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RAD 350 Chapter 5 X-ray Imaging System
Three MAIN components of an x-ray imaging system: x-ray tube, operating console and high
voltage generator.
-kVp range is 25-150 {50-125 realistic diagnostic range, not including mammography}
-mA range 100-1500
Table-tube relationships may vary with type of unit, but ALL have the same essentials – RAD;
RAD-fluoro; fluoro; C-arm, tomography
-table MUST be radiolucent – most modern tables are made of carbon fiber and have a
max weight of 500 lbs (up from 300 pounds just a few years ago!)
-table components are:
-bucky tray (Potter-bucky diaphragm)
-grid (most table grids are 8:1/most used for portables are 5:1 grid ratios)
-closing bucky slot hinge on fluoro units to help control scatter radiation
-fluoro tables usually tilt 90/30 degrees
Operating console – consists of ALL LOW VOLTAGE components and controls:
Quantity – mA/mAs (amount of radiation produced) increases in mA will increase the
number of electrons available to be “boiled off” the filament via THERMIONIC EMISSION.
FILAMENT CURRENT is usually 3-6 AMPS in order to have enough electrons to boil off.
REMEMBER – tube current flow is measured in MILLI-AMPS and the polarity is negative to
positive!
QUALITY – kVp/kV (how penetrating the beam is = energy level)
-use OPTIMUM penetration techniques” that are based upon atomic mass density
of the patient/contrast agent being used
-kVp energy changes are made via an AUTOTRANSFORMER – which uses one
winding with variable “tabs” to enable one to select the varied kVp’s
-Increasing kVp’s will increase the energy of the x-ray photons produced
Transformer law is the relationship of windings on the PRIMARY (left side) to the
SECONDARY (right side)
-REMEMBER: transformers only work with A/C current and the step up or
down is related to the number of windings – primary to secondary
TIMERS – devices that set the time the x-ray exposure is made; ALL are in the low
voltage side of the x-ray circuit EXCEPT the mAs timer (as mAs – milliamp seconds = mA
X time ant eh mAs timer is the MEASUREMENT OF ACUTAL ELECTRON FLOW IN THE XRAY TUBE!)
-mechanical timer – like an “egg timer” least complicated and least accurate
-synchronous timer – a motor running on 60 cycle per second (60 Hz), and drives a
shaft at exactly 60 revolutions per second – therefore MIN EXPOSRUE TIME IS
1/60 second
-ELECTRONIC TIMERS are the MOST accurate as well as most comoplicated BUT
can have minimum times as short as 1ms.
- Automatic Exposure Control – DEVICE THAT MEASURES THE AMOUNT OF
RADIATION exiting the patient (either in the form of an ionization chamber
between the patient and the image receptor/film OR a fluorescent screen and
photodiode below the image receptor
-Film/Image receptor/imaging plate
Line (voltage) compensator – to ensure the incoming voltage into the x-ray unit
(before being stepped up) is the proper voltage – MOST ARE AUTOMATIC NOW
(autotransformers) but you may still see some older units that can be manually
adjusted
HIGH VOLTAGE GENERATOR/HIGH VOLTAGE TRANSFORMER/ FILAMENT TRANSFORMER AND
RECTIFIERS
The high voltage transformer is used to lower current (amps) and raise the voltage to
KILOVOLTS – Remember that just as high as the voltage is STEPPED UP, it LOWERS the
amps a like amount to milliamps = 1/1,000 of an amp
A RECTIFIER is used to convert PULSED A/C in to PULSED D/C. A series of A/C channeling
diodes are used to “give” the A/C path in BOTH the above the sine wave horizontal line and
below the line enabling the 60 cycle/second current to use both the + and the – sides of the
electrical pulses – giving the machine120 impulses per second of USEABLE POWER
Unrectified voltage is only able to use the “above the horizontal line impulses” or 60 impulses
per second – half wave rectified. If the x-ray tube ITSELF is the rectifier (as a diode) it is
termed SELF RECTIFICATION!
Full wave rectification is where FOUR diodes are used together to effectively give electricity a
+ and a – path to flow – BUT in the x-ray tube ALWAYS the current goes from cathode (-)side
to anode (+) side. THE +/- POLARITY DOES NOT CHANGE!
Phases of power:
Single phase – one sine wave of A/C current
Three phase = three A/C sine waves are superimposed upon one another in the same
wire, BUT are out of phase with each other (sine waves are NOT the same up and down
motion on top of one another – just slightly out of phase) and ELIMINATE the drop in impulse
to the horizontal line (energy NEVER drops to zero).
Ripple – ½ or full wave rectified drops to the horizontal sine wave line (100% ripple). Three
phase NEVER drops to zero. The LESS RIPPLE, the higher the energy!
See figure 5-29 on page 100 for ripple sine waves
Half wave = 100%; full wave 100%; 3 phase 6 pulse = 13 %; 3 phase 12 pulse = 4%; hi
frequency = <1%
Power = Current X Potential
Watt = Amps X Volts
See page 88 figure 5-6 and page 100 figure 5-30