Advanced Image Acquisition- unit 1- chapter 23 technical considerations in digital imaging

what type of technical factors should be used for digital imaging?

high kVp and low mAs

why should high kVp be used in digital imaging?

high kVp ensures adequate penetrability of the body part

why should low mAs be used in digital imaging?

low mAs minimizes radiation exposure

why should a short exposure time be used?

a short exposure time minimizes motion

what is the total amount of exposure reaching the detector called?

the signal

what happens if mAs is too low?

quantum mottle may be introduced

do most under exposures demonstrate a significant amount of noise?

no most underexposures do not

what does the computer do when a digital image is underexposed?

it attempts to adjust the image to make the anatomy visible

what does visible quantum mottle indicate?

an insufficient amount of x-rays have reached the IR

what does insufficient signal mean?

there is insufficient remnant radiation reaching the image receptor

what happens to quantum mottle when adequate exposure reached the image receptor?

the mottle does not actually go away but it is overwhelmed by the good signal

what does sufficient exposure ensure?

a high signal to noise ratio

what can excessive mAs produce?

detector saturation also called data drop

do most digital overexposures demonstrate saturation?

no

what does detector saturation indicate?

multiple times more exposure than necessary was used

how can detector saturation be compared to buckets filled with water?

like full buckets spilling water into neighboring buckets excessive electrical charge overwhelms the receptors storage capacity and spill across an area of the detector plate

what happens to pixel values during detector saturation?

the readouts reach their maximum values, so there is no longer a distinction between pixels and therefore no image to process

approx how much exposure may be required to cause saturation?

about 8-10 timed the normal exposure, some require more

is detector saturation common?

no

why can most digital overexposures still provide acceptable images?

the computer system can usually compensate for the overexposure so the final image has acceptable image quality

what is used to reference image quality and level of exposure?

the exposure indicator number EI#

are EI calculations the same for every manufacturer?

no

what should be established as a standard care for exposure indicators?

target EI numbers

who often provides target EI numbers?

equipment manufacturer

what are the 2 historically proposed exposure technique systems?

fixed kVp with variable mAs and variable kVp with fixed mAs

which technique system is best suited for digital receptors?

the fixed kVp system

how does fixed kVp system work?

it uses an optimum kVp for various body parts

why does the fixed kVp system use an optimum kVp for each body part?

to ensure proper beam penetration

when using a fixed kVp system, by how much should mAs generally be changed?

in increments of at least 30%

what is the suggested optimal kVp range for infant extremities?

50-60 kVp

what is the suggested optimal kVp range for adult extremities?

65-75 kVp

what is the suggested optimal kVp range for bucky extremities?

75-90 kVp

what is the suggested optimal kVp range for an AP spine?

85-95 kVp

what is the suggested optimal kVp range for a lateral cervical spine?

85-100 kVp

what is the suggest optimal kVp range for a lateral lumbar spine?

85-100 kVp

what is the suggested optimal kVp range for chest?

110-130 kVp

what us the suggested optimal kVp range for skull?

80-90 kVp

how does the variable kVp system work?

it establishes a threshold minimum kVp and increases kVp for each 2 cm change in part thickness

what is the typical threshold for minimum kVp in a variable kVp system?

30 kVp

when using the variable kVp system, when is kVp increased?

for each 2 cm change in part thickness

what is the formally for calculating kVp in the variable kVp system?

30 kVp + (2 kVp x part thickenss in cm) = new kVp

why is digital post processing tolerant of overexposure?

saturation is generally only seen at extreme levels of overexposure

does underexposure of overexposure have less exposure latitude?

underexposure

what is digital image dose creep?

the tendency for exposure techniques to gradually increase because digital systems tolerate overexposure while underexposure can produce quantum mottle

why can dose creep occur?

techs may increase exposure techniques to avoid quantum mottle because overexposure is often compensated for by digital system

According to ASRT best practices, what technical factors should technologists use?

The highest kVp within the optimal range for the position and part, coupled with the lowest mAs needed to provide adequate exposure to the image receptor.

Why is keeping exposure near the Target Exposure Index (EIT) important?

