MRI compatibility of position-sensitive photomultiplier depth-of-interaction PET detectors modules for in-line multimodality preclinical studies

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Abstract

This work addresses the feasibility of a small-animal, in-line PET/MR system based on Position-Sensitive Photo Multiplier Tubes (PS-PMTs). To this end, we measured the effects of static magnetic fields on the PS-PMTs performance in order to explore the minimal tandem separation between the PET and MR subsystems to preserve their respective performances. We concluded that it is possible to achieve minimal degradation of the PET scanner performance (after a system recalibration) if the magnetic field strength influencing the PET detectors is less than 1 mT and if it is oriented perpendicularly to the longitudinal axis of the tube. Therefore, we predict that it will be possible to maintain the PET image quality if it is placed outside the 1 mT line.

Introduction

First publications about combined MRI/PET imagers date from 1997 and since then the number of designs has steadily grown [1], [2], [3], [4]. Substantial efforts were focused on overcoming the interference between the two imaging systems when they are coaxially integrated in a single hybrid unit [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Other approaches avoided this close integration and proposed an in-line, tandem combination of the PET and the MRI systems with a revolving bed in between [17]. While this solution has the advantage that none of the imaging systems (the PET or the MRI) suffer performance degradation due to the proximity of the other one, the resulting device occupies a large space, requires a complex mechanism for patient transport and cannot acquire simultaneous PET/MR scans. On the contrary, space is not a mayor issue when using low-field, permanent based MRI preclinical imagers. In this case both systems can be closely attached since the MRI system fringe field can be considered negligible outside the magnet itself. However, the worse quality of the low-field MR images and the impossibility of doing simultaneous acquisitions are still two clear drawbacks for certain applications.

Our approach makes use of state-of-the-art commercial preclinical scanners: an MRI system (Bruker Biospec 70/20USR) and a small-animal PET (SEDECAL Argus PET/CT) [18], placing both systems as close as possible to each other with their axial axes perfectly aligned. In this way it is possible to use a common, straightforward bed-moving mechanism to transport the animal between the two scanners. Simultaneous data acquisition is not possible, but the exquisite image quality of both systems is preserved. Since the static magnetic fringe field of the MRI system can affect the PS-PMT behavior [19], we evaluated the detector performance when operated under these conditions, as well as the potential degradation of final PET image quality.

Section snippets

Materials and methods

A test-bench was built using two Argus detector modules placed 12 cm apart and connected with a PET data acquisition system working in coincidence mode. The Argus detectors consist of a phoswich (two-layer scintillator with a 7 mm long LYSO and an 8 mm long GSO crystals, [20]) optically glued to a Hamamatsu R8520-00-C12 PS-PMT and with a signal pre-amplification board. The coincidence acquisition system is based on a 622 LeCroy's NIM logic unit and a 2×6-channel 12-bit ADC/TDC module (A&D

Results

Intrinsic spatial resolution: Fig. 4 illustrates different examples of the crystal map degradation for different fringe field intensities and orientations. The upper panel shows raw images for the XZ axis and the bottom panel shows the crystals maps and the phoswich diagrams for the Y axis. Field-flood images are distorted but mapping of the 13×13 phoswich crystals is still feasible for intensities lower than 3 mT, provided that only the Y axis component of the field is present. If a sizeable X

Conclusions

We tested the effect of static magnetic fields on the PS-PMTs DOI PET detectors of the Argus PET scanner. The results show that these detectors can withstand magnetic fields up to 1 mT if the main component of the field is perpendicular to the longitudinal axis of the tube, but do not maintain its performance if the magnetic field orientation is parallel to said axis. In the PET/MR tandem configuration presented here the parallel component of the magnetic fringe field impinging in the PS-PMT is

Acknowledgment

This work was supported in part by projects CENIT-AMIT Ingenio 2010, TEC2011-28972 (Ministerio de Ciencia e Innovación) ARTEMIS S2009/DPI-1802 (Comunidad de Madrid, Fondo Social Europeo), and EU IMI Joint Undertaking PreDiCT-TB 115337.

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