The entry slice phenomenon arise in MRI when blood with unsaturated spins flows in the observed slice(s). These spins will emit a strong signal, because of their unsaturated status (flow related enhancement). The number of slices affected depends on the flowvelocity and the slice thickness; the direction of flow determines which slices are affected.
Time of FlightMRA is based on this entry slice phenomenon.
Displaced reduplications of image in phase encoding direction
REASON
Motion, heartbeat, respiration
HELP
Triggering, breath hold, pharmaceuticals to reduce bowel motion
Ghosting artifacts are in the most cases caused by movements (e.g., respiratory motion, bowel motion, arterial pulsations, swallowing, and heartbeat) and appear in the phase encoding direction.
Image Guidance
Ghosting artifacts can be reduced by respiratory and cardiac triggering, the use of breath holding pulse sequences, flow compensation or presaturation pulses, depending on their origin. To reduce bowel motion also pharmaceuticals, such as glucagon or scopolamine are useful. This will decrease artifacts from both peristalsis and breathing.
(I MRA) In MR imaging, inflowing non-saturated fluid gives a higher signal intensity than stationary tissue. This effect makes it especially useful for imaging of flowing blood. Other factors such as susceptibility and spinsaturation, can affect the signal of the blood within the vessels. Furthermore turbulence is part of normal blood flow and can decrease signal intensity.
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