Fundamentals of Digital Radiology
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Transcript of Fundamentals of Digital Radiology
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Fundamentals of Digital
Radiology
George DavidMedical College of Georgia
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So what is “Digital”?
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What we mean by DigitalDigital
Digital RadiographsPACS
Picture Archival & Communication Systems
Reading from Monitors
Filmless DepartmentFilmless DepartmentFilmless DepartmentFilmless Department
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What we really mean by DigitalDigital
No more No more File File
Room!!!Room!!!
No more No more File File
Room!!!Room!!!
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Digital Image FormationPlace mesh over
image
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Digital Image FormationAssign each square
(pixelpixel) a number based on density
Numbers form the digital image 194
73
22
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Digital Image FormationThe 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 ShadesEach number of a digital image corresponds to
a gray shade for one picture element or pixelpixel
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So what is a digital digital image?
Image stored as 2D array of #’s representing some image attribute such asoptical densityx-ray attenuationecho intensitymagnetization
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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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
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 you’ve got the same numbers ...
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
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 Copiesthen you have an identical copy
=
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Digital CopiesDigital copies are identicalAll digital images are originals
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Image MatrixDoubling the matrix dimension
quadruples the # pixels
111 87
118 155
125 25 311 111
199 192 85 69
77 103 118 139
145 301 256 223
2 X 2 Matrix4 pixels 4 X 4 Matrix
16 pixels
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Image Matrix
A 10242 matrix compared to a 5122 matrix quadruplesdisk storage
requirements image transmission
timedigital image
manipulation
Matrix # Pixels 512 X 512 => 262,1441024 X1024 => 1,048,5762048 X2048 => 4,194,304
Doubling the matrix dimension quadruples # pixels
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Matrix Size & Resolution
More pixels = better spatial resolutionMore pixels = better spatial resolution
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The BitFundamental unit of Fundamental unit of
computer storagecomputer storageOnly 2 allowable valuesOnly 2 allowable values
0 1
Computers do all operations with 0’s & 1’s
BUT
Computers group bits together
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Special Binary Digit Grouping Terms
NibbleNibble4 binary bits (0101)
ByteByte8 binary bits (1000 1011)
WordWord16 binary bits (1100 0100 1100 0101)
Double WordDouble Word32 binary bits
(1110 0100 0000 1011 0101 0101 1110 0101)
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Abbreviations Review
BitBit (binary digit)Smallest binary unit; has value 0 or 1 only
ByteByte8 bits
KilobyteKilobyte210 or 1024 bytessometimes rounded to 1000 bytes
MegabyteMegabyte213 or 1,048,576 bytes or 1024 kilobytessometimes rounded to 1,000,000 bytes or
1,000 kilobytes
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# of unique values which can be # of unique values which can be represented by 1 bitrepresented by 1 bit
2 unique combinations / values
1
2
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# of unique values which can be # of unique values which can be represented by 2 bitsrepresented by 2 bits
4 unique combinations / values2
1
3
4
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# of unique values which can be # of unique values which can be represented by 3 bitsrepresented by 3 bits
8 unique combinations / values
2
1
3
4
6
5
7
8
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Digital Image Bit Depth
the number of computer bits (1’s or 0’s) available to store each pixel value
1 2 3 ...8
0, 100, 01, 10, 11000, 001, 010, 011, 100, 101, 110, 111...00000000, 00000001, ... 11111111
2 1 = 22 2 = 42 3 = 8...2 8 = 256
Bits Values # Values
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Digital Image Bit Depth
bit depth indicates # of possible brightness levels for a pixel
presentation of brightness levelspixel values assigned brightness levelsbrightness 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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Computer StorageStorage = # 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 Size
Related to both matrix size & bit depthhigher (finer) matrix requires more storage
doubling matrix size quadruples image sizehigher 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
11 22 33 44 55 66 77 88
99 1100
1111
1122
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Image Compressionreduction of digital image storage size by
application of algorithmfor example, repetitive data could be
represented by data value and # repetitions rather than by repeating value
37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37, 37
(20) 37’s
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Image Compression
Image Decompressioncalculating 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 TypesReversibleReversible Compression
Image decompresses to original pixel valuesLow compression ratios only
Non-reversableNon-reversable CompressionDecompressed image’s pixel values not
necessarily identical to originalmuch higher compression ratios possiblevariation from original image may or may
not be visible or clinically significant
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Non-Reversable Compressionvariation from original image generally
increases with increasing compression ratiobut a higher compression ratio means
less storage requirementsvariation less noticeable for dynamic
(moving) images than for still images such as radiographs
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Computed Radiography (CR)Re-usable metal imaging plates replace
film & cassetteUses conventional bucky & x-ray
equipment
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CR Exposure & Readout
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CR Readout
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Another View: CR Operation
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Computer Radiography (CR)plate is
photostimulable phosphor
radiation traps electrons in high energy states
higher statesform latent image
Higher EnergyElectronState
Lower EnergyElectronState
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X-RayPhoton
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Photon pumpselectron tohigher energy state
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Reading Imaging Platereader scans plate
with laserlaser releases
electrons trapped inhigh energystates
electrons fall to lowenergy states
electrons give upenergy as visible light
light intensity ismeasure of incident radiation
Laser Beam
Higher EnergyElectronState
Lower EnergyElectronState
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Lower Energy Electron State
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Reading Imaging PlateReader scans
plate with laser light using rotating mirror
Film pulled through scanner by rollers
Light given off by plate measured byPM tube &recorded by computer
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Laser & Emitted Light are Different ColorsPhosphor stimulated by laser lightIntensity of emitted light indicates amount of
radiation incident on phosphor at each locationOnly color of light emitted by phosphor
