Besign-directed measurements of B1 heterogeneity and spin-lattice relaxation for 8 Tesla MRI
The current trend toward magnetic resonance imaging (MRI) above 4 Tesla requires design-directed research before the benefits of increased signal to noise ratio (SNR) associated with high field MRI can be realized clinically. In this work the well-known radiofrequency (RF) heterogeneity is explored with respect to imaging the human brain and performing T1 measurements. T1 data are presented in a number of applications including in-situ measurements and relaxivity measurements of gadolinium diethelene pentaacetic acid (Gd-DTPA) in protein solutions. Many authors have noted spatial variation in the excitation produced by RF pulses in MRI experiments. It is generally agreed that this heterogeneity is a result of the interaction between the magnetic field (B1) associated with the RF excitation and the electromagnetic properties of the sample, namely the dielectric constant and conductivity. After reviewing theoretical approaches and experimental methods used to study the B1 field, experimental data are presented for the field distribution in a uniform, spherical phantom. The fields are shown over a range of six of the operating modes of the transverse electromagnetic (TEM) coil. Human brain images and field distributions are shown for a normal volunteer for three of these operating modes. These images suggest that coils may be tuned to a particular mode to target a specific anatomical region. MRI techniques can provide quantitative measurements in addition to anatomic images. The robustness of one such measurement, spin-lattice relaxation rate, is explored in light of the B1 field heterogeneity noted above. Phantom and in-situ data are presented for common methods of determining the exponential time constant, T1, associated with spin-lattice relaxation. Numerical simulations and experimental data show that the Inversion Recovery (IR) method, while time consuming, provides the most stable T1 measurements in light of B1 heterogeneity. Finally, relaxivity data are presented for the contrast agent Gd-DTPA. The 8 Tesla data in this work support the argument that the presence of biomacromolecules increases the relaxivity of Gd-DTPA.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:mri ultra high field relaxivity
Date of Publication:01/01/2004