Objectives: The purpose of this study was to develop and validate a computer model to produce realistic simulated computed radiography (CR) chest images using CT data sets of real patients. Methods: Anatomical noise, which is the limiting factor in determining pathology in chest radiography, is realistically simulated by the CT data, and frequency-dependent noise has been added post-digitally reconstructed radiograph (DRR) generation to simulate exposure reduction. Realistic scatter and scatter fractions were measured in images of a chest phantom acquired on the CR system simulated by the computer model and added post-DRR calculation. Results: The model has been validated with a phantom and patients and shown to provide predictions of signal-to-noise ratios (SNRs), tissue-to-rib ratios (TRRs: a measure of soft tissue pixel value to that of rib) and pixel value histograms that lie within the range of values measured with patients and the phantom. The maximum difference in measured SNR to that calculated was 10%. TRR values differed by a maximum of 1.3%. Conclusion: Experienced image evaluators have responded positively to the DRR images, are satisfied they contain adequate anatomical features and have deemed them clinically acceptable. Therefore, the computer model can be used by image evaluators to grade chest images presented at different tube potentials and doses in order to optimise image quality and patient dose for clinical CR chest radiographs without the need for repeat patient exposures.

Objective: The purpose of this study was to examine the correlation between the quality of visually graded patient (clinical) chest images and a quantitative assessment of chest phantom (physical) images acquired with a computed radiography (CR) imaging system. Methods:The results of a previously published study, in which four experienced image evaluators graded computer-simulated posteroanterior chest images using a visual grading analysis scoring (VGAS) scheme, were used for the clinical image quality measurement. Contrast-to-noise ratio (CNR) and effective dose efficiency (eDE) were used as physical image quality metrics measured in a uniform chest phantom. Although optimal values of these physical metrics for chest radiography were not derived in this work, their correlation with VGAS in images acquired without an antiscatter grid across the diagnostic range of X-ray tube voltages was determined using Pearson's correlation coefficient.Results: Clinical and physical image quality metrics increased with decreasing tube voltage. Statistically significant correlations between VGAS and CNR (R50.87, p,0.033) and eDE (R50.77, p,0.008) were observed. Conclusion:Medical physics experts may use the physical image quality metrics described here in quality assurance programmes and

Objectives: The purpose of this study was to derive an optimum radiographic technique for computed radiography (CR) chest imaging using a digitally reconstructed radiograph computer simulator. The simulator is capable of producing CR chest radiographs of adults with various tube potentials, receptor doses and scatter rejection. Methods: Four experienced image evaluators graded images of average and obese adult patients at different potentials (average-sized, n550; obese, n520), receptor doses (n510) and scatter rejection techniques (average-sized, n520; obese, n520). The quality of the images was evaluated using visually graded analysis. The influence of rib contrast was also assessed. Results: For average-sized patients, image quality improved when tube potential was reduced compared with the reference (102 kVp). No scatter rejection was indicated. For obese patients, it has been shown that an antiscatter grid is indicated, and should be used in conjunction with as low a tube potential as possible (while allowing exposure times ,20 ms). It is also possible to reduce receptor air kerma by 50% without adversely influencing image quality. Rib contrast did not interfere at any tube potential. Conclusions: A virtual clinical trial has been performed with simulated chest CR images. Results indicate that low tube potentials (,102 kVp) are optimal for average and obese adults, the former acquired without scatter rejection, the latter with an antiscatter grid. Lower receptor (and therefore patient doses) than those used clinically are possible while maintaining adequate image quality.

The development of size-based CBCT protocols that use the planning CT scan (with AEC) to determine which protocol is appropriate ensures adequate image quality whilst minimizing patient radiation dose.

Higher doses are delivered over a short duration for stereotactic ablative radiotherapy (SABR) and as a result individual fraction times are significantly higher compared with conventional radiotherapy. Furthermore, many lung SABR patients are elderly with associated co-morbidities and may not be able to retain their treatment position adequately. These patients benefit from faster treatment deliveries which can be achieved by using flattening filter free (FFF) beams. To determine a clinically appropriate FFF energy for accurate delivery, 15 previously delivered flattened 6 MV lung SABR plans were re-planned at 6 FFF and 10 FFF, with organ at risk (OAR) and target dose-volume statistics examined for significance. A two half arc technique, the Monitor Unit Objective Function and the AcurosXB algorithm were employed within the Eclipse TM planning system (V11, Varian Medical System). The deliverability of these FFF plans was verified by physical measurement on a TrueBeam TM (V2.5, Varian Medical System) using the Compass TM dosimetry system (V3.1, IBA Dosimetry) in addition to the usual treatment planning system comparisons. Acceptable plans were produced for all beam energies. 6 FFF provided statistically significant OAR sparing compared to 6 FF and 10 FFF. However, absolute dose differences were not clinically significant and doses were well within recommended clinical tolerances. Skin sparing was superior in the 10 FFF plans. Overall, reduction in treatment delivery time of 61% and 55% was found when using 10 FFF and 6 FFF respectively compared to 6 FF. A 15% reduction in the average treatment time was achieved with 10 FFF when compared to 6 FFF. Treatment delivery verification measurements were compared with clinically delivered 6 FF plans and no significant differences in the deliverability were seen between the plans. As a result of this study 10 FFF has been implemented for SABR lung planning locally.

