Positron Emission Tomography, Computed Tomography (PET-CT)

Positron emission tomography/computed tomography (PET-CT) is a combined nuclear medicine (PET) and radiology (CT) imaging technique that uses cross-sectional imaging to very precisely localize the distribution pattern of radioactive substances (tracers). The integration of PET and CT in a single operation is a significant technical advance, which first became available for everyday clinical use in 2001 with a PET-CT scanner. PET is a function-oriented examination in which radioactively labeled tracers are introduced into the metabolism of specific cells (e.g., tumor cells) and subsequently detected (determined with the aid of detectors). The CT examination performed at the same time allows the functionally conspicuous findings of the PET to be precisely assigned anatomically. For this purpose, the molecular and morphological image data are digitally fused after the examination, so that an improved diagnostic conclusion is achieved. The evaluation is usually performed interdisciplinary by a nuclear medicine physician as well as radiologists.

Indications (areas of application)

The most important indication for performing PET-CT is tumors. Depending on the tumor origin, different radiopharmaceuticals are used, so that nowadays almost all tumor types can be imaged with the help of PET. PET-CT is not suitable as a screening method for tumor detection. It is relevant in the clinic in the following situations:

Staging of tumors: the accumulation of the tracer in tumors compared with normal tissue and the high spatial resolution allow imaging of very small malignant processes (e.g., lymph node metastases). In addition, there is also the possibility of a whole-body examination, so that the requirements for a method suitable as tumor staging (detection of tumor extent) are met.
CUP (“cancer of unknown primary”): in the CUP syndrome, a metastasis is discovered without the original tumor being known. PET-CT is also a possible method to search for the primary tumor in this case.
Therapy stratification during chemotherapy/determination of therapy success: after chemotherapy or radiotherapy has been performed, PET-CT can be used to assess the tumor’s response to therapy based on its reduced (therapy success) or constant/increased (no therapy success) metabolic activity.

Various tumors are amenable to PET-CT diagnostics:

Bronchial carcinoma (lung cancer; for primary non-small cell and small cell lung carcinoma) and for solitary pulmonary nodules.
Hodgkin’s lymphoma – malignant neoplasm (malignant neoplasm) of the lymphatic system with possible involvement of other organs.
Colon carcinoma (colon cancer)
Head and neck tumors [PET-MRI equally accurate]
Bone and soft tissue tumors
Lymphomas (including for initial staging whether bone marrow involvement is present).
Mammary carcinoma (breast cancer).
Malignant melanoma (black skin cancer)
Neuroendocrine tumors (NET) – localization: depending on the localization are distinguished: the bronchus carcinoid, thymus carcinoid, appendix carcinoid, ileum carcinoid, rectum carcinoid, duodenal carcinoid, as well as the gastric carcinoid; about 80 percent of the tumors are located in the terminal ileum or appendix.
Esophageal carcinoma (esophageal cancer).
Ovarian cancer (ovarian cancer)
Pancreatic carcinoma (pancreatic cancer)
Prostate carcinoma (prostate cancer)
Sarcoma
Thyroid carcinoma (thyroid cancer)
Tumors of the skeletal system

Another indication area for PET-CT is neuromedicine. Due to the possibility of a functional examination of brain receptors, degenerative brain diseases can be differentially diagnosed at an early stage:

Early differential diagnosis of Parkinson’s disease.
Early diagnosis of multisystem degenerations (synonym: multisystem atrophies, MSA); these are also called multisystem degenerations. These are clinical pictures in which various structures and systems of the central nervous system (CNS) regress simultaneously. This results in the clinical picture of Parkinson’s disease (secondary Parkinson’s ‘syndromes).These disorders include: Shy-Drager syndrome; striatonigral degeneration; Steele-Richardson-Olszewski syndrome; combination of amyotrophic lateral sclerosis (ALS) with dementia and Parkinson’s disease; olivopontocerebellar atrophy.
Early detection of Huntington’s disease (synonyms: Huntington’s disease major (Huntington’s); Huntington’s chorea; Huntington’s disease; older name: St. Vitus’ dance) – incurable hereditary disease of the brain.

