Image Acquisition kV + distance

4 factors that affect image acquisition

mA

time

kV

distance

kV

kilovoltage

kV affects?

primary = quality

secondary = quantity (bc INC kV = INC speed of e- / penetrating power = more XR photons hit IR, doesn't actually generate more xray photons)

quality synonyms

beam energy, penetrability

quantity synonyms

beam intensity, exposure, amount of xr

how does kV work?

INC kV = INC speed of electrons as they hit anode with more force

hitting anode with more force = higher energy xr photons produced

higher energy beam/quality will cause more photons to penetrate thru patient and hit IR, therefore increasing quantity

INC kV = INC speed of electrons = INC energy = INC quality AND quantity

kV controls?

radiographic contrast

% change to perceive a change in optical density?

need at least a 4% change in kV to perceive a change in optical density

(multiply by 0.04 to find out the amount of kV needed to make a perceivable change)

INC kV = --- image contrast

INC kV = lower contrast (long scale, more grays)

DEC kV = --- image contrast

DEC kV = higher contrast (short scale, less grays)

? allows us to have good images with bad technical factors

digital post processing

prefer over or underexposed images?

overexposed because post processing cannot add more xr photons to an image

long scale

low contrast

more grays

short scale

high contrast

less grays, more black and white

low contrast desired for ?

chest and abdomen exams bc the organs are different densities

high contrast desired for ?

extremities because mostly just skin and bone, not much soft tissue

--- kV, --- mAs better for tube life and pt dose?

HIGH kV

LOW mAs

better for tube life because it uses less heat

reduces patient exposure

15% rule

to double the exposure of an image, increase kV by 15% (same as x2 mAs) MULTIPLY BY 1.15

to half the exposure of an image, decrease kV by 15% (same as 1/2 the mAs) MULTIPLY BY 0.85

when to use 15% rule vs 4% change kV?

15% rule when need drastic exposure change (double or half exposure)

4% rule when image is almost optimal, but need a minor tweak

maintaining exposure

if kV is INC 15%, DEC mAs by half to maintain same exposure

used only when image is diagnostic quality but exposure number is incorrect or kV values increased for reduction in patient exposure

high kV techniques

reduce pt exposure

reduce the xr tube heat production

heat unit equation for more efficient generators (3 phase, 12 pulse)

HU = mAs x time x kV x 1.41

inverse square law

the intensity of radiation is inversely proportional to the square of the distance

inverse square law eq

i1/i2 = d2^2/d1^2

xr in air

roentgen

R or mR

INC distance = --- intensity

INC distance = DEC intensity

DEC distance = --- intensity

DEC distance = INC intensity

direct square law

mAs is directly proportional to the square of the distance

if you increase distance you will increase mAs to maintain same amount of photons hitting the IR

direct square law equation

mAs1/mAs2 = d1^2/d2^2

how to solve for greatest amount of exposure

rank each technical factor from smallest to greatest

add the rank numbers

greatest number = greatest amt of exposure

smallest number = smallest amt of exposure

low mA + high kV = --- HU

low mA + high kV = low HU

high mA + low kV = --- HU

high mA + low kV = high HU