Samuel R. Mazin

Inverse-geometry volumetric CT system with multiple detector arrays for wide field-of-view imaging

Current volumetric computed tomography (CT) methods require seconds to acquire a thick volume (>8 cm) with high resolution. Inverse-geometry CT (IGCT) is a new system geometry under investigation that is anticipated to be able to image a thick volume in a single gantry rotation with isotropic resolution and no cone-beam artifacts. IGCT employs a large array of source spots opposite a smaller detector array. The in-plane field of view (FOV) is primarily determined by the size of the source array, in much the same way that the FOV is determined by the size of the detector array in a conventional CT system. Thus, the size of the source array can be a limitation on the achievable FOV. We propose adding additional detector arrays, spaced apart laterally, to increase the in-plane FOV while still using a modestly sized source array. We determine optimal detector placement to maximize the FOV while obtaining relatively uniform sampling. We also demonstrate low wasted radiation of the proposed system through design and simulation of a pre-patient collimator. Reconstructions from simulated projection data show no artifacts when combining the data from the detector arrays. Finally, to demonstrate feasibility of the concept, an anthropomorphic thorax phantom containing a porcine heart was scanned on a prototype table-top system. The reconstructed axial images demonstrate a 45 cm in-plane FOV using a 23 cm source array.

Emission guided radiation therapy for lung and prostate cancers: a feasibility study on a digital patient.

PURPOSE Accurate tumor tracking remains a challenge in current radiation therapy. Many strategies including image guided radiation therapy alleviate the problem to certain extents. The authors propose a new modality called emission guided radiation therapy (EGRT) to accurately and directly track the tumor based on its biological signature. This work is to demonstrate the feasibility of EGRT under two clinical scenarios using a 4D digital patient model. METHODS EGRT uses lines of response (LORs) from positron emission events to direct beamlets of therapeutic radiation through the emission sites inside a tumor. This is accomplished by a radiation delivery system consisting of a Linac and positron emission tomography (PET) detectors on a fast rotating closed-ring gantry. During the treatment of radiotracer-administrated cancer patients, PET detectors collect LORs from tumor uptake sites and the Linac responds in nearly real-time with beamlets of radiation along the same LOR paths. Moving tumors are therefore treated with a high targeting accuracy. Based on the EGRT concept, the authors design a treatment method with additional modulation algorithms including attenuation correction and an integrated boost scheme. Performance is evaluated using simulations of a lung tumor case with 3D motion and a prostate tumor case with setup errors. The emission process is simulated by Geant4 Application for Tomographic Emission package (GATE) and Linac dose delivery is simulated using a voxel-based Monte Carlo algorithm (VMC++). RESULTS In the lung case with attenuation correction, compared to a conventional helical treatment, EGRT achieves a 41% relative increase in dose to 95% of the gross tumor volume (GTV) and a 55% increase to 50% of the GTV. All dose distributions are normalized for the same dose to the lung. In the prostate case with the integrated boost and no setup error, EGRT yields a 19% and 55% relative dose increase to 95% and 50% of the GTV, respectively, when all methods are normalized for the same dose to the rectum. In the prostate case with integrated boost where setup error is present, EGRT contributes a 21% and 52% relative dose increase to 95% and 50% of the GTV, respectively. CONCLUSIONS As a new radiation therapy modality with inherent tumor tracking, EGRT has the potential to substantially improve targeting in radiation therapy in the presence of intrafractional and interfractional motion.

A comparison of four algorithms for metal artifact reduction in CT imaging

Streak artifacts caused by the presence of metal have been a significant problem in CT imaging since its inception in 1972. With the fast evolving medical device industry, the number of metal objects implanted in patients is increasing annually. This correlates directly with an increased likelihood of encountering metal in a patient CT scan, thus necessitating the need for an effective and reproducible metal artifact reduction (MAR) algorithm. Previous comparisons between MAR algorithms have typically only evaluated a small number of patients and a limited range of metal implants. Although the results of many methods are promising, the reproducibility of these results is key to providing more tangible evidence of their effectiveness. This study presents a direct comparison between the performances, assessed by board certified radiologists, of four MAR algorithms: 3 non-iterative and one iterative method, all applied and compared to the original clinical images. The results of the evaluation indicated a negative mean score in almost all uses for two of the non-iterative methods, signifying an overall decrease in the diagnostic quality of the images, generally due to perceived loss of detail. One non-iterative algorithm showed a slight improvement. The iterative algorithm was superior in all studies by producing a considerable improvement in all uses.

Fourier rebinning algorithm for inverse geometry CT

Inverse geometry computed tomography (IGCT) is a new type of volumetric CT geometry that employs a large array of x-ray sources opposite a smaller detector array. Volumetric coverage and high isotropic resolution produce very large data sets and therefore require a computationally efficient three-dimensional reconstruction algorithm. The purpose of this work was to adapt and evaluate a fast algorithm based on Defrises Fourier rebinning (FORE), originally developed for positron emission tomography. The results were compared with the average of FDK reconstructions from each source row. The FORE algorithm is an order of magnitude faster than the FDK-type method for the case of 11 source rows. In the center of the field-of-view both algorithms exhibited the same resolution and noise performance. FORE exhibited some resolution loss (and less noise) in the periphery of the field-of-view. FORE appears to be a fast and reasonably accurate reconstruction method for IGCT.

