Image Quality Optimisation and Dose Management in CT, SPECT/CT, and PET/CT

University dissertation from Lund University

Abstract: The significant increase in the use of CT, alone or combined with SPECT or PET, has raised concerns about patient radiation exposure and the consequent increased risk of malignancy later in life. The overall objective of this work was to evaluate and optimise different approaches for minimising patient radiation absorbed dose and maintaining or improving image quality in CT, SPECT/CT, and PET/CT. One way to achieve optimisation is to use automatic exposure control (AEC) systems in CT. The reduction in radiation absorbed dose ranges from 35-60% for an anthropomorphic chest phantom, depending on the system and AEC settings. The variation in image noise among images obtained along the scanning direction is lower when using the AEC systems, but the image noise generally increases. Evaluation of an abdominal CT protocol, aimed to reduce age-specific risks, demonstrated that an increased amount of intravenous contrast medium can compensate for a reduced radiation absorbed dose and vice versa, maintaining the signal-to-noise ratio in the liver and contrast-to-noise ratio for a hypothetical hypovascular liver metastasis. Subjective image quality was affected by an increased noise level in the images, but was judged to be acceptable in all investigated patient groups except the one with the lowest radiation absorbed dose. The effective dose was reduced by 57% in the youngest patient group and the amount of intravenous contrast medium was reduced by 18% in the elderly group. Organ and effective doses to the patient using a cone-beam O-arm system in spinal surgery were estimated using the Monte Carlo technique. The highest estimated absorbed doses were in the breast and lungs when scanning the thoracic spine, and stomach when scanning the lumbar spine. The effective dose was reduced to 1.5-2.4 mSv, which is 5 times lower than the scan settings recommended by the manufacturer for intra-operative imaging of small patients, without a negative impact on image quality in regard to the information required for spinal surgery. A new patent-filed phantom, the MADEIRA phantom, has been developed for the investigation of spatial resolution, partial volume effect (intensity diffusion), and detectability in nuclear medicine tomography. The phantom has been tested and shows potential as a useful and important practical tool in the evaluation and optimisation work. New image reconstruction methods are constantly being developed. Three different reconstruction methods for SPECT were optimised and compared in a visual assessment of 123I-MIBG. Based on a rank-order study, Siemens Flash 3D using 32 (4 h post-injection) and 16 (24 h post-injection) equivalent iterations were found to be the preferable reconstruction algorithms.

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