An investigation of the estimation of ejection fractions and cardiac volumes by a quantitative gated SPECT software package in simulated gated SPECT images.

BACKGROUND

The purpose of this investigation was to determine the accuracy of the estimation of ejection fractions (EFs) and left ventricular volumes from a commercially available software package (Quantitative Gated SPECT [QGS]) as a function of different true EFs, count level in the acquisitions, severity and location of perfusion defects, increasing hepatic activity, and modified wall motion.

METHODS AND RESULTS

The dynamic mathematic cardiac-torso digital phantom was used to create three-dimensional source and attenuation maps representing the distribution of a technetium-99m-labeled cardiac perfusion agent in the chest. Three hearts with varying end-systolic volumes were used to investigate different EFs. Perfusion defects were created as localized uptake within selected portions of the cardiac walls, scaled to the desired fraction of the normal wall uptake, and subtracted from the normal distribution. The hepatic uptake was increased up to five times of the normal heart uptake to investigate the influence of a "hot" liver. Alteration of lateral wall motion was also investigated. A three-dimensional projector that included the influence of distance-dependent spatial resolution and nonuniform attenuation was then used to create projection images. The projections were scaled to the desired acquisition count level, and Poisson noise was added. Automatic determination of EF slightly overestimated the true EF for normal count levels by 3% to 7% of the true EF and underestimated the true EF by up to 9% for very low count levels for 180-degree reconstructions. The accuracy for determining the volumes was not as high as for the EFs (an average error of 12% was observed). The calculated EFs were relatively accurate for perfusion defects of 50% or less. When perfusion defects exceeded 50%, extracardiac counts were included in the heart contours, causing larger underestimations of EF. With removal of the extracardiac counts, the EFs increased. With a hepatic uptake of two or more times the heart uptake, no meaningful EF could be obtained. Either drawing a single region of interest for every slice or use of the manual mode with constrain option could remarkably improve the estimation. The accuracy of the calculation of EF and volumes for the heart with stationary wall was fairly high but decreased significantly when coupled with perfusion defects.

CONCLUSION

It is concluded that the QGS program evaluates the functional parameter of EF accurately. The biggest limitations occurred in determining the appropriate cardiac contour if areas with very high extracardiac counts were present in the heart slices, and when a greater than 50% decrease occurred in uptake for perfusion defects.