Improving emission-computed-tomography quantification by Compton-scatter rejection through offset windows

Abstract

Historically in nuclear medicine the acceptance window for total-energy pulse-height has been set to maximize contrast and resolution in the image. Recently, there has been increased emphasis on quantification of digital images to calculate, for example, left ventricular volume or tumor radiation absorbed dose. In this paper, we consider specifically what improvements can be made in emission computed tomography simply by employing a window which is offset to the high-energy side of the photopeak in order to reduce the contribution of Compton scattered gamma-rays. The window is offset so as to reduce the count rate for a source in air by 20%. Two lucite phantoms were measured. One was a short cylinder filled with a uniformly-distributed solution of 99mTc in water. The other was a head phantom with a 0.6 cm3 "tumor" containing 99mTc. Water surrounding the tumor could be nonradioactive or contain a dilute background activity. Absolute calibration was accomplished by imaging the simulated tumor in the air-filled phantom. With the offset window, calculated tumor activity is only 3% and 7% high without and with background respectively compared to 20% and 26% high with the symmetric window. However, total activity of the entire slice is still 37% high even without background. For the cylinder containing uniform activity, the error in specific activity drops from 30% to 18% with the offset window. Therefore, an asymmetric window significantly improves quantification, and for certain cases such as an isolated tumor, may be sufficiently accurate without further correction.