PET - Positron Emission Tomography

PET - Positron Emission Tomography

What is PET

There PET (Tomography ad Positron Emission) is an imaging technique that quantifies radioactivity in the body. It uses the injection of radiopharmaceuticals into the body to study their accumulation in organs and pathological tissues. It can therefore be defined as a "functional" imaging method as it studies the absorption of a drug into the tissues. PET allows the reconstruction of a three-dimensional image, and is often combined with CT (PET-CT), in order to match the functional image of the PET with the precision of the anatomical image of the TC, computed tomography. PET can also be combined with MRI, but this happens less frequently.

Physical principles

At the beginning of the examination a radiopharmaceutical, usually the fluoro-deoxy-D-glucose (F18FDG) is injected intravenously. It is a compound similar to normal glucose, the sugar that all cells in the body absorb for energy. FDG is also absorbed by cells, in greater quantities the faster their metabolism. Tumors are clusters of cells with very high metabolism, and therefore they absorb very high quantities of this compound. Fluorodeoxyglucose is converted into a compound that accumulates in cells, and can be detected and quantified, as it is a radioactive compound.

This radioactive compound decays by releasing positrons (hence the name of PET), particles that are released, interact with electrons forming high-energy photons, which are detected in turn by special detectors. At this point the detectors allow to obtain an image.

Similar to the functioning of the PET is that of the SPECT (Single Photon Emission Tomography), which also bases its operation on the decay of a radioactive tracer, which however decays by releasing gamma radiation, detected directly by the device.

PET CT (sometimes erroneously referred to as PET CT) consists of two different imaging methods, which are however performed by a single device, with a single exam.

For the physical principles of CT, see this page on computed tomography.

Uses of PET

The indications of PET and PET-CT are different. More often PET is performed together with CT, to allow a better evaluation of all structures, since PET alone has poor resolution.

There PET it can be used:

  • in oncology, for the diagnosis of cancer, the determination of its extent and response to therapy, and to detect any relapses. Often the Total body PET, with scanning of all the parts of the body, in order to highlight areas of possible anomalous uptake of the radiopharmaceutical.
  • in the neurological field, for the early diagnosis of Alzheimer's disease, in the study of epilepsies and their triggering causes, and to study certain areas of the cerebral cortex and their functions
  • in cardiology, for the study of the heart muscle and its metabolism: a heart attack can damage the myocardium, reducing metabolism in an evident way.

There PET-CT is used:

  • to define the extent of a tumor when it can be radically treated. This is because the technique allows to accurately establish the extent of the disease before treatment, in order to study the response. Also in this case the Total body PET-CT, with whole body scan, for a complete staging of the disease.
  • in patients with previous cancers and dubious standard imaging investigations, which seem to indicate relapse but without certainty, PET-CT can resolve the doubt
  • to evaluate a lesion of an unclear nature, for example a solitary lung nodule.
  • to guide a biopsy, for example in the case of pleural mesothelioma
Positron Emission Tomography

PET image of the brain of a patient with Alzheimer's disease: functional loss of the temporal lobe can be noted.

Comparison between PET and other diagnostic methods

PET cannot be considered a first level diagnostic method, unlike X-ray and ultrasound, partly because it has high costs, and also because it has much more restricted indications (it is applied above all in oncology), while X-ray and ultrasound have costs lower and larger uses.

Compared to other advanced imaging methods (MRI, CT), PET has the particularity of being a functional rather than morphological imaging method, based on tissue metabolism. This is important because the metabolic alterations of the tissues occur before the morphological ones. The latter take longer to become evident. So a recently appeared tumor, may not be visible on CT if small, but is more easily visible on PET scan.

Hence the importance for an early diagnosis. For the rest, however, PET has more restricted applications than CT and MRI. It is more useful when combined with one of these two methods, rather than alone. How CT exposes the patient to ionizing radiation (the radiation dose is similar for a single PET or CT scan). Unlike CT and MRI it always requires the administration of a drug to be performed, while CT and MRI can be done without contrast medium.

Compared to SPECT, PET is more expensive. The tracers used in the two methods are different, the SPECT exploits drugs based on iodine or technetium that decay release gamma radiation. The PET allows to visualize the structures with greater contrast and spatial resolution. The two methods are otherwise similar, both allow functional imaging and can be combined with MRI and CT.

Finally, the operating principle of scintigraphy is similar to those of PET and SPECT, always based on the administration of radioisotopes and on the uptake of gamma rays by special detectors. Unlike PET and SPECT, however, scintigraphy images are not three-dimensional.

Contraindications of PET

PET is a safe test with no known side effects. However, given the injection of a radioactive drug, PET is contraindicated in pregnant women. Allergic reactions to fluoro-deoxy-D-glucose are not known or reported in the literature. In some patients, after the administration of the radioactive drug, it is advisable to limit contact with pregnant women and children for a few days (for example for teachers), because the drug metabolism may be slow and these subjects may be more sensitive to contact with people who have been given a radiopharmaceutical.

In cases where a PET-CT or a PET-MRI is performed, the contraindications for the two examinations (CT or MRI) are valid, which can be performed with relative contrast liquid.

Limitations of PET

Like other imaging methods, PET also has some limitations.

The greatest is represented by the appearance of motion artifacts. It means that any involuntary movements of the patient during the examination will generate errors in the image which will be less useful and reliable.

The presence of some movement artifacts cannot be avoided: the heart, intestines and lungs are organs that are always in motion. Computers built into machines can partially correct these artifacts.

Implants and prostheses also generate artifacts linked to their high density, their presence must be reported before performing any imaging examination.

Some structures, although not tumors, can pick up a high amount of radiopharmaceutical, because they have cells with high metabolism, or cells in rapid duplication. This is the case of abscesses, areas of inflammation or reactions to any foreign bodies that can become evident as areas of marked uptake. The bladder is always evident and clearly visible when performing a PET scan, because the radiopharmaceutical after being metabolized and expelled by the kidney accumulates there before being expelled with the urine.

Performing a PET scan has the further limitation of requiring complex exam preparation, not common to other imaging exams.

PET: exam preparation

Patients will be asked to fast for 4-6 hours prior to the examination, to minimize the absorption of fluoro-deoxy-D-glucose from the heart and increase the absorption by any tumors. Before the exam, avoid drinking alcohol or caffeinated drinks. Patients can drink water without any problems.

Before injecting the radioactive drug, the blood glucose level will be measured. Values below 150 mg / dl of blood glucose are desirable for conducting the test. It is essential that the patient has good blood glucose control (this can be a problem in diabetic patients), because uptake of the drug into the tissues is reduced by glucose. In the 24 hours before the examination and after the injection of the drug, patients should avoid strenuous physical activity, to prevent the drug from being absorbed by the muscle, distorting the examination.

After the injection of the radiopharmaceutical, the patient's movement and even speech must be limited, again to avoid metabolization by the muscle.

The examination begins approximately 60 minutes after the injection. It is important to take a comfortable position at the beginning of the examination, because the PET scan takes 30-45 minutes, during which it is important that the patient does not move, to avoid the appearance of artifacts. If a PET-CT is done, the CT exam is done just before the PET and takes only 60-70 seconds.

Sources:

  1. Positron emission tomography, A.Prof Frank Gaillard and Dr Aditya Shetty et al.
  2. PET-CT indications, Dr Dan J Bell and Dr Ki Yap et al.
  3. An Introduction to PET-CT Imaging, Vibhu Kapoor, MD, Barry M. McCook, MD, and Frank S. Torok, MD

 

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