Review of Radiological Imaging - X-ray Production (Lecture 002)

I. GeneratorsIntroduction An x-ray generator provides power to the x-ray tube. X-ray generators also control the x-ray energy, exposure duration, and total exposure required for a particular examination. Generators contain high-voltage transformers, filament transformers, and rectifier circuits. Generators also include electronic circuits for manual and automatic exposure control, as well as voltage and current meters. In the United States, the electric power supply from utility companies is normally 120 volts (V) alternating current (AC), which oscillates at a frequency of 60 cycles per second (60 Hz). Generators for x-ray systems in radiology use higher voltages (440 V). A generator increases the voltage and rectifies the waveform from AC to direct current (DC). Generators permit x-ray operators to control three key parameters of x-ray operation:x-ray tube voltage (kilovolts, or kV), which affects the x-ray energy; tube current (milliamperes, or rnA), which affects the radiation quantity; and exposure time (seconds). Voltage is applied across the x-ray tube, and current flows through the x-ray tube. The power dissipated equals the product of tube voltage (V) in kilovolts and of current in milliamperes (I), or Vx I, and is measured in kilowatts (kW). Typical transformer ratings in x-ray departments are 100 kV and 800 mA, which correspond to a power of 80 kW.Transformers One major requirement of a generator is to produce high voltages, which can exceed 100,000 V. A transformer changes the size of the input voltage and is used to produce high and low voltages. Step-up transformers increase the voltage, and step-down transformers decrease the voltage. If two wire coils are wrapped around a common iron core, current in the primary coil produces a current in the secondary coil by electromagnetic induction. The voltages in the two circuits (Vpand Vs) are proportional to the number of turns in the two coils (Np and Ns), expressed mathematically as Np/Ns= Vp/Vs, where "p" refers to the primary and "s" to the secondary coils. The product of the voltage (V) and current (I) is equal to the power and must be equal in the two circuits (conservation of energy) if there are no additional losses. For an ideal transformer, the power in the primary and secondary circuits will be equal, so that Vpx Ip= Vsx Is The step-up transformers used in x-ray generators have a secondary coil with many more turns (500:1) to produce a high voltage across the tube. Generators also have a step-down transformer with fewer turns in the secondary coil for the x-ray tube filament circuit, which only requires about 10 V. An autotransformer permits adjustment of the output voltage, using movable contactsto change the number of windings in the circuit.Rectification The electric current from an AC power supply flows alternately in both directions, resulting in a voltage waveform shaped like a sine wave. Rectification changes the AC voltage into DC voltage across the x-ray tube. Rectification is achieved using diodes, which only permit current to flow in one direction. With half-wave rectification, one direction of current is eliminated. In full-wave rectification (achieved using a minimum of four diodes), two pulses per cycle are produced. Single-phase generators use a bridge rectifier circuit that directs the alternating flow of high-voltage electrons so that flow is always from cathode to anode. Single-phase generators have been replaced by three-phase generators for use in diagnostic radiology but may be encountered in dental x-ray units. Three-phase generators obtain power from three lines of current, each 120 degreesout of phase with the others. Diodes arc arranged in combinations of delta and wye circuits to produce six-and 12-pulse outputs. Modem high-frequency inverter generators transform AC input into low-voltage DC, then into high-frequency AC, and finally into high voltage AC waveforms that are rectified to yield an approximately constant waveform. High-frequency generators are smaller and more efficient than are three-phase generators.Voltage waveform Voltage waveform is a plot of voltage over time. A constant high voltage is desired across the x-ray tube for x-ray production, but in practice, there is some variation in the voltage, which is called ripple. The peak voltage or kilovolt peak (kVp) is the maximum voltage that crosses the x-ray tube during a complete waveform cycle. The voltage waveform ripple is the maximum voltage minus the minimum voltage per cycle expressed as a percentage of the maximum voltage. Single-phase half-and full-wave rectified systems have 100% ripple. Three-phase six-pulse systems typically have 14% ripple, and 12-pulse systems have approximately 4% ripple. High-frequency generators have ripple comparable to that of 12-pulse systems. A low ripple is desirable because a more constant voltage is produced. Fig. 2.1 shows the waveforms for different types of generators and their corresponding ripple valueII. X-Ray