Influence of Minimum Count in Brain Perfusion SPECT: Phantom and Clinical Studies

The count per pixel in brain perfusion SPECT images depends on the administered dose, acquisition time, and patient condition and sometimes become low in daily clinical studies. The aim of this study was to evaluate the effect of different acquisition counts on qualitative images and statistical imaging analysis and to determine the minimum count necessary for accurate examinations. Methods: We performed a brain phantom experiment simulating normal accumulation of ^99mTc-ethyl cysteinate dimer as a brain uptake of 5.5%. The SPECT data were acquired in a continuous repetitive rotation. Ten types of SPECT images with different acquisition counts were created by varying the number of rotations added. We used normalized mean squared error and visual analysis. For the clinical study, we used images of 25 patients. The images were acquired in a continuous repetitive rotation, and we created 6 brain images with different acquisition counts by varying the number of rotations added from 1 to 6. The contrast-to-noise ratio was calculated from the mean counts within regions of interest in gray and white matter. In addition, the severity, extent, and ratio of disease-specific regions were evaluated as indices of statistical imaging analysis. Results: For the phantom study, the curve of normalized mean squared error tended to converge from approximately 23.6 counts per pixel. Furthermore, the visual score showed that images with 23.6 counts per pixel or less were barely diagnosable. For the clinical study, the contrast-to-noise ratio was significantly decreased at 11.5 counts per pixel or less. Severity and extent tended to increase with decreasing acquisition counts, and a significant increase was shown at 5.9 counts per pixel. On the other hand, there was no significant difference in ratios among different acquisition counts. Conclusion: On the basis of a comprehensive assessment of phantom and clinical studies, we suggest that 23.6 counts per pixel or more are necessary to maintain the quality of qualitative images and to accurately calculate indices of statistical imaging analysis.