Different types of imaging devices and principles

1. Digital X-Ray Imaging

2. Digital Radiography
o Screen film cassettes are replaced by digital radiographic image
receptors
o Radiography is the last modality to make the transition to
digital acquisition
o Screen film system produces excellent image quality ; large
FOV and high spatial resolution of radiography require digital
images to contain large amounts of data
o Disadvantages of large size of digital radiographic images are
a. They require lots of space on digital storage media
b. They require high network bandwidth in PACS
c. They require costly high luminance and high resolution
monitors for display

3. PACS
Picture Archiving and Communication System (PACS) is
a technology which provides economical storage and
convenient access to images from multiple modalities
4 Major Components
 Imaging machines
 Secure network for the distribution and exchange of
patient images
 Workstations or mobile devices for viewing, processing
and interpreting images
 Electronic archives for storing & retrieving images and
related documentation, reports

4. PACS and DICOM
• Electronic images and reports are transmitted
digitally via PACS; this eliminates the need to
manually file, retrieve, or transport film jackets,
the folders used to store and protect X-ray film.
• The universal format for PACS image storage
and transfer is DICOM

5. PACS and DICOM

6. DICOM
 DICOM (Digital Imaging and Communications in
Medicine) is a standard for handling, storing, printing, and
transmitting information in medical imaging.
 It includes a file format definition and a network
communications protocol.
 The communication protocol is an application protocol
that uses TCP/IP to communicate between systems.
DICOM enables the integration of scanners, servers,
workstations and network hardware from multiple
manufacturers into a PACS
 The National Electrical Manufacturers Association
(NEMA) holds the copyright to this standard

7. Digital System Technologies
 Computed radiography(CR)
 Direct Radiography (DR)
o CCD
o Indirect detection flat panel systems
o Direct detection flat panel systems
Computed Radiography is an imaging system
comprised of
1. Photostimulable storage phosphor
2. CR reader
3. Digital electronics

8. Computed Radiography
• Computed radiography (CR) is a marketing term for
photostimulable phosphor (PSP) detector systems.
• When x-rays are absorbed by photostimulable phosphors,
some light is promptly emitted; much of absorbed energy
is trapped in the PSP screen and can be read out later
• CR imaging plates are made of BaFBr and BaFI
• The imaging plate is exposed in a procedure identical to
screen-film radiography and the CR cassette is then
brought to a CR reader unit
• The cassette is moved into the reader unit and the
imaging plate is mechanically removed

9. Computed Radiography
o The digital image generated by the CR Reader is
temporarily stored on a local hard disk
o The imaging plate is a completely analog device, but it is
read out by analog and digital electronic technique.
o The imaging plate is translated along the readout stage in
the vertical direction (the y direction)
o Scanning laser beam interrogates the plate horizontally
(the x direction).
o The red laser light strikes the imaging phosphor at a
location (x,y)

10. Illustration of C R Hardware

11. Computed Radiography
oTrapped energy from the x-ray exposure at that
location is released from the imaging plate.
o A fraction of the emitted light travels through the
fiber optic light guide and reaches PMT
o The electronic signal that is produced by the PMT is
digitized and mapped onto a pixel matrix
o For every spatial location (x,y), a corresponding
gray scale value is determined

12. Computed Radiography
o Light released from the imaging plate is of a different
colour than the stimulating laser light
o Optical filter is mounted in front of the PMT
Working of stimulable phosphors
o Typical imaging plates are composed of about 85%
BaFBr and 15% BaFI activated with a small quantity of
Europium
o Doping creates defects in the BaFBr crystals that allows
electrons to be trapped more efficiently

13. Computed Radiography
oThe absorbed energy excites electrons associated with the Eu
atoms, causing divalent Eu atoms to be oxidized
oThe excited electrons become mobile and some fraction of
them interact with F centre which traps these electrons in a
higher energy metastable state
oThe latent image that exists on the imaging plate after x-ray
exposure exists as billions of electrons trapped in F-centers
oDuring laser light scanning of the imaging plate, the energy of
the red light is transferred to the electrons.

14. Computed Radiography
o Electrons gain enough energy to reach the
conduction band
oThese electrons then become de-excited by releasing
blue-green light
oTo erase the latent image the plate is exposed to a
very bright light source, which flushes almost all of
the metastable electrons to their ground state,
emptying most of the F-centers.

16. Computed Radiography
CR vs. Screen-film system
One advantage of CR over screen
film radiography is the much
larger dynamic range. Exposure
latitude is much wider.
So, we don’t have to take retakes
for under exposure or lower
exposure. (Hence patient
exposure is saved)
They can be used for portable
examinations.

17. Direct Radiography (DR)
• Refers to the acquisition and capture of the x-ray
image without user intervention
• Indirect detector: conversion of x-rays into light and
then light into photoelectrons
• Direct detector: conversion of X-rays into electron-
hole pairs with direct signal capture

19. Charge-Coupled Devices
• Charge-coupled devices (CCDs) produce high quality images from
visible light exposure
• CCD chip is an integrated circuit made of crystalline silicon
•It has an array of discrete detector electronics etched into its surface
• The silicon surface of a CCD chip is photosensitive
• As visible light falls on each pixel, electrons are liberated and build
up in the pixel
• The electrons are kept in each pixel because there are electronic
barriers 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

