Transcript ENTC 4390
ENTC 4390 PRODUCTION OF X RAYS INTRODUCTION To produce medical images with x rays, a source is required that 1. Produces enough x rays in a short time 2. Allows the user to vary the x-ray energy 3. Provides x rays in a reproducible fashion 4. Meets standards of safety and economy of operation Currently, the only practical sources of x rays are radioactive isotopes, nuclear reactions such as fission and fusion, and particle accelerators. Only special-purpose particle accelerators known as x-ray tubes meet all the requirements mentioned above. • In x-ray tubes, bremsstrahlung and characteristic x rays are produced as high-speed electrons interact in a target. A heated filament releases electrons that are accelerated across a high voltage onto a target. • The stream of accelerated electrons is referred to as the tube current. X rays are produced as the electrons interact in the target. • The x rays emerge from the target in all directions but are restricted by collimators to form a useful beam of x ray’s. • A vacuum is maintained inside the glass envelope of the x-ray tube to prevent the electrons from interacting with gas molecules. ELECTRON SOURCE A metal with a high melting point is required for the filament of an x-ray tube. • Tungsten filaments (melting point of tungsten • 3370 C) are used in most x-ray tubes. A current of a few amperes heats the filament, and electrons are liberated at a rate that increases with the filament current • The filament is mounted within a negatively charged focusing cup. • Collectively, these elements are termed the cathode assembly. The focusing cup, also called the cathode block, surrounds the filament and shapes the electron beam width. • The voltage applied to the cathode block is typically the same as that applied to the filament. • This shapes the lines of electrical potential to focus the electron beam to produce a small interaction area (focal spot) on the anode. • A biased x-ray tube uses an insulated focusing cup with a more negative voltage (about 100 V less) than the filament. • This creates a tighter electric field around the filament, which reduces spread of the beam and results in a smaller focal spot width. Although the width of the focusing cup slot determines the focal spot width, the filament length determines the focal spot length. • • X-ray tubes for diagnostic imaging typically have two filaments of different lengths, each in a slot machined into the focusing cup. Selection of one or the other filaments determines the area of the electron distribution (small or large focal spot) on the target. The filament current determines the filament temperature and thus the rate of thermionic electron emission. • • • As the electrical resistance to the filament current heats the filament, electrons are emitted from its surface. When no voltage is applied between the anode and the cathode of the x-ray tube, an electron cloud, also called a space charge cloud, builds around the filament. Applying a positive high voltage to the anode with respect to the cathode accelerates the electrons toward the anode and produces a tube current. • Small changes in the filament current can produce relatively large changes in the rube current . The existence of the space charge cloud shields the electric field for tube voltages of 40 kVp and lower, and only a portion of the free electrons are instantaneously accelerated to the anode. • When this happens, the operation of the x-ray tube is space charge limited, which places an upper limit on the tube current, regardless of the filament current. Above 40 kVp, the space charge cloud effect is overcome by the applied potential difference and the tube current is limited only by the emission of electrons from the filament. • Therefore, the filament current controls the tube current in a predictable way (emission-limited operation). The tube current is five to ten times less than the filament current in the emission-limited range. • Higher kVp produces slightly higher tube current for the same filament current; • For example, at 5 A filament current, 80 kVp produces • 800 mA and 120 kVp produces ~1,100 mA, approximately as kVp15. • Beyond a certain kVp, saturation occurs whereby all of the emitted electrons are accelerated toward the anode and a further increase in kVp does not significantly increase the tube current. Anode The anode is a metal target electrode that is maintained at a positive potential difference relative to the cathode. Electrons striking the anode deposit most of their energy as heat, with a small fraction emitted as x-rays. • Consequently. the production of x-rays, in quantities necessary for acceptable image quality, generates a large amount of heat in the anode. To avoid heat damage to the x-ray tube, the rate of x-ray production must be limited. • Tungsten (W, Z = 74) is the most widely used anode material because of