Multi-component apparent diffusion coefficients in human brain.
The signal decay with increasing b-factor at fixed echo time from brain tissue in vivo has been measured using a line scan Stejskal-Tanner spin echo diffusion approach in eight healthy adult volunteers. The use of a 175 ms echo time and maximum gradient strengths of 10 mT/m allowed 64 b-factors to be sampled, ranging from 5 to 6000 s/ mm2, a maximum some three times larger than that typically used for diffusion imaging. The signal decay with b-factor over this extended range showed a decidedly non-exponential behavior well-suited to biexponential modeling. Statistical analyses of the fitted biexponential parameters from over 125 brain voxels (15 x 15 x 1 mm3 volume) per volunteer yielded a mean volume fraction of 0.74 which decayed with a typical apparent diffusion coefficient around 1.4 microm2/ms. The remaining fraction had an apparent diffusion coefficient of approximately 0.25 microm2/ms. Simple models which might explain the non-exponential behavior, such as intra- and extracellular water compartmentation with slow exchange, appear inadequate for a complete description. For typical diffusion imaging with b-factors below 2000 s/mm2, the standard model of monoexponential signal decay with b-factor, apparent diffusion coefficient values around 0.7 microm2/ms, and a sensitivity to diffusion gradient direction may appear appropriate. Over a more extended but readily accessible b-factor range, however, the complexity of brain signal decay with b-factor increases, offering a greater parametrization of the water diffusion process for tissue characterization.
Mulkern
R V
RV
Department of Radiology, Children's Hospital, Harvard Medical School, Boston, MA 02115, USA. mulkern@bwh.harvard.edu
Gudbjartsson
H
H
Westin
C F
CF
Zengingonul
H P
HP
Gartner
W
W
Guttmann
C R
CR
Robertson
R L
RL
Kyriakos
W
W
Schwartz
R
R
Holtzman
D
D
Jolesz
F A
FA
Maier
S E
SE
D000328:Adult; D001921:Brain / Q000033:anatomy & histology / Q000502:physiology*; D004058:Diffusion; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D019047:Phantoms, Imaging; D012815:Signal Processing, Computer-Assisted