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FSL  August 2008

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Subject:

Re: BOLD sensitivity

From:

Mark Cohen <[log in to unmask]>

Reply-To:

FSL - FMRIB's Software Library <[log in to unmask]>

Date:

Fri, 8 Aug 2008 05:04:29 +0100

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If I may chime on on this. It is a very interesting area to work on but to my knowledge 
there are no published solutions. When we think of BOLD sensitivity there are a host of 
factors - physiological and instrumental - that play parts. Because, for example, the 
vascular reactivity may differ by region, some of these contributors are deeply buried. 
Nevertheless, some thoughts might help. It is well accepted that a paramagnetic field 
pertuber (e.g., deoxyHgb) will decrease in R2* (1/T2*) that is approximately linear with 
concentration. It is not too difficult to show that this will propagate to the observed signal 
intensity in such a way that the fractional signal change (% signal change) is very close to 
linear over a broad concentration range and over a broad range of tissue T2* values. 
Therefore, measured as % signal change, the BOLD sensitivity to [deoxyHgb] is close to 
uniform. However, the _statistical_ sensitivity is another matter.

In MRI, the instrumentation noise is more or less constant for all locations. Thus, the signal 
to noise ratio, and therefore the detection sensitivity is roughly proportional to the absolute 
image intensity. Finally, you should be aware that in most cases, the noise per voxel is (and 
ideally should be) dominated by physiologically-based signal fluctuations. One crude test for 
this would be to calculate the variance (or std dev) by region. If the noise is dominated by 
physiology, the stdev should be approximately proportional to the signal intensity.

I am working on the manuscript, but I hope this helps in the meantime.

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