Abstract
1767
Objectives: Due its favorable chemical characteristics Iodine-123 is increasingly used in SPECT. Iodine-123 emits photons of 159 keV used for imaging and high energy photons causing an error in the projection data primarily by penetration of the collimator and backscattering into the crystal with an energy close to the photons used for imaging. The Triple Energy Window (TEW) method corrects for both scatter and backscatter, but uses two narrow noise sensitive energy windows. Therefore broader energy windows are preferred in low count (e.g. dynamic) studies. For this purpose the Double Energy Window (DEW) approach was extended with a backscatter correction. Methods: Measurements were performed on a 3-headed IRIX camera (Philips Medical) with LEGP collimators. In the DEW approach the scatter part of the energy spectrum (111-143 keV) is subtracted from the imaging window (143-175 keV). Both the Scatter (S) and the Imaging (I) window need to be corrected for backscatter. Backscatter correction was performed by subtracting a backscatter (B) window (184-216 keV) from both Scatter and Imaging window. Final images were reconstructed from I-k1*B-k2*C=I-(k1-k2*k3)*B-k2*S, where C denotes Compton scattering, k1 and k3 are backscatter multipliers for I and S, and k2 is the empirical DEW scatter multiplier which was assumed to be 0.5. The backscatter multipliers k1 and k3 were determined by minimizing the background in the projection data of the Imaging and Scatter window, respectively. Reconstruction was performed with and without (back)scatter corrections in 128x128 matrices (2.33 mm pixel size) using FBP, a low pass 4th order Butterworth filter at 0.3 Nyquist and uniform attenuation correction. Results: Human studies of both the serotonin (I-123-ADAM) and dopamine (PE2I-123) transport system were analyzed. The backscatter multipliers k1 and k3 were measured to be 1.1 and 1.0 for both studies. In the serotonin study the contrast between brain and ventricles improved with 29%, and the contrast between hot spots and brain with 8%. In the dopamine study the contrast between striatum and brain increased with 63%. Conclusions: The DEW approach combined with the subtraction of a backscatter window corrects quantitatively for the deterioration of SPECT-images caused by scatter and backscatter, and is suitable for low count SPECT-studies.
- Society of Nuclear Medicine, Inc.