amp and instru circuit [compatibility mode]

8/12/2019 Amp and Instru Circuit [Compatibility Mode]

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Detection Systems

Introduction

Amplifier u se e g t na yzers

Display Mode

Cathode Ray Tubes


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Introduction


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Preamplifier

To amplify the relatively small signal fromthe detectors

To match the impedance levels between the

To shape the signal pulse for optimal


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A few points

= -

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

The amplification for scintillation detectors is small (5-20)

because the signals from the detectors have already been

am lified b hoto-multi l 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 detectorto maximise the signal to noise ratio (often in single unit).


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Amplifiers

o amp y e s sma s gna s rom e

preamplifier (1-1000).

To reshape the slow decaying pulse from

preamplifier into a narrow one (for highcount rate and increasing the S/N ratio etc,).

Requirements for shaping: preserve the

input signal information such as pulse

hei ht and rise time.


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RC Shaping

Differentiate circuit: The output is a rapid rising pulse with decay constant d

=RC which is smaller than that in preamplifier.

The amplitude of output is proportional to the rising.

It discriminates against low frequency noise.

output pulse rises with time constant. V=Vo(1-e-t/RC).

It discriminates against high frequency noise.


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RC Shaping (continued)

Differentiate plus Integration circuit: The out ut am litude is determined b the

input

Time constant is shortened (0.25-5 sec forscintillation and semiconductor detectors, in

contrast to 50-500 sec in the preamplifier).

Only one polarity (except for some smallnegative overshoot at the end)


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RC Shaping (continued)

Double differentiation plus integration circuit Out ut is bi olar

Shorter rising time and longer total duration

than unipolar output

Preferred for high counting rate


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Baseline Shift and Pulse Pileup

Baseline Shift is caused by the negativecom onent of the out ut at the end of the

pulse)

rates that they fall on top of each other.


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A few Points

Baseline shift and pulse pileup are caused by high countingrates

ot pro ems can e re uce y s orten ng t me constant

but also reduce the energy resolution and S/N ratio.

constant (0.025-0.5 u sec) are commonly used for NaI(Tl)

detectors

Un po ar an onger t me constant (0.5-8 u sec) orsemiconductor detectors (achieve high energy resolution).


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Pulse-Height Analyzers

Basic Functions

Time Methods

u t -c anne na yzers


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Basic Function

The amplitude of output signal isro ortional to the ener of the radiation

event detected

certain amplitude resulted in counting of

A certain energy range or interval is called


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Single Channel Analyzers

Counting only those within a single energy range Com osed of three arts: Lower Level

Discriminator (LLD), Upper Level Discriminator

(ULD) and Anticoincidence Percentage window: a certain percentage of the

windows central voltage.

A single channel analyzer without ULD is a circuitcalled discriminator


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Medicine applications There are a number of timing methods available but two of

ose are o en use n nuc ear me c ne: ea ng-e ge an zero-

crossing.

Leading-edge uses the rising portion of the input pulse to triggerthe 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 (4nsec for NaI(Tl)).


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Multichannel Analyzers

Simultaneous recording of multiple energyradiations.

The principle of the popular Multichannel

Analyzer (MCA) is different from the singlechannel analyzer

The center of the Multichannel analyzer is the

ana og- o- g a conver er A memory is required for the sorting of energy

c anne s energy ranges, energy spec rum .


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Analog-to-Digital Converter

Two types of ADC are used in nuclear medicine for MCAand the interface between scintillation cameras and

approximation

Both require time for the conversion which could be abottle neck for the time resolution but is not a major

problem for nuclear medicine application

which means that the more bits the more accurate but

requires more time and memory.


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Ramp ADC

RC circuitry and clock oscillator Di h r in im r r i n l h m li

of the input pulse (radiation energy)

counted in a counting circuit

proportional to the discharging time which in

turn ro ortional the radiation ener .


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Successive Approximation

The input pulse is compared with one-halfof the full scale

The comparison voltage is then either

initial level depending on whether the pulse

level.

.


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Time to Amplitude Converter


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Scalers and Timers

A device that only counts pulses is called ascaler

An auxiliary device that controls the scaler

.


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Analog Ratemeters

A analog ratemeter is used to determine theaverage 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 timeconstant which can be adjusted (change the

capac tor


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Coincidence Unit


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Cathode Ray Tube (CRT)

Electron Gun

Phosphor-coated Display Screens

ocus an r g tness ontro s

Colour Cathode Ray Tubes


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Electron Gun

Cathode: Hot filament-Tungsten, thoriated tungsten, nickel

coated with oxides of barium and strontium, etc

ontro gr : a cape w t a o e n ts centre an a negat ve

potential applied to control the passage of electrons.

shape. Positive potential applied to accelerate the electrons.

Focusing anode: a second anode that further shapes the

e ectron eam. A negat ve potent a s app e to compressand focus the beam of electrons.


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Deflection Plates

Deflection plates are used for the positioning theelectron beams on the screen: electronstatic or

electromagnetic types.

Electrostatic type applies voltages to the two setsof plates. Used for small screen with fast speed.

Electromagenetic type uses two sets of wire coil.

se or arge screen w a s ower spee .


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-Screen

Electrons strike the screen (glass coated withphosphorescent materials) and release

phosphorescent light.

Persistence time: the lifetime of the light emissionfrom the phosphor.

Persistence scope: long persistence time up to a

ew m nu es. ompose o s orage mes an oogun etc. Used as visual monitor for patient

.


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Focus and Brightness Control

Second anode in the CRT controls thefocus. It is a otentiometer that varies the

potential applied to the anode.

electron intensity. Increasing the voltage


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Colour Cathode Ray Tube

Three electron guns produce differentelectron beams onto arra s of individual

phosphors which in turn, produce three

colours red reen and blue. A total of 64 colours can be produced by

.


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Oscilloscopes

Oscilloscope is composed of a CRT, a signal amplifier and

a time-sweep generator. It is used for displaying signal

.

The signal amplifier is used to amplify the small signals to

be displayed which is connected to the vertical deflectionplates.

The sweep signal is applied to horizontal deflection plates

speed and is repeated.

Often used in cardiac studies for nuclear medicine


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Monitors

A CRT tube with the two deflection plates controlled by

constant frequency time-sweep generators.

.

Active or retrace sweeps: the electron gun is on or off.

Most TV monitors use interlacing. The two sets of scan linesare called fields and the two interlaced fields is called frame.

Each frame takes 1/30 or 1/25 sec depending on the

fre uenc of the electricit .

The resolution depends on the number of lines (65%) for thevertical direction and the changing rate of the brightness

ur ng e or zon a sweep.


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amp and instru circuit [compatibility mode]

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