Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Therefore, the ourpur exposure increases by approximately 78% (Fig. 5-36).An increase in k Vp increases the efficiency of x-ray production and the quantity andquality of the x-ray beam.Changes in the kVp must be compensated by corresponding changes in mAsto maintain the same exposure. At 80 kVp, 1.78 units of exposure occur for every1 unit of exposure at 60 kVp. To achieve the original 1 unit of exposure, the mAsmust be adjusted to 1/1.78 = 0.56 times the original mAs, which is a reduction of44%. An additional consideration of technique adjustment concerns the x-rayattenuation characteristics of the patient. To achieve equal transmitted exposurethrough a typical patient (e.g., 20 cm tissue), the mAs varies with the fifth powerof the kVp ratio:( kV Pl)5 X mAs l = mAszkVpzAccording to Equation 5-8, if a 60-kVp exposure requires 40 mAs for aproper exposure through a typical adult patient, at 80 kVp the proper mAs isapproximately( 60)5 80 X 40 mAs == 9.5 mAsor abour one fourth of the original mAs. The value of the exponent (betweenfour and five) depends on the thickness and attenuation characteristics of thepatient.3. The tube current (mA) is equal to the number of electrons flowing from thecathode to the anode per unit time. The exposure of the beam for a given kVpand filtration is proportional to the tube current.4. The exposure time is the duration of x-ray production. The quantity of x-raysis directly proportional to the product of tube current and exposure time(mAs).5. The beam filtration modifies the quantity and quality of the x-ray beam by selectivelyremoving the low-energy photons in the spectrum. This reduces the pho-X-ray output spectrum(Equal tube current)Q)(.)100000c 80000Q)::JG=C00.t::a.Q)>6000040000~ 20000 .Q)0:::0040 60 80Energy (keV)FIGURE 5-36. X-ray tubeoutput intensity variesstrongly with tube voltage(kVp). In this example, thesame tube current and exposuretimes (mAs) are comparedfor 60 to 120 kVp. Therelative area under eachspectrum roughly follows asquared dependence (characteristicradiation is ignored).