2721 Equipment Design for Radiation Protection

Radiation Safety Features of Radiographic Equipment, Devices, and Accessories

• All necessary measures to ensure that radiographic equipment operates safely to ensure radiation protection for

  • Patients

  • All personnel

• Every diagnostic imaging system must have a

  • Protective tube housing

  • Correctly functioning control panel

• A radiographic exam table and other devices and accessories must be designed to reduce patient radiation dose

Diagnostic Protective Tube Housing

• Must protect the patient and personnel from leakage radiation by restricting the emission of x-rays to the area of the primary beam

• Must be lead lined so that the radiation leakage through the portion of the housing away from the useful beam, measured at 1 m (3 ft) does not exceed 1mGy an/hr (100 mR/hr) when the tube is operated at its highest voltage at the highest current that allows for continuous operation

• The housing also serves as a shield against electric shock from high voltage entering the x-ray tube and facilitates cooling of the tube

Control Panel

• Where you select technical factors; mA, mAs, kVp are selected

• Must be located behind a suitable barrier that has a radiation absorbent window which protects the RT and allows viewing of patient

• Must indicate the conditions of exposure and when the tube is energized

• X-ray control panels could be touch screen; they provide audible tone and visible light when exposure begins/ends

Examination Table

• Must be strong enough to support pts whose weight exceeds 300 lbs

• Must be uniform in thickness (may be curved or flat)

• Must be a floating table top

• May have x-ray from underneath (fluoro)

• Must be as radiolucent as possible

• Carbon fiber materials used

SID Indicator

• Equipment must have indicator that will accurately measure the distance from the anode focal spot to the image receptor

• Examples: Tape measure, lasers, digital readout

• Must be accurate to within 2% of the indicated SID

• Distance and centering indicators must be accurate within 2% and 1% of the SID distance

X-Ray Beam Limitation Devices

• Collimation;

  • Reduces the quantity of tissue exposed to radiation

  • Reduce dose to patient

  • Reduces the amount of scatter production

  • Improves the overall quality of the image

Light Localizing Variable Aperture Rectangular Collimators

• Most versatile, used most often, most accurate collimator

• Close to exit window of x-ray tube, permanently mounted

• Contains

  • 1st set (upper) shutters: Mounted as close as possible to the tube window to reduce off focus or stem radiation (rad emitted from parts of tube other than focal spot)

  • 2nd set (lower) shutters: Located below level of light source and mirror; consists of two lead plates perpendicular to each other; further restricts beam to area of interest

  • A light source to illuminate the x-ray field and permit centering

  • A mirror to deflect the light beam toward the patient

Skin Sparing

Skin surface should be at least 15 cm below the collimator

Luminance (brightness of a surface)

• Quantifies the intensity of a light source

• The localizing light beam should adequately outline the margins of the radiographic beam on the area of interest on all patients

• Primary unit is the candela per square meter (nit)

• Taken at 40” a light meter reading will be at least 15 foot candles

• A foot candle is about 10.76 candela per square meter (nit)

Coincidence-Between Radiographic Beam and Localizing Light Beam

• When using a rectangular collimator, good coincidence between radiation beam and localizing light beam is essential to eliminate collimator cut off of the body structures being irradiated

• Alignment and congruence: (Respectively) The sum of the cross table and along the table alignment differences and the sum of the length and width dimensions of the radiographic and light beams must correspond to within 2% of SID

• *Coincide: correspond in position; meet or intersect

Positive Beam Limitation

• Adjusted to the selected IR size. Border should be visible on all four sides when field is limited to clinical areas of interest; PBL had override button/key

• Radiographer must ensure basic principle of radiation protection: collimating the radiographic beam so that it is no larger than the IR

• 2% of SID with PBL (some states 3%)

