The Prime Factors

Prime factors

Affect x-ray emission and are under DIRECT CONTROL OF RADIOGRAPHER

milliamperage-second (mAs)

kilovoltage (kV)

distance (d)

quantitative factors

-milliamperage-second (primary quantitative)

-kilovoltage

-distance (further away, radiation spreads out more)

-filtration (more stuff in between)

Qualitative factors

how strong it is

Kilovoltage

Filtration

mA

quantity of electrical current flowing through a circuit

how is mA described as

the rate representing the number of electrons passing down a wire per second

mA control of machine

selection which taps different series of resistors in filament circuit to control the intensity of the flow of electricity

when selecting higher mA

greater flow rate of electricity

more electrons in shorter amount of time

Intensity=

quantity

the intensity rate of x-ray beam is

directly controlled by the mA sstation set at console

Number of x-rays created is a product of

number of electrons crossing tube and amount of time electrons are allowed to cross (mAs)

change in mA or mAs

affects the number of electrons

BUT NOT the kinetic energy of electrons flowing from cathode to anode

mAs

product of mA and exposure time (s)

if either mA station or set exposure time is doubled

the total radiation exposure is doubled to both the detector and the patient

mA and exposure time are

inversely proportional

if mA is doubled

time is halved

mAs=

mAs

formula for mAs

mA1xs1=mA2xs2

mAs is the primary controller of

quantity/intensity of x-ray photons (directly proportional)

in film, mAs was

density (directly proportional)

mAs and exposure relationship

direct

if mAs goes up, detector exposure/exposure index goes up

Insufficient mAs produces

underexposed image

quantum mottle

excessive mAs produces

overexposed image

saturation

harder to discern in digital systems but will provide inappropriate EI numbers

kilovoltage (keV or kV)

measure of the electrical force or pressure behind a current of electricity, which causes it to flow

-measure of electrical energy

-greater the potential difference, the more pressur exerted

Due to AC electricity

kV is measured in terms of average value/peak value

(Kilovolt peak KVP)

X-ray beams with higher average energy

Capable of penetrating through different types of tissue

Most important function of kVp

to provide at least partial penetration through all tissues to be recorded

kVp is primary controller of

total quality of x-ray photons

kVp in film

contrast

kVp impacts this as well indirectly

quantity of photons due to more interactions by incident electrons in target material

UNDESIRED SIDE AFFECT

kVp is primary controller of differences in radiographic densities/IR exposures, known as

image contrast (brightness)

As we increase kVp

Increased penetrability, which will result in image with less contrast (think of an abdomen)

15% rule

an increase in kVp by 15% will cause a doubling in exposure'

a decrease by 15% will cause a halving of exposure

If maintaining a particular exposure is desired, but more penetration is needed

increase kVp by 15%

cut mAs in half

kVp adjustments should NOT

be used to control radiographic exposure

Do not oversimplify 15% rule to

10-kVp rule, stating that with every 10 kVp, changes in exposure occur by factor of 2

since kVp controls both quality and quantity of x-ray photons

there will be some increase in patient exposure

using 15% rule to increase kVp increasess

patient exposure by 1/3 (28-38% avg 33%)

however if using 15% rule and cutting mAs in half

then exposure is offset by this 50% cut in quantity of photonss

Using 15% kvp rule averages a

net savings in patient dose of about 1/3

net result for apply 15% rule is exposure averaging about 67% of the original

distance

x-ray photons from a point source begin to spread out or diverge with distance

-causes decrease in intensity

x-ray beam is

divergent

x-ray intensity is measured with a dosimeter

previously recorded in roentgens (R or mR)

now recorded in milliGray (mGy)

more distance

decrease in intensity

As SID is increased and collimators are closed further to maintain field size

Further cutting photons and decreasing intensity

Inverse square law

As distance increases, beam intensity decreases

Amount of exposure received is less

The intensity of radiation at a given distance from the point source iss

inversely proportional to the square of the distance

Inverse square law concerning distance and intensity formula

I1/I2=D2squared/D1squared

I1= original intensity (mR)

I2= new intensity (mR)

D1= original distance

D2= new distance

The concentration of radiation will be

inversely proportional to the square of the distance (at twice the distance, the radiation will be 1/2squared or 1/4 as intense)

Doubling the distance

allows the x-ray beam to spread out over a fourfold area, reducing exposure intensity to one quarter the original

isotropically- evenly all the way around

mAs and detector exposure relationship to distance

As distance increase

Intensity decreases

Decreases IR exposure

Direct Square Law

formula for mAs, detector exposure relationship, and distance

direct square law

-to compensate for SID changes

-determines amount of mAs necessary to provide enough photonss to create an image after SID changes

Direct relationship is necessary to

compensate for the changes in intensity and IR exposure

Direct square law is also known as

exposure maintenance formula

and density maintenance formula (for film)

Exposure maintenance formula

mAs1/mAs2=D1^2/D2^2

or

mAs2= mAs1 x D2^2/ D1^2

Exposure maintenance dissected

Direct square law also applies to each individual component of mAs- ma or s separately

mA1/mA2= D1^2/D2^2

T1/T2= D1^2/D2^2

Increasing SID

-reduces patient dose

-increases spatial resolution

-reduces magnification

-because of reduced magnification of anatomy within field, more of body part can be included with projection

Technique changes mobile radiography application

with digital imaging systems

the relationship of kvp being contrast and mAs being density has been decoupled

brightness and contrast are now controlled primarily through

post-processing

milliamperage

measures the rate of electricity flowing through the X-ray tube, and controls the x-rays emitted from it

mA stations at console

select from diffrent resistors to control amount of amperage flowing through the filament to maintain a steady space charge boiled off by thermionic emission

mAs overall

controls the totaly amount of x-rays delivered from x-ray tube during exposure

mAs is preferred controlling factor for

total exposure/intensity

overexposure from excessive mAs

is not apparent in digital images, and can only be monitored by checking the expossure indicator readout

mAs is not considered a factor in controlling

contrast

mAs has no direct relationship with recognizability factors such as

sharpness, magnificaation, distortion

however, shorter exposure times make unsharpness due to motion less likely to occur

the predominance of different tissues within a body part

determine minimum kVp that should be used

no amount of radiation intensity can ever compensate for

insufficient penetration of x-ray beam

a 15% change in kVp

alters intensity of radiation reaching detector by a factor of 2

optimum kVp

level well above the minimum needed for sufficient penetration, which strikes a balance between saving patient exposure and preventing excessive scatter

kvp has no direct impact on the geometrical aspects of image

sharpnesss, magnification, distortion

increasing SID

reduced exposure intensity at detector by inverse square of distance, because x-rays spread out isotropically

when should technique be adjusted to compensate SID

any change greater than 15%

adjustment for radiographic technique for changes in SID follows

direct square law

rule of thumb for distance changes from 40'' to 72''

adjust technique by 3x

increased SID

if increase in SSD

because source to skin distance changes by a greater ratio than SID

increased SID can be used to reduce patient skin dose