Fundamental Principles of Exposure

Exposure errors

... can adversely affect the quality of the digital image

>+3

DI:

- excessive radiation exposure

- repeat only if relevant anatomy is clipped or "burned out"

- Require immediate management follow-up

+1 to +3.0

DI:

- overexposure

- repeat only if relevant anatomy is clipped or "burned out"

-0.5 to +0.5

DI:

Target range

<-1.0

DI:

- Underexposure

- Consult radiologist for repeat

<-3.0

DI:

Repeat

F

T or F: mAs affects image brightness

Kilovoltage-peak (kVp)

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

lower

Use of kVp for computed radiography should be lower compared to film-screen radiography

50-60

Infant extremities kVp

65-75

Adult extremities kVp

75-90

Bucky extremities kVp

85-95

AP spine kVp

85-100

Cervical kVp

85-100

Thoracic kVp

85-100

Lumbar kVp

110-130

Chest kVp

80-90

Skull kVp

Higher kVp and lower mAs

... kVp and ... mAs are now recommended with digital detectors because of reduction in entrance skin exposure

Fixed kVp systems

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

Fixed kVp systems

The rule is to double or halve mAs for every 5 cm of thickness

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

kVpₙₑᵥᵥ = (2kVp x part) + 30kVp

Formula for new kVp using variable kVp systems

Fixed kVp systems

... systems are better suited for use with digital image receptors

Collimation

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

Source-to-Image Receptor Distance (SID)

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

object-to-image receptor distance (OID)

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

Decreased

The presence of OID may result in ... beam intensity

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

In computed radiography, ... phosphors have faster speed and absorbs more radiation

Thinner

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

Scatter radiation

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

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

Unwanted exposure or fog

... or ... on the image does not provide information about the Anatomic area of interest

Increased mAs

Maintaining the exposure to the image receptor when imaging a thicker part requires an ...

Increases, decreases

As thickness increases, 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

Automatic Exposure Control (AEC)

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

Automatic Exposure Control (AEC)

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

Receptor exposure

Ionization chambers measure ...

Lead shielding

... should never be placed over any portion of an active ionization chamber