three-dimensional shaded-surface reconstruction is not used in routine practice. Greater efforts are required on the part of the physician than for multiplanar reformation and MIP reconstruction. Projection in a caudocranial view or slightly oblique caudocranial view of data sets using an MIP algorithm from a few images is rapid and helpful to assess visceral vessels such as renal and celiomesenteric arteries.
The sensitivity and specificity of dual-slice helical CT to show renal artery stenosis exceeding 50% were 91% and 100%, respectively. The sensitivity increased to 100% when only stenosis greater than 75% was considered. These data are similar to those reported by other authors in patients with abdominal aortic aneurysm using collimation as thin as 2 or 3 mm. Zeman et al. [9] used single-slice helical CT with variable collimation and reported 100% sensitivity in the detection of renal artery stenosis exceeding 70% in a series of 23 patients. Van Hoe et al. [10] studied renal artery stenosis greater than or equal to 70% in 33 patients and obtained a sensitivity of 94% and a specificity of 96%. However, in our study, CT overestimated three grade 1 stenoses as grade 2. We believe that precise quantification of the degree of stenosis probably requires thinner effective slice thickness as experimentally demonstrated by Brink et al. [13]. However, detecting renal artery stenosis did not affect surgical management in our series; only one patient required an aortorenal bypass at surgery and one patient with grade 2 stenosis of the renal artery and medically uncontrolled hypertension underwent percutaneous transluminal angioplasty after surgery.
Dual-slice helical CT accurately revealed celiac and superior mesenteric artery stenoses. Celiac and mesenteric artery occlusive disease occurred in 10-41% of patients with abdominal aortic aneurysm [14, 15], and such patients are rarely symptomatic [16]. We observed 11 occlusive lesions in nine patients (19%). However, only two patients (4%) required reimplantation of the inferior mesenteric artery, with an aortosuperior mesenteric artery bypass graft in one patient. We agree with Nuno et al. [16] that correction of celiac and mesenteric lesions is rarely necessary. We found that dual-slice helical CT correlated better with surgery than DSA correlated with surgery in detecting inferior mesenteric artery patency. Opacification of the inferior mesenteric artery during angiography in supine patients with a large abdominal aneurysm may be difficult because the inferior mesenteric artery originates from the anterior wall of the aneurysm. Probably a larger volume of contrast material than that used in our study is required in some patients to obtain homogenous opacification of the entire aneurysmal sac.
To our knowledge, evaluation of iliac and femoral artery occlusive disease by helical CT in patients with abdominal aortic aneurysm has not been previously reported. It is important to assess iliac and femoral arteries in these patients and to reveal the extent of abdominal aneurysms, isolated aneurysms, and occlusive disease. The reported prevalence of iliac occlusive disease in these patients is as high as 22-29% [17]. Our results show that CT accurately reveals aneurysms and evaluates stenoses exceeding 50%. These findings have directly affected our surgical treatment. An aortobifemoral bypass graft has been the treatment of choice in patients with associated arterial occlusive disease. Moreover, the geometry and diameter of femoroiliac arteries, which can potentially be obtained by CT [18], are important to assess in patients undergoing endovascular treatment of the aneurysm.
In conclusion, dual-slice helical CT angiography is a substantial improvement over helical CT that appears useful and efficient for the pretherapy assessment of abdominal aortic aneurysm. Morphologic evaluation of the aneurysm and associated visceral and iliofemoral artery disease could be provided with acceptable accuracy. The development of a faster imaging scanner system (multislice and multiarray detectors) with submillimeter isotropic imaging is expected to improve vessel assessment, with optimal spatial and temporal resolution, especially in patients with abdominal aortic aneurysm.
Acknowledgments
We thank Lorna Saint-Ange for editing the manuscript and Didier Joseph for
photographic assistance.
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