Biomedical subjects

Jung-Jiin Hsu

Publications and source records attributed to Jung-Jiin Hsu.

5 recordsLinked to original sources

PubMed2006

Encoding to the longitudinal magnetization for MR imaging and flow velocity mapping.

Phase-encoding to the longitudinal magnetization is implemented by adding encoding gradient pulses in the evolution period tau of the NMR pulse sequence 90 degrees+x-tau-90 degrees-x. This work focuses on the effect of the spin-lattice relaxation and its removal and on the constraint that the 90 degrees-x pulse can only transform the phase of the transverse magnetization partially to the longitudinal magnetization. Theoretical analysis shows that the encoded phase information and the spin-lattice relaxation effect are separable and the latter is identical in each repetition in collecting phase-encoding data. Thus the relaxation effect can be eliminated by subtracting a second data set whose phase information is inverted or by alternating the polarity of the relaxation contribution. From data with partial phase information, Fourier-transform image reconstruction results in mirror aliasing in which the two halves of the Fourier spectrum of positive and negative coordinates overlap. Removal of mirror aliasing requires imaging data of the orthogonal component. Nevertheless mirror aliasing is not necessarily a problem, depending on the subject of study. Phase-encoding to the longitudinal magnetization for spatial MRI and flow velocity mapping are demonstrated using the rotating ultra-fast imaging sequence (RUFIS).

Algorithms↗

PubMed2006

Rapid MRI method for mapping the longitudinal relaxation time.

A novel method for mapping the longitudinal relaxation time in a clinically acceptable time is developed based on a recent proposal [J.-J. Hsu, I.J. Lowe, Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization, J. Magn. Reson. 169 (2004) 270-278] and the speed of the spiral pulse sequence. The method acquires multiple curve-fitting samples with one RF pulse train. It does not require RF pulses of specific flip angles (e.g., 90 degrees or 180 degrees ), nor are the long recovery waiting time and the measurement of the magnetization at thermal equilibrium needed. Given the value of the flip angle, the curve fitting is semi-logarithmic and not computationally intensive. On a heterogeneous phantom, the average percentage difference between measurements of the present method and those of an inversion-recovery method is below 2.7%. In mapping the human brain, the present method, for example, can obtain four curve-fitting samples for five 128 x 128 slices in less than 3.2s and the results are in agreement with other studies in the literature.

Benchmarking↗

PubMed2005

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 operator's 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.

Artifacts↗

PubMed2004

Spin-lattice relaxation and a fast T1-map acquisition method in MRI with transient-state magnetization.

The magnetization under the spin-lattice relaxation and the nuclear magnetic resonance radiofrequency (RF) pulses is calculated for a signal RF pulse train and for a sequence of multiple RF pulse-trains. It is assumed that the transverse magnetization is zero when each RF pulse is applied. The result expressions can be grouped into two terms: a decay term, which is proportional to the initial magnetization M0, and a recovery term, which has no M0 dependence but strongly depends on the spin-lattice relaxation and the equilibrium magnetization Meq. In magnetic resonance pulse sequences using magnetization in transient state, the recovery term produces artifacts and can seriously degrade the function of the preparation sequence for slice selection, contrast weighting, phase encoding, etc. This work shows that the detrimental effect can be removed by signal averaging in an eliminative fashion. A novel fast data acquisition method for constructing the spin-lattice relaxation (T1) map is introduced. The method has two features: (i) By using eliminative averaging, the curve to fit the T1 value is a decay exponential function rather than a recovery one as in conventional techniques; therefore, the measurement of Meq is not required and the result is less susceptible to the accuracy of the inversion RF pulse. (ii) The decay exponential curve is sampled by using a sequence of multiple pulse-trains. An image is reconstructed from each train and represents a sample point of the curve. Hence a single imaging sequence can yield multiple sample points needed for fitting the T1 value in contrast to conventional techniques that require repeating the imaging sequence for various delay values but obtain only one sample point from each repetition.

Algorithms↗

PubMed2002

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.

Artifacts↗