Absolute Quantitation of Diffusion Constants in Human Stroke

Abstract

Background and Purpose Animal studies have shown that MR diffusion imaging can outline acute ischemic regions before irreversible damage (infarction) occurs. To study evolution of ischemic lesions in humans, it is therefore important to quantify absolute diffusion constants ( D values), but quantitation has not been reproducible among different clinics. These problems are explained, and a method for reproducible quantitation is suggested. Methods Diffusion-weighted and absolute diffusion images were acquired, and the absolute apparent diffusion constants in three orthogonal spatial directions ( D xx , D yy , and D zz ) were measured. These were combined to calculate images of the orientation-independent apparent diffusion parameter D av =1/3 Trace{D̄}=1/3( D xx + D yy + D zz ). Values of the individual diffusion constants and D av were evaluated in 6 patients and 6 normal volunteers. Results Patient data show that comparison of diffusion constants between contralateral and ipsilateral hemispheres after ischemia may give results varying by more than 100% depending on orientation. Findings in normal-appearing regions 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)×10 –3 mm 2 /s, with a small intersubject variation, whereas D xx , D yy , and D zz vary strongly. Hemispheric ratios (ipsilateral/contralateral [I/C]) in these subjects were (I/C) D av =1.00±0.05, (I/C) D xx =1.02±0.15, (I/C) D yy =1.07±0.24, and (I/C) D zz =0.96±0.28. The individual subjects in this group all had an (I/C) D av within 10% of unity, while the other three ratios showed intersubject variations as large as 100%. Conclusions (I/C) D av ratios are a reliable means to quantitate changes in absolute diffusion constants for the study of stroke evolution independent of tissue orientation, gradient orientation, and diffusion time. The use of these ratios will enable reproducible intersubject and interclinic quantitation.