Fundamentals of Digital Radiology

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Fundamentals of Digital Radiology
George David Medical College of Georgia
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• So what is Digital?

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What we mean by Digital
Filmless Department

• Digital Radiographs
• PACS
• Picture Archival Communication Systems
• Reading from Monitors

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What we really mean by Digital
No more File Room!!!
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Expectations for Digital

• All images available on-line
• new images
• old / comparison images
• images from all modalities
• Convenience
• Eliminate file room headaches
• lost / stolen films
• Reduce report turn-around time

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Important Considerations

• Resolution needed
• Changing role of radiologists with referring
  physicians
• everyone has instant access to all images
• Security
• Cost

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Digital Image Formation
Clinical Image
Screen Wire Mesh
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Digital Image Formation

• Place mesh over image

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Digital Image Formation

• Assign each square (pixel) a number based on
  density
• Numbers form the digital image

194
73
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Digital Image Formation

• The finer the mesh, the better the digital
  rendering

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What is this?
12 X 9 Matrix
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Same object, smaller squares
24 X 18 Matrix
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Same object, smaller squares
48 X 36 Matrix
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Same object, smaller squares
96 X 72 Matrix
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Same object, smaller squares
192 X 144 Matrix
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Numbers / Gray Shades

• Each number of a digital image corresponds to a
  gray shade for one picture element or pixel

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So what is a digital image?

• Image stored as 2D array of s representing some
  image attribute such as
• optical density
• x-ray attenuation
• echo intensity
• magnetization

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Computer Storage

• 125, 25, 311, 111, 182, 222, 176, 199, 192, 85,
  69, 133, 149, 112, 77, 103, 118, 139, 154, 125,
  120, 145, 301, 256, 223, 287, 256, 225, 178, 322,
  325, 299, 353, 333, 300

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Digital Copies

If youve got the same numbers ...
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Digital Copies

• then you have an identical copy

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Copies

• Digital copies are identical
• All digital images are originals
• Film copies never identical to original
• may be substantially degraded
• loss of original information

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The Bit

• Fundamental unit of computer storage
• Only 2 allowable values
• 0
• 1
• Computers do all operations with 0s
  1sBUTComputers group bits together

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Special Binary Digit Grouping Terms

• Nibble
• 4 binary bits (0101)
• Byte
• 8 binary bits (1000 1011)
• Word
• 16 binary bits (1100 0100 1100 0101)
• Double Word
• 32 binary bits(1110 0100 0000 1011 0101 0101
  1110 0101)

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Abbreviations Review

• Bit (binary digit)
• Smallest binary unit has value 0 or 1 only
• Byte
• 8 bits
• Kilobyte
• 210 or 1024 bytes
• sometimes rounded to 1000 bytes
• Megabyte
• 213 or 1,048,576 bytes or 1024 kilobytes
• sometimes rounded to 1,000,000 bytes or 1,000
  kilobytes

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Computer Storage

• Storage Pixels X Bytes/Pixel
• Example 512 X 512 pixels 1 Byte / Pixel
• 512 X 512 pixel array
• pixels 512 X 512 262,144 pixels
• Storage 262,144 pixels X 1 byte / pixel
  262,144 bytes 256 KBytes .25 MBytes

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Image Matrix

• Doubling the matrix dimension quadruples the
  pixels

2 X 2 Matrix 4 pixels
4 X 4 Matrix 16 pixels
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Image Matrix
Doubling the matrix dimension quadruples pixels

• A 10242 matrix compared to a 5122 matrix
  quadruples
• disk storage requirements
• image transmission time
• digital image manipulation

Matrix Pixels 512 X 512 gt
262,144 1024 X1024 gt 1,048,576 2048 X2048 gt
4,194,304
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Matrix Size Resolution

• More pixels better spatial resolution

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of unique values which can be represented by 1
bit
2 unique combinations / values
1
2
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of unique values which can be represented by 2
bits
1
2
4 unique combinations / values
3
4
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of unique values which can be represented by 3
bits
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1
6
2
7
3
8
4
8 unique combinations / values
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Digital Image Bit Depth

