Computed Radiography Cr Digital Radiography Dr3148

transcript

George DavidAssociate Professor of Radiology

Medical College of Georgia

Computed Radiography (CR)Computed Radiography (CR)

• Re-usable metal imaging plates replace film & cassette

• Uses conventional bucky & x-ray equipment

CR Exposure & ReadoutCR Exposure & Readout

CR ReadoutCR Readout

Another View: CR OperationAnother View: CR Operation

Computer Radiography (CR)Computer Radiography (CR)

• plate is photostimulable phosphor

• radiation traps electrons in high energy states

• higher statesform latent image

Higher EnergyElectronState

Lower EnergyElectronState

X-RayPhoton

Photon pumpselectron tohigher energy state

Reading Imaging PlateReading Imaging Plate• reader scans plate with laser• laser 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

Lower Energy Electron State

Reading Imaging PlateReading Imaging Plate• Reader 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 ColorsLaser & Emitted Light are Different Colors

• Phosphor stimulated by laser light• Intensity of emitted light indicates amount of radiation

incident on phosphor at each location• Only color of light emitted by phosphor measured by PMT

CR OperationCR 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 Phosphor LayerCR Phosphor Layer• Phosphor balanced for

x-ray absorption characteristics light output laser light scatterscreen thickness

• Above variables affectelectronic noise image resolution propertiesspeed of imaging system

• Overcoat protects plate from physical damage

CR ResolutionCR Resolution

• Small cassettes have better spatial resolutionSmaller pixelsMore pixels / mm

CR ThroughputCR 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 LatitudeCR Latitude

• Much 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 LatitudeFilm Screen vs. CR Latitude

CR Latitude: .01 –

100 mR

CR Very Sensitive to ScatterCR Very Sensitive to Scatter

Digital Radiography (DR)Digital Radiography (DR)

• Digital bucky

• Incorporated into x-ray equipment

Digital Radiography (DR)

• Receptor provides direct digital output

• No processor / reader requiredImages available in < 15 secondsMuch less work for technologist

Direct vs. IndirectDirect vs. Indirect

TFT = THIN-FILM TRANSISTOR ARRAY

“Direct” DR“Direct” DR• X-ray energy

converted directly to electrical signal

• X-rays interact with semiconductor materialAmorphous selenium

• X-rays converted directly into electrical chargeNo intermediate

“Indirect” DR“Indirect” DR

• X-ray strike scintillator producing light

• Photodiode array converts light to electrons

Indirect DRIndirect DR

• Light spreads can limit spatial resolution

• Can be controlled by “channeling”

• Winning in the marketplace

Digital Radiography (DR)

• Potentially lower patient dose than CR

• High latitude as for CR

• Digital bucky fragileFirst DR portables coming

to market

SummarySummary

• DR becoming industry leader in radiographic imaging

• DR images displayed & stored in about 8 seconds

• DR has faster throughputUp to 2-4 times faster than traditional

screen-film-darkroom technology

Raw Data ImageRaw Data Image

• Unprocessed 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)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

Look Up Table (LUT)Look Up Table (LUT)

• Converts a raw data pixel value to a processed pixel value

• “Original” raw data pixel value indicates amount of radiation falling on pixel

Image SegmentationImage Segmentation

• Computer must establish location of collimated border of image

• Computer then defines anatomic region

• Finished image produced by tone scalingRequires histogram analysis of

anatomic region

HistogramHistogram

• Graph 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

LUT can Simulate Appearance of FilmLUT can Simulate

Appearance of Film

LUT SelectionLUT Selection

• LUT calculated by algorithm depends onBody partprojection

• User can also alter LUT manually

LUT SelectionLUT Selection

• Monitors on CR reader or DR console compared to reading workstations havelower resolutionpoorer qualityRecommended that

LUT not be manually modified

Film/Screen Limited LatitudeFilm/Screen Limited Latitude

• Film use has little ambiguity about proper radiation exposure

Should I Worry?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 noisy

• Overexposures look GOOD!!!

CR / DR LatitudeCR / DR Latitude

DANGER Will

Robinson!!!

Exposure CreepExposure Creep:Tendency of radiographs toward higher-then-necessary exposures

• No detrimental effect on image quality

• Desire to see less noise on radiographs

• Increased exposure latitude

• No one complains

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

optimum by looking at my image?

