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Background and purpose Cone Beam Computed Tomography (CBCT) is routinely used in radiotherapy to identify the position of the target volume. The aim of this study was to determine whether the CBCT dose, when followed by the treatment, influences the therapeutic outcomes as determined by in-vitro clonogenic cell survival in a radiobiological experiment. Materials and methods Human cell lines, four cancer and one normal, were exposed to a 6 MV photon beam, produced by a linear accelerator. For half of each sample, a prior imaging dose was delivered using the on-board CBCT. A sample size of n = 103 was used to achieve statistical power. Results The experimental group of cell lines exposed to CBCT imaging prior to treatment exhibited a reduction in mean cancer cell survival of ~17 times (p = 0.02) greater than predicted from the average dose response and equivalent to more than 5% of the therapeutic dose, compared to 11 times greater than predicted for normal cells (n.s.). Conclusion The greater than predicted reduction in survival resulting from the additional CBCT dose is consistent with radiation-induced bystander effects.

Investigate the impact of acquisition time and reconstruction parameters on single-photon emission computed tomography/computed tomography (SPECT/CT) image quality with the ultimate aim of finding the shortest possible acquisition time for clinical whole-body SPECT/CT (WB-SPECT/CT) while maintaining image quality METHODS: The National Electrical Manufacturers Association (NEMA) image quality measurements were performed on a SPECT/CT imaging system using a NEMA International Electrotechnical Commission (IEC) phantom with spherical inserts of varying diameter (10-37 mm), filled with

Introduction The aim of this work is to report diagnostic reference levels (DRLs) for hybrid positron emission tomography and x-ray computed tomography (PET/CT) exams in Australia (AU) for Queensland (QLD) and Western Australia (WA) (AU QLD/WA) and New Zealand (NZ). Methods Two-structured booklets were designed to collect dose information, patient demographics, equipment details and acquisition protocols for fluoride-18 fluorodeoxyglucose (18 F-FDG) PET/CT procedures, and any additional diagnostic CT routinely performed as part of 18 F-FDG whole-body examination. The DRL was reported based on the 75th percentile and achievable dose for 18 F-FDG, CT dose index volume (CTDIvol ) and dose length product (DLP). The effective dose and total effective dose was reported for 18 F-FDG whole-body PET/CT examination. Also, the effective dose was reported separately for identified additional diagnostic CT. Results The findings of this study show that the current DRL for 18 F-FDG in AU QLD/WA and NZ was 333.75 MBq and 332.87 MBq, respectively. The reported AU QLD/WA CTDIvol and DLP associated with 18 F-FDG whole-body PET/CT examinations from vertex to thigh (VT) was 4.41 mGy and 474 mGy.cm. In NZ, the reported VT CTDIvol and DLP was 13.07 mGy and 1319.05 mGy.cm. The effective dose for 18 F-FDG and CT component was 5.6 mSv and 4.7 mSv for AU QLD/WA. For NZ, the effective dose was 5.7 mSv and 10.9 mSv for 18 F-FDG and CT component. The total effective dose delivered from the 18 F-FDG whole-body scan from the AU QLD/WA PET/CT centres (10.44 mSv) were lower than the radiation doses delivered from the NZ (16.65 mSv). Conclusions The current DRLs were proposed for AU QLD/WA and NZ for 18 F-FDG whole-body PET/CT examinations. Variations existed in the current practice of AU QLD/WA and NZ PET/CT examinations. There is a need to optimize the radiation doses delivered from PET/CT examinations.

Purpose To investigate the possibility of reducing the injected activity for whole-body [18F]FDG-PET/CT studies of paediatric oncology patients and to assess the usefulness of time-of-flight (TOF) acquisition on PET image quality at reduced count levels. Procedures Twenty-nine paediatric oncology patients (12F/17M, 3–18 years old (median age 13y), weight 45±20 kg, BMI 19±4 kg/m2), who underwent routine whole-body PET/CT examinations on a Siemens Biograph mCT TrueV system with TOF capability (555ps) were included in this study. The mean injected activity was 156 ± 45 MBq (3.8 ± 0.8 kg/MBq) and scaled to patient weight. The raw data was collected in listmode (LM) format and pre-processed to simulate reduced levels of [18F]FDG activity (75, 50, 35, 20 and 10% of the original counts) by randomly removing events from the original LM data. All data were reconstructed using the vendor-specific e7-tools with standard OSEM only, with OSEM plus resolution recovery (PSF). The reconstructions were repeated with added TOF (TOF) and PSF+TOF. The benefit of TOF together with the reduced count levels was evaluated by calculating the gains in signal-to-noise ratio (SNR) in the liver and contrast-to-noise ratio (CNR) in all PET-positive lesions before and after TOF employed at every simulated reduced count level. Finally, the PSF+TOF images at 50, 75 and 100% of counts were evaluated clinically on a 5-point scale by three nuclear medicine physicians. Results The visual inspection of the reconstructed images did not reveal significant differences in image quality between 75 and 100% count levels for PSF+TOF. The improvements in SNR and CNR were the greatest for TOF reconstruction and PSF combined. Both SNR and CNR gains did increase linearly with the patients BMI for both OSEM only and PSF reconstruction. These benefits were observed until reducing the counts to 50 and 35% for SNR and CNR, respectively. Conclusions The benefit of using TOF was noticeable when using 50% or greater of the counts when evaluating the CNR and SNR. For [18F]FDG-PET/CT, whole-body paediatric imaging the injected activity can be reduced to 75% of the original dose without compromising PET image quality.

Purpose The purpose of this study was to investigate the effect of increasing iterative reconstruction (IR) algorithm strength at different tube voltages in coronary computed tomography angiography (CCTA) protocols using a three‐dimensional (3D)‐printed and Catphan® 500 phantoms. Methods A 3D‐printed cardiac insert and Catphan 500 phantoms were scanned using CCTA protocols at 120 and 100 kVp tube voltages. All CT acquisitions were reconstructed using filtered back projection (FBP) and Adaptive Statistical Iterative Reconstruction (ASIR) algorithm at 40% and 60% strengths. Image quality characteristics such as image noise, signal–noise ratio (SNR), contrast–noise ratio (CNR), high spatial resolution, and low contrast resolution were analyzed. Results There was no significant difference (P > 0.05) between 120 and 100 kVp measures for image noise for FBP vs ASIR 60% (16.6 ± 3.8 vs 16.7 ± 4.8), SNR of ASIR 40% vs ASIR 60% (27.3 ± 5.4 vs 26.4 ± 4.8), and CNR of FBP vs ASIR 40% (31.3 ± 3.9 vs 30.1 ± 4.3), respectively. Based on the Modulation Transfer Function (MTF) analysis, there was a minimal change of image quality for each tube voltage but increases when higher strengths of ASIR were used. The best measure of low contrast detectability was observed at ASIR 60% at 120 kVp. Conclusions Changing the IR strength has yielded different image quality noise characteristics. In this study, the use of 100 kVp and ASIR 60% yielded comparable image quality noise characteristics to the standard CCTA protocols using 120 kVp of ASIR 40%. A combination of 3D‐printed and Catphan® 500 phantoms could be used to perform CT dose optimization protocols.

