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. 2019 Feb;81(2):1434-1446.
doi: 10.1002/mrm.27444. Epub 2018 Sep 9.

Performance evaluation of RF coils integrated with an RF-penetrable PET insert for simultaneous PET/MRI

Brian J Lee  1   2 , Ronald D Watkins  1 , Keum Sil Lee  1 , Chen-Ming Chang  1   3 , Craig S Levin  1   4   5   6
Affiliations

Performance evaluation of RF coils integrated with an RF-penetrable PET insert for simultaneous PET/MRI

Brian J Lee et al. Magn Reson Med. 2019 Feb.

Abstract

Purpose: An "RF-penetrable" PET insert that allows the MR body coil to be used for RF transmission was developed to make it easier for an existing MR center to achieve simultaneous PET/MRI. This study focuses on experiments and analyses to study PET/RF coil configurations for simultaneous PET/MR studies.

Methods: To investigate the appropriate RF coil design, a transmit/receive (TX/RX) birdcage coil and an RX-only phased-array coil (TX from body coil), both fitting inside the PET ring were built and characterized. For MR performance evaluation, B1 field uniformity and MR image SNR were calculated. PET photon attenuation due to each coil was studied by means of CT-based attenuation maps and reconstructed PET images.

Results: When using the RX-only phased-array coil (TX from body coil), compared with the TX/RX birdcage coil, the B1 field uniformity and the MR image (gradient echo and fast spin echo) SNR increased by 2.4±4.8%, 386.1±62.3%, and 205.0±56.5%, respectively. Although some components of the coil were distributed within the PET FOV, no significant PET photon attenuation was shown in the CT-based attenuation map and reconstructed PET images.

Conclusion: RF coil configurations for an RF-penetrable PET insert for simultaneous PET/MRI were studied. The RX-only phased-array coil (TX from body coil) outperformed the TX/RX birdcage coil with improved MR performance as well as negligible PET photon attenuation.

Keywords: PET insert; PET/MR; RF coil; RF-penetrable; instrumentation.

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Figures

Figure 1
Figure 1
The RF-penetrable PET insert on the patient bed of a 3T MR system. Each of the 16 detector modules comprising the 32 cm inner diameter PET ring are shielded inside 17.5 μm thick copper cages that extend 20 cm axially. The entire insert is powered by batteries. The PET electrical signals are multiplexed and converted to optical signals, which are taken out of the MR system through 20 m length optical fibers.
Figure 2
Figure 2
(A) Schematics of the simultaneous PET/RF coil insert placed inside an MR system. The PET scintillation crystals are placed at the axial edge of the PET ring prototype, which is placed at the MR system isocenter, therefore, some part of the RF coil (19 cm length, 29 cm diameter) protrudes axially from the PET ring. Photos of a custom (B) quadrature TX/RX birdcage coil and (C) 8 channel RX-only phased-array coil built for the studies reported in this paper.
Figure 3
Figure 3
Schematic of the resolution and normalization phantom. The resolution phantom consists of 5.2, 4.2, 3.2, and 2.8 mm diameter hot rods and 4.2 mm cold rods in a 6.5 cm diameter cylinder. The gap between rods is the same as the diameter of the rod. The normalization phantom is a uniform cylinder phantom with the same dimension as the resolution phantom.
Figure 4
Figure 4
The B1 field maps (center slice), axial slice amplitudes (flip angles) and field uniformities are plotted for all configurations.
Figure 5
Figure 5
The center slices of GRE and FSE sequences MR images and SNR maps were plotted for all configurations.
Figure 6
Figure 6
CT-based attenuation maps of the custom TX/RX birdcage coil and 8 channel phased-array RX-only coil. The attenuation maps that align with the sensitive axial FOV of the PET system are shown in the red-dotted box.
Figure 7
Figure 7
Sum of all 28 reconstructed PET image slices and associated CNR of the rods in the sum image for each coil configuration.
See this image and copyright information in PMC

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