Digital radiography-Avinesh Shrestha

INTRODUCTION
 Historically, digital radiography referred to specialized modalities that produced digital images.(CT, MRI,
Nuclear Medicine, Ultrasound) which began in 1970’s
 Since then, the use of digital technology in diagnostic medical imaging is rapidly expanding
 Now, Digital radiography (DR) is a term used to describe general radiography when the radiographic images
are in digital form and are capable of being displayed on a computer monitor where digital X-ray sensors
are used instead of traditional photographic film
 Here Radiation detectors produce output as electrical signal which is proportional to the radiation intensity
and hence, digital image is acquired.
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HISTORY
 Digital imaging modalities, such as CT, ultrasound, & nuclear
medicine gained widespread acceptance in 1970s.
 The first form of digital imaging, Digital Subtraction
Angiography, was introduced in 1977 and put to clinical use
in 1980.
 In CR, storage phosphor image plates were first used to
record general radiographs in 1980.
 The direct capture of x-rays for digital images was introduced
with DDR which used CCD, in 1990.
 Introduced in 2001,The flat-panel thin-film transistor
detectors could expose & display images in near real time.
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ADVANTAGE
 Principal advantage of DR over film/screen (F/S) radiography is the increased dynamic range and the
feature of Image enhancement/Post manipulation for better interpretation.
Also:
 Superior gray-scale resolution: uses 256 colors of gray compared to 16-25 shades on film
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ADVANTAGE
 Exposure reduction: 50-80% less radiation with no loss of image quality
 Fast: instantaneous viewing and interpretation
 Cost effectiveness over time: less film and processing solutions
 Environmental: No chemicals to dispose of
 Efficiency: More productive and versatility of storing/sending images, Repeat examinations
are reduced
 Image enhancement/manipulation for better interpretation: Contrast, colorization,
magnification, sharpness, and image orientation 4/5/2016
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DISADVANTAGE
 Cost: High set up cost depending on manufacturer and features
 Technology changes: System may become obsolete and no longer has support
 Artifacts: unique to CR/DR can be introduced in the digital image acquisition and /or
retrieval process.
 The spatial resolution of DR image recording systems is lower than that of F/S image
recording systems however ;The impact of such a lower spatial resolution system on
clinical performance is not significant.
 Dose-creep: since , exposure latitude is wide , high exposure technique may be used
which increases the patient dose which is called dose-creep. This can be reduced by
exposure indicator or exposure index which gives the user feedback about the actual
dose. 4/5/2016
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COMPONENTS OF DR
 Capture element: in which the X- ray is captured. In CR, it is PSP & in DR it may be
CsI., GdSO or a-Se.
 Coupling element: which transfers x-ray generated signal to the collection
element. It may be a lens or fiber optic assembly, a contact layer or a-Se.
 Collection element: may be a photodiode, CCD or TFT. The photodiode & CCD are
light-sensitive whereas TFT is charge sensitive.
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CHARGED COUPLED DEVICE
 Originally invented by bell laboratories in 1969
 The basic CCD consists of a series of metal oxide semiconductor capacitors with
discrete pixel electronics that are fabricated very close together on the semiconductor
surface
 Is an integrated circuit made of crystalline silicon.
 The silicon surface of CCD chip is photosensitive.
 CCDs are used as indirect type of digital receptor.
 Thermally generated electrons are indistinguishable from photo-generated electrons .
They constitute a noise source known as ‘Dark Current’ and it is important that CCDs are
kept cold to reduce their number. 4/5/2016
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CHARGED COUPLED DEVICE
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WORKING OF CCD
 Light – incoming photons falls onto the surface of a pixel.
 Result- generates free electrons in the silicon of the CCD due to the
photoelectric effect, proportional to the number of photons striking it.
These electrons collect in little packets.
 Each pixel contains 3 electrodes as shown. The e-s are kept in each pixel
because of these electronic barriers (voltage) on each side of the pixel
during exposure.
 Once the CCD chip has been exposed, the electronic charge that resides in
each pixel is read out. Charge is readout in “bucket brigade” fashion ,i.e.;
The electronic charge is shifted pixel by pixel by appropriate control of
voltage level at the boundaries of each pixel.
