Proton spectroscopic imaging of the human prostate at 7 T

Abstract

The sensitivity of proton MR Spectroscopic Imaging ((1)H-MRSI) of the prostate can be optimized by using the high magnetic field strength of 7 T in combination with an endorectal coil. In the work described in this paper we introduce an endorectal transceiver at 7 T, validate its safety for in vivo use and apply a pulse sequence, optimized for three-dimensional (3D) (1)H-MRSI of the human prostate at 7 T. A transmit/receive endorectal RF coil was adapted from a commercially available 3 T endorectal receive-only coil and validated to remain within safety guidelines for radiofrequency (RF) power deposition using numerical models, MR thermometry of phantoms, and in vivo temperature measurements. The (1)H-MRSI pulse sequence used adiabatic slice selective refocusing pulses and frequency-selective water and lipid suppression to selectively obtain the relevant metabolite signals from the prostate. Quantum mechanical simulations were used to adjust the inter-pulse timing for optimal detection of the strongly coupled spin system of citrate resulting in an echo time of 56 ms. Using this endorectal transceiver and pulse sequence with slice selective adiabatic refocusing pulses, 3D (1)H-MRSI of the human prostate is feasible at 7 T with a repetition time of 2 s. The optimized inter-pulse timing enables the absorptive detection of resonances of spins from spermine and citrate in phase with creatine and choline. These potential tumor markers may improve the in vivo detection, localization, and assessment of prostate cancer.

Uniform prostate imaging and spectroscopy at 7 T: comparison between a microstrip array and an endorectal coil

“…However, spectroscopic images could be made with the endorectal transceiver using a semi-LASER sequence with MEGA [Mescher Garwood (named after the authors)] water and lipid suppression with FOV ¼ 50 Â 40 Â 50 mm 3 , acquisition matrix of 7 Â 7 Â 8 and TR/TE ¼ 1.6/56 ms (5). Clear resonances of choline, creatine and citrate can be seen (Fig.…”

“…For spectroscopic imaging of the prostate with the eight-element microstrip array, conventional pulses are not suitable, as these pulses cannot obtain a sufficiently high amplitude within the available power and SAR limitations to achieve sufficient bandwidth. Adiabatic sliceselective refocusing pulses that have a much larger bandwidth were therefore considered for volume-selective spin manipulation, similar to Klomp et al (5).…”

7 Tesla MR Imaging: Opportunities and Challenges

“…Klomp et al demonstrated the successful detection of resonances of spins of potential tumor markers like creatine and choline [63]. They applied an endorectal transceiver coil system and a pulse sequence with slice selective adiabatic refocusing pulses [63].…”

“…Klomp et al demonstrated the successful detection of resonances of spins of potential tumor markers like creatine and choline [63]. They applied an endorectal transceiver coil system and a pulse sequence with slice selective adiabatic refocusing pulses [63]. Recent publications have mainly focused on the optimization of coil systems for ultra-high-field prostate imaging, as deeply localized organs are known to be challenging with respect to spin excitation and signal reception [64,65].…”

DCE-MRI, DW-MRI, and MRS in Cancer

“…For mpMRI, full 3D acquisition is usually most appropriate. For coupled spin systems, it is essential to select the acquisition echo time to achieve the optimal in-phase signals (eg, for lactate 27 and citrate in prostate 28 ). The repetition time should be chosen with reference to expected metabolite T 1 to achieve optimal SNR per unit time.…”