Introduction An ideal organ‐specific insert phantom should be able to simulate the anatomical features with appropriate appearances in the resultant computed tomography (CT) images. This study investigated a 3D printing technology to develop a novel and cost‐effective cardiac insert phantom derived from volumetric CT image datasets of anthropomorphic chest phantom. Methods Cardiac insert volumes were segmented from CT image datasets, derived from an anthropomorphic chest phantom of Lungman N‐01 (Kyoto Kagaku, Japan). These segmented datasets were converted to a virtual 3D‐isosurface of heart‐shaped shell, while two other removable inserts were included using computer‐aided design (CAD) software program. This newly designed cardiac insert phantom was later printed by using a fused deposition modelling (FDM) process via a Creatbot DM Plus 3D printer. Then, several selected filling materials, such as contrast media, oil, water and jelly, were loaded into designated spaces in the 3D‐printed phantom. The 3D‐printed cardiac insert phantom was positioned within the anthropomorphic chest phantom and 30 repeated CT acquisitions performed using a multi‐detector scanner at 120‐kVp tube potential. Attenuation (Hounsfield Unit, HU) values were measured and compared to the image datasets of real‐patient and Catphan® 500 phantom. Results The output of the 3D‐printed cardiac insert phantom was a solid acrylic plastic material, which was strong, light in weight and cost‐effective. HU values of the filling materials were comparable to the image datasets of real‐patient and Catphan® 500 phantom. Conclusions A novel and cost‐effective cardiac insert phantom for anthropomorphic chest phantom was developed using volumetric CT image datasets with a 3D printer. Hence, this suggested the printing methodology could be applied to generate other phantoms for CT imaging studies.

Respiratory motion degrades the detection and quantification capabilities of PET/CT imaging. Moreover, mismatch between a fast helical CT image and a time-averaged PET image due to respiratory motion results in additional attenuation correction artifacts and inaccurate localization. Current motion compensation approaches typically have 3 limitations: the mismatch among respiration-gated PET images and the CT attenuation correction (CTAC) map can introduce artifacts in the gated PET reconstructions that can subsequently affect the accuracy of the motion estimation; sinogram-based correction approaches do not correct for intragate motion due to intracycle and intercycle breathing variations; and the mismatch between the PET motion compensation reference gate and the CT image can cause an additional CT-mismatch artifact. In this study, we established a motion correction framework to address these limitations. Methods: In the proposed framework, the combined emission–transmission reconstruction algorithm was used for phase-matched gated PET reconstructions to facilitate the motion model building. An event-by-event nonrigid respiratory motion compensation method with correlations between internal organ motion and external respiratory signals was used to correct both intracycle and intercycle breathing variations. The PET reference gate was automatically determined by a newly proposed CT-matching algorithm. We applied the new framework to 13 human datasets with 3 different radiotracers and 323 lesions and compared its performance with CTAC and non–attenuation correction (NAC) approaches. Validation using 4-dimensional CT was performed for one lung cancer dataset. Results: For the 10 (18)F-FDG studies, the proposed method outperformed (P < 0.006) both the CTAC and the NAC methods in terms of region-of-interest–based SUV(mean), SUV(max), and SUV ratio improvements over no motion correction (SUV(mean): 19.9% vs. 14.0% vs. 13.2%; SUV(max): 15.5% vs. 10.8% vs. 10.6%; SUV ratio: 24.1% vs. 17.6% vs. 16.2%, for the proposed, CTAC, and NAC methods, respectively). The proposed method increased SUV ratios over no motion correction for 94.4% of lesions, compared with 84.8% and 86.4% using the CTAC and NAC methods, respectively. For the 2 (18)F-fluoropropyl-(+)-dihydrotetrabenazine studies, the proposed method reduced the CT-mismatch artifacts in the lower lung where the CTAC approach failed and maintained the quantification accuracy of bone marrow where the NAC approach failed. For the (18)F-FMISO study, the proposed method outperformed both the CTAC and the NAC methods in terms of motion estimation accuracy at 2 lung lesion locations. Conclusion: The proposed PET/CT respiratory event-by-event motion-correction framework with motion information derived from matched attenuation-corrected PET data provides image quality superior to that of the CTAC and NAC methods for multiple tracers.

Cardiac computed tomography angiography (CCTA) is a commonly used diagnostic tool for cardiovascular disease. Despite constant improvements to imaging technologies, the radiation dose to patients from CCTA remains a concern when using this procedure. There remains a need for optimisation of CCTA procedures and accurate dose monitoring to reduce the potential risk of cancer. Establishing diagnostic reference levels (DRLs) allows for the assessment of radiation dose variations, enabling strategies aimed at standardising doses across radiological centres. This systematic review explores the literature on CCTA methodologies that have been used to establish DRLs. A search was carried out using the Web of Science, SCOPUS, Medline, CINAHL and EMBASE databases. Reference lists of published articles were also assessed to identify further articles. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology was employed to evaluate articles for relevance. Articles were included if they assessed DRLs in CCTA. The search resulted in 448 articles, of which, six were included after a thorough screening process. The literature demonstrates a wide dose variation in reported CCTA DRLs ranging from 671 to 1510 mGy cm in DLP. Where reported, CTDIvol DRLs ranged from 26 to 70 mGy. Differences were found in the methodologies used for establishing CCTA DRLs, including the sampling methodology used for identifying suitable patients and scanning protocols. This current review emphasises the need for an international standardisation for DRLs establishment methods, to provide a more comparable global measurement of dose variations across CT sites.

