IMAGE CHARACTERISTICS , FOR RADIOGRAPHY

1. Basic Principles of Digital
Radiography
DIGITAL IMAGE CHARACTERISTICS
Pixels
 A digital is made up of 2D array
of numbers called a matrix. The
matrix consists of column (M)
and rows (N) that define small
regions called picture elements
or pixels.
NOTE:
 Each square is called
pixel. In order for us to
identify which pixel we are
using, we right-handed
coordinate system
 Right-hand coordinate
system-used to describe
digital images in the
spatial location domain.
The exact location of a
pixel can be found using
columns and rows.
 The pixel’s coordinate in
the picture is 9.4 meaning
column 9 and row 4.
 The pixels that generally make
up the matrix are generally
square. Each pixel contains a
number (discrete value) that
represents a brightness level.
 Each pixel contains a unique
number that represents the
brightness.
 The numbers represent tissue
characteristics being imaged.
NOTE
 For example, in radiography
and CT, these number are
related to atomic number
and mass density of the
tissues, and in MRI they
represent other characteristic
of tissues, such as proton
density and relaxation
times.
 The numbers representation
varies from each modality to
another. As I said awhile ago,
CT and radiography, each
pixel contains the brightness
of each organ depending on
its atomic number and mass
density of tissues, while
MRI represents proton
density and relaxation time.
 In digital radiography, each
pixel represents the
brightness level of a tissue
or an organ.
 The pixel size can be calculated
according to the following
relationship:
PIZEL SIZE= FOV/Matrix size.

2. NOTE:
 If you have both of the factor
such as FOV/Matrix size you can
readily calculate for pixel size.
 For digital imaging modalities,
the larger the matrix size, the
smaller the pixel size (for the
same FOV) and the better
spatial resolution.
NOTE:
 As you can see the picture with
the same FOV, as you can see
the pixels the first picture is
smaller and its getting larger as
the progress with the images. As
you can see the image resolution
in the first picture is much better
than the other larger pixel size.
The first picture has smaller pixel
size and same FOV. The smaller
pixel size, the spatial
resolution is better compared to
larger pixel size.
MATRIX SIZE
 Generally diagnostic digital
images are rectangular in
shape (portrait orientation).
When imaging a patient with a
digital imaging modality, the
operator selects the matrix size,
sometimes referred to as the
field of view (FOV).
NOTE:
 FOV can be synonymous to
matrix size but for clarity, the
FOV, if magzoom ka ng picture,
the FOV is lesser (smaller)
kaysa magzoom out ka sa isa ka
picture.
 When you zoom a picture, your
FOV is smaller because because
small areas are covered unlike
when you zoom out, the FOV is
larger because it covers a larger
area. But when it comes to
matrix size, it depends on the
number of pixels that is
present in the matrix.
 The larger the matrix size, and
smaller the pixels, the better
spatial resolution.
 The larger the pixel size then
larger pajud imong matrix the
lower the spatial resolution.
 Your spatial resolution is pixel
dependent or pixel emitted.
 The smaller the pixel size, the
higher the resolution.

3. Matrix Size
 As images becomes larger, they
require more processing time,
storage space and take more
time to be transmitted to a remote
location.
Note:
 If we choose to download a file
that has one 1GB when
compared to downloading of file
that has only 10 megabytes, we
can differentiate that 1GB is
larger and it will require faster
internet connection than that of
the 10-megabyte file. It is the
same with the files in digital
radiography, as the images
become larger, it requires more
processing time, storage
space, since we have a larger
image when compared to small
image, it will take up more
storage and take more time to be
transmitted to a remote location.
 The transferring of image from
one area to another would be
difficult if the image has a large
matrix size.
Spatial Resolution
 Spatial Resolution refers to the
ability to image small objects that
have high subject contrast, such
as bone-soft tissue interface, a
breast microcalcification, or a
calcified lung nodule.
Note: Image A has a better resolution
since we can see the borders of the
bones rather than Image B since Image
B is pixilated. The image quality in
image A is also better than image B.
Bit Depth
 The number of bits per pixel is
the bit depth.
o In each pixel there is a bit.
This bit represents the
shades of gray that will be
produce in an image for
each pixel.
 Because this binary number
system used the base 2, kbits=2k.
Therefore, each pixel will have 2k
gray levels.
o Letter k represents how
many bits or pixels are
present.
 For example, in a digital image
with bit depth of 2, each pixel will
have 22 (4) gray levels.
Note: Since nakabase 2 tayo sa bit
depth ang base lang dyud is dapat 2

