The ABCs of CTs and MRIs
in Neurological Practice


Gary E Cordingley, MD, PhD
Have you ever wondered how computed tomographic (CT) and magnetic
resonance imaging (MRI) scans are made? Admittedly, some of it occurs
by magic, but the truth can be told about the rest of the process.
Before computed tomographic (CT) scans became available in the 1970s, there was no good method for
imaging the brain. The available methods and technologies struck around the target without quite hitting the
bull's-eye.

We had skull X-rays which imaged the bony brain-case, but not the brain itself. We had arteriograms which
imaged the insides of blood-vessels supplying the brain. We had nuclear brain scans which imaged chunks of
brain that were recently damaged. We had a particularly nasty test called a pneumoencephalogram (PEG) in
which the doctor squirted air through a spinal tap needle and encouraged it to bubble around and inside the
brain by turning the patient every which-a-way -- including upside-down -- while X-ray pictures showed where
the air could and couldn't go. Finally, the most accurate method was not a physical picture at all, but a
mind's-eye picture within the brain of an examining neurologist. Yet diagnoses still got made and patients did
get treated.

CT scans revolutionized the practice of neurology. It's not that the other methods disappeared (well, yes, PEGs
thankfully did disappear) but that CT scans vastly improved the accuracy of diagnosis and treatment. Even
when CT scans didn't show the disease itself (e.g. multiple sclerosis or a fresh stroke) they assisted the
diagnostic process by proving the absence of a brain tumor, abscess or hemorrhage that were also on the list
of diagnostic possibilities.

CT scans did (and still do) this by sending x-ray beams through the head at various angles and collecting the
x-ray beams on the opposite side that were not absorbed by the head.
Then magic occurs.  A series of images
appear on a computer monitor or on x-ray film as if the head had been run through a giant salami-cutter and
the slices were laid out flat and in sequence.

On CT pictures the different parts of the head are displayed in various shades of gray according to how much
they absorb X-rays. The skull-bone absorbs X-rays the most and shows as the whitest component. At the other
end of the gray-scale, the watery spaces in and around the brain absorb X-rays the least and show as the
blackest components. The brain itself is somewhere in between, showing up in the mid-gray range. Abnormal
components, like brain tumors and blood-collections, are identified not just by appearing in their own shades of
gray, but also by their locations and shapes. Creating a second set of slices after the patient receives an
infusion of intravenous dye provides an additional dimension to imaging not unlike that provided by the older,
nuclear scans.

Then in the 1980s magnetic resonance imaging (MRI) scans burst upon the scene and astonished the medical
community by not just imaging the brain itself, but by doing so in a brand-new way. Instead of imaging the
extent to which the head's different components absorb X-rays, MRIs instead focus on water-molecules. To be
more precise, MRIs image the rate at which spinning hydrogen-atoms of water molecules within different parts
of the brain either line-up or fall out or alignment with a strong magnetic field. These differing rates of
magnetization or de-magnetization are fed into a computer.
Then magic occurs yet again. A series of slice-like
images is created and displayed on a computer-screen or X-ray-type film in shades of gray. Abnormal
structures, like brain-tumors or the plaques of multiple sclerosis, are displayed in their own shades of gray and
are also recognizable by their shapes and locations. Obtaining another set of images after intravenous
administration of gadolinium -- the MRI equivalent of X-ray dye -- also adds diagnostic information.

One of the virtues of MRI pictures is that they are based on physical principles totally different from those
responsible for creating CT pictures. Thus, the MRI is good (or not so good) at showing different things than
CTs. Another virtue is that MRIs can slice and dice the brain at different angles, while CTs slices are limited to
just the horizontal plane. Yet another virtue of MRIs is that they are much better than CTs at imaging most
diseases of the spine. Finally, MRIs are much more flexible than CTs: new bells, whistles and capabilities are
being added all the time.

To the patient, the experiences of having a CT and of having an MRI greatly resemble each other. In both
cases the patient lies horizontally on a flat table that moves into and out of an opening in the scanner that
resembles a giant doughnut-hole. The doughnut-hole in the MRI machine is narrower, so claustrophobic
patients need to inform their doctors if this might be a problem. The MRI machine is also noisier: a loud sound
is created each time its radio-frequency coils turn on and off. For each kind of scan the technologist might stick
a needle in the patient's vein to administer contrast-material.

Both tests are otherwise painless and are very safe with certain exceptions. Pregnant women who need a scan
might have to do without one for fear of exposing the fetus to excessive X-rays in the case of the CT scan or to
an excessive magnetic field in the case of the MRI. If push comes to shove, the woman is more likely to receive
a CT scan because her abdomen can be draped with a lead shield that blocks passage of most x-rays, while
there is no good method for blocking the magnetic field produced by an MRI machine.

A circumstance in which MRIs are simply not done is when the patient has a cardiac pacemaker. This is
because the MRI machine's magnet might disrupt the pacemaker and stop the heart. No image is so necessary
and valuable that this risk would be worth taking. Another circumstance in which an MRI is avoided is when the
patient is critically ill. An unstable patient can be adequately monitored and supported while receiving a CT
scan, but not while receiving an MRI.

Depending on the nature of the patient's problem, the doctor will usually order just one of the two types of
scans and not the other, but in selected cases the magic of both kinds of scan might be needed.


(C) 2005 by Gary Cordingley