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

Abstract

Purpose In vivo 1H and 31P 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 1H and 31P spectroscopic imaging of the prostate using a 31P and 1H endorectal radiofrequency coil (ERC) in combination with a multitransmit body array at 7 Tesla (T).MethodsAn ERC with a 31P transceiver loop coil and 1H receive (Rx) asymmetric microstrip (31P/1H ERC) was designed, constructed and tested in combination with an external 8‐channel 1H 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 1H 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 1H and 31P spectroscopic imaging.ResultsElectromagnetic field simulations of the 31P loop coil showed no differences in the E‐ and B‐fields of the 31P/1H ERC compared with a previously safety validated ERC without 1H microstrip. The hotspot of the specific absorption rate (SAR) at the feed point of the 31P/1H ERC loop coil was 9.42 W/kg when transmitting on 31P at 1 W. Additional in vivo measurements showed a maximum temperature increase at the SAR hotspot of 0.7°C over 6 min on 31P at 1.9 W transmit (Tx) power, indicating safe maximum power levels. When transmitting with the external 1H 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 1H microstrip of the ERC provided higher SNR than the 8CH. The total coil combination allowed acquisition of an mpMRI protocol and the assessment of 31P and 1H metabolites of the prostate in all test subjects.ConclusionWe developed a setup with a 31P transceiver and 1H 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 1H and 31P MRSI at 7T in patients with prostate cancer within one MR examination.

Ultra-high-field MR in Prostate cancer: Feasibility and Potential

“…To allow 3D coverage of the entire prostate, the combination of a body array transmit coil and ERC for reception was tested by Lagemaat et al [ 41 ]. This combination of a transmit surface array coil and receive ERC was able to provide a homogeneous B 1 + field in the prostate with detection at high SNR [ 41 , 56 ]. Patients were subjected to an mpMRI examination, consisting of T 2 w, DWI, and 1 H-MRSI.…”

“…In a 7 T mpMRI feasibility study by Philips et al [ 56 ], using a 31 P/ 1 H ERC with an external 1 H transceiver body array and spectral-spatial refocusing pulses, 1 H-MRSI spectra were acquired from three patients. Adequate SNR was seen from apex to base of the prostate resulting in spectra with well distinguishable choline and spermine signals, while citrate in some cases suffered from lipid contamination.…”

“…Ratios including the total choline could then serve to characterize prostate cancer and remove ERC coil profile effects [ 57 ]. As the spectral-spatial pulses result in high spermine signals in the 1 H spectra [ 53 , 56 ], the value of these spermine signals for prostate cancer characterization should be investigated further, as biopsy studies have shown a significant decrease in spermine concentration with increasing tumor grade [ 59 ].…”

“…As T 2 w imaging of the prostate by a transceiver ERC suffers from B 1 inhomogeneities, the combination of an ERC with a 1 H external coil array would be optimal for functional and anatomical imaging. In a study by Philips et al, this strategy was implemented by combining an ERC with a 31 P transceiver loop-coil and 1 H receive (Rx) asymmetric microstrip ( 31 P/ 1 H ERC) with an external 1 H transceiver body array coil [ 56 ]. After extensive safety testing, the total coil combination allowed acquisition of a multinuclear mpMRI protocol, consisting of high-resolution T 2 w imaging and DWI without artifacts and the assessment of 31 P and 1 H metabolites of the prostate in three patients with histology proven prostate cancer.…”

“…31 P ( c , d ) spectra are shown of the cancer lesion (red circle) as well as a location within healthy tissue (blue circle). Adapted from Philips et al [ 56 ] …”