das basics

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Author: amuhammad-rezky-sulfajri

Post on 13-Jan-2017




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Med TrainingScan System
Fan beam scanner All SOMATOMS are fan beam scanners with rotating Tube-Detector-System. The detector array consists of solid state or Xenon detector elements.
Detector elements The detector elements measure the radiation intensities during a scan.
DAS/DMS The electronic system that transfers the detector element signal to the image processor is called DAS(Data Acquisition System) or DMS(Data Measurement system.
The signal processing steps of the DAS are:
Analog to digital conversion
The Solid State Detector
Detector The solid state detector ( also called scintillation crystal detector) consists of individual detector elements. Each element, in turn, consists of encapsulated scintillation crystals fixed to the surface of light sensitive diodes. These elements are mounted on a printed circuit board.
The Xenon Gas Detector
Detector The gas detector work on the principle of the ionization chamber.The anode-and cathode plates are mounted inside the aluminium housing. Every two anodes and their corresponding cathode form one ionization chamber.the number of ionization chambers depends on the scanner type.
Detector element One detector element(=channel) is the volume between two plates on the same potential.
CS TC 22
Principle of DAS
The Monitor Detector
Signal Processing
Detector element The radiation detected by the detector element is converted into a current.
I-- >U amplifier A current- to-voltage converter is used to make it possible to measure the very small currents. The voltage is proportional to the radiation intensity detected by the detector element.
Integration amplifier The signal is integrated.
PGA/FPA The integrated signal is fed into the FPA(Floating Point Amplifier). Also the word PGA(programmable gain amplifier) is used. The FPA(=PGA) is an amplifier which selects its gain automatically. The gain selection 1.8 or 64 depends on the FPA input signal i.e., signal amplitude itself.
The gain that was used for each signal is transmitted to the SMI in two FPA bits.
DMS Hardware
Detector connection The radiation detected by the detector element is converted into a current. Because the current flow is very small in magnitude, the detector connection to the integrator, filter or hybrid boards must be very close to prevent any noise pick up.
Integrator boards Each boards has to convert the very small currents generated by the detector elements into voltages suitable for AD conversion. The current(theoretical range 0...2µA) is converted into a voltage that could theoretically be between 0 and 10V. Practically, the complete range is never used, in order to avoid saturation problems.
Analog bus The outputs of the boards from an analog bus system which is connected to one or more A/D converter(s).
Monitor channel The monitor channel, i.e., the signal of the monitor detector is connected to and converted by one or all ADC(s).
ADC(s) The ADC outputs are connected to the transmitter. The latter performs a parallel to serial conversion and sends the data to the image processor, via a serial data path.
Test data For test purposes, it is possible to send test data to the image processor. Digital info from a ROM in the DMS are sent instead of detector signal using the correct software, this test can verify, if the SMI and the serial data transmission line work or not.
FPA(PGA), Floating Point (Programmable Gain) Amplifier
ADC, Analog to Digital Converter
Output Latch
Autozero Circuit
The analog busses of the integrator and monitor boards are connected to the inputs of the FPA.
FPA The FPA(PGA)is an amplifier which selects its gain automatically. The selected gain can be 1,8 or 64. The gain used is indicated by two bits called ”FPA bits”.
ADC The FPA output is connected to the ADC. The ADC converts the analog value to a 16 bit digital information, but only the highest 14 bits are used.
Dynamic range of DAS Using this configuration, i.e., a PGA with steps of 1,8 and 64 followed by a 16 bit ADC transmitting only the 14MSB, the overall dynamic range of the DMS is 1 to 1.000.000. This means that a signal can be measured with a precision of 1ppm and still deliver the very high speed required to measure 768 elements, with double integrators giving 1536 signals plus monitor values plus start/stop words etc. for every reading and each reading needing only app. 750ns(values for Plus 4)
Latch The 14 bits and the two FPA are temporarily stored in the output latch before they are transferred, via the 16 bit ADC bus, to the parallel to serial converter.
Floating-Point A/D - Conversion
Jittering One possible source of problems is the jittering LSB. If the input voltage is just on the edge between either activating or not activating the LBS, the result of the conversion has an error of 50%! If the LSB is not active, the output would indicate 0-voltage. While, if used, the output would indicate the full level the LSB stands for. Oversampling is excluded in CT, because the required ADC-speed is too high. The solution is not to transmit the 2LSB, so that effectively only 14 bit get transmitted. Now, even the LSB out of these 14 is significant without allowing for the jitter-effect, because internally inside the ADC, 2 more bits were used.
PDA(FPA) 214 individual steps are not sufficient for the required precision. So, before sending the analog input to the DC, a PGA(FPA) amplifies the signal, the factor of 1,8,or 64 depending on the signal amplitude itself.
resolution The resulting resolution can be calculated to be better than one part per million(significant, not ±½ LSB!).
610µV ÷64 is app. 9.5µV
(result÷max. PGA-factor)