Understanding Medical Imaging

What is radiation?

Radiation is energy that travels as invisible waves or particles. We receive radiation from our environment. Food, water, light, and air have small amounts of naturally occurring radiation and we are exposed to radiation that comes out of the ground and that comes from the air around us, and that comes from the solar system. There are many types of radiation, including radio waves, magnetic fields, microwaves, and cell phones -- all of these types have low energy. Ultrasound and MRI use radiowaves to create medical images. Ionizing radiation has higher energy. It used in x-rays, PET, and CT scans. The higher the energy, the more potential to harm tissues and organs.

Every year each person in the world receives a dose of about 2.4 mSv from natural sources and 0.6 mSv from medical imaging for diagnoses.

Medical imaging tests (such as x-ray and CT scans) that use shorter wave (ionizing) radiation have the potential to cause harm.

What is medical radiation?

Medical imaging takes pictures of internal parts of the body. The image helps your doctor make a diagnosis and guide your treatment. Medical imaging gets information without using invasive methods like surgery or biopsy using either ionizing, or non-ionizing radiation. Below you will see examples of different types of equipment and the types of images they can produce.

Nuclear medicine: As part of a nuclear medicine procedure, a small amount of radioactive material, or tracer, is placed in the body. This may be swallowed, inhaled, or most commonly, injected into your vein. You would then lie down under a camera that would detect the gamma rays emitted by the radioactive material inside your body. This is how we use high-energy ionizing radiation to create images. Usually, nuclear medicine tests provide informatino about how an organ is working (functioning) or provides information about biologic activity.

X-ray (ionizing): This method sends radiation through a section of the body to a film or detecting screen.

Dense body parts such as bone block or deflect the radiation from reaching the screen. They appear lighter than less dense parts such as muscle or air-filled lungs.

CT (computed tomography, also called CAT, for computer-assisted or computerized axial tomography) (ionizing): This is an advanced x-ray method. CT is a series of x-rays, or a single, long x-ray. The patient lies on a platform that moves through a donut-shaped scanner as x-rays are taken.

All the x-ray information is collected and processed by computer software. The result is a three-dimensional, cross-sectional image. Colors show tissues with different densities.

PET (positron emission tomography) (ionizing): This is a specific type of nuclear medicine test that tells about how well part of your body is functioning by observing metabolic processes. Very often PET scanning is combined with CT or MRI scanning to get even  more detailed information about where the radioactive material goes in the body. Typically, PET is used to assess if there are cancer cells in the body (cancer metastasis), or to evaluate the function of the heart, or to evaluate the brain.

Non-ionizing radiation methods: These methods use lower-energy radiation than nuclear medicine techniques and carry less potential for harm.

Ultrasonography (non-ionizing): In this method, sound waves sent from a probe bounce off tissues and back to the probe. Different types of tissues reflect the sound waves differently. The ultrasound machine uses this information to create a real-time ultrasound image. One of the advantages of ultrasound is that a technologist can assess the patient (for pain from the location of symptoms) at the same time that imaging is done.

Ultrasound images are best known for checking embryonic and fetal progress during pregnancy. They are also used to look for problems of joints and organs including the heart.


MRI (magnetic resonance imaging) (ionizing): This method uses magnetic fields and radiowaves to produce images as patient lies in an MRI machine. Different tissues respond uniquely as the magnetic and radiowave conditions change. Computer software uses these differences to make a cross-sectional image of a body part.

Why might my doctor suggest medical imaging?

Medical imaging can provide information for:

  • Diagnosis of an injury such as a broken bone or illness such as pneumonia. 
  • Screening, such as for breast, lung, or colorectal cancer.
  • Checking how well a treatment, for example chemotherapy, is working.

Why should I ask questions before getting medical imaging?

Deciding about any medical procedure means thinking about possible benefits and harms. The risk of any individual scan is very low. However, not all medical imaging is necessary. We know this because some hospitals and clinics use imaging much more often than others for people with the same conditions without getting better results for their patients. At a patient-centered health care system, someone will explain to you why a procedure is necessary and what you will learn from the results. If you don’t get an explanation, ask a few simple questions.