On-off ParadigmsWhy do you have to do an "on-off" comparison? Why not just measure the absolute BOLD signal instead?
|
|
The raw MR BOLD signal is not absolute but affected by numerous technical and patient-specific factors. Technical factors include field strength, amplifier gain, number and location of head coil receiver elements, pulse sequence (SE or GRE), slice acquisition order, sequence timing parameters (TR/TE/α), and voxel size. Patient-related factors include hematocrit, respiratory rate, head size, age, gender, hormonal status, and medications (including caffeine).
The BOLD signal is typically expressed in arbitrary units (A.U.s) or as a percent change from baseline. Although limited successes at BOLD calibration have been achieved in some research laboratories (see Advanced Discussion), for the most part BOLD signals cannot be easily converted into absolute units, such as blood flow per minute or number of depolarization spikes per second.
Hence for most BOLD/fMRI studies the signal from an "activated" brain region must be differentiated from "non-activated" areas using statistical techniques based on changes measured between "on task" and "off task" states.
The BOLD signal is typically expressed in arbitrary units (A.U.s) or as a percent change from baseline. Although limited successes at BOLD calibration have been achieved in some research laboratories (see Advanced Discussion), for the most part BOLD signals cannot be easily converted into absolute units, such as blood flow per minute or number of depolarization spikes per second.
Hence for most BOLD/fMRI studies the signal from an "activated" brain region must be differentiated from "non-activated" areas using statistical techniques based on changes measured between "on task" and "off task" states.
References
Blockley NP, Griffeth VEM, Simon AB, Buxton RB. A review of calibrated blood oxygenation level-dependent (BOLD) methods for the measurement of task-induced changes in brain oxygen metabolism. NMR Biomed 2013; 26:987-1003.
Hoge RD. Calibrated fMRI. Neuroimage 2012; 62:930-937. (review)
Lu H, Golay X, Pekar JJ, van Zijl PCM. Functional magnetic resonance imaging based on changes in vascular space occupancy. Magn. Reson. Med. 2003; 50: 263–274.
Blockley NP, Griffeth VEM, Simon AB, Buxton RB. A review of calibrated blood oxygenation level-dependent (BOLD) methods for the measurement of task-induced changes in brain oxygen metabolism. NMR Biomed 2013; 26:987-1003.
Hoge RD. Calibrated fMRI. Neuroimage 2012; 62:930-937. (review)
Lu H, Golay X, Pekar JJ, van Zijl PCM. Functional magnetic resonance imaging based on changes in vascular space occupancy. Magn. Reson. Med. 2003; 50: 263–274.
Related Questions
How do you design a BOLD/fMRI study?
How do you design a BOLD/fMRI study?