4. and since bit depth is also 2 = 22 = 4
gray levels.
Notes:
For the first image the bits per pixel
is 1 and the its shades of gray
consists of only 2 and there only 2
colors present the black and white
which projects a HIGH CONTRAST
image.
But with the 24 it will provide us with
16 shades of gray. It is a much
better image than with 1 bit per
pixel.
o As the number goes
LARGER, the image
provided would have a better
image.
Contrast Resolution
 Contrast Resolution is the
ability to distinguish
anatomical structures of
similar subject contrast such as
liver-spleen and gray matter-
white matter.
 The actual size of the objects that
can imaged is always smaller
under conditions of high
subject contrast than under
conditions of low subject contrast.
o If the images provided are
HIGH SUBJECT
CONTRAST the actual size
of the object being imaged is
SMALLER while if the image
is LOW SUBJECT
CONTRAST the actual size
of the object being imaged is
LARGER.
Note: The liver and the kidney has
almost the same contrast resolution.
Note
Both the spatial and contrast
resolution focuses on detail
however SPATIAL RESOLUTION
most likely compared to blur.
o Higher number of pixels
the better is the spatial
resolution
o Smaller number of pixels
it would appear blurred or
low spatial resolution.

5. CONTRAST RESOLUTION
o More shades of gray the
more detail you can see or
an INCREASE in detail
o The higher the contrast
resolution is, the higher
the detail, the higher the
shades of gray.
o Contrast resolution and
bit depth are somewhat
connected.
Digital Receptor
Digital receptor is the device that
intercepts the x-ray beam after it has
passed through the patient’s body
and produces an image in digital form,
that is, a matrix of pixels, each with a
numerical value.
This replaces the cassette containing
intensifying screens and film that is used
in non-digital, film-screen radiography.
Note: Computed radiography much more
widely available than screen film
radiography because it much easier to
work with and lesser work load for the
staff compared with film-screen that you
have to process films on the darkroom,
you have to have a passbox, you have to
have an automatic processor. With the
computed radiography and digital
radiography, it is much easier for the
radiologic technologist to process the
images.
DIGITAL RECEPTORS THAT HAVE
BEEN USED FOR AGES
Charge-Coupled Device (CCD)
Charge-Coupled Device (CCD) is the
light sensing element for most digital
cameras
Has three principal advantageous
imaging characteristics
- Sensitivity
- Dynamic Range
- Size
Charge-Coupled Device (CCD) is a
silicon-based semiconductor that
converts visible light into electrical
signal
“Indirect (DR) because it still needs to be
converted into light through the use of a
phosphor, needs to be converted into
another type of energy before it can be
converted into digital signal.”
Sensitivity is the ability of the CCD to
detect and respond to very low levels of
visible light. The sensitivity is important
for photographing the heavens through a
telescope and for low patient radiation
dose in digital imaging.
Note: Since we are in the digital imaging
field, sensitivity should be higher or the
digital receptors we should use must be
sensitive enough. Compared to screen
speed, in film-screen radiography, dapat
high-screen speed talaga sya because
this will result to lower patient dose but
with the sacrifice of spatial resolution.
Dynamic Range is the ability of the
Charged-Coupled Device (CCD) to
respond to a wide range of light intensity,
from very dim to very bright.
Note: it doesn’t matter if is produced with
low level of light or very bright light
because the CCD has wide dynamic
range which can respond to low and high
level of light.

