Philip Halsted

Design and development of an MR-compatible PET scanner for imaging small animals

An MR compatible PET system has been designed and is currently under construction. It will consist of four concentric rings of LSO crystals, each coupled to one of eight multi-channel photomultiplier tubes via 3.5 m optical fibres. The photomultiplier tubes may be located outside the main magnetic field of the MR scanner. A highly reproducible method has been established to optimise the amount of scintillation light that reaches the PM tubes, as this factor will heavily influence the scanner performance. Two small sections of the scanner, each containing 4 by 4 crystal arrays, demonstrated good flood position histograms with all sixteen channels clearly identifiable. The light loss through a fibre of length of 3.25 m was approximately 70%. The spatial resolution of the two arrays in coincidence was measured at 1.6 mm (FWHM). The temporal resolution of one array in coincidence with a single LSO crystal was measured to be 10.9 ns. A technique for improving sampling at the centre of the field of view within the scanner has also been investigated, whereby the concentric rings are offset with respect to one another. An offset of one quarter of the crystal width between layers results in significantly improved sampling. These results indicate that the scanner will be capable of carrying out the studies for which it has been designed.

Simultaneous PET–MR acquisition and MR-derived motion fields for correction of non-rigid motion in PET

ObjectivePositron emission tomography (PET) provides an accurate measurement of radiotracer concentration in vivo, but performance can be limited by subject motion which degrades spatial resolution and quantitative accuracy. This effect may become a limiting factor for PET studies in the body as PET scanner technology improves. In this work, we propose a new approach to address this problem by employing motion information from images measured simultaneously using a magnetic resonance (MR) scanner.MethodsThe approach is demonstrated using an MR-compatible PET scanner and PET–MR acquisition with a purpose-designed phantom capable of non-rigid deformations. Measured, simultaneously acquired MR data were used to correct for motion in PET, and results were compared with those obtained using motion information from PET images alone.ResultsMotion artefacts were significantly reduced and the PET image quality and quantification was significantly improved by the use of MR motion fields, whilst the use of PET-only motion information was less successful.ConclusionsCombined PET–MR acquisitions potentially allow PET motion compensation in whole-body acquisitions without prolonging PET acquisition time or increasing radiation dose. This, to the best of our knowledge, is the first study to demonstrate that simultaneously acquired MR data can be used to estimate and correct for the effects of non-rigid motion in PET.

Gamma shielding materials for MR-compatible PET

As for standard positron emission tomography (PET) scanners, MR-compatible PET scanners will require gamma shielding to suppress the influence of activity outside the PET field of view (FOV). Suitable materials must have very specific properties, including magnetic properties close to those of water, high density, high atomic number, and ideally a low conductivity. In order to identify potential suitable materials, we have selected several heavy-metal-based candidates based on the available data for magnetic and shielding properties. These materials include several nonferromagnetic metals and metal oxides, two scintillating crystals (bismuth germanate and lead tungstate) and two metal/epoxy compounds. The magnetic resonance imaging (MRI) compatibility of these materials was assessed under various conditions, both on a human and a small-animal MRI scanner. In parallel, we assessed the shielding efficiency at 661 keV of the most promising candidates. These experiments showed that there is a range of possibilities for the design of MR-compatible gamma shields. Lead has acceptable magnetic compatibility but can induce significant conductivity-related artefacts. Heavy-metal-based minerals are fully insulating and hot-pressed lead monoxide showed good MR compatibility combined with good shielding properties. Other possibilities include the use of lead based powders and heavy-metal oxide composites.

Performance Evaluation of an MRI-Compatible Pre-Clinical PET System Using Long Optical Fibers

We have designed and constructed an MR-compatible PET system for fully simultaneous PET/MR studies of small animals. The scanner uses long optical fibers to distance the magnetic field sensitive PET PMTs from the high magnetic field at the center of an MR scanner. It is a single slice system with an inner diameter of 7 cm. A full evaluation of the performance of the PET system and the results of an MR compatibility assessment in a Philips Achieva whole body 3 T MRI scanner are presented. The reconstructed resolution of the PET scanner is 1.5 mm at the center falling to 2.5 mm at the edge of the field of view; the system sensitivity is 0.95%; the count rate is linear up to an activity of 6 MBq (~4 kcps) and the scatter fraction is 42% which can be reduced to 26% using MR-compatible gamma shields. Simultaneous PET/MR images of phantoms and a mouse have been acquired. The system is highly MR compatible, as demonstrated here, showing no degradation in performance of either the MR or PET system in the presence of the other modality. The system will be used to demonstrate novel pre-clinical applications of simultaneous PET/MR.

