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Comparison of CT angiography with conventional arterial angiography in aortoiliac occlusive disease

Department for Diagnostic Imaging, North Staffordshire Hospital NHS Trust, Newcastle Road, Stoke on Trent ST4 6QG, UK

	Abstract

Top Abstract Introduction Subjects and methods Results Discussion References

 
This study compared the results of conventional and CT angiography of the aortoiliac segment in 35 patients with occlusive disease. Disease severity was graded into five categories. Two assessors independently assessed the segment from the distal abdominal aorta to the proximal superficial femoral artery and a consensus was formed. There was interobserver agreement in 87% of segments for conventional angiography and in 78% for CT angiography. Comparison of the two modalities gave identical grading in 84% of segments. Dissimilar grading was found in 16%, with a roughly equal number of undergrading and overgrading. CT angiography has the advantages of being minimally invasive, requiring only an intravenous injection of contrast medium and imaging surrounding soft tissues. Multiplanar reconstruction aids the visualization of asymmetrical stenoses, and collateral blood supply is readily appreciated. However, CT angiography may fail to demonstrate short stenoses owing to limited z-axis resolution.

	Introduction

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Angiography via an arterial catheter is the gold standard in imaging of the arterial system of the lower limbs. It provides high resolution imaging of the entire lower limb vascular tree and allows percutaneous vascular intervention at the same sitting. However, it requires arterial puncture with its attendant complications. Furthermore, it can fail to demonstrate eccentric stenoses. Spiral CT angiography is an alternative method of vascular imaging [1]. Arterial puncture is not required and eccentric stenoses are well demonstrated [2–4]. The main problem of CT angiography is the limited z-axis resolution, which may result in underestimation or complete failure to visualize short stenoses.

We present a retrospective comparison between conventional arterial angiography of the aortoiliac vessels and CT angiography of the aortoiliac segments.

	Subjects and methods

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35 patients with clinical features of aortoiliac disease who had been studied by CT angiography and conventional angiography between January 1996 and July 1998 were retrospectively reviewed.

CT angiograms were performed on a Picker PQ5000 (Marconi, Cleveland, OH) and a Toshiba Express GX Aspire (Toshiba Co, Tokyo, Japan). Contrast medium was injected through a 20 G intravenous cannula situated in the antecubital fossa. After a timed study, the definitive procedure delineated from below the renal arteries to the femoral bifurcation. Slice thickness was 4–5 mm, table speed 8 mm s^-1 and contrast medium flow rate 3 ml s^-1. The duration of the injection was 5 s greater than the scan time to ensure good opacification of the vessels.

Data were processed on a workstation. Slices were indexed at 2 mm. Curved coronal and sagittal multiplanar reconstructions of the aorta and each iliac vessel were obtained. Conventional angiography was performed using a standard angiographic technique with 80 ml Ultravist 370 injected via a 5 F pigtail catheter at 8 ml s^-1 into the aorta just above the origin of the renal vessels. Imaging was carried out using a Sireskop 30 screening table. A brachial approach was used when femoral access was impossible.

The conventional aortoiliac angiographic images and the CT axial and reconstructed angiographic images were both assessed. Each study was divided into nine segments: aorta; left and right common iliac arteries; left and right external iliac and common femoral arteries; and left and right superficial femoral arteries.

Each vascular segment was classified according to the degree of disease. The classification was chosen to reflect clinical significance:

1. Normal or insignificant stenosis <50%.
   
2. Equivocal stenosis.
   
3. Significant stenosis >50%.
   
4. Occlusion.
   
5. Aneurysm.
   

Each study was interpreted independently by two radiologists blinded to the result of the other imaging modality and a consensus view was formed. In cases of incompletely visualized segments (part of the infrarenal aorta and proximal superficial femoral artery), comparison was made between the parts visualized on both studies.

	Results

Top Abstract Introduction Subjects and methods Results Discussion References

 
35 patients were studied by both modalities. 28 of these underwent perfemoral angiography and a brachial or axillary approach was used in 7 patients. No significant complications occurred. In 13 patients CT angiography stopped short of the femoral bifurcation and therefore the origin of the superficial femoral artery was not shown. A total of 219 vascular segments were assessed by both modalities (35 aortic segments, 70 common iliac segments, 70 external iliac/common femoral artery segments, and 44 proximal superficial femoral artery segments).