Reduction of truncation artifacts in rapid 3D articular cartilage imaging.

To reduce Gibbs ringing artifact in three‐dimensional (3D) articular knee cartilage imaging with linear prediction (LP).

Geometry analysis of an inverse-geometry volumetric CT system with multiple detector arrays

An inverse-geometry volumetric CT (IGCT) system for imaging in a single fast rotation without cone-beam artifacts is being developed. It employs a large scanned source array and a smaller detector array. For a single-source/single-detector implementation, the FOV is limited to a fraction of the source size. Here we explore options to increase the FOV without increasing the source size by using multiple detectors spaced apart laterally to increase the range of radial distances sampled. We also look at multiple source array systems for faster scans. To properly reconstruct the FOV, Radon space must be sufficiently covered and sampled in a uniform manner. Optimal placement of the detectors relative to the source was determined analytically given system constraints (5cm detector width, 25cm source width, 45cm source-to-isocenter distance). For a 1x3 system (three detectors per source) detector spacing (DS) was 18deg and source-to-detector distances (SDD) were 113, 100 and 113cm to provide optimum Radon sampling and a FOV of 44cm. For multiple-source systems, maximum angular spacing between sources cannot exceed 125deg since detectors corresponding to one source cannot be occluded by a second source. Therefore, for 2x3 and 3x3 systems using the above DS and SDD, optimum spacing between sources is 115deg and 61deg respectively, requiring minimum scan rotations of 115deg and 107deg. Also, a 3x3 system can be much faster for full 360deg dataset scans than a 2x3 system (120deg vs. 245deg). We found that a significantly increased FOV can be achieved while maintaining uniform radial sampling as well as a substantial reduction in scan time using several different geometries. Further multi-parameter optimization is underway.

The potential of positron emission tomography for intratreatment dynamic lung tumor tracking: A phantom study

PURPOSE This study aims to evaluate the potential and feasibility of positron emission tomography for dynamic lung tumor tracking during radiation treatment. The authors propose a center of mass (CoM) tumor tracking algorithm using gated-PET images combined with a respiratory monitor and investigate the geometric accuracy of the proposed algorithm. METHODS The proposed PET dynamic lung tumor tracking algorithm estimated the target position information through the CoM of the segmented target volume on gated PET images reconstructed from accumulated coincidence events. The information was continuously updated throughout a scan based on the assumption that real-time processing was supported (actual processing time at each frame ≈ 10 s). External respiratory motion and list-mode PET data were acquired from a phantom programmed to move with measured respiratory traces (external respiratory motion and internal target motion) from human subjects, for which the ground truth target position was known as a function of time. The phantom was cylindrical with six hollow sphere targets (10, 13, 17, 22, 28, and 37 mm in diameter). The measured respiratory traces consisted of two sets: (1) 1D-measured motion from ten healthy volunteers and (2) 3D-measured motion from four lung cancer patients. The authors evaluated the geometric accuracy of the proposed algorithm by quantifying estimation errors (Euclidean distance) between the actual motion of targets (1D-motion and 3D-motion traces) and CoM trajectories estimated by the proposed algorithm as a function of time. RESULTS The time-averaged error of 1D-motion traces over all trajectories of all targets was 1.6 mm. The error trajectories decreased with time as coincidence events were accumulated. The overall error trajectory of 1D-motion traces converged to within 2 mm in approximately 90 s. As expected, more accurate results were obtained for larger targets. For example, for the 37 mm target, the average error over all 1D-motion traces was 1.1 mm; and for the 10 mm target, the average error over all 1D-motion traces was 2.8 mm. The overall time-averaged error of 3D-motion traces was 1.6 mm, which was comparable to that of the 1D-motion traces. There were small variations in the errors between the 3D-motion traces, although the motion trajectories were very different. The accuracy of the estimates was consistent for all targets except for the smallest. CONCLUSIONS The authors developed an algorithm for dynamic lung tumor tracking using list-mode PET data and a respiratory motion signal, and demonstrated proof-of-principle for PET-guided lung tumor tracking. The overall tracking error in phantom studies is less than 2 mm.

