PhD student, University of Manitoba
Evaluation of geometrical variables on the performance of positron emission tomography scanners
Medical imaging modalities can be divided into two groups: anatomical (e.g. magnetic resonance imaging or MRI) and functional imaging (e.g. positron emission tomography or PET) modalities. While an anatomical modality produces images regarding the structure of the body, functional imaging reveals information on the metabolism of each organ. PET is one of the most widely-used functional modalities which has clinical applications in areas such as oncology and neurology. To perform a PET scan, a radiotracer (a positron emitter radionuclide bound to a biologically active molecule such as glucose) is injected into the patient's body such that it accumulates in specific organs. As the decay of the radionuclide happens, gamma photons will be emitted which are being detected by rings of gamma detectors around the patient. The exact spatial and temporal information of the detected photons provides a basis for reconstructing an image of the radiotracer distribution which in turn represents the functionality of the target organ. The images obtained from a PET scan are typically difficult to interpret due to the absence of anatomical landmarks. Therefore, PET acquisition is often done along with an anatomical modality acquisition which is termed 'hybrid imaging'. The anatomical images provide invaluable information that can be used in clinical diagnosis. Also, the acquired anatomical images can be used to improve the quantitative accuracy of the reconstructed PET images. Simultaneous acquisition of PET and MRI (PET/MRI) is one of the most sophisticated techniques in state-of-the-art hybrid imaging. One of the approaches to simultaneous PET/MRI is to build a so-called 'PET insert' for an existing MRI machine. A PET insert is a PET scanner that can be retrofit into an existing MRI machine to facilitate simultaneous PET and MRI imaging. A PET insert has several advantages over a fully integrated PET/MRI, such as: it can be designed and built for any existing standalone MRI scanner and it is cost-effective. We are now designing a human brain-dedicated PET insert that can be retrofit into the Siemens Magnetom 7 Tesla Brain scanner (an ultra-high field brain-dedicated MRI scanner) located in London, Ontario, Canada. The specific goal of the project is to find the optimum set of geometrical variables for the PET insert 's detectors that can maximize the system resolution while maintaining other performance metrics of the scanner as high as possible.
Abstract: The silicon photomultiplier (Si-PM) is a promising photo-detector for PET for use in magnetic resonance imaging (MRI) systems because it has high gain and is insensitive to static magnetic fields. Recently we developed a Si-PM-based depth-of-interaction PET system for small animals and performed simultaneous measurements by combining the Si-PM-based PET and the 0.15 T permanent MRI to test the interferences between the Si-PM-based PET and an MRI. When the Si-PM was inside the MRI and installed around the radio frequency (RF) coil of the MRI, significant noise from the RF sequence of the MRI was observed in the analog signals of the PET detectors. However, we did not observe any artifacts in the PET images; fluctuation increased in the count rate of the Si-PM-based PET system. On the MRI side, there was significant degradation of the signal-to-noise ratio (S/N) in the MRI images compared with those without PET. By applying noise reduction procedures, the degradation of the S/N was reduced. With this condition, simultaneous measurements of a rat brain using a Si-PM-based PET and an MRI were made with some degradation in the MRI images. We conclude that simultaneous measurements are possible using Si-PM-based PET and MRI.
Pub.: 16 Dec '11, Pinned: 27 Jun '17
Abstract: The combination of functional and morphological imaging technologies such as positron emission tomography (PET) and X-ray computed tomography (CT) has shown its value in the clinical and preclinical field. However, CT provides only very limited soft-tissue contrast and exposes the examined patient or laboratory animal to a high X-ray radiation dose. In comparison to CT, magnetic resonance tomography (MRI) provides excellent soft-tissue contrast and allows for nuclear magnetic resonance spectroscopy (NMRS) or functional MRI (fMRI). Thus, the combination of PET and MRI has been pursued for several years. First approaches have succeeded using conventional photo multiplier tube (PMT) technology together with light fibers to transfer scintillation light away from the high magnetic field. Latest PET/MRI developments use solid-state light detectors that can be operated even at high magnetic fields. Initial pilot studies with prototype animal PET/MRI systems have shown promising results by combining high resolution morphology with multifunctional information isochronously.
Pub.: 22 Mar '08, Pinned: 27 Jun '17
Abstract: Recently, positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of cancer. Combined PET and X-ray computed tomography (PET-CT) scanners are now the modality of choice in cancer treatment planning. More recently, the combination of PET and magnetic resonance imaging (MRI) is being explored in many sites. Combining PET and MRI has presented many challenges since the photo-multiplier tubes (PMT) in PET do not function in high magnetic fields, and conventional PET detectors distort MRI images. Solid state light sensors like avalanche photo-diodes (APDs) and more recently silicon photo-multipliers (SiPMs) are much less sensitive to magnetic fields thus easing the compatibility issues. This paper presents the results of a group of Canadian scientists who are developing a PET detector ring which fits inside a high field small animal MRI scanner with the goal of providing simultaneous PET and MRI images of small rodents used in pre-clinical medical research. We discuss the evolution of both the crystal blocks (which detect annihilation photons from positron decay) and the SiPM array performance in the last four years which together combine to deliver significant system performance in terms of speed, energy and timing resolution.
Pub.: 15 Aug '14, Pinned: 27 Jun '17
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