6. At very low x-ray exposure, the response
of a Charge-Coupled Device (CCD)
system is greater than that of the screen
film. This should result in lower patient
dose during DR.
Note: Charge-Coupled Device (CCD) is
more sensitive than the intensifying
screen of the film-screen radiography.
Patient dose is lower because Charge-
Coupled Device (CCD) is very sensitive,
it will respond to a very low level of light
produced by the phosphor compared to
intensifying screen or film-screen
radiography.
Charge-Coupled Device (CCD) is very
small, making it highly adaptable to DR in
its various forms.
Measures approximately 1 to 2 cm, but
the pixel size is an exceptional 100 x 100
μm.
Charge-Coupled Devices (CCDs) can be
tiled to receive the light from an area x-
ray beam as it interacts with a scintillation
phosphor such as cesium Iodide (Csl).
“A compact device and it is very small in
size but it can provide 100 x 100 pixel
size. Smaller pixel size can provide a
better spatial resolution for the image”
“Coupling element of CCD – Fiber optic
taper”
“Fiber optic taper connects the emitted
light from the cesium Iodide phosphor
towards the CCD array.”
DONE CHARGED-COUPLED DEVICE
DONE CHARGED-COUPLED DEVICE
DONE CHARGED-COUPLED DEVICE
Complimentary Metal Oxide Systems
(CMOS)
It is still indirect because it needs a
phosphor to convert light into a digital
signal or electrical signal.
When x-rays interact with the scintillator,
x-rays are converted into light and the
semiconductor within the system
converts the visible light into an electrical
signal.
When compared to a CCD, CMOS is:
- Susceptible to noise
- Light intensity is lower
- Uses less energy
- Are inexpensive
- Tend to have lower quality
“Images produced by a CMOS has an
increase in image noise”
“Light intensity is lower meaning more x-
rays are needed to produce // in order for
the light to increase to and it can be
detected by the CMOS”
It uses less electrical energy and
inexpensive.
More likely, CMOS is the knock-off
version of CCD
“CMOS is the lower or budget version of
CCD”
“Sacrifice: Lower image quality = higher
patient dose”
INDIRECT DR: CESIUM
IODIDE/AMORPHOUS SILICON
An early application of DR is involved
the use of CsI to capture the x-ray as
well as the transmission of the resulting
scintillation light to a collection element.

7. The collection element is a silicon
sandwiched as a TFT.
-Silicon is a semiconductor that is
usually grown as a crystal. When
identified as amorphous silicon is not
crystalline but is a fluid that can be
painted onto a supporting surface.
NOTES:
 The combination of cesium iodide
and amorphous silicon, the
phosphor will be the cesium
iodide
 When x-ray interacts with cesium
iodide, cesium iodide converts
the x-ray into light, and then it will
transmit the light into the
amorphous silicon which acts as
TFT (Thin film transistor)
 We are using SILICON as a
material for our thin film transistor
(TFT), it shouldn’t be crystal since
originally the silicon is crystal
 but it cannot be installed to our
collecting element or to the DR
system if it is crystal form.
 That’s why its transformed into
amorphous silicon in order for it
to be painted, so that it would be
installed properly and easily in
digital radiography system.
-The DR image receptor is fabricated
into individual pixels. Each pixel has
light-sensitive face of a-Si with a
capacitor and a TFT embedded.
- CsI/a-Si is an indirect DR process by
which x-rays are converted first to light
and then to electrical signal.
NOTES:
 Each square is a pixel, that
contains the phase of amorphous
silicon TFT which is the yellow
one and the storage capacitor
which is the white one.
 Before the TFT or the amorphous
silicon can transform light into x-
rays, the x-ray must interact with
cesium iodide in order for it to be
transformed into light.
-The percentage of the pixel face that is
sensitive to x-rays is the fill factor. The
fill factor is approximately 80%;
therefore 20% of the x-rays beam does
not contribute to the image.
NOTES:
 Pixel phase is not composed
entirely of amorphous silicon,
some of it have thin film transistor
(TFT) and storage capacitor,
which does not provide a
perfect 100% film factor
 Hindi ma-convert lahat ng x-rays
into electrical signal because of
the existence of TFT and storage
capacitor.
 20% can be provided if it has
been converted into light towards
the storage capacitor and TFT
-This represents one of the dilemmas for
DR. As a pixel size is reduced, spatial
resolution improves but at the
expense of the patient radiation dose.
With smaller pixels, the fill factor is
reduced, and x-ray intensity must be
increased to maintain adequate signal
strength.

