The best scatter mitigation strategy was found to be a combination of a grid with selectivity larger than 9, combined with iterative scatter estimation. None of the investigated grids required increasing the imaging dose. The PSF determined using proposed method leads to image quality improvements results for all but one of the investigated scenarios.

The evaluated fiber-interspaced grid improved the image quality of the CBCT system for broad range of imaging conditions. Clinical scans show significant improvement in soft tissue visibility and uniformity without the need to increase the imaging dose.

Four dimensional cone beam computed tomography (4DCBCT) is an image guidance strategy used for patient positioning in radiotherapy. In conventional implementations of 4DCBCT, a constant gantry speed and a constant projection pulse rate are used. Unfortunately, this leads to higher imaging doses than are necessary because a large number of redundant projections are acquired. In theoretical studies, we have previously demonstrated that by suppressing redundant projections the imaging dose can be reduced by 40-50% for a majority of patients with little reduction in image quality. The aim of this study was to experimentally realise the projection suppression technique, which we have called Respiratory Triggered 4DCBCT (RT-4DCBCT). A real-time control system was developed that takes the respiratory signal as input and computes whether to acquire, or suppress, the next projection trigger during 4DCBCT acquisition. The CIRS dynamic thorax phantom was programmed with a 2 cm peak-to-peak motion and periods ranging from 2 to 8 s. Image quality was assessed by computing the edge response width of a 3 cm imaging insert placed in the phantom as well as the signal to noise ratio of the phantoms tissue and the contrast to noise ratio between the phantoms lung and tissue. The standard deviation in the superior-inferior direction of the 3 cm imaging insert was used to assess intra-phase bin displacement variations with a higher standard deviation implying more motion blur. The 4DCBCT imaging dose was reduced by 8.6%, 41%, 54%, 70% and 77% for patients with 2, 3, 4, 6 and 8 s breathing periods respectively when compared to conventional 4DCBCT. The standard deviation of the intra-phase bin displacement variation of the 3 cm imaging insert was reduced by between 13% and 43% indicating a more consistent position for the projections within respiratory phases. For the 4 s breathing period, the edge response width was reduced by 39% (0.8 mm) with only a 6-7% decrease in the signal to noise and contrast to noise ratios. RT-4DCBCT has been experimentally realised and reduced to practice on a linear accelerator with a measurable imaging dose reductions over conventional 4DCBCT and little degradation in image quality.

Background and purpose: The scatter induced image quality degradation of cone-beam computed tomography (CBCT) prevents more advanced applications in radiotherapy. We evaluated the dose calculation accuracy on CBCT of various disease sites using different scatter mitigation strategies. Materials and methods: CBCT scans of two patient cohorts (C1, C2) were reconstructed using a uniform (USC) and an iterative scatter correction (ISC) method, combined with an anti-scatter grid (ASG). Head and neck (H&N), lung, pelvic region, and prostate patients were included. To achieve a high accuracy Hounsfield unit and physical density calibrations were performed. The dose distributions of the original treatment plans were analyzed with the γ evaluation method using criteria of 1%/2 mm using the planning CT as the reference. The investigated parameters were the mean γ (γ mean ), the points in agreement (P γ≤1 ) and the 99th percentile (γ 1% ). Results: Significant differences between USC and ISC in C1 were found for the lung and prostate, where the latter using the ISC produced the best results with medians of 0.38, 98%, and 1.1 for γ mean , P γ≤1 and γ 1% , respectively. For C2 the ISC with ASG showed an improvement for all imaging sites. The lung demonstrated the largest relative increase in accuracy with improvements between 48% and 54% for the medians of γ mean , P γ≤1 and γ 1% . Conclusions: The introduced method demonstrated high dosimetric accuracy for H&N, prostate and pelvic region if an ASG is applied. A significantly lower accuracy was seen for lung. The ISC yielded a higher robustness against scatter variations than the USC.

Abstract. Four dimensional cone beam computed tomography (4DCBCT) uses a constant gantry speed and imaging frequency that are independent of the patient's breathing rate. Using a technique called Respiratory Motion Guided 4DCBCT (RMG-4DCBCT), we have previously demonstrated that by varying the gantry speed and imaging frequency, in response to changes in the patient's real-time respiratory signal, the imaging dose can be reduced by 50-70%. RMG-4DCBCT optimally computes a patient specific gantry trajectory to eliminate streaking artefacts and projection clustering that is inherent in 4DCBCT imaging. The gantry trajectory is continuously updated as projection data is acquired and the patient's breathing changes. The aim of this study was to realise RMG-4DCBCT for the first time on a linear accelerator.To change the gantry speed in real-time a potentiometer under microcontroller control was used to adjust the current supplied to an Elekta Synergy's gantry motor. A real-time feedback loop was developed on the microcontroller to modulate the gantry speed and projection acquisition in response to the real-time respiratory signal so that either 40, RMG-4DCBCT 40 , or 60, RMG-4DCBCT 60 , uniformly spaced projections were acquired in 10 phase bins. Images of the CIRS dynamic Thorax phantom were acquired with sinusoidal breathing periods ranging from 2s to 8s together with two breathing traces from lung cancer patients. Image quality was assessed using the contrast to noise ratio (CNR) and edge response width (ERW).For the average patient, with a 3.8s breathing period, the imaging time and image dose were reduced by 37% and 70% respectively. Across all respiratory rates, RMG-4DCBCT 40 had a CNR in the range of 6.5 to 7.5, and RMG-4DCBCT 60 had a CNR between 8.7 and 9.7, indicating that RMG-4DCBCT allows consistent and controllable CNR. In comparison, the CNR for conventional 4DCBCT drops from 20.4 to 6.2 as the breathing rate increases from 2s to 8s. With RMG-4DCBCT, the ERW in the direction of motion of the imaging insert decreases from 2.1mm to 1.1mm as the breathing rate increases from 2s to 8s while for conventional 4DCBCT the ERW increases from 1.9mm to 2.5mm.Image quality can be controlled during 4DCBCT acquisition by varying the gantry speed and the projection acquisition in response to the patient's real-time respiratory signal. However, although the image sharpness, i.e., ERW, is improved with RMG-4DCBCT, the ERW depends on the patient's breathing rate and breathing regularity.