exam 1 -- filtration and patient dose

filtration

• process of eliminating undesirable low-energy x-ray photons by insertion of absorbing materials into primary beam

• allows radiographer to shape emission spectrum

  • reduces quantity and increases quality of the primary beam

• low energy photons cannot penetrate the part

  • only contribute to patient dose

filter

any material designed to selectively absorb photons from the x-ray beam

aluminum

• most common and standard filtering material other than glass, oil, copper, tin

• expressed as Al/Eq

hardening of the beam

• removes low energy “soft” photons

• increases average beam energy

soft tissue penetration

requires approximately 30-40 kiloelectronvolt (keV) photons

half-value layer (HVL)

• filtration needed to reduce beam to one half its original intensity

• for the purpose of quality control of x-ray beam

types of filtration

inherent filtration, added filtration, compound filtration, compensation filtration, total filtration

inherent filtration

• glass envelope

• dielectric oil bath (also cools/insulates tube)

• glass window of housing

• about 0.5 to 1 mm Al/Eq

• tube aging increases this type of filtration

  • vaporized tungsten coats tube window

  • HNL testing important

added filtration

• any filtration outside tube and housing

  • collimator

    • typically provides 1 mm Al/Eq due to silver on collimator mirror

  • other

    • additional added aluminum

compound filtration

• K-edge filters

  • two or more materials

  • each layer absorbs characteristic photons created in previous layer

• Tin, copper, aluminum in Thoraeus filter

  • characteristic photons produced by aluminum are 1.5 keV

  • these are absorbed by air between filter and patient

  • thoraeus filter in radiation therapy

compensation filter

• evens radiographic density with parts that have uneven tissue thickness or densities

  • e.g. wedge for foot or T-spine, trough for CXR

  • Al, lead-plastic etc.

total filtration

• inherent + added

• does not take into account any compound or compensating filtration

effect on patient dose

• ideally, filtration would only remove low-energy photons

• some high energy photons are removed

• results in decrease in radiographic density that must be compensated for with increase in technique

• overall, patient dose reduction

exposure

• radiation intensity in air

• measured in Roengtens (R) — traditional unit

  • SI unit: coulomb/kilogram (C/kg)

dose

• amount of radiation absorbed

• measured in rad

entrance skin exposure (ESE)

• patient receives the highest exposure at the entrance of skin

• maximum exposure to body

  • calculated at minimum SOD

• better to overestimate exposure rather than underestimate

• requires a calculation of mR/mAs

  • we measure exposure for patient dose estimation

• varies between x-ray machines and x-ray tubes

diagnostic radiography mR/mAs charts

• tube output in mR divided by mAs at particular SID

• typically calculated at 40’’ SID (100cm)

• varies based upon kVp

  • higher kVp’s produce a higher mR/mAs

how to estimate ESE

• apply inverse square law

  • derive mR₁ from mR/mAs chart

  • derive SOD from SID and object to receptor distance (OID)

• select the correct kVp to identify mR/mAs based upon kVp

fluoroscopic R/min charts

• ESE for fluoroscopic equipment

  • measured in R/min

• FDA limits fluoroscopic exposure rates

  • standard fluoroscopy

    • 11.5 R/min (10cGy/min)

  • high level control fluoroscopy

    • 23 R/min (20cGy/min)

reducing patient dose with communication

• radiographers must appear confident to gain a patient’s confidence

• earning a patient’s confidence results in more cooperation

• demonstrates competence and professionalism

reducing patient dose with positioning

• radiographic projection

  • different projections of body part can yield differing ESE and absorbed dose values

    • AP female pelvis versus PA female pelvis

      • lower ovarian exposure

    • PA skull versus AP skull

      • lower exposure to lens of eye

• immobilization

immobilization

• elimination of movement essential to reducing patient dose

  • reduces retakes due to motion artifacts

• improves image quality and visible spatial resolution

reducing patient dose with technical factors

interrelationship with prime factors: kilovoltage range, milliamperage time, distance, focal spot size, filtration

field size, gonad shielding, subject part density, grids

digital image receptor system, film/screen receptor systems, film processing

interrelationship of prime factors

all factors influence total dose

• (mAs)(kVp)² / d²

kilovoltage range

• increase in kVp without compensation in mAs

  • increase patient dose

• increase in kVp with compensation in mAs

  • decrease patient dose

best kVp and mAs combination if image quality is good

high kVp, low mAs

milliamperage and time

• increase in mAs without compensation from other technical factors

  • increases patient dose

• mAs at lowest level possible will reduce patient dose

distance

• SID of SOD increases

  • results in ESE decrease

• decrease in OID will increase SOD

  • therefore, ESE decreases

filtration

increasing in filtration results in an overall ESE reduction and reduction of patient dose

field size

• decrease in primary beam size

  • decrease in patient dose

gonad shielding

• accurate use of shielding will decrease patient dose

• three major types:

