phys lecture 5 digitalradiography -...
TRANSCRIPT
Digital Radiography
PHYS Lecture
Carlos Vinhais
Departamento de FísicaInstituto Superior de Engenharia do Porto
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Overview
• Digital imaging• Film-screen vs Imaging Plate
• Materials for Digital Detectors• Detectors in Digital Imaging
• Computed Radiography (CR)• Photostimulable Phosphor
• Digital Radiography (DR)• Indirect DR and Direct DR• Charge Coupled Devices• Flat Panel Detectors• Thin-Film Transistors
• Image Processing
• Digital Mammography (FFDM)
• Temporal Subtraction
• Digital Subtraction Angiography (DSA)
• Dual-Energy Subtraction
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Digital Imaging
• Common Digital Modalities:
• Digital Chest Radiograph 4096 x 4096 x 12 bit• CT 512 x 512 x 12 bit• SPECT 128 x 128 x 8 bit• MRI 256 x 256 x 8 bit• US 512 x 512 x 8-24 bits
• Highest Quality Viewing Station
• 2k x 2k x 12 bits
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Digital Imaging
• Eliminate film• No processing, darkroom, film room,...
• Image archive, retrieve, transmission• Eliminate lost/missing films
• Higher image dynamic range:• Wider exposure latitude• Higher image signal-to-noise ratio (contrast)
• Imediate image• Computer image enhancement• Potencially lower dose
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Film-screen vs Imaging Plate
• Film-screen• Non-linear characteristics• Contrast compression• Under/over exposures
• Imaging Plate• linear characteristics• Wide exposure range• Exposure safety
• Dynamic Range
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Materials for Digital Detectors
• Ideal Material
• photoelectric interactions → high Z, matched to photon spectrum exiting patient
• adequate thickness to absorb a large number of x-rays, but not so thick as to adversely impact spatial resolution
• low amount of x-ray energy required to produce a light photon or electron (signal)
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Materials for Digital Detectors
• gas detectors (Xe)• X-ray →e-
→ ADC
• Photoconductors (Se, CdTe, HgI2, PbI2)• X-ray → e-
→ TFT → ADC
• Scintillators/phosphors (CsI, Gd2O2S)• X-ray→ e-
→Visible Light (VL)→ e-→ TFT → ADC
• Photostimulable phosphors (BaFBr)• X-ray → e-
→ F+/F → laser → e-→ VL → PMT → ADC
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Detectors in Digital Imaging
• Solid-state materials
• Electrons arranged in bands with conduction band usually empty
• Solid-state detectors
• Photoconductor – charge collected and measured directly
• Scintillator (phosphor) – some deposited energy converted to visible light
• Photostimulable phosphors – energy stored in electron traps
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Digital Technologies
• Computed Radiography (CR)• Digital Radiography (DR)
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Computed Radiography (CR)
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Computed Radiography (CR)
• Photostimulable Phosphor (PSP)Barium fluorohalide85% BaFBr:Eu + 15% BaFI:Eu
• e- from Eu2+ liberated through absorption of x-rays
• Liberated e- fall from the conduction band into ‘trapping sites’ near F-centers
• Low energy laser light (700 nm) stimulation
• e- repromoted into the conduction band
• Recombination of e- with Eu3+ ions and emission of blue –green (450-550 nm) visible light (VL)
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Computed Radiography (CR)
• Imaging plate (IP) made of PSP is exposed identically to SF radiography
• IP in CR cassette taken to CR reader where the IP is separated from cassette
• IP is transferred across a stage with stepping motors and scanned by a laser beam (~700 nm) swept across the IP by a rotating polygonal mirror
• Light emitted from the IP is collected by a fiber-optic bundle and funneled into a photomultiplier tube (PMT) that converts VL into e- current
X-ray → e-→ F+/F → laser →
e- → VL → PMT → ADC → RAM
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Computed Radiography (CR)
• Electronic signal output from PMT input to an ADC
• Digital output from ADC stored
• Raster swept out by rotating polygonal mirror and stage stepping motors produces I(t) into PMT which eventually translates into the stored DV(x,y):
• IP exposed to bright light to erase any remaining trapped e- (~50%)
• IP mechanically reinserted into cassette ready for use
• 200µm and 100µm pixel size (14”x17”:1780x2160 and 3560x4320, respectively) X-ray → e-
→ F+/F → laser → e- → VL → PMT → ADC → RAM
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Computed Radiography (CR)
• IP dynamic range about 100x that of SF
• Very wide latitude → flat contrast
• Image processing required:• Enhance contrast• Spatial-frequency filtering
• CR’s wide latitude and image processing capabilities produce reasonable OD or DV for either under or overexposed exams
• Portable radiography: where the tight exposure limits of SF are hard to achieve
• Underexposed → ↑ quantum mottle• overexposed → unnecessary patient dose
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Computed Radiography (CR)
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Digital Radiography (DR)
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Digital Radiography (DR)
• Indirect DR
• create visible light photons from x-rays with scintillator then produce electrons with photodiodes
