Transmission imaging and attenuation correction for the microPET/spl reg/ P4 tomograph

Abstract

Imaging with Positron Emission Tomography (PET) requires compensation for attenuation of the annihilation photons in subject tissues. Although the magnitude of this correction for small subjects is significantly smaller than for humans, it Is important for a quantitatively accurate representation of the tracer distribution. Attenuation correction (AC) has been implemented on the Concorde microPET/spl reg/ P4 scanner using a Ge-68 point source (PS) that spirals through the field of view under computer control. Transmission (TX) scans of a rat-sized phantom were acquired with this PS and this tomograph by varying acquisition parameters including: acquisition mode (coincidence vs. singles), energy window, coincidence timing window, PS radius, PS activity, and acquisition time. We also evaluated the quality of post-injection TX images. Transmission data were processed by reconstruction of the log of the attenuation sinograms with 3D filtered backprojection. The attenuation coefficient images were subsequently analyzed with regions of interest in the uniform region of the phantom. Despite a significant amount of scatter contribution, singles data acquisition was found to have the best signal-to-noise ratio (SNR). We found that using higher PS activity or smaller PS radius resulted in a dramatic improvement in the SNR. A combination of both could lead to short acquisition times appropriate for TX imaging in a high throughput imaging facility. Attenuation correction sinograms were created by forward projecting through a smoothed TX image, in which the water-equivalent tissues were scaled to the appropriate /spl mu/-values. Noise levels in the corrected and uncorrected emission images indicate that no significant noise was introduced from the correction. A rat was imaged postinjection in the heart region. Singles TX images were used to create AC sinograms after emission contamination subtraction using a mock scan. The results are encouraging and demonstrate the feasibility of using PET TX images in AC.