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


KTVZ

Four Jefferson Co. youth OD on cold medicine, hospitalized
KTVZ
The Jefferson County District Attorney's Office and Juvenile Department and Madras Police warned parents Tuesday about a spike in juvenile abuse of over-the-counter cold medicines containing dextromethorphan -- which put four youth in the hospital with ...



Forbes

Medicine and Dentistry: Undoing an Illogical 150-Year Divide
Forbes
It's no secret that medicine and dentistry have evolved an unnatural professional separation that is helpful to no one and often causes unnecessary suffering on the part of patients. But we're in a moment—driven by our greater understanding of the ...



Mylan recalls batches of blood pressure medicine in US
Reuters
(Reuters) - Mylan NV said on Tuesday it would recall certain batches of blood pressure medicine valsartan in the United States after they were found to contain a probable cancer-causing impurity, the latest recall amid heightened safety concerns globally.

and more »


Healthcare IT News

Center for Connected Medicine polls top health systems about 2019 priorities
Healthcare IT News
The Center for Connected Medicine polled IT executives across 38 health systems for its 2019 Top of Mind survey. Cybersecurity continues to be the biggest concern across the industry, with telehealth and interoperability not far behind. Those topics ...

and more »


PharmaTimes

Mirati joins CR UK's Stratified Medicine Programme
PharmaTimes
Mirati Therapeutics has joined Cancer Research UK's Stratified Medicine Programme, bringing new investigational treatment options to eligible patients with advanced lung cancer. The partnership will make Mirati's experimental drug, sitravatinib ...



AAFP News (blog)

Bustling Clerkship Shows Family Medicine Is Never Boring
AAFP News (blog)
Not only was family medicine the first specialty I was ever interested in, it was my first third-year clerkship in medical school, too. Although I started medical school thinking family medicine was the specialty for me, it wasn't until my clinical ...



Elko Daily Free Press

Huynh joins UNR Med Elko Family Medicine Residency Program
Elko Daily Free Press
The Elko Family Medicine Residency Program aims to increase the primary care physician workforce in Elko and the surrounding areas, through the training of resident physicians under the supervision of experienced UNR Med faculty physicians.



OncLive

Dr. Borgen on Precision Medicine in Breast Cancer
OncLive
The TAILORx trial, which was discussed at the 2018 ASCO Annual Meeting, is a perfect example of precision medicine making its way into this space, according to Borgen. Data from this trial suggested that by applying the precision medicine concept in ...



News3LV

Pulmonary Medicine
News3LV
Pulmonary Medicine. OptumCare Lung and Allergy Care. Chronic obstructive pulmonary disease, or “COPD” is a chronic inflammatory lung disease that causes obstructed airflow from the lungs. Symptoms include breathing difficulty, cough, mucus production ...

and more »


Flathead Beacon

Zinke to Appeal Ruling in Badger-Two Medicine
Flathead Beacon
Attorneys representing the U.S. Department of the Interior, tribal and environmental groups Tuesday filed a notice of appeal challenging a federal judge's decision to reinstate the last remaining oil and gas leases on the Badger-Two Medicine, an area ...


Google News

home | site map | Dr. Thad Thomas
© 2006