Digital imaging
Defined as
- any imaging acquisition process that produces an electronic image that can be VIEWED and MANIPULATED on a computer
- these digital images can be sent through a computer network to various locations as needed
Computer
Used throughout the department for a number of purposes: front desk, tech work area, radiologists reading room
-computer set up chosen to fulfill purpose
-basically consists of:
🔹input devices
🔹output devices
🔹processing devices
Input and output devices
Input devices: keyboards, mice, microphones, barcode readers, touch screens, image scanners
Output devices: monitors, printers (paper and laser film), speakers
Monitors
Two main types -CRT (cathode ray tube) -LCD (liquid crystal display) Third type in development -OLED (organic light: emitting diode)
Pixels
-individual controllable set of dot triads
-dot triad (grouping of dots)
-basic picture element on display
-resolution: # of pixels on a display
🔹higher resolution = higher # of pixels
-arranged in a matrix that represents the pixel intensity to be displayed
-dot pitch: how close the dots are in the pixel
🔹the smaller the dot pitch, the higher the resolution
Monitors
CRT
-refresh rate
-how fast the monitor rewrites the screen or the number of times the image is redrawn on the display/second, controls the ‘flicker’ seen by user, higher refresh rate = less flicker
LCD
-response rate
-the amount of time for crystal to go from off state to on and vice versa, slower response time will cause blurring during viewing of dynamic images
Aspect ratio
Ratio of the width of the monitor to the height of the monitor
CRT
- consists of a cathode and anode in a vacuum tube
- works like an x-ray tube (electrons sent to the monitors anode which is a sheet of glass coated with phosphors)
- electrons strike phosphors causing glass to emit a colour determined by the intensity of the interaction and area with which electrons interacted
- electron beam starts in the upper left corner and scans across the glass from side to side, top to bottom
- after reaching the bottom, it starts again at the top left
LCD
-images are produced by shining or reflecting light through a layer of liquid crystal and a series of colour filters
-made of two pieces of polarized glass with a liquid crystal layer in between the two
-light passes through the first layer, and when a current is applied to the crystal layer, it aligns and allows light varying intensities through to the next layer
🔹colour filters in that layer of glass form the colours and images seen on the display
Digital imaging
- a digital image is one that has been converted into numerical values for transmission or processing
- requires a means to acquire the image formation (DR,CR)
- information gathered by scanning an area using array detection
- array processing is when information is received from an entire area at once
Array processors
-special computer that functions as a peripheral device
-uses its own memory to perform simultaneous mathematical operations at extremely high speeds
🔹compiles all the image information
-allows us to see the image instantaneously
Factors that remain unchanged in digital imaging
- collimation
- patient positioning
- projections
Digital is better because
- wider range of exposure
- linear response
- greater range of densities
- easily manipulated with computer
- post processing
Characteristics of a digital image
-digital image begins as an analog signal
-critical characteristics of a digital signal are:
🔹spatial resoluton
🔹contrast resolution
🔹noise
🔹dose efficiency (of the IR)
Pixel size
- size of pixel is directly related to the amount of spatial resolution or detail in the image
- the smaller the pixel, the greater the detail
- can be affected by matrix size
Pixel bit depth
-each pixel contains bits of information
-is the # of bits within a pixel
Ex: bit depth = 8 means 2^8 (meaning 256 shades of grey can be produced by that pixel
-higher the bit depth, the more grey levels can be displayed by the pixel
🔹more shades of grey gives more accurate representation of the image
Bit systems
8 bit system is 2^8 (256 shades of grey)
10 bit system is 2^10 (1024 shades of grey)
12 bit system is 2^12 (4096 shades of grey)
14 bit system is 2^14 (16384 shades of grey)
Matrix
-image is recorded as a matrix
-described in numerical values
-divided into many small cubes/boxes
🔹each square region is a pixel
🔹assigned one binary value that is equal to the radiation exposure received
🔹binary value = shade of grey
Matrix
- ⬆️ matrix size = ⬆️ resolution
- larger the matrix = smaller the pixels (if the same size detector)
- matrix size is not the same as screen size
- pixel size controls resolution
Dynamic range
-the ability to respond to varying levels of exposure
🔹dynamic range wider for digital vs film/screen
-the range of grey values that can be displayed
-human eye can differentiate about 30 shades
-dynamic range of digital imaging systems is identified by the bit capacity of each pixel
-although the response of digital system can produce a huge range of greys, limited by human visual system (can only see about 30 of them)
-through the use of window and level, any region/range of greys can be rendered into the 30 shades of grey that the human eye can see
-white is designated as 1
Windowing
- allows us to see only a portion of the entire dynamic range
- raw data image will show the full dynamic range (appears very low contrast, as so many shades of grey are displayed)
Window level
- DENSITY (BRIGHTNESS) enhancement
- controls image BRIGHTNESS
- brightness is controlled by varying the numerical value of each pixel
- direct relationship: ⬇️ window level = ⬆️ image brightness
Altering image brightness
- INCREASING the brightness of the image: all pixels are raised to a higher value by the same factor
- DECREASING the brightness of the image: all pixels are decreased to a lower value by the same factor
window width
- CONTRAST ENHANCEMENT
- controls image CONTRAST
- sometimes called grey scale expansion or compression
- inverse relationship: ⬇️ window width = ⬆️ image contrast
- varies the dynamic range (grey scale) of the densities to adjust the contrast
- NARROW WW: ⬇️ window width = ⬆️ in contrast
- WIDE WW: ⬆️ window width = ⬇️ in contrast
- controls the visibility of detail
- any value outside of the chosen window width is lost from diagnosis
-
Introduction To Equiptment4
-
The Tube14
-
Anode Heel Effect And Line Focus Principle14
-
Electron Binding Energies18
-
Emission Spectrum27
-
Density40
-
Density Part II35
-
Collimation And The Beam17
-
Generators And The X-ray Circuit29
-
Grids13
-
Digital Imaging25
-
APR And AEC17
-
Tube Conservation4
-
Contrast22
-
Sampling, Histograms, LUT, And Exposure Indicators23
-
PACS54
-
Post Processing And Artifacts14
-
Detail And Distortion24
-
Exposure Factor Selection And Technique Charts5
-
Midterm Review47
-
Final Review35
-
Repeat And Reject Analysis3
-
CR And The CR Reader7
