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Both intraoral and panoramic x-rays machines have the following exposure factor controls: milliamperage (mA), kilovoltage (kVp) and time. The primary difference between the two types of machines is the control of exposure parameters. Typically, intraoral x-ray units usually have fixed mA and kVp controls while the exposure is altered by adjusting the time for specific intraoral projections and regions of the jaws. Panoramic x-ray unit exposure is controlled by adjusting complimentary parameters; the exposure time is fixed while the kVp and the mA are adjusted according to patient size, stature, and bone density. Although the principles of operation are identical, the exposure control panel is more complex in its format.
- Milliamperage (mA) Control - regulates the low voltage electrical supply by adjusting the number of electrons flowing in the electrical circuit. Altering the milliamperage setting influences the quantity of x-rays produced and image density or darkness. A 20% difference is required to visibly alter image density.
- Kilovoltage (kVp) Control – regulates the high voltage electrical circuit by adjusting the potential difference between the electrodes. Altering the kilovoltage setting influences the quality or penetration of the x-rays produced and image contrast or differences in density. A 5% difference is required to visibly alter image density.
- Time Control – regulates the period over which electrons are released from the cathode. Altering the time setting influences the quantity of x-rays and image density or darkness in intraoral radiography. The exposure time in panoramic imaging is fixed for a specific unit and the entire exposure cycle ranges from 16 to 20 seconds in length.
Automatic Exposure Control (AEC), a feature of some panoramic x-ray machines, measures the amount of radiation that reaches the image receptor and terminates the preset exposure when the receptor has received the required radiation intensity to produce an acceptable diagnostic image.2,7 AEC serves to adjust the amount of radiation delivered to the patient as well as optimize image contrast and density.