• the number of computer bits (1s or 0s)
  available to store each pixel value

Values
Bits
Values
1 2 3 . . . 8
0, 1 00, 01, 10, 11 000, 001, 010, 011, 100, 101,
110, 111 . . . 00000000, 00000001, ... 11111111
2 1 2 2 2 4 2 3 8 . . . 2 8 256
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Digital Image Bit Depth

• bit depth indicates of possible brightness
  levels for a pixel
• presentation of brightness levels
• pixel values assigned brightness levels
• brightness levels can be manipulated without
  affecting image data
• window
• level

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Bit Depth Contrast Resolution

• The more bits per pixel the more possible gray
  shades and the better contrast resolution.

2 bit 4 grade shades
8 bits 256 grade shades
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Image Size

• Related to both matrix size bit depth
• higher (finer) matrix requires more storage
• doubling matrix size quadruples image size
• higher bit depth requires more storage
• doubling bit depth theoretically doubles image
  size
• Computer may require storage in multiples of 8
  bits (bytes)
• 10 or 12 bits stored in 16 bit slot
• alters image size requirements

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Image Compression

• reduction of digital image storage size by
  application of algorithm
• for example, repetitive data could be represented
  by data value and repetitions rather than by
  repeating value

jpg
37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37,
37, 37, 37, 37, 37, 37, 37, 37
gif
(20) 37s
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Image Compression

• Image Decompression
• calculating original digital image from
  previously compressed data
• Compression Ratio
• original image size-----------------------
  ---------compressed image size
• ratio depends upon
• data to be compressed
• algorithm

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Compression Types

• Reversible Compression
• Image decompresses to original pixel values
• Low compression ratios only
• Non-reversable Compression
• Decompressed images pixel values not necessarily
  identical to original
• much higher compression ratios possible
• variation from original image may or may not be
  visible or clinically significant

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Non-Reversable Compression

• variation from original image generally increases
  with increasing compression ratio
• but a higher compression ratio means less storage
  requirements
• variation less noticeable for dynamic (moving)
  images than for still images such as radiographs

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Where do Digital Images Come From?

• Direct Digital
• MRI
• CT
• Computed Radiography (CR)
• direct digital TV output (CCD TVs)
• Digitization of an Analog Image
• Film Digitizers
• Video Frame Grabbers (requires good low noise
  video signal)
• teleradiology TV looking at viewbox
• digitizing of fluoroscopic video

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Film Digitizer

• films stacked in input tray
• film pulled through digitizer
• CCD measures transmission from collimated light
  source
• optical density

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Film Digitizers

• Expensive
• faster models cost more
• Patient Demographics
• input manually
• DICOM assistance
• Digitizers can have artifacts

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Computer Radiography (CR)

• Re-usable metal imaging plates replace film
  cassette

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Computer Radiography (CR)

• plate is photostimulable phosphor
• radiation traps electrons in high energy states
• higher statesform latent image

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Reading Imaging Plate

• plate scanned with laser
• Releases electrons trapped inhigh energy states
• electrons fall to lowenergy states
• electrons give upenergy as visible light
• light intensity ismeasure of incident radiation

Lower Energy Electron State
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Reading Imaging Plate

• Reader scans plate with laser light using
  rotating mirror
• Plate pulled through scanner by rollers
• Light emitted by plate measured by PM tube
  recorded by computer

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Plate Travel
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CR Operation

• after read-out, plate erased using bright light
• plate can be re-used
• digital image can be
• printed on film
• Read on-line

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CR Comments

• Throughput
• CR reader must finish reading one plate before
  reading next
• Film processors run films back-to- back
• Latitude
• Plate responds to many decades of input exposure
• Much greater latitude than screen/film
• Computer scales input exposure to viewable
  densities
• Unlike film, receptor separate from viewer

