Commentary

The following is the edited text of the discussion session that followed the presentation of these data by Winchester et al at the SCVIR 23rd Annual Meeting held February 28th - March 5, 1998 in San Francisco, California.

Presenter: Dr. Priscilla Winchester, Cornell University, "MR Digital Subtraction Angiography of the Lower Extremities Comparison with X-Ray Angiography"

Discussant: Dr. Martin Prince, University of Michigan

Dr. Prince: Thank you for that excellent talk. I would like to make a few comments and then I have a number of questions. After that if the audience has some questions, we could entertain those.

This is truly a revolutionary ad- vance. We think of MR as taking really a long time to acquire im- ages, typically on the order of min- utes. If you buy a scanner with the absolute best gradient performance, which will cost quite a lot of money, then sometimes you can get an MR angio sequence done fast enough to complete it within a breath hold. But, this paper presents a way now of acquiring large field of view an- giographic images with a frame rate of 1.5 seconds per image, which is starting now to approach what is possible with conventional angiogra- phy. I think this is a real tour de- force which is going to set the stage for MR just continuing to expand its capability for vascular diagnosis. We generally think as MR gets faster the image quality becomes poorer, but the authors have clev- erly utilized gadolinium in order to buy back the signal to noise lost from less signal averaging. So, one of my questions is how did you de- cide upon the gadolinium dose and do you think that you could have even better quality by increasing your dose further?

Dr. Winchester: We based our dose on results from normal volun- teers. We found that doses greater than 5-7 mL of gadolinium did not substantially improve image qual- ity.

Dr. Prince: If you first inject once and you are not happy with the quality, can you then inject again, similar to in conventional angio where you might inject many times during the study?

Dr. Winchester: Yes, and as a matter of fact we do repeat acquisi- tions where there is gross patient motion.

Dr. Prince: Can you give us a sense as to, on the images where the MR showed more vessels distal to occlusive diseases, why that was the case. Why was MR showing more of the vessels than angio?

Dr. Winchester: I think for two reasons, first it is an intravenous technique and in some of those ex- amples if we has positioned the x- ray catheter higher up, we would have opacified more collaterals but due to renal insufficiency we did not want to give the patient too much contrast at the time. The sec- ond reason is that we are not relv- ing on displacing the blood column with contrast to get an image of the vessel; we just need a small amount of gadolinium to be present to im- age the vessel.

Dr. Prince: I have one final question. Can you give us a sense as to why in one situation you may want to do 3D imaging, whereas for example in the pelvis, and what is the penalty that you have to pay for

3D imaging and why in another sit- uation you will do the 2D imaging?

Dr. Winchester: We think that 3D imaging is great for pelvic and ~roximal femoral vessels because of its higher signal to noise ratio than 2D imaging and because of intrinsic d e ~ t h resolution. Two-dimensional imaging has the advantage in smaller vessels that you do not have ~ a r t i a l volume effects because of th; inplane spatial resolution of 2D currently being higher than 3D (approximately 1 mm by 1 mm for 2D and 2 mm or greater by 3D im- aging). So in the smaller vessels you decrease partial volume averag- ing. In addition, the temporal reso- lution of 2D MRDSA avoids venous enhancement. We find that in our 3D images, especially in distal calf segments that we get substantial venous enhancement in the images. For these reasons we currently do a hybrid of 3D bolus chase to the knee and then 2D imaging from the knee down, analogous to selective x-ray angiograms.

Dr. Prince: Is this a technique that you can now perform routinely as part of the clinical care of pa- tients a t Cornell?

Dr. Winchester: Yes. The sur- geons at our institution feel very comfortable with this technique. In some clinical situations. such as su- perficial femoral artery occlusion with good distal runoff, they often operate without a confirmatory x- ray angiogram. Since we can per- form the exam in under thirtv min- utes, it is becoming more and more popular.