Affiliation:
1. From the Department of Radiology, The Johns Hopkins Medical Institutions, Baltimore, Md.
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.
Publisher
Ovid Technologies (Wolters Kluwer Health)
Subject
Advanced and Specialized Nursing,Cardiology and Cardiovascular Medicine,Neurology (clinical)
Reference32 articles.
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3. Fast magnetic resonance diffusion-weighted imaging of acute human stroke
4. Acute human stroke studied by whole brain echo planar diffusion-weighted magnetic resonance imaging
5. Warach S Benfield A Edelman RR. Time course of abnormal apparent diffusion coefficient in human stroke. In: Proceedings of the 3rd meeting of the Society of Magnetic Resonance in Medicine; August 19-25 1995; Nice France. Abstract page 82.
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