Total Variation Regularization in I-123 Ioflupane SPECT Reconstruction

Objectives: I-123 Ioflupane SPECT has been helpful for diagnosis of parkinsonism and assessment of dopamine transporter (DaT) density. However, due to the low efficiency of the Single Photon Emission Tomography (SPECT) camera, I-123 Ioflupane SPECT requires long imaging time, which can result in patient discomfort and motion. The aim of this study is to evaluate OSEM (Ordered Subsets Expectation Maximization) algorithm with Total Variation (TV) regularization to decrease imaging time.

Methods: Sinogram from a normal clinical I-123 Ioflupane SPECT scan was used to obtain total real sinogram counts. Virtual phantom I-123 Ioflupane SPECT images were constructed with Striatal Binding Ratios (SBR = (striatal/occipital)-1) of 2.0 for normal DaT density and 1.0 for abnormal DaT density. Total counts in virtual sinograms were normalized to total counts from real sinogram. Virtual sinograms were reconstructed with OSEM algorithm and OSEM + TV using 128 and 64 projections (P) and time per projection (t/P) of 30 s, 15 s and 7.5 s. Optimal Normalized Mean Squared Error (NMSE) and maximum Signal to Noise Ratio (SNR = (mean striatal activity)/(standard deviation of adjacent background brain activity)) for greater than 4 iterations were calculated and compared for 128 x 128 reconstructed images.

Results: For phantom with SBR of 2.0 (normal DaT), baseline (t = 32 min) OSEM reconstruction (P = 128, t/P = 15 s) demonstrated NMSE of 0.094 and SNR of 7.48, while half-time (t = 16 min) OSEM + TV reconstruction (P = 128, t/P = 7.5 s, beta = 1.8 ) yielded NMSE of 0.078 and SNR of 9.09. For baseline (t = 32 min) OSEM reconstruction (P = 64, t/P = 30 s) NMSE was 0.087 and SNR of 8.25, while for half-time (t = 16 min) OSEM +TV reconstruction (P = 64, t/P = 15 s, beta = 1.8) NMSE was 0.076 and SNR was 8.63. For phantom with SBR of 1.0 (abnormal DaT), baseline (t = 32 min) OSEM reconstruction (P = 128, t/P = 15 s) demonstrated NMSE of 0.076 and SNR of 6.56, while half-time (t = 16 min) OSEM reconstruction (P = 128, t/P = 7.5 s , beta = 1.8 ) yielded NMSE 0.074 of and SNR of 6.40. For baseline (t = 32 min) OSEM reconstruction (P = 64, t/P = 30 s) NMSE was 0.079 and SNR was 6.26, while for half-time (t = 16 min) OSEM +TV reconstruction (P = 64, t/P = 15 s, beta = 1.8) NMSE was 0.056 and SNR was 7.07.

Conclusion: Half-time SPECT imaging with OSEM + TV reconstruction demonstrated lower NMSE and higher SNR compared with baseline SPECT imaging with OSEM reconstruction for SPECT images with normal and abnormal DaT density and may be a useful tool for decreasing patient imaging time or radiation dose. Research Support: Galvanizing Engineering in Medicine (GEM) grant from the Clinical and Translational Research Institute (CTRI) at University of California, San Diego.