Diagnostic X-ray Circuits PDF

Medical Imaging Instrumentation Subject Code: HTI28101 Diagnostic X-ray Circuits Dr. Jung Sun YOO  Learning Objectives By the end of the series of two lectures, you should be able to: 1. Describe an overview of X-ray genera...

Medical Imaging Instrumentation Subject Code: HTI28101 Diagnostic X-ray Circuits Dr. Jung Sun YOO  Learning Objectives By the end of the series of two lectures, you should be able to: 1. Describe an overview of X-ray generators 2. Discuss the requirement of electrical power for X-ray tube 3. Identify the components of the operating console in the X-ray imaging system 4. Explain the principle of autotransformer and filament circuit 5. Explain the principle and the types of exposure timer 6. Explain the principle and the types of high-voltage generator 7. Relate the differences among single-phase, three-phase, and high-frequency power 8. Recognize the relationship between voltage ripples and X-ray quantity and quality X-ray Imaging System A diagnostic X-ray irradiation equipment has four basic components: ❖ X-ray tube X-ray Production ❖ Operating console (control console) X-ray Circuit (X-ray Generator) ❖ High-voltage generator ❖ Cooling and shielding system X-ray Tube Housing X-ray Imaging System ❖ X-ray tube: examination room ❖ Operating console: adjoining room with a protective barrier ❖ High-voltage generator: equipment cabinet positioned against a wall X-ray Imaging System X-ray generator (X-ray circuits) X-ray Tube X-ray Imaging System :current X from power suitable for X-ray source generation 1. provide high enough current to filament turn filament circuit. 2. control exposure time thru timer circuit 3. high voltage : high KE of e-high Ex-ray 220V > - X enough X-ray generator Power source X-ray tube (X-ray circuit) Aims 3-6A 1. Filament circuit – mA: Supplies power to heat the cathode filament and generate electrons for > - tube current 2. Timer circuit – s: Controls the duration of x-ray production 3. High voltage circuit – kVp: Supplies high voltage to accelerate the electrons from cathode to anode 30kUp-150kUp Main X-ray Circuit (X-ray Generator) Two divisions of the main x-ray circuit: low voltage ❖ Operating console (control console) section – Primary section - Incoming current/voltage 220V ① - Autotransformer change V to - Exposure timer/switch 10 safety in - control better low-voltage to in ② - Primary winding of the step-up transformer (high-voltage transformer) step-down ❖ High-voltage section – Secondary section - Secondary winding of the step-up transformer (high-voltage transformer) - Full-wave rectification circuits always A C. from cathode. > D C -.. anode e move to anode generate e effectively : X - Wiring leading to & from the X-ray tube - cathode effective generate X-ray : - X -heat in X-ray generation easily burn thin filament Electrical Power Source – Incoming Line Power ❖ Electric power provides in Hong Kong: - 220 V X enough - 50 Hz (alternating current, AC) ❖ Electric power may be 110 or 120 V and 60 Hz in the U.S. and Canada. Requirement of Electrical Power for X-ray Tube ❖ In order to produce X-rays, an X-ray tube requires a high voltage. - 30-150 kV is required for clinical application 220V - e.g. Chest X-ray? 150 kV - e.g. mammography? 30 kV ❖ The voltage available from the main supply is far too low! ❖ We need X-ray generator with AC transformer to increase the voltage, a step-up transformer. Operator Console ❖ Selection of control parameters - Tube voltage (kV) - Tube current (mA) - Exposure time (s) or Tube current (mA) x Exposure time (s) = mAs hand small focal spot 4 spatial resolution ❖ Focal spot selections : : ❖ Automatic exposure control HEL ❖ Pre-programmed techniques ❖ Fluoroscopy system real time imaging - Automatic control system for kV & mA : consistent signal > - brightness Overall features of operating console Overall features of operating console 10 section ② 50Hz 220V ① ↳ measure incoming power : X COV always fluctuation > - unstable X-ray spectrum ↳ autotransformer change current to exactly zzor => stable voltage Line Compensator ❖ X-ray imaging system:  220V ❖ Voltage supply to X-ray unit may vary within 5%, i.e., 209V ~231V ❖ So, what’s the problem? - Large variation in the X-ray beam makes consistent production of high-quality image difficult. ❖ Line Compensator - Measures the voltage provided and adjusts that voltage to precisely 220V - The control is usually multi-station and wired to the autotransformer. Quiz – Line Compensation Line compensation __________. A. Adjusts the line frequency to 60Hz B. Compensates for