The x ray imaging system

1. Chapter 8 The X-ray Imaging System

The Console is the part of the machine that the operator controls the operation of the x-ray machine.

All machine console are a little different but there are always similarities. The console is where we control x-ray tube current and voltage.

The console will have controls for:

mA and time or mAs

Focal Spot

Line Voltage Compensation

Automatic Exposure Control

We will be using the symbols to define the circuits in the x-ray machine

At the bottom left is the controls for line voltage compensation.

Most machine are designed to operate at 220 volts while some will work with 110 volts or 440 volts

The power company often cannot provide exactly 220 volts at all times.

Elevators and Air Conditioners may reduce the voltage available for the x-ray unit.

Older machine have a meter to monitor the line voltageattached to the autotransformer.

The operator can adjust the taps on the transformer to account for low or high incoming voltage.

More modern units automatically adjusts for the incoming power so a meter is not provided.

Often over looked by the operator.

Results in improper exposure.

The autotransformer is designed to supply voltage of varying magnitude to several different circuits of the x-ray machine including both the filament circuit and high voltage circuits.

The autotransformer has only one winding and one core.

The single winding has a number of connection or electric taps.

Most consoles will have one or two knobs thatchange the taps on the autotransformer for major and minor kVp.

Modern units have a LED readout of kVp.

Setting the desired kVp will determine the voltage applied to the step-up transformer in the high voltage section of the machine.

If a meter is provided, it is placed across the output terminals of the autotransformer and therefore it reads voltage and not kVp. The scale will read in kVp.

The tube current, the number of electrons crossing from the cathode to anode per second is measured inmilliapmeres (mA).

The quantity of electrons isdetermined by filament temperature.

The filament normally operates at currents between 3 and 6 A.

TheTube Currentis controlled through a separate circuit called thefilament circuit

Voltage is provided by taps of the autotransformer. This voltage is reduced with preciseresisters to a value corresponding to the mA stations available.

Tube current is usually not continuously variable, usually only currents of 50, 100, 150, 200 & 300 mA and higher are provided.

Newer units are continuously variable.

The voltage is then delivered to the filament transformer. The filament transformer lowers the voltage so it is called a step down transformer.

The selection of the small or large filament is connected to the mA selection or as a separate control.

For any given radiographic examination, the number of x-rays reaching the image receptor is directly related to the tube current and the time that the tube in energized.

The timer circuit is separate from the other main circuits.

It consists of a mechanical or electronic device whose action is to make and break the high voltage across the tube on the primary side of the high voltage section.

There are five types of timers:

Mechanical Timers

Synchronous Timers

Electronic Timers

mAs Timers

Phototimers

Very simple device that has a clock mechanism.

Operator turns the dial to the desired time. As it unwinds, the exposure is made.

Can be used for exposure time longer than 250 milliseconds.

Very old machine and dental units.

Mostsophisticated, complicated and most accurate timer.

Consists of complex circuit based upon the time required to charge a capacitor through a variable resister.

Depending upon the incoming power accurate to 1 ms. Most units have this type timer.

Most modern machine are designed to accurately control the tube current and exposure time.

The product of mA and time (mAs) determines the number of x-ray photons emitted and the density on the film.

A special type of timer monitors the product of mA and terminates the exposure when the desired mAs has been attained.

This is a mAs timer.

Designed to provide the shortest exposure and the highest safe tube current for the given filament.

Some have the ability to change mA manually.

Since it monitors the actual tube current, it is on the secondary side of the H.V. Circuit

Units here have mAs timers.

APR or Anatomically Programs Timers have computers that store the technical factors in the machine.

Select the view and enter the patient size and the machine is ready!!!!

Aphototimerthat measures the quantity of radiation reaching the receptor and terminates the exposure when sufficient radiation needed to produce the correct density on the film.

Offered in addition to a manual timer.

There are two types of phototimers:

1. Photomultiplier tube that reads a fluorescent screen behind the film.

2. Ion chamber between the grid and film.

Ion Chambers is used on most modern x-ray units.

It is flat and radiolucent so it will not interfere with the image. Multiple chambers can be used to optimize the image.

Commonly referred to as Automatic Exposure Control or AEC.

Widely used in Medical Radiography.

Used at our Benton Clinic.

With AEC, the operator can select:

Where to read the radiation.

The desired film density

kVp and backup mAs

Many operators do not measure the patient and set a arbitrary back up mAs or time.

Ideally, the patient is measured and the back up mAs is set at 2X the normal mAs.

This allows the AEC to adjust exposure for the patients habitus and area density.

Radiation is measured at the center of the film or off to the sides of the film.

The center is read for most radiography and especially for the spine.

The sides are read for PA chest, abdomen and rib radiography.

The console will also have the exposure button or buttons.

The prep button is depressed to prepare the tube for exposure.

The rotor will spin up to 3400 RPM.

A green light will let you know that the machine is ready to make the exposure.

The exposure button is then depressed and the exposure is initiated.

The button must be held down until the exposure is complete.

If your finger slips off the button, the exposure is terminated.