The interobserver agreement was 87% for conventional angiography and 78% for CT angiography. Comparing the consensus results of CT angiography with conventional angiography, there was agreement in 84% (183 of the 219 segments). In the remaining 36 cases (16%), CT angiography suggested tighter stenosis than conventional angiography in 21 (9.6%) cases and less tight stenosis in 15 (6.8%) cases. There was disagreement between the studies of greater than one category in 8 (3.7%) cases. CT suggested lesser disease in 3 (1.4%) of these cases, twice owing to short stenoses missed on CT and once owing to collateral filling not shown on conventional angiography giving rise to an erroneous impression of occlusion of a segment. The 5 (2.3%) cases in which CT suggested more severe disease were owing to eccentric stenoses and calcification. Conventional angiography was interpreted in two cases as showing insignificant stenosis, which was significant on CT angiography. In these cases there were eccentric stenoses not optimally shown on single angiographic projections. In three cases, conventional angiography was interpreted as significant stenosis, which was deemed as occlusion on CT angiography. These cases showed heavy mural calcification on CT, which made identification of the lumen difficult, leading to overgrading.

14 aneurysmal segments were identified, all of which were identified on CT. Six of these, including three aortic aneurysms, were not appreciated on conventional angiography. Table 1 shows a direct comparison of conventional and CT angiography assessment of aortoiliac disease.

	Discussion

Top Abstract Introduction Subjects and methods Results Discussion References

Non-invasive imaging can provide an accurate representation of the extent of arterial disease, which, in combination with clinical information, enables optimal planning of subsequent arterial procedures while minimizing the requirement for arterial puncture.

Several non-invasive imaging modalities exist. Doppler ultrasound, which is widely available and free from side effects, is particularly suited to imaging of the femoropopliteal and calf vessels [6, 7]. However, imaging of the aortoiliac segment is frequently compromised owing to overlying bowel gas.

MR angiography is a valuable technique in the assessment of arteries of the pelvis and lower limbs [8, 9]. This technique is non-invasive and requires no ionizing radiation. Access to MR remains limited and a significant minority of patients do not tolerate MRI.

CT angiography is a three-dimensional technique that provides information about the imaged vessels and adjacent structures. It requires only venous vascular access and is an outpatient examination with minimal risk. Post-processing with adequate windowing and the use of curved multiplanar reconstructions generally enables confident discrimination between vascular calcification and intravascular contrast medium. Operator dependency has been cited as a problem with curved multiplanar reconstructions [2–4], but this was not found to be a difficulty in this study. Earlier studies used maximum intensity projections, but we found these to be of limited use because the vascular lumen is frequently obscured by mural calcification (Figures 1 and 2). CT angiography demonstrates eccentric stenoses, which can be missed on conventional angiography. Such stenoses were a frequent finding within the aortoiliac segment in the present study (Figures 3 and 4), CT angiography can also demonstrate retrograde filling of vessels distal to an occlusion by collaterals with high origin, i.e. higher than the site of the catheter in conventional angiography. It is therefore more accurate than conventional angiography in determining the length of long occlusions. The excellent demonstration of co-existent aneurysms is a further advantage of CT angiography (Figure 1). CT angiography does however have some limitations. Very short stenoses may by missed on CT as its resolution in the z-axis is considerably inferior to conventional angiography (Figure 5) Furthermore, differentiation between tight stenosis and occlusion can be difficult in patients with very heavy vascular calcification. Despite these pitfalls, this study demonstrates that CT angiography is a reliable technique in the assessment of aortoiliac occlusive disease, which is a region where Doppler imaging is frequently unsatisfactory.

View larger version (100K): [in this window] [in a new window]   Figure 1. (a) CT angiography demonstrates complete occlusion of the left common iliac, the external iliac and the common femoral arteries as well as an aneurysm of the distal abdominal aorta (open arrow). (b) The aortic lumen appears normal on conventional angiography and calcification of the aneurysm is not visible. Early venous filling mimicks external iliac artery patency.

View larger version (95K): [in this window] [in a new window]   Figure 2. Concordance of findings: stenosis of the origin of the right external iliac artery (arrow) on(a) conventional angiography and (b,c) CT angiography.

View larger version (71K): [in this window] [in a new window]   Figure 3. Short eccentric stenosis (arrow) seen in the distal abdominal aorta on (a) conventional angiography lateral view and (b) coronal multiplanar reconstruction of a CT angiography. (c) The sagittal multiplanar reconstruction demonstrates a short but tight stenosis (arrow). These images demonstrate the value of reconstructing in two planes.

View larger version (86K): [in this window] [in a new window]   Figure 4. Concentric stenosis at the origin of the left common iliac artery (arrow) is well demonstrated on conventional angiography and the two multiplanar reconstructions of CT angiography (a,b). A further minor stenosis of the left external iliac artery (open arrow) was better appreciated on the coronal reconstruction (b).

View larger version (101K): [in this window] [in a new window]   Figure 5. Short stenosis at the origin of the left common iliac artery (arrow) is well seen on conventional angiography (a) but not confidently demonstrated on CT angiography (b). This case highlights the poor z-axis resolution of CT angiography.

Received for publication January 24, 2000. Revision received August 29, 2000. Accepted for publication September 25, 2000.

	References

Top Abstract Introduction Subjects and methods Results Discussion References

 

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