Inversion recovery (IR) sequences typically employ magnitude reconstruction to translate the MR signal into pixel intensity. In this method pixel brightness depends only on the magnitude of a tissue's longitudinal magnetization, not its polarity. In other words, two tissues whose magnetization at time TI point in equal but opposite directions along the z-axis will be rendered the same shade of gray and thus be indistinguishable.
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The technical details of creating PSIR images vary from vendor to vendor. All methods require creation of baseline phase maps by obtaining reference data at a time when longitudinal magnetization has fully recovered. For CMR this is typically means acquiring reference data during an R-R interval in which no inversion or other imaging pulses have been applied. Complex weights are then computed for each coil and applied to the data. Further processing involving spatial smoothing, seed growing, noise removal, and surface coil intensity normalization may also be embodied in the final PSIR image.
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What is phase-sensitive IR and why does it have a gray background?
How do you set the TI value to optimize myocardial contrast enhancement?