10 kVp rule - an anthropomorphic pelvis phantom imaging study using a CR system : impact on image quality and effective dose using AEC and manual mode

Purpose: This study aims to investigate the influence of tube potential (kVp) variation in relation to perceptual image quality and effective dose for pelvis using automatic exposure control (AEC) and non-AEC in a Computed Radiography (CR) system. Methods and Materials: To determine the effects of using AEC and non-AEC by applying the 10 kVp rule in two experiments using an anthropomorphic pelvis phantom. Images were acquired using 10kVp increments (60-120kVp) for both experiments. The first experiment, based on seven AEC combinations, produced 49 images. The mean mAs from each kVp increment were used as a baseline for the second experiment producing 35 images. A total of 84 images were produced and a panel of 5 experienced observers participated for the image scoring using the 2AFC visual grading software. PCXMC software was used to estimate the effective dose. Results: A decrease in perceptual image quality as the kVp increases was observed both in non-AEC and AEC experiments, however no significant statistical differences (p>0.05) were found. Image quality scores from all observers at 10 kVp increments for all mAs values using non-AEC mode demonstrates a better score up to 90kVp. Effective dose results show a statistical significant decrease (p=0.000) on the 75th quartile from 0.3 mSv at 60 kVp to 0.1 mSv at 120kVp when applying the 10 kVp rule in non-AEC mode. Conclusion(s): No significant reduction in perceptual image quality is observed when increasing kVp whilst a marked and significant effective dose reduction is observed

Optimising the number of thermoluminescent dosimeters required for the measurement of effective dose for computed tomography attenuation correction data in SPECT/CT myocardial perfusion imaging

Background The use of thermoluminescent dosimeters (TLDs) is regarded as time consuming and laborious. As part of our dosimetry research it was necessary to optimise the use of our resources, both physical and time. Experimental work was carried out to develop a method that allowed for a reduction in the number of TLDs needed for accurate effective dose measurement. For this work specific reference to computed tomography attenuation correction (CTAC) for myocardial perfusion imaging (MPI) acquisitions is made although it is proposed that the developed method could be applied to dose assessments using TLDs. Research to measure and compare the effective dose from CTAC for MPI was to be carried out using an ATOM 701 dosimetry phantom, Harshaw 3500 manual TLD reader and TLD-100s. Method To establish the areas of the phantom where dose measurements should be carried out, a batch calibrated TLD-100 dosimeters were placed along the centre of the phantom. A simulated CTAC for MPI was performed. After reading the distribution of the dose was recorded and areas where dose levels were below the sensitivity threshold dose of 50μGy were noted. To test the effect of excluding dose measurement for some areas on the final calculation of effective dose and the time taken to acquire the data a repeat acquisition was performed with the full complement of TLDs placed in the phantom in organ locations recommended by the manufacturer. The time taken for loading, unloading and reading was recorded. Effective dose and organ doses were calculated. The calculation was repeated with TLDs outside the established range excluded and the potential time saved calculated. Results Excluding TLDs from areas where doses were below the 50μGy threshold resulted in 82 fewer TLDs being used (268–186) leading to a time saving of around 2h per batch. The results of the experiment showed that effective dose measurements were 1.75% lower with the reduced chipset and organ dose measurements were not significantly different (p>0.10). Conclusion It is proposed that this methodology could be applied to TLD dosimetry work to establish the areas that should be included in the measurements. In some cases significant savings in time could be made

Computed tomography imaging with the Adaptive Statistical Iterative Reconstruction (ASIR) algorithm: dependence of image quality on the blending level of reconstruction

Computed tomography (CT) is a useful and widely employed imaging technique, which represents the largest source of population exposure to ionizing radiation in industrialized countries. Adaptive Statistical Iterative Reconstruction (ASIR) is an iterative reconstruction algorithm with the potential to allow reduction of radiation exposure while preserving diagnostic information. The aim of this phantom study was to assess the performance of ASIR, in terms of a number of image quality indices, when different reconstruction blending levels are employed. CT images of the Catphan-504 phantom were reconstructed using conventional filtered back-projection (FBP) and ASIR with reconstruction blending levels of 20, 40, 60, 80, and 100%. Noise, noise power spectrum (NPS), contrast-to-noise ratio (CNR) and modulation transfer function (MTF) were estimated for different scanning parameters and contrast objects. Noise decreased and CNR increased non-linearly up to 50 and 100%, respectively, with increasing blending level of reconstruction. Also, ASIR has proven to modify the NPS curve shape. The MTF of ASIR reconstructed images depended on tube load/contrast and decreased with increasing blending level of reconstruction. In particular, for low radiation exposure and low contrast acquisitions, ASIR showed lower performance than FBP, in terms of spatial resolution for all blending levels of reconstruction. CT image quality varies substantially with the blending level of reconstruction. ASIR has the potential to reduce noise whilst maintaining diagnostic information in low radiation exposure CT imaging. Given the opposite variation of CNR and spatial resolution with the blending level of reconstruction, it is recommended to use an optimal value of this parameter for each specific clinical application