Performance evaluation and MR compatibility assessment of an optical fibre based pre-clinical MR-compatible PET scanner

We have designed and constructed a small animal MR-compatible PET system for fully simultaneous MR/PET acquisitions. The scanner uses long optical fibres to distance the field sensitive PET PMTs from the high magnetic field at the centre of an MR scanner. The system has been designed to operate inside a number of different high field MRI scanners. We present a detailed performance evaluation and high field MR compatibility assessment of the PET system. The PET-only performance results are encouraging. We report a reconstructed transaxial resolution of 1.49mm at the centre of the field of view which falls off gradually to 2.52mm at 26.5mm from the centre. The average axial resolution is 2.7mm. The slice sensitivity to a point source is 0.95%, when corrected for both scatter and randoms. The count rate is linear up to an activity of 6 MBq (∼ 5kcps). The scatter fraction was 42% which could be reduced to 26% using MR compatible gamma shields. We have produced images of various hotspot phantoms that demonstrate good image quality. We assessed the MR compatibility of the PET system in a Philips Achieva human 3T scanner. We found there were no artefacts or distortions imposed on either the PET 2D flood position histograms of the PET PMTs or the PET images when the PET system was operated inside the MRI scanner. We have demonstrated that the MRI scanner did not pick up any significant levels of RF interference from the PET electronics by acquiring data across the entire frequency range of the RF head coil whilst the PET was inside the MRI scanner acquiring data. We have characterised the effects of the PET scanner materials on the main magnetic field of the MR scanner using a field map of a large uniform phantom with the PET scanner in the MRI scanner. We found that there were no significant distortions seen in MR images when the PET was located in the MR FOV. We acquired simultaneous MR and PET images of a mouse brain using a dedicated small animal coil in the 3T human system, which demonstrated good image quality. We now plan to use the system to demonstrate novel pre-clinical applications of simultaneous PET/MR.

An MR Compatible LSO-PET Scanner for Molecular Imaging Studies

We present a detailed system description of an MR compatible PET scanner for small animal molecular imaging studies, which has recently been constructed. The system images a single slice and has an inner diameter 7.5 cm. It consists of 4 concentric rings of 104 LSO crystals each with nominal dimensions of 2 mm (tangential) by 3 mm (axial) by 5 mm (radial). The crystal ring is positioned at the centre of an MR scanner, and 3.5 m long optical fibres transport the scintillation light to eight multi-channel PM tubes situated in a low field region in which they can function correctly. Position encoding boards convert the large number of signals from these multichannel photomultiplier tubes to four position encoded signals. The electronics are standard NIM modules and can be housed in just two racks, to allow the system to be relocated easily for operation in a range of MR scanners. The signals are digitized on ADC cards in a PC and the raw data converted into a sinogram using an interpolation method prior to reconstruction. We present some preliminary results from the PET system. The system will now be fully evaluated and its performance assessed within the MR environment.

Design and development of phantoms capable of continuous motion during simultaneous PET-MR acquisitions

Positron Emission Tomography (PET) is an imaging method affected by motion artifacts. PET-MR simultaneous acquisition may provide a means to correct for the effects of motion. Evaluation and development of motion correction techniques requires the development of phantoms capable for continuous and realistic motion. For this study, phantoms have been developed for sequential or simultaneous PET-MR motion-correction experiments. The phantoms contain different levels of deformation. While the first phantom is consisted of rigid materials, the new approach is the use of polyvinyl alcohol (PVA) cryogel phantom for this study. The elasticity of the PVA cryogel, combined with the motion by a computer guided motor unit provides the necessary complexity and accuracy of motion, required for sequential or simultaneous PET-MR experiments with motion incorporation. These phantoms, combined with the already established methodology, could provide the advantage to address issues such as the optimization of gating and motion estimation/correction algorithms in PET.

