A multitransmit external body array combined with a (1) H and (31) P endorectal coil to enable a multiparametric and multimetabolic MRI examination of the prostate at 7T

Item does not contain fulltextPURPOSE: In vivo (1) H and (31) P magnetic resonance spectroscopic imaging (MRSI) provide complementary information on the biology of prostate cancer. In this work we demonstrate the feasibility of performing multiparametric imaging (mpMRI) and (1) H and (31) P spectroscopic imaging of the prostate using a (31) P and (1) H endorectal radiofrequency coil (ERC) in combination with a multitransmit body array at 7 Tesla (T). METHODS: An ERC with a (31) P transceiver loop coil and (1) H receive (Rx) asymmetric microstrip ((31) P/(1) H ERC) was designed, constructed and tested in combination with an external 8-channel (1) H transceiver body array coil (8CH). Electromagnetic field simulations and measurements and in vivo temperature measurements of the ERC were performed for safety validation. In addition, the signal-to-noise (SNR) benefit of the (1) H microstrip with respect to the 8CH was evaluated. Finally, the feasibility of the setup was tested in one volunteer and three patients with prostate cancer by performing T2 -weighted and diffusion-weighted imaging in combination with (1) H and (31) P spectroscopic imaging. RESULTS: Electromagnetic field simulations of the (31) P loop coil showed no differences in the E- and B-fields of the (31) P/(1) H ERC compared with a previously safety validated ERC without (1) H microstrip. The hotspot of the specific absorption rate (SAR) at the feed point of the (31) P/(1) H ERC loop coil was 9.42 W/kg when transmitting on (31) P at 1 W. Additional in vivo measurements showed a maximum temperature increase at the SAR hotspot of 0.7 degrees C over 6 min on (31) P at 1.9 W transmit (Tx) power, indicating safe maximum power levels. When transmitting with the external (1) H body array at 40W for 2:30 min, the temperature increase around the ERC was < 0.3 degrees C. Up to 3.5 cm into the prostate the (1) H microstrip of the ERC provided higher SNR than the 8CH. The total coil combination allowed acquisition of an mpMRI protocol and the assessment of (31) P and (1) H metabolites of the prostate in all test subjects. CONCLUSION: We developed a setup with a (31) P transceiver and (1) H Rx endorectal coil in combination with an 8-channel transceiver external body array coil and demonstrated its safety and feasibility for obtaining multiparametric imaging and (1) H and (31) P MRSI at 7T in patients with prostate cancer within one MR examination

New excitation concepts for ultra-high-field human MRI

Contains fulltext : 111196.pdf (publisher's version ) (Open Access)Radboud Universiteit Nijmegen, 28 mei 2013Promotores : Norris, D.G., Ladd, M.E.122 p

Multi-echo fMRI of the cortical laminae in humans at 7 T.

Recent developments in ultra high field MRI and receiver coil technology have opened up the possibility of laminar fMRI in humans. This could offer greater insight into human brain function by elucidating both the interaction between brain regions on the basis of laminar activation patterns associated with input and output, and the interactions between laminae in a specific region. We used very high isotropic spatial resolution (0.75 mm voxel size), multi-echo acquisition (gradient-echo) in a 7 T fMRI study of human primary visual cortex (V1) and novel data analysis techniques to quantitatively investigate the echo time dependence of laminar profiles, laminar activation, and physiological noise distributions over an extended region of cortex. We found T(2)* profiles to be explicable in terms of variations in myelin content. Laminar activation profiles vary with echo time (TE): at short TE the highest signal changes are measured at the pial surface; this maximum shifts into grey matter at longer TEs. The top layers peak latest as these have the longest transverse relaxation time. Theoretical simulations and experiment suggest that the intravascular contribution to functional signal changes is significant even at long TE. Based on a temporal noise analysis we argue that the (physiological) noise contributions will ameliorate differences in sensitivity between the layers in a statistical analysis, and correlates with laminar blood volume distribution. We also show that even at this high spatial resolution the physiological noise limit to sensitivity is reached within V1, implying that cortical sub-regions can be examined with this technique

High resolution MR imaging of pelvic lymph nodes at 7 Tesla

Item does not contain fulltextPURPOSE: Pelvic lymph node (PLN) metastases are often smaller than 5 mm and difficult to detect. This work presents a method to perform PLN imaging with ultrahigh-field MRI, using spectrally selective excitation to acquire water and lipid-selective imaging at high spatial resolution. METHODS: A 3D water-selective multigradient echo (mGRE) sequence and lipid-selective gradient echo (GRE) sequence were tested in six healthy volunteers on a 7 Tesla (T) MRI system, using time interleaved acquisition of modes (TIAMO) to improve image homogeneity. The size distribution of the first 10 iliac PLNs detected in each volunteer was determined, and the contrast-to-noise ratio (CNR) of these lymph nodes (LNs) was compared with the individual mGRE images, sum-of-squares echo addition, and computed T2*-weighted images derived from the T2* fits. RESULTS: LN imaging was acquired robustly at ultrahigh field with high resolution and homogeneous lipid or water-selective contrast. PLNs down to 1.5-mm short axis were detected with mean +/- standard error of the mean (SEM) short and long axes of 2.2 +/- 0.1 and 3.7 +/- 0.2 mm, respectively. Computed T2*-weighted imaging allowed flexibility in T2* contrast while featuring a CNR up to 90% of the sum-of-squares echo addition. CONCLUSION: Ultrahigh-field MRI in combination with TIAMO and frequency-selective excitation enables high-resolution, large field-of-view MRI of the lower abdomen, and may ultimately be suitable for detecting small PLN metastases. Magn Reson Med 78:1020-1028, 2017. (c) 2016 International Society for Magnetic Resonance in Medicine

Time-interleaved acquisition of modes:an analysis of SAR and image contrast implications

As the magnetic field strength and therefore the operational frequency in MRI are increased, the radiofrequency wavelength approaches the size of the human head/body, resulting in wave effects which cause signal decreases and dropouts. Especially, whole-body imaging at 7 T and higher is therefore challenging. Recently, an acquisition scheme called time-interleaved acquisition of modes has been proposed to tackle the inhomogeneity problems in high-field MRI. The basic premise is to excite two (or more) different B 1+ modes using static radiofrequency shimming in an interleaved acquisition, where the complementary radiofrequency patterns of the two modes can be exploited to improve overall signal homogeneity. In this work, the impact of time-interleaved acquisition of mode on image contrast as well as on time-averaged specific absorption rate is addressed in detail. Time-interleaved acquisition of mode is superior in B 1+ homogeneity compared with conventional radiofrequency shimming while being highly specific absorption rate efficient. Time-interleaved acquisition of modes can enable almost homogeneous high-field imaging throughout the entire field of view in PD, T(2) , and T(2) *-weighted imaging and, if a specified homogeneity criterion is met, in T(1) -weighted imaging as well

7T ultra-high field body MR imaging with an 8-channel transmit/32-channel receive radiofrequency coil array

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