Production ProcessesIntroduction Diagnostic x-rays are produced when electrons with energies of 20 to 150 kilo electron volts (keV) are stopped in matter, producing electromagnetic radiation in the form of x-rays. Electrons accelerated to the positive anode gain a kinetic energy of V eV, determined solely by the value of the applied voltage (V). The kinetic energy of electrons is transformed into heat and x-rays when the electrons strike the anode. Electrons only penetrate tens of micrometers (mm) into the anode before losing their energy by ionization and excitation of electrons in the anode material. Energetic electrons loose their energy in matter by excitation, in which electrons are energized to higher energy states; ionization, in which an outer-shell electron is removed; and radiation, in which the energy loss is converted directly to a photon. X-rays are generated by two different processes known as bremsstrahlung (radiation) and characteristic x-ray production (ionization). Most incident electrons interact with outer-shell electrons (excitation and ionization). Energy lost in the form of excitation and ionization appears as heat in the anode. The efficiency of x-ray production is approximately kV x Z x 10-6and is approximately 1% for materials with high atomic numbers (2) at 100 kVp. A graph of x-ray tube output showing the number of photons at each x-ray energy is called a spectrum.Bremsstrahlung radiation Bremsstrahlung (braking) x-rays are produced when incident electrons interact with nuclear electric fields, which slow them down (brake) and change their direction. Fig. 2.2 shows a bremsstrahlung process in which a fraction of the initial electron kinetic energy is emitted as an x-ray photon. Bremsstrahlung x-rays produce a continuous spectrum of radiation, up to a maximum energy determined by the maximum kinetic energy of the incident electron. The closer the electron passes to the nucleus, the greater the interaction of the incident electron with the nucleus, and the higher the energy of the resulting x-ray. Maximum photon energies correspond to minimum x-ray wavelengths. The majority of x-rays produced in x-ray tubes are via the bremsstrahlung process. Bremsstrahlung x-ray production increases with the accelerating voltage (kV) and the atomic number (Z) of the anode.Characteristic radiation Characteristic radiation is the result of ionization and is produced when inner-shell electrons of the anode target are ejected by the incident electrons. To eject a bound atomic electron, the incident electron must have energy greater than the binding energy. The resultant vacancy is filled by an outer-shell electron, and the energy difference is emitted as characteristic radiation (e.g., K-shell x-rays, L-shell x-rays), as shown in Fig. 2.3. Characteristic x-rays occur only at discrete energy levels, unlike the continuous energy spectrum of bremsstrahlung. After the ejection of a K-shell electron, the excess energy may also be emitted as an Auger electron. Each anode material emits characteristic x-rays of a given energy, as listed in Table 2.1. K-shell characteristic x-ray energies are always slightly lower than the K-shell binding energy. (Table 1.4 lists K-shell binding energies). K-shell electrons are ejected only if incident electrons have energies greater than the K-shell binding energy. For tungsten, K-shell characteristic x-rays are only produced when the applied voltage exceeds 69.5 kV (K-shell binding energy is 69.5 keV) For molybdenum, K-shell characteristic x-rays are only produced when the applied voltage exceeds 20 kV. L-shell radiation also accompanies K-shell radiation, but because L-shell characteristic x-rays have very low energies, they are absorbed by the glass of the x-ray tube. Only K-shell characteristic x-rays are important in diagnostic radiology.Quantity Intensity refers to the quantity or number of x-ray photons produced. Intensity is affected by the generator type, beam filtration, and distance from the beam (inverse square law). For conventional radiography with a tungsten target, the characteristic radiation produced accounts for up to 10% of the x-ray beam intensity. X-ray output is directly proportional to the current (rnA), and to exposure time (seconds). The product of the tube current (rnA) and exposure time (seconds) is known as the mAs, and the x-ray tube output is proportional to the mAs. Doubling the current at constant exposure time has the same effect as doubling the exposure time at constant tube current. Doubling the mAs doubles the number of x-rays emitted but does not change the energy spectrum. Fig. 2.4A shows how the number of photons at each energy level increases when the tube current is increased, but the spectrum shape does not change. The quantity of x-rays produced can also be increased by increasing the kVp, but this also changes the quality or shape of the x-ray spectrum, as shown in Fig. 2.4B. The quantity (intensity) of x-ray production is approximately proportional to the square of the tube potential.