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CCD Bucket Brigade Analogy
RAIN (PHOTONS)
BUCKETS (PIXELS)
VERTICAL
CONVEYOR
BELTS
(CCD COLUMNS)
HORIZONTAL
CONVEYOR BELT
(SERIAL REGISTER)
MEASURING
CYLINDER
(OUTPUT
AMPLIFIER)
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Exposure finished, buckets(Pixels) now contain samples of rain(Photons)
collected as charge.
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Conveyor belt starts turning and transfers buckets(Pixels).The Rain(Photons) collected on the vertical conveyer belt as
charge is tipped into buckets on the horizontal conveyor.
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Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn into the measuring
cylinder(output amplifier) .
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`
After each bucket has been measured, the measuring cylinder is emptied , ready for the next
bucket load.
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A new set of empty buckets is set up on the horizontal conveyor and the process is
repeated.
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Eventually all the buckets have been measured, the CCD has been read out.
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ADVANTAGES OF CCD
 The main advantage of the CCD is the extremely fast discharge time which is useful in cardiac
catheterization where high-speed imaging is critical to visualizing blood flow.
 Sensitivity-ability to detect & respond to very low levels of visible light. Hence reduces the patient
dose.
 Dynamic range- ability to respond wide range of light intensity. Radiation response of CCD is linear.
 Size- its size is very small that makes it highly adaptable to DR in its various forms. It measures 1 to 2
cm.
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DISADVANTAGE
 The principal disadvantage of CCDs is that they are physically small and consequently can
image only a small region.
 When large FOV is needed to image, it is impossible to image the light into the surface of
CCD chip without losing the light photons.
 The amount of light lost is proportional to demagnification factor (input area of
screen/output area of CCD chip)
 The assembly of multiple CCD (mosaic CCD) is used to overcome this problem.
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PMT
 Photomultiplier tubes are members of vacuum tubes, are extremely sensitive detectors of
light in the ultraviolet, visible and near infrared ranges of the electromagnetic spectrum.
 These detectors multiply the current produced by incident
light by as much as 100 million times ,in multiple dynodes
stages, enabling individual photons to be detected when
the incident flux of light is very low.
 Their operation depends on two topics
 The photoelectric effect, in which incoming photons strike
a metal surface, and eject an electron from the surface.
 Emission of electrons from a metal following the metal being struck by a previous
electron. Providing the incoming electron has sufficient kinetic energy, more than one
electron is emitted, the actual number being a strong function of the incident electron's
kinetic energy. 4/5/2016
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Contd….
 A photomultiplier consists of five main parts:
 a cathode which is coated with a material of low work function
 An anode to collect electrons
 A series of dynodes between the cathode and the anode
 An external power supply which produces an electric field between the cathode and first dynode,
between the dynodes, and between the last dynode and the anode.
 An external current meter to measure the number of electrons collected at the anode, and a
recorder to collect the information.
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PHOTODIODE
 A photodiode is a semiconductor device capable of photo-detection and
then converting light into either current or voltage, depending upon the
mode of operation.
 Photodiodes are similar to regular semiconductor diodes except that they
can be exposed (to detect UV or X-rays) to allow light to reach the sensitive
part of the device.
 There are many types of photodiode (PN photodiode, PIN photodiode, Avalanche photodiode,
Schottky photodiode) designed for use but specifically PIN photodiode is used to increase the speed
of response.
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NEWER COLLECTION ELEMENTS
 Recent Advancement is focusing on Complimentary-Metal-Oxide Semiconductors(CMOS)
technology and are being used instead of CCD
 These are newer design of Transistors.
 CMOS is a type of MOSFET(Metal-Oxide-Semiconductor Field- Effect Transistor) which are
basic electronic devices used to direct and control logic signals in IC design.
