Lecture 7: X-Ray Imaging System PDF

RMI 213 Principles of medical imaging Lecture 7 The X-ray Imaging System Slide 1 fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Identify and explain the functions...

RMI 213 Principles of medical imaging Lecture 7 The X-ray Imaging System Slide 1 fchs.ac.ae Learning Outcomes At the conclusion of this lecture, associated tutorial and practical session (if relevant), you will be able to: 1. Identify and explain the functions of the components of the x-ray imaging system operating console 2. Explain the operation of the high-voltage generator 3. Describe the differences between single-phase, three- phase, and high-frequency power 4. Discuss the importance of voltage ripple to x-ray quantity and quality 5. Determine the power rating of an x-ray imaging system Slide 2 fchs.ac.ae Prescribed Text Bushong, S.C., Radiologic Science for Technologists, 10th edition, Mosby/Elsevier; St Louis, 2012, pages 84-103. Notes: 1. Each lecture in this course will relate very closely to a specific set of pages in the above text. It is strongly recommended that students read the pages indicated prior to coming to the lecture. 2. The students outcomes listed at the commencement of each lecture are essentially those found in the prescribed text for the relevant chapter. Slide 3 fchs.ac.ae  https://www.youtube.com/watch?v=uWfx1KZP Tp8&ab_channel=RadiologyTutorials Diagnostic X-ray Systems While there are many types of x-ray systems, they are usually characterized by the energy of x-rays produced; 1. The Peak voltage applied to the x-ray tube, and 2. The electron beam current inside the tube vacuum We talk of “25 kVp” and “125 kVP” x-ray systems, and We also refer to tube current, say 100 mA or 1000 mA We also quote exposure times, in milliseconds (s); e.g. 10 ms Your focus is Primarily the patient, correct positioning to get the most useful image for a meaningful diagnosis the first time, at the minimum ionising radiation dose Slide 4 fchs.ac.ae Operating Console Your starting point is the console to control voltage (kVp) and current (mAs) Radiation quantity: is the total number of x-ray photons in the diagnostic beam; this depends on kVP and mAs Usually expressed in mR or mR/mAs, where R = roentgen (for US systems) Also expressed in ionizing charge per unit mass of air, with 1 R = 2.58  10-4 C/kg (preferred as the SI unit, outside the US) Radiation quality: refers to the penetrating ability of the x-ray photons and is determined by the kVP... but this also affects the quantity! Can be measured by the HVL (see later) Slide 5 fchs.ac.ae Mains Power The mains power (in the UAE) is 220 VRMS An x-ray set requires a large DC voltage generated from the mains by rectification, amplification and smoothing (there is always a small amount of AC ripple) Bushong, Figure 5-4, page 87 The power system may be single-phase or 3-phase Slide 6 fchs.ac.ae  https://www.youtube.com/watch?v=xZgFK4G0 n_0&ab_channel=RADIATIONPHYSICSRADIATIO NBIOLOGY Transformers in x-ray machine 1. High Tension transformer 2. Filament transformer 3. Auto transformer The high tension transformer and Filament transformer in side the high tension tank The Auto transformer located in side the control panel 7 fchs.ac.ae Transformer An A.C. device used to change high voltage low current A.C. into low voltage high current A.C. and vice-versa without changing the frequency In brief, 1. Transfers electric power from one circuit to another 2. It does so without a change of frequency 3. It accomplishes this by electromagnetic induction 4. Where the two electric circuits are in mutual inductive influence of each other. Slide 8 fchs.ac.ae Principle of operation It is based on principle of MUTUAL INDUCTION. According to which an E.M.F. is induced in a coil when current in the neighboring coil changes. Slide 9 fchs.ac.ae Constructional detail : Shell type Windings are wrapped around the center leg of a laminated core. Slide 10 fchs.ac.ae Core type Windings are wrapped around two sides of a laminated square core. Slide 11 fchs.ac.ae Transformers in x-ray machine Bushong, Figure 5-32, page 102 Slide 12 fchs.ac.ae High Tension transformer Is Step up transformer. The primary wending number is lesser than secondary wending. Supply the anode circuit. This transformer is responsible for producing the high voltages necessary for x-ray production. Turns ratio of 1:500 or 1:1000 13 fchs.ac.ae fchs.ac.ae  Filament transformer Is Step down transformer. The secondary wending number is lesser than primary wending. Supply the filament circuit. Lower voltage and increase current Allows for thermionic emission to occur at the filament. 15 fchs.ac.ae Autotransformer is a single-coil transformer that provide: The means for kVp selection, it supplies power to other parts of the x-ray circuit. Bushong, Figure 5-5, page 88 The tube current,  3 – 5 A, depends on the temperature of the filament Slide 16 fchs.ac.ae Autotransformer – Example Calculation Q1. If the output across the relevant windings of the autotransformer is switchable between 0 and 120 VRMS, and the turns ratio is 1500, what is the range of high voltage transformer output? What are the “relevant windings”? The transformer input, vin Use formula vout/vin = NS/NP (= turns ratio) Rearrange: vout = vin × turns ratio Substitute: vout-min = 0 V × 1500, vout-max = 120V × 1500 Answer: vout has the range 0 V to 180,000 V (RMS) Slide 17 fchs.ac.ae  Autotransformer A simple autotransformer consists of a single coil of wire wrapped around an iron core. It has a fixed number of turns, two lines on the input side and two lines on the output side. 18 fchs.ac.ae  Autotransformer Function Used to give variable and stable voltage to primary wending of the (H.T.T) 19 