Quality X-ray beams in diagnostic radiology are polychromatic and consist of a range of photon energies. Quality refers to effective photon energy of the x-rays produced, and relates to their ability to penetrate the patient. The quality of an x-ray beam is obtained from the effective x-ray energy of the xray spectrum. The effective photon energy is taken to be between one third and one half of the maximum photon energy. Increasing the peak voltage (kVp) increases the x-ray tube output, peak energy, and mean energy of the beam. This increases the beam quality as shown in Fig. 2.4B. Increasing beam quality increases x-ray beam penetrating power because the average photon energy is higher. A rule of thumb is that increasing the peak voltage by 15% has the same effect on film density as that of doubling the mAs. For example, changing tube voltage by 10 kVp (from 65 to 75 kVp) normally has the same effect on film density as doubling the mAs. Reducing the voltage waveform ripple increases average photon energy and thus x-ray beam quality. Increasing x-ray tube filtration also increases beam quality, as low-energy photons are preferentially removed from the x-ray beam (beam hardening). Table 2.2 lists typical x-ray outputs as a function of x-ray tube voltage and filtration.III. X-Ray TubesIntroduction The x-ray tube converts the electric power from the generator into x-ray photons. Fig. 2.5 is a diagram of a radiographic x-ray tube. X-ray tubes contain a negatively charged cathode containing the filament that serves as an electron source. The anode is positively charged and includes the target where x-rays are produced. The anode may bc stationary or rotating. in which case the tube also contains a rotor and stator. The anode and cathode are contained in an evacuated envelope to prevent the electrons from colliding with gas molecules. The envelope is contained in a tube housing that protects and insulates the tube and provides shielding to prevent leakage radiation. The housing contains an oil bath to provide electrical insulation and help cool the tube. The primary x-rays exit through a window in the tube housing. The x-ray window may be a thinner area in the glass or a different material such as beryllium, which absorbs fewer low-energy x-rays (used in mammography).

Filaments The filament is the source of electrons that are accelerated toward the anode to produce x-rays. The filament is usually made of coiled tungsten wire, with modem x-ray tubes having two filaments to allow a choice of two focal spot sizes. A focusing cup or cathode block surrounds the filament and helps direct the electrons toward the target. Typical voltages across the x-ray tube filament are 10 V, and currents through the cathode filament are 4 A. The power dissipated from the filament (I x V) is typically 40 W. The high resistance in the filament causes temperature to rise (>2200C), resulting in the thermionic emission of electrons. Electrons emitted from a heated filament form a negative cloud around the filament called a space charge, which prevents further emission of electrons. The tube current is the flow of electrons from the filament to the anode; this occurs when a negative potential is applied to the filament (cathode) and a positive potential is applied to the anode. Tube current is in the range of 1 to 1,000 rnA. At low peak voltage, the potential is insufficient to cause all the electrons to be pulled away from the filament, and a residual space charge remains (space charge limited operation). At the saturation voltage, all electrons are immediately pulled away from the filament,and the tube current is maximized. Above 40 kVp, the filament current is proportional to and determines the tube current(emission-limited operation).

