Digital Image Processing

Types of Digital Radiolography System

1) Computer Radiography (CR)

2) Digital Radiography (DR)

Computed Radiography (CR) (Basic info)

• Used photostimulable imaging plates (Pst, IP), inside cassette which is then procedded

Digital Radiography (DR) (types)

• Direct conversions: No scientillator, ONLY photodector

• Indirect Conversions: uses CR and DR equipment. Includes a charged-coupled device or amorphous silicon with a film transistor.

Digital Formation formation

Electronic devices that produce radiographic images use analog language. However computers use binary language and would therefore need to be processed over to binary using:

ADC

ADC (Analog-To-Digital) (Steps)

1) Sampling

2) Quantification

Sample (ADC)

• First step

• Analog values measureed at a chosen frequency on an analog wave form

Quanification (ADC)

• Second step

• Analog values assigned a discrete analog value

Computer memory/power expressed in total bytes

• Megabytes

• Gigabytes

• Terabytes

• etc.

Matrix

• Made up of pixels and voxels

• Field of view (FOV)

• Spatial resolution is dependent of this (size)(greater the matrix size, the better the spatial resolution)

Pixel

• Picture element

• Inversely related to spatial resolution

• Sample frequency (pixel/mm)

• Has bit depth

Pixel dependent on

• Matrix size

• Image receptor

• Size

Bit depth

• How much gray scale is it capable of showing

• Greater bit depth = increases the size of the data set/volume of data for the image

EQUATION:

2^(bit depth)

DICOM

Digital Imaging and Communications in Medicine:

• Standard for imaging pictures

Voxel

• Most important for CT or MRI

• The volume of elements

Image File Size (Affected by)

• Pixel size

• Matrix

• Bit depth shades

Image Processing information (types)

• Point Processing operation

• Local Processing operations

• Geometric Processing Operations

Point Processing Operations (Includes what)

• Formation of histograms (histogram equalization)

• Look-up table & windowing (for contrast and brightness)

LUT (Look-up-Table)

• Adjustments to image contrast

• Values assigned to data points (pixels) in the histogram

• Produces contrast. Look, according to the reference contrast for exam view

• Varys with exam/manufactures

• Can NOT be compensated for with LUTs

• Image display can be adjusted from the LUT

• Adjustment similar to changing the D(log)E curve of film emulsion.

Histogram

• Shape depends on anatomy/exposure

• Used to determine pixel values of interest (VOI) and exposed

• Arranges signals from min to max values

• Graphical computation of signal values of a data set

VOI

Values of interest

Histogram Equalization

• Rescaling if overexposed or underexposed to appear like a good histogram

• Wide histogram = higher contrast

• Narrow histogram = low contrast

• If higher comparative # = bright/light

• If lower comparative # = dark/dim

ACR

American college of radiology

• Forms standard procedure for histogram radiation exposure

Local Processing Operations (

Kernal

• Processing which mathematical calculation are applied to pixels until all pixels are subjected

Local processing operations (include)

• High-pass filtering

• Low-pass filtering

• Spatial location filtering

High-pass filtering (local processing operations)

Removes low spatial frequency and produce sharper image (edge enhancing)

Low-pass filtering (local processing operations)

Intentionally blurs (reducing noise)

Should NOT be viewed in normal or high pass mode.

Spatial location filtering (local processing operations)

• Kernel applied repeatedly to each pixel in the matrix to weight values or apply a coefficient across the matrix.

Geometric Processing Operations

• Changing the position or orientation of pixels

• Allows rotation/magnification

Digital Image quality (Types of resolution)

• Spatial resolution

• Density resolution

• Contrast resolution

Spatial resolution

Matrix size increases, pixel size decrease, spatial resolution increases

Density resolution

Gray scale bit depth increases, sensity resolution increases

Contrast reolution

Low contrast resolution (LCR)

• Ability to represent small energy values in data set

• CR/DR receptors hav excellent LCR (great dynamic range)

Noise (types)

• Electronic (system) noise

• Quantum noise

• Ambient noise

Electronic (system) noise

• Results from undesirable signals from the digital system

• Random background info detected. Does NOT contribute to image quality

Quantum Noise

• Insufficient quantity of photons from improperly put exposure levels (grainy images)

Ambient Noise

Background radiation

SNR

• Signal-to-Noise

• To measure noise. A higher value correlates to little noise.

Image noise has an inverse relationship to contrast. Therefore, the more contrast, the less noise.

DQE

Detective Quantum Efficiency

Detective Quantum Efficiency (DQE)

Measure sensitivity and accuracy to which IR converts input to output.

Detective Quantum Efficiency (DQE) (Affected how)

• SNR

• Quantum noise

• System noise

Exposure Index (EI)

• Calculated using histogram values and pixel values of interest (PVOI) mid points

• Varies between vendors

• Responsible for understanding and applying EI values (Assess lmage quaility and ALARA compliance)

  • No universal system

ALARA

As low as reasonably achievable

Deviation Index (DI)

• Created by AAPM and IEC

• Standardizes exposure values between manufacturers using El’s

• Uses department ELT values

• Comparison between acquired exposure (KIND) and target (KTGT)

• Expressed as a scale of DI values