What Is the Difference between an Ultrasound and a CT Scan?

Ultrasound and computer tomography (CT) scans are both diagnostic tools that allow doctors to view internal systems of patients without invasive surgery. An ultrasound and a CT scan can be indicated in many different situations, and can sometimes be used interchangeably for diagnostic purposes. Though both tools can be used to visualize internal systems and diagnose similar conditions, there are several distinctions between the two.

One primary difference between an ultrasound and a CT scan is functional technology. An ultrasound uses high-frequency sound waves, which bounce off internal organs and create echoes. These echoes can then be transformed into a two-dimensional greyscale image for viewing. A CT scan is essentially a targeted X-ray machine that takes several two-dimensional X-rays around a single point of axis, thus allowing the creation of a 3D image.

Another major difference between these two procedures is the type of equipment used. Ultrasound machines are usually mobile systems that connect a wand, known as a transducer, to a monitor screen. The doctor or ultrasound technician manually passes the wand over the body area being examined, creating a real-time image on the screen, as well as allowing still images to be saved. CT machines are large, stationary devices that usually consist of a large, doughnut-shaped apparatus surrounding a table. The patient must lie still on the table while the imaging system takes the X-rays.

A doctor choosing between an ultrasound and a CT scan for diagnosis may need to consider several important factors. While both techniques can be used to diagnose similar conditions, the radiation exposure necessary to CT scans may present a risk to some patients. Pregnant women and children are usually advised against CT procedures, since there are health risks related to the level of radiation exposure. Some doctors also advise women of childbearing age to avoid CT scans of the abdominal region, since there is a small chance that the radiation could adversely affect fertility or cause harm if a woman does not know she is pregnant.

Given the radiation risk, if both an ultrasound and a CT scan can offer similarly effective imaging results, a doctor may choose to order an ultrasound first. This process can also be easier on the patient, as ultrasounds are generally less expensive than CT scans, and may have a shorter waiting period. If, however, a CT scan is rated at a higher sensitivity for a certain condition, doctors may choose to opt for a CT. A CT scan may also be ordered if an ultrasound provides inconclusive results, in order to examine the issue from another angle.