Medical Equipment lec 8

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. Radiation detectors for quantitative
measurement
as measuring exposure, dose, dose rate,
and count rate.
 Ionization chambers
 Geiger- Muller Tubes (counters)
 Solid scintillation detectors
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4. Gas-filled Ionization
Chambers
 Ion pairs are produced as energy is
deposited in a medium (gas) by incident
ionizing radiation.
 Ion pairs are attracted towards charged
electrodes placed in the medium with a
“drift velocity”, depending on the voltage
difference and distance between
electrodes, the type and pressure of the
gas between the electrodes,.
 The voltage between electrodes is
increased until all ion pairs produced by
the incident radiation are collected.
 An electrical current is produced as ion
pairs are collected by the electrodes.
 Electrons collected by the anode of an
ionization chamber constitute a direct current.

5. Saturation current
At the saturation voltage, the electrodes collect all ion pairs produced by
the radiation. No observed increase in ionization current as the voltage
between electrodes is raised a few hundred volts above
saturation:“ionization chamber plateau”
ionization chamber plateau Signal
amplification
Ionization chambers
are operated at a
voltage below that
which causes signal
amplification.

6. Uses of Ionization Chambers:
Examples
 The activity of a radioactive liquid sample prepared
for administration to patients often is determined by
placing the syringe containing the sample into a well-
type ionization chamber referred to as an isotope
calibrator. The volume of the sample to be
administered is computed from the measured activity.
 Portable survey instruments are used in nuclear
medicine to monitor exposure rates in the vicinity of
radioactive sources and patients receiving therapeutic
quantities of radioactive material.
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7. Geiger- Muller Detectors (counters)
 If the potential difference between the electrodes in a
gas-filled detector exceeds a certain limit (region of
limited proportionality), the incident radiation results in
a sudden, almost complete ionization of the counting
gas in the vicinity of the anode.
 Because of this process, the number of ion pairs
collected by the electrodes is independent of the
amount of ionization produced directly by the impinging
radiation.
 Hence, voltage pulses (usually 1 to 10 V) emerging
from the detector are similar in size and independent of
the type and amount of radiation that initiates the 1

8. Geiger- Muller Detectors
(counters)
 The range of voltages over which pulse
signals from the detector are independent
of the type and amount of radiation
entering the detector is referred to as the
Geiger–Muller (G–M)region.
 The detection efficiency of a G-M counter is
about 1% for x and γ rays and nearly 100%
for charged α- and β-particles.
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9. Geiger- Muller Detectors
(counters)
The response of the detector as a function of time after an ionizing event.

10. Geiger- Muller Detectors
(counters)
 Dead time: during which the detector is
completely unresponsive to additional radiation.
 When an ionizing event is initiated in a G–M
detector, an avalanche of electrons is created along
the entire length of the anode. The residual positive
ions require 200 μs or longer “dead time “to migrate
to the cathode.
 An ionizing event occurring within the “recovery
time” produces a voltage pulse that is smaller
than normal.
 The “resolving time” is the time between an
ionizing event and second event that furnishes
a pulse large enough to pass the discriminator
and be counted. 10

11. Solid Scintillation detector
 Gas-filled chambers are not efficient detectors
for x- and γ -ray photons:
 These radiations pass through the low-density gas
without interacting.
 The probability of x- and γ -ray interaction is
increased if a solid detector with a high density and
atomic number is used.
 An alternative method is solid scintillation
detectors.
 About 20 to 30 photons of light are released for
every keV of photon absorbed energy.

12. Solid Scintillation detector
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13. Principles of Scintillation Detection
 When an x or γ-ray interacts within a scintillation crystal,
energy due to incident radiation is absorbed, resulting in
electrons raised from one energy state to a state of higher
energy.
 Light is released as these electrons return to the lower
energy state. (Photoelectric effect)
 The light impinges upon a photosensitive surface
(photocathode) in a photomultiplier tube. Electrons
released from this surface constitute an electrical signal.
 The number of electrons is multiplied by various stages
(dynodes) of the photomultiplier tube, and a signal is
provided at the photomultiplier anode that may be amplified
and counted. 1

