John E. Gillam

Noise evaluation of Compton camera imaging for proton therapy

Compton Cameras emerged as an alternative for real-time dose monitoring techniques for Particle Therapy (PT), based on the detection of prompt-gammas. As a consequence of the Compton scattering process, the gamma origin point can be restricted onto the surface of a cone (Compton cone). Through image reconstruction techniques, the distribution of the gamma emitters can be estimated, using cone-surfaces backprojections of the Compton cones through the image space, along with more sophisticated statistical methods to improve the image quality. To calculate the Compton cone required for image reconstruction, either two interactions, the last being photoelectric absorption, or three scatter interactions are needed. Because of the high energy of the photons in PT the first option might not be adequate, as the photon is not absorbed in general. However, the second option is less efficient. That is the reason to resort to spectral reconstructions, where the incoming γ energy is considered as a variable in the reconstruction inverse problem. Jointly with prompt gamma, secondary neutrons and scattered photons, not strongly correlated with the dose map, can also reach the imaging detector and produce false events. These events deteriorate the image quality. Also, high intensity beams can produce particle accumulation in the camera, which lead to an increase of random coincidences, meaning events which gather measurements from different incoming particles. The noise scenario is expected to be different if double or triple events are used, and consequently, the reconstructed images can be affected differently by spurious data. The aim of the present work is to study the effect of false events in the reconstructed image, evaluating their impact in the determination of the beam particle ranges. A simulation study that includes misidentified events (neutrons and random coincidences) in the final image of a Compton Telescope for PT monitoring is presented. The complete chain of detection, from the beam particle entering a phantom to the event classification, is simulated using FLUKA. The range determination is later estimated from the reconstructed image obtained from a two and three-event algorithm based on Maximum Likelihood Expectation Maximization. The neutron background and random coincidences due to a therapeutic-like time structure are analyzed for mono-energetic proton beams. The time structure of the beam is included in the simulations, which will affect the rate of particles entering the detector.

Simulated one-pass list-mode: an approach to on-the-fly system matrix calculation

In the development of prototype systems for positron emission tomography a valid and robust image reconstruction algorithm is required. However, prototypes often employ novel detector and system geometries which may change rapidly under optimization. In addition, developing systems generally produce highly granular, or possibly continuous detection domains which require some level of on-the-fly calculation for retention of measurement precision. In this investigation a new method of on-the-fly system matrix calculation is proposed that provides advantages in application to such list-mode systems in terms of flexibility in system modeling. The new method is easily adaptable to complicated system geometries and available computational resources. Detection uncertainty models are used as random number generators to produce ensembles of possible photon trajectories at image reconstruction time for each datum in the measurement list. However, the result of this approach is that the system matrix elements change at each iteration in a non-repetitive manner. The resulting algorithm is considered the simulation of a one-pass list (SOPL) which is generated and the list traversed during image reconstruction. SOPL alters the system matrix in use at each iteration and so behavior within the maximum likelihood-expectation maximization algorithm was investigated. A two-pixel system and a small two dimensional imaging model are used to illustrate the process and quantify aspects of the algorithm. The two-dimensional imaging system showed that, while incurring a penalty in image resolution, in comparison to a non-random equal-computation counterpart, SOPL provides much enhanced noise properties. In addition, enhancement in system matrix quality is straightforward (by increasing the number of samples in the ensemble) so that the resolution penalty can be recovered when desired while retaining improvement in noise properties. Finally the approach is tested and validated against a standard (highly accurate) system matrix using experimental data from a prototype system--the AX-PET.

Performance of MACACO Compton telescope for ion-beam therapy monitoring: first test with proton beams

In order to exploit the advantages of ion-beam therapy in a clinical setting, delivery verification techniques are necessary to detect deviations from the planned treatment. Efforts are currently oriented towards the development of devices for real-time range monitoring. Among the different detector concepts proposed, Compton cameras are employed to detect prompt gammas and represent a valid candidate for real-time range verification. We present the first on-beam test of MACACO, a Compton telescope (multi-layer Compton camera) based on lanthanum bromide crystals and silicon photo-multipliers. The Compton telescope was first characterized through measurements and Monte Carlo simulations. The detector linearity was measured employing (22)Na and Am-Be sources, obtaining about 10% deviation from linearity at 3.44 MeV. A spectral image reconstruction algorithm was tested on synthetic data. Point-like sources emitting gamma rays with energy between 2 and 7 MeV were reconstructed with 3-5 mm resolution. The two-layer Compton telescope was employed to measure radiation emitted from a beam of 150 MeV protons impinging on a cylindrical PMMA target. Bragg-peak shifts were achieved via adjustment of the PMMA target location and the resulting measurements used during image reconstruction. Reconstructed Bragg peak profiles proved sufficient to observe peak-location differences within 10 mm demonstrating the potential of the MACACO Compton Telescope as a monitoring device for ion-beam therapy.

