Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

600500~5 4001:~•..=:l()300Q).0=:lf- 2001000With falling load generator:Maximum variable mA x 0.1 seeWithout falling load generator:400 mA x 0.125 see\. Falling load eurveFIGURE 5-35. The fallingloadgenerator control circuitprovides a continuouslydecreasing mA with time, inorder to deliver the maximalcontinuous x-ray outputpossible within power depositioncapabilities of thefocal spot. This generatorcircuit is commonly used inconjunction with automaticexposure control (AEC)devices to provide the shortestexposure time possiblefor a given kVp and mAstechnique. The example inthe figure shows that, for a50-mAs exposure, thefalling-load generator candeliver the same radiation in0.10 second that wouldrequire 0.125 second with aconstant tube current.The output of an x-ray tube is often described by the terms quality, quantity, andexposure. Quality describes the penetrability of an x-ray beam, with higher energyx-ray photons having a larger HVL and higher "quality." Quantity refers to thenumber of photons comprising the beam. Exposure, defined in Chapter 3, is nearlyproportional to the energy fluence of the x-ray beam and therefore has quality andquantity associated characteristics. X-ray production efficiency, exposure, quality,and quantity are determined by six major factors: x-ray tube target material, voltage,current, exposure time, beam filtration, and generator waveform.1. The target (anode) material affects the efficiency of bremsstrahlung radiation production,with output exposure roughly proportional to atomic number. Incidentelectrons are more likely to have radiative interactions in higher-Z materials (seeEquation 5-1). The energies of characteristic x-rays produced in the targetdepend on the target material. Therefore, the target material affects the quantityof bremsstrahlung photons and the quality of the characteristic radiation.2. Tube voltage (kVp) determines the maximum energy in the bremsstrahlungspectrum and affects the quality of the output spectrum. In addition, the efficiencyof x-ray production is directly related to tube voltage. Exposure is approximatelyproportional to the square of the kVp in the diagnostic energy range:Exposure oc kVp2 [5-7]For example, according to Equation 5-7, the relative exposure of a beam generatedwith 80 kVp compared with that of 60 kVp for the same tube current andexposure time is calculated as follows:( 80)2 60 == 1.78