Wingchi E. Kwok

Functional MRI study of auditory and visual oddball tasks

To seek neural sources of endogenous event-related potentials, brain activations related to rare target stimuli detection in auditory and visual oddball tasks were imaged using a high temporal resolution functional MRI technique. There were multiple modality specific and modality non-specific activations. Auditory specific activations were seen in the bilateral transverse temporal gyri and posterior superior temporal planes while visual specific activations were seen in the bilateral occipital lobes and their junctions with the temporal lobes. Modality non-specific activations were seen in multiple areas including the bilateral parietal and temporal association areas, bilateral prefrontal cortex, bilateral premotor areas, bilateral supplementary motor areas and anterior cingulate gyrus. Results were consistent with previous intracranial evoked potential recording studies, and supported the multiple generator theory of the endogenous event-related potentials.

A four-element phased array coil for high resolution and parallel MR imaging of the knee.

A four-element phased array coil for MR imaging of the knee was designed, built and tested for clinical use at 1.5 Tesla. In routine imaging, it provides over twofold increase in signal-to-noise (SNR) compared to two commercially available knee coils, and supports higher spatial image resolution. The phased array knee coil was also tested for its compatibility with parallel MR imaging that reduces imaging time by several folds over conventional MR technique. Results obtained using SiMultaneous Acquisition of Spatial Harmonics (SMASH) technique shows that our phased array knee coil can be used with parallel MR imaging. These improvements may enhance knee diagnosis with higher image quality and reduced scan time.

3D interleaved water and fat image acquisition with chemical-shift correction.

A new technique, 3D interleaved water and fat image acquisition with chemical‐shift correction (3‐DIWFAC), was developed to acquire 3D water and fat images in a single acquisition time and to combine the water and fat images to produce chemical‐shift‐free images. A 3D gradient‐recalled‐echo (GRE) sequence was implemented with a 1‐3‐3‐1 binomial Shinnar‐Le Roux spatial–spectral excitation, and with interleaved phase‐encoding lines that alternate between water and fat excitations separated by half TR. Water‐only and fat‐only images were then realigned to remove chemical shift artifacts. Results from phantoms and human subjects demonstrated that the image contrast was the same as in the regular GRE sequence. With the chemical shift corrected, the shadow artifacts often seen at water and fat boundaries were removed. Since this sequence simultaneously provides water‐only images showing cartilage and bone lesions, and water‐fat images that depict soft tissue anatomy, it may be clinically useful in musculoskeletal imaging. Magn Reson Med 44:322–330, 2000.

Interleaved water and fat dual‐echo spin echo imaging with intrinsic chemical‐shift elimination

A new technique was developed to simultaneously acquire water and fat dual‐echo spin echo images in a single acquisition period. Chemical shifts between water and fat images are intrinsically eliminated, and the images are combined to form water‐plus‐fat image. In vivo water‐only images show fat suppression superior to that of conventional spin echo images. This technique may be clinically useful for musculoskeletal imaging. J. Magn. Reson. Imaging 2001;13:318–323.

A volume adjustable four-coil phased array for high resolution MR imaging of the hip

A four-coil phased array was specifically designed and built for MR imaging of the hip at 1.5 T. Its RF and imaging properties were evaluated using phantom and in-vivo studies and the results were compared to those of three different commercial coils commonly used for hip imaging. Our coil gave a significantly higher S/N at anatomic locations commonly evaluated for hip diagnosis. The increased S/N supports higher image spatial resolution and improves the visualization of fractures and lateral injuries.

High‐resolution uniform MR imaging of finger joints using a dedicated RF coil at 3T

To develop a dedicated radiofrequency (RF) coil for high‐resolution magnetic resonance imaging (MRI) of finger joints at 3T to improve diagnostic evaluation of arthritic diseases.

High‐resolution interleaved water‐fat MR imaging of finger joints with chemical‐shift elimination

To study the use of an interleaved water‐fat (IWF) sequence with a custom‐made radiofrequency (RF) coil for high‐resolution imaging of arthritic finger joints.

A non-ionizing technique for three-dimensional measurement of the lumbar spine

Comprehensive assessments of scoliotic deformity and spinal instability require repetitive three-dimensional (3D) measurements of motion segments at different functional postures. However, accurate 3D measurement of the spine is a challenging task. In this paper, we present a novel, non-invasive, non-ionizing technique to quantify 3D poses of lumbar motion segments in terms of clinically meaningful anatomical coordinates. The technique used ultra-short echo time (UTE) magnetic resonance (MR) images to construct subject-specific geometrical models of individual vertebrae and registered them with 3D ultrasound dataset acquired during pose measurements. A hierarchical registration approach was used to minimize the detrimental effects of speckle noise and artifacts within soft tissues on registration accuracy. The technique was validated using a human dry bone specimen as well as a fresh porcine cadaver. Registration errors were determined by comparing with a gold standard fiducial-based registration. Results showed that the technique is accurate and reliable with bias in sub-degree and sub-millimeter level (except for the flexion-extension of the porcine cadaver experiment, which was -1.74°), and average precision of 1.11° in rotation and 0.86mm in position for the human dry bone experiment, and 1.26° and 1.23mm for the porcine cadaver experiment. Given its non-ionizing nature, the UTE MR-ultrasound registration technique is particularly useful for repeated measurements and longitudinal follow-up. With further refinement and validation, it could be a powerful tool for 3D spinal assessment.

Simultaneous water and fat MRI with chemical-shift correction

We have developed a new MRI technique that simultaneously acquires both water and fat images in a single acquisition time, and combines the water and fat images to produce chemical-shift free water-fat images. The new technique was implemented on a 3D gradient echo (GRE), and 2D dual-echo spin echo (SE) and GRE imaging sequences at 1.5 Tesla. Binomial spatial-spectral selective Shinnar-Le Roux RF and gradient excitation pulses were used for excitation. In the GRE sequences, the phase-encoding lines in the slice selection direction were interlaced with alternate water and fat excitation separated by half repetition time (TR). For the 2D SE sequences, the water signal was first excited and acquired, followed by fat signal excitation. A non-spectral selective /spl pi/ pulse then refocused the spins and generated the first echo for fat and the second echo for water. The water-only and fat-only images were reconstructed on a workstation where they were realigned and combined to form chemical-shift free images. Results from phantoms and human subjects demonstrated that there were no cross-talk between the water and fat excitation, and the image contrast was the same as the regular sequences. With the chemical shift corrected, shadow artifacts often seen at water and fat boundaries were removed. Since this technique simultaneously provides water-only images showing soft tissue and bone pathology, and water-fat images depicting the anatomy, it could be useful in evaluation of musculoskeletal conditions. The superb selectivity for water signal resulting from this technique may also enhance tumor conspicuity in brain and other organs.

A method of switching the signal in an MRI phantom based on trace ion currents

A method for electrically changing the hydrogen nuclear magnetic resonance (NMR) signal intensity in a magnetic resonance imaging (MRI) phantom is presented. The method is based on creating local magnetic field inhomogeneities from impurity ion currents in a polar hydrocarbon. The effect is demonstrated using the propylene carbonate on an NMR spectrometer and an MRI scanner. This effect is largest when the electric field is applied perpendicular to the static magnetic field in magnetic resonance, and is linear with applied voltage. The applicability of a switchable signal in an MRI phantom is demonstrated with a spin-echo, echo planar imaging sequence where the MRI signal is changed between blocks of 10 images in a series of 200 images. This technique may find applications in inter and intra platform fMRI quality control.