measured by PMT
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CR Operation
after read-out, plate erased using a bright light
plate can be erased virtually without limit
Plate life defined not by erasure cycles but by physical wear
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CR ResolutionSmall cassettes have better spatial
resolutionSmaller pixelsMore pixels / mm
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CR Throughput
Generally slower than film processing
CR reader must finish reading one plate before starting to read the next
Film processors can run films back to back
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CR LatitudeMuch greater
latitude than screen/film
Plate responds to many decades of input exposureunder / overexposures
unlikely
Computer scale inputs exposure to viewable densitiesUnlike film, receptor
separate from viewer
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Film Screen vs. CR Latitude
CR Latitude: .01 – 100 mR
100
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Digital Radiography (DR)Digital
buckyIncorporate
d into x-ray equipment
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Digital Radiography (DR)Receptor provides direct digital
outputNo processor / reader required
Images available in < 15 secondsMuch less work for technologist
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Direct vs. Indirect
TFT = THIN-FILM TRANSISTOR ARRAY
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Digital Radiography (DR)Potentially lower patient dose than CRHigh latitude as for CRDigital bucky fragile
First DR portables comingto market
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Raw Data ImageUnprocessed image as read
from receptorCR
Intensity data from PMT’s as a result of scanning plate with laser
DR Raw Data read directly from TFT array
Not a readable diagnostic image
Requires computer post-processingSpecific software algorithms must be
applied to image prior to presenting it as finished radiograph
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Enhancing Raw Image (Image Segmentation)
1. Identify collimated image border
2. Separate raw radiation from anatomy
3. Apply appropriate tone-scale to image
Done with look-up table (LUTLUT)
This process is specific to a
particular body part and
projection
*
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Image SegmentationComputer must establish location of collimated border of image
• Computer then defines anatomic region
• Finished image produced by tone scalingRequires histogram analysis of
anatomic region
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HistogramGraph showing
how much of image is exposed at various levels
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Tone ScalingPost-Processing
Body part & projection-specific algorithms determine average exposureMust correctly identify anatomical region
LUT computed to display image with properDensityContrast
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Film/Screen Limited Latitude
Film use has little ambiguity about proper radiation exposure
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Should I Worry?In CR & DR, image density is no longer a reliable indicator of exposure factor control.
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Almost impossible to under or overexpose CR / DR
Underexposures look noisyOverexposures look GOOD!!!
CR / DR Latitude
DANGER Will
Robinson!!!
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So how do I know if exposure is optimum by looking at my image?
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Exposure Index
Each manufacturer provides feedback to technologist on exposure to digital receptor
Displayed on CR reader monitor Displayed on workstations
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Calculated Exposure Index Affected by
X-Ray technique selectionImproper centering of image on
cassetteImproper selection of study or
projectionPlacing two or more views on same
cassetteCan cause image to appear dark
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Phototimed Phantom Image
75 kVp88 mAs2460 EI
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Let’s Approximately Double mAs
75 kVp88 mAs2460 EI
• 75 kVp• 160 mAs• 2680 EI
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Let’s Go Crazy
75 kVp88 mAs2460 EI
• 75 kVp• 640 mAs• 3300 EI
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How Low Can You Go? Cut mAs in Half!
75 kVp88 mAs2460 EI
• 75 kVp• 40 mAs• 2060 EI
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Let’s Go Crazy Low
75 kVp8 mAs1380 EI
• 75 kVp• 1 mAs• 550 EI
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CR ArtifactsPhysical damage to imaging plates
Cracks, scuffs, scratchesContaminationDust / dirt
Dirt in readerHighly sensitive to scatter radiation
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DR ArtifactsDead detector elementsSpatial variations in background signal &
gainGrid interferenceSoftware can help correct for above
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Shifting Gears:Fluoroscopy Issues
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Digital Video SourcesDR type image receptorConventional Image Intensifier with
Video Signal Digitized (“Frame Grabber”)
Image
Tube
X-RayInput
ImageTube TV
Amplfier
Analogto
DigitalConvert
er
DigitalMemory
(Computer)
Lens System
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Digital Spot FilmFrame grabber digitizes imageDigital image saved by computerRadiographic Technique used
required to control quantum noise
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Last Image HoldComputer displays last fluoro image
before radiation shut off.Image noisier than for digital spot
Image made at fluoroscopic technique / intensity
Allows operator to review static processes without keeping beam onideal for teaching environmentsideal for orthopedic applications such as hip
pinning
Less radiation than digital spot
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Fluoro Frame AveragingConventional fluoro only displays current
frameFrame averaging allows computer to
average current with user-selectable number of previous framesAverages current frame & history
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Fluoro Frame Averaging Tradeoff
Advantage:Reduces quantum noise
DisadvantageBecause history frames are averaged with
current frame, any motion can result in lag
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Other Fluoro FeaturesReal-time Edge Enhancement / Image
FilteringOption of using lower frame rates (15,
7.5, 3.75 fps rather than 30)computer displays last frame until next
one reduces flicker
Lowers patient and scatter exposure Exposure proportional to frame rate
dynamic studies may be jumpy
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Digital SubtractionImmediate replay of runFree selection of mask
before or after bolus>1 frame may be averaged for
maskNote
subtraction adds noise
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Digital Image Manipulationson-screen measurements
distancesanglesvolumes/areasstenosis
image annotationpeak 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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Digital Possibilities
Multi-modality imaging / Image fusionPET/CT
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DR & Energy Subtraction
2 images taken milliseconds apart at 2 kVp’s
Combine / subtract images
Soft Tissue Image Bone Image
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DR Mobile Units See image immediatelyWireless transmission of
images
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Other Possibilities
TomosynthesisMulti-slice linear tomography from
one exposure seriesHistogram Equalization
Use computer to provide approximately equal density to various areas of image.
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The End
?