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 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 200 – 240 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 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? • 63.6 • 65 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 & pulses/sec 360 9 X-Ray Circuit 10 3 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 circuits – @220 volts delivered to primary side from incoming line voltage • Voltage compensation automatic on 1o side – Follows the transformer law 11 Autotransformer • 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 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 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 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 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 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 505,000 cycles/sec – Nearly constant potential waveform – Smaller in design – Increased radiation quality and quantity – Lower patient dose – Increase x-ray tube life 25 8 Voltage Ripple 26 X-ray Circuit Diagram 27 9