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
Latest News
BioImagene CEO, Dr. Ajit Singh, Speaks On Enabling Personalized ... MarketWatch - Digital pathology, a key enabler for personalized medicine, will aid clinicians in managing and interpreting data from a whole new set of diagnostic tests ... Aperio Introduces SecondSlide(TM) Digital Slide Sharing Network |
 eFluxMedia | Medicine for the Job Market New York Times, United States - By JONATHAN GRUBER A CENTRAL feature of Barack Obama’s presidential campaign was an aggressive plan to expand health insurance coverage by subsidizing ... New Report Shows Slower Premium Growth, But Increasing Pressure to ... Health Corps Research database from the US Library of Medicine |
 eFluxMedia | American College of Surgeons Comments on Institute of Medicine ... MarketWatch - CHICAGO, Dec 03, 2008 /PRNewswire-USNewswire via COMTEX/ -- The American College of Surgeons (ACS) today recognized the Institute of Medicine (IOM) for its ... BMC Internal Medicine Residency Program Achieves National ... Report urges more sleep for medical residents Medical Residents Must Sleep After 16 Hours, Experts Urge |
Research and Markets: 2008 Integrative Medicine and Dietary ... MarketWatch - In this 32-page issue, we offer a breakdown of 2007 US Complementary and Alternative Medicine sales data and provides a detailed analysis of new integrative ... |
 Afrik.com | 34 Nigerian Children Dead From Tainted Medicine Voice of America - By VOA News Nigerian health officials say they are flying in doses of an antidote for a poisoned teething medicine that has now killed 34 infants. ... 'Toxic' syrup kills Nigerian babies Teething mixture kills more Nigerian babies Nigeria infant drug toll hits 34, antidote flown in |
Research database from the US Library of Medicine The Plain Dealer - cleveland.com, OH - Search PubMed, Clinicaltrials.gov, the CDC and a dozen other medical sites - all at once. AP MEDICAL VIDEO by Michael O'Malley and Joan Mazzolini/Plain ... |
A New Index of Preventive Medicine Companies Motley Fool - The accompanying table [click to enlarge] presents statistics and the top 15 rated companies in the ETF Innovators [ETFI] Preventive Medicine Index. ... |
American Academy of Dermatology Issues Position Statement on Vitamin D MarketWatch - The currently recommended adequate intake levels established by the Institute of Medicine may be revised upward due to evolving research on the increasing ... |
NIGERIA: Jos displaced grapple with food, water, medicine shortages IRINnews.org, NY - JOS, 4 December 2008 (IRIN) - Water, medicine and food supplies are running low for an estimated 10000 people displaced by violence in Jos, northern Nigeria ... |
 TheMedGuru | "Rogue" stem cell clinics exploit hope: report Reuters - "The direct-to-consumer portrayal of stem cell medicine is optimistic and unsupported by published evidence," Timothy Caulfield of the University of Alberta ... Scientists warn of “rogue” online stem cell clinics Internet stem-cell clinics 'exploiting' MS and Parkinson's patients The International Society for Stem Cell Research Releases New ... |
Resources
-
Sorry Currently Unavailable