 A MOSFET has four terminals: gate, source, drain, and substrate (body), and according to the
nature of doping into these terminals MOSFET are classified as n-MOS or p-MOS or C-MOS
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Contd…
SUBSTRATE(BODY)
DRAINSOURCE
GATE
(Polysilicon)
INSULATOR(SiO)
G
DS
BASIC DESIGN OF MOSFET
B
N-MOS
SUBSTRATE(BODY)
(p-type)
DRAIN
(n-type)
SOURCE
(n-type)
GATE
(Polysilicon)
INSULATOR(SiO)
G
DS
B
SUBSTRATE(BODY)
(n-type)
DRAIN
(p-type)
SOURCE
(p-type)
GATE
(Polysilicon)
INSULATOR(SiO)
G
DS
P-MOS
B
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P-type
N-type
N-type
N-type P-type P-type
GATE
(Polysilicon)
INSULATOR(SiO)
GATE
(Polysilicon)
INSULATOR(SiO)
G
G
G
D
D
S
SDS
N-MOS P-MOS
C-MOS
B B
Coming to CMOS, it is a complementary MOSFET meaning it has both PMOS and NMOS connected somehow to each
other. Hence any circuit which uses both PMOS and NMOS is a CMOS circuit.
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CMOS VS CCD
 Modern CMOS sensors use a more specialized technology and advancements have eradicated the
difference in light sensitivity between a CCD and CMOS sensor at a given price point.
 The disadvantages of CCD sensors are that they are analog components that require more electronic
circuitry outside the sensor, they are more expensive to produce, and can consume up to 100 times
more power than CMOS sensors which can lead to heat issues, which not only impacts image quality
negatively, but also increases the cost.
 Unlike the CCD sensor, the CMOS chip incorporates amplifiers and A/D-converters, which lowers the
cost since it contains all the logics needed to produce an image, i.e. ,Every CMOS pixel contains
conversion electronics.
 Compared to CCD sensors, CMOS sensors have better integration possibilities and more functions.
However, this addition of circuitry inside the chip can lead to a risk of more structured noise, such as
stripes and other patterns.
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Contd…
 Nowadays flat panel X-ray detector (FPD) components employ an innovative CMOS sensor
design to provide a new level of performance and reliability for X-ray systems.
 Its major advantages are high frame rates, low noise, high reliability, no image lag, high
spatial resolution, smaller system size. a faster readout, and lower power consumption
 Clinical benefits in medical applications include lower radiation dose to the patient without
compromising image quality.
 CMOS Detectors are more flexible, more stable, more sensitive, and faster than TFT-based
flat panel detectors while producing higher resolution images.
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FLAT PANEL DETECTORS
 The flat panel comprises a large number of individual detector
elements, each one capable of storing charge in response to x-ray
exposure.
 During exposure, charge is built up in each detector element and is
held there by the capacitor. After exposure, the charge in each
detector element is read out using electronics .
 Because each detector element has a transistor(a simple electronic
switch that has three electrical connections the gate, the source,
and the drain) and the device is manufactured using thin-film
deposition technology,so these flat panel systems are also called
thin-film transistor (TFT) image receptors.
 It may be of Direct or Indirect in nature which categorize the DR
into Direct DR and Indirect DR.
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 Each detector element in a flat panel detector has a
transistor associated with it; the source is the
capacitor that stores the charge accumulated during
exposure, the drain is connected to the readout
lines(vertical wires) , and the gate is connected to the
horizontal wires.
 Negative voltage applied to the gate causes the
switch to be turned off(no conduction from source
to drain), whereas when a positive voltage is applied
to the gate the switch is turned on (source is
connected to drain).
R1C1(
A)
R1C2(
B)
R2C2(
D)
R2C1(
C)
S1 S2
Multiplexer
Digitizer
ScanControl
Charge Amplifier
R1
R2
C1 C2
Detector element
(charge collector)
R=Gate lines
C=Readout line(Drain)
TFT
S
G
D
TFT
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R1C1(A) R1C2(B)
R2C2(D)R2C1(C)
S1 S2
Multiplexer
Digitizer
ScanControl
Charge Amplifier
R1
R2
C1 C2
illustrates the readout process used for flat
panel detector arrays. four detector elements
are shown (A through D). Two gate lines
(rows R1, R2) and Two readout lines
(columns C1,C2) are illustrated.
Detector element
(charge collector/source)
R=Gate lines
C=Readout line(Drain)
S
G
D
TFT
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SCANNED PROJECTION RADIOGRAPHY
 Developed by CT vendors to facilitate patient positioning after the
introduction of 3rd gen CT scanners.