Current pharmacokinetic models, such as the linear parametric neurotransmitter PET (lp-ntPET) model have been developed to detect and quantify transient changes in receptor occupancy caused by variations in the concentration of endogenous neurotransmitters. However, it often performs poorly when applied at the voxel level due to high statistical noise. In this paper, we propose a new method to detect transient changes in neurotransmitter concentration in dynamic PET data using deep learning. Activation onset time and response magnitude of neurotransmitter were directly estimated using a convolution neural network (CNN) and compared to the lpntPET model. Computer simulations, as well as realistic GATE simulations were used to generate dynamic PET data, representing a [11C]raclopride study, with a known range of activation onset times and response magnitudes, across a wide range of noise levels. Results showed that the proposed neural network had better quantitative performance in estimating activation onset time and response magnitude than the conventional lp-ntPET method, especially where noise is high.

Objective: The aim of this study is to report a local diagnostic reference level (DRL) for paediatric whole-body (WB) fludeoxyglucose (18F-FDG) positron emission tomography (PET) CT examinations. Methods: The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) national DRL (NDRL) age category (0–4 years and 5–14 years), the International Commission on Radiological Protection age category (ICRP age) (1–5, >5–10, and >10–15 years), and European guideline weight category ( EG weight) (18F FDG PET/CT studies. Two-structured questionnaires were designed to collect dose data, patient demographics, equipment details, and acquisition protocols for WB 18F-FDG PET/CT procedures. The local DRL was based on the median 18F-FDG administered activity (MBq), dose–length product (DLP), and the CT dose index volume (CTDIvol), values. The effective dose (E) was also calculated and reported. Results: The local DRLs for 18F-FDG administered activity, CTDIvol and DLP values based on ARPANSA age and ICRP age were increased from lower to higher age categories. For the EG weight category, the local DRL for 18F-FDG administered activity, CTDIvol and DLP values were increased from the low EG weight category to the high EG weight category. The mean administered activity in our study based on ICRP age category >1–5, >5–10, and >10–15 years is 79.97, 119.40, and 176.04 MBq, which is lower than the mean administered activity reported in the North American Consensus guideline published in 2010 (99, 166, and 286 MBq) and European Association of Nuclear Medicine and Dosage Card (version 1.5.2008) (120, 189, and 302 MBq). However, the mean administered activity in our study based on ICRP age category Conclusion: The determined local DRL values for the radiation doses associated with WB 18F FDG PET/CT examinations are differed considerably between the ARPANSA and ICRP age category and EG weight category. Although, the determined 18F-FDG value for ICRP < 1 year is in good agreement with available publish data, it is preferable to optimise the 18F-FDG administered activity while preserving the diagnostic image quality. Advances in knowledge: The local DRL value determined from WB 18F-FDG PET/CT examinations may help to establish the ARPANSA NDRL for WB FDG 18F-PET/CT examinations.

AIM To evaluate the diagnostic performance of whole-body (WB) integrated single photon emission tomography (SPECT)/computed tomography (CT) in detecting bone metastasis (BM) and to investigate whether WB-SPECT/CT offered any additional benefit value compared to planar bone scintigraphy (PBS) with 99mTc-hydroxy-methylene diphosphonate or 99mTc methylene diphosphonate. MATERIALS AND METHODS Medline, EMBASE, SCOPUS, Web of Science, and CINAHL were searched systematically up to 28 August 2019. All studies using histopathological analysis and/or follow-up imaging and clinical data as the reference standard were eligible for inclusion. RESULTS Eleven studies (1,611 patients) were analysed. Based on patient analysis, the sensitivity, specificity, and area under the curve (AUC) of WB-SPECT/CT were 92% (92% confidence interval [CI], 89–95%), 95% (95% CI, 94–96%), and 0.9835, respectively, in the case of negative equivocal findings for BM, and 94% (95% CI, 91–96%), 94% (95% CI, 92–95%), and 0.9790, respectively, when regarded positive. On a lesion basis, these parameters were 91% (95% CI, 89–94%), 96% (95% CI, 94–97%), and 0.9906, respectively, in the case negative equivocal findings, and 92% (95% CI, 89–94%), 95% (95% CI, 94–97%), and 0.9898, respectively, when regarded positive. Comparing 1,265 patients from eight studies, higher sensitivity (92% versus 74%, p=0.04) and specificity for WB-SPECT/CT against PBS (93% versus 80%, p=0.01) in the case of positive equivocal findings; however, when regarded negative, WB-SPECT/CT demonstrated higher sensitivity (91% versus 70%, p=0.01), but no significant difference was apparent in specificity (94% versus 89%, p=0.07). CONCLUSION Compared to PBS, WB-SPECT/CT had superior diagnostic accuracy in BM detection and exhibited a more reliable performance with less equivocal results.

Image quality of 68Ga Positron Emission Tomography (PET) studies is impacted by its relatively high positron range compared with 18F, thus limiting spatial resolution. This simulation study describes a post-reconstruction 2-dimensional (2D) positron range correction (post-PRC) using the Richardson-Lucy (RL) de-blurring method for 68Ga. The method proposed is based on a 2-dimensional de-blurring kernel derived in bone, soft-tissue and lung medium. and the kernel was applied to the simulated image of the NEMA Image Quality phantom geometry with a target-to-background ratio of 8:1. The post-PRC method can recover spatial resolution loss by approximately 1%, 9% and 56% in bone, water and lung medium, respectively. However, the proposed de-blurring method increases image noise, particularly in the lung medium. This convenient post-processing de-blurring approach does not require raw data or modification to the reconstruction algorithm allowing it to be adopted to the reconstructed volumes of any PET scanner.

The aim of this work is to report diagnostic reference levels (DRLs) for hybrid positron emission tomography and x-ray computed tomography (PET/CT) exams in Australia (AU) for Queensland (QLD) and Western Australia (WA) (AU QLD/WA) and New Zealand (NZ).Two-structured booklets were designed to collect dose information, patient demographics, equipment details and acquisition protocols for fluoride-18 fluorodeoxyglucose (The findings of this study show that the current DRL forThe current DRLs were proposed for AU QLD/WA and NZ for

The aim of this systematic review is to investigate the national diagnostic reference level (NDRL) methods for positron emission tomography/computed tomography (PET/CT) and single photon emission tomography/computed tomography (SPECT/CT) procedures. A search strategy was based on the preferred, reporting items for systematic review and meta-analysis (PRISMA). Relevant articles retrieved from Medline, Scopus, Web of Science, Embase, Cinahl, and Google Scholar published up to October 2017. The search yielded 1,057 articles. Fourteen articles were included in the review after a screening process. Relevant information from the selected articles were summarised and analysed. Discrepancies were found between the methodologies utilised to establish and report both PET/CT and SPECT/CT NDRLs, e.g. patient sampling and administered activity. Further research should focus on reporting more NDRLs for hybrid PET/CT and SPECT/CT examinations, and establish a robust NDRL standard for the CT portion associated with PET/CT and SPECT/CT examinations. This review provides updated NDRL reommndations to deliver more comparable international radation doses for administered activity and CT dose across PET/CT and SPECT/CT clinics.

