Mitigation of External Radiation Exposures

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Madeline McGee
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1 Mitigation of External Radiation Exposures The three (3) major principles to assist with maintaining doses ALARA are :- 1) Time Minimizing the time of exposure directly reduces radiation dose.

2 2) Distance Doubling the distance between your body and the radiation source will divide the radiation exposure by a factor of 4.

3 3) Shielding Using absorber materials such as Plexiglas for beta particles and lead for X-rays and gamma rays is an effective way to reduce radiation exposures.

4 ALARA Investigation Levels are: External Radiation Exposures There are two types of ALARA investigation levels for external occupational radiation exposure as indicated by a dosimeter. A- Quarterly Investigation Levels (3 months) Based on 2.5 % of any applicable occupational limit B- Annual Investigation Levels (12 months) Based on 10 % of any applicable occupational limit and is related to an individual worker s year-to-date cumulative dose.

5 Instructions for worker exceeds an ALARA investigation dose level 1. If a radiation worker s dosimetry indicates that an investigation level has been exceeded, 2. a notification is sent to the worker and their doses are closely monitored for the remainder of the calendar year.

6 Procedures to Minimize Doses to Patients 1. The operator shall use the minimum exposure time consistent with obtaining a good quality film. This includes using the fastest film 2. Unnecessary repeat films shall be avoided by using a proper technique and reliable and consistent processing. 3. Films holders and bitewing films should be used to save the patient having to hold the film 4. Particular care should be taken with women who may be or are pregnant. A protective apron shall be provided to protect the fetus. 5. Clinical records shall include details of all X-ray examinations carried out.

7 Protection of the Public The direct beam shall not be directed through doors or windows or wooden floors behind which persons may be situated. Normally, only small levels of scattered radiation are detectable on the inner walls of dental surgeries. There is therefore no need for heavy structural shielding to protect from scattered radiation

8 Chapter 4 Positron Emission Tomography (PET and PET/CT) Imaging

9 Positron emission tomography (PET) has been in existence since the 1970s due in large part to the pioneering work of Michael Phelps, Michel Ter-Pogossian, PET imaging can be obtained in 1. Two-dimensional (2D) 2. Three dimensional (3D).

10 PET is based on the physical properties of certain radioactive isotopes known as positron emitters. these radionuclides emit positrons rather than gamma photons when they undergo radioactive decay. Positron decay is a type of beta decay in which a positively charged particle, known as a beta+ particle (denoted β+), is emitted from a proton-rich nucleus as that nucleus attempts to become more stable.

11 β+ particle are positively charged electron. A negatively charged beta particle, sometimes called A negatron, or β particle, β particles is identical to an electron except that its origin is the nucleus rather than the electron cloud surrounding the nucleus. β particles do not play a role in PET.

12 Unlike conventional nuclear medicine imaging with gamma-emitting radionuclides, The photons imaged in PET do not directly come from the nuclei that are undergoing decay. Nor are the positrons being imaged. Because positrons are particles that carry a positive charge, they travel only a very short distance, usually no more than a millimeter or two, before encountering a negatively charged electron. When a positron and electron collide, the particles are annihilated, and according to the conservation of matter and energy,

13 the annihilation of the electron and positron results in The creation of two high energy gamma photons that travel approximately180 degrees from one another. These high energy annihilation photons are not detected efficiently by a conventional gamma camera, and a specialized ring of detectors is used. Their simultaneous detection using short timing intervals is called coincident detection.

14 Photon pairs that do not arrive at opposite points along the PET detector ring at the same time (within a few nanoseconds) are ignored by the PET scanner. This action is called discrimination and helps improve localization of true coincident events. The energy of each coincident photon produced by the annihilation reaction is approximately 511keV, which is much greater than the less energetic 140 kev photons emitted by 99mTc.

Radionuclide Imaging MII Positron Emission Tomography (PET)

Radionuclide Imaging MII Positron Emission Tomography (PET) Radionuclide Imaging MII 3073 Positron Emission Tomography (PET) Positron (β + ) emission Positron is an electron with positive charge. Positron-emitting radionuclides are most commonly produced in cyclotron