Image Production: Equipment and Image Acquisition
Core concepts
In the tube, the cathode filament emits electrons; the actual focal spot is the bombarded area and the effective focal spot is its projection. A smaller effective focal spot sharpens detail but concentrates heat. Anode angle and the line-focus principle explain the anode heel effect: the beam is less intense on the anode side, so place thicker anatomy under the cathode.
mA × time = mAs, which controls photon quantity and therefore receptor exposure and patient dose. kVp controls penetration and photon energy and influences contrast: higher kVp generally means lower contrast, more scatter, and lower patient dose for equal receptor exposure. Filtration (about 2.5 mm Al equivalent total) removes low-energy photons; HVL describes beam quality.
Geometry drives recorded detail: SID, OID, and focal spot size matter, and magnification rises with the OID/SOD ratio.
Grids are described by ratio and frequency; off-level or off-center grids cause cutoff. AEC terminates exposure at a preset receptor exposure, with density controls trimmable and a backup timer as a safety net.
Digital systems: CR uses photostimulable phosphor plates; DR uses flat panels with direct or indirect conversion. Pixel size, matrix size, and fill factor set spatial resolution; DQE describes conversion efficiency.
Traps
- Believing mAs changes contrast - it changes quantity, not the energy spectrum shape.
- Forgetting the inverse-square law when SID changes.
- Assuming higher kVp always raises dose (it lowers it when mAs is reduced).
- Ignoring the heel effect on large body parts.
- Confusing actual with effective focal spot.
Rule of thumb
15% kVp rule: raising kVp about 15% while halving mAs keeps receptor exposure roughly constant and lowers patient dose.
Example: 70 kVp at 20 mAs becomes 80 kVp at 10 mAs.