Image Resolution

Image resolution depends in a complicated way on factors such as the magnet strength (measured in units called tesla (abbreviated as T)) of the MRI scanner, imaging time and the performance capabilities of the magnetic field gradient system. In general, higher magnetic field strength systems also have better performance in the gradient systems.  At NIRL, our projects obtain images from 1.5T, 3T and  4T magnetic resonance systems.

Additionally, slice thickness and gap between the slices play a significant role in image resolution. Thinner slices allow for better volumetric analysis. Previous neuroimaging studies often used slices 5mm in thickness with significant gaps in between slices. Modern scanners offer much thinner slices with no gaps in between.

Finally, current image scanning protocols and image analysis programs work together so that images can be analyzed and visualized in three dimensional formats.

Proton Density Image (click on image for larger view)

A Proton Density (PD) Image is a reflection of the density of protons in different tissue types. A PD Image shows strong image intensity and a good signal-to-noise ratio.. Cerebrospinal Fluid (CSF) is the brightest area in a PD image. Next brightest is gray matter and the darkest brain tissue is white matter. This image type is especially suited to differentiating between gray and white matter. Subtle differences in intensity of brighter areas also help to distinguish CSF from lesions. The image below shows intensity representations of some tissue types in a PD image:

T₁-weighted Image (click on image for larger view)

In a T₁-weighted Image the tissue brightness is related to the time it takes for the protons to relax back to equilibrium to have the nuclear magnetization realigned parallel to the main magnetic field. In the T₁-weighted image, CSF is the darkest area and white matter is the brightest region with gray matter in between. Therefore, the T₁-weighted image appears to have brighnesses that are the opposite of both the PD image (above) and T₂-weighted images (below).

T₂-weighted Image (click on image for larger view)

In a T₂-weighted image the tissue brightness is related to the time it takes for the component of the nuclear magnetization which is perpendicular to the magnetic field to disappear. In such an image the CSF appears very bright and the white matter is darker. The gray and white matter have lower contrast than in a PD or T₁ weighted image.. Lesions usually appear bright on T₂.

FLAIR (Fluid-Attenuated Inversion Recovery) Image (click on image for larger view)

FLAIR images have a very high sensitivity to lesions, especially in the periventricular area since the cerebrospinal fluid is suppressed during the data acquisition.  While the contrast between gray and white matter is not very high, the contrast between white matter and lesions is quite large.  Lesions in different areas of the brain have been related to diseases such as MS and depression.

Diffusion Image

Diffusion MRI measures the diffusion of water molecules in biological tissues. In an isotropic medium (inside a glass of water, for example) molecules naturally move according to Brownian motion; however, in biological tissues the diffusion is often anisotropic or direction dependent. For example, a molecule inside the axon of a neuron will move along the axis of the neural fiber. Diffusion MRI, therefore, is a tool used for identifying the brain's neural tracts, a new area of application that is called tractography.

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