phys lecture 5 DigitalRadiography -...

Digital Radiography

PHYS Lecture

Carlos Vinhais

Departamento de FísicaInstituto Superior de Engenharia do Porto

[email protected]

Carlos Vinhais 2

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

Carlos Vinhais 3

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

Carlos Vinhais 4

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

Carlos Vinhais 5

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

Carlos Vinhais 6

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)

Carlos Vinhais 7

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

Carlos Vinhais 8

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

Carlos Vinhais 9

Digital Technologies

• Computed Radiography (CR)• Digital Radiography (DR)

Carlos Vinhais 10

Computed Radiography (CR)

Carlos Vinhais 11


• 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)

Carlos Vinhais 12


• 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

Carlos Vinhais 13


• 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

Carlos Vinhais 14


• 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

Carlos Vinhais 15


Carlos Vinhais 16

Digital Radiography (DR)

Carlos Vinhais 17


• 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

Carlos Vinhais 18


• 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

Carlos Vinhais 19

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

Carlos Vinhais 20

Charge Coupled Devices (CCD)

lens coupling

ImageIntensifier

Fiberoptic

coupling

Readout

Carlos Vinhais 21

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

Carlos Vinhais 22

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

Carlos Vinhais 23

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

Carlos Vinhais 24

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

Carlos Vinhais 25

Digital Mammography

Digital Detector Film-Screen

Carlos Vinhais 26

Image Processing

• Most common operations based on mathematical convolution

• Convolution kernels:• Soft tissue – smoothing• Bone – edge enhancement

Carlos Vinhais 27

Image Processing

Contrast Enhanced Edge Sharpening Both

Carlos Vinhais 28

Image Processing

original11x11

smooth

edge enhance

edgeminussmooth

Carlos Vinhais 29

Image Processing

histogram equalization

Carlos Vinhais 30

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

Carlos Vinhais 31

Digital Subtraction Angiography (DSA)

Carlos Vinhais 32

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

Carlos Vinhais 33


Two-pulse single

detector

One-pulsesandwiched

detector

Carlos Vinhais 34


HighLow

Carlos Vinhais 35


Soft tissueBone

Carlos Vinhais 36


• 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!

phys lecture 5 DigitalRadiography -...

Documents

Transcript of phys lecture 5 DigitalRadiography -...