fchs.ac.ae  Autotransformer Pre-reading voltmeter and line voltage compensation. Measures incoming voltage and allows adjustments on the autotransformer so that the incoming line voltage remains constant. Usually automatic on modern x-ray machines. kV selector provides pre-determined voltage to primary turnings of the transformer. fchs.ac.ae Exposure Times For an exposure the number of x-rays is dependent on the kVP and the mAs Usually the kVP is set to suit the patient section (and determines the image contrast – see later), and Usually the mAs is adjusted to get the correct overall image brightness Timing is normally done as a function of the mains period, T = 20 ms in the UAE 220 VRMS, or by other means Note: USA texts will have local values for mains power, VRMS = 110 V, frequency, f = 60 Hz and period, T = 16.7 ms Slide 21 fchs.ac.ae Automatic Exposure Control An alternative scheme is the automatic exposure control, AEC, that measures the ionisation between the patient and the image receptor AEC must be correctly calibrated before use The AEC system can use an ionisation detector or a photo diode array Bushong, Figure 5-10, page 92 Slide 22 fchs.ac.ae High Voltage Generator The 220 VRMS mains voltage has a peak voltage, VP , of about 220  2 , or VP = 310 V. It is this value that has to be amplified to the x-ray set kVP The autotransformer starts this process Note: In UAE, T = 1/50 s 1/50 s (Bushong diagram 1/50 s opposite is modified) Bushong, Figure 5-13, page 93 Slide 23 fchs.ac.ae High Voltage Generator – Example Calculation Q2. To power a 25 kVP x-ray tube in UAE, what must be the transformer windings ratio? (You may assume that, to a reasonable approximation, there is no voltage loss in the remaining parts of the circuit) We are given the desired kVP , so must use peak mains voltage, VP = 310 V Use formula vout/vin = NS/NP (= turns ratio) Substitute: turns ratio = vout/vin = 25000 V/310 V Answer: turns ratio = 80.6; say turns ratio = 81. Q3. To power a 125 kVP x-ray tube, what must be the transformer windings ratio? Using the same method, Answer: turns ratio = 403 Slide 24 fchs.ac.ae Rectification We convert the AC voltage signal to a uni-directional (all positive) voltage 1/50 s 1/50 s This is done using a full-wave rectifier. The rectifier is a ‘diode bridge’ 1/50 s 1/50 s Note (again): In UAE, T = 1/50 s (Bushong diagram opposite is modified) Bushong, Figure 5-20, page 97 Slide 25 fchs.ac.ae Slide 26 fchs.ac.ae Process of rectification Conversion of AC to DC Half-wave Full-wave Dedicated circuits Contain diodes Semiconductor Thermionic Power rating Supply current to circuits Slide 27 fchs.ac.ae  Only the positive half- Half-wave rectification cycle of the AC supply is allowed to pass Slide 28 fchs.ac.ae  Half-wave rectifier circuit Simple! Single diode ‘Load’ DC circuit…not really getting DC! Vac Load Slide 29 fchs.ac.ae Slide 30 fchs.ac.ae  Full-wave rectification Positive AND negative half-cycles converted to ALL positive Slide 31 fchs.ac.ae Full-wave rectifier circuit Bridge circuit Load Circuit requiring DC Vac Load Slide 32 fchs.ac.ae Slide 33 fchs.ac.ae Capacitive Smoothing When a time-varying voltage is applied across an R-C series circuit, then any time-variation across the capacitor is ‘smoothed out’ Across the capacitor, you get a much smoother voltage Across the resistor is the remainder of the voltage VR + VC = total rectified voltage VR + VC = VTotal , so clearly VR must vary between 0 and –ve values VC VTotal Bushong, Figure 5-31, page 101 Slide 34 fchs.ac.ae Capacitor Discharge Generator Portable x-ray systems often use the strategy of capacitive smoothing, alone Here a capacitor is charged up to the required kVP and then discharged during the exposure, so kVP varies Bushong, Figure 5-26, page 98 Slide 35 fchs.ac.ae Smoothed output Un-smoothed Large ripple voltage Not true DC Low-value smoothing capacitor RloadC small High-value smoothing capacitor RloadC big Slide 36 fchs.ac.ae Voltage Smoothing At this stage, the voltage is still time-varying The tube needs, effectively, a time-invariant (true DC) voltage This is done using capacitors, as shown below Bushong, Figure 5-25, page 98 Slide 37 fchs.ac.ae Single Phase Power Single-phase power is the norm as far as domestic supply is concerned This is a single voltage, in the form of a sinusoid, at the house, v = 310 sin(2ft) = 310 sin(2f/T) V, with f = 50 Hz and T = 1/f = 20 ms To decrease the time-varying part, we can use 3-phase power: Here, in every 20 ms, there are 3 peaks in the voltage After full wave rectification there are 6 peaks every 20 ms in the voltage waveform Slide 38 fchs.ac.ae Single Phase Power Single phase: 1 voltage peak per 20 ms 3-phase: 3 voltage peaks per 20 ms After full wave rectification, there are 6 voltage peaks per 20 ms (and significant smoothing prior to the capacitor smoothing) Bushong, Figure 5-23, page 97 Slide 39 fchs.ac.ae High Frequency Generators In x-ray power supplies, the mains frequency itself is dramatically increased from 50 Hz to 400 Hz or more This frequency change does not materially affect the basic circuits, but It does materially improve the final smoothed voltage Bushong, Figure 5-29, page 100 applied to the tube Slide 40 fchs.ac.ae Final Tube Voltage Supply The final voltage applied to the tube has a time- dependence over a single exposure like that shown in the bottom diagram, where the ripple is less than 1% The beam current in the tube has the same time- dependence Bushong, Figure 5-29, page 100 Slide 41 fchs.ac.ae  Voltage Waveforms Single phase Rectification Type Ripple Half wave 100% Full wave 100% Rectified three phase Rectification Type Ripple Three phase, 6 pulse 13% Three phase, 12 pulse 4% Three phase, high frequency