Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

Power is the rate of energy production or expenditure per unit time. The 51 unitof power is the watt (W), which is defined as 1 joule m of energy per second. Forelectrical devices, power is equal to the product of voltage and current:Because a volt is defined as 1 joule per coulomb and an ampere is 1 coulomb persecond,1 watt = 1 volt X 1 ampereBecause the power output is equal to the power input (for an ideal transformer), theproduct of voltage and current in the primary circuit is equal to that in the secondarycircuit:where Ir is the input current on the primary side and Is is the output current on thesecondary side.Therefore, a decrease in current must accompany an increase in voltage, andvice versa. Equations 5-3 and 5-5 describe ideal transformer performance. Powerlosses due to inefficient coupling cause both the voltage and current on the secondaryside of the transformer to be less than predicted by these equations.The high-voltage section of an x-ray generator contains a step-up transformer,typically with a primary-to-secondary turns ratio of 1:500 to 1: 1,000.Within this range, a tube voltage of 100 kVp requires an input peak voltage of200 V to 100 V, respectively. The center of the secondary winding is usually connectedto ground potential ("center tapped to ground"). Ground potential is theelectrical potential of the earth. Center tapping to ground does not affect themaximum potential difference applied between the anode and cathode of the x-ray tube, but it limits the maximum voltage at any point in the circuit relativeto ground to one half of the peak voltage applied to the tube. Therefore, themaximum voltage at any point in the circuit for a center-tapped transformer of150 kVp is -75 kVp or +75 kVp, relative to ground. This reduces electrical insulationrequirements and improves electrical safety. In some x-ray tube designs(e.g., mammography), the anode is maintained at the same potential as the bodyof the insert, which is maintained at ground potential. Even though this placesthe cathode at peak negative voltage with respect to ground, the low kVp (lessthan 50 kVp) used in mammography does not present a big electrical insulationproblem.