Week 7: Generators and Charts Flashcards
Single-phase power: Produced by a generator, single-phase 60 Hz AC changes directions ____ times per second
120
Single-phase power: With 60 Hz AC, there are two voltage half-cycles (2 pulses) in _____ of a second
1/60th
With 60 Hz AC, there are ___ positive half-cycles (60 pulses) and ___negative half-cycles (60 pulses) in 1 second.
60, 60
Single phase image
Direct Current (DC)
• Electrons flow in only one direction
• The voltage does NOT fluctuate and
maintains a constant potential until the circuit switch is opened and voltage goes back to 0
• DC is the most efficient power type, but as electrons travel long distances, they lose energy and the voltage drop increases the further current flows in one direction
Three-phase power
• With complex electrical engineering, three-phase power can be created where three single-phase voltage waveforms can be superimposed over each other
• Creates a voltage waveform that
maintains a nearly constant potential (minimal voltage drop from the peak amplitude)
• There are six voltage half-cycles (6 pulses) per 1/60th of a second. There are 360 pulses in 1 second
Three-phase Power: There are __ voltage half-cycles ( __ pulses) per 1/60th of a second. There are ____ pulses in 1 second
6, 6, 360
Single-phase, half-wave rectification (single-phase, one-pulse) (1Φ 1P)
• Single phase 60 Hz AC is rectified so that only the positive half-cycles (positive pulses) of voltage are transmitted through the x-ray tube during an exposure
• This can be created through self-rectification, or by adding 1-2 rectifiers to the high-voltage circuit with the x-ray tube
• HWR wastes half of the supply of power and requires twice the exposure time
• The pulsed x-ray output of a HWR machine occurs 60 times per second (60 x-ray pulses per second)
The pulsed x-ray output of a HWR machine occurs ___ times per second (___ x-ray pulses per second)
60, 60
Single-phase, full-wave rectification (single-phase, two-pulse) (1Φ 2P)
• Single phase 60 Hz AC is rectified so that both the positive and negative half cycles (pulses) of AC can be transmitted through the x-ray tube, which allows for half of the exposure time when compared to HWR
• This can be created by adding at least 4 rectifiers to the high-voltage circuit with the x-ray tube
• The double efficiency of FWR permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used with FWR.
• The pulsed x-ray output of a FWR machine occurs 120 times per second (120 x-ray pulses per second)
The pulsed x-ray output of a FWR machine occurs ____ times per second (____ x-ray pulses per second)
120, 120
Three-phase, six-pulse (3f 6P) (3Φ 6P)
• When full-wave rectification is applied to three-phase power, it produces 6 pulses in 1/60th of a second, or 360 pulses per second through the x-ray tube
• Notice how voltage supplied to the x-ray tube does not drop to 0 like it does with the single-phase generators that are HWR or FWR
• The efficiency of 3Φ 6P permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used when compared to single-phase generators
Three-phase, six-pulse (3f 6P) (3Φ 6P): When full-wave rectification is applied to three-phase power, it produces __ pulses in 1/60th of a second, or ____ pulses per second through the x-ray tube
6, 360
Three-phase, twelve-pulse (3f 12P) (3Φ 12P)
• With further improvements in engineering, 3Φ 12P generators can produce 12 pulses in 1/60th of a second, or 720 pulses per second through the x-ray tube
• Notice how voltage supplied to the x-ray tube does not drop to 0 like it does with the single-phase generators that are HWR and FWR, or as much from peak amplitude as the 3Φ 6P generator
• The efficiency of 3Φ 12P permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used when compared to single-phase generators and 3Φ 6P generators
Three-phase, twelve-pulse (3f 12P) (3Φ 12P): With further improvements in engineering, 3Φ 12P generators can produce ___ pulses in 1/60th of a second, or ____ pulses per second through the x-ray tube
12, 720
High-frequency (HF)
• HF generators produce a nearly constant potential voltage waveform that most closely resembles DC
• The efficiency of HF permits an increase in power output capabilities of the radiographic equipment. Higher mA and kVp can be used when compared to all the other radiology generators
• HF offers the best image quality with the least amount of patient dose because voltage barely fluctuates during an exposure, resulting in fewer lower energy x-rays that leave the tube
• HF is common with fixed radiography, mammography, and CT
Capacitor discharge generator
