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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 Controls
• The console will have controls for:
• mA and time or mAs
• kVp
• Focal Spot
• Line Voltage Compensation
• Automatic Exposure Control
Symbols Used to Draw Circuits
• We will be using the symbols to define the circuits in the x-ray machine
Console Circuits
Line Compensation
• 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
Line Compensation
• 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.
Line Compensation
• Older machine have a meter to monitor the line voltage attached to the autotransformer.
• The operator can adjust the taps on the transformer to account for low or high incoming voltage.
Line Compensation
• 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.
Autotransformer
• 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.
Autotransformer
• The autotransformer has only one winding and one core.
• The single winding has a number of connection or electric taps.
kVp Adjustment
• Most consoles will have one or two knobs that change the taps on the autotransformer for major and minor kVp.
• Modern units have a LED readout of kVp.
kVp Adjustment
• Setting the desired kVp will determine the voltage applied to the step-up transformer in the high voltage section of the machine.
kVp Adjustment
• 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.
mA Control
• The tube current, the number of electrons crossing from the cathode to anode per second is measured in milliapmeres (mA).
• The quantity of electrons is determined by filament temperature.
mA Control
• The filament normally operates at currents between 3 and 6 A.
• The Tube Current is controlled through a separate circuit called the filament circuit
mA Control
• Voltage is provided by taps of the autotransformer. This voltage is reduced with precise resisters to a value corresponding to the mA stations available.
mA Control
• 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.
mA Control
• The voltage is then delivered to the filament transformer. The filament transformer lowers the voltage so it is called a step down transformer.
mA Control
• The selection of the small or large filament is connected to the mA selection or as a separate control.
Exposure Timers
• 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.
Exposure Timers
• 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.
Types of Timers
• There are five types of timers:
• Mechanical Timers
• Synchronous Timers
• Electronic Timers
• mAs Timers
• Phototimers
Mechanical Timers
• 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.
Electronic Timers
• Most sophisticated, 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.
mAs Timers
• 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.
mAs Timer
• 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.
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.
mAs Timer
• Since it monitors the actual tube current, it is on the secondary side of the H.V. Circuit
• Units here have mAs timers.
mAs Timer
• 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!!!!
Phototimers
• A phototimer that 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.
Phototimers
• 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.
Phototimers
• 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.
Phototimers
• Commonly referred to as Automatic Exposure Control or AEC.
• Widely used in Medical Radiography.
• Used at our Benton Clinic.
AEC Console
• With AEC, the operator can select:
• Where to read the radiation.
• The desired film density
• kVp and backup mAs
AEC Console
• 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.
AEC Console
• This allows the AEC to adjust exposure for the patient’s habitus and area density.
• Radiation is measured at the center of the film or off to the sides of the film.
AEC Console
• The center is read for most radiography and especially for the spine.
• The sides are read for PA chest, abdomen and rib radiography.
Other functions on the Control Console.
• 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.
Exposure Button
• 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.
Exposure Button
• The button must be held down until the exposure is complete.
• If your finger slips off the button, the exposure is terminated.
Exposure Button
• The exposure control buttons are referred to as a”Dead man Switch”
• After the buttons are released, the rotor motor reverses and the rotor reduces speed.
Exposure Button
• During the exposure you will hear an audible tone so you will know that the exposure is in progress.
Chapter 8 High Voltage Section
• 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.
High Voltage Section
• It consists of three primary sections:– High voltage step up
transformer– Filament Transformer– Rectifiers ( Diodes)
• All components immersed in oil.
High Voltage Transformer
• 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.
High Voltage Transformer
• 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.
Voltage Rectification
• 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.
Half-Wave Rectification
• Sometimes the x-ray tube alone will work as the diode this is called self-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 Rectification
• 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.
Three-Phase Power
• If three phases of power are combines with the phase off by 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.
High Frequency Generator
• 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-phase power.
Types of X-ray Generators
• The type of generator will determine the efficiency of the machine.
Wave Forms of Different Generator Types
• As the ripple effect decreases, the efficiency increases.
• There is one more type of generator. It uses is called stored energy.
Stored Energy Generators
• 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.
Stored energy or Capacitor Discharge Generators
• 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 a falling load generator.
Generator Types Pros & Cons
• 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.
The Basic X-ray Circuits
• Circuits that make up the basic x-ray machine.
Other Parts of the X-ray Room
• The tube is suspended on the tube stand.
• The tube stand may be wall and floor mounted or ceiling suspended.Locks are provided for horizontal and vertical movement.
Other Parts of the X-ray Room
• 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).
Other Parts of the X-ray Room
• 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.
Other Parts of the X-ray Room
• X-ray tables may be 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.
Collimator and Angle Indicator
• The tube stand also has an angle indicator attached parallel to the tube.
• There are views that will require tube angles.
Collimator and Angle Indicator
• The Collimator is attached to the x-ray tube below the glass window where the useful beam is emitted.
• Lead shutters are used to restrict the beam.
Collimator and Angle Indicator
• 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.
Other items that may be in a x-ray room.
• Fluoroscopy Equipment: Allows dynamic imaging of the body.
• Consists of:
• Image intensifier with television camera and monitor.
• Spot-film device for making radiographs or
• Motion picture camera or digital imaging.
Image Intensifier & Fluoroscopy
• 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.
Image Intensifier & Fluoroscopy
• Plain film radiography uses up to several hundred mA and fractions 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.
Image Intensifier & Fluoroscopy
• Shortly after WW2, Bell Laboratories invented the photomultiplier tube. This was developed into the modern image intensifier.
• The multiplication of the light emitted by a input fluorescent screen is picked up by a cesium photocathode and converted into electrons.
Image Intensifier & Fluoroscopy
• 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.
Image Intensifier & Fluoroscopy
• The output phosphor can be viewed via mirror optics or 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:
Uses of Fluoroscopy
• 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.
Uses of Fluoroscopy
• 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.
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