The use of proton spectroscopic imaging in prostate cancer at clinical field strengths of 1.5 and 3T will be discussed. Moreover, latest developments at 3T, as well as the possibilities of 7T are shown for proton as well as phosphorous spectroscopic imaging, culminating in the ultimate multi-parametric MRI examination...
Prostate Cancer: Proton & Beyond
The use of MR imaging and functional MR techniques in the management of prostate cancer has gradually gained importance during the last decade. Already in the late nineties it was recognized that the excellent soft tissue contrast provided by MR could be beneficial in assessing prostate abnormalities (1). Nowadays prostate MR consist of a multi-parametric MR examination (mpMRI) combining, next to conventional anatomic T2W MR imaging, functional MR techniques such as diffusion-weighted imaging (DWI), dynamic contrast-enhanced MR imaging (DCE-MRI) and proton MR spectroscopic imaging (1H-MRSI). The latter, due to the lack of postprocessing automation and the infamous reputation of being ‘difficult’, is not part of current clinical guidelines for mpMRI (PIRADS 2.0). Surely, there is room for clinical research using 1H-MRSI and spectroscopy of other nuclei to better understand the disease.
The step from 1.5 to 3 Tesla largely increased the number of opportunities for prostate MR and opened up the possibilities of multiparametric MRI for detection, localization, staging and risk stratification of prostate cancer (2). Faster proton spectroscopic imaging schemes (weighted sampling, flyback EPI, spiral sampling of k-space(3)) and low-power adiabatic refocusing pulses (4) provide more flexibility in choosing spatial resolution and total acquisition time, and can provide robust metabolic maps. Increasing the magnetic field strength further may bring additional answers to some of the existing fundamental or clinical questions in prostate cancer. Ultra high field MR is of particular interest for imaging and spectroscopy techniques that are SNR limited at the clinical field strengths of 1.5 or 3 Tesla, such as non-proton imaging and spectroscopy.
1H-spectroscopic imaging has been explored in the prostate at 7T. A transmit receive endorectal coil provided improved peak transmit and receive B1 but reduced B1 homogeneity compared to a transmit-receive external surface array. The rapid decrease of B1+ with increasing distance from the coil limits the advantage of peak B1+ performance and severely compromised anatomical imaging with conventional RF pulses (5,6). However, for 1H-MRSI this coil was very well suited because of the high B1+ close to the coil. To overcome the large B1+ inhomogeneities and to reduce or prevent chemical shift displacements artifacts the first 3D 1H-MRSI sequences for 7 Tesla were semi or fully adiabatic (semi-LASER or LASER) (6,7).
For excitation with large body coils challenges exist at a magnetic field strength of 7 T. The short RF-wavelength of the excitation and refocusing pulses leads to very significant flip angle and electric field variations, causing local hot-spots in SAR inside the body. Numerical computations of the RF field distribution and the corresponding SAR can be performed by using detailed computer-aided design models of the coil(s) and the tissues in the field of view of the coil(s). All models have their limitations, by assuming electrical and thermal properties and tissue dimensions, but they are generally considered conservative regarding safety. To overcome signal drop out at the location of the prostate due to destructive interference of the B1+ field, multi transmit-receive surface array coils can be used in combination with a driving scheme to adjust the phase (and magnitude if necessary) of the transmitting elements within the array (B1+ shimming (8)). Adding a (detunable) endorectal coil tuned to the 1H frequency provides the possibility to perform 1H MRSI at an unprecedented spatial resolution with non-adiabatic excitation and refocusing schemes. Some pulse sequence modifications are necessary to deal with volume selection schemes that stay within allowed RF power depositions (9). However, with this combined coil setup it is feasible to obtain 7 Tesla T2W TSE images with high SNR of the full prostate region with an external transmit-receive array coil (10), and combine this with 1H-MRSI at a high spatial resolution.
The chemical shift dispersion at 7 Tesla allows the design of chemical shift selective pulses that can effectively refocus spermine spins, which can be added to the semi-LASER sequence to obtain maximum signal from all metabolites of interest in the prostate (11). Next to field and TE related increase in SNR this can be an advantage of 7 Tesla 1H-MRSI with respect to 1.5 or 3 Tesla 1H-MRSI (12), if we can better understand the differences in metabolite ratios at these different field strengths (13). At 7 Tesla, the sensitivity of MR of nuclei other than protons may become high enough to be of clinical relevance. Extensive safety validations were performed to secure a safe 31P endorectal coil combination with an external 1H array for an in vivo exam (14). After characterization of relaxation times and NOE effects of 31P signals in the prostate (15), the first results of 31P MRSI of the prostate at a relevant spatial resolution in patients with prostate cancer within a clinically acceptable measurement time have been performed (16). A prototype 1H/31P endorectal coil combination is now in use for a complete experimental mpMRI with 1H and 31P MRSI examination (this conference).
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