X-ray Imaging System. X-Ray Circuit. Principles of Imaging Science II (RAD 120) X-ray Imaging System Circuitry

Transcription

1 Principles of Imaging Science II (RAD 120) X-ray Imaging System Circuitry X-ray Imaging System Operating console Set x-ray tube current (quantity) and voltage (quality) Controls line compensation, kvp, ma and exposure time (mas) via digital meters AEC devices 2 X-Ray Circuit 3 1

2 Operating Console RT controls & meters are located within the low voltage side (primary) of the x-ray circuit Reduce shock potential 4 Operating Console - Line Compensation Incoming Line Voltage (Mains) Electricity supply in US is 60 Hz AC, with a nominal rms of volts Polarity reverses 120 times/sec Voltages constantly fluctuate as resistors are activated in circuit in accordance with Ohm s Law Can vary +5% affecting x-ray production Supplied in the form of a three-phase power cycle 5 Single-Phase Power A. Voltage drops to zero with every change in direction Frequency of sine wave is determined by # of cycles/sec (cps) 60 Hz X 2 directions = 120 changes/sec B. Full wave rectified circuit (DC pulsating) Produces no x-ray photons 120X/sec 7 2

3 Single-Phase Power 1o RMS V of a single-phase sinusoidal wave is approximately 70.7% of peak voltage Calculate the rms voltage of a single-phase sine wave with 90 kvp peak? kvp peak? 46 Inefficient, solve by using three-phase power 8 Three-Phase Power 3o Supplied by power co As each wave peak begins to drop toward 0, voltage is boosted back to peak by next phase A. Sum of phasing never drops to 0 Produces 3 pulses/half cycle 6 pulses/hz & 360 pulses/sec 9 X-Ray Circuit 10 3

4 Autotransformer Single winding around an iron core Operates on self-induction principle Good for controlling voltage on low voltage side of x-ray circuit Supplies precise voltage to the high voltage (secondary) and filament volts delivered to primary side from incoming line voltage Voltage compensation automatic on 1 o side Follows the transformer law 11 A, A1: primary connections that conduct input power to Autotransformer C: Increases voltage due to proximity to end and number of turns encased by the connections E: Decreases voltage Autotransformer 12 Kilovolt Peak (kvp) Selector kvp selector on output side kvp major, kvp minor controls 220 volts delivered to primary side output voltage of autotransformer is usually between 100 to 400 volts Output voltage is then delivered to input side of step up transformer for x-ray tube operation 13 4

5 Milliamperage (ma) Selector X-ray tube current is controlled by the filament circuit Thermionic emission is based upon temp of the filament measured in amperes (A) Filaments operate at 3 6 amps, 6-12 volts Fixed ma stations as resistors Falling load generators Max ma, drops 14 Milliamperage (ma) Selector Voltage from ma station is delivered to filament transformer (Step-Down) Lower voltage, higher current to filament ma meter measures x-ray tube current Placed in the center on output side of high voltage transformer May be placed on control console 15 Exposure Timers Determines exposure duration Connected on the primary side of the high voltage transformer Types: Electronic: Most common, microprocessor controlled. Short time 1ms Good for multiple sequence imaging mas Timer: Electronic timer monitors tube current and is on the output side of the high voltage transformer. Uses the shortest exposure time for mas selected Used in falling load generators Designed to work in 3 phase or high frequency generators Kvp, ma regulated separately Exposure begins at highest ma, then decreases Permits better use of acceptable x-ray tube limits; less costly AEC 16 5

6 Voltage Rectification Required for x-ray tube operation Process of changing alternating current (AC) to pulsating direct current (DC) A rectifier functions by allowing current to flow through it in one direction only Electron flow in the x-ray tube must be from cathode to anode 17 Semiconductors Modern method of rectification N type & P type semiconductors are used N-type have loosely bound electrons that flow easily between the atom s conduction bands Silicon/Phosphorus, Silicon/Arsenic P-type have electron traps (positive holes) that attract and hold electrons instead of allowing them to move to another atom Silicon/Boron, Silicon/Gallium 18 Diode Joining of n-type and p-type semiconductors Electrons are attracted toward positive charge and move through the n-type material to the junction between the semiconductors. Additional electrons move in to replace electrons that migrated At the junction, electrons are attracted to the positive holes keeping a continuous electric potential in one direction only 19 6

7 Solid-State Semiconductor Diode 20 Types of Rectification Half-wave Rectification Single rectifier that suppresses the negative half of the alternating cycle No steady flow of current Energy loss in form of heat 21 Neg Cycle Types of Rectification Full-Wave Rectification Four rectifiers that are arranged to allow electron flow from negative - positive Uses all the current flow from the AC source Rise and fall of current potential Rippling of current produces lower energy x-rays Pos Cycle 22 7

8 Full-Wave Rectification Positive Half Cycle Diodes A & D permit electron flow during the positive half cycle Diode C cannot conduct electrons Negative Half Cycle Diodes B & C permit electron flow Diodes A & D block electron flow 23 Three-phase generators Incorporates three out-ofphase currents to produce a steadier DC, eliminating ripple Three coils of wire are wrapped around the generator core. 24 High-frequency DC X-ray circuitry is designed to increase the standard 60 cycle frequency to 50-5,000 cycles/sec Nearly constant potential waveform Smaller in design Increased radiation quality and quantity Lower patient dose Increase x-ray tube life 25 8

9 Voltage Ripple 26 X-ray Circuit Diagram 27 9