forces attract and decelerate an electron that changes its direction and velocity, resulting in the radiative emission of an x-ray photon (i.e., bremsstrahlung radiation).
▪ FIGURE 6-1 Minimum requirements for x-ray production include a source and target of electrons, an evacuated envelope, and connection of the electrodes to a high-voltage source.
electron no. 2). A direct impact with the target nucleus stops an electron and converts all its kinetic energy into an equivalent energy x-ray photon (see Fig. 6-2, electron no. 1), resulting in the highest bremsstrahlung x-ray energy.
of tungsten (W, Z = 74); in mammography, molybdenum (Mo, Z = 42) and rhodium (Rh, Z = 45) are also used. As an example, for 100-keV electrons impinging on a tungsten target, the ratio of radiative to collisional losses is (100 × 74)/820,000 ≅ 0.009 ≅ 0.9%; at this tube potential of 100 kV, more than 99% of the incident electron energy striking the anode is converted to heat. Due to the low efficiency of x-ray production, x-ray tube design is largely driven by heat dissipation concerns.
TABLE 6-1 ELECTRON BINDING ENERGIES (keV) OF COMMON X-RAY TUBE TARGET MATERIALS
TABLE 6-2 K-SHELL CHARACTERISTIC X-RAY ENERGIES (keV) OF COMMON X-RAY TUBE TARGET MATERIALS
▪ FIGURE 6-5 The filtered spectrum of bremsstrahlung and characteristic radiation from a tungsten target with a potential difference of 90 kV illustrates specific characteristic radiation energies from Kα and Kβ transitions. Filtration (the preferential removal of low-energy photons as they traverse matter) is discussed in Section 6.5.
▪ FIGURE 6-6 A diagram of the major components of a modern x-ray tube and housing assembly is shown.
filament as the repulsive force of the negative charge of emitted electrons equals the thermionic emission force. When x-ray tube voltage is applied, electrons from the filament are accelerated toward the anode, and represent the x-ray tube current. Note that the x-ray tube current and filament current are not the same but are nonlinearly related, as shown in Figure 6-9. For most diagnostic acquisitions, the x-ray fluence is emission-limited, and thus the filament circuit controls the x-ray tube current, which in turn controls the x-ray output. In situations where an already high filament current is needed to produce a higher tube current, the space charge cloud surrounding the filament emitter limits the further emission of electrons from the filament surface. In this case, the tube current cannot be increased by increases in the filament current, particularly at lower kV settings, and x-ray output is space charge limited. Higher tube potentials enable larger x-ray tube current for the same filament current; for instance, for the same filament current of 5 A at 80 kV, a tube current of 800 mA is produced, whereas at 120 kV a tube current of about 1,100 mA results.
for pulsed fluoroscopy and in angiography systems to rapidly and precisely turn on and turn off the x-ray beam. Conventional x-ray generator voltage switching endures a build-up lag to get to peak voltage and a decay lag to return to 0 voltage, chiefly due to capacitance effects in the high-voltage cables. The pulse width is lengthened, and the x-ray beam energy is lowered, which results in extra patient dose and degradation of fast-moving objects in the images.
space charge limitations of conventional cathode emitters, but also requires focusing coils to maintain a tight distribution of electrons when impacting the anode (see Fig. 6-25).
▪ FIGURE 6-11 A flat emitting surface cathode made of tungsten (3 mm × 10 mm active area) is used in many computed tomography x-ray tubes. It can handle the large centrifugal forces occurring at fast rotation speeds of modern CT systems and provide an efficient work function and large surface area for realizing the high tube current requirements of CT and interventional angiography. Magnetic and electrostatic coils are used to focus the electrons along the drift path to the anode, providing the flexibility to position the focal spot distribution on the anode and adjust focal spot size. (Also see Fig. 6-25.)
x-ray tube insert. Before an exposure, the stator/rotor induction motor is energized to spin the anode, and after a short delay, rotation speeds of 3,000 to 3,600 (low speed) or 9,000 to 10,000 (high speed) revolutions per minute (rpm) are achieved. X-ray tube-generator systems for general radiography are designed such that the x-ray tube voltage is applied only when the anode is at full speed, causing a short delay (1 to 2 seconds [s]) prior to x-ray exposure when the button is pushed by the technologist.
▪ FIGURE 6-12 The anode of a fixed anode x-ray tube consists of a tungsten insert mounted in a copper block. Generated heat is removed from the tungsten target by conduction into the copper block.
is employed to reduce geometric blurring; the second design is beneficial when high exposure rate is needed for shortest exposure times using higher mA. However, a small anode angle limits the usable x-ray beam coverage at a given focal spot-to-detector distance, because of beam cutoff on the anode side of the projected x-ray field. In addition, field coverage is smaller for shorter focal spot-to-detector distances (Fig. 6-15). Therefore, the optimal x-ray tube anode angle depends on the clinical imaging application. A small anode angle (˜7° to 9°) is desirable for limited field-of-view devices, such as CT scanners with narrow collimation in the A-C dimension, and fluoroscopy C-arm devices where field coverage is determined by the small image receptor diameter (e.g., 23 cm) and focal spot-to-detector distance of 100 cm. Larger anode angles (˜12° to 15°) are necessary for general radiographic imaging to achieve large field area coverage (e.g., 43 cm length of a digital detector) at typical focal spot-to-detector distances of 100 cm.
and small (bottom row) focal spots with a typical “bi-gaussian” intensity distribution. Correcting for the known image magnification allows one to estimate the focal spot dimensions. The slit camera consists of a highly attenuating metal plate with an extremely thin slit, typically 10 µm wide, and is the primary method described in the IEC 60336 standard to evaluate focal spot dimensions. Slit images are shown in Figure 6-17F. The star pattern test tool (Fig. 6-17G) contains a metal (lead) attenuator radial spoke pattern 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 (location of the outermost unresolved spokes as shown by the arrows in Fig. 6-17G) on the image allows the calculation of the effective focal spot dimensions in the directions perpendicular and parallel to the A-C axis. A large focal spot will have a greater blur diameter than a small focal spot, as shown in the figure. For a quick and simple estimate of focal spot size, a resolution bar pattern with discrete line pairs can be employed (Fig. 6-17H). Bar pattern images demonstrate the effective resolution parallel and perpendicular to the A-C axis for a given magnification geometry, determined by the bar pattern that can be resolved. If a focal spot dimension is determined to be out of tolerance using this method, one of the methods approved by the IEC standard should be used for more precise evaluation.