Transcript Instruments for Radiation Detection and Measurement

Instruments for Radiation
Detection
and Measurement
Lab # 4
• 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
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 registred 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
•
•
•
•
•
•
•
•
•
•
also known as a gamma camera
consists of :
Collimator
Detector
X, Y positioning circuit
PM tubes
Preamplifiers
Linear amplifiers
PHA
Display or storage
Collimator
• classification of collimators used in scintillation cameras
depends primarily on
– The type of focusing
– The thickness of the holes
• Depending on the type of focusing
– parallel hole
– Pinholet
– Converging
– 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 highresolution, all-purpose, and 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
• NaI(Tl) crystals used as the detector
• 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 g rays
and hence the sensitivity of the detector
X, Y Positioning Circuit
• When a g ray interacts in the crystal, its exact location is
determined by the X, Y positioning circuit
• Many PM tubes are mounted on the NaI(Tl) crystal in
scintillation cameras
• After g-ray interaction in the crystal, a maximum amount
of light will be received by the PM tube nearest to the
point of interaction
Pulse Height Analyzer
• circuit that sums up the output of all PM tubes to produce
a pulse known as the Z pulse that represents the energy
of a g ray
• The SCA analyzes the amplitude of the Z pulses and
selects only those of desired energy by the use of
appropriate peak energy and percent window settings
• In many scintillation cameras, the energy selection is
made automatically by pushbutton type isotope selectors
designated for different radionuclides such as 131I,
99mTc
Pulse Height Analyzer
• 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
– Single Photon Emission Computed Tomography
– Positron Emission Tomography
Tomographic Imagers
• mathematical algorithms, to reconstruct the
images at distinct focal planes (slices).