Poster abstracts

Poster number 33 submitted by Xiangyu Yang

Postprocessing correction for quadratic B0 Inhomogeneity in R2* Measurement at Ultrahigh Field

Xiangyu Yang (Radiology Department, the Ohio State University), Petra Schmalbrock (Radiology Department, the Ohio State University), Steffen Sammet (Radiology Department, the Ohio State University), Michael V. Knopp (Radiology Department, the Ohio State University)

Abstract:
Introduction
Clinical studies have suggested that the transverse relaxation rate R2* is a good measure of tissue iron content [1, 2]. However, R2* measurement is subject to B0-inhomogeneity-induced error in clinical scanners. At low field (1.5T and below), B0 inhomogeneity can either be ignored or approximated by a linear gradient, which corresponds to a sinc modulation [3]. Its nonlinearity becomes proportionally more important with the main magnetic field. In this study, we proposed a method to correct for quadratic B0 inhomogeneity and demonstrated it in a phantom study.
Materials and Methods
When the B0 inhomogeneity has the form: δB0 = B0 × (αz² + βz) across a slice with slice thickness 2z₀, the FID signal can be solved analytically as an ideal exponential decay modulated by a squared sum of sine and cosine Fresnel integrals.
B0 map and EPI test mode data were collected on a phantom built with 0.125~1mM MnCl₂ solutions at a Philips Achieva 7T scanner. Three commonly used models were fitted to the data: monoexponential, monoexponential-plus-constant, and sinc-modulated exponential. R2* of MnCl₂ solutions follow a linear relationship with Mn²⁺ concentrations. The slope coefficients were calculated in the presence and absence of B0 inhomogeneity, and compared by the relative differences (RD): RD = (absolute difference) / mean. All calculations were done with Matlab (Mathworks, Natick, MA).
Results
RD’s of the three commonly used models are: 43%, 39%, 47%, and 10%. RD of the quadratic correction method is only 1/4 to 1/5 of those of the three commonly used models.
Discussion and Conclusion
We have demonstrated that B0 inhomogeneity induced effect can be remedied by measuring B0 map and conducting a quadratic correction.

References:
[1] Haacke et al, MRI 2005, 23: 1-25;
[2] Wood et al, Blood 2005, 106: 1460-1465;
[3] Fernandez-Seara et al, MRM 2000, 44:358-366;

Keywords: MRI, T2, Postprocessing correction