THE X RAY IMAGING SYSTEM
If a filament transformer has a turns ratio of 0.05 and 200 mA is supplied to the primary side of the transformer, what will be the secondary current?
0.05:1 1/0.05=20 20x200mA=4000 4A
X ray Machine
1. Main power source (220V) AC 2.Line monitor ( measures voltage going into machine, it detects ↑ or ↓ maintain 220V for Autotransformer) 3. Autotransformer ("Self induction" 1 coil supplies voltage, works off of AC selects KV, Receives power first, variable turns ratio, single winding functions as primary and second sides) 4.Major kVp Selector/ meter (Pre-reading volt meter, monitors V expected once sent to high voltage step up transformer) 5./6.Timing circuit and selectors( terminates exposure and controls length) 7./8. Step up transformer ( increases volts~kilovolts, Mutual induction P & S. More turns on S. V↑ A↓, AC between 500:1 1000:1 9.Line compensator (Adjust incoming voltage fluctuations to a standard) 10.mA selector [rheostat] (regulates amperage and thermionic emission (discharge of electrons from heated surface) connected to filament circuit) 11. P/12.S Step down (V↓, A↑, more turns on P, mutual induction, AC controlled by mA selector & provides electrons for x-ray current, also heats up filaments to boil off electrons 13. Cathode (-) 14. Anode (+) 15. Diodes (allow power flow in 1 direction, can be used as a rectifier, from cathode to anode 15.Bridge Rectifier (changes AC →DC, solid state semiconductors) 15. mA Meter (Shows selected mA & measures tube current in miliamps) -2 circuit diagram -High voltage circuit (DC) -Filament circuit low voltage (control circuit) (AC) -filaments is made of tungsten found on negative side of xray its is 1-2 cm in length -Operating console allows manipulation of high voltage section through low voltage section
The shortest exposure possible with single-phase equipment is 1/120 s. How many milliseconds is that?
1/120=0.008 0.008/1^-3=8 ms
A radiographic technique calls for a 400mA, 1/20 S exposure. What is the mAs?
1/20 S=0.05S 400mAX0.05S= 20mAs
Operation at 300 mA for 1/20 s is equivalent to operation at 900 mA for _______
1/20=0.05 0.05X300mA=15 mAs 15mAs/900mA= 0.01666667 17ms
If 60 Hz AC power is full wave rectified output voltage consists of
120 pulses per second
220V is supplied to 800 primary turns of an autotransformer. what will be the output voltage across 200 secondary turns
200/800=0.25 220VX0.25=55V 55V
A radiographic technique calls for 86 kVp/200 mAs using the 800 mA station. what is the exposure
200mAs/800mAs=0.25 0.25/1^-3=250ms
Three Phase power
30 power simply means that there are 3 power sources and those waveforms are 120 out of phase with each other
How many overlapping pulses are generated in 1 s for three phase, six pulse power?
360
In order to have full wave-rectification, there must be __________
4 diodes
If an xray imaging system is operated at 600mA,50 ms, the total mAs will be _________
50^-3 X600 mA=30mAs
If a single rectifier is inserted into a circuit that conducts 60Hz Ac so that it suppresses the positive portion of the waveform, then the output wave form will contain_________
60 negative pulses per second
With an automatic exposure control (AEC) device__________
A back up timer is required
Single 0 (phase) Half Wave Rectification
A single AC waveform Two diodes route the electron flow to always go from cathode to anode
Single 0 (phase) Full wave Rectification
A single AC waveform Four diodes route the electron flow to always go from cathode to anode
High-Voltage Transformer
A step-up transformer *between 500:1 and 1000:1 Ac waveform
Automatic Exposure Control
AEC measures xray intensity that penetrates the patient and terminates exposure upon reaching a specific measure There are 2 modes *Scintillator with photodiode *Ion chamber ~note that the position of the detectors
The principal purpose of the autotransformer is to __________
Adjust voltage
A semiconductor diode:
Allows current to flow only from p type of material to n type
Operating Console
Allows manipulation of the high voltage section through the low voltage section -Intensity~ number of photons (quantity) -Energy~ penetrating ability of the photons (quality)
The line voltage compensator corrects for variations in line voltage by __________
Altering the Np of the autotransformer
A change in the voltage waveform from the primary side to the secondary side of the high voltage transformer produces a change in________
Amplitude
Near the p-n junction of a semiconductor diode, one will find
An n type of material containing excess electrons
What is the component that provides power to the console and high voltage section ?
Autotransformer
Which of the following provides a variable turns ratio
Autotransformer
Transformers
Autotransformer Law Vs/Vp=Ns/Np Autotransformer -Single winding serving as both Ns and Np -The only transformer that allows a change in the turns ratio by allowing a change in the number of coils and tapped on secondary side
An automatic exposure control (AEC) device _______________
Can use ionization chamber between the patient and image receptor
The kVp meter is the _________
Control console
A rectifier ________
Converts AC to DC
Thermionic emission refers to _________
Electron emission from a heated source
A posterior anterior (PA) chest requires a technique of 125kVp at 4 mAs. The total number of electrons used to make the exposure is ____
Electron=6.3X10^18 (4)(6.3X10^18) 2.5X10^16
A filament current of 5A is necessary for thermionic emission. what electron flow is this
Electron=6.3X10^18 5(6.3X10^18) 3.15X10^19
The autotransformer operates on the principle of __________
Faradays law
The control of focal spot size is depends on ______
Filament that is energized
Autotransformer
Has a variable turns ratio Single winding functions as primary and second sides self induction
The filament transformer produces low voltage required to heat the filament. It is located in the ____________
High voltage section
The voltage supplied to an xray imaging system is 220V. the voltage used by the xray tube is produced by __________
High voltage transformer
The autotransformer can be used to do __________
Increase kVp
The principal purpose of the high voltage transformer is to do what ?
