Quantitative T2 in the occipital lobe: the role of the CPMG refocusing rate.
OBJECTIVE
To investigate the dependence of occipital gray and white matter T(2) on the Carr-Purcell-Meiboom-Gill (CPMG) refocusing interval, thereby testing the basis of a novel functional magnetic resonance imaging (fMRI) method for blood volume quantification, and addressing recent questions surrounding T(2) contrast in the occipital lobe.


METHODS
A CPMG sequence with 1 x 1 x 5 mm(3) resolution was used to quantify T(2) in a single axial slice at the midlevel of the occipital lobe in 23 healthy adult volunteers. Refocusing intervals of 8, 11, and 22 msec were compared. A Bayesian classifier was used to classify a 1 x 1 x 1 mm(3) T(1)-weighted three-dimensional data set into gray matter, white matter, and cerebrospinal fluid, with an average 95% a posteriori probability used as the threshold for inclusion into a tissue-specific region of interest (ROI).


RESULTS
The usual T(2) contrast between the gray and white matter (i.e., T(2GM) > T(2WM)) was observed, with a highly significant effect of tissue type on the estimated T(2) (P < 10(-5)). The observed T(2) gradually decreased with increasing refocusing interval, for a decrease of 3.3 +/- 1.5 msec in gray matter and 3.0 +/- 1.5 msec in white matter between the 8 and 22 msec refocusing interval acquisitions.


CONCLUSIONS
The observed T(2) shortening is consistent with the effect of the dramatic decrease in T(2) of partly deoxygenated blood on this range of refocusing rates.
Stefanovic
Bojana
B
McConnell Brain Imaging Centre, Montreal Neurological Institute, McGill University, Montreal, Canada. bojana@bic.mni.mcgill.ca
Sled
John G
JG
Pike
G Bruce
GB
D000328:Adult; D001811:Blood Volume Determination / Q000379:methods; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*; D009778:Occipital Lobe / Q000033:anatomy & histology*