how does it work?
Introduction
A PET/CT scanner consists of a PET scanner combined with a CT scanner and their separate images are 'fused' together. A typical scanner has the CT modality first followed by the PET section. CT complements PET by providing accurate physical localization of a tumour in relation to surrounding tissue, thereby maximising the information available from the cancer. The CT scan also provides what is called 'attenuation correction' to provide valuable information about exactly how much glucose uptake a tumour has for example.
Typically a patient will have been asked to fast for a number of hours prior to arrival at the centre. He or she is typically injected with 4.5MBq/kg of a glucose based radiopharmaceutical about an hour before scanning and then is asked to void their bladder just prior to imaging. This powerpoint slide show shows the general principle of PET CT scanning.
More technical descriptions of the PET technology:
- What is a positron?
- Influence of imaging positrons in the body
- PET detector principles
- Modes of operation for PET scanning
what is a positron?
As mentioned before PET is an acronym for 'Positron Emission Tomography'. However we don't actually measure or image positrons, we image what happens to the positrons in the body when they combine with nearby electrons to create two back to back 511keV photons.
The positron itself was first discovered by Professor Carl Anderson.
He performed 1500 cloud chamber photographs of cosmic rays and observed a particle that he could not explain in 15 cases! By applying a vertical magnetic field across his cloud chamber, which was super saturated with water vapour, he could deflect charged particles and observe their tracks. He introduced a lead plate and saw the track of positrons as they passed through. From this he deduced that he had in fact discovered the positron which was the opposite of the electron... the first anti-matter particle!
Professor Carl Anderson was awarded the Nobel Prize in 1936 for his great achievement. Due to his pioneering work, and others of a similar calibre, we are able to perform PET scans today to visualize cancers with great sensitivity and help save lifes by early detection and monitoring disease during treatments.
The downloadable powerpoint slide show illustrates his technique for discovering the positron.
Positrons in the body
The influence of positrons on PET imaging within the body are highlighted by this powerpoint slide show
If a patient has been injected with FDG then there is a good likelihood that some enhanced uptake will occur at a cancer cell. A positron may be emitted from this site by 18F as it decays. This positron will have a possible range of energies and will travel a short distance, typically 0.2mm on average for 18F isotope before combining with an electron. The shorter the distance travelled the greater the accuracy of imaging and for 18F this is not a problem. Due to the conservation of energy and momentum the mass of the electron and its anti-matter partner the positron will be converted to 2 rays of energy called photons, emitted at 180 degrees apart, each with an energy of 511keV. Any of you who have watched 'Star Trek' will be familiar with this concept of annihilation!
It is these two 511keV photons that are detected and contribute towards making an image in PET scanners. As many tumours are avid for FDG uptake they are relatively easy to visualize.
PET detector principles
PET scanning works because a positron that is emitted from a cancer cell will combine with an electron nearby and annihilate to create two 511keV photons that are 180 degrees apart. Once these two back to back 511keV photons have been emitted they can pass through the body and then interact in two opposing PET camera detectors and be recorded in order to contribute towards the PET image. The detectors in PET tend to be arranged in rings around the patient to increase sensitivity. This powerpoint slide show illustrates how PET scanner detection occurs.
The beauty of PET scanning is that the two 511keV photons must be collected within a small time window, of the order of nano seconds! This is called 'coincidence counting' and this form of 'electronic collimation' makes PET such a sensitive process.
PET mode of data acquisition
There are 2 routine clinical ways of scanning with PET systems, namely by operating the scanners in 2D or 3D mode. As mentioned before PET relies upon 'electronic collimation' and the detection of two 511keV photons within a small coincidence time window, of the order of nano seconds.
However in addition to this we can also use physical collimation between the rings of detectors and the patient. By doing this we reduce what is called 'scattered radiation' that degrades image quality. An image of the collimators being retracted and revealing the detectors below is shown.
The drawback to using physical collimators is the reduction in sensitivity as one rejects many pairs of 511keV photons as a result. However the remaining signals lead to high quality images when acquiring data in 2D. In 3D imaging the physical collimators are fully retracted leaving all possible 511keV pair photon tracks but more processing is required to make use of good signals.
Summary
The great advantage of 3D scanning is the potential for reduced injected activity and scanning times in comparison with 2D imaging whilst still maintaining the same image quality. However PET systems must cope with dead time issues and additional noise from scatter and randoms that degrade image quality. There is still great debate as to the pros and cons of each type of PET acquisition mode.
2D Imaging
Here is an image showing this effect. The green blocks represent the opposing pairs of scintillation crystals in the PET camera and the short black horizontal lines represent physical collimators which reject scattered radiation emitted from the body. The red horizontal lines represent the actual paths of the two 511 keV photons recorded in opposite pairs of detectors. The graph to the right shows that we get a uniform response across the detectors to emitted radiation, which is to be desired. Of course we reject much signal in 2D.
3D Imaging
Compared with 2D imaging this mode greatly increases the sensitivity of the PET scanner (>5 times typical). It is noticed that the detector response in 3D is not uniform but more 'peaked' (orange graph to the right). For adequate sampling this requires that more bed positions need to be scanned in 3D compared with 2D for the same body sections. In 2D acquisition mode we may scan a patient in 5 or 6 bed positions as they pass through the PET scanner but in 3D mode it may require 7 or 8 beds for example.
However now the PET system must be able to cope with the additional scattered radiation that degrades image quality. In addition the scanner must be able to cope with the extra signals being emitted from the patient so dead time (time required to process information) must be managed properly so the system is not paralysed. Furthermore the increased signal received will also contribute to a form of noise called 'randoms' which increases as the square of the signal on opposing detectors. So if we get twice the number of counts recorded from each detector this will lead to 4 times the noise level in images.
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