Anode The anode is the positive electrode in the x-ray tube. Electrons striking the target, located in the anode, produce heat and x-rays. Tungsten is the most common target material because of its high atomic number (Z= 74) and melting point. Rhenium is often added to reduce the pitting and cracking caused by overheating. Molybdenum (Z = 42) and rhodium (Z = 45) are used for targets in mammography. A stationary anode usually consists of a tungsten target embedded in a copper block. Although copper is a good heat conductor, heat dissipation is limited. Stationary anodes are used in some C-arm x-ray units. A rotating anode greatly increases the effective target area used during an exposure and therefore raises the heat capacity. To maintain the vacuum required inside the x-ray tube, rotating anodes use an electricinduction motor. The rotor (inside the envelope) turns in response to the changing electric current in the stator electric windings (outside the envelope).Focal spots The focal spot is the apparent source of x-rays in the tube. Focal spot size is a result of the filament shape, focusing cup, and electric field created between the cathode and anode. Focal spots must be small to produce sharp images, but large enough to tolerate a high heat loading without melting the target. The focal spot size enlarges as milliamperes increase owing to the repulsion of adjacentelectrons. This effect is called blooming. The line focus principle is used to permit larger heat loading while minimizing size of the focal spot by orienting the anode at a small angle to the direction of x-ray beam irradiating the patient (inset in Fig. 2.5). The anode angle is the angle between the target surface and the central beam. Typical anode angles range from about 7 to 20 degrees. Radiation field coverage increases with increased target angle. The focal spot size is the dimension of the x-ray source as viewed from the image. Focal spot sizes, as quoted by manufacturers of x-ray tubes, range from about 0.1 1.2 mm. Focal spot dimensions, as quoted by manufacturers, are nominal values. Focal spot sizes can be measured using pinhole cameras, star or bar test patterns, or slit cameras. Measured focal spot sizes may be up to 50% larger than the nominal values listed in Table 2.3. A large focal spot is favored when a short exposure time is the priority. A small focal spot is preferred when spatial resolution is a priority.IV. Tube LoadingX-ray techniques In manual mode, the operator selects the x-ray tube voltage, x-ray tube current, and exposure time on the generator control panel. In automatic exposure control (AEC) mode, the operator selects the x-ray voltage and the desired film density, and the generator circuits control the exposure time current (mAs). The x-ray tube output is directly proportional to the x-ray tube current. A typical current for radiography is 100 to 1,000 rnA. Typical radiographic exposure times are between tens and hundreds of millisecond. Typical tube current exposure times in radiography are tens of mAs. In fluoroscopy, tube currents are typically between 1 and 5 mA. For small body parts, such as the extremities, x-ray tube voltages are generally 55 to 65 kVp. Most radiographic and fluoroscopy imaging is performed at x-ray tube voltages between 70 and 90 kVp. Higher voltages may be used for larger patients. Chest radiography is often performed at higher x-ray tube voltages of about 120 kVp. High voltages (> 100 kVp) are also used in some fluoroscopy performed with barium contrast agents to provide sufficient penetration.Energy deposition Only about 1% of the electric energy supplied to the x-ray tube is converted to x-rays. Approximately 99% of the electrical energy supplied to an x-ray tube is converted to heat. The amount of heat energy deposited during an x-ray exposure is known as the tube loading. X-ray tube loading depends on the tube voltage, voltage waveform, tube current, exposure time, and number of exposures. For a constant x-ray tube voltage (V) and current (I), the energy deposited during an x-ray exposure is V x I x t joules, where t is the exposure time measured in seconds. This energy is temporarily stored in the anode, which has a heat capacity of several hundred thousand joules. X-ray tube loading is assessed by using an x-ray tube rating chart and an anode thermalcharacteristics chart. -If the voltage is not constant, the calculation of energy deposition in joules is more complicated. For systems with single-phase power supplies and full-wave rectification, the quantitykVp X rnA X time is termed heat units. One heat unit = 0.74 J (1 J = 1.35 heat units).Tube rating The rating of an x-ray tube is based on maximum allowable kilowatts (kW) at an exposure time of 0.1 second. For example, a tube with a rating of 80 kW (80,000 W) tolerates a maximum exposure of 80 kVp and 1,000 rnA at 0.1 second. Typical x-ray tube ratings are between 5 and 100 kW and depend on focal spot size. The loading on the focal spot, anode, and x-ray tube housing must be considered to ensure none of these components overheats. In radiography, power loadings are typically 80 to 100 kW for a large focal spot size and 20 kW for the small focal