Transcript Nuclear Medicine Physics and Instrumentation I, Part 1

Pulse-Height Analyzers
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Basic Functions
Single Channel Analyzers
Time Methods
Multi-channel Analyzers
Basic Function
• The amplitude of output signal is
proportional to the energy of the radiation
event detected
• Selective counting of those pulses within
certain amplitude resulted in counting of
selective energy range
• A certain energy range or interval is called
energy channel
Single Channel Analyzers
• Counting only those within a single energy range
• Composed of three parts: Lower Level
Discriminator (LLD), Upper Level Discriminator
(ULD) and Anticoincidence
• Percentage window: a certain percentage of the
window’s central voltage.
• A single channel analyzer without ULD is a circuit
called discriminator
Timing Method
• Determine the timing of radiation event is important in Nuclear
Medicine applications
• There are a number of timing methods available but two of
those are often used in nuclear medicine: leading-edge and zerocrossing.
• Leading-edge uses the rising portion of the input pulse to trigger
the lower level discriminator which depends on the pulse
amplitude (suffer certain amount of inaccuracy--5 to 50 nsec for
NaI(Tl)).
• Zero-crossing requires bipolar pulses and is more accurate (4
nsec for NaI(Tl)).
Multichannel Analyzers
• Simultaneous recording of multiple energy
radiations.
• The principle of the popular Multichannel
Analyzer (MCA) is different from the single
channel analyzer
• The center of the Multichannel analyzer is the
analog-to-digital converter (ADC)
• A memory is required for the sorting of energy
channels (energy ranges, energy spectrum).
Analog-to-Digital Converter
• Two types of ADC are used in nuclear medicine for MCA
and the interface between scintillation cameras and
computers: Wilkinson or Ramp converter and successive
approximation
• Both require time for the conversion which could be a
“bottle neck” for the time resolution but is not a major
problem for nuclear medicine application
• Both of the converters use binary number representation
which means that the more bits the more accurate but
requires more time and memory.
Ramp ADC
• RC circuitry and clock oscillator
• Discharging time proportional to the amplitude
of the input pulse (radiation energy)
• Clock oscillator produces pulse train that are
counted in a counting circuit
• The number of the clock pulses counted are
proportional to the discharging time which in
turn proportional the radiation energy).
Successive Approximation
• The input pulse is compared with one-half
of the full scale
• The comparison voltage is then either
increased or decreased by one half of its
initial level depending on whether the pulse
amplitude did or did not exceed the initial
level.
• The process is repeated for several steps.
Time to Amplitude Converter
Scalers and Timers
• A device that only counts pulses is called a
scaler
• An auxiliary device that controls the scaler
counting time is called timer.
Analog Ratemeters
• A analog ratemeter is used to determine the
average number of events occurring per unit time.
The average is determined continuously rather
than over discrete counting time
• Linear vs logarithmic ratemeters: V0=knQRp vs
V0=klog(nQRp) - wider range of counting rate
• Ratemeter responds to the rate change has a time
constant which can be adjusted (change the
capacitor)
Coincidence Unit
Cathode Ray Tube (CRT)
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Electron Gun
Deflection Plates
Phosphor-coated Display Screens
Focus and Brightness Controls
Colour Cathode Ray Tubes
Electron Gun
• Cathode: Hot filament-Tungsten, thoriated tungsten, nickel
coated with oxides of barium and strontium, etc
• Control grid: a cape with a hole in its centre and a negative
potential applied to control the passage of electrons.
• Accelerating anode: similar to control grid but reverse in
shape. Positive potential applied to accelerate the electrons.
• Focusing anode: a second anode that further shapes the
electron beam. A negative potential is applied to compress
and focus the beam of electrons.
Deflection Plates
• Deflection plates are used for the positioning the
electron beams on the screen: electronstatic or
electromagnetic types.
• Electrostatic type applies voltages to the two sets
of plates. Used for small screen with fast speed.
• Electromagenetic type uses two sets of wire coil.
Used for large screen with a slower speed.
Phosphor-coated Display
Screen
• Electrons strike the screen (glass coated with
phosphorescent materials) and release
phosphorescent light.
• Persistence time: the lifetime of the light emission
from the phosphor.
• Persistence scope: long persistence time up to a
few minutes. Composed of storage mesh and flood
gun etc. Used as visual monitor for patient
positioning with the gamma camera.
Focus and Brightness Control
• Second anode in the CRT controls the
focus. It is a potentiometer that varies the
potential applied to the anode.
• The control grid controls the brightness or
electron intensity. Increasing the voltage
(negative) decreases the intensity
Colour Cathode Ray Tube
• Three electron guns produce different
electron beams onto arrays of individual
phosphors which in turn, produce three
colours, red, green and blue.
• A total of 64 colours can be produced by
mixing the three colours for human eyes.
Oscilloscopes
• Oscilloscope is composed of a CRT, a signal amplifier and
a time-sweep generator. It is used for displaying signal
amplitude or frequency as a function of time.
• The signal amplifier is used to amplify the small signals to
be displayed which is connected to the vertical deflection
plates.
• The sweep signal is applied to horizontal deflection plates
which sweeps the electron across the screen at a constant
speed and is repeated.
• Often used in cardiac studies for nuclear medicine
Television or Computer
Monitors
• A CRT tube with the two deflection plates controlled by
constant frequency time-sweep generators.
• Electron gun controls the intensity at each point.
• Active or retrace sweeps: the electron gun is on or off.
• Most TV monitors use interlacing. The two sets of scan lines
are called fields and the two interlaced fields is called frame.
Each frame takes 1/30 or 1/25 sec depending on the
frequency of the electricity.
• The resolution depends on the number of lines (65%) for the
vertical direction and the changing rate of the brightness
during the horizontal sweep.