A Compton imaging algorithm for on-line monitoring in hadron therapy

Hadron therapy, a subject of study by the ENVISION project, promises to provide enhanced accuracy in the treatment of cancer. The Bragg-peak, characteristic of the hadron-beam structure provides larger dose to the tumor while being able to spare surrounding tissue - even tissues in the beam-path, beyond the tumor-site. However, increased dose gradients require more precise treatment, as small beam misalignment can result in dose to healthy, often delicate, surrounding tissue. The requirement for accuracy necessitates imaging during therapy, yet the lack of a transmitted beam makes this difficult. The particulate beam interacts with the target material producing neutrons, positron emitting isotopes and a broad spectra of gamma radiation. Photons from positron-annihilation allow in-beam PET to provide on-line measurements of dose deposition during therapy. However, ib-PET suffers from low statistics and lost projections due to low sensitivity and detector constraints respectively. Instead, Compton imaging of gamma radiation is proposed to provide on-line monitoring for hadron therapy. Compton imaging suffers similarly from low statistics, especially, as is the case here, when incident energy is unknown. To surmount this problem, a method of Compton image reconstruction is proposed and tested using simulated data, which reconstructs incident energy along with the spatial variation in emission density. Through incident energy estimation, a larger range of measurements are available for image-reconstruction - greatly increasing the sensitivity of the system. It is shown in this preliminary study that, even with few statistics, a reasonable estimate of the beam path is calculable.

Positron emission imaging using acquired cone-surfaces from opposing compton cameras

The Compton camera, a method of electronic collimation, introduces angular resolution into a gamma-ray imaging system without the need of a collimating device. Compton kinematics are used to deduce the incident trajectory of the photon, resulting in a cone-surface of possible source locations. So far this type of system has been successfully employed only as a single photon detection device. However, it has been proposed that positron emission tomography may also be enhanced by implementing the Compton camera. Cone-surfaces acquired from the collinear gamma rays produced in positron annihilation may be used to validate the resulting line of response. Moreover, single sided detections need no longer be redundant in image reconstruction due to the ability to perform cone-surface reconstruction in tandem with reconstruction from LoRs

PET Reconstruction From Truncated Projections Using Total-Variation Regularization for Hadron Therapy Monitoring

Hadron therapy exploits the properties of ion beams to treat tumors by maximizing the dose released to the target and sparing healthy tissue. With hadron beams, the dose distribution shows a relatively low entrance dose which rises sharply at the end of the range, providing the characteristic Bragg peak that drops quickly thereafter. It is of critical importance in order not to damage surrounding healthy tissues and/or avoid targeting underdosage to know where the delivered dose profile ends-the location of the Bragg peak. During hadron therapy, short-lived β+-emitters are produced along the beam path, their distribution being correlated with the delivered dose. Following positron annihilation, two photons are emitted, which can be detected using a positron emission tomography (PET) scanner. The low yield of emitters, their short half-life, and the wash out from the target region make the use of PET, even only a few minutes after hadron irradiation, a challenging application. In-beam PET represents a potential candidate to estimate the distribution of β+-emitters during or immediately after irradiation, at the cost of truncation effects and degraded image quality due to the partial rings required of the PET scanner. Time-of-flight (ToF) information can potentially be used to compensate for truncation effects and to enhance image contrast. However, the highly demanding timing performance required in ToF-PET makes this option costly. Alternatively, the use of maximum-a-posteriori- expectation-maximization (MAP-EM), including total variation (TV) in the cost function, produces images with low noise, while preserving spatial resolution. In this paper, we compare data reconstructed with maximum-likelihood-expectation-maximization (ML-EM) and MAP-EM using TV as prior, and the impact of including ToF information, from data acquired with a complete and a partial-ring PET scanner, of simulated hadron beams interacting with a polymethyl methacrylate (PMMA) target. The results show that MAP-EM, in the absence of ToF information, produces lower noise images and more similar data compared to the simulated β+ distributions than ML-EM with ToF information in the order of 200-600 ps. The investigation is extended to the combination of MAP-EM and ToF information to study the limit of performance using both approaches.

