A multitransmit external body array combined with a 1 H and 31 P endorectal coil to enable a multiparametric and multimetabolic MRI examination of the prostate at 7T

Abstract

Purpose: In vivo ¹ H and ³¹ P magnetic resonance spectroscopic imaging (MRSI) provide complementary information on the biology of prostate cancer. In this work we demonstrate the feasibility of performing multiparametric imaging (mpMRI) and ¹ H and ³¹ P spectroscopic imaging of the prostate using a ³¹ P and ¹ H endorectal radiofrequency coil (ERC) in combination with a multitransmit body array at 7 Tesla (T).

Methods: An ERC with a ³¹ P transceiver loop coil and ¹ H receive (Rx) asymmetric microstrip (³¹ P/¹ H ERC) was designed, constructed and tested in combination with an external 8-channel ¹ H transceiver body array coil (8CH). Electromagnetic field simulations and measurements and in vivo temperature measurements of the ERC were performed for safety validation. In addition, the signal-to-noise (SNR) benefit of the ¹ H microstrip with respect to the 8CH was evaluated. Finally, the feasibility of the setup was tested in one volunteer and three patients with prostate cancer by performing T₂ -weighted and diffusion-weighted imaging in combination with ¹ H and ³¹ P spectroscopic imaging.

Results: Electromagnetic field simulations of the ³¹ P loop coil showed no differences in the E- and B-fields of the ³¹ P/¹ H ERC compared with a previously safety validated ERC without ¹ H microstrip. The hotspot of the specific absorption rate (SAR) at the feed point of the ³¹ P/¹ H ERC loop coil was 9.42 W/kg when transmitting on ³¹ P at 1 W. Additional in vivo measurements showed a maximum temperature increase at the SAR hotspot of 0.7°C over 6 min on ³¹ P at 1.9 W transmit (Tx) power, indicating safe maximum power levels. When transmitting with the external ¹ H body array at 40W for 2:30 min, the temperature increase around the ERC was < 0.3°C. Up to 3.5 cm into the prostate the ¹ H microstrip of the ERC provided higher SNR than the 8CH. The total coil combination allowed acquisition of an mpMRI protocol and the assessment of ³¹ P and ¹ H metabolites of the prostate in all test subjects.

Conclusion: We developed a setup with a ³¹ P transceiver and ¹ H Rx endorectal coil in combination with an 8-channel transceiver external body array coil and demonstrated its safety and feasibility for obtaining multiparametric imaging and ¹ H and ³¹ P MRSI at 7T in patients with prostate cancer within one MR examination.

Keywords: 1H; 31P; endorectal coil; prostate cancer; spectroscopy.