Surface Quadrupolar Relaxation (SQUARE) Contrast in Pulmonary MRI with Hyperpolarized ⁸³Kr

This chapter explains the concepts behind hyperpolarized (HP) ⁸³Kr magnetic resonance imaging (MRI) and how surface quadrupolar relaxation (SQUARE) contrast, that originates from the nuclear electric charge distribution of this spin I=9/2 noble gas isotope, can be utilized to probe lung surfaces. Work with model surfaces illustrates how ⁸³Kr SQUARE T₁ times are affected by the surface-to-volume ratio (S/V) and by the chemical composition of the surface. Measurements of SQUARE in ex vivo lungs, enabled through a custom designed ventilator that allows for the lungs to "actively inhale" HP ⁸³Kr, demonstrate the high reproducibility of the method that is surprisingly little affected by the inhalation volume. Finally, MRI SQUARE contrast of an elastase model of emphysema shows a strong correlation between SQUARE parameters measured ex vivo and the presence of the disease model in rat lungs. The prospects of future clinical applications of this novel MRI contrast are presented.