Other Magnetic Resonance Methods

The biomedical application of magnetic resonance methods is continuing to advance at a rapid pace. Therefore, a brief summary of several emerging methodologies is provided.

Diffusion Weighted Imaging

Diffusion weighted imaging (DWI) provides a means to obtain images which are sensitive to the molecular motion of water (Le Bihan et al., 1992). DWI capitalizes on the fact that as water diffuses across magnetic field gradients, MR signals from water molecules tend to dephase and to disappear (relax) more quickly. The rate at which this dephasing occurs is proportional to the strength of the magnetic field gradient as well as the rate of diffusion.

DWI has recently become a popular method to evaluate individuals who are in the early stages of a cerebral infarction (Warach et al., 1992). During episodes of cerebral ischemia, water protons diffuse more slowly and this may be reflected using DWI. Evaluating the extent to which DWI may be used to monitor the evolution of strokes, as well as the effects of putative therapies, is currently an area of active research (Lo et al., 1994).

Magnetic Resonance Angiography

Magnetic resonance angiography (MRA) is a noninvasive technique that uses the magnetic resonance signal from flowing blood to create an image of cerebral vasculature. Several techniques can be used to acquire MRA images, including phase contrast angiography and three-dimensional time of flight (3DTOF) angiography. With the 3DTOF technique, stationary tissue (e.g., brain) has its magnetization saturated and produces minimal signal. Tissue that is in motion at slow velocities (venous blood) also gives off minimal signal. Only blood flowing at high velocity (arterial blood) gives off substantial MR signal. Acquisition parameters can be programmed so that selective images of blood flow in arteries or veins can be made. MRA techniques are highly sensitive to blood flow perturbations which reduce vessel signal intensity. When blood flows past a vessel narrowing (stenotic region), a portion slows down because of local turbulence, while a portion (toward the middle of the vessel) speeds up. The decelerated blood has its magnetization saturated and also loses phase coherence, while the accelerated blood loses phase coherence, and these effects result in lower blood flow signal.

The 3DTOF technique acquires multiple thin slice (~1.5 mm thick) images of brain which are used to reconstruct composite images of the brain vascular system. The composite images are known as maximum intensity projections (MlPs) and include only the brightest pixels from each source image which presumably are from flowing blood. It is possible to produce maximum intensity projection images in many different orientations using 3DTOF data. Angiographic techniques may be particularly useful in characterizing the blood flow abnormalities associated with acute and chronic drug abuse.

Relaxometry

By varying the image acquisition parameters, TR (pulse repetition time) and TE (echo time), it is possible to construct images of the proton relaxation times T1 and T2. The development of methods for echo planar imaging makes it possible to collect images with a large number of TR and/or TE values quite rapidly and, thereby, to estimate localized T1 and T2 values very reliably. This relaxation time mapping is sometimes referred to as relaxometry.

Changes in relaxation times may reflect anomalous cerebral development, as recently demonstrated in schizophrenia (Andreasen et al., 1991; Williamson et al., 1992; Yurgelun-Todd et al., 1995). Alternatively, alterations in cerebral perfusion may lead to small changes in T2 which can be detected using echo planar imaging (Teicher et al., 2000). In the area of substance abuse, relaxation time measurements have also been used to assess brain hydration. In general, as brain water content decreases, relaxation times become shorter.

Magnetization Transfer

Within the brain, water molecules often interact with other, larger molecules due to dipolar effects, such as hydrogen bonding. Water which is "bonded" to a larger molecule will have very different magnetic resonance properties than "free" water. In particular, bonded water tends to have very short relaxation times and, in a magnetic resonance spectrum, a very wide resonance peak. Thus, irradiation at a frequency which is quite different from the resonance frequency of the free water pool can sometimes excite the bonded water pool. This effect is referred to as magnetization transfer (MT). Assessment of MT effects may be used to characterize the interactions of both water and ethanol with brain macromolecules.

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