rectification C. Is necessary for proper exposure timing D. Is necessary to convert AC to DC O E. Is required to stabilize voltage Transformer ❖ X-ray generator must provide high voltage to the X-ray tube - 30-150 kV change > - Changing B-field flux ❖ Transformer to convert a low B- (E) / > - -V = > - > - L voltage (220V) into a high voltage > - - V (30-150kV) by electromagnetic induction ❖ Input AC power on the primary v = windings induces a voltage on the VpIp = VsIs I = t secondary windings that is E P = VI proportional to the input voltage = and ratio of the number of turns Electromagnetic Induction Transformer Relationships Law of Transformers: Power output = Power input Vp/Vs = Np/Ns = /Ip Is Primary coil Secondary coil VpIp = VsIs Np, Vp, Ip Ns, Vs, Is Amplitude High Step-up Transformers: Vs > Vp voltage Ns > Np Is < Ip generator Isolation Transformers: Vs = Vp Ns = Np Is = Ip 220V > 12V - Step-down Transformers: Vs < Vp Filament Ns < Np Is > Ip circuit Autotransformer if at step-up > - change voltage Ns : Np = 500 , need 100kVp Step-up fixed Np : Ns 500 = Up = zo0V : autotransformer Fe 220V > - 2001 ↳ output from 1000 - 400V Step-down : filament circuit Autotransformer: a single winding, to supply a precise voltage to the filament circuit & the high-voltage circuit It is much safer and easier to control a low voltage and then increase it than increase a low voltage to the kilovolt level and then control its magnitude! Quiz – Autotransformer The principal purpose of the autotransformer is to ___________. O A. Adjust voltage B. Increase voltage C. Rectify voltage D. Reduce voltage E. Stabilize voltage Autotransformer VS = 60 Law of Transformers = Law of Autotransformers Vp/Vs = Np/Ns Is/Ip UPIp = VSIs Quiz - Autotransformer Question: If the autotransformer in the Figure 5-7 is supplied with 220V to the primary connections AA’, which enclose 500 windings, 2201 what is the secondary voltage across BB’ (500 windings), CB’ (700 windings), and DE (200 500 windings)? BB' = 220V isolation CB1700 x 220V = 308V step-up DE 200 XLIOV = 88V step-down Adjustment of Kilovolt Peak (kVp) larger interval smaller interval ↑ ↑ ❖ Major kVp, minor kVp - To provide the required kilovolt peak precisely ❖ kVp meter measure low-voltage to show kVp (high-voltage) - Across the output terminals of the autotransformer X500 - - Reads voltage, not kVp - Registers kilovolts because of the known multiplication factor of the turns ratio ❖ Prereading kVp meter known kVp 64 exposure - The kVp meter registers even though no exposure is being made and the circuit has no current. Voltage? 209- 220V 100- → input to the high- - To allow the voltage to be monitored 231V 400V voltage step-up before an exposure transformer Quiz – Autotransformer Which of the following is directly connected to the autotransformer? A. Filament O B. kVp meter C. mA meter > - filament current - tube current X exactly proportional D. Rectifier low v : space-charge effect E. X-ray tube Adjustment of Milliamperage (mA) ❖ X-ray tube current - Measured in milliamperes (mA) - The number of electrons emitted by the filament is determined by the temperature of the filament. Heat ❖ Filament current - Measured in amperes (A), normally 3-6 A - As filament current increases, the filament temperature increases, and more electrons are released by thermionic emission. ❖ Filament circuit: control of X-ray tube current (mA)  Filament Current Thermionic emission: when a metal is heated, its atoms absorb thermal energy and some electrons escape the metal surface. X-ray tube current arg p Face Filament Circuit ↑ grounding ↳ safety high voltage part need measure filament current to estimate tube current Filament circuit 220V ILV Filament transformer Vs b Is4 3-6A Filament Circuit Filament X-ray circuit consists of two parts: ❖ mA Selector - It is a Rheostat (variable resistor). It can adjust resistance so that select mA (100, 200, or 300mA, and higher). - It is represented by the mA stations on the control panel. ❖ Filament transformer - Filament heating isolation step-down transformer - Filament current must be in A, not mA! 3 6 A Ist - 12V VsA - The voltage supplied to the filament via 150V 5- 1A autotransformer should be relatively low (12 V) 0 to provide enough current to heat the. 220V filament. - Primary windings: thin copper, 0.5-1A, ~150V - Secondary windings: thick, 5-8A, ~12V mA Meter ❖ mA Meter - Direct monitoring of X-ray tube current (mA) - Connected at the center of the grounding secondary winding of the high-voltage step-up transformer (zero volts due to alternating secondary voltage) - No