Radiation Dosimetry and Survey Instruments

Dosimetry

• Radiation dosimeters are used for dosimetry
• Field survey instruments are Geiger-Mueller survey devices, scintillation detection devices, and ionization chamber instruments
• Personal monitoring devices include optically stimulated luminescence (OSL) dosimeters, film badge dosimeters, thermoluminescent dosimeters (TLD), and pocket dosimeters

Field Survey instruments

• Gas filled detectors ionize gas atoms as radiation passes through
• Electrons released during ionization are detected as an electrical signal proportional to radiation intensity
• Gas filled detectors measure radiation intensity and detect radioactive contamination
• Ion chambers operate in the ionization region
• Portable survey instruments are often ionization (IO) Chambers, primarily being used for radiation surveys
• IO chambers can measure radiation intensities from 10 microgray to several thousand gray per hour
• Precise calibration of the output intensity of diagnostic x-ray imaging systems is done with ionization chamber dosimeters
• Ionization Chamber Dosimeters can be used to evaluate equipment performance, leakage radiation/scatter, and patient exposure
• Chambers enclose a known volume of air
• Chargeable electrodes are positioned inside to attract electrons freed by ionization
• Electrometers measure the difference in electrode charge before and after exposure, displaying results in R or C/kg
• Proportional counters accelerate electrons more rapidly increasing the probability of additional ionization when chamber voltage increases above the ionization region, this is proportional region
• Proportional counters are sensitive instruments used primarily to assay small quantities of radioactivity
• Proportional counters can distinguish between alpha and beta radiation, but are not useful in radiography
• Geiger counters operate in the Geiger-Muller (G-M) region on the voltage response curve
• A cascade of secondary electrons is produced when a single ionizing event occurs if the voltage is high enough
• Geiger counters add a quenching agent to the gas to enable subsequent ionizing events to be detected
• Geiger counters are used in nuclear medicine laboratories for contamination control
• Geiger counters are difficult to calibrate, making them not useful as dosimeters
• Geiger counters are sensitive instruments that can detect and indicate single ionizing events
• Geiger counters are primarily used to detect the presence of radiation rather than exact measurements
• Geiger counters are most effective with particulate radiation and least effective with x-ray or gamma radiation
• Some Geiger counters even have an audio amplifier and speaker for audible crackle of individual ionization
• Most Geiger instruments are limited to less than 1 mGya/hour

Scintillation Detection Devices

• Scintillation detectors use a scintillator with a device that can convey light to an electric signal
• Scintillators emit visible light when exposed to radiation
• The amount of light emitted is proportional to the amount of energy absorbed
• Scintillation detectors most often indicate individual ionizing events and are incorporated into fixed or portable radiation detection devices
• Scintillation crystals are enclosed in aluminum with a polished inner surface in contact with the crystal which allows the light flash to be reflected internally to the one face of a crystal not enclosed when being used as radiation detectors
• Aluminum containment is necessary to seal the crystal hermetically, in turn preventing the crystal from encountering air or moisture
• Many scintillation crystals are hygroscopic and absorb moisture; moisture absorption causes the crystals to swell and crack
• Cracked crystals produce an interface that reflects and attenuates scintillation losing usefulness
• Thallium-activated sodium iodide and thallium-activated cesium iodide are the most widely used scintillation phosphors
• Activator atoms of thallium are impurities grown into the crystal to control the emitted light spectrum and to enhance its intensity
• Both types of crystals have been incorporated into CT imaging system detector arrays
• Scintillation detectors are sensitive devices for X-rays and Gamma rays
• Measuring radiation intensifies is possible with scintillation detectors, even when as low as single photon interactions
• This results in scintillation detectors being used a portable radiation detectors

