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Part 4 Principles of Patient Radiation Protection and ALARA
THIS ARTICLE EXPIRES ON FEBUARY 1, 2007. THE POST TEST MUST BE COMPLETED BEFORE 12:00 A.M. (CENTRAL TIME ZONE) ON FEBRUARY 1, 2007
Author: Nicholas Joseph Jr. R.T.(R), M.S., written on Sunday September 12th 2004 - 10:22 AM Credits: 2.5
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Discuss how the Cardinal Principles of radiation protection and ALARA are used to protect patients from excessive radiation exposure.
State how the primary fluoroscopist can limit exposure time to patients and others during dynamic imaging procedures.
Discuss the FDA’s role in setting standards for manufacturing of x-ray equipment.
Differentiate between primary beam filtration and leakage radiation and state the standards for maximum leakage radiation set by the FDA.
State the minimum filtration requirements set by the FDA and NCRP for primary beam filtration for units operating at various kVp ratings.
Discuss why filtration of the primary beam with greater than 3.0 mm aluminum equivalency is not recommended by the NCRP.
Discuss the use of various types of compensation filters to reduce repeat radiographs to obtain proper density and contrast throughout the radiographic image.
Discuss how the selection of technical factors of mAs and kVp can help reduce patient exposure dose.
State the mAs/intensity formula and discuss its role in reducing patient radiation dose.
Solve simple mAs/intensity problems when 3 of 4 factors are known.
State the relationship of kVp to radiation exposure reduction using the kVp/intensity formula.
List 5 potential fates of a photon when it interacts with the tissues of the patient during imaging.
Discuss how positive beam limitation (PBL) and automatic exposure control (AEC) can reduce excess radiation exposure of the patient.
Discuss how AEC functions to reduce patient exposure.
State the useful range of densities for diagnostic imaging.
List 2 components of AEC apparatus and discuss their locations and functions.
Discuss how to properly adjust AEC controls at the console and discuss the role of the sensors and density settings.
Define minimal response time (MRT) and discuss how it can be a cause of excessive exposure of small pediatric patients.
State the maximum back-up timer and mAs settings permitted by the FDA.
List 3 common types of beam restrictors and state which is the most efficient and which is the most effective when their use is appropriate.
Define light field-radiation field congruency and state the NCRP/FDA standards for beam alignment.
State the NCRP ruling for shielding of the gonads of patients in the childbearing years.
State the proper placement(s) of a lead shield for patients undergoing stationary and C-arm type fluoroscopy procedures.
State the NCRP standards for closeness of the tube to skin distance for single exposure, stationary fluoroscopy, and mobile fluoroscopy imaging.
List 5 standards for fluoroscopy equipment operation that are patient and personnel radiation safety measures.
State the standards for accuracy of x-ray equipment that affect patient exposure dose.
List 3 common measurements of patient dose and discuss their meaning.
Federal regulations and the establishment of the ALARA principle.
Primary beam filtration mainly removes those photons with less than 40 keV.
Beam filtration is measured in HVL which is defined as that quantity of a material that reduces the intensity of the beam by one half.
The production of x-rays is very inefficient, 99% of the kinetic energy of the projectile electron is converted to heat, and <1% converted to x-rays.
According to NCRP regulation leakage radiation from the x-ray tube may not exceed100 mR/hr at 1 meter.
Total filtration requirements of the NCRP:
<50 kVp……….0.5 mm Al
50-70 kVp……..1.0 mm Al
>70 kVp………..2.5 mm Al
Mammography units: 30 m Mo or 60 m Rh.
For pediatric imaging the primary beam is usually filtered with 0.2 mm Cu.
A compensation filter is used to produce even radiographic tones over uneven patient tissue densities.
The relationship between mAs and radiation intensity is direct, that is, if the mAs is doubled the radiation intensity will also double.
The roentgen (R) the unit of radiation exposure.
According to the 50/15 rule a doubling of the mAs is equivalent to a 15% decrease in the kVp; a halving of the mAs is equivalent to a 15% increase in the kVp.
The kVp has a greater effect on radiation intensity than does mAs; a 50%
increase in mAs has less intensity than a 50% increase in kVp.
The intensity of radiation changes by the square of the kVp.
Increasing kVp raises the average energy of photons, decreases P.E. effect, causes more scatter to exit the patient’s body, and causes more transmitted radiation to contribute to image formation.