This study aims to assess patient radiation dose from cardiac computed tomography angiography (CCTA) with the aim of proposing a national diagnostic reference levels (NDRLs) for CCTA procedures in Australia. A questionnaire was used to retrospectively gather baseline information related to CCTA scanning and patient parameters in CT centres across the country. The 75th percentile of both volumetric CT dose index (CTDIvol) and dose length-product (DLP) was used as DRL values for CCTA. A DRL for CT calcium scoring test was also determined. NDRLs were compared with international published data. Data sets of 338 patients from nine CT centres were used for analysis. The CCTA DRL for the CTDIvol and the DLP were 22 mGy and 268 mGy cm, respectively. The CT calcium scoring test DRL for DLP was 137 mGy cm. The DRL values for CCTA in Australia have been recommended for the first time. DRLs are lower than those in most published studies due to the implementation of dose-saving technologies such as prospective ECG-gated mode and iterative reconstruction algorithms. Considerable variations remain in patient doses between hospitals for the most frequently used CCTA protocols, indicating the potential for DRLs to prompt dose optimisation strategies in CT facilities.

OBJECTIVE: This work proposes the use of mammographic breast density (MBD) to estimate actual glandular dose (AGD), and assesses how AGD compares to mean glandular dose (MGD) estimated using Dance et al method. METHODS: A retrospective sample of anonymised mammograms (52,405) was retrieved from a central database. Technical parameters and patient characteristics were exported from the Digital Imaging and Communication in Medicine (DICOM) header using third party software. LIBRA (Laboratory for Individualized Breast Radiodensity Assessment) software package (University of Pennsylvania, Philadelphia, USA) was used to estimate MBDs for each mammogram included in the data set. MGD was estimated using Dance et al method, while AGD was calculated by replacing Dance et al standard glandularities with LIBRA estimated MBDs. A linear regression analysis was used to assess the association between MGD and AGD, and a Bland-Altman analysis was performed to assess their mean difference. RESULTS: The final data set included 31,097 mammograms from 7728 females. MGD, AGD, and MBD medians were 1.53 , 1.62 mGy and 8% respectively. When stratified per breast thickness ranges, median MBDs were lower than Dance’s standard glandularities. There was a strong positive correlation (R(2) = 0.987, p < 0.0001) between MGD and AGD although the Bland-Altman analysis revealed a small statistically significant bias of 0.087 mGy between MGD and AGD (p < 0.001). CONCLUSION: AGD estimated from MBD is highly correlated to MGD from Dance method, albeit the Dance method underestimates dose at smaller CBTs. ADVANCES IN KNOWLEDGE: Our work should provide a stepping-stone towards an individualised dose estimation using automated clinical measures of MBD.

Cardiac computed tomography angiography (CCTA) is a commonly used diagnostic imaging tool for cardiovascular disease. Despite constant improvements to imaging technologies, the radiation dose to patients remains a concern when using this procedure. Diagnostic reference levels (DRLs) are used as a trigger to identify and alert individual facilities that are using high doses during CT. This study aims to assess patient radiation dose and establish new national DRLs (NDRL) associated with CCTA in Saudi clinical practices. A structured booklet survey was designed for recording patient and scanning protocols during CCTA procedures. The data were collected retrospectively from the participating centres. NDRLs for CCTA were defined as the 75th and 25th of volumetric CT dose index (CTDIvol) and dose length product (DLP). Specific DRLs based on two main ECG-gating modes were also proposed. Data sets related to 197 CCTAs with a mean weight of 77 kg were analysed in detail. The DRL values for CTDIvol and DLP for prospective gating mode and retrospective gating mode were 29 and 62 mGy and 393 and 1057 mGy cm, respectively. NDRLs for CCTA in Saudi Arabia are comparable or slightly lower than European DRLs due to the current use of dose-saving technology. There are major variations in patient doses during CCTA due to differences in CT scanners, scanning modes and departmental CCTA protocols.

International audience; Awake and/or freely moving small animal single photon emission imaging allows the continuous study of molecules exhibiting slow kinetics without the need to restrain or anaesthetise the animals. Estimating motion free projections in freely moving small animal planar imaging can be considered as a limited angle tomography problem, except that we wish to estimate the 2D planar projections rather than the 3D volume, where the angular sampling in all three axes depends on the rotational motion of the animal. In this study, we hypothesise that the motion corrected planar projections estimated by reconstructing an estimate of the 3D volume using an iterative motion compensating reconstruction algorithm and integrating it along the projection path, will closely match the true, motion-less, planar distribution regardless of the object motion. We tested this hypothesis for the case of rigid motion using Monte-Carlo simulations and experimental phantom data based on a dual opposed detector system, where object motion was modelled with 6 degrees of freedom. In addition, we investigated the quantitative accuracy of the regional activity extracted from the geometric mean of opposing motion corrected planar projections. Results showed that it is feasible to estimate qualitatively accurate motion-corrected projections for a wide range of motions around all 3 axes. Errors in the geometric mean estimates of regional activity were relatively small and within 10% of expected true values. In addition, quantitative regional errors were dependent on the observed motion, as well as on the surrounding activity of overlapping organs. We conclude that both qualitatively and quantitatively accurate motion-free projections of the tracer distribution in a rigidly moving object can be estimated from dual opposed detectors using a correction approach within an iterative reconstruction framework and we expect this approach can be extended to the case of non-rigid motion.

