X-Message-Number: 5227
From:  (Brian Wowk)
Newsgroups: sci.cryonics
Subject: Re: High definition MRI
Date: 20 Nov 1995 19:34:31 GMT
Message-ID: <>
References: <48pjlo$>

In <48pjlo$> yvan Bozzonetti <> 

>There is a simple way to upgrade the signal to noise level: Take data for a

>longer time. With a N-times longer duration you get a square root better 

>ratio. If you go from 100 seconds to 10^8 s, you have a 1000 times improvement.

	Of course.  I was refuting the original poster's assertion that
computer power increase (not scan time increase) can increase SNR.
>There is an even better solution: Use a high polarization gas, for example
>hyperpolarized He3 or Xe and get a millions times stronger signal.

	I attended a plenary lecture on this subject in Nice, France
this past summer.  It is indeed very interesting work.  However it's
not going to revolutionize SNR in MRI.

	Yes, laser excitation can be used to prepare He3 or Xe nuclei
to give a million times more signal than H1 nuclei (protons).  However
there are 1000 times more protons in blood than dissolved He3 or Xe,
so there is only a ~1000 times signal increase.  However this increase
is only available *for one excitation*.  Hyperpolarized nuclei do
not undergo T1 recovery like protons.  One 90 degree excitation will
destroy the hyperpolarization forever, ending the imaging experiment.
This means that hyperpolarized gas imaging must use very small flip
angles, which further reduces the SNR to about the same as ordinary
protons in tissue.

	Hyperpolarized gas allows lungs to be imaged in a new way,
but it's not going to substantially increase resolution of brain

---Brian Wowk           

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