Session D08: From Particle Physics to Brain Imaging and Radiotherapy

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TBD   

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Imaging is central to radiation therapy (RT) to define the clinical target volume, guide the delivery of radiation and assess the response to treatment. Over the past 60 years, RT has evolved from using textbook anatomic information and simple projection radiographs to employing the full gamut of modern imaging technologies. Most notably, the advent of several imaging technologies, particularly cone-beam CT, in the early 2000’s provided 3D anatomic information of the patient at time of treatment and ushered in the current era of image guided radiation therapy. Their increasing use also brings to light the complexities of organ motion and disease response during the course of treatment and necessitates treatment modification, thus adaptive radiation therapy (ART). The challenges of ART are multi-faceted as are their solutions. Fast imaging and delivery are needed to mitigate the effects of organ motion. Molecular imaging, such as using positron emission tomography, provides metabolic and biologic information about the tumor and its environment, so does functional imaging using magnetic resonance imaging with the added advantage of superb anatomic contrast. At present, ART is a highly active and exciting area of research where advances are made by bridging multidisciplinary expertise in computational modeling, physics, engineering and laboratory discovery research.   

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Hypothesis: Dedicated high performing brain imagers will in a vital way accelerate the development of the cure for dementia, assist with prevention/reversal, as finally safe low-dose early detection accurate molecular screening will become possible. This is the main rationale why scientists and engineers should get involved in this activity. With the beginning of the new era of revolutionary total body dynamic molecular PET Explorer imagers, the previously articulated need to develop dedicated imagers for breast, prostate, heart, etc may slowly disappear, except in the specialized cases in treatment guidance and monitoring, such as in proton therapy. Part of the reason is high cost of the dedicated systems but also an intriguing emerging opportunity that long axial length Explorer PET scanners can be equipped with “magnifying” inserts that can locally boost the spatial resolution and sensitivity, as per the so-called “virtual pinhole concept”. However, the exception is the brain imager. The varied asymmetric geometry possible in the head-surrounding designs, still gives an exciting opportunity to the PET imager developers to compete for the “best” brain imaging PET system. And we are far away from that goal. We all want to produce “high quality” dynamic molecular PET brain images at as low as practical injected radiation doses, and at low cost. Many new designs are being proposed and being built at this time around the world. These designs mostly fall in two categories: (1) the mini-Explorer cylindrical type and (2) compact helmet type, both with large angular brain coverage assuring high PET detection sensitivity. To further improve sensitivity there is a push now to achieve better than 100 ps or even 50 ps FWHM timing performance. Paul Lecoq from CERN and others from the HEP community formulated the vision/goal of reaching 10 ps FWHM TOF resolution, equivalent to 1.5 mm resolution in space, allowing for non-tomographic open geometry PET imaging. Several groups are working on such concepts, new detection materials, etc. In this race “to save lives through better diagnosis”, any new ideas from the expert instrumentation community (not only the medical imaging experts) are highly encouraged, as there is expected great impact on brain imaging once such high-performance and robust, economical (dissemination ready) designs are developed. Ideally, brain imagers of the next generation will have high sensitivity and will reach spatial resolution approaching the predicted physical limit due to the positron range and the non-collinearity between the emission directions of the two emitted annihilation photons, of about just over 1 mm FWHM.   

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Positron emission tomography (PET) is a nuclear medicine modality enabling in-vivo imaging of a range of biological processes. The time-of-flight (TOF) measurement can be used to improve the signal-to-noise ratio of reconstructed images. In the last decade, the TOF resolution of PET scanners improved from about 500 ps FWHM to 200 ps FWHM with important consequences for the clinical capabilities. The most significant remaining contribution limiting the TOF resolution are the temporal properties of the light production in scintillators used for PET detectors. This limitation can be avoided by basing the TOF measurement on detection of Cherenkov photons, which are produced promptly. However, with energy of the annihilation gammas used in PET, only about 10 Cherenkov photons are produced, placing high performance requirements on the other main PET detector component, the photodetector. In this contribution, the Cherenkov based TOF-PET method and work exploring the performance of Cherenkov based detectors will be presented. Using lead fluoride (PbF$_2$), a pure Cherenkov radiator, instead of traditionally used scintillation crystals, a TOF resolution of 71 ps FWHM was experimentally demonstrated. With a module of 4$\times$4 Cherenkov based detectors a gamma detection efficiency of up to 35\% was measured. Simulations in GATE software show that a Cherenkov based TOF-PET scanner can achieve image quality comparable to current state-of-the-art. Also, lead fluoride is considerably less expensive than scintillator materials currently used, opening a possibility for lower cost PET detectors. This is of special interest for the emerging total-body PET scanners, which are pushing the length of the scanner to cover the whole human body.