Medicine Information

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 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

Gary Cordingley, MD, PhD, is a clinical neurologist, teacher and researcher who works in Athens, Ohio. For more health-related articles see his website at: http://www.cordingleyneurology.com


MORE RESOURCES:
This RSS feed URL is deprecated, please update. New URLs can be found in the footers at https://news.google.com/news


Salt Lake Tribune

Commentary: The Tribune was wrong. Medicine often involves a risk to the patient.
Salt Lake Tribune
(Courtesy of the Utah Department of Health) A health department initiative called Talk to Your Pharmacist placed stickers on bottles to prompt prescribers to talk to their pharmacist about the risk of using opioids. By Lynn Webster | For The Tribune ...



The Guardian

Adam Kay: 'If I had kids I would put them off studying medicine'
The Guardian
Kay had enjoyed a longstanding interest in comedy, performing in medical student revues, and later in corporate gigs for pharmaceutical companies. He'd also been on Radio 4 and performed at the Edinburgh festival, so comedy was the obvious way to go ...



Florida Flambeau

FSU College of Medicine hosts World AIDS Day vigil
Florida Flambeau
From 2008 to 2010, there were about 32 new HIV cases and 19 new AIDS cases in Leon County. During this time period, Leon County reported 1,392 cases of gonorrhea, chlamydia and syphilis. Gonorrhea and syphilis rates are higher in this county than ...

and more »


The Hindu

'Exciting careers beyond engineering and medicine'
The Hindu
In a State where a huge chunk of student population go towards engineering and medicine, choosing a course other than the two requires a certain aptitude. It's the aptitude that matters when choosing a career, experts from various fields advised ...



KFSN-TV

New Treatment for Neuropathy: Medicine's Next Big Thing?
KFSN-TV
Thirty percent of all Americans will be affected by peripheral neuropathy, a condition that impacts nerves leading to the arms and legs. (KFSN). KFSN. By Margot Kim. Saturday, December 09, 2017 11:48PM. Thirty percent of all Americans will be affected ...



Rethink Healthcare: Government hospitals must provide quality, affordable medicine
Economic Times (blog)
Delhi government's decision to cancel the licence of Max hospital, Shalimar Bagh, for negligence resulting in the death of a premature newborn – the baby was declared dead by the hospital even though he was alive at the time he was given to his parents ...

and more »


New York Times

What Doctors Should Ignore
New York Times
Those variants are enriched in people of African ancestry. Girish N. Nadkarni, a kidney specialist at Icahn School of Medicine at Mount Sinai in New York City, explained to me that scientists think this may be because those variants protect against the ...



Daily Mail

HEALTH: Nature's medicine for sore joints
Daily Mail
When financier John Carey, then 35, woke up one night in 2002 with a searing pain in the ball of his left foot, the last thing on his mind was gout. 'I'd been training hard the day before – I practise judo and ran marathons – and thought I must have ...



HuffPost

Medicine And Why Net Neutrality Matters
HuffPost
Recently, the library at Einstein, where I work as a medical librarian, experienced a momentary internet outage. In that instant, everything seemed to stop in its tracks. Medical students preparing to take their STEP 1 Board exams couldn't finish their ...

and more »


EurekAlert (press release)

Landmark CAR-T cancer study published in the New England Journal of Medicine
EurekAlert (press release)
IMAGE: This is how CAR-T cell therapy harnesses a patient's immune system to fight cancer. view more. Credit: Loyola Medicine. MAYWOOD, IL - Loyola University Medical Center is the only Chicago center that participated in the pivotal clinical trial of ...
Loyola Medicine Oncologist Patrick Stiff, MD, Co-Author of Landmark @NEJM CAR-T Study, Offers Measured ...Newswise (press release)

all 5 news articles »

Google News

home | site map | Dr. Thad Thomas
© 2006