Imaging Equipment and Image Acquisition Techniques
Intraoral fundamentals
Know the geometry: focal spot, object, and receptor, and how source–object and object–receptor distances affect magnification and sharpness. The paralleling technique minimizes distortion using a long cone and receptor parallel to the tooth; bisecting-angle technique relies on the central ray perpendicular to the bisector and produces more dimensional distortion. Understand kVp/mA/time selection for bitewings and periapicals, and the role of rectangular collimation in dose reduction.
Panoramic imaging
The panoramic unit uses a rotating beam and a moving receptor through a focal trough (image layer). Objects within the trough are sharp; objects outside are blurred or ghosted. Realize that panoramic images magnify (~20–30%) and distort, so they are unsuitable for precise measurement. Common errors: patient positioning (chin up/down, head rotated), which shifts the focal trough and creates overlapping or ghost images; lead apron and thyroid collar placement (collar can shadow the mandible).
Extraoral and digital acquisition
Know screen–film vs photostimulable phosphor (PSP) vs direct/indirect digital sensors, their resolution and dose characteristics, and the difference between spatial resolution (lp/mm) and contrast resolution. Understand automatic exposure control in CBCT and the trade-off between voxel size, FOV, and dose.
Traps
Questions test whether you know the panoramic focal trough is a curved layer, not a plane; whether magnification is real vs apparent; and whether collimation reduces or increases dose. Positioning-error questions often hinge on which structure is blurred vs ghosted.
Rule of thumb
If an object is sharp on a panoramic image, it lies in the focal trough; if blurred, it is outside the trough; if it appears twice (ghost), it is a high-density structure outside the trough.