The exposure control buttons are referred to as aDead man Switch

After the buttons are released, the rotor motor reverses and the rotor reduces speed.

During the exposure you will hear an audible tone so you will know that the exposure is in progress.

The high voltage section converts low voltage from incoming power to kilo-voltage of the correct wave form.

It is usually enclosed in a large metal container in the x-ray room.

It consists of three primary sections:

High voltage step up transformer

Filament Transformer

Rectifiers ( Diodes)

All components immersed in oil.

The high voltage transformer is a step-up transformer.

There will be more winding on the secondary side compared to the primary side.

The ratio of windings is referred to as the turns ratio.

The only difference between the primary and secondary waveforms is the amplitude.

The turn ratio for most x-ray high voltage transformers is between 500 and 1000.

Incoming Volts converted to output: Kilovolts.

Transformers operate with alternating current.

Remember that x-ray tubes operate on direct voltage ( electron moving in one direction).

To convert AC to DC we use rectifiers.

Sometimes the x-ray tube alone will work as the diode this is calledself-rectification .

When one or two diodes are placed in the circuit that stops the negative flow of electrons it is called Half Wave Rectification.

60 pulses per second.

Full wave rectified x-ray machines contain at least four diodes.

It changes the polarity of the negative half of the wave.

This allows 120 pulses of x-ray per second.

The exposure time can be cut in half compared to half-wave systems.

If three phases of power are combines with the phase offby one step, the normal reduction of voltage back to zero is removed. Commonly called the Ripple.

Technical factor cut in half due to more efficient power.

Too expensive got office use.

By changing the frequency from 60 Hz to a higher frequency of 500 to 1000, the ripple is reduced to less than 1%.

Single phase machine operating on 220 volts and even 110 volts are more efficient that machine operating on three-phasepower.

The type of generator will determine the efficiency of the machine.

As the ripple effect decreases, the efficiency increases.

There is one more type of generator. It usesis called stored energy.

If 220 volt power is not available, the operator may choose a stored energy machine.

A battery charger is powered by typical house hold current.

If produces direct current.

There is a short charging time before the exposure can be made.

The disadvantage to the design is a drop in power at the end of the exposure of about 1 kV/mAs. This is called afalling load generator .

Single phase half or self rectified: Cheap but not efficient. Full wave rectified better.

Three phase: Expensive to install but cheaper to maintain. Too costly for most offices. 6 pulse less costly than 12 pulse

High Frequency: very efficient and works with single or three phase power.

Stored energy: works on conventional 110 volt power but batteries must be replaced.

Circuits that make up the basic x-ray machine.

The tube is suspended on thetube stand .

The tube stand may be wall and floor mounted or ceiling suspended.Locks are provided for horizontal and vertical movement.

When the tube is angled toward the wall grid holder, the horizontal lock allow us to set the distance between the tube and the film (SID).

When the tube is aimed at the table, the vertical lock allows us to set the SID.

Hanging on the wall grid cabinet is the non-Bucky film holder.

It allows erect non-grid films.

X-ray tables maybe bolted to the floor or mobile. The table will also have a grid cabinet for grid radiography.

We will discuss grids in greater detail next week.

The tube stand also has an angle indicator attached parallel to the tube.

There are views that will require tube angles.

The Collimatoris attached to the x-ray tube below the glass window where the useful beam is emitted.

Lead shutters are used to restrict the beam.

A mirror and light source allows us to restrict the beam to the area of interest.

Collimation is our greatest tool in keeping patient exposure as low as possible.

Fluoroscopy Equipment: Allows dynamic imaging ofthe body.

Consists of:

Image intensifier with television camera and monitor.

Spot-film device for making radiographs or

Motion picture camera or digital imaging.

Thomas Edison invented the fluoroscope in 1896. Early units consisted of a fluorescent hand held viewer that the doctor held in from of the patient during continuous exposure.

This resulted in the first x-ray death.

Dose is still relatively high compared to plain film radiography.

Plain film radiography uses up to several hundredmA andfractions of seconds.

Fluoroscopy tubes operate at less than 5 mA but for minutes. 2 to 4 mA is normal.

In California Fluoroscopy is beyond the chiropractic scope of practice.

Shortly after WW2, Bell Laboratories invented the photomultiplier tube. This was developed into the modern image intensifier.

The multiplication of the light emitted by ainput fluorescent screen is picked up by a cesium photocathode and converted into electrons.

A potential of about 25,000 volts is maintained between the photocathode and the anode.

There are electronic optics and electrostatic focusing lenses between the photocathode and output phosphor.

The output phosphor can be viewed via mirror opticsor a video monitoring system.

A Videotape recorded can be placed into the video chain.

Fluoroscopy allows the evaluation of the internal structures in motion. Normal uses include:

Dynamic spinal imaging of range of motion and with contrast called myelograms.

Dynamic studies of joints with or without contrast media.

Studies of the digestive system.

Studies of arteries and blood flow called angiography.

When connected to a computer, for digital fluoroscopy and spot films.

With digital fluoroscopy, digital angiography is possible.

By over-lapping an image without contrast, digital subtraction angiography is performed where the bone is removed.

Education

The x ray imaging system