ABSOLUTE QUANTITATION OF DIFFUSION CONSTANTS IN HUMAN STROKE

Background and Purpose Animal studies have shown that MR diffusion ima ging can outline acute ischemic regions before irreversible damage (in farction) occurs. To study evolution of ischemic lesions in humans, it is therefore important to quantify absolute diffusion constants (D va lues), but quantitation has not been reproducible among different clin ics. These problems are explained, and a method for reproducible quant itation is suggested. Methods Diffusion-weighted and absolute diffusio n images were acquired, and the absolute apparent diffusion constants in three orthogonal spatial directions (D-xx, D-yy, and D-zz) were mea sured. These were combined to calculate images of the orientation-inde pendent apparent diffusion parameter D-av=1/3 Trace{D-=}=1/3(D-xx+D-yy +D-zz). Values of the individual diffusion constants and D-av were eva luated in 6 patients and 6 normal volunteers. Results Patient data sho w that comparison of diffusion constants between contralateral and ips ilateral hemispheres after ischemia may give results varying by more t han 100% depending on orientation. Findings in normal-appearing region s containing a mixture of gray and white matter in patients (n=5) and in normal volunteers (n=6) show that D-av=(0.92+/-0.11)x10(-3) mm(2)/s , with a small intersubject variation, whereas D-xx, D-yy, and D-zz va ry strongly. Hemispheric ratios (ipsilateralicontralateral [I/C]) in t hese subjects were (I/C)(Dav)=1.00+/-0.05, (I/C)(Dxx)=1.02+/-0.15, (I/ C)(Dyy)=1.07+/-0.24, and (I/C)(Dzz)=0.96+/-0.28. The individual subjec ts in this group all had an (I/C)(Dav) within 10% of unity, while the other three ratios showed intersubject variations as large as 100%. Co nclusions (I/C)(Dav) ratios are a reliable means to quantitate changes in absolute diffusion constants for the study of stroke evolution ind ependent of tissue orientation, gradient orientation, and diffusion ti me. The use of these ratios will enable reproducible intersubject and interclinic quantitation.