D Multifield image intensifier tubes are usually either dual-field or tri-field and are designed this way in order to permit magnification imaging. As voltage is applied to the electrostatic focusing lenses, the focal point moves back—closer to the input phosphor—and a smaller portion of the input phosphor is utilized. As a result, the FOV decreases and magnification increases, producing better spatial resolution. At the same time, brightness is decreased requiring an increase in mA (therefore increased patient dose). This increase in mA increases image quality. It can be likened to an increase in signal-to-noise ratio
In the parieto-orbital projection, the patient is PA with the acanthomeatal line perpendicular to the IR. The head rests on the zygoma, nose, and chin, and the MSP should form a 53° angle with the IR (37° with the central ray). Radiographically, the optic canal should appear in the lower outer quadrant of the orbit. Incorrect rotation of the MSP results in lateral displacement, and incorrect positioning of the baseline results in longitudinal displacement.