Biomedical subjects
Publications and source records attributed to M Hedley.
The phase of an image obtained with many magnetic resonance imaging techniques is related to some physical variable of interest. This phase needs to be unwrapped, which is complicated by the presence of noise and multiple objects of irregular shape. A new two-dimensional phase unwrapping algorithm is presented, along with simulation results.
Patient motion during data acquisition in magnetic resonance imaging causes artifacts in the reconstructed image, which for two-dimensional Fourier transform imaging techniques appear as blurring and ghost repetitions of the moving structures. While the problem with intra-view effects has been effectively addressed using gradient moment nulling techniques, there is no corresponding technique for inter-view effects with equal effectiveness and general applicability. A number of techniques have been proposed for correcting the inter-view effects, and these may be divided into those that minimise the corruption of the data, and those that post-process the data to restore the image. The techniques in the former category are briefly reviewed, then those in the latter category are examined in detail. These are analysed in terms of motion model, model parameter estimation, and data correction.
In magnetic resonance imaging using two-dimensional Fourier transform techniques, motion causes ghosting in the phase-encoded direction and loss of image resolution. Although numerous techniques have been proposed to suppress these motion artifacts, they are a continuing problem. In this paper a new technique is presented to reduce the artifact from motion along the slice selection axis. It is shown that such motion causes an amplitude modulation of the data, and an iterative algorithm is developed to reduce this modulation by post-processing the data using signal processing techniques. The algorithm is tested on simulated data, and shown to perform well, even with significant levels of noise added to the data.
The H-2D region in the major histocompatibility complex (MHC) of the BALB/c mouse includes at least five class I genes: Dd, D2d, D3d, D4d, and Ld. The pattern of expression and functions of the D2d, D3d, and D4d genes are not known. To examine the expression of D2d we obtained mouse L cells transfected with a wildtype D2d gene or an exon shufled D2d/D2d/Dd gene that contained exons encoding the alpha 1 and alpha 2 domains of D2d and the alpha 3 domain of H-2Dd. Analysis of the mRNA in transfected cells suggested that two forms of the message were generated at approximately equal abundance by alternative splicing. Several tissues were analyzed and shown to express both forms of the D2d mRNA although the highest levels were found in lymphoid organs. Tumor lines also expressed D2d mRNA but exhibited differing ratios of alternative versus normally spliced message. This ratio was also affected by treatment of cells with Con A supernatants that contain interferon or infection with vesicular stomatitis virus (VSV).
The quality of Magnetic Resonance imaging systems has improved to the point that the motion is the most important limitation in many examinations. Gross movements of the patient cause a ghost artifact in the image which can interfere with the diagnosis. In this paper a new method is presented to suppress the translational motion artifact caused by gross movements. The method uses postprocessing on a standard two-dimensional Fourier transform image. It is shown that translational motion causes an additional phase factor in the detected signal and that this phase error can be removed using an iterative algorithm of generalised projections. The method has been tested using computer simulations and successfully removed most of the artifact, even in the presence of noise.