Objective: This study aimed to examine the associated factors for dose variation and influence cardiac CT angiography (CCTA) dose benchmarks in current CT imaging centres. Methods: A questionnaire was distributed to CT centres across Australia and Saudi Arabia. All participating centres collected data for adults who underwent a CCTA procedure. The questionnaire gathered information about the examination protocol, scanning parameters, patient parameters, and volume CT dose index (CTDI vol) and dose–length product (DLP). A stepwise regression analysis was performed to assess the contribution of tube voltage (kV), padding time technique, cross-sectional area (CSA) of chest and weight to DLP. Results: A total of 17 CT centres provided data for 423 CCTA examinations. The median CTDIvol, DLP and effective dose were 18 mGy, 256 mGy.cm and 5.2 mSv respectively. There was a statistically significant difference in DLP between retrospective and prospective ECG-gating modes (p = 0.001). Median DLP from CCTA using padding technique was 61% higher than CCTA without padding (p = 0.001). The stepwise regression showed that kV was the most significant predictor of DLP followed by padding technique then CSA while patient weight did not statistically significantly predict DLP. Correlation analysis showed a strong positive correlation between weight and CSA (r = 0.78), and there was a moderate positive correlation between weight and DLP (r = 0.42), as well as CSA and DLP (r = 0.48). Conclusion: Findings show radiation dose variations for CCTA. The associated factors for dose variation found in this study are scanning mode, kV, padding time technique and CSA of the chest. This results support the need to include CSA measurements in future dose survey and for setting DRLs. Advances in knowledge: The study provides baseline information that helps to understand the associated factors for dose variations and high doses within and between centres performing CCTA.

To establish the extent to which test set reading can represent actual clinical reporting in screening mammography.Institutional ethics approval was granted, and informed consent was obtained from each participating screen reader. The need for informed consent with respect to the use of patient materials was waived. Two hundred mammographic examinations were selected from examinations reported by 10 individual expert screen readers, resulting in 10 reader-specific test sets. Data generated from actual clinical reports were compared with three test set conditions: clinical test set reading with prior images, laboratory test set reading with prior images, and laboratory test set reading without prior images. A further set of five expert screen readers was asked to interpret a common set of images in two identical test set conditions to establish a baseline for intraobserver variability. Confidence scores (from 1 to 4) were assigned to the respective decisions made by readers. Region-of-interest (ROI) figures of merit (FOMs) and side-specific sensitivity and specificity were described for the actual clinical reporting of each reader-specific test set and were compared with those for the three test set conditions. Agreement between pairs of readings was performed by using the Kendall coefficient of concordance.Moderate or acceptable levels of agreement were evident (W = 0.69-0.73, P.01) when describing group performance between actual clinical reporting and test set conditions that were reasonably close to the established baseline (W = 0.77, P.01) and were lowest when prior images were excluded. Higher median values for ROI FOMs were demonstrated for the test set conditions than for the actual clinical reporting values; this was possibly linked to changes in sensitivity.Reasonable levels of agreement between actual clinical reporting and test set conditions can be achieved, although inflated sensitivity may be evident with test set conditions.

This study measured reading workstation monitors and the viewing environment currently available within BreastScreen New South Wales (BSNSW) centres to determine levels of adherence to national and international guidelines. Thirteen workstations from four BSNSW service centres were assessed using the American Association of Physicists in Medicine Task Group 18 Quality Control test pattern. Reading workstation monitor performance and ambient light levels when interpreting screening mammographic images were assessed using spectroradiometer CS-2000 and chroma meter CL-200. Overall, radiologic monitors within BSNSW were operating at good acceptable levels. Some non-adherence to published guidelines included the percentage difference in maximum luminance between pairs of primary monitors at individual workstations (61.5 % or 30.8 % of workstations depending on specific guidelines), maximum luminance (23.1 % of workstations), luminance non-uniformity (11.5 % of workstations) and minimum luminance (3.8 % of workstations). A number of ambient light measurements did not comply with the only available evidence-based guideline relevant to the methodology used in this study. Larger ambient light variations across sites are shown when monitors were switched off, suggesting that differences in ambient lighting between sites can be masked when a standard mammogram is displayed for photometric measurements. Overall, BSNSW demonstrated good adherence to available guidelines, although some non-compliance has been shown. Recently updated United Kingdom and Australian guidelines should help reduce confusion generated by the plethora and sometimes dated nature of currently available recommendations.

Multiple pinholes are advantageous for maximizing the use of the available field of view (FOV) of compact small animal single photon emission computed tomography (SPECT) detectors. However, when the pinholes are aligned axially to optimize imaging of extended objects, such as rodents, multiplexing of the pinhole projections can give rise to inconsistent data which leads to 'ghost point' artefacts in the reconstructed volume. A novel four pinhole collimator with a baffle was designed and implemented to eliminate these inconsistent projections. Simulation and physical phantom studies were performed to investigate artefacts from axially aligned pinholes and the efficacy of the baffle in removing inconsistent data and, thus, reducing reconstruction artefacts. SPECT was performed using a Defrise phantom to investigate the impact of collimator design on FOV utilization and axial blurring effects. Multiple pinhole SPECT acquired with a baffle had fewer artefacts and improved quantitative accuracy when compared to SPECT acquired without a baffle. The use of four pinholes positioned in a square maximized the available FOV, increased acquisition sensitivity and reduced axial blurring effects. These findings support the use of a baffle to eliminate inconsistent projection data arising from axially aligned pinholes and improve small animal SPECT reconstructions.

In iterative reconstruction of pinhole SPECT data, the forward and back projection processes are often performed using the ray tracing method. Ray tracing is computationally efficient, but it has the drawback of poor reconstruction quality due to the missing voxel effect and textural artefacts. In this paper, the pinhole projection process was modelled starting from consideration of all the main factors affecting pinhole projection, such as voxel shape, penetration of the pinhole edges and detector response. Next, approximations were made to reduce the computational speed and the effect of the approximations on reconstructed image accuracy was evaluated in simulation and phantom experiments and compared with the ray tracing algorithm. When used in conjunction with the ML-EM algorithm, the proposed model improved reconstructed image accuracy compared with the ray tracing method and achieved comparable computational efficiency. Therefore, the proposed projection model is a practical alternative to the ray tracing algorithm for pinhole SPECT reconstruction.

Group work has many benefits for a student's professional development but it is difficult to determine the individual contributions to the group assessment tasks. Peer assessment of an individual's contribution to group work can be used to encourage student participation. It is important that the method of peer assessment is fair and that the students' submissions be treated confidentially. A model for peer assessment of individual contributions to the group assessment is described. Students who did not participate adequately in the group were penalised resulting in a reduced individual grade. Perceptions of the peer assessment method are reported for students enrolled ( n =169) in the subject ‘ Medical Radiations Project '. The questionnaire showed a positive student response towards the peer assessment model.