• typically lower spatial resolution than direct DR and lower dose efficiency than direct DR due to limiting the phosphor thickness so as not to adversely impact spatial resolution
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Digital Radiography (DR)
• Direct DR
• directly create electrons from absorbed x-rays
• typically higher spatial resolution than indirect DR
• higher dose efficiency than indirect DR due to electric field lines constraining electron lateral drift
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Charge Coupled Devices (CCD)
• Form images from visible light• Videocams & digital cameras
• Each picture element (pixel) a photosensitive ‘bucket’ (PE)• Electrons accumulate in individual pixel cells• Accumulated charge read out pixel by pixel
• After exposure, the elements electronically readout via ‘shiftand-read’ logic and digitized
• Requires coupling between light source and CCD• Fluoroscopy and cine-angiography, digital cineradiography• Digital biopsy system (phosphor screen)
• 1K and 2K CCDs used
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Charge Coupled Devices (CCD)
lens coupling
ImageIntensifier
Fiberoptic
coupling
Readout
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Indirect Flat Panel Detectors
• Photodetector coupled to x-ray intensifying screen to generate VL photons from an x-ray exposure
• Gd2O2S or CsI• CsI grown in columnar crystals to improve
efficiency
• EK: Cs = 36 keV, I = 33.2 keV
• X-rays absorbed in screen give off visible light
• Visible light absorbed in photodetector• Fill factor determines efficiency
• Each element of the array (pixel) consists of transistor (readout) electronics and a photodetector area
• Detector size determines best spatial resolution• 125 µm -> 4 cycles/mm• 100 µm -> 5 cycles/mm
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Thin-Film Transistors (TFT)
• After the exposure is complete and the e- have been stored in the photodetection area (capacitor), rows in the TFT are scanned, activating the transistor gates
• Transistor source (connected to photodetector capacitors is shunted through the drain to associated charge amplifiers
• Amplified signal from each pixel then digitized and stored
X-ray→e-→VL→e →TFT→ADC→RAM
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Direct Flat Panel Detectors
• Use a layer of photoconductive material (e.g., α-Se) atop a TFT array
• e- released in the detector layer from x-ray interactions used to form the image directly
• High degree of e- directionality through application of E field
• Photoconductive material can be made thick w/o significant degradation of spatial resolution
• Photoconductive materials• Selenium (Z=34, EK = 12.7 keV)• CdTe, HgI2 and PbI2
X-ray→e-→TFT→ADC→RAM
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Digital Mammography
• Full-Field Digital Mammography (FFDM)
• Mosaic of CCD detectors• TFT flat panel detectors• Slot-scan detector
• 1D detector array
• Digital Imaging Detector
• Large dynamic range• Reasonable spatial• resolution (300 µm)• Expensive ~ $300k
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Digital Mammography
Digital Detector Film-Screen
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Image Processing
• Most common operations based on mathematical convolution
• Convolution kernels:• Soft tissue – smoothing• Bone – edge enhancement
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Image Processing
Contrast Enhanced Edge Sharpening Both
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Image Processing
original11x11
smooth
edge enhance
edgeminussmooth
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Image Processing
histogram equalization
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Temporal Subtraction
• Mask (background) subtracted from images during/post contrast injection
• Motion can cause misregistration artifacts
• Digital value proportional to contrast concentration and vessel thickness
Is = ln(Im) – ln(Ic) = µvessel · tvessel
• Temporal subtraction works best when time differences between images is short
• Possible to spatially warp images taken over a longer period of time
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Digital Subtraction Angiography (DSA)
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Dual-Energy Subtraction
• Exploits differences between the Z of bone (Zeff ≈ 13) and soft tissue (Zeff ≈ 7.6)
• Images taken either at two different kVp (two-shot), or
• One image (one-shot) taken with energy separation provided by a filter (sandwich)
Iout = ln (Ilow) – R · ln (Ihigh)
where R is altered to produce soft-tissue predominant or bone predominant images
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Dual-Energy Subtraction
Two-pulse single
detector
One-pulsesandwiched
detector
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Dual-Energy Subtraction
HighLow
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Dual-Energy Subtraction
Soft tissueBone
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Dual-Energy Subtraction
• MacMahon H. “Dual-energy and temporal subtraction digital chest radiography”. In: Samei E, Flynn MJ, eds. Syllabus: Advances in Digital Radiography: Categorical Course in Diagnostic Radiology Physics. Oak Brook, Ill: RSNA Publications; 2003: 181-188.
• Ho JT, Kruger RA, Sorenson JA. “Comparison of dual and single exposure techniques in dual-energy chest radiography”. Med Phys. 1989; 16:202-208.
End of Lecture!