PET/CT

Standardized Uptake Value (SUV)

• How much radiotracer has been taken up by the tissue

• SUV activity concentration in tissue / injective activity aka body size

• SUV max

  • Highest value in the voxel regardless of size

  • more susceptible to noise

• SUV mean

  • Averages value from multiple voxels of the region of interest

  • Less sensitive to noise

Factors Affecting Quantification of Tracer Uptake

• Weight & Body size

• Blood glucose

• Post-injection uptake time

• Respiratory motion

• Technical factors associated with obtaining the image itself (FOV, use of contrast, etc.)

Protocols

• All PET/CTs should include these

  • Non-attenuation corrected

  • Regular body CT

  • Attenuation corrected PET series

• Protocols

  • Skull base to mid-thigh

  • Vertex of skull to mid-thigh

  • Whole body

Mechanism of Action

• FDG-glucose will not undergo glycolysis

• The amount of glucose in cell is regulated by two items

  • Number of GLUT transporters

  • Level of hexokinase in the cell

• Insulin causes increased muscle uptake of glucose

  • Need to time the insulin use in diabetics before PET is obtained

• Note you can have hyperinsulinemia with a normal glucose level, classically if a person recently ate a few hours before the exam the glucose will likely be normal but insulin will be high

• Ideally want glucose between 70-200

• When insulin is high then the glucose will be diverted to other areas and falsely dilute its transport to the cancer so even if the cancer is hot it is likely not as hot as it should be because the glucose tracer is being diverted to muscle and other areas

• Avoid exercise at least a day before scan —> causes increase glucose uptake by muscles

Normal Distribution of FDG-PET

• Highest uptake in brain

  • Higher in gray matter

  • White matter only has axons so no glucose transporters present and therefore lower uptake

• Low levels of uptake

  • Blood vessels

  • Salivary glands

  • Lymph nodes

• Uptake in liver used as the background level of metabolism

• Spleen should be equal to or lower in level of uptake compared to the liver

• Kidney

  • Will have high levels in collecting system

  • If have kidney disease may not have any in collecting system

FDG Patterns of Bone

• Diffuse, homogenous uptake in bone

  • Look for G-CSF use —> stimulate bone marrow

  • Spleen will also typically have diffuse uptake as it is stimulated by these drugs too

    • Size of spleen is typically normal if the diffuse uptake is caused by the drugs

      • If diffuse uptake and enlarged consider lymphoma

• Diffuse, heterogenous, multifocal in bone

  • Look for lymphoma

Benign Findings & Pitfalls

Benign Lesions which may have increase tracer uptake

Pleurodesis

• Will be FDG-avid basically from chronic inflammation

• Will see associated density on the non-con

• Mesothelioma may look similar but should be more nodular and throughout the pleura, and should also typically be bilateral

Vocal Fold/Cord

• Focal unilateral uptake within the vocal fold/cord is likely secondary to entrapment of the recurrent laryngeal nerve caused the contralateral side to work harder and therefore have increased tracer uptake

• Same phenomenon can be seen with radiation changes to the chest/mediastinum/neck region

Infected ICD

Brown Fat

• Mostly seen in the neck, cervicothoracic paraspinal soft tissues, clavicular region and mediastinum

  • Can also see in peri-renal area (looks like adrenal gland but obv is not the adrenal gland)

• When determining if brown fat or abnormal look for

  • Symmetry

  • Look for no lesions and just fat on the regular non-con CT

  • Look for no surrounding abnormal LN

• How to reduce this

  • Keep room warm

  • Give benzodiazepines - shuts down pathway

Key Notes:

• For non FDG PETs (Dotatae and PSMA), the SUV has nothing to do with actual tracer uptake it has to do with cell surface receptor level

  • So the worse the cancer gets, the more amorphous and fucked up the cells become so they no longer look like the normal organ tissue and therefore SUV will decrease

• For stating disease progression

  • Needs to be >20% increase in size and > 5mm increased —> BOTH

• What is PET good at vs MR and Contrasted CT

  • Solid organ mets —> PET is best

  • Vascular invasion and peritoneal carcinomatosis —> CECT is best

  • Shit in brain —> brain is best

Tumors that are PET cold

• Broncheoalveolar carcinoma

• Carcinoid

• Islet cell tumors

• RCC

• HCC (varies)

  • Has enzyme that removes phosphate which makes FDG not bright or some shit

• Prostate

• Anything mucinous

Thyroid Uptake

• Diffuse uptake —> Hashimoto

• Focal uptake —> anything (cancer or anything)

  • Need to get US

References:

Cases

Ga 68-Dotatate PET

General

• Basically a somatostatin analog

• Used to evaluate for

  • Neuroendocrine tumors

  • Meningiomas

  • Sarcoid

• Compared to FDG PET

  • No brain uptake

  • + uptake in pituitary gland (looks like Indian bindi)

  • Spleen also hot

• Physiologic uptake in adrenal glands, liver, pancreas, spleen

  • Note that in pancreas it is in uncinate process, so looks somewhat focal normally - question asking what part of pancreas so need to know

General

• Special PET used in patients with prostate cancer who have previously had radiation therapy or prostatectomy to look for recurrence

• Lytic lesions tend to have strong uptake and sclerotic lesions tend to have little to no uptake

  • If there is a sclerotic lesion with no uptake then you need bone specific evaluation with skeletal scintigraphy

• Homogenous and diffuse uptake in the pancreas is a normal finding, do not be tricked into picking pancreatitis

Spine and pelvic mets