PET-CT

  Position emission tomography - computed tomography (better know by its acronym PET-CT) is a medical imaging device which combines in a single gantry system both a Positron Emission Tomography (PET) and an x-ray Computed Tomography, so that images acquired from both devices can be taken at the same time from the patient and combined into a single superposed (co-registered) image. Thus, functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning. Two- and three-dimensional image reconstruction may be rendered as a function of a common software and control system.

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PET-CT has revolutionized many fields of medical diagnosis, by adding precision of anatomic localization to functional imaging, which was previously lacking from pure PET imaging. For example, in oncology, surgical planning, radiation therapy and cancer staging have been changing rapidly under the influence of PET-CT availability, to the extent that many diagnostic imaging procedures and centers have been gradually abandoning conventional PET devices and substituting them by PET-CTs. Although the combined device is considerably more expensive, it has the advantage of providing both functions as stand-alone examinations, being, in fact, two devices in one.

The only other obstacle to a wider dissemination of PET-CT is the difficulty and cost of producing and transporting the radiopharmaceuticals used for PET imaging, which are usually extremely short-lived (for instance, the half life of radioactive fluor₁₈ used to trace glucose metabolism (using fluorodeoxyglucose -- FDG) is two hours only. Its production requires a very expensive synchrotron as well as a production line for the radiopharmaceuticals.

Procedure for FDG imaging

An example of how PET-CT works in the diagnostic work-up of FDG metabolic mapping follows:

• Before the exam, the patient undergoes a minimum of 8-hour fasting and rest;
• In the day of the exam, the patient rests lying for a minimum of 15 min, in order to quiet down muscular activity, which might be interpreted as abnormal metabolism;
• An intravenous bolus injection of a dose of recently produced 2-FDG or 3-FDG is made, usually by a vein in one of the arms. Dosage ranges from 0,1 to 0,2 mCi per kg of body weight;
• After one or two hours, the patient is placed into the PET-CT device, usually lying in a supine position with his/her arms resting at the sides, or brought together above the head, depending on the main region of interest (ROI)
• An automatic bed moves head first into the gantry, first obtaining a topogram, also called a scout view, which is a kind of whole body flat sagital section, obtained with the X-ray tube fixed into the upper position.
• The operator uses the PET-CT computer console to identify the patient and examination, delimit the caudal and rostral limits of the body scan onto the scout view, selects the scanning parameters and starts the image acquisition period, which follows without human intervention;
• The patient is automatically moved head first into the CT gantry, and the x-ray tomogram is acquired;
• Now the patient is automatically moved through the PET gantry, which is mounted in parallel with the CT gantry, and the PET slices are acquired;
• The patient may now leave the device, and the PET-CT software starts reconstructing and aligning the PET and CT images.

A whole body scan, which usually is made from mid-thights to the top of the head, takes about 40 min. FDG imaging protocols acquires slices with a thickness of 2 to 3 mm. Hypermetabolic lesions are shown as false color-coded pixels or voxels onto the gray-value coded CT images. Standard Uptake Values are calculated by the software for each hypermetabolic region detected in the image. It provides a quantification of size of the lesion, since functional imaging does not provide a precise anatomical estimate of its extent. The CT can be used for that, when the lesion is also visualized in its images (this is not always the case when hypermetabolic lesions are not accompanied by anatomical changes).

FDG doses in quantities sufficient to carry out 4-5 examinations are delivered daily, twice or more times per day, by the provider to the diagnostic imaging center.

For uses in stereotactic radiation therapy of cancer, special fiducial marks are placed in the patient's body before acquiring the PET-CT images. The slices thus acquired may be transferred digitally to a linear accelerator which is used to perform precise bombardment of the target areas using high energy photons (radiosurgery).

See also

• Positron emission tomography
• Single photon emission computed tomography
• Neuroimaging
• Imaging in cancer

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