On the optimization of imaging protocol for the mapping of cerebrovascular reactivity.
BACKGROUND
To devise an improved blood-oxygen-level-dependent (BOLD) imaging protocol for cerebrovascular reactivity (CVR) measurement that can remove a known artifact of negative values.


METHODS
Theoretical and simulation studies were first performed to understand the biophysical mechanism of the negative CVR signals, through which improved BOLD sequence parameters were proposed. This was achieved by equating signal intensities between cerebrospinal fluid and blood, by means of shortening the echo time (TE) of the BOLD sequence. Then, 10 healthy volunteers were recruited to participate in an experimental study, in which we compared the CVR results of two versions of the optimized ("Opt1" and "Opt2") protocols with that of the standard protocol at 3 Tesla. Two sessions were performed for each subject to test the reproducibility of all three protocols.


RESULTS
Experimental results demonstrated that the optimized protocols resulted in elimination of negative-CVR voxels. Quantitative CVR results were compared across protocols, which show that the optimized protocols yielded smaller CVR values (Opt1: 0.16 ± 0.01 %BOLD/mmHg CO2 ; Opt2: 0.15 ± 0.01 %BOLD/mmHg CO2 ) than (P < 0.001) the standard protocol (0.21 ± 0.01 %BOLD/mmHg CO2 ), but the CNR was comparable (P = 0.1) to the standard protocol. The coefficient-of-variation between repetitions was found to be 5.6 ± 1.4%, 6.3 ± 1.6%, and 6.9 ± 0.9% for the three protocols, but there were no significant differences (P = 0.65).


CONCLUSIONS
Based on the theoretical and experimental results obtained from this study, we suggest that the use of a TE shorter than those used in fMRI is necessary to minimize negative artifact in CVR results.
Department of Radiology, Johns Hopkins University, Baltimore, Maryland, USA.|Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, Texas, USA.|Department of Bioengineering, University of Texas at Arlington, Arlington, Texas, USA.
Harshan
H
Ravi
Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, Texas, USA.|Department of Bioengineering, University of Texas at Arlington, Arlington, Texas, USA.
Binu P
BP
Thomas
Department of Radiology, Johns Hopkins University, Baltimore, Maryland, USA.|Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, Texas, USA.
Shin-Lei
SL
Peng
Department of Bioengineering, University of Texas at Arlington, Arlington, Texas, USA.
Hanli
H
Liu
Department of Radiology, Johns Hopkins University, Baltimore, Maryland, USA.|Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, Texas, USA.
Hanzhang
H
Lu
D000328:Adult; D000465:Algorithms; D000704:Analysis of Variance; D016477:Artifacts; D001921:Brain / Q000473:pathology; D001931:Brain Mapping; D002245:Carbon Dioxide / Q000097:blood; D002555:Cerebrospinal Fluid; D002560:Cerebrovascular Circulation / Q000502:physiology*; D003198:Computer Simulation; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D006935:Hypercapnia; D008279:Magnetic Resonance Imaging; D008297:Male; D008962:Models, Theoretical; D010100:Oxygen / Q000737:chemistry; D015203:Reproducibility of Results; D055815:Young Adult
D002245:Carbon Dioxide; D010100:Oxygen
CO2; blood vessels; brain; cerebrospinal fluid; cerebrovascular reactivity; gas inhalation