PET-CT Flashcards
Basic Positron Emission Tomography Physics
What does a PET radiotracer emit?
What does it meet with? When they meet _______ occurs creating two ____ keV ________ traveling nearly ____ degrees apart.
The two high-energy _____ traveling in ________ directions are simultaneously detected by a circular crystal, which can determine that the two ______ arrived coincidentally.
- A positron emission tomography (PET) radiotracer emits a positron that travels a small local distance within the tissue. After meeting an electron, both the positron and electron annihilate and create two 511 keV photons traveling nearly 180 degrees apart.
- The two high-energy photons traveling in opposite directions are simultaneously detected by a circular crystal, which can determine that the two photons arrived coincidentally.
PET Radiotracer
What positron emitter is used? What is its 1/2 life?
What glucose analog is used that competes with glucose for transport into cells by the ______ transporters.
After becoming phosphorylated by _______, the _______ cannot undergo glycolysis and is effectively _______ in cells.
- Fluorine-18 (F-18) is a positron emitter, half-life 110 minutes.
- F-18-fluorodeoxyglucose (F-18 FDG) is a glucose analog that competes with glucose for transport into cells by the GLUT 1 and 3 transporters. After becoming phosphorylated by hexokinase, FDG-phosphate cannot undergo glycolysis and is effectively trapped in cells.
Radiotracer Uptake Quantification
SUV stands for what and is used to roughly quantify ____ uptake?
SUV is proportional to?
What are the factors that can affect the SUV of an ROI?
Can you exclude malignancy with SUV values?
What is the preferred approach to absolute SUV values?
- The standardized uptake value (SUV) roughly quantifies FDG uptake.
- SUV is proportional to (ROI activity x body weight) / administered activity.
- SUV of a region of interest can vary significantly depending on numerous factors including specific equipment, time elapsed since FDG administered, amount of tracer extravasation, muscle uptake, glucose and insulin levels at time of injection, etc.
- Malignancy can never be definitively diagnosed or excluded using SUV as the sole criterion.
- A preferred approach to absolute SUV values is to use a ratio of background liver, cerebellum, or basal ganglia to compare with a region of interest.
- For instance, using the basal ganglia uptake as a baseline, relative SUV of a region of interest <20% is “mild”; 20-60% is “moderate”, and >60% is “intense” uptake.
PET’s CT Correlation
What is an important exception as to why a CT wouldn’t be performed with a PET?
The CT is performed with a _____ dose than a diagnostic-quality CT.
The CT is used for __________ and __________.
What may cause artifacts of FDG uptake due to miscalculation of attenuation correction?
- Most modern PET exams are performed together with a CT as a PET-CT. One important exception is in the pediatric population, where PET may be performed in isolation to reduce the risk of radiation exposure from CT.
- The CT exam is often performed with a lower dose than a diagnostic-quality CT. The CT protocol varies by institution (e.g., whether intravenous contrast is administered).
- The CT is used for anatomic localization and attenuation correction.
- Very dense retained oral contrast (or dense metallic objects such as joint prostheses) may cause artifacts of FDG uptake due to miscalculation of attenuation correction.
PET Patient Preparation
What is the FDG uptake in both normal and pathological tissues dependent on?
Elevated insulin levels will cause ________ muscle uptake and _________ sensitivity for detecting mildly PET-avid lesions.
Can patients eat before a PET/CT? What should the glucose level be?
After injection of the F-18 FDG, what does the patient need to do? What can happen if the patient doesn’t do this?
- The uptake of FDG in both normal and pathological tissues is dependent on the serum glucose and insulin levels.
- Elevated insulin levels will cause increased muscle uptake and decreased sensitivity for detecting mildly PET-avid lesions.
- Patients should be NPO for at least four hours to allow insulin to reach a basal level.
- Blood glucose should be below 200 mg/dl, preferably below 150.
- After injection of F-18 FDG, the patient should rest in a quiet room for 60 minutes.
- If the patient talks, the vocal cords may show FDG uptake.
- If the patient walks, the muscles may show FDG uptake.
Normal FDG Distribution
Brain
KUB
Salivary glands, tonsils, thyroid
Liver
Bowel (what can cause increase colonic and to a lesser extent small bowel FDG uptake?
Heart
Muscles (what can cause increased muscle uptake?
Brown fat (where is this located and what can cause an increased uptake in this tissue?
- Brain: The brain has intense FDG uptake. Brains love sugar! Despite intense uptake, with appropriate windowing, excellent detail of the cortex, basal ganglia, and cerebellum can be seen.
- Kidneys, ureters, and bladder: FDG is concentrated in the urine, with very intense uptake in the renal collecting system, ureters, and bladder.
- Salivary glands, tonsils, thyroid: mild to moderate symmetric uptake.
- Liver: The liver is moderately FDG avid and typically shows inhomogeneous uptake.
- Bowel: Diffuse mild to moderate uptake is normal. Focal uptake within the bowel, however, should be regarded with suspicion.
- Note that metformin can increase colonic, and to a lesser extent, small bowel FDG uptake.
- Heart: Uptake is totally variable, depending on insulin/glucose levels. The heart prefers fatty acids but will use glucose if available.
- Muscles: FDG uptake is normally low. However, elevated insulin levels or recent exercise can cause increased muscle uptake.
- Brown fat: Brown fat is metabolically active adipose tissue typically found in the supraclavicular region and intercostal spaces that can be mild to moderately FDG avid, especially if the patient is cold.
PET-CT and Lung Cancer
What is the role of PET-CT in lung cancer?
Typically what group of lung cancers are evaluated using PET-CT?