part of the mA meter is in contact with the high voltage (Safe!) Timing Circuit ❖ X-ray exposure is related with X-ray tube current and exposure time. ❖ The timer circuit is separate from other main circuits ❖ On the primary side of high voltage transformer → To “make” and “break” the high voltage across the X-ray tube, where the voltage is lower. → Safer and easier! Timing Circuit Exposure Timers – 1. Mechanical Timers ❖ Very simple device that has a clock mechanism. Used for old machine and dental units. ❖ Operator turns the dial to the desired time. As it unwinds, the exposure is made. ❖ Can be used for exposure time longer than 250 ms up to 1hour Exposure Timers – 2. Synchronous Timers ❖ Electrical current in US = 60Hz, Europ = 50 Hz C ❖ A synchronous motor that turns a shaft (at 60 rps). 60 rotations Is S S S ❖ Times are a subdivision of 60 rps, i.e., 1/60, 1/30, 1/20. (Obsolete) ❖ Minimum exposure time is 1/60 s (17 ms). > - X good enough for real time exposure/OT/pediatrics ❖ Not for serial exposures as it must be reset after each exposure. Not good. Why? Exposure Timers – 3. Electronic Timers ❖ Most sophisticated and most accurate! ❖ It is based on the time required to charge a capacitor through a variable resistor. ↑ time resolution ❖ Accurate down to 1ms, rapid serial exposures ❖ Suitable for interventional radiology procedures (angiography) Good for what V = IR examination? R4 It ++ ee- Exposure Timers – 4. mAs Timers ❖ They measure product of tube current (mA) and exposure time. ❖ They are used with falling load generators; the exposure begins at maximum mA, and the mA drops as the anode heats. The result is minimum exposure time. ❖ To provide the highest safe tube current for the shortest exposure for any mAs selected ❖ On the secondary side of high voltage transformer (the only timer located in the secondary circuit) Falling-Load Generator ❖ To ensure the shortest possible exposure time ❖ The mA starts at the highest possible setting and “falls” throughout the exposure based on maximum heat loading capacity of the tube. - The rate of drop follows the cooling characteristics of the X-ray tube anode. - Tube life can be shorten due to repetitive use of high mA. - However, exposure time will be reduced to minimize movement error. - Interventional radiology ↓ : cooling Exposure Timers – 4. mAs Timers The mAs timer must monitor the actual tube current!  phototimer ionization chamber Exposure Timers – 5. Automatic Exposure Control (AEC) 3 AEC cell locations on the vertical bucky of a DR unit Once a predetermined amount of radiation is transmitted through a patient, the X-ray exposure is terminated. Control of exposure time, thus control of total radiation exposure to the image receptor (IR) Radiographer still selects kVp, mA, grid, but not exposure time Phototimer Exit-type device The detectors are positioned behind the image receptor (IR). Radiation must exit the IR before it is measured by the detectors. Design of Phototimer Type AEC Fluorescent screen (converting X-rays into blue visible light) + photoelectric effect photomultiplier tube (or photodiode, converting visible light into electricity) + timer circuit (capacitor & exposure terminating switch) Ionization Chamber Entrance-type device The detectors are positioned in front of the image receptor (IR). Radiation can interact with the detectors just before interacting with the IR. Radiolucent – no interference with the radiographic image Design of Ionization Chamber Type AEC Hollow cell (air will be ionized by radiation, creating electricity) + timer circuit (capacitor & exposure terminating switch) Phototimer vs Ionization Chamber Common: converting X-rays into electricity and automatic stopping the exposure when the predetermined radiation amount is acquired i dosimeter Ionization Chamber type AEC systems are less delicated and less accurate, but they are less prone to failure compared with Phototimer type AEC systems. IC types are more common today. Quiz – Timing Circuits Which of the following is WRONG regarding timing circuits of X-ray circuits? A. Mechanical timer is very simple device and used in old X-ray machines and ~ dental units. O B. Synchronous timer is useful for serial exposure because its temporal resolution is very good. Y60s = 17mS C. Electronic timer is the most sophisticated and the most accurate timer with 1ms minimum exposure. ~ D. mAs timer is used with falling load generator to minimize exposure time. ~ E. Automatic exposure control has two different types, i.e., phototimer and ionization chamber. ~ Autotransformer does line compensation & kVp selection. High Voltage Generator Responsible for increasing the output voltage from the autotransformer to the kVp necessary for X-ray production! ❖ High-voltage transformer Trectification ❖ Rectifiers * Immersed in oil to prevent sparking Quiz – High Voltage Generator Oil is used in the high-voltage section of an X-ray imaging system for which of the following functions? O A. Electrical insulation B. Reduction of rotor friction C. Reduction of voltage ripple D. Thermal conduction E. Voltage rectification High Voltage Transformer ❖ Step-up transformer ❖ Secondary voltage is much higher than the primary voltage. ❖ The number of secondary windings is greater than the number of primary windings. ❖ Current is reduced proportionately. 200V 100kV Ep = UpIp = Es = UsIs The usual turns ratio (Ns:Np) – 500:1 ~ 1000:1 v= - N Is ❖ Primary and secondary waveforms Np < Np < V - Both sides are sinusoidal (AC) > Is [p - Amplitude is different! (The primary voltage is measured in volts (V) and + the secondary voltage is measured in 1 + It kilovolts (kV).) - The primary current is measured in Primary current Secondary current amperes (A), and the secondary In amperes (A) In milliamperes (mA) current is measured in milliamperes (mA). Quiz - High Voltage Transformer Question: The turns ratio of a high-voltage transformer is 700:1, and the supply voltage is peaked at 120 V. What is the secondary voltage supplied to the X-ray tube? : 700 Vs = 84kVp  V Voltage Rectification ~ post ❖ 60 Hz: The current changes direction 120 times each second. + AC  DC - Electrons from Cathode  Anode To avoid back-propagation Voltage applied to tube Voltage Rectification ❖ Rectification - Process of converting alternating current (AC) to direct current (DC) - X-ray tube: Electron flow should be in one direction from the cathode to the anode! The cathode cannot withstand the tremendous heat if electrons flow in the reverse direction, from anode to cathode. ❖ Rectifier - Electronic device to allow current flow in only one direction - Located in the high-voltage section 1111 stop period > X - effective X-ray production - Half-wave rectifier - Full-wave rectifier Rectifier – Vacuum Tube Diode used in old days Valve tube X-ray > - tube Electronic device containing 2 electrodes Vacuum Tube Rectifier Anode -cathode t S electron flow t Anode e & - cathode E- e-attracted by the cathode Rectifier – P-N Junction Diode t - ph Solid-state rectifier Semiconductor diode: p-n junction diode – only one direction holes e e fill the holes + - - - e > = stay in N junction t e - Half-wave Rectification + t P H - First Half Cycle: Closed circuit Voltage applied to tube N P Tube current (mA) allowed - - - + - - P i - Second Half Cycle:  Open circuit No voltage applied to tube np- No tube current (mA) 0, 1, or 2 diodes + 60 X-ray pulses/second + Half-wave rectification Disadvantage Wasted Used Half-wave rectifier wastes half the supply of power. Voltage applied to x-ray tube It requires twice the exposure time. mA waveform X-Rays Produced Full-wave rectification Four diodes, 120 X-ray pulses/second Exposure time is half of the half-wave rectifier circuit First Half Cycle Second Half Cycle + r - n - Pt n p+ - X p - X n - Pt n - e- 1 L e- Pt n - X p- n+ p- X n - Pt - e- n+ ↳ + Voltage applied to tube (also mA waveform)  : only high enough krp leads to X-ray product X-ray Full-wave rectification - The output voltage across the X-ray tube is always positive. No gaps! M St The cathode of the X-ray tube is always negative and the anode is positive. [ ↑ [ j pt n+ H- p- & ↑ ↑ X pt ht n - p- - > 7 V > > & V & p- n - ht pt [ L [ N p- n - N+ L p+ Y S V V [ J Quiz – Rectification 1- Full-wave rectification _________. & 0 % min & max > - amplitude A. Has less ripple compared with half-wave rectification X-ray tube B. Is one example of self-rectification C. Produces higher kVp than half-wave rectification O D. Requires at least four rectifiers E. Requires at least 12 rectifiers  Pulses: number of peaks per 1/60 second X-ray Generator Circuit Design Three-phase may be rectified to provide with 6 pulses using 6 rectifiers, 6 pulses with 12 rectifiers or 12 pulses with 12 rectifiers ❖ Single-phase, single-pulse (half-wave rectified) - Dental and some mobile systems, Low power ❖ Single-phase, 2-pulse (full-wave rectified) high ripple 100 % : X continuous high V - Low and medium power ❖ Three-phase, 6-pulse - Medium and high power ❖ Three-phase, 12-pulse - Two shifted three-phase system is used, High power up to 150kW ❖ High-frequency - To convert 5-60Hz AC wave into high frequency DC wave (kHz