Positron Emission Tomography (PET) is a non-invasive technique for imaging inside of the body. It sits alongside other non-invasive imaging techniques such as ultrasound, X-ray imaging and MRIs.
PET scans are set apart from the other non-invasive techniques in that it can be used to measure changes in metabolic processes, and in other physiological activities including blood flow, regional chemical composition, and absorption. This is done by providing a patient with a radiotracer – typically in the form of a radioisotope attached to a drug – which undergoes a specific type of radioactive decay (beta plus decay) that releases a positron inside the body (the P in PET).
The positron will quickly come into contact with an electron, held in an atom of the body, and the pair will annihilate each other emitting (the E in PET) energy in the form of two gamma rays traveling in opposite directions. These gamma rays are then captured by two gamma cameras and used to build a 3D image of the body by sections, which is referred to as tomography (the T in PET).
Whole body PET scans
In October 2023, the National PET Imaging Platform (NPIP) was launched to bring total-body PET to the UK for the first time, with a centre in London and another in Scotland both of which are expected to become operational this year. The whole body scan systems that will be used have been developed by Siemens.
Why are these whole body PET scans better than conventional PET scanning methods? The benefits are highlighted most clearly when we consider the example of parametric PET imaging.
Parametric PET imaging images radiotracer kinetics over time, looking at a particular volume of the patient’s body for a known length of time to measure how the concentration of the radiotracer changes over the measurement time period. Blood input function, which characterizes the concentration of the radiotracer in the blood over time, is a key component in parametric PET.
However, PET scanners have a limited field of view (FOV) that is typically shorter than a patient’s body. It is therefore not possible to capture images of the entire body in the same image capture session. Previous attempts to overcome this drawback have led to the development of continuous bed motion (CBM) PET systems capable of performing whole body PET scans.
A CBM PET system works by moving the bed on which the patient is lying with respect to the PET scanner at a constant rate. As the patient is moved with respect to the PET scanner, images may be captured in the form of “slices” which show a thin cross-section of the patient’s body. These image slices are then pieced together to form a complete image of the patient's body.
The drawback of this method, particularly in the application of parametric PET scans, is that the image slices are all acquired at separate times from the moment the patient was provided with the radiotracer. If this is not accounted for in the processing of the PET scan data, the accuracy of the parametric PET scan is significantly reduced.
A fundamental part of accurately tracking this information is down to the sampling of bed tags, which are coordinate pairs carrying both the position and time information of the bed during the scan. The reason that these tags cannot simply be linked to bed position is that the motion of the bed is not necessarily consistent during the scan, which means that the position of the bed as a function of time cannot be easily calculated.
The accurate coordinates containing both position and time information are then associated with the boundaries of each image slice captured by the scanner and the additional time information is used in the interpretation of the captured image data.
The net result of this development is an improvement in the accuracy of CBM PET scans and opens up a new range of different scanning modalities that can be used for imaging the body.
The field of medical imaging is one of continual innovation and this is just one example of the significant developments occurring in this area. Patent filing data in the technical field of medical imaging and, in particular, in the field of PET imaging, highlights some interesting innovation trends.
The field of spectroscopic radiation diagnosis, which includes PET scanning, has been a source of a considerable amount of patent filings over the last 25 years with a peak in filings occurring around 2011. Also, the field of patient positioning for radiation diagnosis, which includes the CBM systems discussed above, has seen a steadily increasing number of patent filings over the same time period, suggesting that the peak is still yet to come. Whilst it is true that patient positioning techniques are likely to be applicable to a wider range of imaging techniques than just PET scans, it is certainly an area of increasing innovation that is critical to the future of PET scans.
