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RT A 2007 WK 4 X-RAY TUBE & EXPOSURES INVERSE SQ LAW CIRCUITRY Objectives X-ray tube review Inverse Square Law Properties of Xrays X-ray Circuit “Prime Factors” X-Ray Properties Are highly penetrating, invisible rays which are a form of electromagnetic radiation. Are electrically neutral and therefore not affected by either electric or magnetic fields X-Ray Properties Can be produced over a wide variety of energies and wavelengths (polyenergetic & heterogeneous). Release very small amounts of heat upon passing through matter. X-Ray Properties Travel in straight lines. Travel at the speed of light, 3 X 108 meters per second in a vacuum. Can ionize matter. X-Ray Properties Cause fluorescence of certain crystals. Cannot be focused by a lens. Affects photographic film. X-Ray Properties Produce chemical and biological changes in matter through ionization and excitation. Produce secondary and scatter radiation. PRIME FACTORS How does the “technique” What is it? How does it affect the “image” PRIME FACTORS • KVP • MAS • DISTANCE • The amount of overall blackening on a radiograph, or of a certain part of the image, is referred to as density. • Density results from two things: • the amount of radiation that reaches a particular area of the film, • and the amount of black metallic silver deposited when the film is developed. Producing optimal radiographs How much of the radiation received by the patient Actually reaches the IR ? About 1% Creating the IMAGE • When x-rays pass through a patient's body, three things can happen: • (1) the x-ray photon is transmitted, passing through the body, interacting with the film, and producing a dark area on the film; • (2) the x-ray photon is absorbed in an area of greater tissue density, producing lighter areas on the film; and • (3) the x-ray photon is scattered and reaches the film causing an overall gray fog. Radiographic Prime factors • The factors principally responsible for xray quality and quantity. • These are mAs, • kVp, • distance (SID). PRIME FACTORS • SID - ADJUSTED IN THE ROOM • KVP - “intensity of beam” CONTAST RANGE FOR PART • MAS – “density of image” CHANGES WITH PT SIZE mAs & kVp Introduction to Technical Factors to Create an image X-ray quantity (mAs) • is a measure of the number of xray photons in the beam. • Also called x-ray output, intensity or exposure. X-ray quality (kVp) • is a measure of the penetrating ability of the x-ray beam • The quantity of electron flow, or current, in the X-ray tube is described in units of milliamperes (mA). • The maximum kinetic energy of the accelerated electrons is defined in terms of kilovolts peak potential (kVp). IMAGES • DENSITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE RADIOGRAPH • CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES Film Screen • Overexposed – Referring to a radiograph that is too dark because too much x-radiation reached the image receptor • Underexposed – Referring to a radiograph that is too light because too little x-radiation reached the image receptor Density on Image • When a radiologist looks at a radiograph, they looks for any anatomic or pathologic change that causes a change from the normal density. • That could be, a solid mass in the lung that stops x-rays from reaching the film and decreases the amount of film darkening in that area (appears “light or white on image” • Or excessive “AIR” – looks BLACK mAs mA X s = mAs Milliamperage • mA • One milliampere is equal to one thousandth of an ampere. • The amount of current supplied to the x-ray tube • Range 10 to 1200 mA Tube current (mA) • Tube current is equal to the number of electrons flowing from the cathode to the anode per unit time • Exposure of the beam for a given kVp and filtration is proportional to the tube current Time • In seconds • How long x-rays will be produced • 0.001 to 6 seconds mAs Changes • at least 20 - 30 % mas change needed to see a visible change in density mAs DOUBLED = DENSITY DOUBLED + 25% + 50 % mas Kilovoltage Peak • kVp • One kilovolt is = to 1000 volts • The amount of voltage selected for the x-ray tube • Range 45 to 120 kVp (diagnostic range) • kVp controls contrast Tube voltage (kVp) • Determines the maximum energy in the beam • spectrum and affects the quality of the output spectrum • Efficiency of x-ray production is directly related to tube voltage Contrastthe differences between blacks to whites • Kilovolts to anode side – kVp (40 -120) • Kilovolts controls how fast the electrons are sent across the tube • kVp – controls CONTRAST on images • Low kvp – more absorbed – black - white • High kvp - more grays on image Influencing factors: kVp 15% rule: 15% kVp = doubling of exposure to the film 15% kVp = halving of exposure to the film 15% rule will always change the contrast of the image because kV is the primary method of changing image contrast. Remember : 15% change ( ) KVP has the same effect as doubling or ½ the MAS on density + 15% kvp - 15% kvp X-Ray Machine • Purpose: – provide a specific current (mA) & voltage (kV) to the x-ray tube – convert electrical energy to electromagnetic energy (x rays) in a controlled manner • control the energy of the x-ray photons • control the number of photons kVp = energy mAs = amount X-RAY CIRCUITRY Introduction to Circuitry Contributions by Mosby, Thompson Publisher, Carlton, Bushberg, and the WWW. How the current gets to the TUBE Current from the outlet Generator+ Tranformers (where the power comes from) Review Handouts •Circuit Board •Symbols •Function high voltage, low current low voltage, high current Bushberg, et al., The Essential Physics of Medical Imaging, 2nd ed., p. 126. © UW and Brent K. Stewart PhD, DABMP 54 AUTOTRANSFORMER • RAISES OR LOWERS THE VOLTAGE • KVP CONTROL TAPS LOCATED • 220 VOLTS INCOMING CONVERTED FROM 100 T0 300 VOLTS TIMER SWITCH • Timer switch ends exposure • Timer – length of exposure set at control panel 3 Divisions of Circuit Board • PRIMARY (CONTROL PANEL) yellow • SECONDARY (HIGH VOLTAGE) blue • FILAMENT (LOW CURRENT) purple TRANSFORMERS (Step Up or Step Down) • Increases the VOLTAGE going to the ANODE side of the tube OR • 110 volts to 110,000 volts • Decreases the CURRENT going to the CATHODE side of the tube • 5 Amps to 50 milliamps X-Ray Tube Circuit Functional Position Control Console Transformers Tube QUESTIONS ?