MR angiography of intracranial aneurysms: proceed, but with caution.

Intracranial aneurysms are believed to be present in up to 8% of the population. Twenty thousand aneurysms rupture each year in the United States. A noninvasive method of screening for these lesions clearly would be of significant benefit. Magnetic resonance (MR) angiography (MRA) has been repeatedly suggested as such a method. The article by Huston et al in this issue of the AJNR ( 1 ) presents enough data on the limitations of this technique to give the more circumspect among us pause. Previous reports comparing MRA with conventional angiography have shown a high level of sensitivity in the detection of aneurysms with the MR methods. The first of these by Ross et al had a true-positive rate of 86% when time-offlight MRA was combined with standard spinecho imaging and the individual MRA partitions as well as the cine maximum intensity projections were viewed (2). However, among the 21 aneurysms, only two were smaller than 5 mm and only 1 of these was detected with MR. Significantly, the false-positive rate among a group of healthy control subjects in this study was zero. More recent studies by Schuierer et al (3) using time of flight alone and Gouliamos et a! ( 4) using both time-of-flight and phase-contrast MRA have shown similar or better results. Although Schuierer did not provide detailed aneurysm size data, the overall detection rate was at least 86%. Nine percent of lesions were questionably seen, and 5% (one lesion, smaller than 1 em) were missed. Gouliamos studied 14 cases and failed to see one 2-mm aneurysm on MRA, for a detection rate of 93%. Interestingly, one 3-mm aneurysm was seen with MRA but not on conventional angiography. In the current work by Huston et a! , 16 patients with 27 aneurysms were examined along with 19 healthy control subjects. The authors reviewed T1 -weighted MR, T2-weighted MR, time-of-flight MRA, and phase-contrast MRA separately and arrived at overall sensitivities of 26%, 48%, 56%, and 44%, respectively , for each method. These numbers are considerably below those previously reported and viewed alone are cause for great concern regarding the value of MR and MRA. Nonetheless , if one scrutinizes the details underlying the broad conclusions, fur ther important observations can be made. To compare directly Huston's conclusions with those of Ross, one could combine the re sults of each of the MR and MRA tests. Using the criteria of two or more readers detecting an aneurysm, 70% of aneurysms would have been identified on one of the four imaging studies. This does not account for the overall improvement in detection sure to result from having all the images available to review together. The whole undoubtedly is greater than the sum of the parts in this analysis, although the authors ' comments regarding information overload in reviewing these cases surely will be echoed by any radiologist who routinely performs these examinations. Of the eight aneurysms not diagnosed, all but one were smaller than 5 mm. Thus , for aneurysms at least 5 mm, the overall true-positive rate is 94%, which in fact is equal to that seen by Ross for the same size range. Unfortunately , aneurysms smaller than 5 mm do rupture , making this statistic of only limited comfort. Although studies have shown no hemorrhages in patients with aneurysms smaller than 3 mm, six of the eight missed aneurysms were larger than or equal to this size, and the overall detection rate for these aneurysms was only 30%. Three of the eight lesions not seen in this study were aneurysms of the carotid siphon, an