Diagnostic X-ray Circuits

• Diagnostic X-ray circuits are crucial for medical imaging, operating within medical imaging instrumentation.

X-Ray Imaging System Components:

• X-ray systems include X-ray production, circuits, tube, and housing.
• The X-ray tube is in the examination room.
• The operating console is in an adjoining room behind a protective barrier.
• The high-voltage generator is positioned against a wall in an equipment cabinet.

Circuit Functions

• X-ray generators, fed by power sources, power the X-ray tube.
• Filament circuits (mA) provide the power to heat the cathode filament, generating electrons.
• Timer circuits (s) control the duration of X-ray production,
• High voltage circuits (kVp) supply the high voltage necessary to accelerate electrons from the cathode to the anode.

Divisions of Main X-Ray Circuit:

• The primary section includes the operating console, autotransformer, and exposure timer.
• The secondary section includes the step-up transformer's secondary winding, rectification circuits, and wiring to the X-ray tube.

Electrical Power and Line Compensator:

• Power in Hong Kong is 220V / 50 Hz.
• The US and Canada use 110 or 120V / 60 Hz.
• X-ray tubes demand high voltage (30-150 kV) for clinical applications.
• Line compensators: compensate for variations in voltage to ensure consistent, high-quality X-ray beam production.
  • They measure and adjust the voltage to precisely 220V.

Operator Console Parameters:

• Includes selection of tube voltage (kV), tube current (mA), and exposure time (s).
• Allows for focal spot selections.
• AFC (Automatic Exposure Control) maintains consistent signal brightness.
• Can be pre-programmed

Autotransformer

• Operates by electromagnetic induction to convert low (220V) to high voltage (30-150kV) for X-ray generation.
• Operates on single winding to supply precise voltage to the filament and high-voltage circuits.
• Input AC power on primary winding will induce a voltage on secondary winding that is proportional to the input voltage and ratio of turns

Kilovolt Peak (kVp) Adjustment:

• Major and minor kVp selectors allow adjusting the required kilovolt peak.
• kVp meters measure low-voltage that reflect high-voltage levels.

Milliamperage (mA) Adjustment:

• The X-ray tube current, measured in milliamperes, depends on the number of electrons emitted by the filament.
• Filament current, typically 3-6 A, controls filament temperature and thermionic emission.

Filament Circuit Details:

• Filament circuits control X-ray tube current (mA).
• Filament X-ray circuits have mA selectors (rheostats) and filament transformers.
• It adjusts resistance to select mA and is featured on many control panels.
• It often includes a heating isolation step-down transformer.

mA Meter

• Allows the direct monitoring of the X-ray tube current (mA).
• It's connected at the center of the secondary winding, where voltage remains at zero due to alternating voltage.

Timing Circuit:

• X-ray exposure is linked to tube current and exposure time.
• Timing circuits, separate from main circuits, are on the primary side of high-voltage transformer.

Exposure Timer Types:

• Mechanical timers are simple, using a clock mechanism suitable for old machines and dental units, useable from 250ms to 1 hour.
• Synchronous timers use a synchronous motor, subdividing time at 60 rps. Times are limited by 60 rps.
• Electronic timers are accurate down to 1ms, for rapid serial exposures for radiology procedures.
• mAs timers measure the product of tube current and exposure time to deliver the highest safe tube.
• AECs (Automatic Exposure Control) terminate X-ray exposure once a pre-determined amount of radiation is sent through a patient.

Automatic Exposure Control (AEC):

• AECs control exposure time for total radiation exposure on the image receptor.
• Radiographers select kVp, mA, and grid, but not exposure time.

AEC Types:

• Phototimers use detectors behind the image receptor to measuring radiation exposure. Screen converts Xrays into visibile light. System stops exposure upon preregistered charge.
• Ionization chambers are in front of the image receptor, using a radiolucent monitor to measure radiation exposure and send signal to stop exposure.

High Voltage Generator

• Responsible for increasing output voltage from auto-transferred to the kVp needed for production of Xrays.
• Employs high-voltage transformer and rectifiers.

High Voltage Transformer Overview

• Secondary voltage exceeds primary, with more secondary windings than primary.
• Current reduces proportionately.
• Waveforms on both sides are sinusoidal (AC).

Voltage Rectification:

• Converts alternating current (AC) to direct current (DC), for electron flow from cathode to anode.

Rectifier Types:

• Valve tubes were in place previously
• Solid-state, semiconductor diodes are in place now.
• Process can isolate P-N Junction diodes to only allow one direction.
• Half wave only enables one current.

Half-Wave Rectification:

• The circuit is closed and voltage applied, allowing tube current in the first half cycle.
• In the second half cycle the circuit is open, preventing voltage application and tube current.
• It wastes one half of the suplied power by requiring exposure time.

Full-Wave Rectification:

• Employs four diodes, generating 120 X-ray pulses/second improving frequency.
• Exposure time is one half of what it would be for a half-wave rectifier circuit.
• The voltage is always positive to lead the X ray tube.

X-Ray Generator Circuit Designs:

• Characteristics vary from single-phase to high-frequency.
• Single-phase power in dental and mobile systems is low.
• Three-phase systems offer medium to high power capabilities with 6 or12 pulses.
• Single phase power creates only pulsed power
• High-frequency generators convert AC to high frequency DC for reproducibility and consistency.

Single Phase Generators

• Generates single waveform of dental and portable devices.
• Is responsible for pulsed X Ray radiation and is only significant near the peak.
• Provides little diagnostic value

Three Phase Generators

• Operates in three waves of evenly spaced power
• Voltage is reliably constant with 6 pulses at every 1/60 second

High-Frequency Features and Advantages:

• Circuits contain high-speed switches that convert DC into a series of square pulses, creating near constant potential.
• They smaller, less costly and efficient.

Understanding Voltage Ripple:

• Voltage ripple refers to the dip in between tube voltage and maximum X-ray ranges.
• Voltage decreases as percentage to the maximum percentage,
• Ideally constant votlage reduces voltage ripple.