20. Bucket brigade CCD analogy

21. Charge-Coupled Devices

22. Charge-Coupled Devices
o CCD cameras are commonly used in medical imaging for
fluoroscopy and digital cineradiography
o The amplified light generated by the image intensifier is focused
with the use of lenses or fibre optics onto the CCD chip
o For small field-of-view applications such as dental radiography
(e.g., 25 X 50 mm detector), an intensifying screen is placed in
front of the CCD chip and the system is exposed.
o The light emitted from the screen is collected by the CCD chip
o Because of the excellent coupling between the screen and the
CCD chip, only a relatively small amount of the light generated in
the screen is wasted

23. Charge-Coupled Devices
o For applications in which the field of view is only
slightly larger than the area of the CCD chip, such as in
digital biopsy systems for mammography, a fiber optic
taper is placed between the intensifying screen and the
CCD
oThe fiber optic taper serves as an efficient lens that
focuses the light produced in the screen onto the
surface of the CCD chip

25. CMOS Detectors
 Complementary Metal-Oxide Semiconductor (CMOS)
light sensitive arrays are an alternative to the CCD arrays
 Both CCD and CMOS image sensors convert light into
electrons by capturing light photons at photosites
 The next step is to quantify the accumulated charge of
each photosite in the image. Here’s where the technologies
start to differ: in a CCD device, the charge is transported
across the chip and read at one corner of the array, and an
analog-to-digital converter turns each photosite’s charge
into a digital value.

26. CMOS Detectors
 In CMOS devices, on the other hand, there are several
transistors at each photosite that amplify and move the
charge using more traditional wires. This makes the sensor
more flexible for different applications, because each
photosite can be read individually.
 A special manufacturing process gives CCD devices the
ability to transport charges across the chip without
distortion, leading to high-quality, highly sensitive
sensors. CMOS chips use more conventional (and
cheaper) manufacturing processes.

27. Flat Panel Detectors
Indirect Detection Flat Panel Systems
• These detectors are based on amorphous silicon TFT/photodiode
arrays coupled to x-ray scintillators
• Commonly used scintillators
are CsI and Gd2O2S
The fundamental x-ray
conversion chain

28. Flat panel detector
 Flat panel detector systems are
pixelated discrete detector systems
 The flat panel comprises a large
number of individual detector
elements, each one capable of
storing charge in response to x-ray
exposure.
 Each detector element has a light-
sensitive region, and a small
corner of it contains the
electronics.

29. Flat panel detector
 The light-sensitive region is a photoconductor, and electrons
are released in the photoconductor region on exposure to
visible light.
 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

30. Flat panel detector
 Flat-panel TFT arrays are made of amorphous silicon,
where lithographic etching techniques are used to deposit
electronic components and connections necessary for x-ray
detector operation.
 Electronic components within each detector element (dexel)
include a TFT, a charge collection electrode, and a storage
capacitor.
 The TFT is an electronic switch that is comprised of three
connections: gate, source and drain.
 Gate and drain lines connect the source and drain of the
TFT’s along the row and columns, respectively.

32. Indirect Detection Flat panel
Step 1: X-ray photons striking the detector are absorbed by a
scintillation or phosphor material in the imaging plate that converts
the incident x-ray photon energy to light.
Step 2: A photosensitive array, made up of small (about 100 to
200 µm) pixels, converts the light into electrical charges. Each pixel
contains a photodiode that absorbs the light from the scintillator and
generates electrical charges. A FET or silicon TFT isolates each
pixel element and reacts like a switch to send the electrical charges
to the image processor.

33. Indirect ?? Detection Flat panel
 The term indirect comes from the fact that x-rays are
absorbed in the screen
 Absorbed energy is then relayed to photo detectors by visible
light photons
 Intensifying screen is layered on the front surface of the flat
panel array
 Light from the back of the screen strikes the detector panel
 Light has to propagate relatively large distances through the
screen resulting in blur; hence CsI screens mostly used

34. Fill Factor
 Because the electronics requires
a certain amount of area on the
dexel, the entire surface area is
not photosensitive.
 This reduces the geometrical
efficiency of light collection of
each dexel to less than 100%.
 The fill factor refers to the
percent of the area of each dexel
that is photosensitive

35. Fill factor
 Size of detector element largely determines spatial resolution
of detector system
 High spatial resolution need smaller detector elements
 The electronics of each detector takes some area
 This decreases light sensitive area of the detector element
 Light collection efficiency decreases as detector elements get
smaller
 Fill factor = light sensitive area/total area of detector element
 Low fill factor implies low contrast resolution

36. Direct Detection Flat Panel Systems
 Made from a layer of photoconductor material on top of a
TFT array; Selenium is commonly used as photoconductor
 The electrons released in the detection layer from x-ray
interactions are used to form the image directly
 With indirect systems, light released in the intensifying
screen diffuses as it propagates to TFT array
 Even with CsI blurring does still occur
 For direct detection system, electrons are the secondary
quanta that carry the signal

37. Direct Detection Flat Panel Systems
 A negative voltage is applied to a thin metallic layer on front
surface of the detector
 Detector elements are held positive
 During exposure, x-ray interactions liberate electrons that
migrate through the Se matrix
 Electrons are collected on the detector elements

38. Direct Detection Flat Panel Systems
 By applying an electric field electrons directionality can be
controlled
 Electric field lines can be locally altered at each detector
element so that sensitive area of the detector element
collects electrons that would otherwise reach the insensitive
area
 Hence effective fill factor increases
 Spatial resolution only limited by the dimensions of the
detector

39. Direct Detection Flat Panel Systems
 Selenium (Z=34) has relatively low attenuation coefficient at
diagnostic x-ray energies
 Selenium detectors are made much thicker than indirect
detection systems to improve detection efficiency
 Mercuric iodide, cadmium telluride and lead iodide are
being studied for use in direct detection flat panel systems