What is PET most useful for in initial staging?
Approximately what percent of patients with a negative metastatic workup by CT will have PET evidence of mets?
Sensitivity and specificity of PET for detecting malignant lymph nodes
___________ is the gold standard for lymph node staging.
If a potentially curative surgery is denied based solely on a PET-positive lymph node, what needs to be done?
- PET-CT plays a role both in the initial staging of patients with lung cancer, and in evaluating response to treatment. Typically, only non small-cell lung cancer is evaluated by PET as small-cell is considered to be metastatic at diagnosis.
- For initial staging, PET-CT is most useful to evaluate local tumor extension and to search for distant metastases.
- Approximately 10% of patients with a negative metastatic workup by CT will have PET evidence of metastasis.
- PET is very sensitive for detecting malignant lymph nodes; however, it is not specific. A negative PET would allow surgery to proceed without further testing. Mediastinoscopy is the gold standard for lymph node staging. Given the lack of PET specificity, PET-positive mediastinal nodes must be followed by mediastinoscopy before potentially curative surgery is denied based solely on the PET.
PET-CT and Solitary Pulmonary Nodule
What is the smallest sized nodule that can be evaluated by PET?
The majority of malignant SPNs are FDG _____. What malignant SPNs may be falsely negative of PET?
Conversely, the majority of benign SPNs are FDG _____. What benign nodules may give a false positive on PET?
Can you diagnose an SPN as benign or malignant based on SUV?
In general, if a nodule is not FDG avid, ________ is reasonable. If the nodule is FDG avid then __________ is preferred.
- Evaluation of a suspicious solitary pulmonary nodule is an indication for PET-CT.
- 8 mm is typically the smallest size nodule that can be reliably evaluated by PET.
- The majority of malignant SPNs are FDG avid. However, low-grade tumors such as minimally invasive adenocarcinoma or carcinoid may not be metabolically active and may be falsely negative on PET.
- Conversely, the majority of benign SPNs are not FDG avid. However, active granulomatous disease (including tuberculosis) may take up FDG and represent a false positive on PET.
- It is never possible to definitively diagnose a nodule as benign or malignant based on SUV.
- In general, if a nodule is not FDG avid, short-term follow-up is reasonable. If the nodule is FDG avid then biopsy or resection is preferred.
What are some of the oncologic indications of PET-CT?
- Lung cancer
- Solitary Pulmonary Nodule
- Colon cancer
- Head and neck cancer
- Thyroid cancer
- Lymphoma
- Breast
- Esophageal cancer
PET-CT and Colon Cancer
What is the role of PET-CT in colon cancer?
What is a common indication for PET-CT with colon cancer?
After initial treatment how long does one wait for a f/u PET-CT and why?
- PET-CT has a limited role in determining local extent of colon cancer due to poor spatial resolution and physiologic bowel uptake, but PET-CT does play a primary role in evaluating metastatic disease in colorectal malignancies. In particular, since isolated hepatic metastases can be resected or ablated, evaluation for extrahepatic metastases is a common indication for PET-CT.
- After initial treatment, follow-up PET-CT is usually delayed approximately 2 months due to a flare phenomenon of increased FDG uptake in the peritreatment period.
PET-CT and H&N Cancer
Role?
Why would evaluating recurrent disease after chemoradiation be limited?
Post-treatment scans are delayed how long s/p treatment?
- PET-CT is often used in the initial staging of head and neck squamous cell carcinoma, especially for evaluation of regional nodal metastases.
- Specificity for evaluating recurrent disease after chemoradiation is limited due to altered anatomy and inflammation from treatment. Usually, post-treatment scans should be delayed 4 months after treatment to minimize these effects.
PET-CT and Thyroid Cancer
In what clinical setting is a PET-CT used in the evaluation of thyroid cancer?
Which thyroid cancers may have this clinical setting?
- PET-CT is used in the clinical setting of a rising thyroglobulin level with negative whole-body radioiodine scans.
- Undifferentiated or medullary thyroid cancers may not take up radioiodine but may be FDG avid.
PET-CT and Lymphoma
Role?
Which lymphomas are FDG avid? Which tend to be less avid?
Diffuse marrow uptake may be due to what possible causes?
Focal increased uptake is likely to represent what?
- PET-CT plays a key role in the staging and restaging of patients with lymphoma.
- Most histological types of lymphoma, including Hodgkin and non-Hodgkin lymphoma, are FDG avid. Some low-grade lymphomas, such as small lymphocytic and mantle cell, tend to be less FDG avid, however.
- Increased marrow uptake in lymphoma patients can be difficult to interpret. Diffuse marrow uptake may be due to granulocyte colony-stimulating factor (G-CSF) stimulation, rebound effect from chemotherapy, or malignant marrow infiltration.
- Focal increased uptake, however, is more likely to represent lymphoma.
What is the role of PET-CT in breast cancer?
- Although used in the staging and response to therapy of recurrent or stage IV breast cancer, PET-CT is not routinely used for patients with stage I-III breast cancer.
PET-CT and Esophageal Cancer
Priamary role in the initial evaluation of these patients?
After neoadjuvant treatment, a decrease in FDG avidity by at least ___% suggests a more favorable prognosis. What about those patients who do not show a decrease in SUV values?
- The primary role of PET-CT in the initial evaluation of patients with esophageal cancer is to identify those with stage IV disease who are not surgical candidates.
- After initial neoadjuvant treatment, a decrease in FDG avidity by at least 30% suggests a more favorable prognosis. In contrast, those patients who do not show a decrease in SUV values can potentially be spared ineffective chemotherapy regimens.
