Exposure foundations

what are the top 2 imaging goals

1. minimize pt exposure

2. optimize image quality

why minimize pt exposure

harmful effect has NO threshold aka you do not know what exp level will trigger a negative effect

why optimize image quality

crucial for diagnosis

how to achieve min pt exposure and optimized image quality (2)

1. set proper exposure settings

2. proper pt positioning

what are the 3 factors of image quality

1. contrast resolution

2. spatial resolution

3. noise

contrast resolution definition

the ability to render SUBTLE differences in greyscale

helps to distinguish between similar density

how well structures with similar attenuation characteristics can be distinguished from one another depends on?

contrast resolution

spatial resolution definition

ability to delineate adjacent structures

how small of an object can be visualized depends on ?

spatial resolution

if you want to minimize blur then maximize ?

spatial resolution

noise definition

how well pertinent info stands out from background information

strong signal = ____ noise

less

what will degrade contrast resolution and spatial resolution?

noise--> impacts ability to visualize structures, lesions, tubes, lines etc.

what are the 2 categories for noise? Which is an under and which is an over exposure? what will they look like?

1. quantum mottle (due to underexposure, can fix by adjusting mAs) --> grainy or mottled appearance

2. scatter (overexposure specifically bc kVP (energy of photons) is too high, is associated with the Compton effect) --> hazy/foggy or washed out appearence

imaging is a balancing act between ___ and ____

optimizing image quality and minimizing pt exposure

optimizing image quality will involve:

1. appropriate detector ____

2. ____ noise

3. better demonstration of ____ and _____

1. exposure

2. minimize

3. contrast resolution and spatial resolution

if an image is too light it is?

if an image is too dark it is?

underexposed

overexposed

even if you over or under expose why might the resulting image be okay?

processing algorithm can "fix" the image--> this makes it easy to overexpose bc you lose visual cues relative to detector exposure

ON TEST

What are the prime factors for impacting image exposure (4)

why are the prime factors considered prime?

1. mA

2. exposure time (seconds)

3. kVp (kilovoltage peak)

4. SID (source to image distance

prime bc they impact the x-ray beam

which prime factors impacts the x-ray bean by changing the QUANTITY of photons produced and emitted from the tube (3-4)

mA, time, kVp

can also say mAs as that is mAxtime

which prime factor impacts the x-ray by the amount of beam divergence

SID

mA definition

measures electrons in motion

the # of electrons flowing across the x-ray rube per second

what is mA relationship with x-ray quantity

direct and proportional

2mA= 2x the # of electrons

seconds definition

the length of time or how long electrons flow across the x-ray tube

what is time relationship with x-ray quantity

direct and proportional

2time= 2x # of electrons

what is the primary controlling factor of x-ray photons produced

mAs

mAs definiton

the total # of electrons flowing across the x-ray tube

what is mAs relationship to x-ray quantity

direct and proportional

2mAs= 2x the # of electrons

mAs examples

1. 200 mA X 0.05s= ?

2. 200mA x 8ms=?

1. 10mAs

2. 1.6mAs (8ms=0.008 sec)

if you want to minimize motion artifact but keep the same mAs you should?

increase mA and DECREASE TIME

kVp defintion

Electric potential (potential difference) - force driving electrons across x-ray tube

X-ray quality controlling factor

kVp

kVp has what relationship with x-ray quantity

direct but NOT proportional

x-rays are ___energenic

if you increase kVp then you ___ the _____ of photons reaching detector

poly

increase the probability

• 70 kVp - ____ photons, ____ average energy

• 75 kVp - ____ photons, ____ average energy

• 70 kVp - fewer photons, lower average energy

• 75 kVp - more photons, higher average energ

min-max of diagnostic kVp

40-150

Since kVp controls x-ray quality (energy), kVp is set according to the ____

anatomic structure

what controls penetration of x-ray beam

kVp

Using the highest kVp appropriate for an anatomic structure allows for a ___ mAs to be used

lower

Using the highest kVp appropriate for an anatomic structure allows for a lower mAs to be used; result:

lower patient exposure with appropriate detector exposure

if kVp is too high then what image characteristic is affected negatively

contrast--> "blow through" image

what is the 15% rule

To maintain detector exposure:

If kVp increases 15%, halve the mAs

If kVp decreases 15%, double the mAs

mAs x kVp=??+

exposure

2 x mAs = 15% _____ kVp

increase

?? mAs = 15% decrease kVp

1/2

80 kVp @ 10mAs

68kVp (about 15% decrease) @ __ mAs to MAINTAIN exposure

20 (double mAs)

70 kVp @ 12mAs

__ kVp @ 6 mAs to MAINTAIN exposure

80.5 (15% increase)

if kVp is increased by 15% what do you do to mAs?

1/2

increase kVp by 15% has the same impact as doing what to mAs

doubling

does source to image distance change the quantity of photons produced?

NO

what does SID change

Changes quantity of photons per unit area due to beam divergence

think: flashlight

As SID increases, patient exposure ??

decreases (inverse square law); therefore, receptor exposure also decreases

Intensity ∝

1 /D ^2

inverse square law formula

I1/I2 = D2^2/D1^2

Original intensity = 200 µGy Original distance = 40" New distance = 60" New intensity =

I2 = 89 µGy

mAs - Distance Formula

𝑚𝐴𝑠1 /𝑚𝐴𝑠2 = 𝐷1^2 /𝐷2^2

To maintain receptor exposure, as SID changes, ___ needs to change

mAs

An acceptable image is produced using 5 mAs at 40". How much mAs will be needed if the distance is changed to 60"?

11 mAs

Object-to-Image Distance- definition + rule of thumb

increase OID that ? # of photons

Distance from anatomic structure to image detector

Rule of thumb: Minimize OID

if you increase OID then you are decrease # of photons

Collimation definition

if you decrease field size then that ? # photons to detector

AKA: Beam restriction

Reduces beam (field size) to area of interest

decreases

what absorbs scatter?

grids

filtration categories

total- regulatory requirements

compensating- balance detector exposure

2 pt factors

part thickness

part composition

part composition can be affected by

Atomic number & mass density of anatomical part

Bone, soft tissue, air

Pathology - additive & subtractive

even though you are using a manual technique you may not select the ?

focal spot size--> automatically set based on selected mA

in AEC what do the cells do

detect remnent exposure

AEC is __ device

timing

in AEC mAs appears ___ exposure

AFTER

in AEC what is the unknown factor

time--> AEC determines for you

system detects if enough remenet radiation has been reached

increase remnant beam then ___ exposure time

decrease remnant beam?

longer

shorter

what happens to exposure time

1,. increase kVp

2. decrease kVp

3. increase SID

4. decrease SID

5. increase mA

6. decrease mA

1. de

2. in

3. in

4. de

5. de

6. in

what happens to exposure time

1.increase part thickness

2. decrease part thickness

3. additive pathology

4. subtractive pathology

1. in

2. de

3. in

4. de

chest x ray and you accidentally select middle cell over spine not side cells over lungs--> exposure ?

over exposure due to increased exposure time

accidentally select cell off center of target bone and end up imaging half bone half soft tissue--> exposure?

underexposed due to decreased exposure time