 It remains in use with virtually all current multi-slice helical CT imaging
systems.
 The patient is positioned on the CT couch and then is driven through
the gantry while the x-ray tube is energized.
 The x-ray tube and the detector array do not rotate but are stationary,
and the result is a digital radiograph.
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Contd…
 In SPR, the x-ray beam is collimated to a fan by pre-
patient collimators. Post-patient image-forming x-rays
likewise are collimated to a fan that corresponds to
the detector array—a scintillation phosphor, usually
NaI or CsI—and is coupled to a linear array of CCDs
through a fiber-optic light path.
 At the present time, SPR is reemerging with some
modification as a promising adjunct to digital
radiographic tomosynthesis (DRT).
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Contd…
A scanned projection radiography of the entire of the body obtained in computed tomography
(Courtesy,Colin Bray,Baylor College Of Medicine))
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COMPUTED RADIOGRAPHY(CR)
 Digital radiography that records radiographic images on photostimulable phosphor plates instead of
film/screen image receptors and the image is acquired in Digital Form.
CR cassette with Imaging Plate(IP) Conventional Cassette with Screen-Film
IP
Screen
Film
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 CR is based on the principle of Photo-stimulable Luminescence.
 The commonly used phosphor is barium fluorohalides: BaFBr (85%) and BaFI (15%):
Eu (europium). (X-ray + Eu+2 = Eu+3 + e–)
 Some light is promptly emitted, but much of the absorbed x-ray energy is trapped in
the PSP screen & can be read later. So, also called storage phosphors screen.
 Cassette is placed in a reader(Digitizer) to capture and analyze the image data.
COMPUTED RADIOGRAPHY(CR)
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 The phosphor can be made as flexible screen, which is
enclosed in a rugged cassette and is called imaging plate
 Imaging plate was first introduced by Fuji, Japan, in 1983 and
is similar to that of a screen-film cassette
 PSPs are grown as linear filaments in the IP (needle-shaped
technology), enhances the absorption of x-rays & limit the
spread of stimulated emission.
IMAGING PLATE
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 PROTECTIVE LAYER: a very thin, tough, clear plastic that
protects the phosphor layer from handling trauma.
 PHOSPHOR LAYER: the active layer of Ba FX: Eu +2 , traps
electrons during exposure
 REFLECTIVE LAYER: It sends light in a forward direction when
released in the cassette reader.
 CONDUCTIVE LAYER: This layer grounds the plate to reduce
static electricity problems and to absorb light to increase
sharpness.
 BASE: is a semi rigid material that provides the imaging sheet
with strength and is a base for coating the other layers.
IMAGING PLATE
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e
X-ray photon
X-ray interaction with valence electron
Excitation of electron into a metastable state
e
He-Ne/solid State Laser
De-Excitation with Photostimulable Luminescence
e
e
Excitation Followed by spontaneous De-excitation with Fluorescence
X-ray photon
X-ray interaction with valence electron
Electron Trap(F-centre)
(Due to the presence of Eu)
What happens In Imaging Plate??
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CR READER(DIGITIZER)
The CR reader represent the mechanical, optical and computer modules.
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MECHANICAL FEATURES
 When the CR cassette is inserted into the CR reader , the IP is removed & is fitted to
a precesion drive mechanism.
 The drive mechanism moves the IP slowly along the long axis of the IP, known as
slow scan mode.
 A deflection device deflects the laser beam back & forth across the IP which is called
fast scan mode.
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 It precisely interrogate each meta-stable e-s of the latent image in the precise fashion.
 Its components include the laser, beam-shaping optics, light-collecting optics, optical filters
& a photodetector.
 The he-ne gas laser is the source of stimulating light which is monochromatic.
 The gas laser has been largely replaced by the solid state laser which produces the light of
longer wavelength which are less likely to interfere with emitted light.
OPTICAL FEATURES
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OPTICAL FEATURES
 The light beam is focused into the reflector by a lens system that keeps the beam diameter
small (less than100micrometer).
 Smaller the laser beam size , higher the spatial resolution of the image.
 Emitted light is focused by fiber optic collection assembly & is directed at the photodetector.
