phy232topic1.ppt

Topic 1. Electronics for Radiation Detection Systems

• Introduction

• Preamplifier

• Amplifier

• Pulse Height Analyzers

• Display Mode

• Cathode Ray Tubes

Introduction

Preamplifier

• To amplify the relatively small signal from the detectors

• To match the impedance levels between the detector and subsequent components

• To shape the signal pulse for optimal subsequent processing

A few points• The output from the preamplifier is V=Vo exp(-t/RC)

where Vo =Q/C and RC is the time constant, typical 20-200 u sec for nuclear medicine detectors.

• The amplification for scintillation detectors is small (5-20) because the signals from the detectors have already been amplified by photo-multiply tubes (105-1010).

• Higher amplification is required for semiconductor detectors (103-104) due to small detector signals.

• Preamplifier is located as close as possible to the detector to maximise the signal to noise ratio (often in single unit).

Amplifiers

• To amplify the still small signals from the preamplifier (1-1000).

• To reshape the slow decaying pulse from preamplifier into a narrow one (for high count rate and increasing the S/N rate etc,).

• Requirements for shaping: preserve the input signal information such as pulse height and rise time.

RC Shaping

• Differentiate circuit: – The output is a rapid rising pulse with decay constant τd

=RC which is smaller than that in preamplifier.

– The amplitude of output is proportional to the rising portion of the input and insensitive to the tails.

– It discriminates against low frequency noise.

• Integration circuit: – output pulse rises with time constant. V=Vo(1-e-t/RC).

– It discriminates against high frequency noise.

RC Shaping (continued)

• Differentiate plus Integration circuit:– The output amplitude is determined by the

input– Time constant is shortened (0.25-5 u sec for

scintillation and semiconductor detectors, in contrast to 50-500 u sec in the preamplifier).

– Only one polarity (except for some small negative overshoot at the end)

RC Shaping (continued)

• Double differentiation plus integration circuit– Output is bipolar– Shorter rising time and longer total duration

than unipolar output– Preferred for high counting rate

Baseline Shift and Pulse Pileup

• Baseline Shift is caused by the negative component of the output (at the end of the pulse)

• Pulse Pileup is caused by high counting rates that they fall on top of each other.

A few Points

• Baseline shift and pulse pileup are caused by high counting rates

• Both problems can be reduced by shortening time constant but also reduce the energy resolution and S/N ratio.

• Double differentiate bipolar amplifier and short time constant (0.025-0.5 u sec) are commonly used for NaI(Tl) detectors

• Unipolar and longer time constant (0.5-8 u sec) for semiconductor detectors (achieve high energy resolution).

Pulse-Height Analyzers

• 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 zero-crossing.

• 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)

• 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.