Medical Equipment lec 7

1. Medical Equipment III
Radiography Detectors
Shereen M. El-Metwally
Associate Professor,
Systems and Biomedical Engineering Department,
Faculty of Engineering - Cairo University
sh.elmetwally@eng1.cu.edu.eg

2. Detectors
 Analog
 Film
 Digital
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3. Radiographic Film
 Few of the original x-rays actually make
the image.
 Remnant Radiation: the fraction of x-
rays that interacts with the x-ray film.
 is the image-forming radiation that passes
completely through the patient.

4. Remnant Radiation
 Remnant radiation
 Output radiation from the
patient
 consists of the transmitted
useful beam and x-rays
scattered away from the
receptor.
 The film is sandwiched
between radiographic
intensifying screens in a
protective cassette.

5. Intensifying Screens & Film
 Radiographic film is similar in construction and
characteristics to photographic film.
 Only about 2% to 6% of the total energy in the
diagnostic x-ray beam is absorbed in the emulsion of an
x-ray film exposed directly to the beam.
 Consequently, direct exposure of film to x rays is a very
inefficient utilization of energy available in the x-ray
beam.
 The intensifying screens convert the x-rays into
visible light. The visible light exposes the
radiographic film.
 Approximately 5% to 20% of the absorbed x-ray energy

6. Intensification factor
 The major advantage of x-
ray intensifying screens is a
reduction of the exposure
required to form an image
of acceptable quality.
 Intensification factor is the
ratio of exposures required
to produce an optical
density of 1.0 without and
with the screen in position.
 A range of 50-100
is common.
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7. Film Construction
 Radiographic Film has two
basic parts.
 Base
 Emulsion
 An adhesive layer attaches
the emulsion to the base.
 The emulsion is enclosed in
a protective layer or
overcoat.
 Most films have two layers
of emulsion so it is referred
to as “Double Emulsion

8. Radiographic Film Base
 Initially x-ray were taken on glass plates.
 In 1918, cellulose nitrate bases film
replaced glass due to a shortage of glass.
 Cellulose nitrate was flammable so x-ray film
was a fire hazard.
 1920, cellulose triacetate or safety base
was introduced. Not as flammable.
 1960’s, polyester base replaced cellulose
triacetate. It is semi-rigid and about 150 to
300 µm thick.

9. Emulsion
 The emulsion is the heart of the film. The x-rays or
light from the intensifying screens interact with the
emulsion and transfer information to the film.
 The emulsion consists of a very homogeneous
mixture of gelatin and silver halide crystals. It is
about 3 to 5 µm thick.
 The gelatin
 clear so it transmits light to the silver halide crystals.
 porous so the processing chemicals can penetrate to the
silver halide crystals.
 primary function of the gelatin is to provide a support
medium for the silver halide crystals by holding them in
place.

10. Silver Halide Crystals
 Tabular shape used most
commonly for general
radiography.
 About 1µm thick for screen
film exposure.
 98% Silver Bromide
 2% Silver Iodide
 The differences in film speed,
contrast and resolution
depend upon the process by
which the silver halide
crystals are manufactured
and by the mixture of these
crystals into the gelatin.

11. The Latent Image
 The interaction between the x-ray or
light photons and the silver halide
crystals produces the “latent image” or
manifest image.
 The “latent image” is the invisible
change in the silver halide crystals.
 This interaction is sometimes referred to
as the “photographic effect”.

12. Producing the Latent Image: Photographic
effectA Electrons are released as energy is absorbed
from the incident radiation.
B Electrons migrate to “sensitivity centers” in
the crystal lattice of the silver halide granules.
C At the sensitivity center, atomic silver is
formed as electrons attract and neutralize
interstitial silver ions in the lattice.
D Small quantities of metallic silver are
deposited in the emulsion, primarily along the
surface of the silver halide granules.
E When the film is placed in a developing
solution, additional silver is deposited at the
sensitivity centers.
F The resulting silver grain is formed.
 No silver is deposited along granules that
are unaffected during film exposure to
radiation. The irradiated and non-irradiated
silver halide granules produce the latent
image.

13. Optical density
 The degree of blackening of a region of the
processed film depends on the amount of free
silver deposited in the region and,
consequently, on the number of x-rays
absorbed in this region.
 This is described as the optical density (OD)
of the region.
 The optical density plotted as a function of the
exposure or [log (exposure)] to the film is
termed the ‘characteristic curve’ or ‘H–D
curve’ for the particular film or film-screen
combination.
 The shape of the characteristic curve is
affected by the quality of the x-ray beam used
(tube voltage) and by the conditions 13
The slope of the straight line
portion is referred to as the
average gradient of the
film.
Often, D1 and D2 are taken as

14. Optical density
 The radiation exposure to an x-ray film should be
sufficient to place the range of optical densities
exhibited by the processed film along the essentially
straight-line portion of the characteristic curve.
 The latitude of the film: The exposure range over
which acceptable optical densities are produced.
 Contrast and latitude are reciprocally related.
 A film with a lower average gradient (more gradual slope
for the characteristic curve) results in a wider latitude or
range of exposures (more shades of gray), and less
contrast.
 A film with a higher average gradient (steep
characteristic curve) results in a shorter latitude (more
blacks and whites and fewer shades of gray), or high 1

15. Types of X-ray Film
 Two main types:
 Screen film used with intensifying screens.
 Film used is most sensitive to the wavelengths of
light emitted by the screens.
 Single emulsion: emulsion on one side of base.
 Double emulsion: is used with two screens.
 Non-screen film or Direct exposure film.
 Film is less sensitive to visible light.
 Special purpose: Duplication, Cine, Dental

16. Screen Film Factors
 Main factors to be considered when
selecting a film:
 Contrast
 Speed
 Resolution
 Crossover
 Spectral matching
 Safelights

17. Contrast
 Most manufacturers offer multiple contrast
levels in their film lines.
 High contrast film has low latitude
 Medium contrast has medium latitude
 Low contrast has high latitude
 High contrast films have small uniform
grains
 Low contrast films have larger grains and
wide range in size.