The aim of this systematic review is to evaluate the radiation dose reduction achieved using iterative reconstruction (IR) compared to filtered back projection (FBP) in coronary CT angiography (CCTA) and assess the impact on diagnostic image quality. A systematic search of seven electronic databases was performed to identify all studies using a developed keywords strategy. A total of 14 studies met the criteria and were included in a review analysis. The results showed that there was a significant reduction in radiation dose when using IR compared to FBP (P 0.05). The mean and standard deviation (SD) difference of CTDIvol and dose-length-product (DLP) were 14.70 ± 6.87 mGy and 186 ± 120 mGy.cm respectively. The mean ± SD difference of effective dose (ED ) was 2.9 ± 1.7 mSv with the range from 1.0 to 5.0 mSv. The assessment of diagnostic image quality showed no significant difference (P 0.05). The mean ± SD difference of image noise, signal-noise ratio (SNR) and contrast-noise ratio (CNR) were 1.05 ± 1.29 HU, 0.88 ± 0.56 and 0.63 ± 1.83 respectively. The mean ± SD percentages of overall image quality scores were 71.79 ± 12.29% (FBP) and 67.31 ± 22.96% (IR). The mean ± SD percentages of coronary segment analysis were 95.43 ± 2.57% (FBP) and 97.19 ± 2.62% (IR). In conclusion, this review analysis shows that CCTA with the use of IR leads to a significant reduction in radiation dose as compared to the use of FBP. Diagnostic image quality of IR at reduced dose (30-41%) is comparable to FBP at standard dose in the diagnosis of CAD.

Molecular imaging refers to the use of non-invasive imaging techniques to detect signals that originate from molecules, often in the form of an injected tracer, and observe their interaction with a specific cellular target in vivo. Differences in the underlying physical principles of these measurement techniques determine the sensitivity, specificity and length of possible observation of the signal, characteristics that have to be traded off according to the biological question under study. Here, we describe the specific characteristics of single photon emission computed tomography (SPECT) relative to other molecular imaging technologies. SPECT is based on the tracer principle and external radiation detection. It is capable of measuring the biodistribution of minute (

One hundred and fifty-four final year health science undergraduates from the disciplines of Medical Radiation Sciences, Speech Pathology and Physiotherapy in the Faculty of Health Sciences, the University of Sydney completed the Australian Time Organisation and Management Scale (ATOMS). Findings identified the strengths and weaknesses of students on the four dimensions included in this analysis. Whilst the results indicated strengths in the area of students' sense of purpose, level of focus and goal-setting ability, areas of weakness were noted, particularly with regard to the use of basic time-management devices (e.g. use of a diary and making lists). How these skills and competences are related to age, gender and discipline is explored. In addition, the degree to which these time-management behaviours can be enhanced (eg with time management intervention programs), the generalizability of the findings to other settings (non-academic), and the development of these skills over time are major issues discussed in the paper.

In a previous simulation study, we demonstrated the feasibility of using coded apertures together with pixelated detectors tors for small animal SPECT. In this paper; we further explore the potential of this approach with a prototype detector and simulated multipinhole apertures. We also investigated the effect of multiplexing due to overlapped projections on convergence properties, image signal-to-noise ratio (SNR) and spatial resolution. The detector comprises a 48/spl times/44 array of NaI(Tl) crystals, each 1 mm/spl times/1 mm/spl times/5 mm on a 1.25-mm pitch. The crystal array is directly coupled to a Hamamatsu R3941 8 cm position sensitive photomultiplier tube. Multipinhole apertures were simulated by performing repeated SPECT acquisitions of the same object with a single tungsten pinhole translated to different positions in the aperture plane. Image reconstruction is based on a three-dimensional ray driven projector which is an extension of a method described for single pinhole SPECT with a displaced center of rotation. Image estimates are updated using the maximum likelihood expectation maximization (ML-EM) algorithm. The effect of multiplexing was to slow convergence and reduce the achievable SNR by approximately 15% compared with nonmultiplexed data (but the result may be achieved in a fraction of the time). The reconstructed resolution obtained with a resolution phantom was 1.5-mm full width at half maximum and there was no appreciable difference between the resolution of multiplexed and nonmultiplexed data. These results encourage us to develop a prototype coded aperture system for high sensitivity, high resolution small animal SPECT.

The kinetics of single photon emitting macromolecules (e.g. antibodies) are typically slower than small molecules, necessitating long or repeat acquisitions. We previously proposed the use of motion tracking and limited angle tomographic reconstruction to characterise tracer kinetics over extended periods in awake rodents. In this study, we explored this approach by imaging a contrast phantom, moved with 6 degrees of freedom, using a prototype preclinical SPECT scanner with parallel-hole collimation and a fixed detector located at 0 and 90 degrees. The position of the phantom was tracked and data were acquired in list mode. Each event was motion-corrected and reconstructed using LM-MLEM. Planar projections were created by summing the reconstructed volume along the x-axis. Line profiles of the contrast phantom were compared for a planar reference projection and projections generated from the motion-free, motion-corrupted and motion-corrected reconstructions. Projections created from the motion-corrected agreed well with the planar reference projection of the stationary object and exhibited similar contrast. Whilst this initial study was limited to rigid motion, it demonstrates the feasibility of motion-corrected planar projections of a moving object.

Single photon emission computed tomography (SPECT) is a tomographic imaging modality based on the radiotracer principle [1]. It is used to measure the 3D distribution of radiolabelled molecules in vivo using very sensitive radiation detectors and mathematical image reconstruction algorithms. Although SPECT has been used as a clinical tool for several decades, it is also well suited to imaging small animal models of human disease, such as laboratory mice and rats, for pre-clinical research. Because of the relatively long physical half-lives of single photon emitters (Table 4.1), SPECT is best suited to the study of macromolecules, such as antibodies and proteins, which have relatively slow rates of accrual at their target sites and slow plasma clearance. Additionally, proteins and antibodies are easily labelled with one of the radioisotopes of iodine (125I, 123I or 131I), or else by attaching a chelating agent incorporating one of the other common single photon emitters with suitable imaging properties, such as 99mTc or 111In. Conversely, the closely related radiotracer technique positron emission tomography (PET), which is discussed in the following chapter, is best suited to the study of small molecules such as synthetic drugs which have relatively fast kinetics in the body. Thus, the two techniques are highly complementary in the pre-clinical research environment.

High-resolution dedicated cardiac SPECT systems with CZT detectors have a great potential to provide 3D molecular breast imaging of sub-cm tumors. In this study, we investigated the feasibility of breast imaging using a dedicated cardiac SPECT system (GE Discovery 530c/570c) by simulating different lesion sizes, positions, and contrasts.

