Medical imaging is a very broad area of medicine and includes many different types of imaging modalities. These include most commonly radiography (x-ray), nuclear medicine, magnetic resonance imaging (MRI), ultrasound, elastography, tomography, positron emitted tomography (PET) and computed tomography (CT), and echocardiography. Other more advanced or exotic imaging studies include photoacoustic imaging and functional near-infrared spectroscopy but these studies are not typically used in everyday medicine.
Relative Radiation Levels of Medical Imaging Modalities
Not all medical imaging studies expose patients to potentially harmful radiation. Only certain types of imaging studies use ionizing radiation to create images for radiologists to interpret. The main modalities in radiology use ionizing radiation are radiography, fluoroscopy, CT, and nuclear medicine studies. Below is a graph charting the relative amount of total population radiation exposure that each of these modalities contributes to the overall radiation exposure of the US population.
As you can see, the majority of population exposure to radiation doesn’t even come from medical imaging. It comes from radon and thoron, which are background radiation contributors that we’re all exposed to on a daily basis. While medical imaging does contribute to population radiation exposure, its a smaller overall contributor than background and natural sources. Of medical imaging, CT scans contribute the most radiation dose overall to the US population. Does this mean that CT scans are the highest dose radiation exam in all of medical imaging? The answer is an emphatic NO!
The amount of radiation exposure to an individual patient from an individual medical imaging study is very difficult to estimate. The radiation dose depends on the body part being imaged, the size/weight of the patient, the technical parameters of the tube current and peak potential (in the case of radiography and CT), or the amount and type of radiotracer given in the setting of nuclear medicine imaging. The table below brings this all to light and compares the doses from each exam and how long it would take to get the same exposure from normal background radiation. If you notice the last one, coronary angiography has the potential to expose patients to radiation levels that would take 5 years to be exposed to the equivalent dose from background radiation! 5 years!