D Strul

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.

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.

Use of an analytical model for optimizing the design of a small-animal PET scanner with DOI capability

The optimization of spatial resolution is a critical issue for small-animal PET scanners, and is often addressed by Monte-Carlo simulation. Analytical models, though less versatile are very fast and their simplicity allows a direct appreciation of the influence of different model parameters. The authors have developed an analytical model for multi-layer PET systems, which provides estimates of the radial and tangential resolution at different positions within the field of view. After a preliminary validation, this model was used to optimize the design of a small single-slice multi-layer PET scanner with depth of interaction capability. The authors found satisfactory agreement between the analytical model and Monte Carlo results for several scanner configurations. The dependence of the resolution on the crystal width, the number of layers, and the crystal layout was determined for a scanner with internal and external diameters at 74 mm and 114 mm respectively. Both simulation methods agreed perfectly on the influence of these parameters. In particular confirming the degree of resolution improvement obtained using multiple-layers of crystals. These results show that an analytical model can provide accurate estimates of the spatial resolution, and can be used to complement or cross-validate Monte Carlo simulations.

MR-compatible shields for 511 keV gamma photons: a feasibility study

An MR-compatible PET scanner capable of acquiring PET and MR images simultaneously will require 511 keV gamma shielding to suppress the influence of activity outside the PET field of view. Suitable materials must have a magnetic permeability close to that of water and air, high density, high Z and ideally a low conductivity. Eleven materials were selected on the basis of the criteria above, including various MRI magnetically-compatible metals and metal oxides, 2 scintillating crystals (bismuth germanate and lead tungstate), and 3 metal/epoxy compounds. Samples of these materials, immersed in a water-filed phantom, were first imaged on a human MRI-scanner to assess their MR-compatibility. A similar protocol was then applied to 3 annular shields, respectively made of red lead oxide, lead powder and bulk lead. Lastly, a water-filled phantom was imaged on a small-animal MRI scanner with the coil within annular shields made of bismuth germanate, sintered lead monoxide or bulk lead. These experiments showed that lead cannot be generally used for MR-compatible gamma shielding, since it yields conductivity-related artefacts or signal loss. The most promising candidates are high-density insulating compounds such as lead oxides, and crystalline scintillator materials such as bismuth germanate.

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.

MR-compatible shields for 511 keV gamma photons

An MR-compatible PET scanner capable of acquiring PET and MR images simultaneously will require 511 keV gamma shielding to suppress the influence of activity outside the PET field of view. Suitable materials must have a magnetic permeability close to that of water and air, high density, high Z and ideally a low conductivity. Eleven materials were selected on the basis of the criteria above, including various MRI magnetically-compatible metals and metal oxides, 2 scintillating crystals (bismuth germanate and lead tungstate), and 3 metal/epoxy compounds. Samples of these materials, immersed in a water-filed phantom, were first imaged on a human MRI-scanner to assess their MR-compatibility. A similar protocol was then applied to 3 annular shields, respectively made of red lead oxide, lead powder and bulk lead. Lastly, a water-filled phantom was imaged on a small-animal MRI scanner with the coil within annular shields made of bismuth germanate, sintered lead monoxide or bulk lead. These experiments showed that lead cannot be generally used for MR-compatible gamma shielding, since it yields conductivity-related artefacts or signal loss. The most promising candidates are high-density insulating compounds such as lead oxides, and crystalline scintillator materials such as bismuth germanate.

Conference Record (Cat. No.00CH37149)

An MR-compatible PET scanner capable of acquiring PET and MR images simultaneously will require 511 keV gamma shielding to suppress the influence of activity outside the PET field of view. Suitable materials must have a magnetic permeability close to that of water and air, high density, high Z and ideally a low conductivity. Eleven materials were selected on the basis of the criteria above, including various MRI magnetically-compatible metals and metal oxides, 2 scintillating crystals (bismuth germanate and lead tungstate), and 3 metal/epoxy compounds. Samples of these materials, immersed in a water-filed phantom, were first imaged on a human MRI-scanner to assess their MR-compatibility. A similar protocol was then applied to 3 annular shields, respectively made of red lead oxide, lead powder and bulk lead. Lastly, a water-filled phantom was imaged on a small-animal MRI scanner with the coil within annular shields made of bismuth germanate, sintered lead monoxide or bulk lead. These experiments showed that lead cannot be generally used for MR-compatible gamma shielding, since it yields conductivity-related artefacts or signal loss. The most promising candidates are high-density insulating compounds such as lead oxides, and crystalline scintillator materials such as bismuth germanate.