Occupational exposure to the technologist is from scatter radiation exiting the patient not from the primary beam itself.
The relative % interactions of photons in tissue tells us that as kVp increases, transmitted radiation increases, scatter exiting the patient increases, and the photoelectric effect decreases.
4 main controls the radiographer uses to reduce patient radiation exposure during x-ray examinations: PBL, AEC, beam restrictors, and shielding.
If an x-ray machine has PBL, it must be accurate to within 2% SID.
The useful range of radiographic densities is between 0.25-2.5 O.D. units.
The 2 main parts of an AEC device is the sensor(s) and the comparator.
The 3 types of sensors commonly found in AEC devices: phototimer, ion chamber, or solid state detector.
The sensor is that part of the AEC device that detects radiation exposure; phototimer type is located behind the cassette and is not radiolucent, ion chambers and solid state detectors are radiolucent and are located between the grid and cassette.
The comparator is the component of the AEC apparatus that stores voltage proportional to the sensing of radiation, and then terminates the exposure at the generator when the preset voltage is reached.
Each density step change on the console (-2,-1,0,+1,+2) represents a 25-30% change in the preset voltage and radiographic density.
For units equipped with AEC its sensors must be accurate to within 10% of the preset value for all combinations of mAs.
The maximum single exposure for automatic exposure control by the FDA is 6 seconds or 600 mAs, which ever comes first.
The minimum response time (MRT) is defined as the minimum exposure time allowed by a particular AEC device. If the MRT is longer than the needed mAs, the patient will be over exposed and the radiographic image too dark.
The purpose of a beam restrictor is to limit the area to which the patient is exposed and represents the single most important radiation safety courtesy to the patient.
The most commonly used and most efficient beam restrictor is the collimator. NCRP and FDA guidelines specify that the light field-radiation field congruency is accurate to within 2% of the SID.
The extension cylinder is the most effective type of beam restrictor because it allows the start point for beam divergence to be closer to the patient without decreasing the SID.
All persons of childbearing age, pediatric patients, and pregnant females should be shielded with lead if the primary beam edge is within 10 cm of the gonads without omitting pertinent anatomy.
During fluoroscopy the following patient protections are required by NCRP:
Fluoroscopy timer must be a 5 minute cumulative timer with audible tone.
Exposure switch must be a dead man type in which pressure must be applied to cause exposure.
SID no less than 12” for mobile and 15” for stationary fluoroscopy.
Generators and timers must be periodically checked and maintained.
The patient must be visible to the technologist at all times.
Lead curtain must be at least 0.25 mm lead equivalency.
All persons in the radiographic room during exposure must wear a lead apron of at least 0.5 mm lead equivalency.
Intermittent beam on-off imaging is recommended as well as use of image hold techniques.
Three ways in which patient dose is expressed: entrance skin exposure (ESE), mean marrow dose (MMD), and gonadal dose (GD).
Mean marrow dose is an estimate of the amount of radiation exposure to the active blood forming organs.
Genetically significant dose (GSD) is an estimated dose to the populations gene pool.
NCRP report # 54, Medical Exposure of Pregnant and Potentially Pregnant Women, states that <5 rads of radiation exposure should be considered a negligible risk.
Fetal exposure risk is determined using the exact gestational age and the radiation dose to the fetus.
ALARA was established under federal code 10CFR35.20. and all measures that can be taken to reduce patient dose to as low as is reasonably achievable must be taken.
References
Bushong, S.C., Radiologic Science for Technologist: Physics, Biology, and Protection, 7th ed., St. Louis, Missouri: Mosby, Inc. 2001.
Saia, D.A., Prep Radiography, 2nd ed., New York, NY, Appleton & Lange Reviews/McGraw-Hill, 1999.
Early, P.J., Principles and Practice of Nuclear Medicine, 2nd ed., St. Louis, Missouri, Mosby Co. 1995.
Seeram, E., Computed Tomography, Philadelphia, Pennsylvania, W.B. Saunders Co. **1994.
Selman, J., The Fundamentals of X-ray and Radium Physics, 6th ed. Springfield, Illinois, Thomas Books, 1980.
Sprawls, P., Physical Principles of Medical Imaging, Rockville, Maryland, 1987, Aspen Publishers, Inc.
