Considerations Regarding the Unintended Radiation Exposure by Charles B. Meinhold

By Charles B. Meinhold

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This longitudinal component is described by an exponential function with the time constant Tl. Besides the proton density (PD = the density of "free," unbound hydrogen nuclei), the different relaxation characteristics of tissues are primarily responsible for producing image contrast in MRI. Spatial encoding is necessary to locate the point from which the received signals originate within the examined volume (here, a water sample). This is done by means of gradient coils, each of which generates an additional magnetic field that is superimposed over the main field and produces a linear change of field strength along its own axis.

The brightness of the color is proportional to the flow velocity. Blood moving toward the transducer is encoded in red; blood moving away from the transducer is encoded in blue. Thus, the transducer is positioned so that arteries appear red and veins appear blue. An additional color can be used to encode a certain velocity range such as the high flow in a tight stenosis. While the carotid arteries can be imaged in both longitudinal and axial views, the vertebral arteries can be imaged only in longitudinal section.

Lengths, areas, and volumes can be measured, however, as in other digital imaging techniques. It should be noted that distortions can occur at the periphery of MR images. These distortions are especially pronounced in EPI. MRI data can also be used for multiplanar reconstructions. If the data sets were acquired with 3-D techniques and a small slice thickness, one can reconstruct images in any desired orientation with the same quality as the original images. Magnetic Resonance Angiography Magnetic resonance angiography (MRA) permits the noninvasive imaging of blood vessels based on the magnetic properties of "flowing" protons (Fig.

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