Jung-Jiin Hsu

Modulation of Subgenual Anterior Cingulate Cortex Activity With Real-Time Neurofeedback

The advent of real‐time neurofeedback techniques has allowed us to begin to map the controllability of sensory and cognitive and, more recently, affective centers in the brain. The subgenual anterior cingulate cortex (sACC) is thought to be involved in generation of affective states and has been implicated in psychopathology. In this study, we examined whether individuals could use real‐time fMRI neurofeedback to modulate sACC activity. Following a localizer task used to identify an sACC region of interest, an experimental group of eight women participated in four scans: (1) a pretraining scan in which they were asked to decrease activity in the sACC without neurofeedback; (2) two training scans in which sACC neurofeedback was presented along with instructions to decrease sACC activity; and (3) a neurofeedback‐free post‐training scan. An additional nine women in a yoked feedback control group saw sACC activity from the participants in the experimental group. Activity in the sACC was significantly reduced during neurofeedback training in the experimental group, but not in the control group. This training effect in the experimental group, however, did not generalize to the neurofeedback‐free post‐training scan. A psychophysiological interaction analysis showed decreased correlation in the experimental group relative to the sham control group between activity in the sACC and the posterior cingulate cortex during neurofeedback training relative to neurofeedback‐free scans. The finding that individuals can down‐modulate the sACC shows that a primary emotion center in which functional abnormality has been strongly implicated in affective disorders can be controlled with the aid of neurofeedback. Hum Brain Mapp, 2010.

Mitigation of susceptibility-induced signal loss in neuroimaging using localized shim coils.

Correction of magnetic field distortions is essential for obtaining accurate brain blood‐oxygen‐level‐dependent functional magnetic resonance imaging (fMRI) activation maps. The present work introduces an active shimming method that utilizes the magnetic field generated by resistive shim coils placed in the mouth to locally homogenize the magnetic field in the inferior portion of the frontal lobe, where the field is most seriously distorted. The shimming field can be optimized in situ patient by patient for the region of interest of the scanner operators choice. The method at 1.5 T is shown to be effective in reducing field inhomogeneity and in recovery of fMRI signal. For example, in a region of interest approximately of 149 cm3, a coil of simple geometry can reduce the root mean square of the magnetic field by more than 50% and the recovered signal increases the extent of activation detected in a breath‐holding fMRI experiment. Magn Reson Med 53:243–248, 2005.

Rapid methods for concurrent measurement of the RF-pulse flip angle and the longitudinal relaxation time

Measuring both the flip angle (FA) and the longitudinal relaxation time T1 is essential in quantitative and longitudinal studies because the signal amplitude is dependent on these quantities. Conventional methods can only measure one of them at a time and require long scan times. In this work, two mutually consistent methods are developed; each can acquire multislice data for determining both the FA and T1 in a scan time about half the time needed for a conventional FA measurement. On the basis of a recent development of longitudinal‐relaxation measurement (Hsu and Lowe, J Magn Reson 2004;169:270–278; Hsu and Glover, J Magn Reson 2006;181:98–106), one of the methods uses RF pulse trains of two FAs whereas the other uses pulse trains of different pulse spacing. When only the FA or T1 is needed, the present methods can still be faster than conventional methods for the needed quantity. In benchmarking with a uniform‐density sample, both methods generate precise T1 values independent of the FA chosen (except at and near 90°). In the demonstration with three normal volunteers at 3 T, the T1 values of frontal and occipital white matter, putamen, and caudate are compared; the T1 values are in agreement with literature values and the intrasubject deviation is 0.2%–2.8%. Magn Reson Med, 61:1319–1325, 2009.

Optimizing saturation–recovery measurements of the longitudinal relaxation rate under time constraints

The saturation–recovery method using two and three recovery times is studied for conditions in which the sum of recovery times is 1.5T1 to 3T1, where T1 is the longitudinal relaxation time. These conditions can reduce scan time considerably for long T1 species and make longitudinal relaxation rate R1 (R1 = 1/T1) mapping for body fluids clinically feasible. Monte Carlo computer simulation is carried out to determine the ideal set of recovery times under various constraints of the sum of recovery times. The ideal set is found to be approximately invariant to the signal‐to‐noise ratio. For the three‐point method, two of the recovery times should be set the same or approximately the same and should be shorter than the third one. Only marginal improvements in accuracy and precision can be achieved by the three‐point method over the two‐point method under a common constraint of the sum of recovery times. Three‐dimensional, high resolution, whole‐brain saturation–recovery scans on volunteers with a fast‐spin‐echo technique (XETA) and completed in a scan time of 10 min generated R1 measurements of cerebrospinal fluid (T1 ∼ 4 s) in agreement with the computer simulation and literature results, which demonstrates the clinical feasibility of applying the two‐point saturation–recovery method for R1 mapping for long relaxation components. Magn Reson Med, 2009.

Signal recovery in free induction decay imaging using a stimulated spin echo

A novel pulse sequence for acquiring a stimulated spin echo (SSE) is devised and implemented to recover the initial portion of the signal that is usually blanked or distorted in MRI using free induction decays (FIDs). The receiver‐phase offset, center of the k‐space, and time frame coordinates of the FID data points can be accurately determined from the SSE. A simple numerical method of signal recovery is formulated which can merge the FID and the SSE data for higher extrapolation accuracy. The application of the signal recovery method is demonstrated using the rotating ultrafast imaging sequence (RUFIS). The result shows the importance of experimentally finding the correct time origin of the FID signal. Magn Reson Med 47:409–414, 2002.

Developments in chlorine detection in concrete using NMR

Monitoring chloride concentration and transport in concrete structures susceptible to corrosion of embedded steel reinforcement is a challenge as difficult as it is important. An embedded sensor based on nuclear magnetic resonance (NMR) would be a good solution to the problem because it would make a non-destructive atom-specific measurement of the presence and concentration of chloride. The important question is the scale of the device required to detect the chloride. Laboratory experiments to detect chloride in a cement matrix using pulse-NMR were conducted to assess the potential of this application; they provided a basis for projecting the scale of a device that would have a good chance of success. The coils were cm-scale and the magnetic field was 2.35 T. NMR signals were obtained from both aqueous chloride solution and samples of both regular and white portland cement. The experiments demonstrated that the signal-to-noise ratio (SNR) for a cm-scale cement sample volume is so small, even after averaging, that sample volumes much lower than that are unlikely to produce measurable signals at fields of 1 T or below. Thus the potential for realizing an embedded NMR-based sensor including the magnet is low. Parametric studies identify feasible alternative coil diameters and magnetic field strengths for detecting chloride ion concentrations in hardened concrete.