Personnel Monitoring Devices

• Film badge dosimeters consist of two pieces of film with varying sensitivities to x-rays contained within a light tight envelope which is placed within a holder that contains several filters
• Filters determine the energy of the incident radiation and are based on the absorption of radiation by a specific thickness of a given material depending on the energy of the radiation
• The film emulsion darkens in proportion to the degree of radiation exposure received when exposed to ionizing radiation
• Film badge dosimeters are capable of measuring exposures over approximately 10 mrem to 2000 mrem
• Some materials emit visible light (glow) when heated, known as thermoluminescence
• Thermoluminescent Dosimeter (TLD) is the emission of light by a thermally stimulated crystal following irradiation and contain small chips of thermoluminescent material
• A portion of the absorbed energy is stored in the crystal structure when exposed to radiation
• The absorbed energy in the chips is released as visible light when heated
• The amount of measured light is proportional to the absorbed radiation dose, and the heating and measurement of the chips are carried out in a TLD analyzer
• TLDs measure doses as low as 50 microgray with modest accuracy; accuracy is very precise at doses exceeding 100 milligray
• TLDs are small, which is advantageous
• With irradiation, the energy absorbed by the TLD remains stored until released as visible light by heat during analysis
• TLDs respond proportionally to dose, but does not respond to individual ionizing events, therefore it cannot be used in a rate meter type of instrument
• Optically Stimulated Luminescence Dosimetry is the most common type of personal monitoring device, and measures radiation that passes through a thin strip of aluminum oxide
• A laser light stimulates the aluminum oxide, which becomes luminescent in proportion to the amount of radiation exposure received
• Advantages of OSL over TLD are that it can report doses within a large range from 1 mrem up, has more sensitive than TLD with a minimum reportable dose of 10 microgray OSL, can go under reanalysis to confirm exposure with no loss of information, and has excellent long term environmental (humidity/temperature) stability
• Pocket dosimeters, also known as direct reading conductive fiber electroscopes, and electronic personal dosimeters are now available
• Pocket dosimeters provide immediate personal dosimetry, and these can be seen in interventional radiology suites
• Personal monitoring devices must only be worn by the individual to which they are assigned, worn in the proper location for the prescribed time, and turned in for processing when due
• The department Radiation Safety Officer (RSO) typically monitors personal monitoring devices, and individuals have a personal responsibility to understand their dose and how to manage it professionally
• Film/TLD badges may be adversely affected by heat, humidity, mechanical pressure, inadvertent exposure to light, and prolonged delay between exposure and processing
• Groups of personal dosimeters are provided with a control and are stored in an unexposed area, providing a level against which the personnel dosimeters are evaluated
• HARP (Healing Arts Radiation Protection Act) requires radiation workers to wear radiation monitoring devices typically between the chest and waist or if wearing a lead apron, at the collar
• Individuals should be aware of their exposure by reviewing their exposure readings on the reports posted in the facility on a routine basis
• The individual responsible for the dosimeters will notify any individual who has acquired a dose within a wearing period

CR (Computed Radiography)

• Photostimuable Imaging Receptors are rigid sheets with layers designed to record and enhance transmission of images from a beam of ionizing radiation
• The protective layer of these receptors insulates the imaging plate from handling trauma
• The phosphor layer holds the photostimuable phosphor is the active component
• The conductor layer grounds the plate to eliminate electrostatic issues and absorbs light to increase sharpness
• The support layer is the base
• The light-shield layer prevents light from erasing data or leaking and decreasing spatial resolution
• Barium fluorohalide bromides and barium fluorohalide iodides with europium activators are the most common phosphors used
• Radiation exposure causes fluorescence of the imaging plate while storing some energy
• This stored energy is used to create an image during the reading process so some electrons excited by the absorbed energy are trapped in the crystal structure of the phosphor at higher energy levels
• This composite of trapped electrons is the latent image stored in the plate which is used to create the image

Computed Radiography Reader

• Once the latent image is created, the imaging plate needs to be read to release stored information
• The latent image is processed by loading the plate into an image reader device, which use laser beams to cause the phosphors to emit the stored latent image in the form of light photons
• Light photons are detected by light-sensitive receptors and converted to a unique digital value for that level of luminescence
• Once the plate is read, it is ready to be erased and used again
• Reading the imaging plate involves a finely focused laser beam
• The laser frees trapped electrons, allowing them to return to a lower energy state, also known as photostimulated luminescence
• The laser beam is directed towards the plate
• Light liberated from the imaging plate is emitted in all directions and is collected by an optical system
• Photodetectors convert the visible light into an electrical signal whose output is in analog form
• The analog signal must be converted into a digital signal before the computer can form an image
• This is accomplished with an analog-to-digital converter (ADC)

