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Learn about photomultiplier tubes, their structure, and how they work. Explore the components and functions of PMTs in detecting radiant energy for PET imaging.
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Lecture 12 • Last Week Summary • Sources of Image Degradation • Quality Control • The Sinogram • Introduction to Image Processing
Photomultiplier tubes • Photomultiplier tubes (or PMTs) are photodetectors which provide extremely high sensitivity and ultra-fast response. A typical PMT consists of a photoemissive cathode (photocathode) followed by focusing electrodes, an electron multiplier and an electron collector (anode) in a vacuum tube.
How the PMT works • When light enters the tube and strikes the photocathode, the photocathode emits photoelectrons into the vacuum. These photoelectrons are then directed by the focusing electrode voltages towards the electron multiplier where electrons are multiplied by the process of secondary emission. The multiplied electrons are collected by the anode as an output signal. Because of secondary-emission multiplication, photomultiplier tubes provide extremely high sensitivity and exceptionally low noise among the photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. A photon striking the photocathode would usually yield the emission of a single electron but the multiplier can create a final output of one million electrons for each electron emitted. This is the gain of the PMT and it is enormous!
PMT Continued • The electron multiplier section consists of nine (or more, some PMTs use up to 19) electrodes called dynodes. Each dynode is charged with about 100 volts more positive charge than the previous dynode in the chain. As electrons are emitted from a previous dynode they are focused to the next dynode by means of this increasing positive voltage. The electrons strike that dynode and are multiplied and the cascade of emitted electrons continues to grow at each dynode. Finally the stream of electrons, which began perhaps as a single electron, is collected by an anode where it appears as an electrical current.
PMT Continued • The entire PMT is powered by a source of about 1000 Volts. The photocathode being the most negative electrode, each dynode is succession is more positive than the last. The potential difference required is easily derived using a resistive voltage divider consisting of a chain of one Megaohm resistors in series. The final dynode has a potential of 1000 volts positive relative to the photocathode. Finally the electron stream is collected by the anode.
Range in mm before annihilation • What is the mean distance traveled before annihilation of the following radionuclides • Rb-82 _______________2.6mm_________________ • N-13 _______________1.4 mm________________ • O-15 _______________1.5mm_________________ • C-11 _______________0.3mm_________________ • F-18 _______________0.2mm_________________
PET Data Acquisition • If 2 photons are simultaneously detected by 2 small detectors, we can infer that the annihilation must have occurred along the line connecting the 2 detectors. This line is referred to as the "line of response," or "LOR." To increase the sensitivity of the scanner, the object is surrounded by a "ring" of small detectors rather than only 2. Such a ring is shown in Figure 1A. To image multiple planes simultaneously, several such rings are placed back-to-back.
PET Data Acquisition • It is therefore convenient to organize the detectors into 2D arrays, called detector "blocks," where the detectors along the x-axis go around the ring and those in the y-direction go axially into the ring. An array of photomultiplier tubes are placed behind the detector block to collect the scintillation light and determine within which detector the event occurred
Quality Control(Performance Measurements) We need to define some basic principles: • Spatial resolution : The ability to differentiate two points in space. • Temporal resolution : The ability to differentiate information in time. • Sensitivity ; Sensitivity is defined as the ability of a detection system to register counts per unit time per dose administered : cts/min/mCi. It can also be measured in counts per second/ unit dose. • Coincidence window : The time during which one coincidence event can be counted. • Scatter : Interaction of radiation with matter, which results in a change of direction of the photon. • Randoms : Two unrelated coincidence events that fall within the coincidence window
Statistical Aspects • Not only does the sampling pose a problem, but the statistical density, or lack thereof (especially in nuclear medicine) poses a “noise problem”. • Noise can be thought of as the small, random fluctuations that appear across the image.
