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10/09/2003
No-spin Doesn't Work
A couple of years ago, I wrote about attending the celebration of
the 125th anniversary of the founding of the chemistry
department at the University of Pittsburgh, where I did my
graduate work in chemistry. A number of Pitt chemistry alumni
were being honored at the dinner that night, among them my
research professor, Ed Wallace. Another honoree was a fellow
who received his Ph.D. at Pitt a dozen years after I did. His
name was Paul Lauterbur. I hadn’t known of him and was
surprised to hear that he was being honored as one of the
inventors of magnetic resonance imaging, MRI. As I mentioned
a couple of weeks ago, an MRI exam confirmed the presence of a
tumor in my kidney that led to the surgery I discussed in last
week’s column.
A few days ago, it was announced that Lauterbur, now at the
University of Illinois at Urbana, and Sir Peter Mansfield, at the
University of Nottingham in Great Britain, had won the 2003
Nobel Prize for medicine for their discoveries related to MRI.
Lauterbur, while at the State University of New York at Stony
Brook, discovered the possibility that 2-dimensional pictures
could be obtained by utilizing a variable magnetic field, thus
laying the basis for the MRI. Mansfield came up with the
finding that signals from the body in an MRI exam could be
analyzed and transformed rapidly into an image. The first MRI
exam on a human being reportedly was performed by Drs.
Raymond Damadian, Larry Minkoff and Michael Goldsmith on
July 3, 1977. It took nearly five hours to produce one image.
Actually, MRI is based on a phenomenon known as nuclear
magnetic resonance, NMR, the discovery of which was
celebrated by the awarding of the 1952 Nobel Prize in physics to
Felix Bloch and Edward Purcell. Strictly speaking, instead of
MRI, we should be talking about NMRI. It’s likely the term
“nuclear” was dropped because of the public’s possible
reluctance to undergo a procedure of a “nuclear” nature.
However, an MRI exam is a benign imaging technique,
especially when compared to X-ray procedures. The difference
is chemistry. X-rays can interact with your body’s atoms’
electrons and electrons are the stuff of chemistry – fiddle with
electrons and you can create free radicals and such, which can be
bad actors in certain cases.
MRI, on the other hand, deals with your atoms’ nuclei and
there’s no chemistry going on there. Let’s take a simplified look
at what’s required for an MRI exam and what kind of nuclei are
required for MRI to work. The key requirement is that the
nucleus has to have a “spin”. If you have an elementary particle
such as an electron, proton or neutron, you can imagine the
particle to be spinning on its axis, like the earth spins on its axis.
In NMR, we’re talking about the nucleus of the atom so we only
have to consider protons and neutrons when it comes to talking
about spinning.
Now, it turns out that when two protons or two neutrons get
together, their spins will pair up and cancel each other. This
means that if your nucleus has an even number of both neutrons
and protons the spins are all paired up and the nucleus has no
spin. If you want to look into your body, we’re a carbon-based
life form so you might think carbon would be the element to look
at. However, most of the carbon in our body has a nucleus with
4 protons and 8 neutrons, both even numbers – no spin.
Fortunately, there’s another element in our body that’s all over
the place. That’s hydrogen, with a nucleus that is simply one
proton. With just a proton, the hydrogen nucleus has a spin.
So, into the MRI machine we go. The MRI machine has a big
magnet and in the core, where you are lying, the magnetic field is
aligned down the center of the core. In your body, the magnetic
field causes your hydrogen atoms’ protons to line up pointing
either to your head or to your feet. Half of the protons point to
the head, half to the feet. This isn’t good because that means the
spins cancel out! But wait, out of a million protons, there are
maybe less than a handful that don’t cancel; that is, there are a
very small number more than a half lined up in one direction.
Percentage wise, not many but since we have so many hydrogen
atoms there will be a sufficient number of protons to give a
signal strong enough to get good pictures.
To go into detail as to how the MRI functions and the signals are
analyzed is a daunting task. In my weakened condition, I’m just
not up to it. Let me just point out that the MRI machine doesn’t
just consist of that one big magnet. There are other so-called
“gradient” magnets that allow the local magnetic field to be
varied and also a device that generates radio waves. What these
magnetic and radio wave sources do is to fiddle with the spins of
the protons in such a way that the spins are pushed out of their
normal alignment in the main magnetic field. When the protons
return back to their normal state, radio waves are emitted that are
detected and processed by the computer to form the MRI images.
For more details on MRI the Web site howstuffworks.com has a
good article by Todd Gould, a Registered Technologist in
Radiography and MRI.
By controlling the local magnetic fields, the position of the body
under study can be changed so that slices of the body are
scanned. Different types of tissues affect the emitted signals
differently, which allows the visualization of abnormal tissues
such as my tumor to show up in the computer-generated image.
Not everyone is eligible for an MRI exam. For example, if you
have a pacemaker it would be adversely affected by the magnetic
field. The magnetic field could also disastrously move around
certain metallic implants such as metal staples used to repair
aneurysms. Even some dental implants are magnetic.
Fortunately, the stainless steel plate in my leg posed no problem.
And don’t try and sneak your wallet with you into the machine.
The magnetically encoded information on your credit cars will be
wiped out! If you’re extremely claustrophobic, the MRI may not
be for you. I personally found that keeping my arms extended
over in back of my head for the half hour or so to be the most
difficult part of the exam.
I hope that neither you nor I will have to have an MRI exam in
the future. However, if we do we should thank Drs. Lauterbur
and Mansfield for their seminal discoveries that have led to one
of medicine’s most valuable diagnostic tools.
Allen F. Bortrum
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