Increase voltage
The filament transformer ___________
Increases current
What type of AEC controls the time of exposure by measuring the ionization of gas ?
Ion chamber
Line Voltage Compensation
Line voltage entering the radioology department or clinic fluctuates with demand on the power company Line voltage compensation alters the voltage entering the x ray system to a standard (E.G-220V) *Small cahnges in low voltage produce large changes in the high voltage section *Line monitor *COmpensator
Xray tube current is usually measured in _________
Milliamperes(mA)
Mammography sometimes requires exposures as long as 1.5 s. this is equivalent to which of the following??
Millisecond=1000 1.5X1000= 1500ms
In half-wave retification, each inverse half cycle in the primary circuit corresponds to how many voltage pulses across the xray tube?
One
The meter that monitors xray tube current is
Physically located on the control console
The designed of fixed mA stations requires the use of ___________
Precision resistors
The output voltage from the autotransformer is ________
Proportional to the turns ratio
Voltage Ripple
Ripple is simply the variation between the peak and minimum voltage in a waveform The lower the voltage ripple, the greater the efficiency of xray production *Higher energy *Higher intensity
Power to the primary side of the high voltage transformer comes from the :
Secondary side of the autotransformer
Synchronous Timers
Synchronous timers rely on the frequency of the AC waveform *60HZ in USA, 50Hz everywhere else In the USA, the shortest time is simply the time for one cycle of the AC waveform *1/60 second Time on these machines is displayed in fractions of a second
Control of Milliamperage (ma)
The filament circuit provides current for the heating of the tube filament mA selector~ allows the selection of a specific value of resistor *mA setting Filament switch~ completes the circuit Filament transformer~ a step down transformer *Produces low voltage and high current on the secondary side Filament selector~allows the selection of small or large filaments(focal spot)
Concerning the transformer used in the xray circuit___________
The high voltage transformer is a step up device
Exposure Timers
The mAs unit is simply the product of the mA (tube current) multiplied by the time in seconds that the tube is energized Various combination of mA and time may produce a specific mAs On your xray unit time may be displayed as seconds, milliseconds, or a fraction of a second
High-Voltage generator
This section contains the high-voltage transformer filament transformer, and rectifiers
In a three phase power source, there are 3 power sources. Although they are exactly the same frequency and amplitude, they are _________
Timed 120 degrees out of phase to each other
What is higher for a single phase high voltage generator than for a three phase high voltage generator
Voltage ripple
With this autotransformer setup, what is the voltage on the secondary side ?
Vs/220V=200/600 Vs=(220)0.33 Vs=73.33V
The filament transformer in the previous question is supplied with 150V to the primary side. What is the secondary voltage
Vs/Vp=Ns/Np 1/20=0.05 0.05X150=7.5V
A filament transformer has 800 primary windings and is supplied with 200 mA. If the secondary coil has 100 windings, what will be the secondary current?
Vs/Vp=Ns/Np 100/800=0.125 200/0.125 1600mA
440V is supplied to 1000 primary turns of an autotransformer. If the desired output voltage is 100V, how many secondary turns must be tapped?
Vs/Vp=Ns/Np Vs=100V Vp=440 Ns=? Np=1000 100V/400V=Ns/1000 0.227272727/1000=227 Ns=227
A filament transformer has a turns ratio of 1:20 what current must be supplied to the primary windings if 5A is required by the filament?
Vs/Vp=Ns/Np Ns/Np=1/20 Vp=5A 1:20=1/20=0.05 0.05X5A=0.25 250mA
Voltage Rectification
We need DC for tube operation *cathode to anode *Rectification~converts AC to DC *Rectifiers (diodes) allow current flow in one direction only (thus,DC) *Rectifiers (solid state diodes) are made from semiconductor material
The filament transformer is designed __
With a turns ratio less than 1
A rectifier ______
can be a semiconductor
In a circuit that contains a single rectifier
electrons flow in one direction but not the other
What is directly connected to the autotransformer
kVp meter
Control of Milliamperage (mA)
mA refers to the current moving through the xray tube, from cathode to anode *Controlled by the controlling the current travelling through the cathode filament *The greater the number of electrons moving through the filament, the more will be available to move through the tube during an x ray exposure
The tube current (electrons crossing from cathode to anode) is governed by the current moving through filament which is controlled by the
mA selector
Operation at 100 mA for 1 s results in_________
mA=10^-3 100^-3(6.3X10^18) 6.3 X10^17 electrons
One coulomb per second (C/s) is equivalent to 1 A, and 1 C is equal to 6.3X10^18 electrons. Therefore, operation at 100mA would result in a current of _______
mA=10^-3 (100^-3)(6.3X10^18) (6.3X10^17 electrons/s)
mAs Timers
mAs timers track the actual tube current *A function of mA x s Used with mobile units *Plug in unit/ set technique/ bank of capacitors is charged *Exposure started/ the voltage drops over time of exposure due to exhausting of capacitor voltage Used in interventional units which short exposure times are required
Full wave rectification_____
requires at least four rectifiers