Beam Limiting Devices

scatter

A change in direction of an xray photon after interacting with matter

Primary radiation

radiation emitted from the xray tube

Remnant radiation

unabsorbed and scattered xrays that exit the patient

Imaging forming xrays

photons that interact with the IR

INC scatter = ?

INC scatter = INC density/exposure

INC scatter = ? contrast

INC scatter = DEC contrast

compton scatter vs coherent/classical scatter

compton scatter = ionization takes place, interacts w valence/outer electron shell

coherent/classical scatter = nonionizing

biggest source of scatter for tech

patient

4 Factors that affect the amount of scatter reaching the IR

field size, thickness of tissue or body habitus, tube potential, anatomy

INC field size = ? scatter

INC field size = INC scatter (direct relationship)

INC thickness/body habitus = ? scatter

INC thickness/body habitus = INC scatter (direct relationship)

INC mas or kv = ? scatter

INC mas or kv = INC scatter (direct relationship)

Specific body part itself ? scatter

Specific body part itself (foot versus abdomen) (more dense body parts have more scatter)

beam limitation

same as collimation

beam limitation = ? pt exposure

DEC Patient exposure

beam limitation = ? image quality and how?

beam limitation = INC image quality by reducing scatter

beam limitation = ? visibility of detail

beam limitation = INC visibility of detail

beam limitation = ? image contrast

beam limitation IMPROVES image contrast

types of beam limiting devices

aperture diaphragms (similar to collimator)

cone

automatic collimator

aperture diaphragms

similar to collimator

simplest, lease expensive

set size, not adjustable (piece of cut out lead)

large amount of blur

aperture diaphragms = ? penumbra

aperture diaphragms = INC penumbra

aperture diaphragm dedicated units

dental units, chest rooms

2 types of cones

flared cone

cylinder cone

flared cone

doesnt really restrict beam, matches divergence of beam

flare is usually larger than the beam

cylinder cone

blur is reduced

circular field

excessive radiation

cone disadvantages (2)

image comes out round and IR is rectangular

if the image fills the entire IR, then parts of the patient may be unnecessarily irradiated

variable aperture

this is what we use

• collimator

• beam limiting device

positive beam limiting device PBL or automatic collimator

limits the radiation field size so it cannot be larger than the IR in the bucky

automatic collimator

• allows for infinite number of field sizes

• provides a light field

• scale for field sizes

• adds to ADDED filtration (1 mm Al collimator box)

• upper/entrance shutters control stem radiation

how many number of field sizes are allowed with automatic collimator?

infinite

automatic collimator adds to what type of filtration?

added filtration (1 mm Al from collimator box)

which shutters of automatic collimator controls stem radiation?

upper/entrance shutters controls stem radiation

which shutters of automatic collimator reduce blur?

lower shutters/lead leaf shutters reduce blur

stem radiation

AKA off focus radiation OR extrafocal radiation

xr photons that are not produced at focal spot

• can be produced in tube housing or evaporated tungsten on glass envelope

which shutters do we control of automatic collimator?

lower shutters AKA lead leaf shutters

what does lower shutter of automatic collimator affect?

lower shutters / lead leaf shutter REDUCE blur

what does upper shutter of automatic collimator affect?

controls stem radiation AKA off focus radiation AKA extrafocal radiation