Aim To examine how the location where reading takes place and the availability of prior images can affect performance in breast test-set reading. Materials and methods Under optimized viewing conditions, 10 expert screen readers each interpreted a reader-specific set of images containing 200 mammographic cases. Readers, randomly divided into two groups read images under one of two pairs of conditions: clinical read with prior images and laboratory read with prior images; laboratory read with prior images and laboratory read without prior images. Region-of-interest (ROI) figure-of-merit (FOM) was analysed using JAFROC software. Breast side-specific sensitivity and specificity were tested using Wilcoxon matched-pairs signed rank tests. Agreement between pairs of readings was measured using Kendall's coefficient of concordance. Results Group performances between test-set readings demonstrated similar ROI FOMs, sensitivity and specificity median values, and acceptable levels of agreement between pairs of readings were shown ( W = 0.75–0.79, p 0.001) for both pairs of reading conditions. On an individual reader level, two readers demonstrated significant decreases ( p 0.05) in ROI FOMs when prior images were unavailable. Reading location had an inconsistent impact on individual performance. Conclusion Reading location and availability of prior images did not significantly alter group performance.

Aim: To investigate the level of agreement between test sets and actual clinical reading Background: The performance of screen readers in detecting breast cancer is being assessed in some countries by using mammographic test sets. However, previous studies have provided little evidence that performance assessed by test sets strongly correlate to performance in clinical reading. Methods: Five clinicians from BreastScreen New South Wales participated in this study. Each clinician was asked to read 200 de-identified mammographic examinations gathered from their own case history within the BreastScreen NSW Digital Imaging Library. All test sets were designed with specific proportions of true positive, true negative, false positive and false negative examinations from the previous actual clinical reads of each reader. A prior mammogram examination for comparison (when available) was also provided for each case. Results: Preliminary analyses have shown that there is a moderate level of agreement (Kappa 0.42−0.56, p < 0.001) between laboratory test sets and actual clinical reading. In addition, a mean increase of 38% in sensitivity in the laboratory test sets as compared to their actual clinical readings was demonstrated. Specificity is similar between the laboratory test sets and actual clinical reading. Conclusion: This study demonstrated a moderate level of agreement between actual clinical reading and test set reading, which suggests that test sets have a role in reflecting clinical performance.

It is well accepted that collimation is a cost-effective dose-reducing tool for X-ray examinations. This phantom-based study investigated the impact of X-ray beam collimation on radiation dose to the lenses of the eyes and thyroid along with the effect on image quality in facial bone radiography.A three-view series (occipitomental, occipitomental 30 and lateral) was investigated, and radiation doses to the lenses and thyroid were measured using an Unfors dosemeter. Images were assessed by six experienced observers using a visual grading analysis and a total of 5400 observations were made.Strict collimation significantly (p0.0001) reduced the radiation dose to the lenses of the eyes and thyroid when using a fixed projection-specific exposure. With a variable exposure technique (fixed exit dose, to simulate the behaviour of an automatic exposure control), while strict collimation was again shown to reduce thyroid dose, higher lens doses were demonstrated when compared with larger fields of exposure. Image quality was found to significantly improve using strict collimation, with observer preference being demonstrated using visual grading characteristic curves.The complexities of optimising radiographic techniques have been shown and the data presented emphasise the importance of examining dose-reducing strategies in a comprehensive way.

The purpose of this article is to review the limitations associated with current methods of assessing reader accuracy in mammography screening programmes. Clinical audit is commonly used as a quality-assurance tool to monitor the performance of screen readers; however, a number of the metrics employed, such as recall rate as a surrogate for specificity, do not always accurately measure the intended clinical feature. Alternatively, standardized screening test sets, which benefit from ease of application, immediacy of results, and quicker assessment of quality improvement plans, suffer from experimental confounders, thus questioning the relevance of these laboratory-type screening test sets to clinical performance. Four key factors that impact on the external validity of screening test sets were identified: the nature and extent of scrutiny of one's action, the artificiality of the environment, the over-simplification of responses, and prevalence of abnormality. The impact of these factors on radiological and other contexts is discussed, and although it is important to acknowledge the benefit of standardized screening test sets, issues relating to the relevance of test sets to clinical activities remain. The degree of correlation between performance based on real-life clinical audit and performances at screen read test sets must be better understood and specific causal agents for any lack of correlation identified.

Objective: The aim of this investigation is to determine the impact of hybrid single photon emission computed tomography/computed tomography (SPECT/CT) on the detection of parathyroid adenoma. Materials and methods: 16 patients presented with suspected parathyroid adenoma localised within the neck. All patients were injected with Tc-99m sestamibi and were scanned with a GE Infinia Hawkeye SPECT/CT. There were six negative and ten positive confirmed cases. Five expert radiologists specializing in nuclear medicine were asked to report on the 16 planar and SPECT data sets and were then asked to report on the same randomly ordered data sets with the addition of CT. Receiver operating characteristic (ROC) analysis was performed using the Dorfman-Berbaum-Metz multireadermulticase methodology and sensitivity and specificity values were generated. A significance level of p ≤ 0.05 was set for all comparisons. Results: ROC analysis demonstrated an AUC of 0.64 and 0.69 for SPECT and SPECT/CT respectively (p = 0.31). Mean sensitivity scores increased from 0.64 to 0.80 (p = 0.17) and specificity scores decreased from 0.57 to 0.40 (p = 0.17) with the addition of the CT data. Conclusion: This preliminary investigation suggests that extra CT information may increase lesion detection as well as false positive rates for SPECT-based investigations of a single parathyroid adenoma. However the difference in diagnostic efficacy between the two groups was not found to be statistically significant therefore requiring further investigation. These findings have implications beyond the clinical situation described here.

The neuroanatomy and physiology of the baboon brain closely resembles that of the human brain and is well suited for evaluating promising new radioligands in non-human primates by PET and SPECT prior to their use in humans. These studies are commonly performed on clinical scanners with 5 mm spatial resolution at best, resulting in sub-optimal images for quantitative analysis. This study assessed the feasibility of using a microPET animal scanner to image the brains of large non-human primates, i.e. papio hamadryas (baboon) at high resolution. Factors affecting image accuracy, including scatter, attenuation and spatial resolution, were measured under conditions approximating a baboon brain and using different reconstruction strategies. Scatter fraction measured 32% at the centre of a 10 cm diameter phantom. Scatter correction increased image contrast by up to 21% but reduced the signal-to-noise ratio. Volume resolution was superior and more uniform using maximum a posteriori (MAP) reconstructed images (3.2-3.6 mm(3) FWHM from centre to 4 cm offset) compared to both 3D ordered subsets expectation maximization (OSEM) (5.6-8.3 mm(3)) and 3D reprojection (3DRP) (5.9-9.1 mm(3)). A pilot (18)F-2-fluoro-2-deoxy-d-glucose ([(18)F]FDG) scan was performed on a healthy female adult baboon. The pilot study demonstrated the ability to adequately resolve cortical and sub-cortical grey matter structures in the baboon brain and improved contrast when images were corrected for attenuation and scatter and reconstructed by MAP. We conclude that high resolution imaging of the baboon brain with microPET is feasible with appropriate choices of reconstruction strategy and corrections for degrading physical effects. Further work to develop suitable correction algorithms for high-resolution large primate imaging is warranted.