8. -Spatial resolution in DR is pixel limited
NOTES:
 Since the fill factor is only 80%,
we have to increase it somehow if
you want to provide a good
spatial resolution.
 An increased in spatial resolution
will provide more exposure and
more patient radiation dose.
 In order to have a good spatial
resolution you must have smaller
pixels.
 If we have smaller pixels, the fill
factor will be reduced and in
order for the fill factor to be
increased we have to increased
x-ray intensity, which provides
adequate signal. Increased
spatial resolution but sacrificed
on the patient radiation dose.
 Spatial resolution depends on the
size of the pixel
 Smaller pixel size, spatial
resolution is INCREASED.
Larger pixel, spatial resolution
is DECREASED.
-What has been described for the CsI/a-
Si image receptor can be repeated for
the GdOS (Gadolinium oxysulfide)
screen-film speed was increased, spatial
resolution was reduced because of light
dispersion in the GdOS
NOTES:
 The alternative of cesium iodide
is GdOS (Gadolinium oxysulfide)
 Gadolinium oxysulfide is one of
the materials for the film
screen/intensifying screen, it can
also be applied in DR as the
phosphor of choice combined
with amorphous silicon
 GdOS’ phosphor speed is
increased, that means that the
image receptor is more sensitive.
Which means there is lower
patient exposure dose.
 Spatial resolution is reduced
because light dispersion of
GdOS is increased
 When compared Cesium iodide
to GdOS, GdOS disperses light
even more which brings spatial
resolution to its lowest when
compared to cesium iodide.
-Such is not the case with DR.
Increasing thickness of GdOS
(Gadolinium oxysulfide) in a DR image
receptor increases the speed of the
system with no compromise in spatial
resolution.
NOTES:
 In screen-film radiography for
specifically in the intensifying
screens, the more we increase
thickness of the phosphor
layer in an intensifying screen
increases its screen speed. An
increased in screen speed
provides lower patient dose which
provides lesser spatial resolution
 increasing the thickness of the
phosphor increases the speed
with no compromise in spatial
resolution.

9.  The more you thicken the
phosphor in the DR, more fast
your image receptor, but spatial
resolution becomes higher with
no compromise.
 In film screen radiography,
intensifying screen, if you are
increasing the thickness of the
phosphor layer, there will be a
sacrifice on the image quality,
because the patient dose is lower
and the film speed is higher.
DIRECT DR: AMORPHOUS SELENIUM
It is called DIRECT DR because
when x-rays interact with the
amorphous selenium the material
of choice for the collecting
element, the amorphous
selenium converts these x-rays
into electrical signal directly.
-This DR modality is identified by some
as direct DR because no scintillation
phosphor is involved. The image-
forming-x-ray beam interacts directly
with amorphous selenium (a-Se),
producing a charged pair.
-The a-Se is both the capture element
and coupling element.
DIRECT DR: AMORPHOUS
SELENIUM
 Why is it called Direct DR?
 It’s because when x-rays
interact with the amorphous
selenium, it converts these
x-rays into electrical signal
directly.
 No need for a phosphor so
that to convert x-rays into
light. But, use amorphous
selenium converts x-rays
directly into electrical
signal.
a-Se is a direct DR process by which x-
rays are converted directly to electric
signal. The a-Se is approximately 200
μm thick and is sandwiched between
charged electrodes. (top electrode and
pixel electrode)
X-rays incident on the a-Se create
electron hole pairs through direct
ionization of selenium.
The created charge is collected by a
storage capacitor and remains there until
the signal is read by the switching action
of the TFT.

