Preliminary evaluation of a combined microPET-MR system

Abstract

There are many motivations for adding simultaneously acquired MR images to PET scanning. The most straight forward are, superior registration of MR and PET images, the addition of morphological detail when there is non-rigid motion and for pre-clinical studies simultaneous imaging could lead to a significant reduction in the time that animals are required to be anesthetised. In addition simultaneous MR has the potential to provide accurate motion correction for PET image reconstruction. For functional imaging simultaneous acquisition is required to assess the subject in the same physiological state, such as acute stroke studies. The elimination of the additional radiation associated with combining CT with PET, by providing anatomic detail with MR, would be a crucial advantage for cancer screening. Combining the two instruments necessitates some engineering tradeoffs, especially associated with the use of the highly developed photomultiplier tube (PMT) used for light amplification, because of its incompatibility with strong magnetic fields. Our approach is to provide a split in the magnet and gradients to locate the magnetic sensitive components, the PMTs, in regions of low magnetic field, leaving only the essential PET components, the scintillator blocks, in the strong magnetic field region. The crystals are coupled to the PMTs by extending the optical fibres. A further advantage accrues by moving the PET electronics out of the region seen by the MR radio-frequency (RF) and gradient coils as electromagnetic interference effects between the PET and MR systems, which could cause artefacts in either modality, are eliminated. Here we describe a preliminary evaluation of the system, which is essentially a microPET Focus-120 located in a 1T split magnet, and compare its performance to previous microPET instruments.

Figure 1

View of the 1T split magnet with the AutoPac (Automatic Positioning System for Animal Beds and Coils, Bruker BioSpin) attached. At the top left hand corner the Faraday screen cover has been removed revealing one quadrant of the PET detectors.

Figure 2

The split gradient coils undergoing bench testing. The current is fed to each half as a serial winding, with water cooling and thermocouples fed separately to each end to avoid obscuring the gap in final operation.

Figure 3

Transmit/receive birdcage RF coil constructed with tuning and match adjustable capacitors outside the PET field of view.

Figure 4

A picture of the crystal blocks being assembled in the magnet gap. The brass ring providing support to the modules can be seen as well as the ends of the brass rods securing the modules azimuthally. The cut angles of 13, 8 and 3 degrees in the fibres can clearly be discerned. The system is made light tight once all modules are inserted and alignment completed.

Figure 5

A comparison of the absolute sensitivity of the R4 and Focus-120 with the PET/MR scanner for a 6ns coincidence window and energy window of 350-650 keV.

Figure 6

A comparison of the volumetric resolution computed from an image reconstructed from all segments for the R4, Focus-120 and PET/MR.

Figure 7

A comparison of the NECR for the R4, Focus-120 and the PET/MR scanner for a 6 ns coincidence window and an energy window of 650 keV.

Figure 8

A comparison of the scatter fraction measured for a rat-sized phantom between the Focus-120 and the PET/MR scanner for an energy window 350-650 keV.

Figure 9

The top image is the PET scan of the micro Derenzo phantom acquired simultaneously with the MR GEFI image shown in the middle. The lower image is the fusion of the two images, with only scaling and linear translation used to achieve superposition.

Figure 10

Simultaneous PET and MR images of an ApoE (genetically modified hyperlipidemic) mouse ~ four hours following injection of 73.6 MBq of 18F-FDG. The PET scan parameters were a 30 min acquisition, 3DFBP reconstruction with normalization but no attenuation and scatter correction. The MR image was acquired in 15 minutes using a ‘gradient echo fast imaging’ (GEFI) sequence.