Instruments for Radiation Detection & Measurement

1. Instruments for Radiation Detection & Measurement PRINCE SATTAM BIN ABDUL AZIZ UNIVERSITY COLLEGE OF PHARMACY Nuclear Pharmacy (PHT 433 ) • Dr. ShahidJamil L13-L14

2. Instruments for Radiation Detection & Measurement L13-L14

3. Instruments for Radiation Detection and Measurement Since we cannot see, smell or taste radiation, thus, in nuclear medicine, it is necessary to ascertain the presence, type, intensity, and energy of radiations emitted by radionuclides, and these are accomplished by radiation-detecting instruments. The two commonly used devices are gas-filled detectors and scintillation detectors L13-L14

4. Detectors The function of the detector is to convert radiation energy into an electrical signal. There are two basic mechanisms for converting this energy: excitation and ionization. In ionization, an electron is stripped from an atom and the electron and resulting ion are electrically charged. These charged particles can be influenced by an electric field to induce a current that can be measured directly or converted into a voltage pulse. 3He neutron detectors, Geiger Mueller, and other gas proportional detectors are examples of ionization detectors. L13-L14

5. In excitation, electrons are excited to a higher energy level and when the vacant electron is filled, electromagnetic radiation is emitted. Scintillation detectors such as NaI, BGO, CsI, Polyvinyl toluene (PVT) plastic scintillator and the neutron sensitive glass fibers are examples of scintillation detectors. L13-L14

6. Gas-Filled Detectors The most common type of instrument is a gas filled radiation detector. This instrument works on the principle that as radiation passes through air or a specific gas, ionization of the molecules in the air occur. L13-L14

7. When a high voltage is placed between two areas of the gas filled space, the positive ions will be attracted to the negative side of the detector (the cathode) and the free electrons will travel to the positive side (the anode). These charges are collected by the anode and cathode which then form a very small current in the wires going to the detector. L13-L14

8. By placing a very sensitive current measuring device between the wires from the cathode and anode, the small current measured and displayed as a signal. The more radiation which enters the chamber, the more current displayed by the instrument. L13-L14

9. The two most commonly used gas-filled detectors are ionization chambers and Geiger-Muller (GM) counters. The difference between the two devices is the operating voltage that is applied between the two electrodes. Ionization chambers are operated at 50 to 300 V Examples are "Cutie-Pie" counters and dose calibrators, which are used for measuring high intensity radiation sources, such as output from x-ray machines (Cutie-Pie) and activity of radiopharmaceuticals (dose calibrators)., The GM counters are operated at around 1000 V. and used for measuring low level beta and gamma radiations. L13-L14

10. Dose calibrator : The dose calibrator is one of the most essential instruments in nuclear medicine for measuring the activity of radionuclides for formulating and dis-pensing radiopharmaceuticals. It is a cylindrically shaped, sealed chamber with a central well and is filled with argon and traces of halogen at high pressure. Its operating voltage is about 150 V. A typical dose. calibrator is shown in Figure L13-L14

11. Geiger-Muller Counters The GM counters are used for the measurement of exposure delivered by a radiation source The GM counter is one of the most sensitive detectors. One end of the detector is made of a thin mica window that allows passage of beta particles and low energy gamma radiations that would otherwise be stopped by the metal cover provided for detection of gamma radiations. L13-L14

12. The readings are given in microroentgen (µR) per hour, milliroentgen (mR) per hour, roentgen (R) per hour, or counts per minute (cpm). Some GM counters are equipped with audible alarms or flashing light alarms that are triggered by radiations above a preset intensity, It is called area monitor and used for area survey for contamination with low level activity. L13-L14

13. Scintillation Detectors The second most common type of radiation detecting instrument is the scintillation detector. The basic principle is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is sodium-iodide. L13-L14

14. The light produced from the scintillation process is reflected through a clear window where it interacts with a photomultiplier tube. The first part of the photomultiplier tube is made of a special material (a photocathode(. L13-L14

15. The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates called dynodes through the application of a positive high voltage. When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. L13-L14

16. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger than it was at the beginning of the tube. At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed by a special instrument. Scintillation detectors are very sensitive radiation instruments and are used for special environmental surveys and as laboratory instruments. L13-L14

17. The practice of Nuclear Medicine includes both diagnostic and therapeutic techniques. Most of the procedures are related to organ imaging using internally distributed radioactive material. There are also diagnostic techniques which quantitatively measure physiologic function (such as thyroid radioiodine uptake or gastric emptying time). Therapeutic procedures that done using radioactive material are Nuclear Medicine Therapy and Radiotherapy (Radiation Oncology(. L13-L14

18. In Nuclear Medicine, The radioactive material is introduced into the body so that it becomes incorporated into some body physiology and cannot be retrieved. In the specialty of Radiotherapy (Radiation Oncology), The radioactive material is used in an encapsulated form which can be retrieved from the body. L13-L14

19. The most common procedures in Nuclear Medicine are organ imaging procedures. The difference between nuclear imaging and radiographic images is that: Radiographsare produced by transmitting an x-ray beam through the patient and recording the shadow. In nuclear imaging, the photon source is the patient so the beam is radiating outward from the site in which the radiopharmaceutical is localized. L13-L14

20. The standard detector system for Nuclear Medicine is a scintillation camera. The scintillation camera uses a sodium iodine crystal to convert the energy of the gamma photon into a flash of light that is detected by a photo-multiplier tube collection which views the side of the scintillating crystal away from the patient. L13-L14

21. There is a collimator between the patient and the scintillating crystal so that the image will record only primary photons moving directly from the organ to the crystal. (A collimator is a focusing equipment with hole that limit the field of view and prevent radiations from outside the field of view to reach the detector). L13-L14

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23. A basic limitation of the scintillation camera is that they show only images in two-dimensional displays. Images of structures in the third dimension, depth, are obscured by the underlying and overlying structures. To solve this problem, obtain images at different angles around the patient such as anterior, posterior, lateral, and sloping projections. However, success of the technique is limited because of the complexity of structures surrounding the organ of interest. L13-L14

24. Tomographic Imagers Tomographic scanners have been devised to delineate the depth of the object of imaging. The common Tomographic technique is computed tomography, which is based on the rigorous mathematical algorithms, to reconstruct the images at distinct focal palnes (slices). Illustrations of four tomographic slices of the heart are shown in Figure. L13-L14

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26. The initial approach to the tomography technique was to focus the image of the object at the plane of interest and at the same time to shadow the images of the objects not in plane. This is called the focal (important) plane method or longitudinal tomography. In nuclear medicine, two types of computed tomography are employed based on the type of radionuclides used: single photon emission computed tomography (SPECT), which uses ƴ-emitting radionuclides such as (99mTc, 123 I, 67Ga) and so forth, and positron emission tomography ( PET), which uses β emiting 11C,13N,15O,18F,68Ga,82Rb. L13-L14

27. Normal Abnormal Tomographic Imagers L13-L14