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Letter to the Editor |
Department of Radiology, Academic Hospital Maastricht, Peter Debijelaan 25, 6229 HX Maastricht, The Netherlands
Editor:
In the review article on contrast material–enhanced magnetic resonance (MR) angiography by Watanabe et al in the January-February 2000 issue of RadioGraphics (1), several technical principles and clinical applications of MR angiography are highlighted. However, the authors completely circumvent the discussion of dynamic subtraction contrast-enhanced MR angiographic techniques other than their own (which relies on multiple injections) for imaging peripheral arterial obstructive disease or bypass grafts. In addition, they only very briefly discuss possible pitfalls of their or others' methods of performing multiple field-of-view (FOV) MR angiography. For instance, they do not caution readers about the need to ensure sufficient overlap between imaging volumes when more than a single FOV is scanned. They also do not discuss potential drawbacks of using multiple injections, such as venous enhancement, soft-tissue enhancement, and residual arterial contrast material present from the previous injection(s), as well as the time it takes to image the peripheral arteries versus the time it takes to perform a multiple FOV examination with a single continuous injection of contrast material. To fully inform clinicians who wish to incorporate dynamic subtraction contrast-enhanced MR angiography into their own practice, we believe clinicians should be aware of all current investigations in this area, such as our work (2) and that of Meaney et al (3), Wang et al (4), and Rofsky et al (5).
References
Department of Radiology, Kurashiki Central Hospital, 1-1-1 Miwa, Kurashiki City, Okayama, 710-8602 Japan
We appreciate Dr Leiner and his colleagues' comments and important criticism of our article, but we believe they must have some misunderstanding of our work (1,2). Leiner et al described excellent work in moving-bed contrast-enhanced MR angiography (3-6). The principle of their technique is nearly identical to that of ours (1-6). The subtraction technique increases the contrast of arterial enhancement. This technique, especially when combined with our method of using multiple injections, allows elimination of venous enhancement, soft-tissue enhancement, and the residual arterial contrast material present from the previous injection (1,2). Sufficient overlap between imaging volumes is handled in a similar way.
One of the major differences between their technique and ours is the injection dose and method. Leiner et al use a slow, continuous injection of a large dose (39 mL) of contrast material (3). In Japan, however, the injection dose of contrast material is limited by the Japanese equivalent of the Food and Drug Administration. The maximum dose approved is 20 mL or 0.1 mmol/kg. Therefore, we developed a method for imaging vessels of the lower half of the body with multiple injections of a small dose of contrast agent. Our method is flexible, since we can use it to depict not only arteries but also portal veins and systemic veins, in contrast to the moving-bed method, which images mainly arteries. By repeating imaging sequences, we can trace the dynamics of the injected contrast material over time and apply various postprocessing techniques, such as double subtraction and addition (1), to demonstrate arteries and veins separately and to display the arteries in a single image, even when the circulation time is prolonged. If imaging sequences were repeated in the moving-bed technique, venous images could also be obtained. However, the dynamics of the injected contrast material could not be traced because of the long interval time (120 seconds) for a sequence.
The advantage of the moving-bed technique is short examination time. Contrast-enhanced MR angiography with our technique takes about 30 minutes for an examination of the lower half of the body, whereas a similar study performed with the moving-bed technique can be completed 15 minutes. Therefore, I am sure that the moving-bed technique is a convenient, excellent method of contrast-enhanced MR angiography when use of a large dose of contrast material is allowed and when the study must focus on arterial imaging.
The problem of both methods is the difficulty in depicting small vessels such as mesenteric arteries, distal renal arteries, and peripheral small arteries, as commented by Dr Baum (7). To overcome this problem, other techniques must be developed. In addition, the nature of gradient-echo sequences has some inherent problems for imaging small vessels. The fat-tissue interface becomes dark because of chemical shift artifact (which occurs both when echo time is set at in-phase and when it is set at out-of-phase), which reduces the ability to see small arteries with contrast-enhanced subtraction MR angiography. Use of a fat suppression technique may be helpful for demonstrating small vessels satisfactorily (8).
MR angiography is still exciting and challenging, and we hope that the best methods of contrast-enhanced MR angiography will be developed and established on the basis of all previous work.
References
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