Biomedical subjects

D C Noll

Publications and source records attributed to D C Noll.

8 recordsLinked to original sources

PubMed1995

Improved assessment of significant activation in functional magnetic resonance imaging (fMRI): use of a cluster-size threshold.

The typical functional magnetic resonance (fMRI) study presents a formidable problem of multiple statistical comparisons (i.e., > 10,000 in a 128 x 128 image). To protect against false positives, investigators have typically relied on decreasing the per pixel false positive probability. This approach incurs an inevitable loss of power to detect statistically significant activity. An alternative approach, which relies on the assumption that areas of true neural activity will tend to stimulate signal changes over contiguous pixels, is presented. If one knows the probability distribution of such cluster sizes as a function of per pixel false positive probability, one can use cluster-size thresholds independently to reject false positives. Both Monte Carlo simulations and fMRI studies of human subjects have been used to verify that this approach can improve statistical power by as much as fivefold over techniques that rely solely on adjusting per pixel false positive probabilities.

Brain↗

PubMed1994

Variable-averaging RARE.

The RARE method and its variants have become popular, rapid-imaging alternatives to conventional spin-echo imaging, particularly for long repetition time proton density and T2-weighted imaging. One variant is to generate both early and late echo images using the same pulse sequence, which has the added benefit of reduced edge artifacts and blurring. Described in this paper is variable-averaging RARE (VA-RARE), a method by which independent amounts of averaging can be set for the early and late echo images generated by a single scanning sequence. Through the use of this method, the signal-to-noise ratio (SNR) of late echo images can be improved without unnecessarily increasing the number of averages for the early echo image, thus saving scanning time. Comparisons to various alternatives are made with respect to scanning time and image quality. Phantom measurements and in vivo images are given to demonstrate the effectiveness of VA-RARE as an efficient method for improving SNR of late echo time images in RARE imaging.

Humans↗

PubMed1993

Functional topographic mapping of the cortical ribbon in human vision with conventional MRI scanners.

The human brain has anatomically distinct areas in which processing is laid out in space at the millimetre level with substantial variation across individuals. Activity occurs along a cortical ribbon 1.5-3 mm thick in response to specific stimuli. Here we report the first use of cortical ribbon analysis on humans using non-invasive functional magnetic resonance imaging techniques performed with a conventional 1.5 T MRI scanner. Changes in activation were detected using T2*-weighted, gradient echo imaging sequences. Subjects observed partial field, flashing checkerboard patterns (left-right, top-bottom, half rings, and wedges). Stimuli produced magnetic resonance signal changes in the 1-8% range, varying at the millimetre scale, which showed contralateral vertically reflected patterns of activation in the visual cortex. To compare the spatial topographies across subjects, computer algorithms were used to control for the subject-unique folding of cortex, providing a flattened cortical ribbon identifying four topographically distinct areas.

Brain Mapping↗

PubMed1993

Consistent projection reconstruction (CPR) techniques for MRI.

Motion of the subject causes inconsistencies in MR scan data which in turn produce streaks in projection reconstruction (PR) scans or ghosts in 2DFT imaging. It is shown that a data manifold consistent with a static object may be extracted from input scan data by applying consistency criteria based on moment or Fourier expansions of the object's projections. The criteria limit the spectrum of azimuthal variation relative to the radial moments or radial spectral components, and their application diminishes k-space inconsistencies from motion or flow by discarding unallowed expansion terms. This consistent projection reconstruction (CPR) algorithm was applied to the reconstruction of projection data from a moving phantom and from human abdominal sections, and in each case provided reduced motion streaks relative to conventional reconstructions. In addition, CPR was found to be effective when used with 2DFT data by resampling the Cartesian data as radial projections, but to a lesser extent.

Abdomen↗

PubMed1992

Deblurring for non-2D Fourier transform magnetic resonance imaging.

For several non-2D Fourier transform imaging methods, off-resonant reconstruction does not just cause geometric distortion, but changes the shape of the point spread function and causes blurring. This effect is well known for projection reconstruction and spiral k-space scanning sequences. We introduce here a method that automatically removes blur introduced by magnetic field inhomogeneity and susceptibility without using a resonant frequency map, making these imaging methods more useful. In this method, the raw data are modulated to several different frequencies and reconstructed to create a series of base images. Determination of degree of blur is done by calculating a focusing measure for each point in each base image and a composite image is then constructed using only the unblurred regions from each base image. This method has been successfully applied to phantom and in vivo images using projection-reconstruction and spiral-scan sequences.

Algorithms↗

PubMed1992

MR imaging of lung parenchyma: a solution to susceptibility.

The authors have developed a pulse sequence for imaging lung parenchyma with projection reconstruction magnetic resonance (MR) imaging that reduces the effects of motion and susceptibility. In this study, the projection reconstruction technique was further modified by optimizing MR signal frequencies for reconstructing the images. This was done by means of one of two methods. With the first method, a susceptibility map was derived from the raw image data and this map was used to indicate the optimal frequencies for reconstructing the images. The second method of susceptibility correction was a postprocessing technique in which the optimal reconstruction frequencies were selected with use of specific focusing criteria to generate the least blurred image. The effect of using susceptibility map correction on a phantom was demonstrated, and both of these methods were used to improve the visibility of pulmonary structures on images of subjects with normal and abnormal lungs.

Humans↗

PubMedDate not supplied

Spiral K-space MR imaging of cortical activation.

Brain function can be mapped with magnetic resonance (MR) imaging sensitized to regional changes in blood oxygenation due to cortical activation. Several MR imaging methods, including conventional imaging and echo-planar imaging, have been successfully used for this purpose. The authors investigated spiral k-space MR imaging, implemented with an unmodified 1.5-T clinical imager, for imaging of cortical activation. A gradient-echo, spiral k-space imaging method was used to measure activation in the primary visual cortex (number sequence task), primary motor cortex (fist-clenching task), and prefrontal cortex (verbal fluency task). Comparison of conventional and spiral k-space imaging in the visual and motor cortex, in which signal-to-noise ratio, voxel size, and imaging time were matched, showed that artifacts were reduced with the spiral k-space method, while the area and degree of activation were similar. The number of sections that could be imaged in a fixed time interval was increased by a factor of four with this implementation of spiral k-space imaging compared with conventional imaging.

Brain Mapping↗

PubMedDate not supplied

Tracking of cyclic motion with phase-contrast cine MR velocity data.

A method of computing trajectories of objects by using velocity data, particularly as acquired with phase-contrast magnetic resonance (MR) imaging, is presented. Starting from a specified location at one time point, the method recursively estimates the trajectory. The effects of measurement noise and eddy current-induced velocity offsets are analyzed. When the motion is periodic, trajectories can be computed by integrating in both the forward and backward temporal directions, and a linear combination of these trajectories minimizes the effect of velocity offsets and maximizes the precision of the combined trajectory. For representative acquisition parameters and signal-to-noise ratios, the limitations due to measurement noise are acceptable. In a phantom with reciprocal rotation, the measured and true trajectories agreed to within 3.3%. Sample trajectory estimates of human myocardial regions are encouraging.

Heart↗