X‐ray Detector Physics Ho Kyung Kim [email protected]Pusan National University Medical Imaging Detectors Review Energy states in atoms are discrete (quantum mechanics) Unstable nuclei (atoms) become stable by emitting radiations Radiation can ionize atoms, resulting in ion pairs (ionizing radiation) Fast electron interaction with target atoms produces bremsstrahlung & characteristic x rays The number of x‐ray photons & their energy are controlled by the tube current & voltage, respectively X‐ray photons interact with matter by the photoelectric absorption (ா )& Compton scattering (ௌ ) processes The interaction probability is characterized by the linear attenuation coefficient • ௧௧ ሺ; , ሻ ൌ ா ሺ; , ሻ ௌ ሺ; , ሻ As a result of interaction, the number & intensity of x‐ray photons are exponentially attenuated with material thickness () or area density () • ൌ 0 ఓ௧ ൌ 0 ሺ ഋ ഐ ሻሺఘ௧ሻ Exposure describes x‐ & ‐ray fields in terms of their ability to ionize air, while the absorbed dose describes the energy imparted to matter by all kinds of ionization radiations 2
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X‐ray Detector Physicsbml.pusan.ac.kr/LectureFrame/Lecture/Graduates/Image... · 2020-03-17 · Intensifying screen Very inefficientphotographic film for capturing x rays • Only
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Energy states in atoms are discrete (quantum mechanics)
Unstable nuclei (atoms) become stable by emitting radiations
Radiation can ionize atoms, resulting in ion pairs (ionizing radiation)
Fast electron interaction with target atoms produces bremsstrahlung & characteristic x rays
The number of x‐ray photons & their energy are controlled by the tube current & voltage, respectively
X‐ray photons interact with matter by the photoelectric absorption (𝜇 ) & Compton scattering (𝜇 ) processes
The interaction probability is characterized by the linear attenuation coefficient
• 𝜇 𝐸; 𝑍, 𝜌 𝜇 𝐸; 𝑍, 𝜌 𝜇 𝐸; 𝑍, 𝜌
As a result of interaction, the number & intensity of x‐ray photons are exponentiallyattenuated with material thickness (𝑡) or area density (𝜌𝑡)
• 𝑁 𝑡 𝑁 0 𝑒 𝑁 0 𝑒
Exposure describes x‐ & ‐ray fields in terms of their ability to ionize air, while the absorbed dose describes the energy imparted to matter by all kinds of ionization radiations
• Measure the number of incident photons as a function of energy (bins)
• (Ideal PCD) reproduces the incident spectrum
• All the ‐ray (imaging) detectors including recent x‐ray detectors for multi‐energy imaging
𝐸 (keV)
𝑁 𝐸
10 (0.04)25 (20.10)
85 (0.35)100 (0.41)
604020 80 100
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Film radiography
Film
• Containing an emulsion with silver halide crystals (e.g., AgBr)
• Absorbed optical photons by the silver halide grains, and then metalized (dark)
• Precipitated metallic silver when developed
• Negative image
• Graininess
‒ The image derived from the silver crystals is not continuous but grainy
‒ The larger the grains, the faster the film becomes dark (amount of photons needed to change a grain into metallic silver upon development is independent of the grain size)
• Speed
‒ Inversely proportional to the amount of light needed to produce a given amount of metallic silver on development
‒ Mainly determined by the silver halide grain size
‒ The larger grain size the higher the speed
‒ How many x‐ray photons are needed to produce a certain density on the film
‒ Speed in the screen‐film system: Reflector improves the speed
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• Contrast
– Plot of the optical density 𝐷 vs. the logarithm of the exposure 𝐸 (called the sensitometric curve)
– 𝐷 log
– A larger slope implies a higher contrast at the cost of a smaller useful exposure range
– gamma: the maximal slope
• Resolution
– Depending on its grain size and the light scattering properties
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A. A light photon removes the outermost electron from a bromide anion. The bromine atom (now uncharged) diffuses out the crystal. The liberated electron wanders through the crystal and is trapped at the sensitivity speck.
B. The speck is now negatively charged.
C. It draws an interstitial silver cation to itself.
D. The electron on the sensitivity speck neutralizes the charge of the silver ion, and the resulting silver atom is deposited there.
E. Another light photon causes the process to repeat. The deposition of 10 or so silver atoms at the sensitivity speck transforms it into a latent image center. A crystal with a latent image center will be transformed into a fleck of pure silver during the development process.
