Transcript Instruments for Radiation Detection and Measurement

Instruments for Radiation
Detection
and Measurement
Lab # 3
• In nuclear medicine it is important to
ascertain the
– Presence
– Type
– Intensity
– Energy of radiations emitted by radionuclides
• Two commonly used devices
– Gas-filled detectors
– Scintillation detectors
Gas-Filled Detectors
• The operation of a gas-filled detector is
based on the ionization of gas
molecules by radiations, followed by
collection of the ion pairs as current
with the application of a voltage
between two electrodes.
• The measured current is primarily
proportional to the applied voltage and
the amount of radiations.
Gas-Filled Detectors
collection of the
ion pairs as
current with
the application
of a voltage
between two
electrodes
ionization of gas
molecules
by radiations
The
measured
current is
primarily
proportional
to the applied
voltage and
the amount
of radiations.
Gas-Filled Detectors
Gas-Filled Detectors
• The two most commonly used gas-filled
detectors are
– Ionization chambers
• Cutie-Pie counters used for measuring high
intensity radiation sources, such as output
from x-ray machines
• Dose calibrators measures the activity of
radiopharmaceuticals
– Geiger-Müller (GM) counters.
Dose Calibrators
• one of the most essential instruments for
measuring the activity of radionuclides
– Cylindrically shaped
– Sealed chamber with a central well
– Filled with argon and traces of halogen at high
pressure
Geiger-Müller (GM) Counters
• One of the most sensitive detectors.
• Used for the measurement of exposure delivered by a
radiation source and called survey meters.
• Primarily used for area survey for contamination with
low-level activity.
• It is usually battery operated.
Scintillation Detecting Instruments
• g-ray detecting equipment
• Most commonly used:
– well counters
– Thyroid probes
– g or scintillation
• All these instruments are g-ray detecting devices
• Consist of:
• Collimator (excluding well counter)
• Sodium iodide detector
• Photomultiplier tube
• Preamplifier
• Pulse height analyzer
• Display or Storage
• Scintillation detectors consist of scintilator emitting
flashes of light after absorbing gamma or x radiation.
• The light photons produced are then converted to an
electrical pulse by means of a photomultiplier tube.
• The pulse is amplified by a linear amplifier, sorted by
a pulse-height analyzer and then registered as a
count.
• Different solid or liquid scintillators are used for
different types of radiation.
• In nuclear medicine, sodium iodide solid crystals
with a trace of thallium NaI(Tl) are used for gamma
and x ray detection.
The light photons will strike the
photocathode of a
g rays from a source interact
in the sodium iodide
photomultiplier
The pulse is first amplified
by a preamplifier and then by a linear amplifier
detector and light photons
are
emitted.
(PM) tube
and a pulse is
generated at the end of the PM
tube.
Scintillation Camera
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also known as a gamma camera
consists of :
Collimator
Detector
X, Y positioning circuit
PM tubes
Preamplifiers
Linear amplifiers
PHA
Display or storage
Collimator
• In all nuclear medicine equipment for imaging a
collimator is attached to the face of a sodium iodide
detector to limit the field of view so that all radiations
from outside the field of view are prevented from
reaching the detector.
• Made of lead and have a number of holes of
different shapes and sizes.
Collimator
• Classification of collimators used in scintillation
cameras depends primarily on
1. The type of focusing
2. The thickness of the holes
• Depending on the type of focusing
1. parallel hole
2. Pinholet
3. Converging
4. Diverging type
Pinhole collimators are used in
imaging small
organs such as thyroid glands
Converging collimators are
employed when
the target organ is smaller than
the size of the detector
Parallel hole collimators are most
commonly used in
diverging
nuclear medicine procedures. collimators are used in imaging
organs such as lungs that are larger
than the
size of the detector
• Parallel hole collimators are classified as
– High-resolution
– All-purpose
– High-sensitivity types.
• The size and number of holes the same for all
these collimator
• The only change is in the thickness.
