BUSHONG: CHAPTER 05: X-RAY IMAGING SYSTEM

What are the two main types of diagnostic imaging systems?

Radiographic/fluoroscopic systems and mobile C-arm units

What voltage and tube current ranges are diagnostic imaging systems usually operated at?

25 to 150 kVp and 100 to 1200 mA

Where is the fluoroscopic x-ray tube usually located?

Under the examination table.

How is the radiographic x-ray tube positioned?

Attached to an overhead movable crane assembly for easy positioning.

What are the two types of movement for patient couches?

- Floating (manually moved)

- Motor-driven (electrically controlled).

What material are patient couches typically made of, and why?

Radiolucent carbon fiber to minimize x-ray absorption and reduce patient dose

It automatically shields the opening to prevent radiation leakage

Bucky slot cover

What tilt functions are available for patient couches?

90 degree foot-side tilt

30 degree head-side tilt

What are the three main components of an x-ray imaging system?

- Operating console

- X-ray tube

- High-voltage generator

Where is the operating console typically placed?

In an adjoining control room behind a protective barrier

Its purpose is to allows patient observation while providing radiation protection

Lead-glass window in Protective barrier

The operating console allows technologists to control, what?

- X-ray quantity (number of photons)

- X-ray quality (beam energy/penetrability)

How is x-ray quantity measured?

in milligray (mGya) or mGya/mAs

How is x-ray quality expressed?

kVp or half value layer

What are the key controls on the operating console?

- kVp selector

- mA selector

- exposure time

- line compensator

Its purpose is to stabilizes input voltage to ~220 V to ensure consistent x-ray output.

Line compensator

It terminates exposure when the optimal dose reaches the detector.

Automatic exposure control (AEC)

What is the backup timer set to in AEC?

1.5x the expected exposure time for safety

It supplies precise voltage to the filament and high-voltage circuits

Autotransformer

How does the autotransformer differ from a conventional transformer?

It uses one winding with multiple taps instead of separate primary and secondary windings

What is autotransformer law states?

the voltage receive and provide by the transformer is directly proportional to the number of turns

What was the dual control system in older consoles?

- Major kVp (coarse adjustment)

- Minor kVp (fine-tuning)

What does the kVp meter measure?

Autotransformer output voltage and projected kVp

Its purpose is steps down voltage to ~12V to heat the filament and provide high current for thermionic emission

Filament transformer

What are the primary and secondary winding specifications of the filament transformer?

- Primary: 0.5-1A at ~150V

- Secondary: 5-8A at ~12V

What is the formula for mAs?

mAs = mA x time

It automatically terminates exposure after ~6 seconds to prevent overexposure.

Guard timer

Where is the timer circuit located?

On the primary side of the high-voltage transformer for safer control

It consist of an electronic device whose action is to make and break the high voltage across the x-ray tube

Timer circuit

It is a precision device designed to drive a shaft at precisely 60 rps.

This timer using a 60Hz (US) or 50Hz (Europe)

Synchronous timer

What is the minimum exposure time for synchronous timers?

1/60 sec (17 ms)

What is the limitation or disadvantage of synchronous timers?

Cannot be used for rapid exposures (requires reset after each exposure)

Where is the mA meter placed for safety?

Connected to the center tap of the high-voltage transformer's secondary winding.

Why is the center tap a 0V reference?

Due to symmetrical AC voltage oscillation (60 Hz).

It is most sophisticated, most complicated, and most accurate timers and ideal for rapid serial exposures

Electronic timers

It allows a wide range of time intervals to be selected and are accurate to intervals as small as 1ms, using capacitor-resistance circuits.

Electronic timers

This timers that terminate exposure when the desired mAs value is attained.

mAs timers

It monitors the product of mA and exposure time

mAs timer

Where are mAs timers located?

On the secondary side of the high-voltage transformer

Its purpose is to increase output voltage from the autotransformer to the kVp needed for x-ray production

High-voltage generator

What are the three primary parts of the high-voltage generator?

High-voltage transformers, filament transformer, and rectifiers.

