What are the differences between various types of imaging technologies?
There are many different types of medical imaging technologies, but the five major types are x-ray (also known as general radiography), computed tomography (CT), magnetic resonance imaging (MRI), ultrasound (also known as sonography) and nuclear medicine. The type of imaging ordered by your physician depends upon many factors. Most important, of course, is the ability of the technology to visualize the anatomical part under study. For example, because MRI can demonstrate soft tissue better than x-rays can, it is usually the technology of choice to image the brain.
How is an x-ray image produced?
An x-ray image, or radiograph, is produced when a small amount of radiation passes through a body part and is recorded on film, video or computer to produce a black-and-white anatomical image. Areas that are difficult for x-rays to penetrate, such as bone, appear white on the x-ray film. Areas that the x-rays penetrate easily, such as the lungs or other areas filled with air, appear black. Soft tissue, vessels and organs appear as various shades of gray on an x-ray image, depending upon their composition and density.
Are x-rays safe?
The x-rays delivered to a patient during a typical diagnostic imaging exam are safe. Radiological technologists use the lowest possible dose of radiation to achieve a quality image. In addition, new techniques and equipment are constantly being developed to decrease the total amount of radiation received by the patient. For example, modern mammography equipment, operated by trained technologists, delivers 1/40th the amount of radiation used 20 years ago.
X-rays are a form of electromagnetic radiation. Sources of naturally occurring "background radiation" are the atmosphere, the Earth’s crust and cosmic rays. Annually, the average person is exposed to about 3 millisieverts (mSv) of background radiation from the environment. Those who live at high altitudes, where the atmosphere is thinner, are exposed to more. By comparison, a typical dental x-ray exposes a patient to approximately 0.06 mSv; a chest x-ray delivers 0.08 mSv; a mammogram delivers about 1.0 mSv; and an x-ray of the thoracic spine delivers 1.5 mSv.
Certain organs are "radiosensitive," which means they are more sensitive to radiation exposure than other parts of the body. Those organs, including the thyroid gland and the male and female reproductive organs, are shielded when they are in the path of the x-ray beam. Because a developing fetus also is radiosensitive, pregnant women should seek a physician’s advice before undergoing an x-ray examination.
Keep in mind that any small risk posed by x-ray examination is far outweighed by the value of the information gained about a patient’s condition.
How does a CT scan work?
Computed tomography (CT) also uses x-rays, but is able to depict anatomy at different levels within the body. This ability, known as cross-sectional imaging, is possible because the x-ray source rotates around the patient during a CT scan, encircling the patient and capturing anatomical detail from many angles. Each rotation of the x-ray beam produces a single cross-sectional "slice" of anatomy, like the slices in a loaf of bread. A computer then creates an image by stacking the individual image slices. Using this technology, physicians can view the inside of anatomic structures, a feat not possible with general radiography.
What is an MRI?
Magnetic resonance imaging (MRI) uses a large magnet, radiofrequencies and a computer to produce images. During an MRI scan, atoms in the patient’s body are exposed to a strong magnetic field. A radiofrequency pulse then is applied to the field, which knocks the atoms out of alignment. When the pulse is turned off, the atoms return to their original position. In the process, they give off signals that are measured by a computer. The computer processes these signals to create detailed images of human anatomy. MRI is effective in visualizing soft tissue, the brain, the joints, the musculoskeletal system and the vascular system.
Will an MRI test hurt?
No. Since MRI is "non-invasive," the exam is painless. However, your doctor may utilize a contrast agent to better visualize a part of your anatomy. If this is the case, you may receive an injection prior to or during the exam.
You will hear a loud knocking or buzzing sound at various intervals throughout your exam. Other than that, you won’t feel a thing.
Will I be claustrophobic in the MRI machine?
A typical exam lasts between 30 and 60 minutes. You should always allow extra time in case the exam lasts longer than expected. Most people have no reaction at all. However, if you have had claustrophobic reactions to enclosed spaces before, let the technologist know prior to the start of your exam as the staff may be able to assist you.
You will be in contact with a technologist at all times. Even when he or she is not in the MRI room, you will be able to talk to him or her by intercom. The technologist is always able to see you through a large patient viewing window. In some cases a friend or family member may stay in the scan room with you during your exam. Please consult your MRI facility for their policy on this matter.
Is there anything that will prevent me from getting an MRI?