• Commonly found on mobile units, a capacitor can be charged by batteries prior to an x-ray exposure. During the exposure, the charge is released (discharged) from the capacitor to form the x-ray tube current needed to produce x-rays
• After an exposure, the capacitor can continue to discharge after the usable exposure creating leakage radiation
• Grid biased x-ray tubes, automatic lead beam stoppers, or both can be used to stop the leakage radiation
• From the start of the exposure to the end of the exposure is termed wavetail cutoff
Battery operated mobile units
• Mobile units can operate on battery supplied AC current
• This AC can be converted to 3Φ 12P or greater frequencies
• These generators do not exhibit leakage radiation and offer the benefits of three-phase or greater x-ray exposures
• These have become popular in mobile radiography
Falling load generator
• Common in interventional radiology when short exposure times are necessary
• During an exposure, the initial tube loading is higher and drops during the exposure
• Notice that 500 mAs was achieved starting with close to 2000 mA that drops during the exposure to help create the 500 mAs needed. Because of this, a 300 ms exposure time was used instead of a 500 ms exposure time
Voltage ripple
• Voltage ripple refers to the percentage drop of voltage from peak amplitude
• The most efficient method of producing x-rays involves the lowest ripple
• Less voltage ripple results in greater radiation quantity and quality
• Quantity is higher because for any projectile electron emitted by the x-ray tube filament, a greater number of x-rays are produced when electron energy is high than when it is low
• Quality is higher because fewer low-energy projectile electrons pass from cathode to anode to produce low energy x-rays. The average x-ray energy of the primary beam is higher as voltage ripple decreases
• As voltage ripple decreases, the exposure to the patient will be less as more photons will have higher energy with less likelihood of photoelectric absorption
• As voltage ripple decreases, lower kVp/mAs is needed for proper IR exposure because of the higher average energy of the primary beam
Phase, Pulse, & Frequency
• Phase refers to the number of distinct wave cycles. Examples- single-phase & three-phase
• Pulse refers to the quantity of voltage pulses per cycle
• Frequency refers to the number of pulses per cycle
Generators contribution to the exposure
• As generator phase increases, this increases the pulses, which increases the frequency of the voltage supplied to the x-ray tube
• These 3 increases will decrease voltage ripple, which will increase the average primary beam energy measured in keV that leaves the tube
• These 3 increases will also increase x-ray intensity, meaning the quantity of photons leaving the tube will increase
Power Ratings
• Generator power ratings are determined by the greatest load the generator is capable of sending to the x-ray tube. Power is expressed in kilowatts because x-ray generators operate in kilovoltage and milliamperage
-P=V x A for 3Φ generators
-P=V x A x .7 for 1Φ generators
-Because 1Φ generators have a lower average photon emission energy, the .7 constant is used
3Φ generators and 1Φ generators Formulas
-P=V x A for 3Φ generators
-P=V x A x .7 for 1Φ generators
-Because 1Φ generators have a lower average photon emission energy, the .7 constant is used
Power ratings example problem: What is the power rating for a 3Φ generator capable of delivering 140 kVp at 1,100 mA to the tube?
140,000 V x 1.1 A= 154,000 W or 154 kW
Power ratings example problem: What is the power rating for a 1Φ generator capable of delivering 140 kVp at 1,100 mA to the tube?
140,000 V x 1.1 A x .7= 107,800 W or 107.8 kW
Heat Units (HU)
• HU describes the thermal energy created inside the tube
• To calculate HU:
• Single-phase HU= kVp x mA x s
• Three-phase & high-frequency HU= kVp x mA x s x 1.4
• Notice how three-phase and HF generators create more heat per exposure due to the low voltage ripple.
Heat Units Formulas
-Single-phase HU= kVp x mA x s
-Three-phase & high-frequency HU= kVp x mA x s x 1.4
Heat Units example: A radiographic exam of the lateral thoracic spine with a single-phase imaging system requires 90 kVp and 60 mAs. How many heat units are generated by this exposure?
90 kVp x 60 mAs= 5, 400 HU
Heat Units example: A radiographic exam of the sinuses requires 4 separate images using a high-frequency generator operated at 80 kVp and 20 mAs. What is the HU generated per image? What is the total heat generated after the 4 exposures?