18. Speed
 Film speed or film sensitivity
 Speed or sensitivity is gained by requiring fewer x-ray or light
photons to form an image.
 The size and shape of the silver halide crystals are the main
factors that determine speed.
 A film with large granules is faster than a film with smaller
granules.
 Faster speed films are almost always double emulsion.
 The light spectrum emitted by an intensifying screen should
correspond closely with the spectral sensitivity of the film used to

19. Resolution
 The spatial resolution of an imaging system is related to
the smallest feature that can be visualized,
Or, is the smallest distance between two features such that
the features can be individually resolved rather than
appearing as one larger shape.
 The most commonly used measure of the spatial
resolution of an imaging system is the modulation transfer
function (MTF) in the spatial frequency domain.
 It measures the response of a system to both low and
high spatial frequencies.
 The spatial frequency of a particular grid of lines is
measured as the number of lines/mm.
 For example: 5 lines/mm for lines spaced 200 µm apart. 1

20. Resolution
 i.e. it exactly reproduces
both the fine structure
(high spatial frequencies)
and areas of relatively
uniform signal intensity
(low spatial frequencies).
 In practice, imaging
systems have a finite
spatial resolution:
 The greater the
attenuation of high
spatial frequencies, the 1
 An ideal system has a constant MTF for all spatial
frequencies,

21. Resolution
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22. Crossover  If film emulsion doesn’t
completely absorb light
emitted from screen, light
passes through film base
and exposes the emulsion
on the other side. This is
referred to as Crossover.
 As light passes through film
base, it can spread and
introduce image blur.
 In modern film-screen
receptors, a dye (light-
absorbing layer) is placed
between film emulsion and
base in order to reduce
crossover.

23. Spectral Matching
 The most important consideration in selecting screen film.
 The material in the screen determines the color of light emitted.
 The light spectrum emitted by an intensifying screen should
correspond closely with the spectral sensitivity of the film used.
 Special dyes are used to match the screens to film.
 Calcium tungstate screen emit blue and blue violet light. Blue-
sensitive films are used.
 Rare-earth screens emit blue-green light. Green-sensitive film
must be used. It is referred to as orthochromatic film.
 If the light spectrum does not match, there will be a significant
loss of speed.
 Kodak Lanex Regular Screens are rated at 400 speed with
orthochromatic film and 200 speed with blue-sensitive film.

24. Safelight
 Working with film in the
darkroom requires special
lighting to avoid exposure of
the film.
 Filters are used to avoid
exposure of the film.
 A red filter is used for blue-
green sensitive film.
 The color is not the only
concern, the wattage of the
bulb and distance from the
counter top is also very
important.
 Maximum wattage is 15w.
 Distance 60 “ from counter top.

25. Special Film Types
 Direct exposure:
Requires 10 to 100 times more exposure. Once
used for small body parts measuring less than
10 cm. No longer used.
 Single emulsion film:
is less sensitive to radiation (low speed). It
requires more exposure.
Used primarily when exceptionally fine detail is
required in the image (e.g., laser printing and
mammography).

26. Special Film Types
 Mammography Film: Only single
emulsion film currently used in modern
radiography.
 Laser Film: Used with a laser printer for
digital radiography, CT and MRI.
Modern units are dry chemical printers.
Similar to laser printers except image is
printed on film.

27. Special Film Types
 Duplication Film: special single
emulsion film used to copy x-ray films.
Sensitive to UV or blue light. Never used
in cassettes.
 Subtraction Film: used in angiography
to do subtraction where the bone is
removed to better visualize the arteries.

28. Handling and Storage of
Radiographic Film
 X-ray film is a sensitive radiation-detector
and it must be handled in an area free of
radiation.
 Film storage must be shielded.
 The darkroom adjacent to the x-ray room must
be shielded.
 If film use is low, more shielding may be
required.
 Heat and Humidity must be controlled.
 Film is sensitive to heat and humidity from the

29. Handling and Storage of
Radiographic Film
 Improper handling of the film will result in poor
image quality due to artifacts.
 Avoid bending, creasing or otherwise rough
handling the film. Avoid sharp objects contacting
the film.
 Hands must be clean and dry.
 Avoid hand creams, lotions or water free hand
cleaners.
 Static electricity or a dirty processor can cause
artifacts.

30. Handling and Storage of
Radiographic Film
 Light will expose the film. Film must be
handled and stored in the dark.
 If low level diffuse light exposes the film, fog is
increased.
 Luminous watches, cell phone and darkroom light
leaks should be avoided.
 Bright light causes gross exposure.
 Shelf life. All films are supplied in boxes with an
expiration date.
 Expired film loses speed and contrast and have
increased fog.

31. Film Processing block
diagram
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32. Digital Detectors
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33. Characteristic curve(Film/
Digital)
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