Simultaneous acquisition of magnetic resonance spectroscopy (MRS) data and positron emission tomography (PET) images with a prototype MR-compatible, small animal PET imager

Abstract

Multi-modality imaging (such as PET-CT) is rapidly becoming a valuable tool in the diagnosis of disease and in the development of new drugs. Functional images produced with PET, fused with anatomical images created by MRI, allow the correlation of form with function. Perhaps more exciting than the combination of anatomical MRI with PET, is the melding of PET with MR spectroscopy (MRS). Thus, two aspects of physiology could be combined in novel ways to produce new insights into the physiology of normal and pathological processes. Our team is developing a system to acquire MRI images and MRS spectra, and PET images contemporaneously. The prototype MR-compatible PET system consists of two opposed detector heads (appropriate in size for small animal imaging), operating in coincidence mode with an active field-of-view of approximately 14 cm in diameter. Each detector consists of an array of LSO detector elements coupled through a 2-m long fiber optic light guide to a single position-sensitive photomultiplier tube. The use of light guides allows these magnetic field-sensitive elements of the PET imager to be positioned outside the strong magnetic field of our 3T MRI scanner. The PET scanner imager was integrated with a 12-cm diameter, 12-leg custom, birdcage coil. Simultaneous MRS spectra and PET images were successfully acquired from a multi-modality phantom consisting of a sphere filled with 17 brain relevant substances and a positron-emitting radionuclide. There were no significant changes in MRI or PET scanner performance when both were present in the MRI magnet bore. This successful initial test demonstrates the potential for using such a multi-modality to obtain complementary MRS and PET data.

Fig. 1

Pictures showing the MRI-PET imaging insert. (A) The multimodality spectroscopy phantom in MRI-PET imaging insert and (B) the insert at its scanning position in the 3T MRI scanner.

Fig. 2

Images from the MRI-PET system. (A) SPGR MRI image of the multi-modality spectroscopy phantom, (B) PET image of the phantom and (C) aligned phantom image.

Fig. 3

PRESS localized 1D spectra acquired from the multi-modality spectroscopy brain phantom. The plot shown in (a) was acquired with the PET scanner component in place; the plot shown in (b) was acquired without the PET scanner in the MRI scanner.

Fig. 4

Localized-correlated spectroscopy (L-COSY) spectrum acquired from the multi-modality sphere with the PET scanner in the MRI scanner. The spectrum shows the diagonal and cross peaks from the various chemical components of the metabolite solution.