range) - Good reproducibility and consistency of tube voltage Single-Phase Generator ❖ Single-Phase Power: a single wave form, a pulsating X-ray beam, dental and mobile systems ❖ Single phase input power results in pulsed X-ray radiation (pulsating X-ray) single phase , I pulse generator ❖ Limitation? Voltage applied to tube - Intensity only significant when voltage is near peak - The potential of the main current drops down to zero with every change of the current flow. Low energy, low penetrability penetration => X , only radiation close → Little diagnostic value Applied to X-ray Tube X-ray Radiation Waveform Jeffective X-ray generation Three-Phase Generator ❖ Three waves of power flowing at evenly spaced intervals from each other - One wave is starting before the previous wave is depleted. Phases 120° apart - The overall waveform never reaches zero. ↓ ripple ❖ The voltage applied to the X-ray tube is nearly constant! 1/60 s - Six pulses per 1/60 second compared with two pulses per 1/60 second with single-phase power. ~ half Mummu full Single Phase Power Three Phase Power Three-Phase Generator ❖ Rectifier circuit Input 3 Phase Voltage - To invert negative voltage - Sending of highest 3 phases to X-ray tube ❖ Three-phase power provides much higher tube ratings than single-phase power ❖ Three-phase power is more efficient than Rectified single-phase power - Shorter exposure is possible - Lower patient exposures ❖ Limitation: the speed of starting an exposure (initiation time) & ending an exposure (extinction time) To X-Ray Tube Quiz – X-ray Generator Which of the following is an advantage of three-phase power over single- phase power? A. Improved spatial resolution B. Increased kVp C. Increased mAs O D. Increased X-ray intensity per mAs ↑ # X-ray 4 photon av. Ex-ray E. Lower capital costmore expensive High-Frequency Generator ❖ Full wave-rectified power at 50Hz (220V, single phase current, HK) is converted to a higher frequency, from 500 to 25,000Hz, and then is transferred to high voltage. - Inverter technology > - expensive air con. - All mammography systems (USA) Smoothed DC - All computed tomography systems 220V, 50Hz (HK) 4V ❖ Full-wave rectification or high-frequency voltage generation is used in almost all stationary X-ray imaging systems. High voltage AC High-Frequency Generator ❖ Inverter circuits - High-speed switches, or choppers, that convert DC into a series of square pulses ❖ Advantages - A nearly constant potential voltage waveform - Much smaller, less costly, and more efficient than 5-60Hz high-voltage generators - Fast exposure switching, in the order of 1ms X-ray Beam Output with Different Generator Design ↑ av. Ex-ray # X-ray 4 Voltage Ripple the difference between the minimum and maximum x-ray tube voltages Ripple expressed as a % of the maximum voltage 80 kVp Ripple = 80 - 72 = 8 kVp OR 72 kVp 8 / 80 =.1 = 10%  Tube Voltage Waveform and Voltage Ripple kV ripple (%) Single phase single pulse 100% Single phase 2-pulse 14% Three phase 6-pulse 4% Three phase 12-pulse 1% High frequency Line voltage 0.01 s 0.02 s Voltage Ripple ↑ Par. ❖ Less voltage ripple results in greater radiation quantity and quality. - Ideal voltage would be constant. - Less voltage ripple means more constant voltage supplied to the X-ray tube. - Less voltage ripple results in greater radiation quantity. (high efficiency of X-ray production) - Less voltage ripple results in greater radiation quality. (fewer low energy projectile electrons pass form cathode to anode to produce low energy X- rays) - High voltage ripple gives lower quality and tube output. Type of Secondary Voltage Waveform Supplied to X-ray Tube X-ray X-ray  consistent Secondary voltage waveforms (left) and X-ray radiation intensity waveforms (right) Quiz – X-ray Generator Which of the following is higher for a single-phase high-voltage generator than for a three-phase high-voltage generator? A. kVp B. Purchase price C. Rotor speed O D. Voltage ripple E. X-ray quality  Quiz – X-ray Generator Which of the following principles of voltage rectification produces the maximum efficiency of X-ray production? A. Four-diode rectification B. Half-wave rectification same O C. High frequency generator D. Self-rectification E. Two-diode rectification References ▪ Bushong S.C. (2016). Radiologic Science for Technologists: Physics, Biology, and Protection. 11th edition. Mosby/Elsevier. Part II – Chapter 5 ▪ Donald T.G. (2011). Principles and Applications of Radiological Physics. 6th edition. Mosby/Elsevier. Part 2 – Chapter 11, Chapter 14, Part 6 - Chapter 28, 29 Thank you very much! 80