10.  When x-rays interact with the
amorphous selenium, electron
holes are created or electron hole
pairs are created because of
direct ionization.
 Direct ionization it removes
directly the outer shell electrons of
amorphous selenium. When that
happens, those charged electrons
will then go to the storage
capacitor, will be stored here until
such time the TFT will open.
 Charged particles from the
storage capacitor go towards the
TFT when the TFT opens.
 (So si TFT, siya na bahala mag
produce ng image which will be
projected here in this glass
substrate)
ADDITIONAL NOTE sa INDIRECT DR:
CESIUM IODIDE/AMORPHOUS
SILICON
When x-rays interact with the phosphor,
the cesium iodide phosphor, the light
produced by the phosphor will be then
directed towards the amorphous silicon
as a TFT. Amorphous silicon functions
as a thin film transistor. This thin film
transistor in indirect DR is the one
transforms x-rays into electrical signal.
Then the electrical signal will then be
converted into an image.
PHOTOSTIMULABLE PHOSPHOR
PLATE (PSP)
The PSP, barium fluorohalide, is
fashioned similarly to a radiographic
intensifying screen. Because the latent
image occurs in the form of metastable
electrons.
 Latent image in film-screen
radiography in the form of silver
grains or the formation of silver
grains, here in photostimulable
phosphor plate in computed
radiography, it is in the form of
metastable electrons.
 When x-rays interact with the
phosphor in the imaging plate,
electrons get excited and they
return from their excited state to
the ground state through the use
of laser light.
 PSP particles are randomly
positioned throughout a binder.
 (so compared to radiographic film,
the binder there is gelatin, binds
the silver halide crystals in place.
For the PSP, wala gi mention ang
unsa na material nasa binder but
it has also the same responsibility
as a radiographic film, the binder
randomly positions and holds the
PSP particles in place.)

11.  The most common phosphors with
characteristics favorable for CR
are barium fluorohalide
bromides and iodides with
activators (BaFBr:Eu and
BaFI:Eu).
 (some materials used in PSP are
barium fluorobromide and
barium flouroiodide.)
The process involves the use of laser
beam to allow the electrons to return to
its ground state after the emission of light
detected by the photodetector.
 So, the coupling element here is
the fiber optics pa rin, same with
the charged couple device,
cesium iodide.
 So, the light-collection optics will
collect the light emitted by the
photostimulable phosphor and
then it will be detected by the
photodetector.
(Exclude SPR, this is dedicated for
Computed Tomography)
 For CR, x-rays interact with
barium fluorohalide or barium
fluorobromide or barium
fluoroiodide. And the light
emitted by the capture element
will be sensed by your fiber optics
or your lens and will be detected
by your collecting element which
is your photodetector. The
coupling element connects the
light emitted from the capture
elements towards the collecting
element which is the
photodetector.
 In indirect DR, we use two types
of phosphor, the cesium iodide
and gadolinium oxysulfide.
When x-rays interact with these
phosphors, fiber optics and
contact layer (amorphous
silicon which is painted onto the
Film Thin Transistor (TFT)) will
connect the light emitted towards
the collecting element which is the
CCD CMOS and TFT
 In direct DR, amorphous
selenium, when it interacts with
the x-rays, converts x-rays into
electrical signals through direct

12. ionization, removal of outer shell
electrons from the amorphous selenium
going to the storage capacitor and then,
when the TFT is open, electrical charge
will go to the TFT and image will be
presented through the glass substrate.
 Remember that TFTs are usually
combined with liquid crystal
display (LCD) that is why in
amorphous selenium, when the
TFT opens, it can readily provide
the image.
After discussing the two modalities
involved in this subject, now we ask
ourselves, what are the differences
between Computed and (Direct) Digital
Radiography?

