Positron emission tomography (PET) is a noninvasive imaging technology that generates 3D medical images by detecting gamma rays emitted when certain radioactively doped sugars are injected into a human body. A PET scan produces digital pictures that can, in many cases, identify the most common forms of cancer, including lung, breast, colorectal, lymphoma, and melanoma. Technically, PET is a medical imaging technology that images the biology of disorders at the molecular level before anatomical changes are visible. The gamma rays are generated when a positron emitted from the radioactive material collides with an electron in tissue. The resulting collision produces a pair of gamma ray photons that emanate from the collision site in opposite directions and are detected by gamma ray detectors arranged around the human body.
The PET system includes signal detection and processing, coincidence processing, line of response (LOR) memory, and image reconstruction. The detector is
a ring located around the gantry bore, which is comprised of an array of thousands of scintillation crystals and hundreds of photomultiplier tubes (PMTs). The
scintillation crystals convert the gamma radiation into light that is detected and amplified by the PMTs. The PMTs’ current output signal is then converted to
a voltage and amplified by a preamplifier, low noise amplifier (LNA), and then goes to a variable gain amplifier (VGA) to compensate for the variability of the
PMTs. The output of the VGA is passed to two paths, one is the data path and the other is the timing path. In the data path, the VGA’s output is filtered and offset
compensated and then passed through to an analog-to-digital converter (ADC). A field programmable gate array (FPGA) is typically used to process the ADC
output data for energy information. In the timing path, the signals from four or more of physically close channels are summed, and this combined signal
is input to an ultrahigh speed comparator. A digital time stamp is generated using the comparator’s output signal and an ultrahigh speed clock to get the timing
information. The coincidence processor needs to find a matching singles event in an opposite detector block based on the energy and timing information.
This is called the line of response. By analyzing tens of thousands of LORs, the back-end image signal processor can construct and display the collision
activity as a 3D image.