Analog to Digital Conversion

• To convert the analog signal into correct components for digital manipulation, the analog signal must be sampled to find the location and size of the signal
• The analog signal must be quantified to determine the average value of the signal in the sample
• Quantification determines the brightness levels or greyscale for the pixels
• The analog signal emitted by the photomultiplier tube has an infinite range of values that ADCs must convert into limited discrete values that can be stored as digital
• Pixel Bit Depth (number of bits that represent an analog signal) determines the number of grey levels
• ADCs determine the gray levels for the analog signal, which is the greyscale of the system
• The number of bits/pixels determine how many gray levels can be present, affecting image brightness/contrast
• Pre-processing in CR includes all processing of raw data performed to correct image flaws
• After the plate has been scanned and data has been sent to the computer, it undergoes pre-processing
• Raw image data are prepared according to the proprietary algoriths of the manufacturer
• Pre-processing operations include exposure field recognition, histogram analysis, and greyscale analysis

Greyscale Analysis

• Greyscale analysis algorithms vary depending on the body part and process the data once the exposed area is determined and the signal histogram is established
• CR systems in the past systems required an accurate selection of anatomical menus before processing. If an incorrect anatomical menu is selected, the image may be processed with incorrect brightness or contrast
• Newer CR systems store the raw data in the workstation, allowing images to be re-processed with the incorrect anatomical menu to be re-processed using the correct anatomical menu but cannot be re-processed once sent to PACS
• Exposure Field Recognition is used to recognize the clinically useful area on the imaging plate to manipulate data and eliminate signals from outside the collimated field
• Greyscale Analysis transforms input data values to output values that will be displayed on the monitor using a grey-scale look-up table with an algebraic equation used for the conversion results that are stored to save processing time
• Histogram Analysis Error results from an image not fitting used reference histogram parameters causing incorrect brightness, contrast and exposure number; can occur when abrupt straight lines of tissue density with prosthetic devices create large tissue density differences

Field Recognition Errors

• Field Recognition Errors occur when the system cannot find the edges of the collimated image causing off focus/scatter radiation in the histogram
• These lead to dark, light and low contrast images and inaccurate exposure numbers
• A CR system look-up table is a graph of the process's pixel values derived from the original image receptor pixel values and allows the display to be adjusted from the LUT to enhance a particular portion of the image or pathology such as fractures
• CR readers can be programmed for different scanning patterns (automatic and semi)
• Automatic: adjusts latitude and sensitivity automatically, works well when the correct parameters are used
• Semi: latitude is fixed, and automatically adjusts sensitivity
• Correct kVp must be utilized-Used when an image is centered by collimation border but is not parallel
• Some CR systems used in dedicated areas can have histogram data modified to enhance particular elements, called Histogram Equalization
• Allows the radiologist to view the normal displacement of a chest along with bone enhanced and soft tissue enhancement
• Post-processing in CR uses image display optimization similar to algorothims that apply exposure field recognition, histogram analysis and the application of LUT along with contrast and brightness optimization and detail enhancement before the the final image data set is presented for video display

Flat Panel Detector Technology

• DR has two classifications: portable detectors can be carried around and integral detectors built into the x-ray table or upright bucky
• The earliest versions of DR detectors required an electrical connection between the panel and x-ray system through wires known as tethers
• Tethers have a fixed length limits positioning and reliability
• Current DR panels are able to communicate wirelessly through wifi technology
• DR panel precautions must be undertaken as the panels can cost anywhere from 30,000 to 80,000 dollars
• Some panels have an internal sensor mechanism to measure the amount of force the panel has encountered during use
• DR systems require routine cleaning between patients and should follow manufaturing guidelines
• Current panels are generally sealed adequately against fluid leakage; precautions taken by the MRT to ensure panel is protected with bodily fluids present
• Design for protection includes Radiation protection features and Protective Tube Housing

