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The acquisition of data is performed by very complex hardware in the PET/CT device. The CT and the PET device can be discussed independently.

Figure 1. PET-CT construction (from Siemens demo material)

The CT scanner is usually a so-called third-generation device, an X-ray source radiating in a fan shape, while there is a curved detector system facing it, and this system rotates around the patient at a high speed. Depending on the price and the age of the PET/CT system, the CT can old-type (slower, with lower resolution, etc.) but it can also be a multislice (16-64), high-resolution, modern, fast device. The extent to which the CT image will be used for diagnostic purposes is obviously an important aspect to consider, because the primary task of the CT in the PET/CT device is – as it has been previously mentioned – to make it possible to localize the PET image as compared to the bones and other organs, which does not contain anatomical information. However, the CT image in itself can contain a great deal of information as well.

Figure 2. Philips CT assembling (from Philips home page)


The question arises whether the gamma radiation originating from the radiopharmaceutical in the patient interferes with CT imaging. After performing a few calculations, we can safely say that it does not. The definition of the ‘activity’ of the CT is the ratio of the energy carried by the photons emitted per second to the energy of one photon. Thus the ‘activity’ at the exit of the X-ray photons is 1E14 Bq, while the maximum of the activity injected in the patient (the activity of the PET isotope) is 1E9 Bq. Therefore, there are at least five orders of magnitude between the activities.
The detector system of the PET consists of a great number of (~3000) scintillation crystals (crystal needles). These form a complete circle and detect in coincidence, as it has been discussed in the previous chapters.

Figure 3 Service of PET-CT (from Scanomed Ltd.)

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