Synopsis

Typically, posterior with anterior array or Large/Medium MSK coils or endorectal coil are used for prostate and rectal imaging. These coils do not provide deep depth, high-resolution and acceleration images. A novel high-resolution flexible phased-array coil for Prostate, Rectal and Pelvis Imaging is presented. This coil contains a posterior base with two lateral flaps and one center flap with wire loops providing a light weight flexible conforming coverage to prostate, rectal and pelvis. This close fitting high density 50-Channel coil provides better diagnostic images and comfort. This coil also introduces new thin flexible materials including two thermoplastic coated thin fabrics.

Introduction

At present, posterior with anterior array or Large/Medium MSK coils or endorectal coil are used for prostate and rectal imaging [1, 2]. A typical anterior array with posterior array does not provide deep depth and high-resolution images due to the large distance of the coil element from the prostate and rectal area. High acceleration factors are also not achievable due to the low SNR. Even though an endorectal coil provides very close proximity to the prostate area, it does not provide patient comfort, volume measurements are not possible, and acceleration is not possible due limited number of receivers. In this paper, a novel flexible high-resolution phased-array coil for Prostate, Rectal and Pelvis Imaging is presented. A prototype of the flexible 50-channel phased-array coil was constructed on fabric materials (fitting more than 99th - percentile US male). The SNR performance was quantitatively evaluated in phantoms and compared to the latest clinical 30-channel phased array with a posterior array built. The performance of the 50-channel flexible coil will be demonstrated in the T2-weighted and highly accelerated diffusion-weighted imaging.

Methods

A range of body sizes were evaluated to ensure this coil provides the flexibility to accommodate the 5^th percentile woman up to the 99^th percentile male. A novel ultra-flexible high-resolution phased-array coil was developed using Air coil loops [3, 4] and flexible materials. The air coil loops are made from highly flexible 1.0 mm wire optimized for zero reactance at 127.7 MHz. The electronics module including the preamplifier, decoupler, matching, and baluns is connected to the new Air coil loop. Nearest neighboring elements were decoupled by overlapping and next nearest neighbor and more distant elements were decoupled by preamplifiers. This coil is optimized to reduce the RF switching time for Silent imaging. Full coverage of the prostate and rectal area was achieved by 50 elements located in a posterior base with two lateral flaps (Left and Right-side) and one center flap. The flexible materials and Air Coil loops provide a light weight flexible conforming coverage (Fig. 1). This coil shall support 1D acceleration factor of 3 (R=3) with ASSET and Multi-Band acceleration factor of 3 (SI) in FOV 30 cm. This coil provides deeper depth, higher resolution and highly accelerated images dues to the patient body proximity and higher channel counts. Air loops are between two thermoplastics coated thin fabrics (0.6mm). The medical grade foil was used as the coil cover sleeve for biocompatibility and a water seal. The SNR and g-factors for R=2, 3, 4 were measured in phantoms. The coil was developed and tested on a 144-channel 3T GE scanner (Signa Premier, GE Healthcare, Aurora, USA).

Results

SNR maps at four axial slices using SOS reconstruction and noise correlation map are shown in Fig. 2. For SNR comparison, Spin Echo images were obtained in phantoms. At the central prostate image region (~7cm from Anus), the novel flexible coil shows 36% better SNR than the 30-channel Air anterior array with posterior array on a loading shell phantom. In the entire prostate image region, the novel flexible coil shows 35% better SNR than the 30-channel Air anterior array with posterior array on a loading shell phantom. On average, in the central and entire pelvis image regions, the novel flexible coil shows ~10% better SNR than the 30-channel Air anterior array with a posterior array on a loading shell phantom (Fig. 3). Figure 4 shows the maximum g-factor results at R= 2, 3 and 4 for the novel flexible coil for 1D acceleration in the axial. Table 1 summarizes the maximum g-factor at 2, 3 and 4 for 1D acceleration in the axial. The benefits of increased channel number for reducing g-factor are clear.

Discussion

The loading shell phantom results of B1 sensitivity maps and g-factors will be compared to the results of in vivo images soon.

Conclusions

This novel 50-channel coil that accommodate the 5^th percentile woman up to the 99^th percentile male provides ultra-flexibility, comfort, and high-resolution imaging. The coil shows good prostate and pelvis coverage, lower g-factors in the axial, sagittal and coronal planes. The loading shell phantom SNR performance in prostate area is ~35% better than Air anterior array with posterior coil. The loading shell phantom SNR performance in pelvis area is similar to Air anterior array with posterior coil.

Acknowledgements

We thank Sandeep Deshmukh from Thomas Jefferson University, YoungHan Lee from Yonsei University and Peder Larson from UCSF for their valuable inputs. We also thank Saban Kurucay, Dan Weyers, Dan Chirayath and Mohamed El-Demerdash from GE Healthcare for their continued support.

References

1. Hricak, H., et al., Carcinoma of the prostate gland: MR imaging with pelvic phased-array coils versus integrated endorectal-pelvic phased-array coils. Radiology, 1994 Dec;193(3):703-9

2. Kim BS., et al., Comparison of pelvic phased-array versus endorectal coil magnetic resonance imaging at 3 Tesla for local staging of prostate cancer. Yonsei Med J., 2012 May;53(3):550-6.

3. P. Rossman et al., Characterization of a new ultra-flexible low profile RF receive coil technology, Proceedings ISMRM 2017, 763.

4. S. Vasanawala et al., Development and Clinical implementation of very light weight and highly flexible Air technology arrays, Proceedings ISMRM 2017, 755.

5. Roemer, P.B., et al., The NMR phased array. MRM, 1990; 16(2):192-225.

6. Pruessmann K.P., et al., SENSE: sensitivity encoding for fast MRI. MRM, 1999;42:952–962.