Some patients with metal implants cannot be safely scanned in the MR environment. People with pacemakers; aneurysm clips, especially in the brain; and neurostimulators generally cannot be scanned. Anyone with surgical pins, shrapnel, plates or other types of metal implants should notify the technologist. You will be required to provide a health history when you arrive for your exam explaining any metallic implants you may have. A doctor will determine if a particular metal implant is approved to be in an MR environment.
What is ultrasound?
Ultrasound uses sound waves to obtain images of organs, vessels and tissues in the body. During an ultrasound examination, a transducer is placed in contact with the patient’s body. It emits high-frequency sound waves that pass through the body, sending back "echoes" as they bounce off organs, vessel walls and tissues. Special computer equipment is used then to convert these echoes into visual data. Ultrasound is ideal for imaging soft tissues, including the heart, blood vessels, uterus and bladder, and it can reveal internal motion such as heart beat and blood flow. It also is used in obstetrics to assess fetal well-being, determine fetal position, verify the diagnosis of multiple gestational sacs (twins, triplets, etc.), and rule out ectopic pregnancy. If the fetus is positioned correctly, the sex also can be determined.
What is nuclear medicine?
Nuclear medicine procedures use small amounts of radioactive materials, or radiopharmaceuticals, to create images of organs, tissue and bone. New technologies in nuclear medicine are also used for three-dimensional and whole-body imaging with no additional radiation exposure other than the initial injection.
Nuclear medicine is unique because it documents function as well as structure. For example, with nuclear medicine you can see how a kidney is functioning, not just what it looks like. Most other diagnostic imaging tests, in comparison, reveal only structure. Radiopharmaceuticals are substances that are attracted to specific organs, bones or tissues. They are introduced into the patient’s body by injection, swallowing or inhalation. The radiopharmaceutical travels through the body, continually producing emissions. A special type of camera transforms these emissions into images and then records the information on a computer screen or on film.
Nuclear medicine procedures are performed to assess the function of nearly every organ. Common nuclear medicine procedures include thyroid studies, brain scans, bone scans, lung scans for blood clots, cardiac stress tests to analyze heart function, and liver and gallbladder procedures to diagnose abnormal function or blockages.
What is radiation therapy?
Radiation therapy uses high-energy radiation to shrink tumors and kill cancer cells. X-rays, gamma rays, and charged particles are types of radiation used for cancer treatment. The radiation may be delivered by a machine outside the body (external-beam radiation therapy), or it may come from radioactive material placed in the body near cancer cells (internal radiation therapy, also called brachytherapy).
About half of all cancer patients receive some type of radiation therapy sometime during the course of their treatment.
Why do patients receive radiation therapy?
Radiation therapy is sometimes given with the hope that the treatment will cure a cancer, either by eliminating a tumour, preventing cancer recurrence, or both. In such cases, radiation therapy may be used alone or in combination with surgery, chemotherapy, or both.
Some examples of palliative radiation therapy are:
- Radiation given to the brain to shrink tumours formed from cancer cells that have spread to the brain from another part of the body (metastases).
- Radiation given to shrink a tumour that is pressing on the spine or growing within a bone, which can cause pain.
- Radiation given to shrink a tumour near the esophagus, which can interfere with a patient’s ability to eat and drink.
How does radiation therapy kill cancer cells?
Radiation therapy kills cancer cells by damaging their DNA — the molecules inside cells that carry genetic information. Radiation therapy can either damage DNA directly or create charged particles (free radicals) within the cells that can in turn damage the DNA.
Cancer cells with their DNA damaged beyond repair stop dividing or die. When the damaged cells die, they are broken down and eliminated by the body’s natural processes.
Radiation therapy can also damage normal cells, leading to side effects. Doctors take potential damage to normal cells into account when planning a course of radiation therapy. The amount of radiation that normal tissue can safely receive is known for all parts of the body. Doctors use this information to make sure radiation does not cause unnecessary tissue damage.
What is a contrast agent?
Bones reproduce well on x-rays and other medical images, but soft tissues of the body can be difficult to see. Contrast agents highlight specific organs or blood vessels, making them more visible on a diagnostic image. Like their name suggests, they provide contrast between various types of tissue. Some contrast agents are designed for the patient to drink, while others are injected, delivered through an intravenous method, or administered into a body cavity. The most common types of contrast used in general radiography have traditionally been air, iodine and barium. Non-iodinated contrast media is increasingly more commonly used.
Air encourages the passage of x-rays through a selected part of the body, while barium and iodine block the passage of x-rays. Special types of contrast agents also are used in magnetic resonance imaging and other imaging examinations.
Because the use of contrast agents carries a small risk of allergic reaction, you should let your physician or the radiological technologist know if you have any allergies.