Recent developments have led to increased interest in the application of borehole nuclear magnetic resonance (NMR) as a probe of petrophysical properties. Of particular importance in this connection is the measurement of the longitudinal relaxation time, T1. As T1 is controlled by the pore surface area, its value may be strongly influenced by the invasion of submicron-sized clay particles found in drilling muds. We have studied this effect by the application of phase encode magnetic resonance imaging (MRI) techniques.
OBJECTIVE: Our objective was to evaluate a new scanning method, MR line scan diffusion imaging, and assess the apparent diffusion coefficient in the brains of healthy subjects and stroke patients.
PURPOSE: The feasibility of a miniature endoluminal magnetic resonance (MR) detection coil was investigated for imaging mural and perimural anatomy of small, tubular structures.
A retrospective analysis of clinical imaging using 2DFT SSFP at 0.14 T is presented. The technique's potential for tissue characterization and its utility for clinical diagnosis were tested by both in vitro measurements of various tissues and in vivo clinical images. Different pulse angles not only influenced image contrast, but also helped characterize lesions, particularly those containing fat. In addition, the pulse angle changed the signal from venous flow perpendicular to the imaged slice.
The addition of a spin-lock preparatory sequence to a Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence provides a method which allows an accurate and simple comparison of T1p and T2 contrast. Sagittal and axial brain images, produced with the application of a three pulse preparatory spin-lock sequence prior to a sixteen-echo CPMG imaging sequence, are compared with images acquired without the spin-lock sequence. The CPMG sequence uses non-selective refocusing pulses. Therefore, observed echo signals accurately reflect T2 relaxation.
In a comparative study more than 35 brain magnetic resonance imaging examinations were analyzed by mapping the CSF flow/motion pattern using the steady state free precession technique with two different flow sensitivity directions. Significant deviations from the normal pattern were seen in ventricular enlargements due to obstruction with no evidence of CSF flow/motion and in normal pressure hydrocephalus with complex flow pattern in lateral ventricles, suggesting a diagnostic potential for this fast imaging technique with sensitivity to very slow flow.
PURPOSE: To calculate age-related and per diopter (D) accommodative changes in crystalline lens and ciliary muscle dimensions in vivo in a single cohort of emmetropic human adults ages 30 to 50 years.
METHODS: The right eyes of 26 emmetropic adults were examined using ultrasonography, phakometry, anterior segment optical coherence tomography, and high resolution magnetic resonance imaging. Accommodation was measured both subjectively and objectively.
Steady-state free precession (SSFP) and particularly the spatially periodic magnetization response with wavelength lambda that results in the presence of an applied gradient is discussed. The maximum SSFP magnetization does not always occur at the Larmor frequency but rather depends on both the phase cycling of the rf pulses and on the rf tip angle. The slow flow sensitivity of SSFP also depends on lambda.
We have proposed a two-dimensional PERiodic-Linear (PERL) magnetic encoding field geometry B(x,y) = g(y)y cos(q(x)x) and a magnetic resonance imaging pulse sequence which incorporates two fields to image a two-dimensional spin density: a standard linear gradient in the x dimension, and the PERL field. Because of its periodicity, the PERL field produces a signal where the phase of the two dimensions is functionally different. The x dimension is encoded linearly, but the y dimension appears as the argument of a sinusoidal phase term.
