TO THE EDITOR:
In the November 1999 issue of The Journal of Nuclear Medicine, Hasegawa et al. (1) reported a direct comparison among PET, SPECT, and dual-head gamma-camera coincidence detection imaging (DCD) to assess myocardial viability with FDG.
We would like to address three issues about the design and methods of the study.
The choice of performing the DCD tomography in 32 steps over 360° penalizes this modality by a factor of 2 in the examination duration, compared with 32 steps over 180° with ultra-high-energy general purpose collimators for SPECT. Furthermore, the gamma camera used in the study (Vertex Plus MCD; ADAC Laboratories, Milpitas, CA) is equipped with a 5/8-in. (15.9-mm) thick crystal, which means that the detection efficiency for the photopeak at 511 keV is approximately 21% for a single detector (2). Thus, the coincidence detection efficiency for DCD is only 21% × 21% = 4.41% (2), which would require more steps to obtain a similar count per image ratio and to allow for a more objective comparison between the 2 modalities. Finally, the 128 × 128-matrix reconstructed data obtained from DCD should have been reduced to a 64 × 64 matrix to obtain the same slice thickness as the SPECT images.
The DCD acquisition started 150 min after SPECT acquisition, occurring with 38.56% of the FDG activity that was initially available for SPECT imaging, because of FDG decay (half-life = 109.7 min).
The delayed DCD acquisition (210 min after injection and 150 min after SPECT) may be reasonable because of the risk of detector saturation by high counting rate, but it assumes that myocardial trapping of FDG is irreversible. Recently, Herrero et al. (3) demonstrated, on a canine model of myocardial glucose usage during hyperinsulinemic–euglycemic clamp, that a reversible myocardial trapping of FDG occurs within the first hour after tracer injection. This delay may therefore alter the myocardial distribution of FDG and consequently the DCD detection in an unpredictable manner.
All these factors acted to penalize DCD and could have influenced the final result of the study: a poor agreement of DCD imaging with PET and a better performance of SPECT. A randomized order for the first acquisition modality (DCD or SPECT) after PET imaging and an optimized DCD acquisition (duration and angle selection) would have been more adequate to sustain the published conclusion.
