CT and MRI Scans in Neurological Practice: A Quick Overview

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 identifed 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