IR Bounce Point Artifact
I've noticed that T1-FLAIR images may show "India ink" outlines. Is this also a chemical shift artifact?
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The black outline of certain anatomic structures sometimes seen on inversion recovery (IR) images is not due to chemical shift, but is caused by a similar mechanism — phase cancellation of the MR signals in boundary pixels.
An example is shown right. Here note the sharp black etched appearance of the brain where it interfaces with cerebrospinal fluid (CSF). This is commonly known as the bounce-point artifact (originally named the "opposed magnetization artifact.") In IR imaging, tissue magnetization (M) is flipped opposite to the main magnetic field and then recovers, passing through zero in the process. As tissues have different T1 values, their recovery curves will be different. In the diagram right, CSF has a longer T1 and recovers more slowly than brain. In magnitude-reconstructed IR only the absolute distance from zero of each tissue's recovery curve determines the displayed signal intensity. The zero crossing point for each tissue is assigned to black and is called the null or bounce point.
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Bounce point artifact
Bounce Point occurs in magnitude-reconstructed IR images at intermediate time (TI) when two substances sharing the same voxel cross the zero axis.
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Consider a pixel on the boundary between CSF and brain containing equal amounts of each material. Let us also assume that the inversion time (TI) has been to a value between the bounce points for brain and CSF where the curve for brain is exactly the same distance above the zero line as the curve for CSF is below the line. At this point the signals for brain and CSF will be of identical magnitude but 180° out of phase. Their signals will exactly cancel, rendering the boundary pixel black.
Even though the bounce point artifact has been described for conventional magnitude corrected IR images, the same phenomenon can be seen with any pulse sequence where an inversion pulse is used. Thus bounce point artifacts may also be seen on such sequences as MP-RAGE, FGATIR, or IR-prepped SPGR.
References
Hearshen DO, Ellis JH, Carson PL, et al. Boundary effects from opposed magnetization artifact in IR images. Radiology 1986; 160:543-7.
Hearshen DO, Ellis JH, Carson PL, et al. Boundary effects from opposed magnetization artifact in IR images. Radiology 1986; 160:543-7.
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