  • flat contact

  • shadow

  • shaped contact

• properly placed gonadal shields significantly reduce patient dose

grids

• higher ratio grid

  • requires increase in mAs

  • increases patient dose

• use the lowest ratio grid necessary to minimize patient dose, without jeopardizing image quality

film/screen image receptor systems

• intensifying screens

  • faster screen speeds reduce dose dramatically

• film

  • increased film speed decreases dose

• film processing

  • when properly maintained, no effect on dose

digital image receptor systems

• respond to 0.01 mR to 100 mR

  • digital systems have a wide dynamic range of exposure response — linear

• extreme exposures not acceptable for image quality

  • produces digital data drop

  • excessive scatter/secondary

• radiographers must be careful not to overexposure image receptor

  • despite ability to correct for this in post processing

discussing radiation risks versus benefit with patients

• minimize patient dose by emphasizing risk

• maximum diagnostic information by emphasizing benefit

• help patients understand dose relative to lifetime risk of daily activities

minimizing patient dose by emphasizing risk

• experienced radiographers can gain stature to permit consultation

• patient advocate for reduction in dose and for additional exams, if needed

• patient’s right to refuse

maximizing diagnostic information by emphasizing benefit

• informed decisions

• comparison of relative radiation risks

factors contributing to compton scatter

• kVp

  • affects the penetrability of beam

• volume of irradiated material

  • field size

  • patient thickness

increased kVp

• affect on interaction:

  • increased transmission

  • decreased photoelectric absorption

  • increased compton scatter

• affect on patient dose:

  • decreased dose

    • decreased photoelectric absorption

    • increase in kVp typically accompanied by reduction in mAs (same exit dose)

• affect on image quality:

  • lower amount of subject contrast (penetration)

  • not as pronounced in digital systems, due to image post-processing

    • image contrast (displayed contrast is controlled by look-up table - applied in imaging processing)

decreased kVp

• affect on interaction:

  • decreased transmission

  • increased photoelectric absorption

  • decreased compton scatter

• affect on patient dose:

  • increased photoelectric absorption

  • decrease in kVp usually accompanied by increase in mAs (kept same exit dose)

    • which increases dose even more

• affect on image quality

  • higher subject contrast

volume of irradiated material

• field size (FS)

• patient thickness

• average tissue density

field size

• increased FS increases volume of tissue irradiated

  • results in increased scatter

• decreased FS decreases beam quantity

  • decreases scatter

  • decreases amount of remnant radiation hitting receptor

  • can increase image noise without mAs compensation

trade off

• by decreasing FS, fewer photons reach image receptor

  • image receptor exposure decreased

  • increase in mAs accompany significant reduction in Fs to maintain image receptor exposure

patient thickness

• thicker body parts produce more scatter

• denser body parts produce more scatter

  • higher electron density present in thicker/denser tissue

  • increased likelihood of interactions occurring, particularly photoelectric

• both of these factors increase number of interactions x-ray beam undergoes as it passes through the body

decreasing patient thickness

• compression devices used to improve spatial resolution and contrast

  • decreases patient thickness

    • thickness becomes more uniform

  • results in lower patient dose

  • brings tissue closer to receptor

    • decreased OID improves spatial resolution

  • routinely used in mammography

  • 
    

control of scatter using beam-restricting devices

• ideally, beam resistors decrease FS to anatomy of interest

  • unnecessary tissue exposure decreases

  • scatter decreases

  • scale of contrast shortens (higher contrast)

  • visibility of details increase

• all good things!!

beam resistors

• aperture diaphragms, cones, and cylinders

  • principal disadvantage of fixed FS

  • rarely used today

• collimators

• ancillary devices

collimator

• modern equipment feature - regulate the field size

• light localizing field light

  • provides indication of midpoint of central ray (CR)

• two sets of shutters that permit infinite number of field sizes

  • length and width of field independently controlled

    • lead shutters

    • permits rectilinear collimation; perpendicular to each other

    • light localizing

collimator and penumbra

• bottom shutters reduce penumbra

  • geometric unsharpness around periphery of image

    • also known as edge unsharpness

  • improves sharpness of recorded image edge

collimator and off-focus radiation

• upper shutters reduce off-focus radiation reaching IR

  • off-focus radiation occurs from areas of x-ray tube other than focal spot areas

  • off-focus radiation produces images beyond exposed field or radiation

    • image shadows

light field

• uses light reflected off mirror to project coverage of x-ray beam

  • proper adjustment of mirror necessary to accurately display location of exposure field

  • light field/x-ray beam coincidence testing should be part of quality control (QC) program

  • needs to be accurate within 2% of SID

features of collimator housing

• central ray must be marked

• some units project location of AEC sensors in light field

• alignment light helps center beam with image receptor

• newer systems will automatically adjust field size with changes in SID

positive beam limitation (PBL) devices

• automatically collimates beam to size of image receptor

• possible to override PBL

  • can reduce beam to smaller field than receptor size

• no longer a federal requirement for x-ray equipment

  • still very popular among technologists

other collimator devices

aperture diaphragm, cones, cylinders

most effective of scatter control: cones

ancillary devices

• lead blockers

  • shields

• lead masks

  • attach to collimator

• check vendor information for digital systems, before using lead blockers or masks