Reply: Comparison of 64-Slice CT with Gated SPECT for Evaluation of Left Ventricular Function

REPLY: We appreciate the important comments regarding our recent comparison of left ventricular function assessment by 64-slice CT versus gated SPECT (1). We agree with Dudley and Kalirai that good correlation does not necessarily indicate good agreement of 2 measurements. Therefore, we have reported Bland and Altman limits of agreement in our study. The latter, however, provide no objective cutoff value above which intermethod agreement is considered significant or insufficient. Therefore, it must remain a matter of clinical judgment to evaluate in each clinical setting whether given limits of agreement are acceptable. In this specific comparison, we believe that the difference may be less relevant at least for values of left ventricular ejection fraction in the higher range than for values in the lower normal range.

Regarding the question of whether nonperfused muscle was included, we should clarify that a transmural scar cannot be depicted by SPECT. We have discussed the fact that one possible explanation for apparent overestimation of left ventricular volumes using CT is that delineation of the myocardial contours by SPECT may be hampered by the presence of severe perfusion defects. This may apply equally to the muscle mass measurement.

In summary, we agree that the statement in our conclusion, namely that the 2 techniques should not be used interchangeably because of variances inherent in the different techniques, should be emphasized and that this statement might possibly be extended, with caution, to left ventricular ejection fraction.

We apologize that the percentage mean difference shown in Table 2 (+1.7%) is erroneously given in the text as SD on the absolute mean difference, which should read 1.1% ± 7.7%. Finally, an erratum has been brought to our attention: The myocardial mass calculated by CT was significantly lower (not higher), compared with gated SPECT (127 ± 24 g vs. 148 ± 37 g; mean difference, 23.0 ± 12.2 g; P < 0.01), as correctly stated in Table 2.