What is Positron emission Tomography (PET) ?
Positron emission tomography & computed tomography (better known by its acronym PET-CT) is a medical imaging device which combines a single gantry system of both a Positron Emission Tomography (PET) and X-Ray Computed Tomography (CT) scanner. The images acquired from this device can be acquired sequentially, in the same session than 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 is precisely aligned or correlated with the anatomic/physical imaging obtained by CT scanning. Two or three-dimensional image reconstruction may be rendered as a function of technologist user software and control systems.
PET-CT has revolutionized many fields of medical diagnosis, by adding precision of anatomic localization to Nuclear Medicine/Molecular 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. Today many diagnostic imaging centers have been gradually abandoning conventional PET devices and substituting them with PET-CT. Although the combined device is more expensive, it has the advantage of providing a unique medical function.
One current obstacle to a wider dissemination of PET-CT is the logistics and cost of producing and transporting the radiopharmaceuticals used for the PET imaging. This is due to the extremely short-lived, the half life of radioactive fluorine18 used to trace glucose metabolism used in fluorodeoxyglucose or FDG is apx two hours. The production requires a very expensive chemical synchrotron as well as a production process of extensive quality control.
Procedure with FDG Radiopharmaceutical
An example of how PET & 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 supine 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. NOTE: An average patient dosage range is 0.1 to 0.2 mCi per kg of body weight.
After one - 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 region of interest (ROI).
An automatic bed moves head first into the gantry, first obtaining a topogram, or "scout" view, which is a kind of whole body flat sagittal 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 from the scout view, selects the scanning parameters and starts the image acquisition period.
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, then the technologist will process the PET-CT data for imaging.
A whole body scan, which usually is made from mid-thigh to the top of the head, takes about 25 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).
For uses in stereotactic radiation therapy of cancer, special fiducial (alignment) 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).