20-09-2012, 01:35 PM
POSITRON EMISSION TOMOGRAPHY-COUMPUTED TOMOGRAPHY (PET-CT)
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ABSTRACT
This tutorial is explaining introduction about how positron emission tomography generates 3D imaging. Imaging procedure of positron emission tomography. The procedure of imaging, history, operation, image reconstruction. Explaining about PET detectors to detect the gamma rays. Limitations of PET scan, advantageous over CT, MRI & X-rays. Different clinical applications such as in lung cancer, breast cancer, prostate cancer, digestive track tumours, some of application in cardiology, & in neurology. PET/CT also being use with combination of CT & MRI scan. Advantageous & disadvantages of pet/ct. Safety and patient preparation before PET scan. Possible risks of PET-CT, limitations & reference.
HISTORY:-
The concept of emission and transmission tomography was introduced by David E. Kuhl, Luke Chapman and Roy Edwards in the late 1950s. One of the factors most responsible for the acceptance of positron imaging was the development of radiopharmaceuticals. In particular, the development of labeled 2-fluorodeoxy-D-glucose (2FDG) by the Brookhaven group under the direction of Al Wolf and Joanna Fowler was a major factor in expanding the scope of PET imaging. The compound was first administered to two normal human volunteers by Abass Alavi in August 1976 at the University of Pennsylvania. Brain images obtained with an ordinary (non-PET) nuclear scanner demonstrated the concentration of FDG in that organ. Later, the substance was used in dedicated positron tomography scanners, to yield the modern procedure. 1.2
INTRODUCTION:-
Positron emission tomography (PET) imaging systems construct 3-D medical images by detecting gamma rays emitted when certain radioactively doped sugars are injected into a patient. Once ingested, these dopes sugars are absorbed by tissues with higher level of activities/metabolism than rest of body. 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 patient. Unlike anatomical imaging techniques like computed tomography (CT), X-ray, and ultrasound, PET imaging provides "functional" information about the human body.
BESIC PRINCIPLE OF PET:
A positron is an antiparticle of an electron with identical mass and charge. After emission, the positron has some kinetic energy, which is lost through multiple collisions with electrons present in the neighbouring tissues. The complete or almost complete loss of energy by the positron results into its combination with electron. This eventually forms a short-lived composition, i.e., positronium. The schematics are shown in Figure 1. The positronium thus created, being short-lived, eventually gets annihilated, converting all its mass into energy and thereby emitting two photons of 511 keV each (which is resting energy of the electron or positron) in opposite direction as depicted in the Figure 1.This ensures conservation of energy and momentum. The unique characteristic of simultaneous emission of two annihilated photons forms the basis for detection and localization of positron emitters using a novel technique called coincidence detection. Scintillation detectors - e.g., bismuth germinate (BGO) or Lutetium Oxyorthosilicate (LSO) - and photomultiplier tubes are placed opposite to the source of positron emitter. The signals are then fed into separate amplifiers and energy discriminating circuits. This process results into detection of a coincidence event, which localizes an annihilation event somewhere along the line joining the two detectors. In a typical PET scanner, there are hundreds of such points of detector banks in the form of ring surrounding the patient.
OPERATION
To conduct the scan, a short-lived radioactive tracer isotope is injected into the living subject (usually into blood circulation). The tracer is chemically incorporated into a biologically active molecule. There is a waiting period while the active molecule becomes concentrated in tissues of interest; then the subject is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour. During the scan a record of tissue concentration is made as the tracer decays.
LOCALISATION OF POSITRON ANNIHILATION EVENT:
The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other; hence, it is possible to localize their source along a straight line of coincidence (also called the line of response, or LOR). In practice, the LOR has a finite width as the emitted photons are not exactly 180 degrees apart. If the resolving time of the detectors is less than 500 picoseconds rather than about 10 nanoseconds, it is possible to localize the event to a segment of a chord, whose length is determined by the detector timing resolution. As the timing resolution improves, the signal (SNR) of the image will improve, requiring fewer events to achieve the same image quality. This technology is not yet common, but it is available on some new systems.
IMAGE RECONSTRUCTION
The raw data collected by a PET scanner are a list of 'coincidence events' representing near-simultaneous detection (typically, within a window of 6 to 12 nanoseconds of each other) of annihilation photons by a pair of detectors. Each coincidence event represents a line in space connecting the two detectors along which the positron emission occurred (i.e., the line of response (LOR)). Modern systems with a higher time resolution (roughly 3 nanoseconds) also use a technique (called "Time-of-flight") where they more precisely decide the difference in time
between the detection of the two photons and can thus localize the point of origin of the annihilation event between the two detectors to within 10 cm. Coincidence events can be grouped into projection images, called sinograms. The sinograms are sorted by the angle of each view and tilt (for 3D images). The sinogram images are analogous to the projections captured by computed tomography (CT) scanners, and can be reconstructed in a similar way. However, the statistics of the data are much worse than those obtained through transmission tomography. A normal PET data set has millions of counts for the whole acquisition, while the CT can reach a few billion counts. As such, PET data suffer from scatter and random events much more dramatically than CT data does.
LIQUID XENON DETECTOR:-
Positron emission tomography (PET) is a functional imaging technique based on detection of gamma rays from positron annihilation. Liquid xenon (LXe) gamma ray detectors are excellent candidates for PET due to copious scintillation and ionization signals which are anti-correlated. Measuring both charge and light leads to an improvement of energy resolution. Combined energy resolutions smaller than 4% FWHM at 662 keV have been reported experimentally. Gamma rays interacting with LXe produce large detectable signals due to the high ionization yield (64e-/keV at high E field) and light output (68 photons/keV at zero E field). The fast scintillation light decay time (2.2ns) allows sub-ns timing resolution. Ionization charge can drift long distances in LXe with little spreading (20μm diffusion for 1 μs drift) allowing sub-mm position resolution. Good energy resolution, sub-ns timing resolution, and 3D sub-mm position resolution result in improved image contrast and spatial resolution, and reduced image artifacts. If a time projection chamber (TPC) is used, the position and energy of the each interaction can be measured and, in case of multiple interactions, the interaction sequence can be reconstructed using Compton event reconstruction algorithms. For high rates, the light signal can be used to roughly localize the position of interaction while the charge is drifting to the anode. LXeTPC detectors have been successfully employed for particle physics, astrophysics, and dark matter detection. Their application to PET, however, remains to be demostrated. In this work we present a novel concept of a PET system for preclinical applications based on LXeTPC detectors. Simulated performance and first results obtained with a sector of the LXe PET system will be discussed.