Simultaneous reconstruction of attenuation and activity in cardiac PET can remove CT misalignment artifacts.

BACKGROUND: Misalignment between positron emission tomography (PET) and computed tomography (CT) data is known to generate artifactual defects in cardiac PET images due to imprecise attenuation correction (AC). In this work, the use of a maximum likelihood attenuation and activity (MLAA) algorithm is proposed to avoid such artifacts in time-of-flight (TOF) PET. METHODS: MLAA was implemented and tested using a thorax/heart phantom and retrospectively on fourteen (13)N-ammonia PET/CT perfusion studies. Global and local misalignments between PET and CT data were generated by shifting matched CT images or using CT data representative of the end-inspiration phase. PET images were reconstructed with MLAA and a 3D-ordered-subsets-expectation-maximization (OSEM)-TOF algorithm. Images obtained with 3D-OSEM-TOF and matched CT were used as references. These images were compared (qualitatively and semi-quantitatively) with those reconstructed with 3D-OSEM-TOF and MLAA for which a misaligned CT was used, respectively, for AC and initialization. RESULTS: Phantom experiment proved the capability of MLAA to converge toward the correct emission and attenuation distributions using, as input, only PET emission data, but convergence was very slow. Initializing MLAA with phantom CT images markedly improved convergence speed. In patient studies, when shifted or end-inspiration CT images were used for AC, 3D-OSEM-TOF reconstructions showed artifacts of increasing severity, size, and frequency with increasing mismatch. Such artifacts were absent in the corresponding MLAA images. CONCLUSION: The proposed implementation of the MLAA algorithm is a feasible and robust technique to avoid AC mismatch artifacts in cardiac PET studies provided that a CT of the source is available, even if poorly aligned.

(11)C-Choline PET/CT as a guide to radiation treatment planning of lymph-node relapses in prostate cancer patients.

PURPOSE: To evaluate, in prostate cancer (PCa) patients the potential of (11)C-choline PET/CT as a guide to helical tomotherapy (HTT) of lymph-node (LN) relapses with simultaneous integrated boost (SIB). The efficacy and feasibility of HTT in terms of acute toxicity were assessed. METHODS: We enrolled 83 PCa patients (mean age 68 years, range 51 - 82 years) with biochemical recurrence after radical primary treatment (mean serum PSA 7.61 ng/ml, range 0.37 - 187.00 ng/ml; PSA0) who showed pathological findings on (11)C-choline PET/CT only at the LN site. (11)C-Choline PET/CT was performed for restaging and then for radiation treatment planning (PET/CT0). Of the 83 patients, 8 experienced further LN relapse, of whom 5 were retreated once and 3 were retreated twice (total 94 radiotherapy treatments). All pelvic and/or abdominal LNs positive on PET/CT0 were treated with high doses using SIB. Doses were in the range 36 - 74 Gy administered in 28 fractions. After the end of HTT (mean 83 days, range 16 - 365 days), serum PSA was measured in all patients (PSA1) and compared with PSA0 to evaluate early biochemical response. In 47 patients PET/CT was repeated (PET/CT1) to assess metabolic responses at the treated areas. Toxicity criteria of the Radiation Therapy Oncology Group (RTOG) were used to assess acute toxicity. RESULTS: PET/CT0 revealed pathological LNs in the pelvis in 49 patients, pathological LNs in the abdomen in 15 patients pathological LNs in both the pelvis and abdomen in 18 patients, and pathological LNs in the pelvis or abdomen and other sites in 12 patients. All these sites were treated with HTT. With respect to PSA0, PSA1 (mean 6.28 ng/ml, range 0.00 - 220.46 ng/ml) showed a complete biochemical response after 66 of the 94 HTT treatments, a partial response after 12 treatments, stable disease after 1 treatment and progression of disease after 15 treatments. Of the 47 patients receiving PET/CT1, 20 showed a complete metabolic response at the treated area, 22 a partial metabolic response, 3 progression of disease and 2 stable disease. HTT with SIB was well tolerated in all patients. Grade 3 acute toxicity in the genitourinary tract was observed in two patients. CONCLUSION: (11)C-Choline PET/CT is a valuable tool for planning and monitoring HTT in LN relapse after primary treatment. High-dose hypofractionated (11)C-choline PET/CT-guided HTT with SIB is well tolerated and is associated with a high early biochemical response rate.