[Show abstract][Hide abstract] ABSTRACT: Conventional T2- and T2*-weighted image contrasts are produced by waiting a TE period for the transverse magnetic resonance (MR) signals to decay to differentiate tissue types with distinct relaxation rates. Significant image signal-to-noise ratio (SNR) is compromised by this contrast-producing process. In this report, a multiple echo frequency-domain image contrast (MEFIC) method is presented. During the conventional TE period, a multiple echo train modulated by T2 or T2* decay is acquired. A third Fourier transform along the echo direction produces an image set with pixel signal intensity modulated by the spectrum of the decay curve. This method simultaneously enhances image contrast with a large increase in SNR. Experimental studies of cerebral vasogenic edema in immature rats and functional MR imaging studies of the human motor cortex have demonstrated that the MEFIC method produces superior image quality over conventional methods for generating T2- and T2* weighted images.
Full-text · Article · Mar 1999 · Magnetic Resonance in Medicine
[Show abstract][Hide abstract] ABSTRACT: Distortion of the static magnetic field inside the human head is dependent on regional tissue susceptibility variations and geometrical shape. These effects result in resonance line broadening and frequency shifts and consequently, intensity and spatial errors in both magnetic resonance imaging (MRI) and magnetic resonance (MR) spectroscopy. To calculate the field distortion due to the susceptibility's geometry, two dimensional (2D) finite element analysis was applied to simulate the field distribution in a 2D model of the human head, placed in a uniform magnetic field. The model contains air-filled cavities and sinuses, and the remainder is treated as water. The magnetic field deviation was evaluated using gray scale plots and histograms of the magnetic field. The shifts in parts/million and broadening of the histograms correspond to the NMR of the sampled region. The field distribution of the human head was also experimentally mapped using the DANTE tagging sequence. The calculated and experimental field maps are in good agreement. Thus, geometric considerations with uniform susceptibilities are sufficient to explain most of the static magnetic field distribution in the human head.
No preview · Article · Aug 1995 · Magnetic Resonance in Medicine