CiteULike: xtizons artefacts

A Method for Calibrating Diffusion Gradients in Diffusion Tensor Imaging.

Yu-Chien Wu — 2007-12-17T21:33:44-00:00

J Comput Assist Tomogr, Vol. 31, No. 6. (r 2007), pp. 984-993.

OBJECTIVE:: To calibrate and correct the gradient errors including gradient amplitude scaling errors, background/imaging gradients, and residual gradients in diffusion tensor imaging (DTI). METHODS:: A calibration protocol using an isotropic phantom was proposed. Gradient errors were estimated by using linear regression analyses on quadratic functions of diffusion gradients along 3 orthogonal directions. A 6-element total effective scaling vector is generated from the calibration protocol to retrospectively correct gradient errors in DTI experiments. RESULTS:: The accuracy of the calibration protocol was within 1% or less in estimating gradient scaling errors. On both the brain study and the computer simulations, the retrospective correction minimized undesirable estimate biases of DTI measurements due to gradient errors. CONCLUSION:: The protocol and retrospective correction are shown to be effective. The method may be used for prospective correction if actual diffusion-gradient waveforms are available. The methodology is expandable to general diffusion imaging schemes.

A nonparametric method for automatic correction of intensity nonuniformity in MRI data.

JG Sled — 2007-04-24T15:11:11-00:00

IEEE Trans Med Imaging, Vol. 17, No. 1. (February 1998), pp. 87-97.

A novel approach to correcting for intensity nonuniformity in magnetic resonance (MR) data is described that achieves high performance without requiring a model of the tissue classes present. The method has the advantage that it can be applied at an early stage in an automated data analysis, before a tissue model is available. Described as nonparametric nonuniform intensity normalization (N3), the method is independent of pulse sequence and insensitive to pathological data that might otherwise violate model assumptions. To eliminate the dependence of the field estimate on anatomy, an iterative approach is employed to estimate both the multiplicative bias field and the distribution of the true tissue intensities. The performance of this method is evaluated using both real and simulated MR data.

Case report: PET/CT, a cautionary tale

Jayson Wang — 2007-08-04T18:40:02-00:00

BMC Cancer, Vol. 7 (03 August 2007), 147.

Spatial and temporal resolution effects on dynamic contrast-enhanced magnetic resonance mammography

Michael Aref — 2007-01-23T09:51:36-00:00

Magnetic Resonance Imaging, Vol. 25, No. 1. (January 2007), pp. 14-34.

We tested the hypothesis that partial volume effects due to poor in-plane resolution and/or low temporal resolution used in clinical dynamic contrast-enhanced magnetic resonance imaging results in erroneous diagnostic information based on inaccurate estimates of tumor contrast agent extravasation and tested whether reduced encoding techniques can correct for dynamic data volume averaging. Image spatial resolution was reduced from 469x469 [mu]m2 to those reported below by selecting a subset of k-space data. We then compared the top five Ktrans/VT "hot spots" obtained from the original data set, 469x469-[mu]m in-plane spatial resolution and an 18-s temporal resolution processed by fast Fourier transform (FFT), with values obtained from data sets having in-plane spatial resolutions of 938x938, 1875x1875 and 2500x2500 [mu]m2 and a temporal resolution of 18 s, or data sets with temporal resolutions of 36, 54 and 72 and a spatial resolution of 469x469 [mu]m2, and found them to statistically differ from the parent data sets. We then tested four different post processing methods for improving the spatial resolution without sacrificing temporal resolution: zero-filled FFT, keyhole, reduced-encoding imaging by generalized-series reconstruction (RIGR) and two-reference RIGR (TRIGR). The top five values of Ktrans/VT obtained from data sets, the in-plane spatial resolutions of which were improved to 469x469 [mu]m2 by zero-filling FFT, Keyhole and RIGR, statistically differed from those obtained from the original 469x469 [mu]m2 FFT parent image data set. Only the 938x938 and 1875x1875 [mu]m2 data sets reconstructed to 469x469 [mu]m2 with TRIGR reconstruction method yielded values of the top five Ktrans/VT hot spots statistically the same as the original parent data set, 469x469 [mu]m2 in-plane spatial and 18-s temporal-resolution FFT. That is, partial volume effects from data sets of different in-plane spatial resolution resulted in statistically different values of the top five Ktrans/VT hot spots relative to a high spatial and temporal resolution data set, and TRIGR reconstruction of these low resolution data sets to high resolution images provided statistically similar values with a savings in temporal resolution of 2 to 4 times.

Accuracy of T1 measurements at high temporal resolution: Feasibility of dynamic measurement of blood T1 after contrast administration

Jie Zheng — 2006-09-13T08:17:34-00:00

Journal of Magnetic Resonance Imaging, Vol. 10, No. 4. (1999), pp. 576-581.

The purpose of this work was to optimize a technique to measure blood T1 dynamically after contrast agent administration with a high temporal resolution. This technique uses a 90° prepared gradient-echo sequence and has a temporal resolution of one T1 measurement per cardiac cycle. The non-ideal excitation slice profiles on the estimation of T1 were evaluated by theoretical simulations and used to obtain corrected blood T1 values. The technique was validated on phantom and in vivo pig studies, which demonstrated significant improvement on the accuracy of the dynamic T1 measurement method after slice profile correction. This technique may find important applications in studying the dynamic blood T1 after injection of various contrast agents. J. Magn. Reson. Imaging 1999;10:576-581. © 1999 Wiley-Liss, Inc.