Exposure IndexExposure Index

• Each manufacturer provides feedback to technologist on exposure to digital receptor

• Displayed on CR reader monitor

• Displayed on workstations

Exposure IndexExposure Index• Measure of radiation received by receptor

below anatomy• Not a direct measure of patient exposure• If exposure index higher than

recommended range, patient overexposed

Exposure Indication Varies between Manufacturers

Receptor Exposure

Kodak EI

Fuji S Number

0.5 1700 4001 2000 2002 2300 1004 2600 50

Fuji“S” number goes down

as exposure goes up!S is half when

exposure doubled

KodakLogarithmic scaleEI goes up 300 when

exposure doubled

Exposure IndexExposure Index• Technologist should strive to keep

exposure index consistent• Kodak recommendation for exposure

index1800 – 2200

• George’s recommendation“Maximum tolerable noise”As low as possible while providing

tolerable noiseThis is not a beauty contest!

Calculated Exposure Index Affected by

• X-Ray technique selection

• Improper centering of image on cassette

• Improper selection of study or projection

• Placing two or more views on same cassetteCan cause image to appear dark

Phototimed Phantom ImagePhototimed Phantom Image

• 75 kVp

• 88 mAs

• 2460 EI

Let’s Approximately Double mAsLet’s Approximately Double mAs

• 75 kVp• 88 mAs• 2460 EI

• 75 kVp• 160 mAs• 2680 EI

Let’s Go CrazyLet’s Go Crazy

• 75 kVp• 88 mAs• 2460 EI

• 75 kVp• 640 mAs• 3300 EI

How Low Can You Go? Cut mAs in Half!How Low Can You Go? Cut mAs in Half!

• 75 kVp• 88 mAs• 2460 EI

• 75 kVp• 40 mAs• 2060 EI

Let’s Go Crazy LowLet’s Go Crazy Low

• 75 kVp• 8 mAs• 1380 EI

• 75 kVp• 1 mAs• 550 EI

CR ArtifactsCR Artifacts

• Physical damage to imaging platesCracks, scuffs, scratchesContaminationDust / dirt

• Dirt in reader• Highly sensitive to scatter

radiation

CR Grid InterferenceCR Grid Interference

• 103 lines / inch grids have same frequency as CR laser scanner. This can cause “Moire” pattern artifact

• Align grid lines perpendicular to scan orientation whenever possibleReduces chances of artifacts caused by laser scanner.

DR ArtifactsDR Artifacts

• Dead detector elements

• Spatial variations in background signal & gain

• Grid interference

• Software can help correct for above

Shifting Gears:Fluoroscopy Issues

Digital Video SourcesDigital Video Sources• DR type image receptor• Conventional Image Intensifier with Video

Signal Digitized (“Frame Grabber”)

X-RayInput

ImageTube TV

Amplfier

Analogto

DigitalConvert

DigitalMemory

(Computer)

Lens System

Digital Spot FilmDigital Spot Film

• Frame grabber digitizes image• Digital image saved by computer• Radiographic Technique used

required to control quantum noise

Last Image HoldLast Image Hold

• Computer displays last fluoro image before radiation shut off.

• Image noisier than for digital spotImage made at fluoroscopic technique / intensity

• Allows operator to review static processes without keeping beam on ideal for teaching environments ideal for orthopedic applications such as hip

pinning

• Less radiation than digital spot

Fluoro Frame Averaging

• Conventional fluoro only displays current frame

• Frame 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 Features

• Real-time Edge Enhancement / Image Filtering

• Option 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

The Future of DigitalThe Future of Digital

DR Mobile Units DR Mobile Units

• See image immediately

• Wireless transmission of images

Other PossibilitiesOther Possibilities

• TomosynthesisMulti-slice linear tomography from

one exposure series

• Histogram EqualizationUse computer to provide

approximately equal density to various areas of image.

DR & Energy SubtractionDR & Energy Subtraction

• 2 images taken milliseconds apart at 2 different kVp’s

• Combine / subtract images

Soft Tissue Image Bone Image

The EndThe End

Computed Radiography Cr Digital Radiography Dr3148

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