spot size In fluoroscopy, power loadings are very low and typically between 100 to 500 W. Increasing the exposure time or using a larger focal spot size may be required to achieve the required x-ray tube output without overheating.X-ray tube heat dissipation X-ray tubes are designed to efficiently dissipate heat. Modem anodes are circular and rotated at high speeds (3,000 to 10,000 rpm) to spread heat loading over a large area. Heat is transferred from the focal spot by radiation to the tube housing and conduction into the anode. Radiation is the primary way that anodes transfer heat from the anode to the housing, as the anode gets white hot during the x-ray exposure. X-ray tubes are usually immersed in oil for electrical insulation and for aiding heat dissipation by convection. Air fans are sometimes used to increase the rate of heat loss. Cooling of the anode and housing are described by thermal characteristics charts. Taking a large number of radiographs, or a prolonged acquisition in computed tomography, can result in the tube overheating. An x-ray tube that has reached the maximum anode heat loading may require several minutes to cool down before additional use. In fluoroscopy, the rate at which heat is deposited into the anode (100 to 500 W) is low enough that the heat dissipation rate will always prevent overheating. Fluoroscopy can normally be performed without the risk of tube overheating.V. Diagnostic X-Ray Beams

Filters The x-ray beam emerging from the x-ray tube may contain a high number of low energy photons. Low-energy photons have a negligible chance of getting through the patient, thereby contributing to patient dose but adding nothing to the image. Some of the very low energy x-rays are stopped as they exit the x-ray tube by the window, which acts as an inherent x-ray beam filter. Beryllium provides very little filtration and is often used as a window in mammography x-ray tubes, which use low-energy photons. Filters are also added to the x-ray tube window to increase the filtration effect, as in Table 2.4. Filters are designed to preferentially absorb low-energy photons. Added filtration does not affect the maximum energy of the x-ray beam spectrum. Added filtration will always reduce the x-ray tube output.

Beam hardening Beam hardening refers to the preferential loss of lower-energy photons from a polychromatic beam with filtration. Beam hardening results in a change in x-ray spectrum shape but not in maximum photon energy. The x-ray beam output is decreased with increased filtration (Table 2.2), but the averagex-ray energy is increased. The x-ray beam becomes more penetrating as mean photon energy increases. Filtered beams with higher mean photon energies are called harder x-ray beams. Beam hardening does not occur with monochromatic x-ray beams. Hard beams are produced at high peak voltages using heavy filtration. Soft beams are produced at low peak voltages using less filtration. Fig. 2.6 shows the effect of filtration on an x-ray spectrum in which low-energy photons are preferentially lost when passing through a filter (or any absorber).Heel effect At typical energy levels used in radiography, x-rays are produced within the anode that travel equally in all directions (isotropic). However, x-rays produced within the anode must pass through a portion of the targetand are attenuated as they pass out of the anode material. This attenuation is greater in the anode direction than in the cathode direction because of differences in the path length within the target. This is known as the heel effect and results in higher x-ray intensity at the cathode end and in lower x-ray intensity at the anode end of the beam. The magnitude of the heel effect depends on the anode angle (Fig. 2.5), source-to-image detector distance, and field size. To reduce the heel effect, the anode angle should be increased, source-to-imagedetector distance increased, and field size decreased. The heel effect can be taken advantage of by placing denser parts of the body at the cathode side and thinner parts at the anode side of the beam. In mammography, for example, the more intense cathode side is used to irradiate the denser chest wall region.Unwanted radiation X-ray tubes are surrounded by lead to absorb unwanted radiation. Leakage radiation is radiation that is transmitted through the x-ray tube housing. Federal regulations require the leakage radiation at 1 m to be no more than mGy/hour (l00 mR/hour). Scattered radiation has been deviated in direction after leaving the tube. The intensity of scattered radiation is typically 0.1 % of the intensity incident on a patient, when measured 1 m from the patient. Stray radiation is the sum of the leakage and scattered radiation.The content of following review is designed by and copyright to Dr. Walter Huda and Dr. Richard SloneThe online-quiz is technically designed and programmed by Dr. Jun Ni at Dept. of Radiology, University of IowaHawkeye Radiology Informatics(HawkRI)Department of Radiology||Carver College of Medicine||The University of IowaCopyright 2007-09, All rights reserved. Legal Notices Credit: Original Javascript was developed byJavaScriptKit.comas developed byJavaScriptKit.com