computational phantom

Methods for segmentation, i.e., Full-segmentation (FS) and Segmentation-rotation (SR), are proposed for maintaining Computed Tomography (CT) number linearity. However, their effectiveness has not yet been tested against noise. This study aimed to evaluate the influence of noise on the accuracy of CT number linearity of the FS and SR methods on American College of Radiology (ACR) CT and computational phantoms. This experimental study utilized two phantoms, ACR CT and computational phantoms. An ACR CT phantom was scanned by a 128-slice CT scanner with various tube currents from 80 to 200 mA to acquire various noises, with other constant parameters. The computational phantom was added by different Gaussian noises between 20 and 120 Hounsfield Units (HU). The CT number linearity was measured by the FS and SR methods, and the accuracy of CT number linearity was computed on two phantoms. The two methods successfully segmented both phantoms at low noise, i.e., less than 60 HU. However, segmentation and measurement of CT number linearity are not accurate on a computational phantom using the FS method for more than 60-HU noise. The SR method is still accurate up to 120 HU of noise.  The SR method outperformed the FS method to measure the CT number linearity due to its endurance in extreme noise.

This study aims to address the radiation exposure incurred by lung scintigraphy in pregnant patients suspected of pulmonary embolism and to investigate the dose variations due to different body habitus of the fetus.
In this respect, seven computational models of pregnant women and fetus in three trimesters of pregnancy were used and Monte Carlo calculations were performed using Monte Carlo n-particle –extended (MCNPX) code version 2.6.0 to assess absorbed dose coefficients. Time-integrated activities for three radiopharmaceuticals considered in this study were also extracted from the available reference biokinetic data.
Fetal dose coefficients (mGy/MBq) for three radiopharmaceuticals labeled with 99mTc were estimated for reference pregnant phantoms at three trimesters of gestation and 10th, and 90th fetal growth percentiles were also considered during the last two trimesters. The results show that the fetal dose coefficients were 2.09 × 10-2, 5.71 × 10-3, and 4.44 × 10-3 mGy/MBq for 99mTc MAA, 8.31 × 10-4, 8.68 × 10-4, and 1.27 × 10-3 mGy/MBq for 99mTc Technegas, and 7.85 × 10-3, 2.42 × 10-3, and 2.66 × 10-3 mGy/MBq for 99mTc DTPA aerosol, respectively. According to the results the factor of fetal body habitus adds variation to the fetal dose within ±15%.
Considering one of the uncertainty components of fetal dose, that is the fetal body habitus, the dose variations were well below the safety threshold for the fetus (the threshold from ICRP Publication 84 for fetal cancer risk). Therefore, to check the safety of the diagnostic examination in terms of radiation dose to the fetus, it is sufficient to take into account the reference dose values in clinical practice.

The effect of region of interest (ROI) size variation on producing accurate noise levels is not yet studied.  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. 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.  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.   A minimum acceptable ROI size is required to maintain the accuracy of noise measurement with a size of 21×21 pixels.