CoMo Science

Medical imaging (Basic science review)

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Radiology is the use of various imaging techniques to understand what is going on in the body. There are many different imaging techniques that are used, including:

  1. X-ray
  2. CT scans
  3. MRI
  4. PET scans
  5. Ultrasound

X-ray

  • An x-ray is an image obtained by exposing a screen to x-rays that have passed through the body. Some of the x-rays are absorbed by the body. Denser regions of the body are more white and less dense regions are darker.
  • X rays are ionizing radiation that can cause cancer.
This is a normal chest x-ray. The bones (spine, clavicles and ribs) are very white because they are very dense. The water in the heart, abdomen and muscles makes those areas white as well, though less than bone. Air in the lungs and a little bit in the stomach is visible as darker areas. The darkest areas are the air outside the body (in the corners of the film) which does not block the x-rays at all.

CT scans

  • CT stands for computer-aided tomography. So, sometimes they are called CAT scans.
  • CT scans take a lot of pictures using x-rays from multiple angles. This exposes the patient to a lot of ionizing radiation1 which can cause cancer.
  • It also provides a very detailed picture of the insides of a patient.
This slice from a CT scan shows a patient’s heart and lungs. It also reveals a small cancer in the right lung (on the left side of the picture).

MRI

  • MRI stands for magnetic resonance imaging
  • MRI does not use ionizing radiation and so will not cause cancer
  • The process of obtaining an MRI is significantly slower than getting a CT scan
This is an MRI of a knee. Notice that the bones do not appear white. This is because the gray scale of the MRI is not based on the density of the tissue the way a CT scan is.

PET scans

  • PET stands for positron emission tomography
  • Positrons are antimatter particles that have the same mass as an electron but opposite (positive) charge
  • When a particle and an antiparticle collide, they destroy each other and turn into gamma rays2.
  • To prepare for a PET scan, a patient is injected with a molecule that has a radioactive atom in it.
  • The molecule is absorbed by cells and are concentrated in cells that require a lot of energy, such as cancer cells, but also the brain and heart. It is also excreted in the urine and so the kidneys and bladder normally are highlighted.
  • When the radioactive atom decays, it emits a positron. The positron soon hits an electron. The positron and electron destroy each other and produce 2 gamma rays. The gamma rays travel in opposite directions.
  • The PET scanner detects the gamma rays and can calculate where they came from in the body by time difference of when they are detected.
Because all body tissues absorb some of the radioactive material, a ghostly image is produced which allows for localization of the cancer, which are the dark spots. Note that the brain, kidneys and bladder are normally highlighted on a PET scan.

Ultrasound

  • An ultrasound image is produced by recording ultrasonic sound waves that bounce off the tissues in the body.
  • Ultrasound is particularly good at imaging certain organs, including the gall bladder and ovaries.
  • Ultrasound is also very good for imaging a fetus because it does not use ionizing radiation.
This is an ultrasound image of a gallbladder. The ultrasound probe is located at the top of the image. The gallbladder is the black hollow area in the middle of the image. The arrow is pointing to a gallstone which is hard and reflects the sound waves. Because of this reflection, the top of the gallstone is bright white and below it is dark because of the “shadow” of the gallstone (because sound is reflected off the top of the gallstone instead of passing through the gallstone, there is no sound for the tissue below to reflect back to the probe).

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