Beyond FLAIR: PRISM (polarity-preserving real inversion solute mapping)—a new Frontier in real inversion recovery MRI for intracranial and sensory organ fluid assessment

Assessing subtle compositional changes in cranial and sensory organ fluids-including cerebrospinal fluid (CSF), ocular humors, and inner ear lymph fluid-is vital for neuroimaging. Heavily T2-weighted Fluid-attenuated Inversion Recovery (FLAIR) is sensitive to T1 changes induced by solutes, but conventional magnitude reconstruction (e.g., HYDROPS subtraction) suffers from artifacts like signal cancellation, paradoxical gadolinium-based contrast agent (GBCA) effects, and motion misregistration. This review summarizes the technical characteristics and diverse applications of PRISM (Polarity-preserving Real Inversion Solute Mapping), a robust 3D-real Inversion Recovery (IR) sequence developed to overcome these limitations. PRISM achieves high T1-sensitivity using phase-sensitive (real) reconstruction with an ultra-long repetition time (TR), allowing whole-brain coverage and simultaneous assessment of CSF, the eyeball, and the inner ear within a clinically feasible time. Critically, its polarity-preserving display depicts fluids lacking T1-shortening solutes with a negative signal (black), clearly differentiating them from bone and air. Clinically, PRISM's utility spans both non-contrast and contrast-enhanced applications. Non-contrast PRISM is useful for assessing CSF protein concentration variations, meningeal lymphatic stasis, or compositional changes indicating inner ear pathologies. When combined with GBCA, time-course PRISM (pre-contrast up to 24 h delayed) uniquely visualizes solute dynamics, providing profound insights into blood-barrier integrity (e.g., blood-labyrinth barrier) and glymphatic waste clearance (e.g., CSF washout, perivascular space enhancement). Moreover, PRISM is a reliable, single-acquisition method for visualizing delayed contrast-enhanced endolymphatic hydrops, bypassing prior subtraction pitfalls. PRISM's ability to detect subtle compositional markers with high resolution positions it as a highly promising MRI advancement. However, to facilitate its routine widespread clinical adoption, further standardization, broader validation, and multi-platform reproducibility are essential. Continued efforts to establish robust protocols will be necessary to realize its potential as a non-invasive tool for screening and monitoring glymphatic function.