Use of xenon and ultrafast CT to measure cerebral blood flow.

I would like to compliment Drs Gobbel, Cann, and Fike for their continued efforts to extract quantitative cerebral blood flow information from computed tomography (CT)based technologies. Because CT continues to be a universally used first screening technology, efforts to expand the information obtained from the initial visit to the CT scanner are warranted. The ability to extract vital cerebral blood flow information from a 30-second high-speed CT study would seem desirable, but the high degree of variability reported in their article ( 1) suggests that this technique would become increasingly unreliable in an injury model. Although xenon-enhanced CT also has limitations, especially its sensitivity to motion during the 5-minute study time, it provides highly useful and stable information that is closely linked to function . Repeat studies at 20-minute intervals are useful in studying normal and abnormal cerebral physiology. As noted by Gobbel et al, early scanning sequences minimize the effects of flow activation which can be induced either by xenon or by carbon dioxide decreases, which often occur during a study. The authors do not mention the article by Marks et al (2), which closely examined the effects of adding carbon dioxide to inhaled mixtures of xenon. This study concluded that the addition of carbon dioxide was contraindicated because carbon dioxide normally falls 2 to 4 mm Hg with xenon inhalation and thereby tends to compensate for the tendency of xenon to raise cerebral blood flow . Thus the addition of carbon dioxide may significantly and falsely raise cerebral blood flow values generated with the xenon-enhanced CT method. Also , the authors may not have been aware of the studies by Good et al (3, 4), which examined errors associated with xenon-enhanced CT in relation to region-ofinterest size. Although the relative error associated with a region of interest measuring 0.3 cm in tissue with flow about 50 mL/ 100 g per minute is about 25%, the error is reduced to 1 O% when regions of interest measuring 1.2 cm are used. Although the ability of ultrafast CT to define asymmetry in flow between hemispheres is of interest, I am concerned that a cerebral blood flow technology that does not provide truly quantitative flow values that are accurate throughout the actual range of normal flow values will not provide the type of clinical information that permits vital decision making. The unique capacity of xenon-enhanced CT to measure flow values of zero, even in the depths of the brain, is one reason this technology may be more clinically useful than ultrafast CT in regional cerebral blood flow.