From: frankm@resonex.com (Frank Muennemann)
Subject: Re: MRI
Organization: Resonex Inc., Sunnyvale CA
Date: Fri, 10 Apr 1992 08:36:37 GMT

In article <1992Apr9.174752.5827@osc.edu> djh@xipe.osc.edu (David Heisterberg) writes:
>In article <167C47B80.M20614@mwvm.mitre.org> M20614@mwvm.mitre.org (Hugh Pritchard) writes:
>>Moreover, MRI scans use radio waves given off by the water molecules in the
>>patient's own body.
>
>Almost.  The energy levels available to hydrogen nuclei ...
>                    ...  What's typically measured then is the absorbtion,
>not emission of RF.

Dead wrong!  Mr. Pritchard has the correct idea: The patient's body
emits the signal from which the image is constructed.  The RF part of
the pulse sequences used in MR imaging consist of (optional)
preparation, (required) excitation and (optional) refocussing.  The
data acquistion occurs while the protons in the patient's body emit
small amounts of RF.  MR imaging is emphatically NOT an absorption 
measurement!

Mr.  Heisterberg's description of MRI, referring to spin states of the
proton in a magnetic field, is essentially quantum-mechanical.  Although
there are some phenomena assiciated with NMR and some kinds of MR
imaging which require an understanding of quantum mechanics, at least 90
percent of the physics involved can be understood in classical terms.
Classical descriptions are much easier for technical as well as
non-technical people to understand, and should therefore be preferred
whenever they correctly describe the behavior of the phenomena under
study.  Here's my stab at it:  (Flame away, those of you who aren't
satisfied with my explanation!)

The conceptually simplest type of pulse sequence is called a 'gradient
echo' sequence.  In this sequence, an RF pulse rotates the protons away
from their preferred 'aligned' orientation.  Gradient fields are applied
during the excitation to restrict the spatial volume which is excited.
These gradient fields are switched at relatively low frequencies, and it
is these which produce the 'knocking' noise you hear when the MRI is
acquiring data.  You can't hear the RF.  The protons emit tiny amounts
of RF as they return to their 'preferred' orientation, even after
the imager has stopped applying RF.  The gradient fields are spatially
non-uniform, and cause the proton spins to become dephased (that is,
they don't all spin in step), and their net signal is very small.

After the excitation, gradient fields with the opposite polarity are
applied to rephase the proton spins.  At the time that most of the
protons get back into step with each other, their signals add
constructively to give an 'echo' which is picked up by a very sensitive
RF receiver.  The echo is what is acquired, and from which the
image is reconstructed.

There's a lot more to MR imaging, but I think the above describes
the basic process of excitation and data acquisition.  Some common
misconceptions I'd like to dispel:

1) There is no nuclear transmutation, and no danger of genetic
   damage to the patient

2) The RF frequency is so low, and the RF antennae are so small, 
   that the fields are essentially "quasistatic," meaning that 
   the term "radiation" is probably inappropriate anyway

3) The main danger to the patient (especially in "high-field"
   scanners) is from tissue heating by the RF. (I hasten to add
   that Resonex builds "mid-field" machines which require 1/16th the
   RF power used by high-field systems like those from G.*. and
   S******.)

4) The machine doesn't "look" at a tiny piece of anatomy with 
   each pulse, and any sensation that something is probing through
   the patient's body is purely due to the patient's imagination

5) Not all MR imagers are built like long, skinny (and scary)
   tubes.  The Resonex scanner is about 5 feet long, and more than
   3 feet wide (on the INSIDE!).  It's also much quieter than most
   other brands

My opinions are my own, and I speak for myself, not Resonex...

Frank Muennemann, Ph.D.
Principal Scientist
Resonex, Inc.
Newsgroups: sci.med