Q: What is the difference between PET, MRI and CT scans? Are they three methods that accomplish the same goal or is there a difference between them?


A: Dear user, PET, magnetic resonance imaging (MRI) and CT are all diagnostic imaging methods, which are used for the study of various diseases. Specifically, PET (Positron Tomography) emission is a nuclear medicine technique that uses radioactive materials to see a patient’s metabolic processes in the body. The tracers used for PET are of different types, but each has such features that allow them to see some particular functions of the human body or of  various tissues. For instance, FDG (fluorodeoxyglucose) labeled with fluorine 18 (radioactive) serves to observe the metabolism of glucose, so PET imaging with FDG provides us with a map of the distribution of the tracer along with the metabolic activity of various tissues. PET is a functional method for observing the function of tissues and organs.

CT scan or Computer Tomography makes another use of radiation. It virtually uses a beam of electrons to produce radiation (photons X) that originate externally to the patient's body to see the density of their tissues and organs. So unlike PET, in which the patient subject to radioactivity, the radiation from the CT comes from the outside. When the radiation crosses the district of the human body under examination (for example during a chest CT scan), photographic images are generated. The principle is the same as an x-ray scan, however, the CT images acquired are at 360 ° around the body (hence the term "tomography"). Each cross section has a different index of attenuation of radiation transmitted from outside, and so the "photo" obtained shows substantial reconstruction of the morphology of the tissues and organs in question. Overall, CT is a morphological method. The ability to inject substances intravenously or orally (contrast media), which change the attenuation of the districts where they are placed, allow an increased accuracy of the CT scan to be seen, differentiating parts of the tissues from their vascularity. 

MRI or Magnetic Resonance, is a test that uses an electromagnetic field to produce various responses or images of the structures inside the body. The response to various stimuli is different for each type of tissue composition and thus allows us  to make a reconstruction even in this cases involving morphology and the function of the organs and bodily tissue. Contrast agents are used, which consequently present different "magnetic" features and allow an increased accuracy of the MRI scan to be seen, differentiating parts of the tissues from their vascularity.

What these three methods provide from a diagnostic point of view is very different: PET observes the function of the tissues, CT defines the appearance of the tissues and MRI does both, although it remains a morphological method. Obviously, the use of one method over the other depends on the disease in question and the patient’s medical history. Best regards.