ABSTRACT
This paper analyzes the effects of intra-scan motion and demonstrates the possibility of correcting them directly in k-space with a new automatic retrospective method. The method is presented for series of 2D acquisitions with Cartesian sampling. Using a reference k-space acquisition (corrected for translations) within the series, intra-scan motion parameters are accurately estimated for each trajectory in k-space of each data set in the series resulting in pseudo-random sample positions. The images are reconstructed with a Bayesian estimator that can handle sparse arbitrary sampling in k-space and reduces intra-scan rotation artefacts to the noise level. The method has been assessed by means of a Monte Carlo study on axial brain images for different signal-to-noise ratios. The accuracy of motion estimates is better than 0.1 degrees for rotation, and 0.1 and 0.05 pixel, respectively, for translation along the read and phase directions for signal-to-noise ratios higher than 6 of the signals on each trajectory. An example of reconstruction from experimental data corrupted by head motion is also given.
Subject(s)
Algorithms , Artifacts , Brain/anatomy & histology , Image Enhancement/methods , Magnetic Resonance Imaging/methods , Motion , Animals , Humans , Models, Biological , Models, Statistical , Quality Control , Reproducibility of Results , Sensitivity and SpecificityABSTRACT
Although the dimensions of the MX-41/AR earphone cushion are specified in ANSI S3.6--1969, nonstandard MX--41/AR cushions are still found on some audiometers. Variations in the diameter of the cushion's center hole are shown to affect the coupler calibration of the earphone and also hearing threshold measurements at high audiometric frequencies.