Digital systems have wide exposure latitude, so staying near the EIT helps prevent unknowingly using more radiation than necessary.

Why was the Deviation Index (DI) created?

Because most Exposure Index values are manufacturer-specific, so a universal standard was needed.

Who created the Deviation Index (DI)?

The American Association of Physicists in Medicine (AAPM).

What are DI values based on?

The Target Exposure Index (EIT).

Where can EI and DI values be permanently stored?

In the DICOM header of the image.

When should the Deviation Index be displayed?

Immediately after every exposure.

What is the formula for Deviation Index?

DI = 10 log₁₀(EI/EIT).

What does EI represent in the DI formula?

The Exposure Indicator readout.

What does EIT represent in the DI formula?

The target EI for the ideal exposure level.

How does DI change with approximately a 25% increase in overexposure?

The DI increases by +1.0.

How does DI change with approximately a 20% decrease in exposure?

The DI decreases by -1.0.

Are changes in DI additive or multiplicative?

Multiplicative

what does positive DI indicate?

overexposure

what does negative DI indicate?

underexposure

What is the target DI range?

-0.5 to +0.5

What exposure range does a DI of -0.5 to +0.5 represent?

Approximately -20% to +25% exposure.

What does a DI greater than +3.0 indicate?

Greater than 100% overexposure and excessive patient exposure.

What should be done with an image having a DI greater than +3.0?

Do not repeat unless gross saturation occurred. Management follow-up may be appropriate.

What does a DI of +1.0 to +3.0 indicate?

Approximately 25% to 100% overexposure.

What should be done with an image having a DI between +1.0 and +3.0?

Repeat only if saturation occurred and make efforts to properly expose in the future.

What does a DI of -1.0 to -3.0 indicate?

The image received approximately 50% to 80% of the target exposure and is underexposed.

What should be done with an image having a DI between -1.0 and -3.0?

Consider repeating if appropriate and consult the radiologist regarding image quality.

What does a DI less than -3.0 indicate?

Less than 50% of the target exposure and excessive underexposure.

What should be done with an image having a DI less than -3.0?

Repeat because quantum mottle is certain.

Should an overexposed digital image automatically be repeated?

No. Overexposures should not be repeated unless saturation occurred.

Should an exposure that received less than 80% of the EIT automatically be repeated?

No. It should not be repeated unless a radiologist determines the level of mottle is unacceptable.

At what exposure level should an image be assumed to be repeatable?

When the exposure is less than 50% of the EIT.

Should a dark digital image automatically be repeated?

No. A dark digital image should not be repeated regardless of EI unless saturation occurred.

Why is a DI greater than +3 concerning even if image quality is acceptable?

It is an ALARA violation and indicates excessive patient exposure.

What is automatic electronic masking?

An automatic process that places a black overlay over areas of the image receptor that were not exposed.

What does automatic electronic masking remove?

Nonclinical data from the image file.

Why is automatic electronic masking used?

To eliminate extraneous brightness from the image edge.

What does electronic masking reduce?

veil glare

What is veil glare?

Veil glare occurs when intense light from a light source floods the eye directly, overstimulates photoreceptors in the retina, and reduces contrast perception.

Can electronic masking be performed manually?

yes

When is manual electronic masking useful?

When automatic masking incorrectly identifies the collimated field.

What are other names for manual electronic masking?

cropping or shuttering

Is electronic masking a substitute for proper collimation?

no

How can electronic masking affect older digital systems?

It can impact the accuracy of the EI number.

Why can cropping anatomy from an image be dangerous?

Incidental findings could be missed.

What is the ASRT position on electronic masking?

A digital image should not be cropped or masked so that it eliminates areas of exposure from the image presented for interpretation.

What is electronic annotation also called?

digital annotation

What is electronic annotation used for?

To indicate image acquisition conditions.

What are examples of information added using electronic annotation?

Upright, decubitus, inspiration, expiration, standing, etc.

Can electronic annotation replace lead markers for laterality?

no

What should be used to indicate patient laterality?

lead R or L an atomic side markers

What is the ASRT best-practice recommendation for anatomic side markers?

Consistently use lead anatomic side markers captured on the original image during the x-ray exposure.