What are the two types of AEC?

Ionization chamber and phototimer.

Where is the ionization chamber placed?

Between the patient and the image receptor (IR).

Where is the phototimer placed?

After the IR, using a light fluorescent screen and photocell.

What is the purpose of the backup timer in AEC?

Prevents overexposure if AEC fails (set to 1.5x expected exposure time).

What is the turns ratio in a high-voltage transformer?

The ratio of secondary to primary windings (500:1 to 1000:1)

What is the purpose of rectifiers?

Convert AC to DC to ensure electrons flow only from cathode to anode.

It is a specialized electronic component with 2 electrodes (anode and cathode) allowing current in one direction only (forward bias)

Diodes

What is a PN junction?

N-type material placed in contact with P-type crystal, forming a diode.

Uses 0, 1-2 diodes to block negative AC cycles, producing 60 pulses/sec.

Half-wave rectification

What is the disadvantage of half-wave rectification?

Wastes 50% of power

doubles exposure time

Uses 4 diodes to invert negative cycles, producing 120 pulses/sec.

Full-wave rectification

Uses the x-ray tube itself as a rectifier, functioning like a half-wave system.

Self-rectification

It allows voltage to drop to zero with each polarity change, producing pulsating x-rays.

Single-phase

What is the ripple percentage for single-phase power?

100%

Uses three overlapping AC waveforms (120° out of phase) for nearly constant voltage.

Three-phase

Results from the manner in which the high voltage step-up transformer is wired into the circuit and its advantage is overall waveform never reaches zero

Three-phase

What is the ripple percentage for three-phase (6-pulse) power?

14%

What is the ripple percentage for three-phase (12-pulse) power?

4%

What is the ripple percentage for high-frequency generators?

~1%

Converts 60Hz AC to high-frequency (500-25,000Hz) AC for smaller, more efficient designs.

High-frequency generator

High-frequency generators operate at how many hertz?

500-25,000 Hz

What are the advantages of high-frequency generators?

- smaller and compact

- increased image quality and decreased px dose

- more efficient

During capacitor discharge generator, the voltage falls approximately

1kV/mAs

Uses NiCd batteries and SCRs to release stored charge for x-ray production.

Capacitor discharge generator

Under of capacitor discharge generator.

It is a special cathode design cuts off electron flow instantly

Grid control

Starts at maximum mA and decreases mA gradually to prevent anode overheating.

Falling load generator

What is the advantage of falling load generators?

Faster exposures and anode protection

Fluctuation in kVp during exposure, affecting x-ray output consistency.

means of characterizing voltage waveforms

percentage variation in voltage

Voltage ripple

What are the advantage of voltage ripple?

- Higher x-ray output

- Better beam quality

- Dose efficiency

What is the power rating for basic radiographic units?

30-50 kW

What is the power rating for interventional suites?

Up to ~150 kW

What is the industry standard for generator power specification?

Maximum mA achievable at 100kVp for 100ms

Why do single-phase systems deliver ~70% of calculated power?

Due to 100% ripple

What is the purpose of the filament circuit?

To heat the filament for thermionic emission

controls x-ray tube current

What is the typical filament current range?

3-6A

What is the purpose of the step-down transformer in the filament circuit?

Reduces voltage to ~12V and increases current for filament heating

What is the purpose of electrical isolation in the filament transformer?

Protects the filament circuit from high voltage

Sets the tube current (quantity of x-rays).

mA selector

Adjusts beam energy (quality of x-rays)

kVp selector

Controls the duration of x-ray production

Exposure timer

Stabilizes input voltage to ~220V for consistent x-ray output.

It measures the voltage provided to the x-ray imaging system

Line compensator

What is the purpose of the high-voltage transformer?

Steps up voltage to the kVp required for x-ray production

What is the purpose of the rectifier circuit?

Converts AC to DC for unidirectional electron flow in the x-ray tube

What is the purpose of the autotransformer in the x-ray circuit?

Supplies variable voltage to the high-voltage transformer

What is the purpose of the power rating in generators?

Indicates maximum x-ray output capability