In prostate cancer (PCa) management multiparametric(mpMRI) proton MRI forms an important tool to assess the aggressiveness and stage [1] of the disease. Advancing this technique to 7 Tesla using a ¹H endorectal coil (ERC) in combination with an 8-channel body coil already showed promising results for imaging and spectroscopy [2]. Combing the mpMRI results with metabolic information derived from ³¹P spectra can add valuable information on the metabolism of the disease.

Surface coils have the disadvantage of an extremely inhomogeneous B1 field which is especially unfavorable for techniques like fast or turbo spin echo imaging (TSE). An earlier described double tuned ERC [3] with TxRx capabilities on ³¹P and ¹H has this drawback. Here we demonstrate a dual element ERC with a ¹H Rx element and a ³¹P TxRx loop combined with an external 8-channel body array for homogeneous B1-shimmed ¹H transmit fields at 7 Tesla.

The ERC is a modified commercially available 3T ERC (Medrad) and consist of a ³¹P TxRx loop [4] and an asymmetric microstrip ¹H receive element. The ¹H receive element is positioned on the rod (fig. 1) of the ERC. The ground plane of the microstrip is rotated around the center of the rod to focus the Rx field. Detuning was done with a series PIN-diode, that adds some extra noise but ensures safe operation if not actively controlled. Via a 4:1 half-wave coax balun and a 75cm cable the element was connected to a preamplifier. A separate 75cm cable was used to connect the ³¹P element to a home-built transmit-receive switch with an integrated preamplifier. A single floating proton cable trap enclosing both cables ensured the absence of common mode currents. For transmitting on proton a multi-transmit 8-channel body coil was used [4,5].

The safety of the setup was ensured by EM-field simulations and measurements and extensive safety measurements on the MR-system.

After patient-specific B0 and B1+ shimming of the prostate with the multi-Tx external coil array a mpMRI protocol was performed with ¹H imaging, and both ¹H and ³¹P spectroscopy. Transversal T2-weighted and diffusion weighted imaging were performed as anatomical and functional references. ¹H-MRSI was done with a PRESS-like (TR=1000ms, TE=135ms, FOV=84x70x70, matrix=12x10x10, 50% Hamming filter, NA=1, true voxel size 0.94 cc, acquisition time 7:01 minutes) sequence with spectral-spatial refocusing pulses [6]. Only the spectral region in the VOI ranging from 2.3-3.3 ppm was refocused. Lipids from outside the prostate were saturated with two saturation bands in the ventral side. The ³¹P MRSI sequence was played out with a non-localized BIR-4 45 degree excitation pulse (TR=1500ms, FOV=120x100x100, matrix=12x10x10, NA=4, true voxel size 4.2cc, acquisition time 13:09 minutes).

Initial measurements were performed on one patient with histologically proven Gleason 4+4 prostate cancer.

Safety tests showed no temperature increase or disturbances in the proton B1+ field of the external body array when the ERC was present. Next to that the EM-field simulations and measurements showed no change in EM-field distributions of the dual-element ERC compared to the ³¹P TxRx ERC (fig 2).

The T2W and ADC images acquired with the ERC clearly show the tumor in the peripheral zone of the patients’ prostate (fig 3a&b). ¹H and ³¹P MRSI voxels representing tumor and healthy tissue were picked based on the 7T images. Because of Hamming filtering of the MRSI k-space matrix, the true voxel shapes after 3D Fourier transform were best represented by spheres, indicated by the colored circles (fig 3c-f).

Proton spectra (fig 3c&d) show elevated total choline (Cho) in the cancer lesion which is almost absent in healthy tissue, in which large polyamine and citrate signals are present. In the right peripheral zone of the prostate (with tumor tissue) the B1-shim was not optimal, resulting in some signal dropout in the ADC image as well as a distorted citrate shape in the shown tumor voxel. The corresponding voxels in the phosphorous spectrum reveal big differences in relative phospho-monoester-and -diester signals between the tumor and the healthy tissue. Glycerophosphocholine (GPC), glycerophosphoethanolamine (GPC) and phosphocholine (PC) were elevated in tumor compared to healthy tissue at equal distance from the ERC (fig 3e&f).

In this work we present a ³¹P/¹H dual-element endorectal coil combined with an 8-channel multi-TxRx body coil capable of acquiring proton imaging, proton spectra and phosphorous spectra of the human prostate in a single scan session at 7 Tesla. Care should be taken for homogeneous B1-shimming of the whole prostate and an optimal balance between voxel size, measurement time and SNR of the ³¹P MRSI experiment.