Emission-Guided Radiation Therapy: Biologic Targeting and Adaptive Treatment

a i r m i P s s h d he past decade has witnessed sigificant growth in applications of tereotactic body radiation therapy SBRT), which involves delivering umoricidal doses of radiation in as ew as 1 to 5 fractions to tumors in he head and neck, lung, liver, panreas, spine, and prostate, with very igh (80%-98%) local control rates eported in a variety of studies. The ncrease in SBRT procedures can be argely attributed to the advent of n-board localization technologies hat include single and stereoscoic x-ray imaging, kilovoltage and egavoltage CT imaging, implantble fiducial transponders, ultraound imaging, and others. Onoard MRI radiotherapy systems re also emerging so that the covted soft tissue contrast of MRI ay be exploited for localization. n addition to target localization, n-board MRI has the potential to ore accurately estimate the dose elivered to sensitive organs during ach treatment. Stereotactic body radiation therpy requires precise localization beause of the very high doses (6-30 y) delivered for each fraction. merican Association of Physicists n Medicine Task Group 101 reently published a report on its recmmendations for SBRT practice 1]. In this report, it was emphaized that SBRT “requires a high evel of confidence in the accuracy f the entire treatment delivery proess.” Therefore, it is essential to

Toward a planning scheme for emission guided radiation therapy (EGRT): FDG based tumor tracking in a metastatic breast cancer patient.

PURPOSE Emission guided radiation therapy (EGRT) is a new modality that uses PET emissions in real-time for direct tumor tracking during radiation delivery. Radiation beamlets are delivered along positron emission tomography (PET) lines of response (LORs) by a fast rotating ring therapy unit consisting of a linear accelerator (Linac) and PET detectors. The feasibility of tumor tracking and a primitive modulation method to compensate for attenuation have been demonstrated using a 4D digital phantom in our prior work. However, the essential capability of achieving dose modulation as in conventional intensity modulated radiation therapy (IMRT) treatments remains absent. In this work, the authors develop a planning scheme for EGRT to accomplish sophisticated intensity modulation based on an IMRT plan while preserving tumor tracking. METHODS The planning scheme utilizes a precomputed LOR response probability distribution to achieve desired IMRT planning modulation with effects of inhomogeneous attenuation and nonuniform background activity distribution accounted for. Evaluation studies are performed on a 4D digital patient with a simulated lung tumor and a clinical patient who has a moving breast cancer metastasis in the lung. The Linac dose delivery is simulated using a voxel-based Monte Carlo algorithm. The IMRT plan is optimized for a planning target volume (PTV) that encompasses the tumor motion using the MOSEK package and a Pinnacle3™ workstation (Philips Healthcare, Fitchburg, WI) for digital and clinical patients, respectively. To obtain the emission data for both patients, the Geant4 application for tomographic emission (GATE) package and a commercial PET scanner are used. As a comparison, 3D and helical IMRT treatments covering the same PTV based on the same IMRT plan are simulated. RESULTS 3D and helical IMRT treatments show similar dose distribution. In the digital patient case, compared with the 3D IMRT treatment, EGRT achieves a 15.1% relative increase in dose to 95% of the gross tumor volume (GTV) and a 31.8% increase to 50% of the GTV. In the patient case, EGRT yields a 15.2% relative increase in dose to 95% of the GTV and a 20.7% increase to 50% of the GTV. The organs at risk (OARs) doses are kept similar or lower for EGRT in both cases. Tumor tracking is observed in the presence of planning modulation in all EGRT treatments. CONCLUSIONS As compared to conventional IMRT treatments, the proposed EGRT planning scheme allows an escalated target dose while keeping dose to the OARs within the same planning limits. With the capabilities of incorporating planning modulation and accurate tumor tracking, EGRT has the potential to greatly improve targeting in radiation therapy and enable a practical and effective implementation of 4D radiation therapy for planning and delivery.

SU‐GG‐J‐03: Emission Guided Radiation Therapy System: A Feasibility Study

Purpose: To evaluate the feasibility of a combined PET‐Linac system for real‐time guidance of radiation therapy for tumors that move due to respiration. Method and Materials: We are developing a treatment system that will simultaneously deliver radiation during PET acquisition. A method to compensate for respiratory motion will be to deliver radiation beam‐lets along individual PET lines‐of‐response (LORs) as they are detected. The system involves rotating a radiationsource and PET detectors on a gantry while dynamically controlling a binary multi‐leaf collimator to deliver the beam‐let responses in a 2D helical mode. Simulations were conducted using GATE to model 300 seconds of PET list‐mode acquisition of a ‘hot’ 3 cm diameter tumor exhibiting 3.7 second periodic 2 cm peak‐to‐peak motion in a ‘warm’ background. A 5 cm planning target volume (PTV) was used as a filter to reject LORs that did not intersect this volume. LORs that intersected the PTV and whose timestamps were within a 500 ms cutoff window were responded to. A voxel‐based Monte Carlo simulation package was used to model the resultant dose distributions comparing the emission guided (EGRT) method with uniform coverage of the PTV. Results: Composite dose volume histograms were calculated using 10 phases of the motion cycle. Dose to the non‐tumor volume was normalized to the same mean value for both scenarios. The EGRT approach exhibited a non‐uniform dose distribution to the tumor compared to uniform PTV coverage. However, even with the non‐uniformity, there was a 30% relative increase in minimum dose to the tumor volume for the EGRT approach. Conclusion: Although non‐uniform dose delivery to the tumor volume needs to be addressed, the feasibility of using PET to guide radiation delivery in real‐time has been demonstrated. Conflict of Interest: SRM and ASN are co‐founders of a company commercializing emission guided radiation therapy.