Producing images with PetScan


PetScan examination

The PetScan examination begins with an intravenous FDG injection approximately one hour before imaging. Strict physical rest should be observed by the patient, without talking, thus allowing the tracer to be absorbed by the zones of interest (cf. below the muscle issue). After infusion, the patient empties his/her bladder and a myorelaxing drug may be administered if the patient is stressed.

Image acquisition is performed via 6 to 8 passages through the PetScan scanner tunnel (with captors). Depending on the examination, the acquisition time (patient immobilisation) may vary from 45 minutes to about 2 hours.

In order to increase image quality, the isotopic examination is coupled to an X-Ray scanner which is integrated in modern machines.

PetScanner
Image courtesy of
thePetscanOnline website

Normal imaging

PetScan interpretation is performed according to the intensity of tracer fixation in the concerned organ, using, in particular, 18F glucose. Glucose is an universal energetic substrate.

Certain normal structures are always visualised:

Less intensively, other organs may be observed:

Normal aspect of 18 FDG PetScan

Diagram courtesy of petscanonline.com website

Due to the absorption of gamma 511 kEv photons by nearby tissue, a specific correction is applied in order to improve the location of very small lesions.

Characterising glucose consumpation

The image produced by the PetScan reflects tracer uptake and then glucose consumption. Complex calculation techniques exist to calculate the quantity of glucose required by the tumour in mg/mn/100 g tumour.

Usually, the SUV (or Standardised Uptake Value) is used, considering the patient's weight and the local tissue concentration of FDG. The minimal values for suspecting a tumour lesion are around 2.5 for lung nodules, 4.4 for mediastinal masses, and 5 for diagnosing relapse.

SUV can also be used to evaluate treatment response.

Image reconstitution

With modern computing techniques, images in transverse axial planes (like scanner) can be built, as well as frontal and sagittal planes. Three dimensional views may also be obtained through various mathematical models.

Spectacular (but generally informative) pictures may also be obtained with patients 'rotating' around a sagittal axis.

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