 Before photo-detection , the light is filtered so that none of the long-wavelength stimulation
light reaches the photo-detector. Filtering improves SNR.
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COMPUTER CONTROL
 The output of the photodetector is a time varying analog signal that is transmitted to a
computer system
 The time-varying analog signal from the photo detector is processed for amplitude, scale,
and compression.
 This shapes the signal before the final image is formed.
 Then, the analog signal is digitized with the help ADC with attention paid to proper
sampling and quantization.
 The computer of the CR reader is in control of the slow scan & the fast scan. 4/5/2016
60

Optical filterPMT
He-Ne/Solid state Laser
Rotating Polygon or Oscillating Mirror
To ADC
Slow scan(IP Direction)
Beam Shaping Optics
INSIDE DIGITIZER
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Bright white fluorescent light
Erasing The IP for reuse
INSIDE DIGITIZER
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DDR
 Direct digital radiography, a term used to describe total electronic image
capturing.
 Eliminates the need for an image plate altogether.
 uses a transistor receiver that captures and converts x-ray energy directly into
digital signal that is seen immediately on monitor then sent to PACS/ printer/
other workstations for viewing.
4/5/2016
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DDR
Indirect
Conversion
CsI,GdSO scintillator
Thin film transistor
CsI
Charge coupled
device
Direct
Conversion
Photoconductor
a-Se
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Direct DR Indirect DR
a-Si photodiode/CCD
(CsI/
GdOs)
Basic Difference of Direct DR and Indirect DR
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Basic Difference of Direct DR and Indirect DR
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INDIRECT DR
 Indirect Conversion with a CCD.
 X-ray energy is converted into light by a scintillator such as Tl doped
cesium iodide. The amount of light emitted is then recorded by the CCD,
and the light is converted into electrical charges.
 It is an array consisting of several CCD chips which forms a detector area
similar to that of a flat-panel detector.
 CCDs can be used for radiography as part of either a lens-coupled CCD
system or a slot-scan CCD system
 CCD-based systems were comparable to flat-panel detectors in terms of
image quality and allowed slightly superior low-contrast visualization.
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Contd…
 Indirect Conversion with a Flat-Panel Detector
 Indirect conversion DR systems are “sandwich” constructions consisting of a
scintillator layer, an amorphous silicon photodiode circuitry layer, and a TFT
array.
 When x-ray photons reach the scintillator, visible light proportional to the
incident energy is emitted and then recorded by an array of photodiodes and
converted to electrical charges.
 These charges are then read out by a TFT array.
 The scintillators usually consist of CsI or GdSO .The advantage of CsI-based
scintillators is that the crystals can be shaped in thin needles, which can be
arranged perpendicular to the surface of the detector.
CsI
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Contd…
 One further advantage of flat-panel detectors is their small size.
 Because CsI based flat-panel detectors are highly vulnerable to mechanical load
because of their fine structure, these systems cannot be used outside of fixed
installations and therefore lack mobility.
 Portable flat-panel detector systems make use of GdOS based scintillators, which are as
resistant to mechanical stress as are storage phosphors .
 Image generation with flat-panel detectors is almost a real-time process, with a time
lapse between exposure and image display of less than 10 seconds.
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DIRECT DR
 Direct conversion requires a photoconductor that converts x-ray
photons into electrical charges by setting electrons free .
 Typical photoconductor materials include amorphous selenium, lead
iodide, lead oxide, thallium bromide, and gadolinium compounds.
The most commonly used element is selenium.
 Selenium-based direct conversion DR systems are equipped with
either a selenium drum or a flat-panel detector.
 A-Se is both capture & coupling element.
 Selenium is a photoconductor that when exposed to radiation alters
its electrical conductivity proportional to the intensity of radiation
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Contd…
 In the former case, a rotating selenium drum, which has a positive electrical surface charge, is
exposed to x-rays. During exposure, a charge pattern proportional to that of the incident x-
rays is generated on the drum surface and is recorded during rotation by an ADC. However,
because of their mechanical design, selenium drum detectors have no mobility at all.
 A newer generation of direct conversion DR systems make use of selenium-based flat-panel
detectors.
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Digital radiography-Avinesh Shrestha

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