An MRI is a test that uses radio wave energy and a magnetic field to create pictures of the inside of the body without any surgery or invasive procedure. It can be used to diagnose many injuries and abnormalities in organs and tissues, such as tumors and cysts, joint injuries, heart conditions, abdominal injuries and disease and abnormalities of the spinal cord and brain.
The MRI scanner itself is a large donut-like machine with a moveable bed inside the hole. There are two rounded magnets that make up the donut. Because MRIs use magnets, they do not have the same potential for harmful radiation exposure that CT scans and X-rays have.
How Does an MRI Work?
The MRI works because the human body is made up mostly of water molecules. The protons within the molecules carry a positive charge and are responsive to magnets. When the first magnet in the MRI scanner is turned on, all of the protons in the body either head towards or away from it. As the second magnet is turned on and off, the protons move back and forth allowing the computer to make an image of your body. You and I can’t feel the tiny protons in our bodies moving, but we can hear a loud knocking sound as the second magnet turns on and off. The technician performing the MRI usually offers to play music to offset the knocking sound.
Types of MRIs
There are different magnets used in MRI scanners, varying from 0.5T to 3T. The higher the number the stronger the magnetic field and the more detailed the results of the MRI. There are also different types to accommodate people who are uncomfortable with confined spaces, such as open MRIs as opposed to wide bore.
- Closed bore MRIs are the original MRIs. The diameter of the hole averages at about 60 centimeters or 2 feet - which is a really small space for the human body to fit into. However, the strength of the magnetic field makes for the most accurate image.
- Open MRIs are much friendlier to people who dislike tight spaces because the sides are wide open. It is not your traditional magnetic donut around the body MRI. Open MRIs allow larger people and those with fears of small spaces to have MRIs but the downside comes with the quality of the images produced. Open MRIs typically only get as strong as 1T. A 1T MRI works for many large sites, but for something with as many tiny and intricate parts as the brain, a stronger magnetic field should be used.
- Wide bore MRIs are the middle ground between open and closed MRIs. They have the standard cylindrical magnet around the body but there is more space within the hole. The average diameter of the opening is 70 centimeters. It may not seem like much, but it allows more people to fit into MRIs with stronger magnetic fields.
In addition to the changes in machines, the way the MRI is administered can vary. MRIs may be given with or without contrast. Contrast liquid is sometimes injected intravenously to enhance the appearance of certain types of body tissue.
SPECT (Single Photon Emission Computer Tomography) Scan
A SPECT scan is a nuclear imaging test that uses radioactive fluid called a tracer and a camera to create 3D images. SPECT scans are generally more sensitive than CT scans or MRIs of the brain, but as sensitive as a PET scan. SPECT scanners are built more like the closed MRI machines with the cylindrical tunnel and tight small space but have two rectangular gamma cameras attached near the opening of the hole.
You are injected intravenously with the tracer 10-20 minutes before the SPECT scan is performed. This allows time for the tracer to reach the brain. The tracer emits gamma rays as it passes through your body, allowing doctors to see how blood flows through your body and how the organs in your body function. The radiation in your body is generally less than that of an X-ray or CT scan.
It functions similarly to a PET scan, but differently in that the tracer stays in your blood stream so it only shows places where the blood flows. SPECT scans are usually cheaper than PET scans too.
SPECT scans can be particularly useful in diagnoses and treatment of brain diseases because of the high resolution. They can tell whether you are vulnerable to Alzheimer’s up to 9 years before symptoms appear. They help doctors monitor whether or not treatment is helping specific areas of the brain. Most importantly, SPECT scans help determine the underlying reasons for problems so you and your family can better understand your situation.
CT (Computerized Tomography) or CAT (Computerized Axial Tomography Scan
A CT scan is a series of X-ray images taken from different angles and combined to form images more detailed than a traditional X-ray. They look a lot like MRI scanners, a donut with a bed in the middle. While you lay on the table, the X-ray tube rotates around you taking images.
CT scans can be useful for virtually any part of the body, including the brain.
They are commonly used to detect internal injuries, tumors, masses, blood clots, fractures, infections; prepare for operations, and monitor the effectiveness of certain treatments.
There is a risk of radiation exposure associated with CT scans but many doctors agree the small risk is outweighed by the benefits of the information obtained.
Like MRIs, sometimes doctors recommend using a contrast liquid to highlight the circulatory system, intestines or other area. The contrast may be given orally or by injection.
PET (Positron Emission Tomography) Scans
PET scans are not typically used for trauma or injury to the brain. They are most commonly used to detect and monitor cancer during treatment.
A PET scan is similar to a SPECT scan in that they both use radioactive tracers to highlight certain parts of the body. The injection used for PET scans, however, is absorbed by the tissues in the body and allows doctors to look for diseased areas.
Doctors often chose a glucose based tracer when monitoring cancer because cancerous cells absorb glucose faster than healthy cells but there are many different radioactive materials that can be used.