80 kVp x 20 mAs x 1.4= 2,240 HU per image
2,240 x 4= 8,960 HU after 4 exposures
Rating charts & anode cooling curves
• Radiographic rating charts allow the technologist to determine:
• If a technique is acceptable or unacceptable (safe or unsafe) for the x-ray tube
• Determine the maximum permissible exposure time
• Determine the maximum allowable kVp
• Determine the highest allowable mA
• Anode cooling curves or charts allows the technologist to determine if the HU accumulated at the anode is acceptable
Rating chart example: Indicate if this technique is acceptable or unacceptable.
Three phase, 60 Hz, .7 mm: 200 mA @ 1/5 Sec., 80 kVp
Use this: https://acrobat.adobe.com/link/reviewuri=urn%3Aaaid%3Ascds%3AUS%3A967cfbeb-5ed0-3b79-bdc8-7e6be2e24f5d
1/5 Sec. and 80 kVp intersect below the 200 mA line.
The technique is acceptable.
Rating Charts example: Determine the maximum permissible exposure time.
Single Phase: 180 Hz, 0.7 mm focus, 200 mA @ 120 kVp
120 kVp intersects with 1 second below the 200 mA line.
1 second is the maximum permissible exposure time.
Rating Charts example: Determine the maximum allowable kVp. Three phase: 60 Hz, 1.4 mm focus: 800 mA @ 1/20 Sec.
1/20 Sec. and 80 kVp intersect below the 800 mA line.
80 kVp is the maximum allowable kVp.
Rating Charts example: Determine the highest allowable mA. Single phase, 60 Hz, 1.4 mm focus: ____ mA @ 1/5 Sec., 100 kVp
1/5 Sec. and 100 kVp intersect below the 400 mA line.
400 mA is the highest allowable mA.
Anode Cooling curve example: How much time is required for the anode to cool from maximum heat capacity to zero?
250,000 HU is the maximum heat capacity for the anode. It takes about 23 minutes for the anode to cool from maximum heat capacity to 0
Anode Cooling curve example: An exposure is made resulting in 100,000 HU. How much time is required for the anode to cool to 0 HU?
23 minutes – 8 minutes = 15 minutes
Anode Cooling curve example: If 225,000 heat units (HU) are generated, how long will it take for the tube to cool to 100,000 heat units (HU)?
8 minutes – 1 minute = 7 minutes
Anode Cooling curve example: Two sequential exposures are to be made, both adding 50,000 HU to the anode. How much time is required between exposures to avoid over-heating?
No need to wait. 250,000 HU is the maximum heat capacity for the anode and these two exposures created 100,000 HU in total.
Anode Cooling curve example: If 225,000 HU are generated, how long must you wait before you can add an additional 200,000 HU and not exceed the 250,000 HU’s capacity?
14 minutes – 1 minute = 13 minutes
Anode Cooling Curve example: A radiographic technique is being considered for an angiogram study. The parameters that will be used are 400 mA @ 1/5 Sec., 80 kVp (Single-phase, 60 Hz, 1.4 mm focus).
The exposure rate will be 8 exposures a second for 5 seconds. How many HU will be created per exposure? How many HU will be created per second? How many total HU will be created for this study?
*80 kVp x 400 mA x 1/5 sec.= 6,400 HU per exposure
*6,400 HU x 8 = 51,200 HU per second
*51,200 x 5 = 256,000 HU for this study
Recommendations for extending tube-life
• Warm up the anode according to the manufacturer’s recommendations to prevent thermal shock (cracking of a cold anode)
• Avoid holding the rotor switch unnecessarily. Double press switches should be completely depressed in one motion.
• Riding the rotor decreases the filament life, deposits vaporized electrons on tube surfaces, decreases the tube vacuum. It also decreases the life of the rotor bearings.
• Lower mA stations should be used when possible (high mA increases filament thermionic emission)
• If two rotor speeds are available, use the lower speed to prevent rotor bearing wear
• Repeated exposures near tube load loading limits should not be made
• Rotating the tube from one position to another rapidly should be avoided to avoid damaging the rotor
• Never use a tube when rotor bearings can be heard because a wobbly anode can cause tube failure