14. Photomultiplier Tube
 Electrons released from photocathode are accelerated to the
first dynode, a positively charged electrode positioned a short
distance from the photocathode.
 For each electron absorbed by the first dynode, three or four
electrons are ejected and accelerated to the second dynode,
where more electrons are released.
 Photomultiplier tubes contain 6 to 14 dynodes with a potential
difference of 100 to 500 V between successive dynodes. 106
to 108 electrons reach the anode for each electron liberated
from the photocathode. The amplification of the signal
depends upon the potential difference between dynodes.
 Electrons collected by the anode are converted to a voltage
pulse. This voltage pulse is delivered to a preamplifier, often
mounted on the photomultiplier tube.
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15. Scintillator emission spectrum
compared to Photocathode spectral
sensitivity
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The spectral sensitivity of the photocathode alloy must match
the wavelength of light emerging from the scintillator crystal.

16. Digital Detectors
Medical Equipment II Spring 2015 Inas A. Yassine 16

17. Film Scanning
 Radiographic films may be digitized after they are
acquired with conventional film/screen systems
 In a typical film digitizer, a laser beam is scanned across
a film. The pattern of optical densities on the film
modulates the transmitted light. A light detector on the
opposite side of the film converts the transmitted laser
light to an electrical signal that is digitized by an analog-
to-digital converter.
 Spatial resolution of a film scanner is determined by
spot size, defined as the size of the laser beam as it
strikes the film. Typically, 100 μm or larger.
 Commercially available laser film scanners have matrix
sizes of at least 2000 × 2000 × 10 or 12 bits. The no. of
bits/pixel define the bit depth. 1

18. Storage Phosphor Plate
 Storage phosphor technology is also known as
computed radiography [CR].
 The image is acquired on a plate containing
crystals of a photo-stimulatable phosphor
capable of storing the energy from an x-ray
exposure.
 When exposed to a strong light source of the
appropriate wavelength, the photo-stimulatable
phosphor re-emits the energy as visible light
that can be detected by a photomultiplier tube
or photodiodes. 1

19. Storage Phosphor Plate
 Thus, the storage phosphor plate records a latent image that
may be read out some time after x-ray exposure. The readout
may be accomplished with a well-collimated laser beam to
yield good resolution.
 Figure shows a computed radiography reader. The reader
consists of a large array of laser diodes and photodiodes.
This array is rapidly moved from right-to-left across the plate
to produce the entire image.
 The recorded electrical signals are digitized with an analog to-
digital converter. Once the image is stored in digital form, it
may be viewed on a high-resolution monitor or printed out on 1

20. Storage Phosphor Plate
 Advantages of storage phosphor or CR systems over
conventional film-screen approaches include:
 The storage phosphor plate simply replaces the screen/film
cassette with no significant change in imaging procedure.
 An improvement in dynamic range.
 Radiographic film operates over possible exposures ranging
corresponding to the straight line region in the characteristic
curve. This range typically causes exposure differences of a
factor of approximately 100.
 The storage phosphor has greater latitude or range of
exposures (more shades of gray). The dynamic range of a
storage phosphor is on the order of 10,000.
 is more “forgiving” if an incorrect exposure is used, leading to
elimination of retakes due to improper technique. 1

21. Phosphor plate cycle
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22. Large-Area Digital Image
Receptors
 Or, large-area flat panel detectors are
of two basic types:
Direct conversion
Indirect conversion
1

23. Direct conversion systems
 The energy of the x rays is converted to
an electrical signal in a single layer of a
material called a photoconductor.
 The thin-film transistor (TFT) array is a
grid of transistors that can be read out
electronically to determine the amount of
electrical charge present above each
transistor.
 The TFT array corresponds to an array of
pixels in the final image, with the number
of transistors equaling the number of
pixels. 1

24. Indirect conversion systems
 A scintillator first converts the
energy of the x-ray photons to
visible light.
 The pattern of visible light
produced can be read out using a
photoconductor fabricated to
convert visible light to electrical
signals.
 Photoconductors used include
charge-coupled devices (CCDs),
or, a two-dimensional array of
photodiodes coupled with a TFT
array. 1

25. Analog/ Digital detectors
 Analog
 Coupled acquisition and display
 Higher resolution
 Limited dynamic range, fixed detector contrast
 Immediate exposure feedback
 Digital
 Separated acquisition and display
 Lower resolution
 Higher dynamic range and noise-limited contrast
 Virtual elimination of retakes due to improper technique.
i.e, is more “forgiving” if an incorrect exposure is used.
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