The continuously improving quality of radiological procedures provides better and better images of structures and functions within the body. However, you may be unclear why your healthcare provider has chosen a specific radiological test. So that you’re capable of having informed conversations about your healthcare provider’s decision, let’s get a basic understanding of imaging techniques. This article will discuss various radiological procedures, help you understand the results, and address radiation exposure concerns.
The numerous types of radiological procedures provide information and images of the body's interior used to diagnose a variety of medical issues.
The type of radiological procedure is based on the type of medical information needed. When the diagnosis may not require a high level of detail, an exam requiring a lower level of radiation exposure can be sufficient.
To obtain images X-rays, CT, PET, and radionuclide scans use ionizing radiation, whereas MRIs and ultrasounds use safe alternative sources of energy.
Ionizing radiation traveling through the body can disrupt DNA on a molecular level and introduces a small associated cancer risk.
Radiologic imaging technologies and radiation
In 1885, the discovery that x-rays could be used to visualize the body’s interior proved momentous for the medical community. Nonetheless, there were limitations to the types of information available. These shortcomings gave rise to the development of other radiologic imaging technologies that can produce highly-detailed views.
The basic requirement for radiological imaging is the generation of energy that enters and passes through structures in the body. Ionizing radiation comes from electromagnetic waves (EMW) that contain high amounts of energy. The greater the frequency (waves per unit time), the more energy they contain.
Many, but not all, radiological procedures to produce images use ionizing radiation.
X-rays have 10,000 times greater frequencies than the regular light we see and thus contain considerably more energy that is sufficient to penetrate the body. Different body tissue types absorb varying amounts of energy and the remaining energy that penetrates and exits the body is what creates an x-ray image (Figure 1).
The energy contained in the x-rays also has the capacity to cause injury to cell contents and even DNA.
Plain x-rays diagnose such conditions as bone fractures, pneumonia, kidney stones, and breast cancer. However, some detected abnormalities will need additional imaging.
Computerized Axial Tomography (CT scans)
CT scans also use x-rays but they create more detailed internal images.
For a CT scan, the patient must lie still on a narrow table within the doughnut shaped CT scanner (Figure 2).
An x-ray machine rapidly revolves around the body while taking multiple pictures in sequential “slices” from multiple angles. A computer processes the information in these “slices” and assembles the images in a manner like stacking coins. The procedure normally only requires minutes to complete.
CT scans overcome many shortfalls of standard x-rays because they can provide more detail, such as distinguishing overlapping tissue or organs (Figure 3).
Medical professionals also use CT scans to identify organ abnormalities and traumas, bone fractures, tumors, other abnormalities throughout the body, including the brain, and heart disease.
Radionuclide scans use radiation similar to x-rays. The image scans reflect organ and tissue function. However, they do not provide structural details like CT scans or other forms of ionizing radiation-based imaging.
Imaging requires injecting a very small amount of a radioactive chemical used as a tracer. While waiting for several hours until the radioisotope reaches its destination, the patient can eat lunch and participate in other normal activities. However, during the procedure when a camera that is extremely sensitive to gamma radiation is suspended over the area to measure levels of the isotope, the patient lies still.
Radionuclide scans may indicate bone defects, tumors, organ scarring or injury, certain endocrine gland disorders, urinary tract abnormalities, active bleeding, and heart function (Figure 4).
These scans are also useful for determining whether cancer has metastasized.
Afterwards patients remain slightly radioactive for a short time, but gamma irradiation is short-lived and are excreted in the urine.
Magnetic Resonance Imaging (MRI)
The MRI is done in a tubular device similar to a CT scanner, but it’s much longer and noisier.
MRI requires no ionizing radiation and, instead, uses energy generated by strong magnetic fields to reorient molecules in the body. When the magnetic field is turned off, the molecules return to their usual states, releasing the energy absorbed from the magnets. The speed and amount of energy released from various body tissues provide very detailed computerized images of organs or structures of interest.
Although the CT scan can be a better method for evaluating bones and organs such as the liver and kidneys, MRI provides better structural detail for the detection and evaluation of cancer, soft tissue injuries, internal bleeding, abnormal breast tissue, brain and spinal cord disorders, nerves, muscles, and tendons.
Ultrasound uses very high frequency sound waves that enable visualization of internal organs and structures. These exams are safe and generate no ionizing radiation.
During the procedure a transducer, or probe, is placed on the skin along with a type of jelly to enhance contact. As the sound waves encounter different structures in the body, they may be absorbed or reflected back to the transducer. A computer translates these images into what you see on the screen.
Ultrasound can track the movements of such things as fetal activity (Figure 6A). An extremely important capability of ultrasound is its use in echocardiography which is an ultrasound of the heart. Physicians can examine the heart’s structure and contractions (Figure 6B) and follow the path of blood through the heart (Figure 6C).
Positron Emission Tomography (PET scans)
PET scans combine radiology techniques to obtain unique images.
To detect the metabolic activity of organs, tissue, and cancer PET uses a radioactive tracer, often a form of glucose. The imaging is done with a machine similar to the types used for CT or MRI. When the computer combines images from the PET scan with those from an MRI or CT scan, detailed anatomic locations can be determined (Figure 7).
These scans can be very effective for detecting early cancer, cancer spread, and brain disease before they are discovered on an MRI or CT scan.
Are there risks associated with ionizing radiation exposure?
Introducing radioactive materials to your body for imaging tests can be unsettling — particularly if exposure may increase cancer risks.
Amounts of exposure vary based on factors such as excess body weight, the type of test, and radiation dosage level. One way to measure these risks is to determine how much additional radiation we are exposed to compared to natural sources in our normal lives (e.g., cosmic radiation and radon gas).
The amount of radiation exposure is expressed as the milli-sievert (mSv). The average annual exposure to natural radiation is around 3mSv. Looking at the more commonly performed studies that use ionizing radiation, the risks from radiation exposure from a single x-ray test are minimal: it exceeds approximately 0.1 from a routine chest x-ray and about 0.4 mSv from a mammogram. Procedures such as radionuclide and CT scans have comparatively more radiation than an x-ray.
The CT scan is the one with the highest exposure, ranging from 2 to 10 or more mSv, depending on the exam. 10 mSv is comparable to around 100 chest x-rays. To put this into perspective, however, the natural lifetime risk of developing a fatal cancer is around 1 in 4, which is equal to 400 in 2,000. The estimated effect of a single 10mSv CT scan increases that risk to 401 in 2,000. Multiple CT scans, of course, would have a greater effect.
- Johns Hopkins Medicine. CT Scan Versus MRI Versus X-Ray: What Type of Imaging Do I Need?
- Medscape. Ionizing Radiation and Medical Imaging.
- FDA. What are the Radiation Risks from CT?
- UVA Radiology and Medical Imaging Blog for Patients. Different Imaging Tests, Explained.