RT 212 COMPUTED AND DIGITAL RADIOGRAPHY PRINCIPLES OF EXPOSURE

RT212 PREFINALS TOPIC 1

TRUE

TRUE OR FALSE: Final digital image quality is ESSENTIALLY a function of TOTAL EXPOSURE to the DETECTOR

TRUE

TRUE OR FALSE: Traditional rules of technique selection do not necessarily apply in digital imaging

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.

➢ Most manufacturers of digital image receptors suggest a range for the exposure indicator based on the imaging procedure

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

True

TRUE OR FALSE: Most manufacturers of digital image receptors suggest a range for the exposure indicator based on the imaging procedure.

>+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

does not affect

mAs _________ image brightness, but what matter is the total amount of exposure incident upon the detector.

Kilovoltage-peak

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

lower

Use of kVp for computed radiography should be _____ 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

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 of __________

Fixed kVp systems

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

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

maximum level

in Fixed kVp systems, the optimal kVp is the _________ that will produce images with appropriate contrast that are consistently within acceptance limits

5 cm of thickness

in Fixed kVp systems, the rule is to double or halve mAs for every _________

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

2 kVp

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

better suited

Fixed kVp systems are ________ 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.

greater

The effect of collimation is _______ when imaging large anatomic areas, performing examinations without a grid, and using a high kVp.

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

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

decreased beam intensity

The presence of OID may result in __________

0

Optimal recorded detail is achieved when OID is ______

Grid

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

Speed

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

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

Thicker phosphors

have faster speed and absorbs more radiation in computed radiography

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 digital and a film image.

It does not provide information about the anatomic area of interest

Compton interactions

Scatter radiation can add unwanted exposure to the radiographic image as a result of __________

Part Thickness

➢ A thick part absorbs more radiation, whereas thin parts transmits more radiation.

➢ Maintaining the exposure to the image receptor when imaging a thicker part required an increased mAs.

➢ As thickness increases, scatter radiation increases, and radiographic contrast decreases.

absorbs more radiation

thick part

transmits more radiation

thin parts

increased mAs

Maintaining the exposure to the image receptor when imaging a thicker part required an _______

increases

decreases

thickness increases

scatter radiation ______

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

increase the amount of scatter

The x-ray beam has increased energy and can ___________ radiation reaching the image receptor.

Automatic Exposure Control (AEC)

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

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

never be placed

Lead shielding should __________ over any portion of an active ionization chamber

Ionization chambers

________ measure receptor exposure

kVp, mAs

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

field size decrease

patient dose decrease

scatter radiation decrease

If Collimation increases:

field size _____

patient dose _____

scatter radiation _____

intensity decrease = mas increase

If SID increases:

intensity _____ = mas _____

magnification increase

spatial resolution decrease

if OID increases:

magnification _____

spatial resolution _____

scatter radiation decrease = mas increase

if Grid increases:

Scatter radiation _____ = mas _____

scatter radiation increases

radiographic contrast decreases

if Thickness increases:

scatter radiation _____

radiographic contrast _____