Junyu Guo

High-resolution DTI with 2D interleaved multislice reduced FOV single-shot diffusion-weighted EPI (2D ss-rFOV-DWEPI)

Diffusion tensor MRI (DTI), using single‐shot 2D diffusion weighted‐EPI (2D ss‐DWEPI), is limited to intracranial (i.c.) applications far from the sinuses and bony structures, due to the severe geometric distortions caused by significant magnetic field inhomogeneities at or near the tissue‐air or tissue‐bone interfaces. Reducing these distortions in single‐shot EPI by shortening the readout period generally requires a reduced field of view (and the potential of wraparound artifact) in the phase‐encoding direction and/or reduced spatial resolution. To resolve the problem, a novel 2D reduced FOV single‐shot diffusion‐weighted EPI (2D ss‐rFOV‐DWEPI) pulse sequence applicable for high resolution diffusion‐weighted MRI of local anatomic regions, such as brainstem, cervical spinal cord, and optic nerve, has been developed. In the proposed technique, time‐efficient interleaved acquisition of multiple slices with a limited FOV was achieved by applying an even number of refocusing 180° pulses with the slice‐selection gradient applied in the phase‐encoding direction. The two refocusing pulses used for each slice acquisition were separated by a short time interval (typically less than 45 ms) required for the 2D EPI echotrain acquisition. The new technique can be useful for high resolution DTI of various anatomies, such as localized brain structures, cervical spinal cord, optic nerve, heart, or other extra‐cerebral organ, where conventional 2D ss‐DWEPI is limited in usage due to the severity of image distortions. Magn Reson Med, 2005.

MR thermometry-based feedback control of efficacy and safety in minimum-time thermal therapies: Phantom and in-vivo evaluations

The experimental validation of a model-based, thermal therapy control system which automatically and simultaneously achieves the specified efficacy and safety objectives of the treatment is reported. MR-thermometry measurements are used in real-time to control the power of a stationary, focused ultrasound transducer in order to achieve the desired treatment outcome in minimum time without violating the imposed safety constraints. Treatment efficacy is quantified in terms of the thermal dose delivered to the target. Normal tissue safety is ensured by automatically maintaining normal tissue temperature below the imposed limit in the user-specified locations. To reflect hardware limitations, constraints on the maximum applied power are also imposed. At the pretreatment stage, MR imaging and thermometry are used to localize the treatment target and identify thermal and actuation models. The results of phantom and canine experiments demonstrate that spatially-distributed, real-time MR temperature measurements enhance ones ability to robustly achieve the desired treatment outcome in minimum time without violating safety constraints. Post-treatment evaluation of the outcome using T2-weighted images of canine muscle showed good spatial correlation between the sonicated area and thermally damaged tissue.

Automatic detection of three-dimensional vascular tree centerlines and bifurcations in high-resolution magnetic resonance angiography

Objectives:We sought to develop a simple and robust algorithm capable of automatically detecting centerlines and bifurcations of a three-dimensional (3D) vascular bed. Materials and Methods:After necessary preprocessing, an appropriate cost function is computed for all vessel voxels and Dijkstras minimum-cost-path algorithm is implemented. By back tracing all the minimum-cost paths, centerlines and bifurcation are detected. The detected paths are then split into segments between adjacent nodes (bifurcations or vessel end-points) and smoothed by curve fitting. Results:Application of the algorithm to both simulated 3D vessels and 3D magnetic resonance angiography (MRA) images of an actual intracranial arterial tree produced well-centered vessel skeletons. Quantitative assessment of the algorithm was performed. For the simulated data, the root mean square error for centerline detection is about half a voxel. For the human intracranial MRA data, the sensitivity, positive predictive value (PPV), and accuracy of bifurcation detection were calculated for different cost functions. The best case gave a sensitivity of 91.4%, a PPV of 91.4%, and an RMS error of 1.7 voxels. Conclusions:To the extent that imperfections are eliminated from the segmented image, the algorithm is effective and robust in automatic and accurate detection of centerlines and bifurcations. The cost function and algorithm used are demonstrated to be an improvement over similar algorithms in the literature.

HASTE Sequence with Parallel Acquisition and T2 Decay Compensation: Application to Carotid Artery Imaging

T2-weighted carotid artery images acquired using the turbo spin-echo (TSE) sequence frequently suffer from motion artifacts due to respiration and blood pulsation. The possibility of using HASTE sequence to achieve motion-free carotid images was investigated. The HASTE sequence suffers from severe blurring artifacts due to signal loss in later echoes due to T2 decay. Combining HASTE with parallel acquisition (PHASTE) decreases the number of echoes acquired and thus effectively reduces the blurring artifact caused by T2 relaxation. Further improvement in image sharpness can be achieved by performing T2 decay compensation before reconstructing the PHASTE data. Preliminary results have shown successful suppression of motion artifacts with PHASTE imaging. The image quality was enhanced relative to the original HASTE image, but was still less sharp than a non-motion-corrupted TSE image.

TSE with average-specific phase encoding ordering for motion detection and artifact suppression

To detect motion‐corrupted measurements in multiaverage turbo‐spin‐echo (TSE) acquisitions and reduce motion artifacts in reconstructed images.

Constrained model-predictive thermal dose control for MRI-guided ultrasound thermal treatments (Invited Paper)

Ultrahigh resolution OCT using broadband light sources achieves improved axial image resolutions of ~2-3 um compared to standard 10 um resolution OCT used in current commercial instruments. High-speed OCT using Fourier/spectral domain detection enables dramatic increases in imaging speeds. 3D OCT retinal imaging is performed in human subjects using high-speed, ultrahigh resolution OCT, and the concept of an OCT fundus image is introduced. Three-dimensional data and high quality cross-sectional images of retinal pathologies are presented. These results show that 3D OCT may be used to improve coverage of the retina, precision of cross-sectional image registration, quality of cross-sectional images, and visualization of subtle changes in retinal topography. 3D OCT imaging and mapping promise to help elucidate the structural changes associated with retinal disease as well as to improve early diagnosis and monitoring of disease progression and response to treatment.