its high melting point and high atomic number. • A tungsten anode can handle substantial heat deposition • without cracking or pitting of its surface. An alloy of 10% rhenium and 90% tungsen provides added resistance to surface damage. • The high atomic number of tungsten provides better bremssrahlung production efficiency compared with low-Z elements. Molybdenum (Mo, Z = 42) and rhodium (Rh, Z = 45) are used as anode materials in mammographic x-ray tubes. • These materials provide useful characteristic x rays for breast imaging. Anode Configurations X-ray tubes have stationary and rotating anode configurations. • The simplest type of x-ray tube has a stationary (i.e., fixed) anode. • It consists of a tungsten insert embedded in a copper block. • The copper serves a dual role: • it supports the tungsten target, and • it removes heat efficiently from the tungsten target. Unfortunately, the small target area limits the heat dissipation race and consequently limits the maximum tube current and thus the x-ray flux. • Many dental x-ray units, portable x-ray machines, and portable fluoroscopy systems use fixed anode x-ray tubes. Despite their increased complexity in design and engineering, rotating anodes are used for most diagnostic x-ray applications, mainly because of their greater heat loading and consequent higher x-ray output capabilities. • Electrons impart their energy on a continuously rotating target, spreading thermal energy over a large area and mass of the anode disk. A bearing-mounted rotor assembly supports the anode disk within the evacuared x-ray tube insert. • The rotor consists of copper bars arranged • around a cylindrical iron core. A series of electromagnets surrounding the rotor outside the x-ray tube envelope makes up the stator, and the combination is known as an induction motor. Rotation speeds are 3,000 to 3,600 (low speed) or 9,000 to 10,000 (high speed) revolutions per minute (rpm). • X-ray machines are designed so that the x-ray tube will not be energized if the anode is not up to full speed; • this is the cause for the short delay (1 to 2 seconds) when the x-ray tube exposure button is pushed. Rotor bearings are heat sensitive and are often the cause of x-ray tube failure. • Bearings are in the high-vacuum environment • of the insert and require special heatinsensitive, nonvolatile lubricants. A molybdenum stem attaches the anode to the rotor/bearing assembly, because molybdenum is a very poor heat conductor and reduces heat transfer from the anode to the bearings. Because it is thermally isolated, the anode must be cooled by radiative emission. • Heat energy is emitted from the hot anode as infrared radiation, which transfers heat to the x-ray tube insert and ultimately to the surrounding oil bath. The focal track area of the rotating anode is equal to the product of the track length (2pr) and the track width (Ar), where r is the radial distance from the track to its center. A rotating anode with a 5-cm focal track radius and a 1-mm track width provides a focal track with an annular area 3/4 times greater than that of a fixed anode with a focal spot area of 1 mm x 1 mm. • The allowable instantaneous heat loading depends on the anode rotation speed and the focal spot area. • Faster rotation speeds distribute the heat load over a greater portion of the focal track area. Anode Angle and Focal Spot Size The anode angle is defined as the angle of the target surface with respect to the central ray in the x-ray field. Anode angles in diagnostic x-ray tubes, other than some mammography tubes, range from 7 to 20 degrees, wirh 12- to 15-degree angles being most common. • Focal spot size is defined in two ways: • The actual focal spot size is the area on the anode that is struck by electrons, and • it is primarily determined by the length of the cathode filament and the width of the focusing cup slot. The effective focal spot size is the length and width of the focal spot as projected down the central ray in the x-ray field. • The effective focal spot width is equal to the • actual focal spot width and therefore is not affected by the anode angle. However, the anode angle causes the effective focal spot length to be smaller than he actual focal spot length. There are three major tradeoffs to consider for the choice of anode angle. A smaller anode angle provides a smaller effective focal spot for the same actual focal area. • A smaller effective focal spot size provides better spatial resolution. However, a small anode angle limits the size of the usable x-ray field owing to cutoff of the beam. • Field coverage is less for short focus-to-detector distances. The optimal anode angle depends on the clinical imaging application. • • A small anode angle (approximately 7 to 9 degrees) is desirable for small field-of-view image receptors, such as cineangiographic and neuroangiographic equipment, where field coverage is