Hi Mark,
Thank you very much for your explanations! I'm looking forward to the
manuskript.
just one more question: In Deichmann et al. 2002 he describes:
Bold sensitivity = TE / Intensity
Would this be a valid approach to calculate BS? Or is this about
another story?
Cheers
Andreas
Am 08.08.2008 um 06:04 schrieb Mark Cohen:
> 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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