# Lecture 5 Intro to digital and CR by J63o2xk8

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```									      FRCR: Physics Lectures
Diagnostic Radiology

Lecture 5
An introduction to digital imaging and
Computed Radiography (CR)

Dr Tim Wood
Clinical Scientist
Overview
• What is a digital image?
– Pixels and bit depth
– The advantages of digital
– The disadvantages
• Computed Radiography (CR)
– The theory of CR
– The advantages and disadvantages
• Digital Radiography (DR)
– The theory of DR
– The advantages and disadvantages
The story so far…
• We know how X-rays are made in the X-ray tube and
how they interact with the patient
• We know how we control the quality and intensity of the
X-ray beam, and hence patient dose, with kVp, mAs,
filtration and distance
• We discussed the main descriptors of image quality
– Contrast
– Spatial Resolution
– Noise
• Discussed ways to improve contrast by minimising
scatter and using contrast agents
• Remember, there is always a balance between patient
dose and image quality – fit for the clinical task!
• Film is a dying medium for X-ray imaging…
Digital imaging
What is a digital image?
• A digital image can be thought of as an array of
pixels (or voxels in 3D imaging) that each take a
discrete value
• The value assigned is dependent on the X-ray
intensity striking it
• Depending on its value, each pixel is assigned a
shade of grey
• Pixel size may determine the limiting spatial
resolution of the system
What is a digital image?
Binary numbers
• Our conventional number system is ‘decimal’
– There are 10 base numbers (including 0)
• Binary system has only two base numbers (1
and 0)
• Computer memory holds a voltage signal that
can either be a 1 or 0
– This is known as a bit
– With 4 bits, 16 (24) possible values can be stored e.g.
0000, 0001, …, 1111
– With 8 bits, 256 (28) possible values can be stored
• Computer memory is described in terms of the
number of bytes (B)
– 1 B = 8 bits
What is a digital image?
• The value is stored in binary format, and the
maximum value is related to the bit depth
– Greater bit depth = more levels of grey in the image =
greater potential contrast in the image
– e.g. 12 bit = 4096 (212) levels of grey
• Bit depth and number of pixels determines the
size of the final image on the hard-drive
– e.g. 512 x 512 image with 4096 levels of grey
requires 0.38 MB, but 1024 x 1024 image requires
1.54 MB!
What is a digital image?
• Image size is important for archiving and
transferring to and from the PACS
• Bit depth is primarily determined by the dynamic
range of the detector
– Typically, 12-bit or 14-bit sufficient for DR

• Image processing, display, artefacts, terminology
and PACS will be dealt with by Craig
Why bother with digital?
• Film has been used since the beginning, so why
are we changing to digital techniques?
– Increased latitude and dynamic range
– Images can be accessed simultaneously at multiple
workstations
– Viewing stations can be set up in any location
– Uses digital archives rather than film libraries
– Images quicker to retrieve and less likely to be lost
– Post processing
– Softcopy reporting – lower cost if do not print
– No need for dangerous processing chemicals
Disadvantages of digital
• Initial cost
• Problems with interconnectivity
• Lack of information and set up of automatic
exposure control (AEC)
• Lack of link between exposure and brightness
– Potential for dose creep (see following slides)
• Human error in choosing exam type and speed
class
• Generally poorer limiting spatial resolution when
compared with film
Dynamic Range - Film
• With conventional film,
too low a dose will
results in a ‘thin’ film
• Too high a dose results
in a very dark film
• Fixed and limited
dynamic range – must
match exposure
parameters to the film
being used
– Gives a measure of
control over patient dose!
Dynamic Range - Digital
• With digital, too low a dose will
still produce a recognisable
image (just a bit noisy!)
• Similarly, too high a dose will
produce a recognisable image
(but with very little noise!)
• Consequences:
– Less retakes = GOOD
– Dose creep = BAD – must pay
special attention to digital imaging
to ensure doses are optimised
Digital Imaging Techniques
•   Computed Tomography (CT)
•   Radionuclide imaging
•   Film scanner
•   Computed radiography (CR)
•   Direct digital radiography (DR)
•   Flat panel fluoroscopy
•   Electronic portal imaging device (EPID)
Computed Radiography
Computed Radiography (CR)
• The most common technique for producing
digital images
• Was the first digital technique available
commercially
• Exploits storage phosphors which emit light that
is proportional to the intensity of the X-rays that
hit it, when they are stimulated by a laser
beam
• Primary reason for being the most common
technique is that it is the cheapest (at least in the
short term)
– Old X-ray sets used for film-screen radiography can
be used, provided exposure factors and AECs are
adjusted for the new type of detector
CR Components
Physical Principles of Computed
Radiography (CR)
• Fluorescence describes the immediate release
of low energy light photons after the absorption
of X-ray photons (exploited in traditional film-
screen radiography)
• Phosphorescence describes the delayed
release of light photon energy. This is the
principle of CR
CR Stage 1: Image Capture
• Image receptor is a laser stimulable phosphor,
known as an image plate (IP)
• Capture image by irradiating an IP in the same
way as conventional film
– Does not need a new X-ray system when replacing
film-screen (just make sure automatic exposure
controls are re-calibrated)
• Typically ~40% of X ray photons are absorbed
• IPs retain majority of absorbed X-ray energy as
a pattern of electrons in meta-stable energy
states
– The spatial distribution of stored electrons is
equivalent to the pattern of absorbed x rays – latent
image
The image plate
• Combination of barium fluorohalide dosed
with europium (BaFX:Eu)
• Halide (X) is 85% Bromide and 15% Iodide
• Powdered phosphor mixed with binder and
laid down on a base with a protective layer
on top (prevent physical damage to the
phosphor layer)
• Similar to intensifying screen
• Held in a light tight cassette similar to film-
screen radiography
Fraction of X ray photons absorbed
as a function of energy

