CT Scan and MRI can detect cancers limited to the anatomy. For example, CT scans and MRIs are only able detecting breast cancer, head, liver, etc but unable describe the organs metabolism, with the PET-Scan, anatomical, metabolic aspects, and spread of cancer are also detected. Furthermore PET Scans detection capability include cancer type, degree of malignancy, locations, and ways of propagation of this deadly disease.
PET also can be use to analyze the results of cancer treatment that has been done. After the treatment of cancer through surgery, it is necessary to check whether there was still some residual cancer remains. For this purpose, PET is the most appropriate method, because in these conditions (after sugery) the presence of cancer is difficult to see physically. In addition, PET also can be used to view the progress of cancer treatment with chemotherapy or radiotherapy or other cancer treatment method that may be has been done. Progress of cancer treatment outcome can be known from metabolic changes in addition to physical changes. For this purpose, the combination of PET and CT provides very valuable information to determine the treatment effectiveness level that has been done.
PET Scans Mechanism
Cancer cells have higher metabolic rates than other cells. One cancer cells characteristic is cancer cells require higher levels of glucose for energy. Positron emission tomography build 3D images of glucose metabolism in cancer cells by detecting gamma rays emitted when radioactive glucose is injected into a patient body. Once ingested, the sugar is absorbed by the tissue was processed with a higher level of activity / metabolic (eg, active tumor).
PET-scan begins by giving an injection of FDG (a radionuclide glucose-based) from the syringe into the patient. As FDG travels through the patient's body that emits gamma radiation, the FDG detected by a gamma camera, from which the chemical activity in cells and organs can be seen. Any abnormal chemical activity that may be sign is the indicator of tumor present.
Gamma rays are produced when a positron emitted from radioactive material collides with electrons in the organ. The resulting collision produces a pair gamma ray photons that coming from the collision site in the opposite direction and detected by gamma ray detectors that placed around the patient. PET detector consists thousands scintillation crystals and hundreds photomultiplier tubes (PMTs) arranged in a circular pattern around the patient. Scintillation crystal is used to converts gamma radiation into light and amplified by the PMTs. The low noise amplitudo (LNA) convert the signals from PMTs into voltage and amplitude. PMT signal generated by a signal pulse is slow. The signal strength from each PMT is determined by integrating the signal into pulses. After the LNA, the system uses a variable-gain amplifier (VGA) to compensate for variability in the sensitivity of the PMTs.
The output from VGA passed through a lowpass filter, offset compensation, and then converted into digital signals in 10 to 12-bit analog-to-digital (ADC converter sampling) with 50Msps to assess 100Msps.
Every signals from each PMT will be added to get ultra high speed signal. A DAC generates a voltage reference comparator to compensate for DC offset. Very high accuracy required to produce a comparator output signal with high-speed time. The output signal from DAC is processed into image processing.
From the results of detection, image reconstruction done to get a picture of the distribution of glucose in the body. PET camera device usually comes with the program for this purpose, so that the image reconstruction can be obtained easily.
PET Scans Camera
PET cameras have better image clarity than the gamma camera, This is because the detection is based on coincidence detection. When a positron is released from the fluorine-18, these particles will soon join the annihilation of electrons.. This annihilation radiation from electromagnetic waves generated by 511 V in the opposite direction (180 degree). Two protons released simultaneously makes it possible to do coincidence detection. On coincidence detection, the signal that captured by the detector will be processed if two signals acquired simultaneously. If there is only one signal captured, then the signal is considered as impurities. Therefore, almost all signals of impurities can be eliminated in this way.
PET cameras have better image clarity than the gamma camera, This is because the detection is based on coincidence detection. When a positron is released from the fluorine-18, these particles will soon join the annihilation of electrons.. This annihilation radiation from electromagnetic waves generated by 511 V in the opposite direction (180 degree). Two protons released simultaneously makes it possible to do coincidence detection. On coincidence detection, the signal that captured by the detector will be processed if two signals acquired simultaneously. If there is only one signal captured, then the signal is considered as impurities. Therefore, almost all signals of impurities can be eliminated in this way.
PET Scan Limitations
The biggest limitation of PET studies is limited availability of radioligands. Being developed more than 60 years ago, and immersed to the clinical environment during the past 20 years based only by studies of glucose metabolism, it will take a lot of efforts to simplify and standardize radiopharmaceutical production methods to introduce new radioligands with competitive prices for the needs of PET imaging. In addition, since PET isotopes are short-lived, they have to be processed near the imaging facility.