Aperture Diaphragm

• Simplest Beam Limitation Device

• Flat piece of lead with a shaped hold cut in its center

• Dimensions of the hold determine size and shape of beam

• Openings can be rectangular, square or round

• Placed directly below the window of the x-ray tube

Cones

• Mostly used for examination of specific areas

  • Head Sinuses Vertebral column

  • Circular metal tubes that attach to x-ray tube housing or variable rectangular collimator

• Limits the x-ray to a predetermined shape and size

• Construction is simple

• Primarily the lower rim of cone that governs beam limitation

Total Filtration

• Reduces exposures to patient skin

• Absorbs most of the lower energy photons which hardens the beam

• Therefore, reducing the overall intensity of the beam

• Two types make up “Total Filtration”

Inherent Filtration

• Glass envelope, insulating oil, and the glass window 0.5 mm thickness of aluminum

• The light localizing variable aperture rectangular collimator provides an additional 1mm aluminum equivalent

Added Filtration

• Consists of sheets of aluminum or equivalent

• Located outside the glass window of the tube housing, above the collimator shutters

• With Image Intensified Fluoro - 3.0 mm AL is preferred (patient dose decreased by 1/4)

• Patient dose is decreased by 25% during fluoro when Al filtration is increased from 1 mm to 3mm

Requirement of Total Filtrations

• 2.5 mm aluminum equivalent for fixed x-ray units operating above 70 kVp is the regulatory standard

• Manufacturers only need to add a 1.0 mm Al eq filter between the tube housing and the collimator in order to meet the requirement

Stationary (Fixed) Radiographic Equipment Tube Potential

• Minimum Total Filtration Required

  • Above 70 kVp - 2.5 mm Al Eq

  • 50-70 kVp - 1.5 mm Al Eq

  • Below 50 - 0.5. mm Al Eq

Mobile Diagnostic Units and Fluoro Equipment

Mobile diagnostic units and fluoro equipment require a minimum of 2.5 mm Al Eq total permanent filtration

Filtration for General Diagnostic Radiology

• Aluminum is the metal most widely selected as a filter material because it effectively removes low energy (soft) x-rays from a polyenergetic (heterogenous) x-ray beam without severely decreasing the x-ray beam intensity

• A diagnostic x ray beam must always ben adequately filtered

• Half value layer (HVL) of the beam must be measured to verity this. A radiologic physicist should obtain this measurement at least once a year and also after and x ray tube is replaced or repairs have been made of the diagnostic x ray tube housing or collimation

• HVL is the thickness of an absorber required to decrease the intensity of the primary beam to 50% of its initial value

• HVL is expressed in millimeters of aluminum

• HVL is a measure of beam quality or effective energy of the x ray beam; a certain minimal HVL is required at a given kVp

Compensating Filters

• May be constructed of aluminum, lead acrylic or other suitable material

• Partially attenuated x rays that are directed toward thinner or less dense areas

• Provides uniform density of part

• Ex: Trough (used in dedicated chest units; think in the center thick laterally) or wedge (Used in foot xrays; thickest part towards toes)

Exposure Reproducibility

• mA reproducibility of +/- 5%

  • If any or all technique factors are changed and then returned to previous value, precisely the same radiation exposure should occur of the exposure should be reproducible

Exposure Linearity

• mA linearity +/- 10% between adjacent stations and +/- 10% overall

• Ability of unit to produce constant radiation output for multiple combinations of mA and exposure time

• The ratio of the difference in mR/mAs values between two successive generator setting to the sum of the mR/mAs values. It must be less than 0.1

• Inspected annually or when a significant change or repair in operating console or high voltage generator components

kVp Accuracy

Must be consistent within +/- 4% (inspected annually or when a significant change in high voltage generator components)

Grids

• Construction: Made of parallel radiopaque strips alternated with low attenuation strips of aluminum

  • Removes scattered x-ray photons that emerge from the patient before they reach the IR

  • Used when part is greater than 10cm thick

• Technical factor requirements

  • Bucky factor: Required increase in exposure

  • Dose increases as a grid is used

  • Pt dose also increases as grid ratio increases

Mobile Radiography

• Patient Protection

  • Same design and recommendations as Fixed Rad. Equipment

  • Use gonadal shielding

  • Vacate area during exposure

  • Announce when exposure is made

  • Provide lead aprons for others

  • Source-to-Skin Distance (SSD) for patient

    • Must be at least 12 in. (30 cm); Should not be < 15 in.