Radiation Protection Features

• Every x-ray tube must be contatined withihn a protective housing that reduces leakage radiation
• The control panel must indicate the conditions of exposure with x-ray beam-on positively and clearly indicated to the radiologist
• The SID indicator that measures the Source to Image Distance must be accurate to 2% of the indicated SID and can be as simple as a taple measure attached to the tube housing
• Light-localized, variable aperture rectangular collimators should be provided along with equipment capable of aligning the center of the x-ray field with the center of the image reception area to within 2%
• Positive-beam Limitation is adjusted so that with any image receptor size and SIDs, the collimator shutters automatically provide an x-ray beam equal to the image receptor
• Each radiographic tube should be provided with a mechanism to ensure proper alignment of the x-ray beam and the image receptor called Beam Alignment
• Radiation absorbing filters (Filtration) must be used to provide a degree of attenuation
• Radiation Output Reproducibility ensures that, for any given radiographic technique, the output radiation intensity should be constant from one exposure to another, called Linearity
• Operator Shield used so that it is not possible to exposure an image receptor while the radiologic technologist stands unprotected outside a fixed protective barrier ( console booth usually)
• With mobile x-ray imaging Sytems, the exposure switch must allow the tech to remain at least 3m from the xray tube during exposure

Protecting Against Primary and Secondary Radiation

• Great attention must be given to the location of the x-ray imaging system when designing protective barriers
• It is often necessary to include protective barriers (sheets of lead) in the walls of the x-ray rooms, and/or shield the floor if the radiology facility is located on an upper floor
• A medical physicist must be consulted for assistance in the design of proper radiation shielding whenever new x-ray facilities are being designed or old ones renovated
• Types of radation and barriers in radiation shielding include Primary radiation, Secondary radiation, Scatter, and Leakage
• Primary radiation is the most intense and hazardous radiation is the useful beam
• a Primary protective barrier is any wall which the primary beam can be directed at, and lead bonded to drywall or wood paneling is used most often as a primary protective barrier; concrete, concrete block, or brick may be used instead of lead
• Secondary radiation consists of Scatter and Leakage Scatter is radiation that results when the useful beam intercepts any object, causing some X-rays to be compton scattered Leakage radiation emitted from the x-ray tube housing in all directions other than that of the useful beam
• Secondary protection barriers are always thinner than primary barriers; lead is not required, the most often used materials are glass, gypsum board or lead acrylic
• Secondary protection Operating console barriers mean the useful beam is never directed at the operating console booth
• The calculated exposure generally impacts the control booth barrier, not to the radiologic technologist
• Radiologic technologists receive most of their occupational radiation exposure during fluoroscopy
• Barrier thickness depends on the distance between the radiation's source and barrier and distance to the adjacent occupied are of that, not the x-ray room's inside wall
• Safety code 35 shielding calculations depend on the use of the area being protected; if a rarely occupied closet or storeroom, shielding would be less than an office or lab occupied 40h/wk, known as the time of occupancy factor
• Controlled Areas are occupied by radiology personnel and patients in procedure rooms and control booths with design limits are for an occupational dose of 20mSv per year
• Uncontrolled Areas can be occupied by anyone and have a max exposure rate set at a public dose of 1 msv/year
• In uncontrolled areas where radiosensitive populations are present, the 0.30 msv/year standard should be used
• Workload Distribution (W) measures equipment operational time or the amount of use of X-ray equipment, indicating workload across a range of operating voltages
• A workload and workload spectrum can be determined by recording and the operating voltage and current-time product of each irradiation taken in each x-ray suite over some time
• Greater number of patient exams performed each week results in thicker shielding being required
• Occupancy Factor(T) is the fraction of time that the area is occupied by the individual who spends the most time at the location while x-ray equipment is operating
• Use Factor (U) defines the fraction of the workload during which the x-ray beam is pointed in the direction under consideration. For a secondary barrier, leakage and scatter radiation are present 100% of the time that the x-ray tube is energized
• For protective barrier calculations, kVp is used as the measure of energy
• Though most modern x-ray imaging systems are designed to operate at up to 150kVp, standard operation is usually assumed at a kVp greater than that used (100kvp for general radiography, 30 for mammo is more likely that the protective barrier is too thick rather than too thin for general machines)
• Comprehensive shielding calculations for large radiological facilities should only be performed by individuals with current knowledge of structural shielding design and acceptable performing methods
• Code 35 Shielding Calculations is completed when the attenuation of the radiation beam by the patient is neglected and the incidence of the radiation beam is always perpendicular to the evaluated barrier, excluding materials other than shielding
• Secondary protective barriers need to shield against leakage and scattered radiation, so determining the barrier thickness is necessary as they are different

