Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

mAand mAscontrolPhototimercircuitsFIGURE 5-25. A modular schematic view shows the basic x-ray generator components. Most systemsare now microprocessor controlled and include service support diagnostics.timer) measures the exposure with the use of radiation detectors located near theimage receptor, which provide feedback to the generator to stop the exposure whenthe proper exposure to the image receptor has been reached. AECs are discussed inmore detail later in this chapter.Many generators have circuitry that is designed to protect the x-ray tubes frompotentially damaging overload conditions. Combinations of kVp, mA, and exposuretime delivering excessive power to the anode are identified by this circuitry, andsuch exposures are prohibited. Heat load monitors calculate the thermal loading onthe x-ray tube anode, based on kVp, mA, and exposure time, and taking intoaccount intervals for cooling. Some x-ray systems are equipped with sensors thatmeasure the temperature of the anode. These systems protect the x-ray tube andhousing from excessive heat buildup by prohibiting exposures that would damagethem. This is particularly important for CT scanners and high-powered interventionalangiography systems.Operator ConsoleAt the operator console, the operator selects the kVp, the mA (proportional to thenumber of x-rays in the beam at a given kVp), the exposure time, and the focal spotsize. The peak kilovoltage (kVp) determines the x-ray beam quality (penetrability),which plays a role in the subject contrast. The x-ray tube current (mA) determinesthe x-ray flux (photons per square centimeter) emitted by the x-ray tube at a givenkVp. The product of tube current (mA) and exposure time (seconds) is expressed asmilliampere-seconds (mAs). Some generators used in radiography allow the selectionof "three-knob" technique (individual selection of kVp, mA, and exposuretime), whereas others only allow "two-knob" technique (individual selection ofkVpand mAs). The selection offocal spot size (i.e., large or small) is usually determinedby the mA setting: low mA selections allow the small focus to be used, and highermA settings require the use of the large focus due to anode heating concerns. On