  • Use low ratio grids

  • Use collimation

  • Use high kVp

  • Avoid repeats

Radiation Safety Features of Digital Imaging Equipment, devices and accessories

• Digital Imaging

  • Use of the computer in almost all imaging modalities:

• Conventional radiography: Analog image

  • Process of producing the conventional radiographic image

  • Disadvantages to the use of this technology?

    • Waiting time for processing

    • Storage

    • Manually retrieving radiographs was time consuming

    • Human error with film jackets and being misfiled or misplaced

• Digital radiography (DR)

  • Process of producing a digital radiographic image:

  • Components of the digital image

  • Resolution (detail) of the digital radiographic image

  • Composition and function of digital radiography image receptors

  • Function of charge-coupled devices (CCDs)

  • Access of DR images

Retake Rates in Digital Radiography

• DR eliminates the need for almost all retakes required because of improper technical selection because image contrast and overall brightness may be manipulated after image acquisition.

• Retake rates for mis-positioning will not be affected.

• Mispositioning retakes should be monitored by an independent quality control technologist at a separate monitor, or a quality control system should be used, whereby the number of images per examination is compared with the number ordered for each technologist.

Fluoroscopy

• Dynamic, active motion for select anatomic structures that produces a real-time image of those structures on a television monitor that works with an imaging intensification system.

• Fluoroscopic Imaging Systems

  • X-ray tube under table and image intensifier and spot film system mounted on a linked system over the patient (C-arm like)

  • Tube above the table: image receptor beneath table

  • Remote control system; tech can fluoro from behind window

• Benefits of Modern Image Intensification

  • Fluoroscopy overall brightness is increased~ 10,000 times the brightness of the discontinued non-image intensifier system

  • Time saver for radiologist-allows them to use normal light as opposed to having to sit in a dark room for 30 mins to adjust their eyes before preforming any fluoro procedure

  • Reduces pt dose because it requires less mA (1.5-2 mA)

Exposure Rate Limitation

• Limits on entrance skin exposure

• Intensity should not exceed 20 mGy/min for each mA of operation at 80 kVp

• Image intensified total limit of Intensity - limited the table top exposure rate of fluoro equipment for routine procedures to 100 mGy/min (10 R/min) when no HLC (high level control) was present. If HLC is present, the max TT intensity allowed is 200 mGy/min

High Dose level controls

• Allows unlimited exposures

• Commonly called HLC (High Level Control) fluoroscopy

• Operating exposure rates are higher

• Provides for visualization of smaller and lower contrast objects

• Used for interventional procedures

• Requires continuous, positive pressure manual operation, continuous audible signal (reminds everyone HLC is in use)

Pulsed/Intermittent Fluoroscopy

• Allow pulsed fluoro (short burst) rather than continuous exposure; patient dose is decreased

• Digital form of fluoro

• Last image hold: stores the image from the last time fluoro button was depressed

Exposure Control

Exposure Switch should be the dead-man-type (if the operator drops dead, the exposure will terminate unless he or she falls on the switch); this applies to the foot pedal or pressure switch on the image intensified tower

Exposure Factors

↑ kVp, ↓ mA reduces pt dose

Control Grids

• Patient dose ↑ when fluoro grid is used.

• Focuses electron beam into the output fluorescent screen by external coils

Bucky-slot Cover

The opening should automatically be covered with at least .25 mm lead equivalent; must be part of the equipment.