Discontinuous
increase due to
K edge of
Barium
The image plate

Protection layer     Protects against mechanical/chemical influences

Phosphor layer       Phosphor atoms

Adhesive layer       Improves mechanical stability
Anti halation layer   Blocks laser beam
White PET          Absorption of scattered light photons

Support layer       Support layer
Between one and two
hundred electrons          These traps are
become trapped per X-ray   artificial faults
photon absorbed            in the crystal
where halogen
vacancies exist

They are
known as
‘F’ or
Colour
Centres
Electron Trapping
• Photon is absorbed by
an electron
• Electron can move
through conduction
band
• They can then be
trapped in Colour
Centres which forms
our latent image
Electron Trapping
• Electrons can
remain trapped in         The fading
an IP for many            characteristic
is shown
hours after the
here
exposure
CR Stage 2: Image Read Out
• Electrons are
actively stimulated
to release their
stored energy
• This is done by
scanning the IP
with an intense
laser beam
CR Stage 2: Image Read Out
• A red Laser is used as this matches the energy
gap between Colour Centre and conduction
band
• Light in the blue end of the visible spectrum is
emitted
• Hence, optical separation of input and output
light photons
– Means a colour filter can be used to prevent laser
photons contaminating the output signal
• Blue light photons are collected via a
photomultiplier tube and digital image is
produced
Read Out
• Stimulation of IPs
with laser causes
trapped electrons
to transfer to
conduction band
• These then relax to
the ground state,
emitting blue light
photons
How Does CR work?
Conduction band

Electron
traps

X Rays are
absorbed

Valence band
Photo stimulated luminescence
Conduction band

Red laser light

Blue
light

Valence band
The read/erasure cycle
• The image plate is removed from the cassette
inside the CR reader
• Scanning or laser achieved with a rotating mirror
• The light guide (with optical filter) directs the
emitted blue light to a photomultiplier tube,
which measures the intensity of the light
(proportional to the number of X-rays absorbed)
• Whilst repeatedly scanning the plate, it is moved
through the laser beam
• Once scanned, the residual signal is removed by
exposing the plate to a very bright light source
(erasure cycle)
• Takes about 30-45 s to read and erase an image
plate
CR Reader cycle

Analogue
output
Dynamic Range
• Film dynamic
range defined by
characteristic
curve  10:1
• CR dynamic range
> 10,000:1
• Linear relationship
between
log(signal) and
log(dose)
Körner M et al. Radiographics 2007;27:675-686
Dynamic Range
• The linear relationship would give a very ‘flat’
image with minimal contrast if signal converted
directly to greyscale
• Hence, need to process the image before
displaying to the reader
• Apply various processing stages (Craig will deal
with this in more detail), and use an optimised
look-up table (LUT) to map the signal intensity to
an appropriate grey level
• However, the reader is able to adjust the window
and level to optimise the presentation of each
individual image
Window and Level
• Window width controls the gradient (gamma) of
the transition from black to white
– Narrow width will change from black to white over a
very limited range of exposure levels – increases
contrast in that range, but everything outside this
range will be completely black or white
• Level determines the (typically) central value of
exposure over which the window width is
defined
• (ImageJ for examples?)
Image quality
• Pixel size limits spatial resolution in CR
– For small plates (detail required) ~5.5 lp/mm
– Large plates (detail not essential) ~3.5 lp/mm
– (Film-screen ~8-12 lp/mm)
• Other limits to resolution in CR;
– Scattering of laser light in the phosphor layer results
in detected light from a larger area than expected
– Divergence of light emitted before detection
• Increases with thickness of phosphor
• Some phosphors have needle-like structure to guide light
(like an optical fibre), but quite brittle so not for general use
CR image quality
Image quality
• Image processing (e.g. edge enhancement) may
improve visibility of fine detail
• Contrast is determined by the image processing
and LUT that is applied (and the window/level
the user decides upon)
Detector Dose Indicators (DDI)
• As discussed, the restricted latitude of film gives
a clear indication of dose
– Too dark a film = overexposure
– Too light a film = underexposure
• Film has ‘in-built’ quality control of exposure
• CR (and all other digital) images will be
presented with the greyscale optimised no
matter what dose is given
– Will always see a recognisable image, but the noise
will vary
• Can result in dose creep
– Increased dose (lower noise) is not punished by the
detection medium, so tendency to go for slowly
increasing image quality – NOT ACCEPTABLE!
DDI
• The DDI has been introduced for digital imaging
as an indication of the level exposure on a broad
region of the detector
• Analogous to the OD of film
• The definition of DDI is manufacturer specific!
–   Some manufacturers have high DDI = underexposure
–   Some the other way round
–   Some are a function of the log of dose
–   Some are linear…
• Manufacturers will provide an indication of
acceptable range of DDI, but local departments
must validate these – DRLs and OPTIMISATION
• Operators should monitor DDI of patient
exposures to ensure doses remain acceptable

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