Medical Equipment: High-Voltage Generator PDF

‫جامعة العين‬ ‫كلية التقنيات الصحية والطبية‬ ‫قسم تقنيات االشعة والسونار‬ conventional Radiological techniques Equipment High-voltage Generator Dr. Hussein A.Dakhild Ph.D. Medical Imaging Technology 1  Hig...

‫جامعة العين‬ ‫كلية التقنيات الصحية والطبية‬ ‫قسم تقنيات االشعة والسونار‬ conventional Radiological techniques Equipment High-voltage Generator Dr. Hussein A.Dakhild Ph.D. Medical Imaging Technology 1  High-voltage Generator The high-voltage generator provides power to the x-ray tube in three possible ways: 1)Single-phase power. 2)Three-phase power. 3)High-frequency power. 1) Single-Phase Power: Single-phase power results in a pulsating x-ray beam. This is caused by the alternate swing in voltage from zero to maximum potential 120 times each second under full-wave rectification. The x-rays produced when the single-phase voltage waveform has a value near zero are of little diagnostic value because of their low energy ;such x-rays have low penetrability. 2  One method of overcoming this deficiency is to generate three simultaneous voltage waveforms that are out of step with one another. Such a manipulation results in three-phase electric power. 2) Three -Phase Power: The engineering required to produce three-phase power involves the manner in which the high-voltage step-up transformer is wired into the circuit. 3 Shown are the voltage waveforms for unrectified single-phase power, unrectified three-phase power, and rectified three-phase power. With three-phase power, multiple voltage waveforms are superimposed on one another, resulting in a waveform that maintains a nearly constant high voltage. There are six pulses per 1/60 s compared with the two pulses characteristic of single- phase power. 4  Voltage waveforms in a three-phase generator 5 3) High-Frequency Generator: High-frequency circuits are finding increasing application in generating high voltage for many x-ray imaging systems. Full-wave rectified power at 60 Hz is converted to a higher frequency, from 500 to 25,000 Hz, and then is transferred to high voltage. 6  One advantage of the high-frequency generator is its size. They are very much smaller than 60 Hz high voltage generators. High-frequency generators produce a nearly constant potential voltage waveform, improving image quality at lower patient radiation dose. This technology was first used with Portable x-ray imaging systems. Now, all Mammography and Computed tomography systems use high-frequency circuits. High-frequency voltage generation uses inverter circuits. An inverter circuit creates a high-frequency AC waveform, which supplies the high- voltage transformer to create a high-voltage, high-frequency waveform. 7  Rectification and smoothing produces high-voltage DC power that charges the high- voltage capacitors placed across the anode and cathode in the x-ray tube circuit. 8 In a high-frequency inverter generator, a single-or three phase AC in put voltage is rectified and smoothed to create a DC waveform. An inverter circuit produces a high-frequency AC waveform as input to the high-voltage transformer. Rectification and capacitance smoothing provide the resultant high-voltage out put waveform, with properties similar to those of a three-phase system. 9  Voltage Ripple: An other way to characterize these voltage waveforms is by voltage ripple. Single-phase power has 100% voltage ripple: The voltage varies from zero to its maximum value. Three-phase, six-pulse power produces voltage with only approximately 14% ripple; consequently, the voltage supplied to the x-ray tube never falls to below 86% of the maximum value. A further improvement in three-phase power results in 12 pulses per cycle rather than 6. Three-phase, 12-pulse power results in only 4% ripple; therefore, the voltage supplied to the x-ray tube does not fall to below 96% of the maximum value. High-frequency generators have approximately 1% ripple and therefore even greater x- ray quantity and quality. 10 11  The most efficient method of x-ray production also involves the waveform with the lowest voltage ripple. The principal advantage with less ripple is the greater radiation quantity and quality that result from the more constant voltage supplied to the x-ray tube. 12  Operating console The part of the x-ray imaging system most familiar to the radiologic technologist is the operating console. The operating console allows the radiologic technologist to control the x-ray tube current and voltage so that the useful x-ray beam is of proper quantity and quality. Radiation quantity refers to the number of x-rays or the intensity of the x-ray beam. Radiation quantity is usually expressed in milliroentgens (mR) or milliroentgens/milliampere-second (mR/mAs). Radiation quality refers to the penetrability of the x-ray beam and is expressed in kilovolt peak (kVp) or, more precisely, half-value layer (HVL). * kVp: the power and strength of the x-ray beam (quality of the x-rays). * mAs: the number of x-ray photons produced by the x-ray tube at the setting selected (quantity of x-rays) 13  the 5 major controls on the operator's console: On/off control, KVp selection, mA selection, time (mAs) selection, Most operating consoles are based on computer technology. Controls and meters are digital, and techniques are selected with a touch screen. and automatic-exposure controls Numeric technique selection is sometimes replaced by icons indicating body part, size, and shape. Many of the features are automatic, but the radiologic technologist must know their purpose and proper use. 14  Milliamperage and Exposure