Nucmed

• There are two main categories of detectors: Gas-filled detectors and Scintillation detectors
• Gas-Filled Detectors: For radioactivity to be detected, it must first interact with matter and release energy
• Gas-Filled Detectors: When radioactivity strikes matter, as with gas molecules inside a detector, the gas ionizes (becomes charged) creating a voltage potential between two electrodes
• Gas-Filled Detectors: This voltage potential is then used as a measure of the radioactivity present
• Three types of Gas-Filled Detectors: Basic Ionization Chamber, Proportional Counters, Geiger-Muller Counter
• Basic Ionization Chamber -- the voltage difference between the electrodes are calibrated to be just high enough to "harvest" all of the ions from the sensitive volume of the chamber, but not high enough that the ions in the chamber are accelerated to the point of creating additional secondary ionizations.
• Basic Ionization Chambers: As a result of this voltage calibration strategy, the current produced in any single event is very small and measurable with any accuracy. Rather, the ionization chamber is used to measure the total current resulting from multiple events over a certain integration time in a given radiation detection system.
• Basic Ionization Chambers: Radiation survey meters such as cutie-pie, some pocket dosimeters and radionuclide dose calibrator are examples of specialized basic ionization.
• Basic Ionization Chambers: The amount of energy converted to electrical current per unit of radioactivity is unique for each radionuclide, and radionuclide dose calibrators must be calibrated for the radionuclide to be measured.
• Proportional Counters: The main difference between a proportional counter and basic ionization chamber is greater applied voltage between electrodes in the former
• Proportional Counters: The higher voltage results in secondary ionization in the sensitive volume of the chamber. The term gas amplification describes this phenomenon.
• Proportional Counters: Gas amplification can result in increased ionization by a factors of 1,000 - 1,000,000
• Proportional Counters: The resulting current pulse is large enough to be measured individually and is proportional to the energy originally deposited in the gas chamber.
• Proportional Counters: The name of the device is based on the proportionality of total ionization to the total energy of the ionizing radiation.
• Proportional Counters: Proportional Chambers do not have wide application in clinical nuclear medicine. They are used in research to detect alpha and beta particles.
• Proportional Counters: One characteristics of proportional counters that makes them particularly useful is their ability to distinguish between alpha and beta radiation
• Geiger-Muller Radiation: The voltage is increased even higher than in the proportional chamber application
• Geiger-Muller Counter: The effect is that nearly all the molecules of the gas are ionized, liberating a large number of electrons. This results in a large electron pulse and detection of single event but not their energy
• Geiger-Muller Counter: The detector may not be capable of responding to a second event if the filling gas has not been restored to its initial condition. Therefore, a quenching agent is added to the filling gas of the Geiger counter to enable the chamber to return to its original condition: subsequent ionizing events can then be detected
• Geiger-Muller Counter: The minimum time between ionizations that can be detected is known as the dead time or resolving time
• The Geiger counter is used for contamination control in nuclear medicine laboratories. They are not particularly useful as dosimeters because they are difficult to calibrate for varying conditions of radiation.
• Survey meter is one of the example of GM counter
• A scintillation detector is a sensitive element used to detect ionizing radiation by observing the emission of light photons induced in a material
• Scintillation detectors is when a light-sensitive is affixed to this material, the flash of light can be changed into small electrical impulses
• Scintillation detectors is the electrical impulses are then amplified so that they may be sorted and counted to determine the amount and nature of radiation striking the scintillating materials.
• Scintillation detectors were used in the development of the first-generation in nuclear medicine scanner, the rectilinear scanner, which was built in 1950
• Modern-Day Gamma Camera : These cameras are scintillation detectors that use a thallium-activated sodium iodide crystal to detect and transform radioactive emissions into light photons.
• Modern-Day Gamma Camera; Through a complex process, these light photons are amplified and their locations are electronically recorded to produce an image that is displayed as hard copy or on computer output systems
• Modern-Day Gamma Camera: Scintillation Cameras with single of multiple crystals are used today
• Modern-Day Gamma Camera: The gamma camera has many components that work together to produce an image. Can be either stationary or mobile
• Mobile gamma cameras are used to perform bedsides studies on patients who cannot be transported to the nuclear medicine department
• The mobile cameras can be moved throughout the hospital, or they may be transported to other sites using a cross-country truck unit.
• Mobile gamma cameras typically have limitations, including a smaller field of view and less detector shielding: thus the types and quality of examination that can be performed are restricted
• Gamma Camera: Collimator, Crystal and Light Pipe; NAI (TL) Crystal, Detector Electronics; Photomultiplier Tube (PMT), Pulse Height Analyzer (PHA), Scaler Timer, Cathode Ray Tube (CRT)
• Gamma Camera Collimator is a shielding device used to limit the angle of entry of radiation
• Gamma Camera Collimator determine a large extent the final image quality obtained from the gamma camera and are therefore one of most important part of the gamma camera
• Types of Collimator: Converging, Diverging, Parallel hole, Pinhole
• Converging Collimator - the holes are not parallel but are angled to converge to a focal point, providing some magnification
• Diverging Collimator - opposite of converging collimator, holes are angled opposite direction in converging collimator which make the image smaller
• Parallel Hole Collimator is the most commonly used collimator. Consists of large number of small holes, separated by the lead septa, which are parallel to each other and usually perpendicular to the crystal