Masaya Hirano

Visualization of hemodynamics in intracranial arteries using time-resolved three-dimensional phase-contrast MRI.

To visualize the hemodynamics of the intracranial arteries using time‐resolved three‐dimensional phase‐contrast (PC)‐MRI (4D‐Flow).

A multi-compartmental SE-BOLD interpretation for stimulus-related signal changes in diffusion-weighted functional MRI.

A new interpretation is proposed for stimulus‐induced signal changes in diffusion‐weighted functional MRI. T2‐weighted spin‐echo echo‐planar images were acquired at different diffusion‐weightings while visual stimulation was presented to human volunteers. The amplitudes of the positive stimulus‐correlated response and post‐stimulus undershoot (PSU) in the functional time‐courses were found to follow different trends as a function of b‐value. Data were analysed using a three‐compartment signal model, with one compartment being purely vascular and the other two dominated by fast‐ and slow‐diffusing molecules in the brain tissue. The diffusion coefficients of the tissue were assumed to be constant throughout the experiments. It is shown that the stimulus‐induced signal changes can be decomposed into independent contributions originating from each of the three compartments. After decomposition, the fast‐diffusion phase displays a substantial PSU, while the slow‐diffusion phase demonstrates a highly reproducible and stimulus‐correlated time‐course with minimal undershoot. The decomposed responses are interpreted in terms of the spin‐echo blood oxygenation level dependent (SE‐BOLD) effect, and it is proposed that the signal produced by fast‐ and slow‐diffusing molecules reflect a sensitivity to susceptibility changes in arteriole/venule‐ and capillary‐sized vessels, respectively. This interpretation suggests that diffusion‐weighted SE‐BOLD imaging may provide subtle information about the haemodynamic and neuronal responses. Copyright

Acoustic Noise Transfer Function in Clinical MRI: A Multicenter Analysis

RATIONALE AND OBJECTIVES Acoustic noise both in terms of its magnitude and frequency during magnetic resonance imaging (MRI) scan is influenced by imaging parameters and pulse sequences. It varies because of many different factors such as structure, materials, and magnetic field strength. The purpose of our study is to evaluate the characteristics of acoustic noise independent of MRI scan protocol by measuring a gradient-pulse-to-acoustic-noise transfer function (GPAN-TF) at various MRI scanners. MATERIALS AND METHODS We measured sound pressure levels in the frequency domain in a 0.4-T, seven 1.5-T, and three 3.0-T clinical MRI systems when applying a simple narrower trapezoidal gradient pulse. We calculated a GPAN-TF [μPa/(mT/m)] in each gradient coil (ie, X, Y, and Z-axis) by the deconvolution process. RESULTS GPAN-TF at a high-frequency range (1000-10,000 Hz) was larger than that at low frequency for all MRI (P<0.01) scanners except for a low static field machine. For high frequency (>1000 Hz), the 3.0-T MRI scanner had a larger GPAN-TF than that of 0.4-T and 1.5-T (P < .01). MR scanner with a vacuum chamber reduced GPAN-TF at a lower frequency (P < .01), but this effect decreased at higher frequency. CONCLUSION GPAN-TF analysis makes it possible to obtain more detailed information on acoustic noise properties among MRI scanners.

Calibrated magnetic resonance hydrometry to quantify pancreatic juice: A preliminary study

To investigate the effectiveness of the calibrated MR hydrometry (CMRH) method for accurately measuring pancreatic secretion.

Signal contributions to heavily diffusion-weighted functional magnetic resonance imaging investigated with multi-SE-EPI acquisitions

Diffusion-weighted (DW) functional magnetic resonance imaging (fMRI) signal changes have been noted as a promising marker of neural activity. Although there is no agreement on the signal origin, the blood oxygen level dependent (BOLD) effect has figured as one of the most likely sources. In order to investigate possible BOLD and non-BOLD contributions to the signal, DW fMRI was performed on normal volunteers using a sequence with two echo-planar acquisitions after pulsed-gradient spin-echo. Along with the changes to the signal amplitude (ΔS/S) measured at both echo-times, this sequence allowed changes to the transverse relaxation rate (ΔR2) to be estimated for multiple b-values during hypercapnia (HC) and visual stimulation (VS). ΔS/S and ΔR2 observed during HC were relatively insensitive to increasing b-value. On the other hand, ΔS/S demonstrated a clear dependence on b-value at both echo-times for VS. In addition, ΔR2 during the latter half of VS was significantly more negative at b=1400s/mm(2) than for the time-courses at lower b-value, but ΔR2 during the post-stimulus undershoot was independent of b-value. The results have been discussed in terms of two models: the standard intravascular-extravascular model for fMRI and a three-compartment model (one intra- and two extravascular compartments). Within these interpretations the results suggest that the majority of the response is linked to changes in transverse relaxation, but possible contributions from other sources may not be ruled out.

Magnetic Resonance Elastography to Observe Deep Areas: Comparison of External Vibration Systems

MRE methods deform the sample using an external vibration system. We have been using a transverse driver, which generates shear waves at the object surface. One of the problems is that shear waves rapidly attenuate at the surface of tissue and do not propagate into the body. In this study, we compared the shear waves generated by transverse and longitudinal drivers. The longitudinal driver was found to induce shear waves deep inside a porcine liver phantom. These results suggest that the longitudinal driver will allow measurement of the shear modulus deep inside the body.