Fundamentals of Digital

Radiology

George DavidMedical College of Georgia

So what is “Digital”?

What we mean by DigitalDigital

Digital RadiographsPACS

Picture Archival & Communication Systems

Reading from Monitors

Filmless DepartmentFilmless DepartmentFilmless DepartmentFilmless Department

What we really mean by DigitalDigital

No more No more File File

Room!!!Room!!!

No more No more File File

Room!!!Room!!!

Digital Image FormationPlace mesh over

image

Digital Image FormationAssign each square

(pixelpixel) a number based on density

Numbers form the digital image 194

73

22

Digital Image FormationThe finer the mesh, the better the digital rendering

What is this?

12 X 9 Matrix

Same object, smaller squares

24 X 18 Matrix

Same object, smaller squares

48 X 36 Matrix

Same object, smaller squares

96 X 72 Matrix

Same object, smaller squares

192 X 144 Matrix

Numbers / Gray ShadesEach number of a digital image corresponds to

a gray shade for one picture element or pixelpixel

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

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

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

Digital Copiesthen you have an identical copy

=

Digital CopiesDigital copies are identicalAll digital images are originals

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

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

Matrix Size & Resolution

More pixels = better spatial resolutionMore pixels = better spatial resolution

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

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)

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

# 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

# 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

# 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

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

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

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

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

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

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

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

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

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

Computed Radiography (CR)Re-usable metal imaging plates replace

film & cassetteUses conventional bucky & x-ray

equipment

                                                                 

CR Exposure & Readout

CR Readout

Another View: CR Operation

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

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

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

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

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

CR ResolutionSmall cassettes have better spatial

resolutionSmaller pixelsMore pixels / mm

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

               

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

               

Film Screen vs. CR Latitude

CR Latitude: .01 – 100 mR

100

Digital Radiography (DR)Digital

buckyIncorporate

d into x-ray equipment

Digital Radiography (DR)Receptor provides direct digital

outputNo processor / reader required

Images available in < 15 secondsMuch less work for technologist

Direct vs. Indirect

TFT = THIN-FILM TRANSISTOR ARRAY

Digital Radiography (DR)Potentially lower patient dose than CRHigh latitude as for CRDigital bucky fragile

First DR portables comingto market

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

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

*

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

HistogramGraph showing

how much of image is exposed at various levels

Tone ScalingPost-Processing

Body part & projection-specific algorithms determine average exposureMust correctly identify anatomical region

LUT computed to display image with properDensityContrast

Film/Screen Limited Latitude

Film use has little ambiguity about proper radiation exposure

Should I Worry?In CR & DR, image density is no longer a reliable indicator of exposure factor control.

Almost impossible to under or overexpose CR / DR

Underexposures look noisyOverexposures look GOOD!!!

CR / DR Latitude

DANGER Will

Robinson!!!

So how do I know if exposure is optimum by looking at my image?

Exposure Index

Each manufacturer provides feedback to technologist on exposure to digital receptor

Displayed on CR reader monitor Displayed on workstations

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

Phototimed Phantom Image

75 kVp88 mAs2460 EI

Let’s Approximately Double mAs

75 kVp88 mAs2460 EI

• 75 kVp• 160 mAs• 2680 EI

Let’s Go Crazy

75 kVp88 mAs2460 EI

• 75 kVp• 640 mAs• 3300 EI

How Low Can You Go? Cut mAs in Half!

75 kVp88 mAs2460 EI

• 75 kVp• 40 mAs• 2060 EI

Let’s Go Crazy Low

75 kVp8 mAs1380 EI

• 75 kVp• 1 mAs• 550 EI

CR ArtifactsPhysical damage to imaging plates

Cracks, scuffs, scratchesContaminationDust / dirt

Dirt in readerHighly sensitive to scatter radiation

DR ArtifactsDead detector elementsSpatial variations in background signal &

gainGrid interferenceSoftware can help correct for above

Shifting Gears:Fluoroscopy Issues

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

Digital Spot FilmFrame grabber digitizes imageDigital image saved by computerRadiographic Technique used

required to control quantum noise

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

Fluoro Frame AveragingConventional fluoro only displays current

frameFrame averaging allows computer to

average current with user-selectable number of previous framesAverages current frame & history

Fluoro Frame Averaging Tradeoff

Advantage:Reduces quantum noise

DisadvantageBecause history frames are averaged with

current frame, any motion can result in lag

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

Digital SubtractionImmediate replay of runFree selection of mask

before or after bolus>1 frame may be averaged for

maskNote

subtraction adds noise

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

Digital Possibilities

Multi-modality imaging / Image fusionPET/CT

DR & Energy Subtraction

2 images taken milliseconds apart at 2 kVp’s

Combine / subtract images

Soft Tissue Image Bone Image

DR Mobile Units See image immediatelyWireless transmission of

images

Other Possibilities

TomosynthesisMulti-slice linear tomography from

one exposure seriesHistogram Equalization

Use computer to provide approximately equal density to various areas of image.

The End

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