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

Abstract

Multiparametric MRI of the prostate at clinical magnetic field strengths (1.5/3 Tesla) has emerged as a reliable noninvasive imaging modality for identifying clinically significant cancer, enabling selective sampling of high-risk regions with MRI-targeted biopsies, and enabling minimally invasive focal treatment options. With increased sensitivity and spectral resolution, ultra-high-field (UHF) MRI (≥ 7 Tesla) holds the promise of imaging and spectroscopy of the prostate with unprecedented detail. However, exploiting the advantages of ultra-high magnetic field is challenging due to inhomogeneity of the radiofrequency field and high local specific absorption rates, raising local heating in the body as a safety concern. In this work, we review various coil designs and acquisition strategies to overcome these challenges and demonstrate the potential of UHF MRI in anatomical, functional and metabolic imaging of the prostate and pelvic lymph nodes. When difficulties with power deposition of many refocusing pulses are overcome and the full potential of metabolic spectroscopic imaging is used, UHF MR(S)I may aid in a better understanding of the development and progression of local prostate cancer. Together with large field-of-view and low-flip-angle anatomical 3D imaging, 7 T MRI can be used in its full strength to characterize different tumor stages and help explain the onset and spatial distribution of metastatic spread.

Keywords: Lymph nodes; Metabolic imaging; Prostate cancer; Ultra-high-field (UHF) MRI; mpMRI.

Fig. 1

T₂w imaging of the prostate in two orientations. Axial (a, b) and sagittal (c, d) slices of the prostate of a patient with prostate cancer at 3 T (a, c) and at 7 T (b, d). The patient had a Gleason 3 + 4 lesion in the right midgland peripheral zone. The 3 T data was acquired with an endorectal coil (Medrad^®, Pittsburgh, PA, US) at a spatial resolution of 0.4 × 0.4 × 3.0 mm (TE 101 ms, acquisition time 4:21 min for each orientation). 7 T data was acquired as in [35] with an external 8-channel transceiver coil after phase shimming at a spatial resolution of 0.75 × 0.75 × 3.0 mm (TE 71 ms, acquisition time 1:30 min for each orientation)

Fig. 2

MRI with an ERC at 7 T of the prostate of a 62-year-old patient with prostate cancer (Gleason score 3 + 3). a Five transversal T₂w turbo spin-echo images from a series of 23 slices from apex to seminal vesicles, resolution 0.3 × 0.3 × 2.0 mm³. b Transversal ADC maps at corresponding locations (5 images from 23 slices), resolution 1.75 × 1.75 × 2.0 mm³. Reprinted with permission from Lagemaat et al. [41]

Fig. 3

GPC + GPE and PC + PE, metabolite maps of a 67-year-old patient with metastatic Gleason 4 + 4 prostate cancer (a, b) overlaid over a T₂w image. ³¹P (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]

Fig. 4

The effect of USPIO nanoparticles at 7 T in the lymph nodes of a 54-year old patient with prostate cancer metastases. a–c Sagittal images of the same location at different computed echo times (TEs). d Lipid-selective image. e The original TE = 8.3 ms image from the multigradient echo water-selective imaging. f Map of fitted R₂* relaxation rates. g Sagittal overview image with inset of location of images a–f. Three lymph nodes accumulated USPIO particles and rapidly lost MR signal intensity (white circles), while one suspicious lymph node without USPIOs retained MR signal intensity with increasing TE (white arrow). The lymph node marked with (#) showed a slow signal decay, with an R₂* value of 80 ± 6 s⁻¹, whereas the lymph node marked with (*) showed fast signal decay, with an R₂* value of 247 ± 25 s⁻¹. Reprinted with permission from Scheenen et al. [64]

Fig. 5

Full FOV and zoomed-in versions of axial and coronal T₂w TSE images of prostates from 4 subjects. Images demonstrate excellent contrast and SNR achieved by phase-only RF shimming. Although providing high peak B₁⁺ and reasonable homogeneity over the prostate, the local RF shim resulted in unmanaged RF fields outside the immediate region of the targeted anatomy that caused spatially varying destructive interference patterns as observed in the unzoomed images. Reprinted with permission from He et al. [69]