Imaging Tests—What’s the Difference?

IMAGING TESTS—WHAT’S THE DIFFERENCE?

Some weeks I have a topic I can’t wait to write about, such as Find a Vaccine—End the Pandemic back on June 6th.  Other weeks, I have no idea what I’m going to discuss until something pops up. This is one of those weeks. I just received my July 2020 issue of the Mayo Clinic Health Letter and the cover article compared the different ways health professionals can view the inside of your body with imaging tests. I didn’t really understand the differences and sat right down and read the article. Now, I’d like to share excerpts from it with you: Picture this—X rays, MRI, CT scans and PET scans

 

 

 

 

 

 

Imaging tests are common, but having your body scanned in a tube can seem more like science fiction than medical science. Read on to learn more about the ways health professionals view the inside of your body, including with X-rays, computerized tomography (CT) scans, magnetic resonance imaging (MRI) and positron emission tomography (PET) scans.

The big picture

Various imaging tests work in different ways, and certain tests are better suited to detect different types of illnesses or conditions. However, they have many commonalities:

• Contrast agents — such as gadolinium, iodine or barium — may be administered via injection, drink or enema to highlight certain areas.
• You’re instructed not to move and sometimes to hold your breath so that the image doesn’t blur.
• MRI, PET scans and CT scans are performed so that cross-sectional images of your body, or “slices” — like a piece of bread — can be viewed from multiple angles and even compiled into a 3D image.
• X-rays, CT scans and PET scans expose you to variable but still relatively low amounts of radiation

X-ray

An X-ray is a quick, common and comparatively inexpensive test that is particularly good at providing images of:

• Skeleton issues, including fractures, joint dislocations, arthritis, dental issues and bone tumors
• Problems in the chest, including pneumonia, tuberculosis, emphysema, lung cancer or an enlarged heart
• Abdominal problems, including digestive system issues and swallowed items (commonly for children)
• Breast cancer — in particular with mammogram, a special type of X-ray used with compression of the breast tissue to evaluate for growths and cancer

How X-rays work

An X-ray machine produces beams of ionizing radiation that pass through your body and onto an X-ray detector. An image is formed because X-rays do not pass through all parts of your body in the same way, and some X-rays are absorbed. Dense materials such as bone absorb many of the X-rays, and therefore show up as white on the image. Fat and muscle are less dense and look gray. The air in your lungs shows up as black.

Getting an X-ray

During an X-ray, a technologist situates your body. This may involve standing or lying on a special table in one or more positions with props such as pillows to help you stay in place.

CT scan

A CT scanner takes many X-rays that a computer assembles into a more complex image than a single X-ray could provide. CT scans may be used to:

• Rapidly inspect people who may have internal injuries from car accidents or other types of trauma
• Plan and direct procedures such as surgery, biopsies and radiation therapy
• Examine the head, heart, chest and gastrointestinal system to evaluate problems such as strokes, blood clots, heart disease, aneurysms, appendicitis, diverticulitis, abscesses and tumors

How CT scans work

The circular machine has an X-ray producer on one side and X-ray detectors on the opposite side. Rotations of the machine create cross-sectional image “slices.” Today, CT scanners can forgo separate “slices” and take X-rays without pausing, resulting in a spiral of images.

Though still low, the amount of radiation is greater than with a plain X-ray because the CT scan is using more X-rays to collect more images.

Getting a CT scan

CT scanners are shaped like a large doughnut standing on its side. You will be positioned on a table that then moves you through the opening. The machine may generate buzzing and whirring noises. You can communicate with the technologist — who is in a separate room — via an intercom.

MRI

MRI is used to capture internal images without using ionizing radiation. Instead, it uses a magnetic field and radio waves. MRI is typically not as quick as a CT scan, but can provide better images of soft tissues such as ligaments and tendons. MRI is commonly used to examine:

• Brain and spinal cord issues, including brain aneurysms, multiple sclerosis, spinal cord tumors, degenerative spine disease, strokes and brain injury
• Heart and blood vessel problems, including aneurysms, blockages in the blood vessels and strokes
• Bone and joint problems, including sports injuries, tendon tears, cartilage damage, torn ligaments, disk problems and bone infections

How MRI works

When you lie inside an MRI machine, the machine’s magnetic field — at many thousands of times the power of the earth’s magnetic field — temporarily realigns positively charged particles known as protons in your body. Radio waves throw off the alignment of these particles, and the machine measures the energy given off by these particles when they fall back into place. This energy varies for different tissues, allowing the MRI machine to create detailed pictures of tissues within the body.

Getting an MRI

An MRI machine also looks like a doughnut standing on its side, but with a longer tube on the inside than with a CT scanner. You will be positioned on a table that then moves you into the tube. People with a fear of enclosed spaces might be given a drug to make them feel less anxious and somewhat sleepy.

Because the MRI machine produces repetitive noises such as tapping and thumping, you might be given earplugs or be offered the option to listen to music to help block the noises. As with a CT scan, a technologist oversees and directs you from another room.

Because MRI machines use powerful magnets, people with metallic devices such as cochlear implants, metal plates or screws, joint replacements, or certain pacemakers might not be able to have an MRI. Ask your doctor or the technologist if this applies to you.

PET scan

PET scans — which are most often combined with CT or MRI — not only produce images of organs and tissues, but also provide information on whether they are working properly by examining activities such as blood flow or sugar metabolism. PET scans may be used to examine:

• Cancer, including finding and tracking cancer and assessing response to cancer treatment
• Heart disease, such as detecting coronary artery disease and revealing areas of decreased blood flow within the heart
• Brain disorders, including assessing dementia and seizures

How PET scans work

A PET scan uses a radioactive drug (tracer) that is usually injected intravenously. Although a PET scan uses radiation, it’s very different from an X-ray or CT scan. During an X-ray or CT, a machine directs radiation through your body to create an image. In a PET scan, the radiation source — the tracer — is within your body and the radiation travels outward to be detected. Though still low, the amount of radiation is greater than with a plain X-ray.

Once it’s injected, the tracer gathers in areas of your body with increased chemical activity, which may correspond to areas of disease. On a PET scan, these areas show up as bright spots. For example, cancer cells show up as bright spots on PET scans because they have a higher metabolism than do normal cells.

Getting a PET scan

The PET scanner is another machine that looks like a giant doughnut standing on its side. PET scanners are often combined with either a CT scanner or MRI machine.

First, you will be given a tracer. Then you’ll wait about 30 minutes or more for the tracer to be absorbed by your body. You will then be positioned on a table that moves you through the opening.

Combining a PET scan with MRI or a CT scan can help make the images easier to interpret.

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Wow, that’s a lot of information! But sooner or later you’ll probably need one or more of these imaging tests. And now you—and I—know what we’re in for.