To determine the effect of noise-reducing innovation-precision imaging (PI)-on image quality and diagnostic efficacy in breast ultrasound.The study, which assessed four levels of PI from zero to three, consisted of two parts: image quality assessment and diagnostic efficacy evaluation. For the first part, 247 sets of ultrasound images displayed at each PI level were evaluated by 6 experienced breast imaging observers, by rating image quality using visual grading analysis on a 1-4 scale. For the diagnostic efficacy part 51 breast lesions were displayed at each PI level and scored 1-6 to generate a receiver operating characteristic (ROC) curve. These images were evaluated by radiologists and sonographers. Analyses were performed using non-parametric Friedman and Wilcoxon signed rank tests and a multireader multicase methodology.Statistically, higher scores of image quality were observed with increased levels of PI than with the zero setting (p 0.001). The ROC analysis did not demonstrate any significant change in diagnostic efficacy, with mean scores for all observers being 0.79, 0.80, 0.81 and 0.81 for settings zero, one, two and three, respectively.This study suggested a perceived improvement in image quality with increasing levels of PI; however, no changes in diagnostic efficacy were noted. The importance of looking at the impact of new imaging technologies in a multifaceted way is emphasized.To our knowledge, this is the first article investigating the impact of the PI algorithm on ultrasound image quality and breast lesion characterization.

MicroPET is a high resolution positron emission tomograph designed to image rodents or small primates. However, the ability to image multiple rodents simultaneously or larger primates such as the baboon would expand its potential applications. The aim of this study was to investigate the factors that determine the feasibility of using the full field of view of the microPET scanner with these applications in mind. Spatial resolution, scatter fraction and attenuation were measured throughout the FOV using phantoms to simulate multiple mice or a large primate brain. The results showed that it is feasible to image larger subjects using the microPET scanner. However, the increased attenuation and scatter require corrections for these effects to ensure quantitatively accurate imaging studies. The variation in spatial resolution throughout the FOV will also need to be taken into account when interpreting the images and methods to improve the uniformity of resolution require further development and evaluation.

In diabetes, extended adrenergic receptor stimulation with hyperglycemia and insulin deficiency is associated with cardiac autonomic dysfunction. Clinically evident diabetic cardiac autonomic neuropathy (CAN) is associated with a poor prognosis. Research studies indicate that autonomic function tests, which are traditionally used to diagnose diabetic CAN, are less sensitive than (123)I-metaiodobenzylguanidine (MIBG) imaging, particularly in the early stages of the disease. This established imaging technique makes use of the noradrenaline analog MIBG, which is radiolabeled with (123)I to assess the noradrenaline uptake-1 mechanism of the sympathetic nervous system. Although scintigraphic studies indicate that long-standing cardiac autonomic dysfunction is permanent, some authors have shown partial reversibility with early metabolic intervention. (123)I-MIBG imaging could therefore have an important clinical role to play in the early diagnosis and treatment monitoring of diabetic CAN.A PubMed/MEDLINE Internet search was performed using MIBG, diabetes, and cardiac autonomic neuropathy as key words.The general expense of (123)I-MIBG imaging, together with the lack of commercial availability of this radiopharmaceutical in the United States, has limited the clinical use of this technique. As such, the clinical role of (123)I-MIBG imaging in the early diagnosis of diabetic CAN has yet to be validated and defined in most regions of the world, and further study is required.

We have designed and constructed a small animal SPECT system based on compact, high resolution detectors and multipinhole apertures. The scanner is currently configured with two detectors mounted on a rotating gantry. Each detector comprises a NaI(Tl) crystal array (1/spl times/1/spl times/5 mm elements), a 12 cm diameter position-sensitive photomultiplier tube (Hamamatsu R3292) and a tungsten aperture with 1 or more pinholes. In this study, we performed phantom experiments to characterise the planar and tomographic performance of the scanner. Intrinsic resolution measured with a highly collimated /sup 99m/Tc point source stepped across the detector face was 1.0 /spl plusmn/ 0;.1 FWHM and 2.9 /spl plusmn/ 0.1 mm FWTM. Energy resolution at 140 keV varied from 14% FWIIM for central crystals to 19% for edge crystals and was 20% FWHM for the whole detector normalised spectrum. Intrinsic uniformity for the central field of view was 2.4% differential and 3.8% integral. Reconstructed spatial resolution was 1.2 mm FWHM at the centre of the field of view and 1.2, 1.7 mm FWHM (radial, tangential) at 10 mm off-axis, using typical geometric parameters for mouse and rat brain imaging. Reconstructed images of a micro deluxe hot rod phantom demonstrate the high resolution of the system and indicate similar resolution and improved signal-to-noise is obtained with a 2 pinhole aperture compared with a single pinhole. We conclude that the performance characteristics of this system make it suitable for high resolution imaging of small laboratory animals.

We have previously demonstrated the potential of multi-pinhole coded apertures when used in conjunction with compact pixelated detectors for high resolution, high sensitivity small animal SPECT. We are now constructing a prototype SPECT system with three such detectors mounted on a rotating gantry. Each detector comprises a 12 cm diameter circular array of NaI(Tl) crystals, each 1/spl times/1/spl times/5 mm/sup 3/ on a 1.25 mm pitch. Each crystal array is coupled to a Hamamatsu R3292 12.7 cm diameter PS-PMT. Signals from the three detectors are read out by a custom subtractive resistive circuit and multiplexed. Data are acquired by a National Instruments PCI-6110E board running on a Macintosh dual processor G4 computer under Kmax software control. Images are reconstructed using a 3D iterative MAP-EM algorithm based on a multi-pinhole forward and back projector. The trade-off between contrast and variance was studied by simulation for a number of multi-pinhole configurations. We also performed initial planar imaging studies on one of the detectors. Our results indicate that spatial resolution approaching 1 mm full width at half maximum (FWHM) can be achieved with a single pinhole aperture, while improved contrast and variance may result from using optimized multi-pinhole apertures.