Time mAs and Quantity of Radiation As the mAs is increased, the quantity of radiation reaching the IR is increased. As the mAs is decreased, the amount of radiation reaching the IR is decreased. Adjusting Milliamperage or Exposure Time 200 mA * 0.1 s= 20 mAs To increase the mAs to 40, you could use ( …….. * ………)=40 mAs ( …….. * ………)=40 mAs As demonstrated in the Mathematical Application, mAs can be doubled by doubling the milliamperage or doubling the exposure time. A change in either milliamperage or exposure time proportionally changes the mAs. To maintain the same mAs, the radiographer must increase the milliamperage and proportionally decrease the exposure time. 15 Milliamperage and Exposure Time Milliamperage and exposure time have an inverse proportional relationship when maintaining the same mAs. Adjusting Milliamperage and Exposure Time to Maintain mAs 200 mA * 100 ms(0.1 s)= 20 mAs To maintain the mAs, use: [(……. )* ……… (……) =20 mAs [(……. )* ……… (……) =20 mAs It is important for the radiographer to determine the amount of mAs needed to produce a diagnostic image. This is not an easy task because there are so many variables that can affect the amount of mAs required. For example, single-phase generators produce less radiation for the same mAs compared with a high-frequency generator 16  A, Radiograph obtained with low mAs showing increased quantum noise. B, Radiograph obtained with high mAs showing decreased quantum noise 17  Kilovoltage Peak The kVp affects the exposure to the IR because it alters the amount and penetrating ability of the x-ray beam. The area of interest must be adequately penetrated before the mAs can be adjusted to produce a quality radiographic image. When adequate penetration is achieved, increasing the kVp further results in more radiation reaching the IR. In addition to affecting the amount of radiation exposure to the IR, the kVp also affects image contrast. kVp and the Radiographic Image Increasing or decreasing the kVp changes the amount of radiation exposure to the IR and the contrast produced within the image. Because kVp affects the amount of radiation reaching the IR, its effect on the digital image is similar to the effect of mAs. Assuming that the anatomic part is adequately penetrated, too much radiation reaching the IR (within reason) produces a digital image with the appropriate level of brightness as a result of computer adjustment during image processing; however, the patient has been overexposed. Similarly, too little radiation reaching the IR (within reason) produces a digital image with the appropriate level of brightness, but the increased quantum noise decreases image quality. Excessive or insufficient radiation exposure to the digital IR, as a result of the mAs or kVp, should be reflected in the exposure indicator value. 18  Excessive Radiation Exposure and Digital Imaging  Although the computer can adjust image brightness for technique exposure errors, routinely using more radiation than required for the procedure in digital radiography unnecessarily increases patient exposure. Even though the digital system can adjust for overexposures, it is an unethical practice to overexpose a patient The kVp has a greater effect on the image when using film-screen knowingly. IRs. Increasing the kVp increases IR exposure and the density produced on a film image, and decreasing the kVp decreases IR exposure and the density produced on a film image 19  Most x-ray imaging systems are designed to operate on 220 V power, although some can operate on 110 V or 440 V. Unfortunately, electric power companies are not capable of providing 220 V accurately and continuously.  The line compensator measures the voltage provided to the x-ray imaging system and adjusts that voltage to precisely 220 V. Older units required technologists to adjust the supply voltage while observing a line voltage meter. Today's x-ray imaging systems have automatic line compensation and hence have no meter. 20  AUTOTRANSFORMER The power supplied to the x-ray imaging system is delivered first to the autotransformer. The voltage supplied from the autotransformer to the high-voltage transformer is variable but controlled. It is much safer and easier to control a low voltage and then increase it than to increase a low voltage to the kilovolt level and then control its magnitude. The autotransformer works on the principle of electromagnetic induction but is very different from the conventional transformer. It has only one winding and one core. This single winding has a number of connections along its length. Two of the connections, A and A′ as shown in the figure, conduct the input power to the autotransformer and are called primary connections. 21  autotransformer law the autotransformer can be Vs/Vp = Ns/Np designed to step up the voltage to approximately twice the input voltage value. Where: Vs: the secondary voltage Vp: the primary voltage Ns: the number of windings enclosed by secondary connections Np: the number of windings enclosed by primary Because the autotransformer operates as an induction device, the connections voltage it receives (the primary voltage) and the voltage it provides Homework: If the autotransformer in the adjacent (the secondary voltage) are related Figure is supplied with 220 V to the primary directly to the number of turns of connections AA′, which enclose 500 windings, what is the transformer enclosed by the the secondary voltage across BB′ (500 windings), CB′ respective connections. The (700 windings), and DE (200 windings)??? autotransformer law is the same as the transformer law. 22 23