Computed tomography (CT) is one of the most important modalities in a radiological department, which produces images with high diagnostic confidence, but in some cases contributes to a high radiation dose to the patient. The radiation dose can be reduced by the use of advanced image reconstruction algorithms. This study was done on a Philips Brilliance iCT with iterative reconstruction iDose4 and model-based iterative reconstruction IMR. The purpose was to investigate the effect on the image quality with thin slice images reconstructed with IMR, compared to standard slice thickness reconstructed with iDose4. Objective measurements of noise and contrast-to-noise ratio were performed using an image quality phantom, an anthropomorphic phantom and clinical cases. Subjective evaluations of low-contrast resolution were performed by observers using an image quality phantom. IMR gives strong noise reduction and enhanced low-contrast and thereby enable selection of thinner slice thickness. Objective evaluation of image noise shows that thin slices reconstructed with IMR provides lower noise than thicker slice images reconstructed with iDose4. With IMR the slice thickness is of less importance for the noise. With thinner slices the partial volume artefacts becomes less pronounced. In conclusion, we have shown that IMR enables reduction of the slice thickness and at the same time maintain or even reduce the noise level compared to iDose4 reconstruction with standard slice thickness. This will subsequently result in an improvement of image quality for images reconstructed with IMR.
The number of CT examinations giving a relatively high patient exposure is increasing. It is therefore important to optimize the imaging conditions at these investigations. Many steps have been taken to reduce the radiation doses in CT examinations. Currently much work is related to iterative image reconstruction methods as alternative to the filtered back
projection method. The aim of this work was to evaluate quality parameters in images from a CT (Philips Brilliance 64)
equipped with the iterative reconstruction method iDose4 using a Catphan 600 phantom with and without body
simulating ring.
CT scans using abdomen protocol were taken with various tube currents and tube voltage and keeping collimation and pitch unchanged for all scans. All collected data were reconstructed with different levels of iDose4 (Level 2, 4, 6) and
traditional filtered back projection. Image quality parameters were evaluated using AutoQA Lite TM (Version 2.3 2007 Iris QA, LLC). Results from the study shows that the iterative reconstruction method decreases the noise with 15-45%
compared with filtered back projection depending on which level of iDose4 is used. The percentage reduction in noise
level is the same with and without body simulating ring. Low contrast was improved with iDose4 and spatial resolution is only marginally affected by the method of reconstruction. However by reducing the image noise, the detectability can
be improved. Our conclusion is that there is great potential to reduce the noise and thereby improve the image quality by using iterative reconstruction methods. This can also be used to lower radiation dose and maintain image quality or
improve image quality.
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