High performance 3D PET reconstruction using spherical basis functions on a polar grid

Statistical iterative methods are a widely used method of image reconstruction in emission tomography. Traditionally, the image space is modelled as a combination of cubic voxels as a matter of simplicity. After reconstruction, images are routinely filtered to reduce statistical noise at the cost of spatial resolution degradation. An alternative to produce lower noise during reconstruction is to model the image space with spherical basis functions. These basis functions overlap in space producing a significantly large number of non-zero elements in the system response matrix (SRM) to store, which additionally leads to long reconstruction times. These two problems are partly overcome by exploiting spherical symmetries, although computation time is still slower compared to non-overlapping basis functions. In this work, we have implemented the reconstruction algorithm using Graphical Processing Unit (GPU) technology for speed and a precomputed Monte-Carlo-calculated SRM for accuracy. The reconstruction time achieved using spherical basis functions on a GPU was 4.3 times faster than the Central Processing Unit (CPU) and 2.5 times faster than a CPU-multi-core parallel implementation using eight cores. Overwriting hazards are minimized by combining a random line of response ordering and constrained atomic writing. Small differences in image quality were observed between implementations.

Sensitivity recovery for the AX-PET prototype using inter-crystal scattering events.

The development of novel detection devices and systems such as the AX-positron emission tomography (PET) demonstrator often introduce or increase the measurement of atypical coincidence events such as inter-crystal scattering (ICS). In more standard systems, ICS events often go undetected and the small measured fraction may be ignored. As the measured quantity of such events in the data increases, so too does the importance of considering them during image reconstruction. Generally, treatment of ICS events will attempt to determine which of the possible candidate lines of response (LoRs) correctly determine the annihilation photon trajectory. However, methods of assessment often have low success rates or are computationally demanding. In this investigation alternative approaches are considered. Experimental data was taken using the AX-PET prototype and a NEMA phantom. Three methods of ICS treatment were assessed--each of which considered all possible candidate LoRs during image reconstruction. Maximum likelihood expectation maximization was used in conjunction with both standard (line-like) and novel (V-like in this investigation) detection responses modeled within the system matrix. The investigation assumed that no information other than interaction locations was available to distinguish between candidates, yet the methods assessed all provided means by which such information could be included. In all cases it was shown that the signal to noise ratio is increased using ICS events. However, only one method, which used full modeling of the ICS response in the system matrix--the V-like model--provided enhancement in all figures of merit assessed in this investigation. Finally, the optimal method of ICS incorporation was demonstrated using data from two small animals measured using the AX-PET demonstrator.

Multichannel DAQ system for SiPM matrices

The use of Silicon Photomultiplier (SiPM) arrays requires the use of multichannel data acquisition (DAQ) systems. For this reason a dedicated DAQ system has been developed for the read-out of several SiPM-based detector layers. The frontend of a system is based on the 64-channel ASIC V ATA64HDR16 from Gamma Medica - Ideas. As first application, the DAQ system will be employed in the construction of Compton telescope for dose monitoring in hadron therapy. However the designed system is suitable for any other devices that need to treat large number of SiPM channels. Tests are presented with SiPM matrix of 16 (4×4) elements each coupled to LYSO and LaBr3 continuous crystals. The complete characterization of the DAQ system and the obtained results are presented.

Simulated One Pass Listmode for fully 3D image reconstruction of Compton camera data

Image reconstruction for Compton camera data can be problematic due to the common trade-off between physically realistic models and speed of computation. In this investigation a novel method of system matrix calculation - Simulated One-Pass Listmode (SOPL) - is extended to incorporate Compton camera data. The method reduces the Cone Surface Response for the Compton camera to an ensemble of Siddon-rays and is conducted in two stages. As part of the ENVISION project for monitoring in hadron therapy, a continuous-crystal Lanthanum Bromide Compton camera has been developed and experimental data acquired. Continuous detection geometries are particularly susceptible to variation in both spatial and spectral resolution over the detection volume and so accurate yet flexible models of detection are particularly important. The SOPL-Compton method was applied via the Maximum Likelihood - Expectation Maximization algorithm to experimental data taken using the prototype device. In this investigation, detection modeling using SOPL-Compton in a two interaction Compton camera is validated and the incorporation of a shift-invariant image-space model confirmed as a useful modification to reduce computational expense. Finally experimental data taken using the prototype LaBr3 Compton camera provide confirmation of the SOPL-Compton approach to system modeling. Results indicate a fast, flexible and accurate algorithm that can easily be extended to alternate and novel detection geometries.