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Intensifying screen
Very inefficient photographic film for capturing x rays
• Only 2% of the incoming x rays contributes to the output image (quantum absorption efficiency)
• Would yield prohibitively large patient dose
• Typically, placed the film b/w two intensifying screens
Screen
• Containing phosphors (Gd2O2S:Tb) with a high quantum absorption efficiency
• Absorbing most of the x‐ray photons
• 25% of QAE or QE of each screen instead of 2% for film
• Converting x rays into visible light (which is scattered in all directions, resulting in image blur)
• Fluorescence
‒ Prompt emission & stop of light and used in intensifying screens
‒ CaWO4, Gd2O2S:Tb, CsI:Tl
• Phosphorescence (or afterglow)
‒ Continuation of light emission (> 10‐8 s)
‒ Undesirable because it causes “ghost” images and image lag (and fogging in film)
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no intensifying screen128 mAs, >12 lp/mm,
fine screen10 mAs, >7 lp/mm,
fast screen1.33 mAs, <5 lp/mm,
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Line pairs per mm
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Image intensifier
Working principle
• Conversion of x rays into visible light by an input phosphor (or fluorescent) screen
• Emission of electrons from a photocathode hit by light
• Accelerated the ejected electrons by a potential difference b/w the cathode and the output
• Focused electron beam to the output phosphor screen by electrostatic or magnetic focusing
• Captured visible light from the phosphor screen by a camera
Capable of producing dynamic image sequences in real time at video rate (a process known as fluoroscopy)
Image degradation
• Less spatial resolution rather than that of a film‐screen system (because of the limited camera resolution)
• Increased noise due to the additional conversions (light electrons light)
• Geometric distortion, called pin‐cushion distortion, toward the borders of the image
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J. A. Seibert | Pediatr. Radiol. | 2006 27
Electronic focusing allows:
• Large FOV
‒ Large coverage
‒ Higher gain due to minification
‒ Pin‐cushion distortion in the periphery of the image (caused by mapping the spherical input phosphor electron image onto the planar output phosphor)
• Small FOV
‒ Magnification
‒ High spatial resolution
‒ Lower gain (or higher patient dose)
J. A. Seibert | Pediatr. Radiol. | 2006 28
PACS
Picture archiving & communications systems
• System for the storage, transfer, & display of radiological images
• Able to include the teleradiology that transmits images for viewing at sites remote from where they are acquired
• Exchange information with:
‒ HIS (hospital information system)
‒ RIS (radiology information system)
‒ EMR (electronic medical record) system
DICOM (digital imaging & communications in medicine)
• Standards to facilitate the transfer of medical images & related information (patients, images, & studies)
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Analog vs. Digital
Mrs. Roentgen, 22 Dec. 1895Taken from I. A. Cunningham’s Slides Me, 22 Sept. 2009
• The conversion from a continuous function to a discrete function retaining only the values at the grid points
17921792 896896 448448 224224
141428285656112112
128 larger pixel
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Quantization
• The conversion from analog samples to discrete‐value samples
8 bits 7 bits 6 bits 5 bits
1 bit2 bits3 bits4 bits
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Digital images
• A set of possible (achromatic) gray levels or (chromatic) colors in a rectangular grid‐point (or pixel) array
• Sampling and quantization (integer)
• Dynamic range: the set of possible gray levels
• Contouring: an artificial looking height map
• How many gray values are needed to produce a continuous‐looking image?
8 bits/pixel 4 bits/pixel
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Why such a long time gap to digitalization?
The size does matter!
• Limited size of available imagers (e.g., CCD, CMOS photodiode arrays)
• Availability in large size wafers
• Marginable production yield in the wafer‐based fabrication process
Radiation hardness of silicon or other materials
17”
14” 17”
17”
CCD, LBNL Taken picture from M. J. Flynn’s Lecture Slides
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Computed radiography
CR based on photostimulable phosphors, introduced in the early 1980s by the Fuji Photo Film Co., has been used until now (and still after)
P. Suetens | Fundamentals of Medical Imaging |Cambridge Univ. Press | 2009R. Schaetzing | RSNA Categorical Course | 2003
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Storage phosphors
Storage phosphors
• Also called photostimulable phosphors
• Photo‐stimulated luminescence
‒ An extreme case of phosphorescence
‒ Released the temporarily stored energy in a form of light by stimulation (laser)
Computed radiography
• Use of the storage phosphor
• Trapped the excited electrons by electron traps (impurities in the scintillator)
• (it takes 8 h to decrease the stored energy by ~25%)
• Extraction of stored energy or latent image by pixelwise scanning with a laser beam
• Released visible light by the de‐excitation of electrons
• Captured light by an optic array and transmitted to a photomultiplier
• Converted analog electrical signal into a digital bit stream by an A/D converter
• Erased any residual image by a strong light source
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Some tricks for large‐area imaging
Utilization of the conventional, small‐size photo‐imagers (e.g., CCD, CMOS)
• With various mechanical motions;
‒ May provide a better image quality due to the scatter rejections
‒ But, can we finish scanning within a single heart beat, and handle the heat load
• By coupling with optics;
• But, very special caution should be devoted when designing optics systems
• e.g. 𝜂 = 1.5% (𝜏 = 0.8, M = 0.5, & F = 1.2)
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222
2
44)1(4M
F
M
MFM
M
M. Mahesh | RadioGraphics | 2004
M. J. Yaffe & J. A. Rowlands | PMB | 1997
Secondary quantum noise
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Scanning radiography: panoramic radiography
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Imaging Dynamic Co., Ltd., Canada
Lens‐coupled DR system
CCD
X-ray
LightLens
Mirror
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• By butting small‐size imagers (mosaic method);
‒ But, should keep the butting‐gap be as small as a pixel pitch
‒ Needed additional image processing techniques for interpolation between gaps and different signal responses between the detector modules
Image courtesy of Dr. T Achterkirchen, Rad-icon Imag. Corp. Image courtesy of Vatech & E-Woo
A pixel
< 50 m
• By stitching small‐size imaging chips (or reticles) in wafer‐process level;
– Ideally, there are no gaps between reticles
– But, also needed an additional image processing technique for different characteristics between reticles due to the nonuniform fabrication process over large area
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Single‐wafer (12”) detector
Image courtesy of Rayence (Vatech)SK Heo et al. | Proc. SPIE | 2011
28 kVp, 100 mAs
24.1 cm
17.1 cm
70 m
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Breakthrough
Large‐area flat‐panel detectors (FPDs)
• Motivated by large‐area AMLCDs and initialized in the mid‐1980s
• Realization of 2D pixel arrays (TFT alone or a combination of TFT plus photodiode in a pixel) on large‐area glass substrate based on amorphous silicon process
‒ Lower fabrication cost compared to the crystalline counterpart
‒ Better radiation hardness
‒ But, worse electrical properties & a high density of charge traps, which may result in image lag & ghosting