Biomedical subjects

Alayar Kangarlu

Publications and source records attributed to Alayar Kangarlu.

5 recordsLinked to original sources

PubMed2006

A statistical framework for the classification of tensor morphologies in diffusion tensor images.

Tractography algorithms for diffusion tensor (DT) images consecutively connect directions of maximal diffusion across neighboring DTs in order to reconstruct the 3-dimensional trajectories of white matter tracts in vivo in the human brain. The performance of these algorithms, however, is strongly influenced by the amount of noise in the images and by the presence of degenerate tensors-- i.e., tensors in which the direction of maximal diffusion is poorly defined. We propose a simple procedure for the classification of tensor morphologies that uses test statistics based on invariant measures of DTs, such as fractional anisotropy, while accounting for the effects of noise on tensor estimates. Examining DT images from seven human subjects, we demonstrate that this procedure validly classifies DTs at each voxel into standard types (nondegenerate DTs, as well as degenerate oblate, prolate or isotropic DTs), and we provide preliminary estimates for the prevalence and spatial distribution of degenerate tensors in these brains. We also show that the P values for test statistics are more sensitive tools for classifying tensor morphologies than are invariant measures of anisotropy alone.

Algorithms↗

PubMed2006

Correction of eddy-current distortions in diffusion tensor images using the known directions and strengths of diffusion gradients.

PURPOSE: To correct eddy-current artifacts in diffusion tensor (DT) images without the need to obtain auxiliary scans for the sole purpose of correction. MATERIALS AND METHODS: DT images are susceptible to distortions caused by eddy currents induced by large diffusion gradients. We propose a new postacquisition correction algorithm that does not require any auxiliary reference scans. It also avoids the problematic procedure of cross-correlating images with significantly different contrasts. A linear model is used to describe the dependence of distortion parameters (translation, scaling, and shear) on the diffusion gradients. The model is solved numerically to provide an individual correction for every diffusion-weighted (DW) image. RESULTS: The assumptions of the linear model were successfully verified in a series of experiments on a silicon oil phantom. The correction obtained for this phantom was compared with correction obtained by a previously published method. The algorithm was then shown to markedly reduce eddy-current distortions in DT images from human subjects. CONCLUSION: The proposed algorithm can accurately correct eddy-current artifacts in DT images. Its principal advantages are that only images with comparable signals and contrasts are cross-correlated, and no additional scans are required.

Algorithms↗

PubMed2005

Effects of coil dimensions and field polarization on RF heating inside a head phantom.

Deterioration of radiofrequency (RF) inhomogeneity with increasing static magnetic field in magnetic resonance imaging (MRI) is one of the fundamental challenges preventing their clinical rendition and posing safety hazards. Variation in RF coil designs could help redistribute RF energy absorption over the imaged object. This work is intended to determine experimentally the difference in RF heating produced within a human head phantom by in situ measurement of RF inhomogeneity as a function of coil design utilized at 8 T. The heating patterns of 1/4 wavelength (long) and 1/8 wavelength 11-cm (short) transverse electromagnetic (TEM) coils loaded with a homogeneous human head phantom at 340 MHz were evaluated. In addition, different transmit/receive (T/R) configurations were used in search for the possibility of "hot-spot" formation. Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast for RF powers corresponding to a specific absorption rate (SAR) of 4.0 W/kg for 10 min. Numerical simulations were performed to study the general RF power deposition patterns in phantoms at 340 MHz including the effects of field polarization. The temperature increases varied from 0 to 0.8 degrees C for the long RF coil, while the short RF coil produced a maximum temperature change of 0.5 degrees C. Similar to ultra high-field electromagnetic simulations, these measurements revealed low peripheral and high deep-tissue heating at 8 T. The findings indicated that the largest temperature changes for both cases were less than 1 degrees C. While these results showed an increase in localized heating due to RF pulses at 8 T, they highlight that RF inhomogeneity could be redistributed using different RF coil designs through which the hot spots could be made cooler.

Equipment Safety↗

PubMed2005

Design and performance issues of RF coils utilized in ultra high field MRI: experimental and numerical evaluations.

In this paper, two TEM resonators were evaluated experimentally and numerically at 8 tesla (T) (340 MHz for 1H imaging). The coils were constructed to be 21.2-cm long (standard) and 11-cm long (a proposed less claustrophobic design). The experimental evaluation was done on a single cadaver using an ultra high field, 8 T, whole-body magnet. The numerical modeling was performed using an in-house finite difference time domain packagethat treats the coil and the load (anatomically detailed human head model) as a single system. The coils were tested with quadrature excitation at different coil alignment positions with respect to human head. For head imaging at 8 T, the overall numerical and experimental results demonstrated that when compared to the longer coil, the shorter coil provides superior signal-to-noise ratio, coil sensitivity, and excite field in the biological regions that lie within both of the coils' structures. A study of the RF (excite/receive fields) homogeneity showed variations in the performance of both coils that are mostly dependant on the region of interest and the position of coil with respect to the head. As such, depending on the application, the shorter coil could be effectively utilized.

Brain↗

PubMed2003

Effect of static magnetic field exposure of up to 8 Tesla on sequential human vital sign measurements.

PURPOSE: To determine if increasing static magnetic field strength exposures up to 8 Tesla (T) affect vital signs or electrocardiograms (ECGs) in normal human volunteers. MATERIALS AND METHODS: We studied 25 normal subjects, consisting of 19 men and six women, ages 24-53 years. The vital signs and ECGs of the subjects were measured 14 times inside and outside the magnetic field. This included the heart rate, respiratory rate, systolic and diastolic blood pressures, finger pulse oxygenation levels, core body temperature via the external auditory canal temperature, and fiber optic core body sublingual temperatures. Inside the magnetic field the vital signs were measured sequentially at field strengths of 8, 6, 4.5, 3, and 1.5 T. RESULTS: The only statistically significant effect of magnetic field strength was observed with systolic blood pressure. An average increase of 3.6 mm Hg in systolic blood pressure was seen with 8 T exposure. ECG rhythm strip analysis demonstrated no significant changes post-exposure. CONCLUSIONS: Normal subjects exposed to varying magnetic field strengths of up to 8 T demonstrated no clinically significant changes in vital signs. Transient ECG artifacts were noted to increase with the field strength.

Adult↗