IEEE Nuclear Science Symposium Conference Record

Combined MRI and PET systems are currently being developed by a number of research groups and also commercially. Most systems have been designed to permit the simultaneous acquisition of MRI and PET data. The approach taken by many groups, is to build an MRI compatible PET insert, that works inside a standard MRI scanner. Unlike other multi-modality imaging systems such as PET/CT, the physical location of the PET scanner within the MRI scanner may vary each time the PET scanner is removed and replaced in the MRI scanner. In order to produce fully aligned PET and MRI images over the same region of the subject, the PET scanner location within the MRI field of view (FOV) is required, as well as the transformation between the PET and the MRI images. We have developed such a technique for our single slice pre-clinical MR-compatible PET system that uses long optical fibres to distance the PET PMTs from the high field at the centre of the MRI scanner. The method uses MRI visible markers attached to the PET scanner at a known position with respect to the PET FOV. In our configuration the markers must be positioned close to MRI compatible PET gamma shields which are used to improve the signal to noise ratio of the PET images by reducing the number of events recorded from activity outside the FOV. These shields are made from BGO and PbO, which have similar magnetic properties to LSO. The main magnetic field of the MRI scanner becomes distorted near to the shields, although they impose little distortion over the useful FOV of the PET system. We have assessed the accuracy with which the MRI visible markers can be used to select the location of the PET imaging slice and to align the PET and MRI images in-plane, using simultaneously acquired data of various phantoms. In order to carry out the in-plane registration, two separate MRI images had to be acquired in which the direction of the MRI frequency-encoding gradient was flipped between the two acquisitions to estimate the in-plane shift in marker location caused by field inhomogeneities imposed by the PET shielding materials. The slice location accuracy and the in-plane registration appear to be accurate to approximately 0.5 mm. We have successfully applied the method to produce simultaneously acquired, registered 18F−PET and MRI images of the mouse neck. A similar method is likely to be required for many future pre-clinical simultaneous PET and MRI studies as it will be essential for situations in which there is little similarity between the PET and MRI images.

A fiducial marker based technique for alignment of simultaneously acquired PET and MRI images

Combined MRI and PET systems are currently being developed by a number of research groups and also commercially. Most systems have been designed to permit the simultaneous acquisition of MRI and PET data. The approach taken by many groups, is to build an MRI compatible PET insert, that works inside a standard MRI scanner. Unlike other multi-modality imaging systems such as PET/CT, the physical location of the PET scanner within the MRI scanner may vary each time the PET scanner is removed and replaced in the MRI scanner. In order to produce fully aligned PET and MRI images over the same region of the subject, the PET scanner location within the MRI field of view (FOV) is required, as well as the transformation between the PET and the MRI images. We have developed such a technique for our single slice pre-clinical MR-compatible PET system that uses long optical fibres to distance the PET PMTs from the high field at the centre of the MRI scanner. The method uses MRI visible markers attached to the PET scanner at a known position with respect to the PET FOV. In our configuration the markers must be positioned close to MRI compatible PET gamma shields which are used to improve the signal to noise ratio of the PET images by reducing the number of events recorded from activity outside the FOV. These shields are made from BGO and PbO, which have similar magnetic properties to LSO. The main magnetic field of the MRI scanner becomes distorted near to the shields, although they impose little distortion over the useful FOV of the PET system. We have assessed the accuracy with which the MRI visible markers can be used to select the location of the PET imaging slice and to align the PET and MRI images in-plane, using simultaneously acquired data of various phantoms. In order to carry out the in-plane registration, two separate MRI images had to be acquired in which the direction of the MRI frequency-encoding gradient was flipped between the two acquisitions to estimate the in-plane shift in marker location caused by field inhomogeneities imposed by the PET shielding materials. The slice location accuracy and the in-plane registration appear to be accurate to approximately 0.5 mm. We have successfully applied the method to produce simultaneously acquired, registered 18F−PET and MRI images of the mouse neck. A similar method is likely to be required for many future pre-clinical simultaneous PET and MRI studies as it will be essential for situations in which there is little similarity between the PET and MRI images.

An MR compatible PET scanner for imaging small animals

An MR compatible PET system has been designed and is currently under construction. It will consist of four concentric rings of LSO crystals, each coupled to one of eight multi-channel photomultiplier tubes via 3.5 m optical fibres. The photomultiplier tubes may be located outside the main magnetic field of the MR scanner. A highly reproducible method has been established to optimise the amount of scintillation light that reaches the PM tubes, as this factor will heavily influence the scanner performance. Two small sections of the scanner, each containing 4 by 4 crystal arrays, demonstrated good flood position histograms with all sixteen channels clearly identifiable. The light loss through a fibre of length of 3.25 m was approximately 70%. The spatial resolution of the two arrays in coincidence was measured at 1.6 mm (FWHM). The temporal resolution of one array in coincidence with a single LSO crystal was measured to be 10.9 ns. A technique for improving sampling at the centre of the field of view within the scanner has also been investigated, whereby the concentric rings are offset with respect to one another. An offset of one quarter of the crystal width between layers results in significantly improved sampling. These results indicate that the scanner will be capable of carrying out the studies for which it has been designed.