A stand-alone positron emission tomography, or PET, scanner typically has a doughnut-shaped hole in the center, so both sides are open, according to RadiologyInfo.org. However, many radiology facilities use scanners that also perform computed tomography procedures. A combination PET/CT scanner has a tunnel-shaped chamber with only one open end.
Radiologists use PET scanners to trace radioactive signals and capture detailed images of organ functions, MedlinePlus states. After administering a radioactive tracer drug to a patient, the doctor waits for the chemical to interact with different types of cellular tissue. During the procedure, the patient lies on a table that slides inside the scanning machine. The scanner detects lingering traces of radioactivity and converts the data into a 3-D image that doctors evaluate on a computer monitor.