RAD370 - DIG. IMAGE. CH2 POWERPOINT

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Digital images

recorded as multiple numeric values and are divided into an array of small elements that can be processed in many different ways

Pixel (picture element)

determined by the matrix size and the FOV

FOV / Matrix Size

Pixel Size formula

Large Image Matrix

increases Spatial Resolution

quantization

takes electrical signals and turns them into digital bits of information.

• Occurs during ADC

• displayed as specific brightness levels on image

bit depth

total number of possible brightness levels that can be assigned to any individual pixel in an image

Pixel bit depth

• The amount of information (bits) within the pixel

• represented by 2^{bit depth}

Increasing collimation

Decreases FOV

Matrix size can be changed without affecting FOV

FOV can be changed without affecting Matrix size

Exposure Index (EI)

• amount of exposure received by IR, not patient

• depends on manufacturer

• AAPM report - standardization will give technologist more confidence in adjusting technical factors while following ALARA

AAPM

American Association of Physicist in America

CR uses

S# or Sensitivity number

• range 100 - 400

DR uses

EI# or Exposure Indicator

• range 100 - 450

• Low S number

• High EI number

overexposure

• High S number

• Low EI number

underexposure

KTSD

Standardization radiation exposure

• general

KIND

Indicated equivalent air kerma

• specific to exposure

KTGT

Target equivalent air kerma value

• optimal exposure for a specific body part and view

actual exposure

KIND

target exposure

KTGT

DI

deviation index

perfect image has DI of

0.0

Brightness

• how dark or light digital image appears on display monitor

• DR brightness of image determined by pixel values

Contrast resolution

ability or the digital system to display subtle changes in the shade of gray

• related to bit depth of pixels in image

Windowing

ability to alter brightness and contrast of digital image (post process)

Window Level

Adjusts brightness and type of tissue to be imaged

Window Width

adjust image contrast and the gray-scale representation of the tissue

High window level

high brightness (light image)

Wider Window Width

low contrast/long scale (more gray shades)

Spatial Resolution

ability of imaging system to demonstrate small details of an object

• sharpness of structural lines in a digital image

• determined by ratio of FOV + Matrix Size

Dynamic Range

The range of exposures that may be captured by a detector

• describes number of gray shades that an imaging system can reproduce

• Ability to respond to varying levels of exposure

• greater ______ will yield greater contrast resolution

• digital imaging has a wide _________

Dynamic Range

MTF

Modulator Transfer Function

“The sum of the components in a recording system cannot be greater than the system as a whole”

Modulator Transfer Function (MTF)

• ability of a system to record available spatial frequencies

• best measurement of spatial resolution

DQE

Detective Quantum Efficiency

Detective Quantum Efficiency

How efficient a system converts a input signal into a useful output image

• DR has the highest

A perfect imaging system would have a MTF of

1

SNR

Signal to Noise Ratio

Signal to Noise Ratio

the average image signal in a given region divided by the noise around that region

SNR Formula

Signal / Noise

Exposure Latitude

The range of exposures which produces quality images

• extent which radiographs can be overexposed or underexposed and still achieve acceptable result

The range of exposure diagnostic image values the image detector is able to produce

• digital imaging has a wide ______

Exposure Latitude

Advantage of having a wide exposure latitude

less repeats

Disadvantage of a wide exposure latitude

Dose Creep (over exposure to patient)

Optimal EI range

100 - 400

Q) The difference between the actual exposure and the target exposure

Q) DI

Q) Measures the efficiency of a system to convert the x-ray input signal into a useful output image

Q) DQE