Mammography

• Approximately 1 in 8 women will develop breast cancer
• Breast Screening Programs have started with Ontario at age 40, using Two view MLO and CC, meaning 2 per breast which is 4 images used to detect unsuspected cancer
• Diagnostic Mammography is perfromed on patients with symptoms or elevated risk factors; generally requires two or three views of each breast because cancer is in the upper lateral compartment due to the presence of lymph nodes (50%)
• Risk of radiation exposure is much lower than mammograms benefits, however, the most sensitive breast tissue to cancer induction by radiation is glandular tissue

Mammography Equipment

• Mammography high frequency generators are all used in new units and eliminate voltage regulation problems Generators also allow allows precise control of kVp, mA and exposure time with excellent Linearity and reproducibility of x-ray exposures
• kVp is usually set to 25-28 so the low-energy x-ray beam can produce high radiographic contrast for the radiologist to delineate normal and diseased breast tissue, because a breast has low inherent contrast and is made up of glands, fibers and fat
• A major disadvantage of using x-rays in the kVp in the 20s range result in high absorption of low-energy x-rays in the breast which contributes significantly to patient dose and must be managed with mAs, typically is 20-100mA when using smaller anodes, smaller focal spots and lower power ratings
• Expspure tips are generally 0.4 up to 1 second for standard projections or 2-4 seconds for magnification projections
• Automatic Exposure Control (AEC) intends to provide consistent image receptor exposure for variations in thickness and density which can affect visualization and subtle breast tissue and cancer diagnosis; without AEC, it is difficult for MRTs to determine exacy breast tissue density composition to time out the exposure
• Use of AECs significantly reduced repeat rates for mammo in relation to exposure and tissue density discrepancies

Mammography Unit Design

• Common units contain a microfocus x-ray tube and Beryllium window in line with a molybdenum anode and filtration
• Units use a Right Angle breast compressor and Low ratio, carbon fiber grid and Automatic Exposure Controls (AEC)
• Mammography tubes make use of a pronounced anode heel effect due to the short SID (Source to Image Distance) and Tube configuration to best position the cathode toward the chest wall (thickest part) and anode towards the nipple (thinner)
• Mammography tube Cathodes contain standard helical-shaped tungsten filaments in a focusing cup; a single filament wire is often used for large and small focal spots but requires applying a negative voltage is to the focusing cup to reduce the size of the electron stream when the small focal spot is engaged
• The high spatial resolution that is typically required calls for substantially smaller focal spot sizes and short SID of 60-76cm decreasing the geometric unsharpess which is managed by a micro-sized focal spot
• Small Focal Spot: 0.1mm for magnification imaging and Large Focal Spot: 0.33 mm for routine imaging

Anode Configuration

• mammography uses Rotating anodes
• Rotating anodes allow enables the use of higher mA which allows exposure time to be under 1 second
• Target angle for tube effectiveness is kept in a range of 22-24 degrees, important for covering the image receptor at SIDs of 60-65cm
• Reference axis target angle smaller and varying from 7.5-12 degrees, is specified in mammography with a beam measured from the center of the x-ray beam because it defines the size of the center focal spot, and should be narrow to allos for greater anode heat capacity
• Utilizing a narrow angle to maintain x-ray coverage on the IR requires x-ray tubes to be tilted and the anode edge to be designed with two distinct surface
• 2 surface configuration: radiation coverage of the film (large focal spot), ability to produce small effective focal spot, placement effective focal spot closer to chest wall
• most Anodes are Molybdenum (solid or graphite backing) however Molydenum has a high melting point, conducts heat well, and require xray energies between 17 and 25keV to differentiate inherent, low contrast breast tissues