Magnification (electron optical magnification)

During Mag. Fluoro. (9 in to 6 in. mode) , mA automatically increases and patient dose ↑ and personnel dose ↑.

Beam Restriction

• Collimate to area of interest, it reduces the integral dose (product of dose & volume of tissue

• An unexposed border must be visible on the TV monitor when the input phosphor of the Image Intensifier is positioned 35 cm above the TT and the collimator is fully open. For automatic collimation, this unexposed border should be visible at all heights above the TT.

Source-to-Skin Distance

• Fixed - Must not be < 15 in. (38 cm)

• Mobile - Must not be < 12 in. (30 cm); Should not be < 15 in. (38 cm)

Half-Value Layer

• The HVL must be measured annually and performed at initial installation by physicist

• For Image Intensified Fluoro. a HVL of 3 - 4 mm Al is considered acceptable at kVp ranges of 80 - 100

Cumulative Timer

• Purpose: To make the radiologist aware of the length of the fluoroscopy time

• The audible/visual (or both) signal indicates a predetermined period of fluoro. beam-on time not to exceed 5 mins. Activated by exposure switch

• The audible/visual (or both) signal should last at least 15 sec. at which time the timer must be manually reset.

Primary Protective Barrier (Direct Beam Absorber)

• Permanently built into unit

• The Image Intensifier Assembly (input phosphor) acts as a primary protective barrier (see lead equivalency below). It must be coupled with the x-ray tube and interlocked so that the fluoroscopic x-ray tube cannot be energized when the image intensifier is in the parked position. The barrier must ensure that the exposure dose does not exceed 2 mrad/hr at 10 cm from the image receptor. The exposure must terminate when the primary protective barrier is removed.

• Lead Equivalency: 1.5 mm - Up to 100 kVp

• Lead equivalency: 1.8 mm - > 100 kVp

• Lead Equivalency: *2.0 mm - Up to 125 kVp (required for image intensifier unit)

C-Arm Fluoroscopy (Mobile)

• 0.5 mm lead Equivalent aprons must be worn by RT

• Minimal Source-to-end-of-Collimator-Distance of 12 inches (30 cm)

• Patient-to-I I-Distance (PID) should be as short as possible

Digital Fluoroscopy

• Decreases patient dose

• Beam-on time must be limited

• Intermittent fluoro

• Collimation

• Pulse progressive System is used-Pulse of rad creates entire image on output phosphor (where electrons interact and produce light)

• Last Image hold

Potential Radiation Responses of Skin from High Dose Fluoro: Early Transient Erythema

• Approx Threshold Dose (rad)

  • 200 (2Gy)

• Approx Time of Onset

  • Hours

Potential Radiation Responses of Skin from High Dose Fluoro: Main Eythema

• Approx Threshold Dose (rad)

  • 600 (6Gy)

• Approx Time of Onset

  • 10 Days

Potential Radiation Responses of Skin from High Dose Fluoro: Temporary Epilation

• Approx Threshold Dose (rad)

  • 300 (3Gy)

• Approx Time of Onset

  • 3 weeks

Potential Radiation Responses of Skin from High Dose Fluoro: Permanent Epilation

• Approx Threshold Dose (rad)

  • 700 (7Gy)

• Approx Time of Onset

  • 3 weeks

Potential Radiation Responses of Skin from High Dose Fluoro: Moist Desquamation

• Approx Threshold Dose (rad)

  • 1500 (15 Gy)

• Approx Time of Onset

  • 4 weeks

Dose area product (DAP)

The quantity that reflects not only the patient radiation dose but also the amount of tissue irradiated.

DAP increases with increased radiation field size

Smaller size = lower DAP

DAP is usually positioned just after the xray beam collimator before the beam enters the patient

The DAP detector is a transmission ionization chamber, parallel plate type, larger than the largest area xray beam

Using DAP to monitor radiation intensity is a good way to implement radiation management procedures and keep patient radiation dose low.