RT212 Fundamental Principles of Exposure

Final digital image quality

is essentially a function of total exposure to the detector

Milliampere-second (mAs)

Digital image receptors can detect a wide range of radiation intensities (wide dynamic range)

Exposure errors can adversely affect the quality of the digital image

Di >+3

Excessive radiation exposure

Repeat only if relevant anatomy is clipped or "burned out"

Require immediate management follow-up

+1 to +3.0

Overexposure

Repeat only if relevant anatomy is clipped or "burned out"

-0.5 to +0.5

Target range

Less than -1.0

Underexposure

Consult radiologist for repeat

Less than -3.0

repeat

total amount of exposure

mas does not affect image brightness, but what does matter is the ______________ incident upon the detector

Kilovoltage-peak (kUp)

It affects the variation in radiation intensities exiting the patient and image contrast

should be lower compared to film-screen radiography

50-60 kVp

infant extremities

65-75 kVp

adult extremities

75-90 kVp

Bucky extremities

85-95 kVp

AP spine

85-100 kVp

Lateral Spine: cervical, Thoracic, Lumbar

110-130 kVp

chest

80-90 kVp

skull

higher kVp

-Smaller signal difference

-Narrower data range

-Display more anatomical data

-Lower dose

reduction in entrance skin exposure, and lower mAs

Higher kVp values are now recommended with digital detectors because ________________________

fixed kVp systems

The optimal kVp is the maximum kVp level that will produce images with appropriate contrast that are consistently within acceptance limits

double or halve mAs for every 5 cm of thickness

rule for fixed kvp systems

Variable kVp systems

The rule is 2 kVp is adjusted per cm of subject thickness, requiring a base of 30 kVp (40-50 kVp can be used) to be used

Fixed kVp systems

are better suited for use with digital image receptor

Collimation

Changes in beam restriction alter the amount of tissue irradiated, affecting the amount of exposure to the image receptor

imaging large anatomic areas

performing examinations without a grid]

high kVp

The effect of collimation is greater when:

Source-to-Image Receptor Distance (SID)

It is the distance between the source of the radiation and the image receptor, and it affects the amount of radiation reaching the patient

Divergence of the x-ray beam and the intensity of the radiation varies at different distances

Object-to-Image Receptor Distance (OID)

It is the distance between the object radiographed and the image receptor

decreased beam intensity

The presence of OID may result in

zero

Optimal recorded detail is achieved when the OID is

Grid

It is a device placed between the anatomic area and the image receptor to absorb scatter radiation exiting the patient

Thicker phosphors

have faster speed and absorbs more radiation

thinner phosphors

have slower speed and absorbs less radiation, producing sharper image quality

Fog

It is produced as a result of scatter radiation reaching the image receptor, and can be visualized on both a digital and a film image

Scatter radiation

can add unwanted exposure to the radiographic image as a result of compton interactions

thick, thin

A ____ part absorbs more radiation, whereas a _____ part transmits more radiation

increased

Maintaining the exposure to the image receptor when imaging thicker part requires an_____ mAs

increases. increases, decreases

As thickness ______ , scatter radiation_______, and radiographic contrast _______

Filtration

It is the removal of low-energy x-rays from the x-ray beam spectrum which does not contribute to image quality

The x-ray beam has increased energy and can increase the amount of scatter radiation reaching the image receptor

Automatic Exposure Control (AEC)

It provides x-rays with consistent receptor exposure, reducing chances of overexposure

It is used to control the amount of radiation reaching the receptors by terminating the length of exposure

ionization chambers

measures receptor exposure

Not necessarily applied in digital imaging but is essential to ALARA compliance and optimum image quality

kVp, mAs

increased collimation, decreased field size, decreased patient dose, decreases scatter radiation

collimation

increased SID, decreased intensity = increased mAs

SID

increased OID, increased magnification, decreased spatial resolution

OID

increased grid, decreased scatter radiation = increased mAs

Grid

increased thickness, increased scatter radiation, decreased contrast

thickness