impact of roi size on the accuracy of noise measurement in ct on computational and acr phantoms

Background: the effect of region of interest (roi) size variation on producing accurate noise levels is not yet studied. objective: this study aimed to evaluate the influence of roi sizes on the accuracy of noise measurement in computed tomography (ct) by using images of a computational and american college of radiology (acr) phantoms. material and methods: in this experimental study, two phantoms were used, including computational and acr phantoms. a computational phantom was developed by using matlab r215a software (mathworks inc., natick, ma natick, ma) with a homogeneously +100 hounsfield unit (hu) value and an added-gaussian noise with various levels of 5, 10, 25, 50, 75, and 100 hu. the acr phantom was scanned with a philips mx-16 slice ct scanner in different slice thicknesses of 1.5, 3, 5, and 7 mm to obtain noise variation. noise measurement was conducted at the center of the phantom images and four locations close to the edge of the phantom images using different roi sizes from 3×3 to 41×41 pixels, with an increased size of 2×2 pixels. results: the use of a minimum roi size of 21×21 pixels shows noise in the range of ±5% ground truth noise. the measured noise increases above the ±5% range if the used roi is smaller than 21×21 pixels. conclusion: a minimum acceptable roi size is required to maintain the accuracy of noise measurement with a size of 21×21 pixels.