The Physics Of PET
The process of producing a PET image places demands on both hardware and software components. Each step of data processing must remain as faithful as possible to the original signal, with the least amplification of noise, highest signal-to-noise ratio and at the same time highest possible resolution. From the moment that an event is registered by a pair of detectors, no more information about that particular annihilation is available. The reconstruction path must be optimally configured to use the information fully and to apply necessary corrections for either physical processes such as scatter and attenuation or to detector artifacts, such as detector efficiency variability. The data path can be understood by examining the following:
- detector configuration
- how an event is registered by the hardware
- how data is stored
- how scatter, attenuation and normalization is applied
- how the projection data is reconstructed to form an image
- how the information about biochemical processes is extracted from the data.
We will illustrate each of these steps, drawing from examples of our own system, the ECAT 953B scanner. Other scanners will vary slightly but the principles are the same.
Once the data is corrected, the sinograms are ready to be reconstructed. The problem at hand is to calculate the original tracer activity in R3 from the projections in R2. The mathematics of this problem have been well known since the early 1900's, when Radon described the famous Radon transform, which was a map, in the case of a function in R2, from R2 to R given by the line integral of the function (two parameters) along some parametrized line (one parameter). More...
The basic unit of a PET detector is a block detector. This is a large piece of crystal, which in the case of the group's scanner is bismuth germenate oxide. The crystal acts as a scintillator, converting high energy photons into visible light. To localize photon entry, the crystal block contains cuts which split it into an 8x8 array of smaller detection crystals. More...
When a decay occurs within the field of view (FOV) of the scanner, two 511 keV photons fly at 180 degrees. Conservation of linear momentum requires that the angle be 180 degrees since the positron and electron were nearly at rest during the annihilation. However, the positron is not exactly at rest when it annihilates, so the angle is slightly off 180 degrees. More...
To understand the basic data structure of a PET scan, it is important to remember that the only information available about the location of the decay event is the two detector coordinates. In other words, whereas we would like an (x,y,z) coordinate of the decay, we can only obtain two of the three degrees of freedom. More...
At this point, the sinogram data is not yet ready for reconstruction. There are several processes that occurred during the scan that must be corrected for, otherwise poor quality images will result. The main corrections that are applied are: normalization, scatter and attenuation. More...
Most of the scanning that the group performs is dynamic, offering the possibility of kinetic analysis. In abnormal states, one not only wants to obtain a qualitative description of tracer activity, but also get a handle on the kinetics involved. Dynamic scans, for example, offer the means of targeting a specific enzymatic reaction and estimating its rate constant. More...