In addition, PET-CT is also used for dynamic studies such as imaging myocardial perfusion (blood flow to the heart muscle) or brain perfusion:

Progress monitoring in lysis therapy (drug therapy to dissolve a blood clot) in the condition after apoplexy (stroke).
Cerebral circulatory disorders – to size the penumbra (as penumbra (lat. : penumbra) is called in a cerebral infarction the area immediately adjacent to the central necrosis zone and still contains viable cells) and to determine the myocardial viability, for example, after myocardial infarction (heart attack).

PSMA(prostate specific membrane antigen) PET/CT can be used for recurrence diagnosis of prostate cancer according to the new S3 guideline from 2017. The procedure is also already used in primary staging (probably less appropriate) and as a substitute for or adjunct to bone scintigraphy required in high-risk patients – before surgery and radiation or during therapy. PSMA-PET-CT is thought to be more sensitive than skeletal scintigraphy (bone scintigraphy) in prostate cancer. According to recent studies, a PSMA-PET-active lesion only correctly detects a tumor by location and number in a maximum of 67%; bone metastases (daughter tumors of a cancer) have been detected by the procedure with a specificity (probability that actually healthy individuals who do not have the disease in question are also detected as healthy by the test) of 68.7-100% (versus 60.8-96.1% by bone scintigram). Note on differential diagnosis: PSMA PET-CT also detects the following diseases; granulomatous diseases such as Wegener’s disease, active tuberculosis, hemangiomas, Paget’s disease, peripheral nerve sheath tumors, schwannomas, and ganglia and fibrous dysplasia.

Before the examination

When using a tracer coupled to glucose (e.g., 18F-FDG), patients should be fasting for at least 4-6 hours before the examination. Serum glucose levels are monitored and should not exceed 6.6mmol/l (120 mg/dl).
To visualize the abdomen or trunk of the body, bowel contrast is required as part of the CT scan. For this purpose, patients receive a drinking solution with water-soluble, iodine-containing contrast medium (e.g., 20 ml Gastrografin in 750 ml mineral water) 60 min before the start of the examination.
Before the examination, the urinary bladder should have been emptied.
For optimal image quality and avoidance of artifacts, patients should lie relaxed and not freeze during the preparation phase and during tracer application.
Combining PET and CT in a single procedure also requires precise definition of the anatomical scope of the examination, patient positioning, and the desired slice thickness for CT.

The procedure

The basis for PET is the tracking of molecules in the patient’s body by positron emission using a positron emitter. Detection (discovery) of positrons is then based on the collision of a positron with an electron, as the collision of charged particles results in annihilation (generation of gamma quanta), which is sufficient for detection. Radionuclides suitable for application are those that can emit positrons in the state of decay. As described earlier, the positrons collide with a nearby electron. The distance at which annihilation occurs is 2 mm on average. Annihilation is a process in which both the positrons and the electrons are destroyed, producing two photons. These photons are part of the electromagnetic radiation and form the so-called annihilation radiation. This radiation impinges on several points of a detector, so that the source of emission can be localized. Since two detectors are located opposite each other, the position can be determined in this way. Sequence of PET and generation of cross-sectional images (CT):

First, a radiopharmaceutical is applied to the patient. These so-called tracers can be labeled by different radioactive substances. The most commonly used are radioactive isotopes of fluorine and carbon. Due to the similarity to the basic molecule, the body is not able to distinguish the radioactive isotopes from the basic element, which results in the isotopes being integrated into both anabolic and catabolic metabolic processes. However, as a result of the short half-life, it is necessary that the production of the isotopes takes place in close proximity to the PET scanner.
After intravenous or inhalation intake of the radiopharmaceutical, the distribution of radioactive isotopes in the fasting patient is waited for, and after about an hour, the actual PET procedure is started. The position of the body must be chosen in such a way that the ring of detectors is in close proximity to the part of the body to be examined. Due to this, for whole-body imaging is necessary to take several body positions.
The detectors already described must be present in a large number to ensure the detection of photons. The method of calculating the collision point of electron and positron is called coincidence method. Each detector represents a combination of scintillation crystal and photomultiplier (special electron tube).
The recording time during an examination depends on both the type of device and the radiopharmaceutical used.
In addition to PET, a computed tomography (CT) scan is performed. It is critical not to change the patient’s positioning during the combined examination (PET and CT) so that subsequent anatomical mapping is possible.