Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

~C1.61.41.2 _~ 1.0::l0 0.8Q).00.62-Q) 0.4"0 0c0.2«03FIGURE 5-9. Relationship of tube current to filament current for various tubevoltages shows a dependence of approximately kVp1.5. For tube voltages 40 kVpand lower, the space charge cloud shields the electric field so that further increasesin filament current do not increase the tube current. This is known as "spacecharge-limited" operation. Above 40 kVp, the filament current limits the tube current;this is known as "emission-limited" operation.current in the emission-limited range. Higher kVp produces slightly higher tubecurrent for the same filament current; for example, at 5 A filament current, 80 kVpproduces ~800 mA and 120 kVp produces ~1,100 mA, approximately as kVpl.5.This relationship does not continue indefinitely. Beyond a certain kVp, saturationoccurs whereby all of the emitted electrons are accelerated toward the anode and afurther increase in kVp does not significantly increase the tube current.The anode is a metal target electrode that is maintained at a positive potential differencerelative to the cathode. Electrons striking the anode deposit the most oftheir energy as heat, with a small fraction emitted as x-rays. Consequently, theproduction of x-rays, in quantities necessary for acceptable image quality, generatesa 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 CW; Z = 74) is the mostwidely used anode material because of its high melting point and high atomicnumber. A tungsten anode can handle substantial heat deposition without crackingor pitting of its surface. An alloy of 10% rhenium and 90% tungsten providesadded resistance to surface damage. The high atomic number of tungsten providesbetter bremsstrahlung production efficiency compared with low-Z elements(see Equation 5-1).Molybdenum (Mo, Z = 42) and rhodium (Rh, Z = 45) are used as anode materialsin mammographic x-ray tubes. These materials provide useful characteristic x-rays for breast imaging (see Table 5-2). Mammographic tubes are described furtherin Chapter 8.