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Digital Radiography (DR)

• Direct digital output
• No processor / reader
• Images available virtually immediately
• Greater throughput

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Digital Radiography (DR)

• Potentially lower patient dose
• High latitude as for CR
• Built into imaging equipment
• Fragile
• requires special attention for use with portable
  x-ray equipment

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Digital Radiography (DR)

• Direct imaging
• X-rays interact with semiconductor material
• Amorphous selenium
• X-rays converted directly into electrical charge
• Indirect imaging
• X-ray strike scintillator producing light
• Photodiode array converts light to electrons

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Digital Radiography (DR)

• Indirect imaging
• uses light and photodiodes
• Light spreads / scatters
• Can degrade resolution
• Both direct indirect read-out digitally
• Charge pattern stored in array
• Analog to digital conversion digitizes charge
  pattern

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Digital Video

• Video Signal Digitized (Frame Grabber)

Image Tube
TV
Lens System
X-Ray Input
Amplfier
Analog to Digital Converter
Digital Memory (Computer)
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Teleradiology

• Frame grabbers used for teleradiology
• Quality depends upon
• TV camera
• viewbox
• can have artifacts from lighting
• matrix size
• affects transmission speeds
• display quality

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Digital Spot Films

• Frame grabber or CCD TV digitizes image
• Radiographic Technique used
• required to control quantum noise
• High-quality camera required
• high signal to noise
• Operationally nice
• allows review of images in exam room
• allows image manipulation
• allows later selection of images for printing

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Digital Fluoroscopy

• TV Image digitized real-time
• Digitized image can be manipulated / enhanced
  real-time

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Last Image Hold

• computer displays last fluoro image after
  radiation shut off
• allows operator to review static processes
  without beam
• ideal for teaching environments
• ideal for orthopedic applications such as hip
  pinning
• Reduces dose for patient, staff

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Frame Averaging

• Normal fluoro
• only current frame displayed
• Frame averaging
• computer calculates average of current
  user-selectable number of previous frames
• reduces quantum noise
• lag results if too many frames averaged

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Other Fluoro Features

• Edge Enhancement / Image Filtering
• real-time
• Can be turned on and off
• Option of using lower frame rates
• computer displays last image until next one
• reduces flicker
• dynamic studies may be jumpy

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Digital Fluoroscopy and Special Procedures / Cath
Lab Applications

• Up to 60 frames per second
• immediate replay
• cine loop replay

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Digital Subtraction

• Immediate replay of run
• Free selection of mask
• before or after bolus
• gt1 frame may be averaged for mask
• Note
• subtraction adds noise

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Registration

• matching anatomy between two images to be
  subtracted
• compensates for motion
• registration adjustments
• often fine adjustment down to 1/10 pixel
• registration types
• translational (left, right, up, down)
• rotational
• warp

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Digital Image Manipulations

• on-screen measurements
• distances
• angles
• volumes/areas
• stenosis
• image annotation
• peak opacification / roadmapping
• peak opacification displays vessels after a test
  injection
• allows visualization of live catheter on top to
  saved image of test injection

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CR/DR Advantages

• post-processing manipulation possible
• tremendous latitude
• virtually no technique repeats
• DR faster than film
• CR operationally slightly slower than film

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Digital Disadvantages

• High up-front expenses
• CR many readers may be required for spread out
  departments
• DR new radiographic equipment required
• High resolution images very large
• Images require
• high speed transmission systems
• massive archival systems required
• Spatial resolution poorer than film
• May be offset by other advantages

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Digital Possibilities

• Multi-modality imaging / Image fusion
• PET/CT

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Digital and other Possibilities

• Tomosynthesis
• Multi-slice tomography from single pass
• Histogram Equalization
• Computer provides approximately equal density to
  various areas in image.

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DR Energy Subtraction

• 2 images taken milliseconds apart at 2 kVps
• Combine / subtract images

Soft Tissue Image
Bone Image
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The End

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