Pulse-Height Analyzers

1. Pulse-Height Analyzers • Basic Functions • Single Channel Analyzers • Time Methods • Multi-channel Analyzers

2. 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

3. 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

5. 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)).

7. 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).

9. 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.

11. 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).

12. 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.

13. Time to Amplitude Converter

14. 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.

16. 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)

19. Coincidence Unit

20. Cathode Ray Tube (CRT) • Electron Gun • Deflection Plates • Phosphor-coated Display Screens • Focus and Brightness Controls • Colour Cathode Ray Tubes

22. 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.

24. 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.

26. 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.

27. 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

28. 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.

30. 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

31. 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.