• High sensitivity collimators are made with
smaller thickness than all-purpose collimators
• High-resolution collimators are made thickest of
all.
Detector
• Sodium iodide crystal doped with a very small amount of
thallium [NaI(Tl)] is most commonly used.
– The choice of NaI(Tl) crystals for g-ray detection is
primarily due to their reasonable density (3.67 g/cm3)
and high atomic number of iodine (Z = 53)
– That result in efficient production of light photons
• Rectangular in shape
• Have the dimension between 33 X 43 cm and 37 X 59
cm with thickness varying between 0.64 cm and 1.9 cm
• The most common thickness is 0.95 cm
• The 0.64-cm thick detectors are usually used in portable
cameras for nuclear cardiac studies
Detector
• Increasing the thickness of a crystal increases
the probability of complete absorption of ɣ rays
and hence the sensitivity of the detector
thickness
absorption
sensitivity
Photomultiplier Tube
• A PM tube consists of
1. Light-sensitive photocathode at one end
2. A series (usually 10) of metallic electrodes called
dynodes in the middle
3. Anode at the other end
– All enclosed in a vacuum glass tube.
• Fixed on to the NaI(Tl) crystal
• The number of PM tubes in the thyroid probe and the
well counter is one whereas in scintillation cameras it
varies from19 to 94 which are attached on the back face
of the NaI(Tl) crystal
Photomultiplier Tube
• When a lightphoton from the NaI(Tl) crystal strikes the
photocathode photoelectrons are emitted and
accelerated toward the immediate dynode
• The accelerated electrons strike the dynode and more
secondary electrons are emitted, which are further
accelerated
• The process of multiplication of secondary electrons
continues until the last dynode is reached, where a pulse
of 105 to 108 electrons is produced
• The pulse is then attracted to the anode and finally
delivered to the preamplifier
Preamplifier
• The pulse from the PM tube is small in amplitude
and must be amplified before further processing.
Linear Amplifier
• The output pulse from the preamplifier is further
amplified and properly shaped by a linear
amplifier.
• The amplified pulse is then delivered to a pulse
height analyzer for analysis as to its voltage.
Pulse Height Analyzer
• Gamma rays of different energies can arise from a
source, either
– from the same radionuclide
– or from different radionuclides
– or due to scattering of grays in the source
• The pulses coming out of the amplifier may be different
in amplitude due to differing g-ray energies
• The pulse height analyzer (PHA) is a device that selects
for counting only those pulses falling within preselected
voltage amplitude intervals and rejects all others
Pulse Height Analyzer
• A pulse height analyzer normally selects only one range of
pulses and is called a single-channel analyzer (SCA).
• A multichannel analyzer (MCA) is a device that can
simultaneously sort out pulses of different energies into a
number of channels.
• In many scintillation cameras, the energy selection is made
automatically by pushbutton type isotope selectors designated
for different radionuclides such as 131I, 99mTc
• In some scintillation cameras, two or three SCAs are used to
select simultaneously two or three g rays of different energies
Display and Storage
• most cameras employ digital computers in
acquiring, storing, and processing of image data
Tomographic Imagers
• limitation of the scintillation cameras is that they depict
images of three-dimensional activity distributions in twodimensional displays
• One way to solve this problem is to obtain images at
different angles around the patient such as anterior,
posterior, lateral, and oblique projections
• Success of the technique is limited because of the
complexity of structures surrounding the organ of interest
1. Single Photon Emission Computed Tomography
2. Positron Emission Tomography
Tomographic Imagers
• Single Photon Emission Computed Tomography
– uses g-emitting radionuclides such as 99mTc, 123I,
67Ga, 111In
• Positron Emission Tomography
• uses beta+ emitting radionuclides such as 11C, 13N, 15O,
18F, 68Ga, 82Rb
Tomographic Imagers
• mathematical algorithms, to reconstruct the
images at distinct focal planes (slices).