Chemical exchange saturation transfer (CEST) is a technique to indirectly detect pools of exchangeable protons through the water signal. To increase its applicability to human studies, it is needed to develop sensitive pulse sequences for rapidly acquiring whole-organ images while adhering to stringent amplifier duty cycle limitations and SAR restrictions. In addition, the interfering effects of direct water saturation (DS) and conventional magnetization transfer contrast (MTC) complicate CEST quantification and need to be reduced as much as possible. It is shown that for protons exchanging with rates of less than 50–100 Hz, such as imaged in amide proton transfer (APT) experiments, these problems can be addressed by using a 3D steady state pulsed acquisition of limited B1 strength (~1 μT). Such an approach exploits the fact that the DS width, MTC magnitude, and SAR increase strongly with B1, while the size of the CEST effect for such protons depends minimally on B1. A short-TR (65 ms) steady state sequence consisting of a brief saturation pulse (25 ms) and a segmented EPI train allowed acquisition of a 3D whole-brain volume in approximately 11 s per saturation frequency, while remaining well within SAR and duty cycle limits. MTC was strongly reduced, but substantial saturation effects were found at frequencies upfield from water, which still confound the use of MT asymmetry analysis. Fortunately, the limited width of the DS signal could be exploited to fit it with a Lorentzian function allowing CEST quantification. APT effects ranged between 1.5 and 2.5% in selected white and gray matter regions. This power and time-efficient 3D pulsed CEST acquisition scheme should aid endogenous CEST quantification at both high and low field.

Purpose: To compare diffusion tensor imaging (DTI) measurements at ultra high field strength (7 Tesla [T]) in human volunteers with DTI measurements performed at 1.5 and 3 Tesla. Materials and Methods:The signal to noise ratio (SNR) and the uncertainty in fitted DTI parameters fractional anisotropy and primary eigenvector are assessed with tractography based regions of interest, measured in nine volunteers at 1.5T, 3T, and 7T with clinically available hardware configurations.Results: An increase in SNR is observed on the 7T system compared with the 1.5 or 3T system. The measured increase in SNR at 7T is larger than expected from field strength alone, indicating the large influence of improved receive coil hardware. Additionally, while the average fractional anisotropy remains relatively constant across field strengths, a decrease in uncertainty in the fitted values for fractional anisotropy and the principal eigenvector of the DTI tensor was found. Increased spatial heterogeneity of signal intensities is observed at 7T.Conclusion: Given the current hardware constraints, DTI at ultra-high field strengths is possible with improved performance in selected regions of interest.

BackgroundThis technical note introduces diffusion-weighted (DW) MR neurography (MRN) of the sacral plexus with unidirectional motion probing gradients (MPGs).MethodsThis is compared with DW MRN with three-directional and six-directional MPGs.Results and conclusionThis paper indicates that DW MRN of the sacral plexus should be performed with unidirectional MPGs.

The strong correlation of MTR and MTR with T and the absence thereof in AREX suggests that much of APT contrast in tumors for the low-power, 3D-acquisition scheme at 7 T originates from the inherent tissue water T -relaxation properties. Magn Reson Med 77:1525-1532, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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