PET/CT Anatomy, Physiology and Attenuation Correction

Separate the two signals in PET/CT

Positron emission tomography/computed tomography (PET/CT) combines a map of radiopharmaceutical distribution with x-ray anatomy. PET detects paired annihilation photons associated with positron emission; CT measures transmitted x-rays. The fused display relates functional uptake to an anatomical structure. A bright PET focus is not automatically a tumor, and a CT abnormality can be important even when it has little tracer uptake.

For the NMTCB CT outline, study the anatomy, physiology, organ-system relationships and attenuation-correction role of CT in the hybrid examination. The CT component may be a low-dose attenuation/localization scan or a diagnostic examination with appropriate image quality and, when indicated, contrast. Those purposes require different acquisition choices. A low-dose localization image does not automatically replace an ordered diagnostic chest or abdominal CT.

Anatomy and organ-system correlation

Use CT to locate uptake in the brain, neck, mediastinum, lungs, liver, bowel, urinary system, skeleton and soft tissues. Distinguish an intrapulmonary focus from a mediastinal node, a bowel focus from an adjacent pelvic node, and a vertebral focus from paraspinal muscle. Inspect axial data with coronal and sagittal views rather than relying only on a rotating PET maximum-intensity projection.

With fluorodeoxyglucose (FDG), normal brain glucose metabolism produces substantial cerebral uptake. Renal excretion commonly produces activity in the kidneys, ureters and bladder. Myocardial and bowel uptake can vary. Muscle use, inflammation and brown adipose tissue can create nonmalignant uptake. These physiologic examples teach why localization matters: an apparent neck node on PET may correspond to activated muscle or brown fat on CT. Other tracers have different biodistributions, so FDG rules must not be applied to every PET agent.

Preparation affects physiology and quantification

FDG imaging uses preparation intended to stabilize the metabolic conditions relevant to the study, including the prescribed fasting, glucose assessment and activity restrictions. This fasting is an FDG protocol requirement, not evidence that every intravenous iodinated contrast examination requires fasting. Record glucose, uptake time, administered activity, injection issues and the information required by the nuclear medicine protocol.

An infiltrated radiopharmaceutical injection can alter delivered activity and quantitative interpretation. Recent therapy, infection, surgery or vigorous activity can change distribution. The technologist should communicate these circumstances so that they are considered when comparing uptake between studies. A change in uptake is not interpretable solely from a displayed numerical value if preparation, timing or reconstruction differ substantially.

CT-based attenuation correction

PET photons can be absorbed or scattered before reaching the detectors. CT provides a spatial map from which the system estimates attenuation at the PET photon energy. The scanner transforms diagnostic-energy CT information to an attenuation map suitable for 511 keV photons. CT HU values are not directly the 511 keV attenuation coefficient, and the transformation must account for tissue behavior.

The attenuation map is used during PET reconstruction to compensate for location-dependent photon loss. It supports quantitative accuracy, but it cannot correct an inaccurate map or a misregistered patient automatically. CT truncation, metal and dense contrast can affect the map and create an apparent uptake abnormality. Inspect non-attenuation-corrected PET when a focus may be a correction artifact; compare the CT and corrected PET before accepting the fused finding.

Respiratory and positional registration

CT is acquired quickly, whereas PET emission data average motion over a longer interval. A deep inspiratory diagnostic chest CT can place the diaphragm differently from the PET data. Misregistration near the lung bases or liver dome can shift apparent lesion location and distort attenuation correction. The attenuation/localization CT often uses shallow or tidal breathing according to the validated protocol, with a separate diagnostic inspiratory series when required.

Keep the patient in a reproducible position and avoid movement between components. Arms above the head may reduce torso streaking when tolerated; arms down may be needed for head/neck tasks or patient limitations. Do not sacrifice safety or force painful positioning to obtain the ideal arrangement. Check registration before the patient leaves when the workflow permits correction of a meaningful problem.

Contrast and dose planning

A diagnostic contrast-enhanced CT may be combined with PET/CT under an approved strategy. Dense oral or intravascular contrast can affect attenuation correction, with the magnitude dependent on the system and protocol. Contrast is not universally prohibited. Verify which CT series supplies the attenuation map and which supplies diagnostic interpretation, and use the validated acquisition order.

The patient receives radiation from both the radiopharmaceutical and CT. CTDIvol and DLP describe the CT component, not the administered tracer dose. Avoid duplicating a diagnostic CT already available when the responsible team can use it appropriately, but do not assume an older scan can supply a current attenuation map without validated registration. Document each component's purpose, technique, limitations and administered materials.

References: EANM FDG PET/CT guideline 2.0, SNMMI/EANM brain FDG PET standard 2.0.