PET-CT imaging is a form of dual-modality imaging that utilizes the advantages of both positron emission tomography (PET) and computed tomography (CT).
PET imaging excels at detailing physiologic or biologic phenomena through the administration of positron-emitting radiopharmaceuticals. The biological distribution of the tracer within the body creates physiologic maps of cellular function and/or molecular expression, dependent on the pharmaceutical.
The concurrently acquired CT scan provides anatomical detail with superior spatial resolution to PET. The co-registration of PET and CT imaging data allows for precise anatomical localization of areas with increased PET tracer uptake 1,2.
The typical PET-CT protocol involves the injection of a given radiopharmaceutical followed by a specified uptake/waiting period. The most commonly used radiopharmaceutical is F-18 fluorodeoxyglucose (FDG). There are, however, multiple novel tracers emerging for different clinical indications, e.g. Ga-68 PSMA for prostate cancer or Ga-68 DOTATATE for neuroendocrine tumors. PET and CT scans are performed sequentially with the patient in the same position to allow co-registration of both sets of images. Images can be displayed side-by-side or fused to overlay the PET data on the CT scan 3,4.
Applications
PET-CT imaging has oncological and non-oncological applications:
- oncological applications 5,6:
- baseline staging
- treatment planning, e.g. prior to radiotherapy or surgery
- assessment of treatment response and changes post-therapy
- assessment for disease stability or progression
- differentiation between benign and malignant lesions
- non-oncological applications 5,8,9:
- cardiology, e.g. myocardial ischemia, cardiac sarcoidosis
- neurology, e.g. dementia and other neurodegenerative disorders, epilepsy/seizure
- vasculitis, e.g. giant cell arteritis
- infection
CT attenuation correction
One of the advantages of hybrid PET-CT imaging involves the use of the CT system for attenuation correction which further improves the spatial and contrast resolution of the PET data. This information is derived from estimating the linear attenuation coefficients of each voxel at 511 keV using CT density numbers 1,4.
Artifacts
Incorrect estimation of the linear attenuation coefficients can lead to attenuation correction artifacts in the resultant PET and CT images. Overestimation of attenuation coefficient can lead to falsely elevated representations of radionuclide concentration. This often occurs with metallic objects in the body, e.g. dense contrast material, prostheses, pacemakers, or body piercings 1,7.
Spatial misregistration artifact results from movement artifact between the non-simultaneous acquisition of the CT and PET images, exacerbated by the long scanning times for the PET phase (including respiratory motion). Both PET and CT components are usually obtained during quiet respiration to minimize discordance. In some circumstances, respiratory gating during the PET acquisition can also be performed to minimize these artifacts 1,7. Patients who are unable to lie still for the typical 20-30 minute acquisition (e.g. due to pain, claustrophobia, or altered mental status) are usually unsuitable for PET-CT.
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