MR Performance in the Presence of a Radio Frequency-Penetrable Positron Emission Tomography (PET) Insert for Simultaneous PET/MRI

Abstract

Despite the great promise of integrated positron emission tomography (PET)/magnetic resonance (MR) imaging to add molecular information to anatomical and functional MR, its potential impact in medicine is diminished by a very high cost, limiting its dissemination. An RF-penetrable PET ring that can be inserted into any existing MR system has been developed to address this issue. Employing optical signal transmission along with battery power enables the PET ring insert to electrically float with respect to the MR system. Then, inter-modular gaps of the PET ring allow the RF transmit field from the standard built-in body coil to penetrate into the PET fields-of-view (FOV) with some attenuation that can be compensated for. MR performance, including RF noise, magnetic susceptibility, RF penetrability through and $B_{1}$ uniformity within the PET insert, and MR image quality, were analyzed with and without the PET ring present. The simulated and experimentally measured RF field attenuation factors with the PET ring present were -2.7 and -3.2 dB, respectively. The magnetic susceptibility effect (0.063 ppm) and noise emitted from the PET ring in the MR receive channel were insignificant. $B_{1}$ homogeneity of a spherical agar phantom within the PET ring FOV dropped by 8.4% and MR image SNR was reduced by 3.5 and 4.3 dB with the PET present for gradient-recalled echo and fast-spin echo, respectively. This paper demonstrates, for the first time, an RF-penetrable PET insert comprising a full ring of operating detectors that achieves simultaneous PET/MR using the standard built-in body coil as the RF transmitter.

Fig. 1.

(A) PET detector Faraday cage (bottom segment 6.2 cm, top segment 7.9 cm, height 4.1 cm, length 21.4 cm) with optical fibers exiting the end through a waveguide. (B) Electro-optical PET detector module comprising 128 LYSO scintillation crystal elements each coupled one-toone to SiPM pixels (the crystals span a 3 cm x 6 cm area); “compressed sensing” multiplexing circuit that multiplexes the 128 SiPM signals to 16 readout channels; (C) Optical transmitter boards each containing two nonmagnetic VCSELs.

Fig. 2.

Conceptual comparison between (TOP) the conventional and (BOTTOM) electrically floating RF-penetrable PET system configurations for simultaneous PET/MR. The electrically floating feature of the latter design, enabled by optical signal transmission and electrically floating batteries, along with short power cables minimizes electromagnetic pick-up noise and promotes a patient-safe environment. In the latter configuration, the RF field from the standard body coil penetrates through small gaps between the PET detector modules to create a uniform B₁ field inside the sensitive field-of-view.

Fig. 3.

RF-penetrable PET ring prototype was inserted into a 3-T MRI. The 16 PET detector modules were populated with detectors/electronics, electro- optical signal transmission components, and powered by electrically floating non-magnetic batteries.

Fig. 4.

SNR comparison of the MR image with PET powered with electrically floating batteries using short (~1 m) and long (~7 m) cables, and a floating/grounded power supply plugged into mains power using ~7 m length cable. Center slices are shown. The air bubble shown in different positions of the phantom is due to repositioning of the phantom between different scan configurations.

Fig. 5.

B₀ field maps without and with operating PET present. Center slices are shown.

Fig. 6.

RF noise spectra with the PET ring (A) not operating (unpowered) and (B) operating. Red lines indicate ±1 standard deviation from mean.

Fig. 7.

3D electromagnetic simulations of B₁ field distribution magnitudes (A) without and (B) with PET. The RF field lines enter through the 1 mm gaps between PET detector modules but do not penetrate the Faraday cages surrounding each module. Center slices are shown.

Fig. 8.

(a) B₁ field map and (b) a 1D B₁ horizontal profile without and with the PET ring, showing RF TX field attenuation. Center slices are shown.

Fig. 9.

RX field attenuation was measured through (A) MR images and (B) SNR maps of GRE and FSE sequences. SNR maps of the EPI sequence were not calculated due to the ghosting artifact. Center slices are shown.

Fig. 10.

Comparison between (a) the RF-penetrability setup and (b) the new prototype currently under construction.