Combined PET and MRI for small-animal imaging
Exploiting a technological advance that enables simultaneous PET and MRI, researchers have acquired functional and morphological data in living mice using an integrated imaging system. They used a PET scanner specifically designed to fit inside an in vivo animal MRI system to image a mouse tumor and to spot tissue differences. They also performed brain imaging, achieving a semiquantitative evaluation of dopamine transporter binding in vivo.
The work was done by a group from Eberhard Karls University of Tübingen in Germany; from the University of California, Davis; from Bruker BioSpin MRI GmbH of Ettlingen, Germany; and from Siemens Preclinical Solutions of Knoxville, Tenn. The technology allowed PET and MRI imaging simultaneously, without either having a strong effect on the other. (For another approach that uses an insert to combine PET and MRI, see the article on p. 49 of the May issue.)
The group’s PET insert used avalanche photodiode arrays instead of the traditional photomultiplier tubes, which are sensitive to magnetic fields. The researchers coupled the photodiodes directly to the back of the positron-detecting scintillator arrays, thus directing the most light to the photodiodes and ensuring the best possible PET signal. A drawback of this arrangement was that some of the electronics were inside the magnetic resonance region, potentially leading to interference between the two imaging systems. However, tests showed that any such effect was minimal.
The researchers used the integrated PET-MRI system to study a mouse bearing a colon cancer of about a cubic centimeter in volume. The size of the tumor was important because an evaluation testing the effectiveness of a proposed therapeutic drug was under way. What’s more, tumors of this size can show variations, with some areas viable and others not.
After injecting a radiotracer, they collected PET images as well as contrast-enhanced static and dynamic MRI. Areas of the tumor that were growing rapidly were picked up easily via PET images because they had relatively high tracer uptake. For areas of low uptake, analysis of magnetic resonance images allowed the researchers to differentiate between tumor cell proliferation, tumor necrosis and inflammation. Tests showed that the PET and MRI data aligned with a spatial accuracy of better than 150 μm.
After injecting a radiolabeled dopamine transporter ligand in another mouse — again making use of the complementary nature of the imaging techniques — they determined tracer uptake with PET in regions of interest determined by MRI.
The researchers noted that it should be feasible to collect magnetic resonance spectroscopy and functional MRI data while using the PET insert. Their initial studies using a construct indicate that simultaneous PET and functional MRI are possible. Thus, they noted, their system allows PET, functional MRI and spectroscopy with morphological MRI.
Nature Medicine, April 2008, pp. 459-465.
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