Simultaneous in vivo positron emission tomography and magnetic resonance imaging

Abstract

Positron emission tomography (PET) and magnetic resonance imaging (MRI) are widely used in vivo imaging technologies with both clinical and biomedical research applications. The strengths of MRI include high-resolution, high-contrast morphologic imaging of soft tissues; the ability to image physiologic parameters such as diffusion and changes in oxygenation level resulting from neuronal stimulation; and the measurement of metabolites using chemical shift imaging. PET images the distribution of biologically targeted radiotracers with high sensitivity, but images generally lack anatomic context and are of lower spatial resolution. Integration of these technologies permits the acquisition of temporally correlated data showing the distribution of PET radiotracers and MRI contrast agents or MR-detectable metabolites, with registration to the underlying anatomy. An MRI-compatible PET scanner has been built for biomedical research applications that allows data from both modalities to be acquired simultaneously. Experiments demonstrate no effect of the MRI system on the spatial resolution of the PET system and <10% reduction in the fraction of radioactive decay events detected by the PET scanner inside the MRI. The signal-to-noise ratio and uniformity of the MR images, with the exception of one particular pulse sequence, were little affected by the presence of the PET scanner. In vivo simultaneous PET and MRI studies were performed in mice. Proof-of-principle in vivo MR spectroscopy and functional MRI experiments were also demonstrated with the combined scanner.

Fig. 1.

MR scanner effect on PET system. (A–C) Detector histograms showing the anticlockwise (A) and clockwise (B) rotations of the crystal maps when compared with the data acquired outside of the magnet (C). (D) PET event rate measured under different conditions: (i) while applying only RF power (with 1,000 ms and 500 ms repetition times) and (ii) while switching the x–z gradients independently (at 100% and 50% power; 400 and 200 mT/m, respectively). Baseline represents the event rate recorded without running MR sequences.

Fig. 2.

PET insert effects on MR imaging. (A and B) SE (A) and GE (B) images of a structured phantom acquired in the presence of the PET insert. (C and D) SNR (C) and uniformity (D) measured for several pulse sequences with and without the PET insert, using a uniform phantom.

Fig. 3.

Simultaneous in vivo PET and MR imaging. (A) Mouse FDG tumor imaging. (Upper Left) PET image, (Upper Right) MR image, and (Lower) fused PET and MR image. One transaxial image slice is shown. (B) Fused PET and MR images of a mouse. Transaxial sections from top of head to bladder are shown. (Scale bars, 5 mm.) The same false-color look-up table is used in both A and B.

Fig. 4.

Advanced MR measurements. (A) In vivo MR spectroscopy; mouse ¹H brain spectrum acquired in the presence of the PET insert. (B) ADC map of an in vivo mouse brain acquired by using a four-shot EPI sequence in the presence of the PET insert. ADC units are 10⁻³ mm²/sec. (Scale bar, 2 mm.)