13. CR (Computed Radiography)
DR (Direct) (Digital
Radiography)
Mechanism
X-rays interact with the
electrons of the phosphor
raising them in a metastable
state. A laser beam is
introduced in order for the
electrons to return to ground
state while emitting visible
light which are then detected
by photodetectors
X-rays are directly converted
into x-rays through direct
ionization of selenium thereby
creating electron hole pairs.
The created charge is
collected by a storage
capacitor.
Recording Medium PSP Plate or Imaging Plate Amorphous Selenium
Processing
Involves the use of a reader
(laser beam is introduced so
that electrons emit visible
light)
No need for processing after x-
rays are bombarded, image
appears immediately on the
screen
Speed Slower Faster
Availability Widely available Only higher level hospitals
Initial Cost
Cheaper. Can adapt to
previous film screen system
Expensive. Needs a new setup
Cassette Rigidity Can withstand rough handling Fragile
Image Quality Lower Better
Workload
Increased work because of
removal of cassette from
stand/bucky then placing on
the reader
Lesser workload because it
only involves changing the
cassette’s position from the
bucky to the vertical stand and
vice versa

14. NOTES:
Mechanism (for CR)
 X-rays interact with the capture
element which is the barium
fluorohalide and then electrons
are raised into metastable state
and laser light or highly
collimated laser beam is
introduced in order for the
electrons to return to their ground
state. And then, with the
transition of electrons into the
ground state, visible light is
emitted which will be transported
by the fiber optics which are
coping elements towards the
collecting element which is the
photodetector
Mechanism (for DR)
Deretso na sya. X-rays are converted
into electrical signals through direct
ionization. Removal of outer shell
electrons into amorphous selenium,
stored into the storage capacitor. And
then, when the film bin transistor opens
or searches open, electrical charges will
go there and then images are displayed
in a glass substrate.
Recording Medium (CR)
-most specifically the barium
fluorohalide or barium fluorobromide or
barium fluoroiodide
Processing (CR)
-Kinahanglan pa ug reader in order for
the image to be displayed.
Processing (DR)
-Once x-rays interact with the
amorphous selenium, images will be
directly displayed on the monitor.
Speed (CR)
-daghan pa kayo kag buhaton, after
exposure you have to feed it on the
reader and then iread pa sya sa reader,
there are so many processes that can
happen.
Speed (DR)
-expose mo lang and the images will be
displayed on the monitor.
Availability (CR)
-available in clinics and hospitals
because film-screens are very tedious,
we have to maintain automatic
processor, we have to buy chemicals
Availability (DR)
-Davao Doctor’s Hospital has direct
radiography
Initial Cost (CR)
-those who want to transform from film-
screen to CR most likely recommended
sa mga manufacturer is the CR, it is
cheaper because maretain pa ang x-ray
tube, maretain pa ang radiographic table
Initial Cost (DR)
-expensive sya kasi you have to dispose
the reader, the cassette, you will
dispose the x-ray tube and all because
you need a new setup

15. Cassette Rigidity (CR)
-although how many times mo sya
mabagsak or matukudan, it can
withstand, it is strong enough to handle
those circumstances
Cassette Rigidity (DR)
-must be very careful in using the DR
because the cassette is 5 million in total
(Davao Doctor’s Hospital)
-one drop it can already be broken.
Image Quality (CR)
-uses phosphor
-converts x-rays into light
-there is still a need for visible light, so
when the visible light is involved, the
image quality is lower.
Image Quality (DR)
-better because it does not involve
intensifying screens
-it does not also involve phosphor which
converts light and then light into digital
signal
Workload (CR)
-medyo tedious sya because you have
to get the cassette from the stand or
from the radiographic table or the bucky
and then you place it to the reader, and
you wait for few minutes in order for it to
appear on the screen.
Workload (DR)
-lesser workload because reader is not
utilized, you don’t have to put the
cassette in the reader since x-rays are
converted directly into electrical signal
and images pop up directly on your
screens. All you have to do is to change
the position of the cassette into vertical
stand, if you are going to change to
radiographic table, just change the
vertical stand to radiographic table