Synopsis

Keywords: RF Arrays & Systems, Body

The purpose of this study is to compare two different coil array designs for prostate imaging. We compared an in-house developed 8ch^pTx/Rx micro stripline element coil array with meanders to a recently introduced, commercially available 8ch^pTx/Rx coil array based on fractionated dipoles for ¹H prostate imaging at 7T. Both coil arrays showed comparable results in the phantom and in vivo examinations. The commercially available coil array performs slightly better in the region of the prostate and is well suited for future prostate imaging at 7T.

Introduction

Ultrahigh-field (UHF) MRI at 7T provides capabilities to better characterize intraprostatic lesions and the potential to improve accuracy in treatment planning. [1] Even standard ¹H imaging at UHF enables the detection of small lesions which are often not depictable at clinical field-strengths of 1.5T and 3T [2,3]. The potential role of prostate imaging at 7T could be in achieving a better understanding of the development and progression of local prostate cancer, the onset and spatial distribution of metastatic spread, and in characterization of different tumor stages and recurrent disease [4]. However, the performance of imaging substantially benefits from RF coil designs fulfilling requirements like efficiency and coupling between individual RF elements [5,6]. Previous work showed that an array of eight transmit dipoles appears to be most favorable for potential use in 7T body MRI [7].In this study, we compared the performance of one custom-built 8-ch^pTx/Rx [8] built of micro stripline elements with meanders named Coil A (Fig. 1left) with a commercially available coil with 8-ch^pTx/Rx built of fractionated dipole antennas (Tesla Dynamic Coils B.V., the Netherlands) named Coil B (Fig. 1right).

Methods

MR imaging was performed on a 7T MRI system (MAGNETOM Terra, Siemens Healthcare GmbH, Germany) equipped with 8ch pTx system. The study was approved by the institutional MR safety board and informed consent was obtained from a single participant (age: 23, sex: male, height/weight: 1.90 m/90 kg). The micro stripline elements of Coil A are 250mm and the fractionated dipoles of Coil B are 316 mm in length. Both coils feature an integrated additional receive array. Coil A has an additional 24ch^{Rx 1}H loop array whereas Coil B has an additional 4^Tx/Rx/24ch^Rx RF array for ³¹P beneath the ¹H Tx elements (Fig. 2). Within the scope of comparison, both additional coil array structures of Coil A and Coil B where deactivated on the scanner, leaving only the 8ch Tx/Rx elements for ¹H activated. All performance comparisons were done with a transmitter reference voltage of 500 V. Quantitative comparisons were performed on the same cylindrical body-sized phantom (fig.2) filled with tissue-simulating liquid (ԑ_r = 46.3, σ = 0.8 Ω⁻¹ m⁻¹) to reflect a typical body load. Flip angle (FA) maps were obtained using the 3DREAM technique [9]. In vivo FA datasets of one subject were acquired with both coil arrays.
SNR maps were obtained from a reference GRE scan using an optimal coil combination weighted by the inverse of the noise covariance matrix obtained from a service sequence on the scanner (coil utils, VE12U). Slice profiles were generated for both coils in the phantom and in vivo (fig.3). Images were evaluated qualitatively regarding artefacts and homogeneity over the prostate region and across slices. After starting with a circular polarized excitation (CP+, 45° phase increment) a phase-only optimization for a B₁^+-efficient excitation within the region-of-interest (ROI) was performed offline in MATLAB R2020a (MathWorks, Natick, MA, USA) based on exported relative B₁⁺ maps [10,11] (fig.4). The phase settings with equal amplitudes were then manually updated on the scanner console. A manually drawn ROI in the prostate was set. The mean FA was measured for both coils in the ROI. Finally, prostate images acquired with Coil B and RF TIAMO shimming [12] are presented (fig.5).

Results

Coil housing comparison shows improved patient comfort due to the integrated foam of Coil B. The RF elements of Coil A are closer to the subject of investigation and show improved SNR in the periphery of the phantom. However, the slice profile and the prostate ROI are comparable with a slight advantage for Coil B. Qualitative measured in vivo FA map in the ROI of both coils in the prostate shows comparable results with the static shimming and 0.9° difference in respect to the chosen ROI.
The B₁⁺field of Coil A and Coil B show similar results in the ROIs. However, when the shimming approach is applied, Coil B has a slight advantage of 2.7% and creates higher B₁⁺field in the chosen ROI. Figure 5 shows the in vivo MRI with the GRE TIAMO sequence of the prostate.

Discussion

We evaluated the Tx/Rx performance of two different 8ch Tx/Rx body RF coils with the goal of enabling 7T prostate imaging and the detection of small lesions with the new, commercially available RF coil in the future. Overall, both coils showed comparable results. The slight advantage of Coil B can be explained through the more morphologically shaped bottom coil former. Coil B shows a very well-integrated coil design that fits into the MAGNETOM Terra MR system, whereas Coil A was originally designed for the patient table of the older MAGNETOM 7T MRI system. Both RF coils yielded satisfying image quality for the prostate region. Workflow improvements were particularly noted with the robust coil design of Coil B since the interface setup is integrated in the upper posterior body part of Coil B. The additional option of enabling the 4^Tx/Rx/24ch^Rx for ³¹P Rx coil array for 7T Coil B is very well suited for multiparametric prostate imaging at 7T in the future. Future work will focus on assessing the reproducibility in larger studies involving subjects with varying body mass indices.

Acknowledgements

The Coil B used in this work was constructed with support from the European Fund for Regional Development EFRO OP-2014-2023-Oost [PROJ-01009].

References

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