Given the increasing use of computed tomography (CT) in the UK over the last 30 years, it is essential to ensure that all imaging protocols are optimised to keep radiation doses as low as reasonably practicable, consistent with the intended clinical task. However, the complexity of modern CT equipment can make this task difficult to achieve in practice. Recent results of local patient dose audits have shown discrepancies between two Philips CT scanners that use the DoseRight 2.0 automatic exposure control (AEC) system in the 'automatic' mode of operation. The use of this system can result in drifting dose and image quality performance over time as it is designed to evolve based on operator technique. The purpose of this study was to develop a practical technique for configuring examination protocols on four CT scanners that use the DoseRight 2.0 AEC system in the 'manual' mode of operation. This method used a uniform phantom to generate reference images which form the basis for how the AEC system calculates exposure factors for any given patient. The results of this study have demonstrated excellent agreement in the configuration of the CT scanners in terms of average patient dose and image quality when using this technique. This work highlights the importance of CT protocol harmonisation in a modern Radiology department to ensure both consistent image quality and radiation dose. Following this study, the average radiation dose for a range of CT examinations has been reduced without any negative impact on clinical image quality.

Effective detective quantum efficiency (eDQE) describes the resolution and noise properties of an imaging system along with scatter and primary transmission, all measured under clinically appropriate conditions. Effective dose efficiency (eDE) is the eDQE normalised to mean glandular dose and has been proposed as a useful metric for the optimisation of clinical imaging systems. The aim of this study was to develop a methodology for measuring eDQE and eDE on a Philips microdose mammography (MDM) L30 photon counting scanning system, and to compare performance with two conventional flat panel systems. A custom made lead-blocker was manufactured to enable the accurate determination of dose measurements, and modulation transfer functions were determined free-in-air at heights of 2, 4 and 6 cm above the breast support platform. eDQE were calculated for a Philips MDM L30, Hologic Dimensions and Siemens Inspiration digital mammography system for 2, 4 and 6 cm thick poly(methyl methacrylate) (PMMA). The beam qualities (target/filter and kilovoltage) assessed were those selected by the automatic exposure control, and anti-scatter grids were used where available. Measurements of eDQE demonstrate significant differences in performance between the slit- and scan-directions for the photon counting imaging system. MTF has been shown to be the limiting factor in the scan-direction, which results in a rapid fall in eDQE at mid-to-high spatial frequencies. A comparison with two flat panel mammography systems demonstrates that this may limit image quality for small details, such as micro-calcifications, which correlates with a more conventional image quality assessment with the CDMAM phantom. eDE has shown the scanning photon counting system offers superior performance for low spatial frequencies, which will be important for the detection of large low contrast masses. Both eDQE and eDE are proposed as useful metrics that should enable optimisation of the Philips MDM L30.

The purpose of this study was to examine the use of three physical image quality metrics in the calibration of an automatic exposure control (AEC) device for chest radiography with a computed radiography (CR) imaging system. The metrics assessed were signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and mean effective noise equivalent quanta (eNEQm), all measured using a uniform chest phantom. Subsequent calibration curves were derived to ensure each metric was held constant across the tube voltage range. Each curve was assessed for its clinical appropriateness by generating computer simulated chest images with correct detector air kermas for each tube voltage, and grading these against reference images which were reconstructed at detector air kermas correct for the constant detector dose indicator (DDI) curve currently programmed into the AEC device. All simulated chest images contained clinically realistic projected anatomy and anatomical noise and were scored by experienced image evaluators. Constant DDI and CNR curves do not appear to provide optimized performance across the diagnostic energy range. Conversely, constant eNEQm and SNR do appear to provide optimized performance, with the latter being the preferred calibration metric given as it is easier to measure in practice. Medical physicists may use the SNR image quality metric described here when setting up and optimizing AEC devices for chest radiography CR systems with a degree of confidence that resulting clinical image quality will be adequate for the required clinical task. However, this must be done with close cooperation of expert image evaluators, to ensure appropriate levels of detector air kerma.