GU Radiology Physics & Instruments RMI216 Lecture 2 PDF

RADIOLOGY PHYSICS& INSTRUMENTS (RMI216) X-RAY TUBE (LEC.2) Dr. Mohammed Sayed Mohammed National Cancer Institute, Cairo University Faculty of Applied Health Sciences, Galala University. Qualified Expert of Radiologic Sciences, Ministry of Health, Egypt. Former Supervisor of Diagnostic Radiology...

RADIOLOGY PHYSICS& INSTRUMENTS (RMI216) X-RAY TUBE (LEC.2) Dr. Mohammed Sayed Mohammed National Cancer Institute, Cairo University Faculty of Applied Health Sciences, Galala University. Qualified Expert of Radiologic Sciences, Ministry of Health, Egypt. Former Supervisor of Diagnostic Radiology Department, College of Applied Medical Sciences, University of Hail, KSA. Former STEM Ambassador, University of Reading, UK. X-ray tube An x-ray tube is an energy converter receiving electrical energy and producing heat and x- rays. For a diagnostic x-ray tube operating at 100 kVp, the production of heat and x-rays is: Heat, 99% X-rays, 1% Electron deceleration (Bremsstrahlung), 0.9% Characteristic radiation through ionization, 0.1% 2 X-ray tubes are designed to minimize heat production and maximize x-ray output. Tubes consist of 2 main elements: a cathode (for electron production) and an anode (for conversion electron energy into x-rays). During the process of x-ray generation, x-ray beams are generated in all directions; however, the useful beam is composed only of those x-rays leaving the lead-shielded tube. All electrical components in an x-ray tube are in a vacuum. The vacuum prevents dispersion of the electrons and ionization, which could damage the filament. Cathode Cathodes produce the electrons necessary for x-ray generation. Cathodes typically consist of a tungsten filament that is heated >2200°C. The filament is surrounded at its cathode end by a negatively charged focusing cup to direct the electrons in a small beam toward the anode. Anode The anode has 2 functions: to convert electron energy into x-rays and to dissipate heat. 3 Heat dissipation is achieved by rotating the angulated target (at approximately 3600 rpm, or 60 cycles/sec). The amount of x-rays produced depends on the atomic number (Z) of the anode material and the energy of electrons. Common anode materials include tungsten (W, Z = 74) and W/rhenium (Re) alloys (90%/10%). These materials are used because of their high melting point and the high yield of x-rays. Mammography units frequently use different anode materials (e.g., molybdenum [Mo]).  FILAMENT It locates in Cathode. The filament is surrounded at its cathode end by a negatively charged focusing cup to direct the electrons in a small beam toward the anode. 4 Focal spot The focal spot is the small area on the anode in which x-rays are produced. The size of the focal spot is determined by the dimensions of the electron beam and the anode angulation. Typical angles are 12° to 20°; however, smaller angles (6°) are used for neuroangiography. Tubes with smaller focal spot size are used when high image quality is essential. 5 Voltage The voltage of an x-ray tube is measured in kVp (peak kilovolts). The kVp determines the maximum energy of the x-rays produced. 100 kVp means that the maximum (peak) voltage across the tube causing electron acceleration is 100,000 V. The term keV (kiloelectron volt) refers to the energy of any individual electron in the beam. When an x-ray tube is operated at 100 kVp, only a few electrons will acquire kinetic energy of 100 keV because the applied voltage usually pulsates between lower values and the maximum (peak) selected. The mean energy of an x-ray beam is approximately one third of its peak energy. For 100 kVp, the mean energy would be 33 to 40 keV. An increase in kVp translates into: 6 Increased photon frequency Increased photon penetration Shortening of photon wavelength Increased anode heat production Decreased skin dose Decreased contrast Film exposure Film exposure is more sensitive to changes in kVp than to changes in mA or exposure time. General rules for selecting kVp include: A 15% increase in kVp doubles exposure at the recording system (e.g., film) and has the same effect on film density as a 100% increase in mA. An increase in kVp of 15% will decrease the contrast, so that doubling of mA is usually preferred. mA generally needs to be at least doubled when changing from a nongrid to a grid technique (depending on 7 field size and patient thickness). Milliampere A milliampere (mA) is a measure of current referring to the number of electrons flowing per second (1 A corresponds to 6.25 × 1018 electrons). The higher the mA, the higher the electron flux and the higher the x-ray production. An increase in mA changes only the amount of x-rays (i.e., the intensity or exposure rate), not the maximum energy of the x-rays produced. Typical mA values range from 25 to 500 on a given x-ray unit. The use of small focal spot sizes (for high resolution) also limits the number of mAs that can be used. General rules for selecting mA include: 8 Use lower mA for smaller focal spots when image detail is important. Select high mA to reduce exposure time (i.e., to limit motion blurring). Select high mA and reduced kVp when high image contrast is desired. Exposure Time Exposure times are set either by the operator (setting of a timer) or by a circuit that terminates the exposure after a selected amount of x-rays have reached the patient. General rules for selecting exposure time include: Short exposure time minimizes blurring. 9 Long exposure times can be used to reduce either mA or kVp when motion is not a problem. Focal spot The focal spot (FS) is the small area on the anode in which x-rays are produced. NEMA specifications require that focal spots of