Biexponential diffusion attenuation in various states of brain tissue: Implications for diffusion‐weighted imaging

Abstract

Diffusion‐weighted single voxel experiments conducted at b‐values up to 1 × 10⁴ smm⁻² yielded biexponential signal attenuation curves for both normal and ischemic brain. The relative fractions of the rapidly and slowly decaying components (f₁, f₂)are f₁ = 0.80 ± 0.02, f₂ = 0.17 ± 0.02 in healthy adult rat brain and f₁ = 0.90 ± 0.02, f₂ = 0.11 ± 0.01 in normal neonatal rat brain, whereas the corresponding values for the postmortem situation are f₁ = 0.69 ± 0.02, f₂ = 0.33 ± 0.02. It is demonstrated that the changes in f₁ and f₂ occur simultaneously to those in the extracellular and intracellular space fractions (f_ex, f_in) during: (i) cell swelling after total circulatory arrest, and (ii) the recovery from N‐methyl‐D‐aspartate induced excitotoxic brain edema evoked by MK‐801, as measured by changes in the electrical impedance. Possible reasons for the discrepancy between the estimated magnitude components and the physiological values are presented and evaluated. Implications of the biexponential signal attenuation curves for diffusion‐weighted imaging experiments are discussed.