Phase Sensitive Inversion Recovery (PSIR)

What is the difference between magnitude and phase-sensitive IR for detecting myocardial enhancement?  

 

Infarcts have shorter T1-values than normal myocardium and recover more quickly due to excessive gadolinium accumulation. Enhancement may be detected by either magnitude or phase-corrected reconstruction by selecting an inversion time (TI) near the null point of normal myocardium.

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. 

Magnitude reconstruction is straightforward to implement and allows selective nulling of normal myocardium by selecting an inversion time (TI) matching its zero-crossing point. This value can be estimated using the "TI scouting" method as described in the prior Q&A. If the wrong TI is picked, however, image contrast between the infarct and viable myocardium may be impaired. This sensitivity to the choice of TI may be overcome by reconstructing the data using a phase-sensitive technique. 

Phase-sensitive Inversion Recovery (PSIR) reconstruction preserves the positive and negative polarities of tissues as they recover from the initial 180°-inversion pulse. In PSIR tissues with more negative longitudinal magnetizations always appear darker than those with more positive magnetizations. Hence contrast-enhancing (infarcted) tissue always has a higher signal than viable myocardium, regardless of the chosen TI. Unlike magnitude reconstruction the PSIR technique gives good results over a relatively broad range of TI's. Although PSIR reconstruction makes it possible to produce excellent quality contrast-enhanced images without using an inversion-time mapping sequence, the PSIR images will be even better if an optimal estimate by TI-scouting is first made.

If TI is not perfectly chosen, detection of contrast enhancement may be impaired using magnitude reconstruction. Phase-sensitive IR is less sensitive to choice of TI as illustrated in this example of of septal ischemia (arrow).

Advanced Discussion (show/hide)»

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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References
     Kellman P, Arai AE, McVeigh ER, Aletras AH. Phase-sensitive inversion recovery for detecting myocardial infarction using gadolinium-delated hyperenhancement. Magn Reson Med 2002; 47:372-383. 
    Huber AM, Schoenberg SO, Hayes C, et al. Phase-sensitive inversion-recovery MR imaging in the detection of myocardial infarction. Radiology 2005; 237:854-860.
    Juan LJ, Crean AM, Wintersperger BJ. Late gadolinium enhancement imaging in assessment of myocardial viability. Techniques and clinical applications. Radiol Clin N Amer 2015; 53:397-411.
     Moran PR, Kumar NG, Karstaedt N, Jackels SC. Tissue contrast enhancement: image reconstruction algorithm selection of TI in inversion recovery MRI. Magn Reson Imaging 1986; 4:229-235.     
     Park HW, Cho MH, Cho ZH. Real-value representation in inversion-recovery NMR imaging by the use of a phase-correction method. Magn Reson Med 1986; 3:15-23.

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Related Questions
     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?  

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