limited by the image intensifier diameter (e.g., 23 cm). Larger anode angles (approximately 12 to 15 degrees) are necessary for general radiographic work to achieve large field area coverage at short focal spotto-image distances. The nominal focal spot size (width and length) is specified at the central ray of the beam. • The central ray is usually a line from the focal spot to the image receptor that is perpendicular to the A-C axis of the x-ray rube and perpendicular o he plane of a properly positioned image receptor. In most radiographic imaging, the central ray bisects the detector field. • X ray mammography is an exception. Tools for measuring focal spot size are • The pinhole camera, • The slit camera, • The star pattern, and • The resolution bar pattern. The pinhole camera uses a very small circular aperture (10 to 30 mm diameter) in a disk made of a thin, highly attenuating metal such as lead, tungsten, or gold. With the pinhole camera positioned on the central axis between the x-ray source and the detector, an image of the focal spot is recorded. The slit camera consists of a plate made of a highly attenuating metal (usually tungsten) with a thin slit, typically 10 mm wide. • In use, the slit camera is positioned above the image receptor, with the center of the slit on the central axis and the slit either parallel or perpendicular to the A-C axis. • Measuring the width of the distribution and correcting for • magnification yields one dimension of the focal spot. A second radiograph, taken with the slit perpendicular to the first, yields the other dimension of the focal spot. The star pattern Test tool contains a radial pattern of lead spokes of diminishing width and spacing on a thin plastic disk. • Imaging the star pattern at a known magnification and measuring the distance between the outermost blur patterns (areas of unresolved spokes) on the image provides an estimate of the resolving power of the focal spot in the directions perpendicular and parallel to the AC axis. • A large focal spot has a greater blur diameter than a small focal spot. • The effective focal spot size can be estimated from the blur pattern diameter and the known magnificacion. A resolution bar pattern is a simple tool for in-the-field evaluation of focal spot size. • Bar pattern images demonstrate the effective resolution parallel and perpendicular to the A-C axis for a given magnification geometry. The focal spot is the volume of target within which electrons are absorbed and are produced. For radiographs of highest clarity, electrons should be absorbed within a small focal spot. • To achieve a small focal spot, the electrons should be emitted from a small or “fine” filament. Radiographic clarity is often reduced by voluntary or involuntary motion of the patient. • This effect can be decreased by using x-ray exposures of high intensity and short duration. The smaller, fine filament is used when radiographs with high detail are desired and short, high-intensity exposures are necessary If high-intensity exposures are needed to limit the blurring effects of motion, the larger, coarse filament is used. TUBE VOLTAGE AND VOLTAGE WAVEFORMS The intensity and energy distribution of x rays emerging from an x-ray tube are influenced by the potential difference (voltage) between the filament and target of the tube. The source of electrical power for radiographic equipment is usually alternating (ac). • This type of electricity is by far the most common form available for use, because it can be transmitted with little energy loss through power lines that span large distances. X-ray tubes are designed to operate at a single polarity, with a positive target (anode) and a negative filament (cathode). • • X-ray production is most efficient (more x rays are produced per unit time) if the potential of the target is always positive and if the voltage between the filament and target is kept at its maximum value In most x-ray equipment, ac is converted to direct current (dc), and the voltage between filament and target is kept at or near its maximum value. • The conversion of ac to dc is called rectification. Two electrical currents flow in an x-ray tube. • The filament current is the flow of electrons • through the filament to raise its temperature and release electrons. The tube current flows from the filament to the anode across the x-ray tube. • The figure illustrates the influence of tube voltage and filament current upon tube current. • One of the factors is space charge. • At low tube voltages, electrons are released from the filament more rapidly than they are accelerated toward the target • The cloud accumulates around the filament. The useful beam of an x-ray tube is composed of photons with an energy distribution that depends on four factors. • Bremsstrahlung x rays are produced with a range of energies even if electrons of a single energy bombard the target.