Image Production and Evaluation
Core concepts
Image production ties together exposure factors (kVp, mAs, SID, OID), density/brightness, contrast, recorded detail, and distortion. kVp controls penetration and contrast scale; mAs controls quantity of photons and density. Short SID and large OID reduce sharpness; angulation and misalignment cause distortion (shape and size). Know the anode heel effect and how it relates to patient orientation. Understand the histogram and exposure indicator concepts at a basic level: underexposure produces noise, overexposure risks dose without added diagnostic value.
Traps
A common trap is assuming more mAs always improves the image - it increases dose and may not fix a penetration problem. Another is confusing spatial resolution with contrast resolution. Questions also test the effect of grid use on contrast and dose, and the difference between size distortion (SID/OID) and shape distortion (angulation/alignment). Watch for wording about motion - it blurs detail and cannot be corrected after the fact.
Worked example / rule of thumb
An image is light with poor penetration of a thick part. Increase kVp modestly first to penetrate, then adjust mAs for density. Rule of thumb: kVp for penetration and contrast, mAs for density, SID/OID for sharpness, alignment for distortion.
Key points
- kVp controls penetration and contrast; mAs controls density.
- Long SID and small OID improve sharpness.
- Angulation and misalignment cause shape distortion.
- Motion blurs detail; prevent it with communication and immobilization.
- Grids improve contrast but increase dose.