ABSTRACT
PURPOSE: CT images are used for patient specific Monte Carlo treatment planning in radionuclide therapy. The authors investigated the impact of tissue classification, CT image segmentation, and CT errors on Monte Carlo calculated absorbed dose estimates in nuclear medicine. METHODS: CT errors as a function of patient size, CT reconstruction, and tube current modulation methods were assessed in a phantom experiment on a clinical CT system. The impact of tissue segmentation methods and CT number variations on EGSnrc Monte Carlo calculated absorbed dose distributions was assessed for 99mTc and 131I in the ICRP/ICRU male phantom and in a patient PET/CT-scanned with 124I prior to radioiodine therapy. RESULTS: CT number variations <20 HU were obtained for whole-body CT examinations at effective CT doses â¼2 mSv. Monte Carlo calculated absorbed doses depended on both the number of media types and accurate calibration of the CT number-to-density conversion ramp. Tissue segmentation by a 13-tissue CT conversion ramp, calibrated by a stoichiometric method, resulted in low (<4%) dose errors in selected organs for both isotopes. CONCLUSIONS: A calibrated CT scanner specific conversion ramp is required for accurate patient specific dosimetry in nuclear medicine. Accurate dosimetry was obtained with a 13-tissue ramp that included five different bone types.
Subject(s)
Image Processing, Computer-Assisted/methods , Monte Carlo Method , Nuclear Medicine , Tomography, X-Ray Computed/methods , Calibration , Humans , Male , Phantoms, Imaging , Radiation Dosage , Radiometry , Radiotherapy Planning, Computer-Assisted , Thyroid Neoplasms/diagnostic imaging , Thyroid Neoplasms/pathology , Thyroid Neoplasms/radiotherapyABSTRACT
Peri-therapeutic (124)I-PET/CT is of interest as guidance for radioiodine therapy. Unfortunately, image quality is complicated by dead time effects and increased random coincidence rates from high (131)I-activities. A series of phantom experiments with clinically relevant (124)I/(131)I-activities were performed on a clinical PET/CT-system. Noise equivalent count rate (NECR) curves and quantitation accuracy were determined from repeated scans performed over several weeks on a decaying NEMA NU-2 1994 cylinder phantom initially filled with 25 MBq (124)I and 1250 MBq (131)I. Six spherical inserts with diameters 10-37 mm were filled with (124)I (0.45 MBq ml(-1)) and (131)I (22 MBq ml(-1)) and placed inside the background of the NEMA/IEC torso phantom. Contrast recovery, background variability and the accuracy of scatter and attenuation corrections were assessed at sphere-to-background activity ratios of 20, 10 and 5. Results were compared to pure (124)I-acquisitions. The quality of (124)I-PET images in the presence of high (131)I-activities was good and image quantification unaffected except at very high count rates. Quantitation accuracy and contrast recovery were uninfluenced at (131)I-activities below 1000 MBq, whereas image noise was slightly increased. The NECR peaked at 550 MBq of (131)I, where it was 2.8 times lower than without (131)I in the phantom. Quantitative peri-therapeutic (124)I-PET is feasible.
Subject(s)
Image Processing, Computer-Assisted/methods , Iodine Radioisotopes , Phantoms, Imaging , Positron-Emission Tomography/methods , Tomography, X-Ray Computed/methods , Whole Body Imaging/methods , Algorithms , Humans , Models, Statistical , Quality ControlABSTRACT
PET/CT with non-pure positron emitters is a highly valuable tool in immuno-PET and for pretherapeutic dosimetry. However, imaging is complicated by prompt gamma coincidences (PGCs) that add an undesired background activity to the images. Time-of-flight (TOF) reconstruction improves lesion detectability in (18)F-PET and can potentially also improve the signal-to-noise ratio in images acquired with non-pure positron emitters. Using the GE Discovery 690 PET/CT system, we evaluated the image quality with (55)Co, (86)Y and (124)I, and the effect of PGC-correction and TOF-reconstruction on image quality and quantitation in a series of phantom studies. PET image quality and quantitation for all isotopes were significantly affected by PGCs. The effect was most severe with (86)Y, and less, but comparable, with (55)Co and (124)I. PGC-correction improved the image quality and the quantitation accuracy dramatically for all isotopes, especially when the activity was limited to a few hot lesions in a warm background. In imaging situations, where high levels of activity were present in the background, activity concentrations were overestimated. TOF-reconstruction improved image quality in isolated lesions but worsened the accuracy of quantitation and uniformity in homogeneous activity distributions. Better modelling of PGCs in the scatter correction can potentially improve the situation.
