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Letters |
Department of Radiology
Gooi-Noord Hospital
1250 CA Blaricum,
The Netherlands
I read with interest the article by Poletti et al., "Subtraction CT Angiography of the Lower Limbs," in the November 2004 issue of the AJR [1].
The authors' images beautifully show the value of subtraction in MDCT. In addition, the images illustrate the importance of the method of contrast medium injection, either intraarterially (IA) or intravenously (IV). When contrast medium IA is delivered in the abdominal aorta, there is only opacification of the collaterals from the abdominal aorta to the femoral vessels. However, delivering contrast medium IV results in opacification of all collaterals leading to the femoral vessels, including those originating from the abdominal wall, such as the inferior epigastric arteries, which in their turn may be fed via collaterals from the thoracic aorta and subclavian (internal mammary [2]) arteries. Consequently, the subtracted (IA) digital subtracted angiogram (DSA) image in Figure 2A shows no contrast medium in the right common femoral artery (the solid arrowhead in Figure 2A points to vessel-wall calcifications of that artery on a nonsubtracted image), whereas the subtracted (IV) MDCT image (Figure 2B) clearly shows its patency.
This influence of the method of contrast medium delivery on vessel patency visibility results in the advice to perform IV DSA (or MDCT) in patients with complete aortoiliac occlusion. In these cases, IV DSA may provide more information than IA DSA, surpassing the "gold standard" [3].
References
Department of Radiology
Division of Emergency
Radiology
University of Geneva
Geneva 1211, Switzerland
Department of
Radiology
University Hospital
Geneva 1211, Switzerland
Department of Radiology
University Hospital
Geneva 1211,
Switzerland
First, we would like to thank Dr. Marsman for his interest in our paper and his thorough analysis of the illustrations. We do agree with his observations and comments, including those concerning the arrowhead on Fig. 2A, which points to wall calcifications rather than to contrast media, since this image is not retrieved in the subtracted series.
We would still add some information relative to the illustrated case. Images of Figure 2A correspond to a relatively early phase after injection of contrast media. Both femoral arteries were visible on the late series, via collaterals from the inferior mesenteric artery. However, they were not as clearly demonstrated as in MDCT because epigastric arteries were not opacified after aortic injection of contrast media. At the level of the knee, no vessel could be depicted on DSA because contrast medium was too diluted, and examination was considered indeterminate in the frame of our study. However, distal vessels could easily be analyzed on both MDCT series. This example substantiates the reported observation that IV DSA or MDCT could be superior to IA DSA in case of complete aortoiliac occlusion, which is a rare situation. However, the quality of IV DSA is limited by the dilution of contrast media: This is the reason why this technique is now abandoned. In our institution, if femoral vessels are not clearly identified on IA DSA, contrast medium is then injected in the proximal aspect of the descending aorta to demonstrate collateral circulation coming from thoracic aorta.
Dr. Marsman's interesting letter reinforces our idea that subtraction MDCT obviates the drawbacks of both diagnostic DSA and nonsubtracted MDCT, and may become an alternative to these techniques once the technical problems discussed in our paper can be resolved.
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