Gamma Camera Components and Operation
Core concepts
A gamma camera comprises a collimator, scintillation crystal (typically NaI(Tl)), light guide, PMT array, and positioning/energy circuitry. The collimator defines spatial resolution and sensitivity: parallel-hole (most common), pinhole (magnification, small organs), converging/diverging, and slant-hole. Septal thickness determines septal penetration; hole length, diameter, and septa trade resolution against sensitivity. The crystal converts gamma energy to light; the PMT array localizes the event via Anger logic (weighted centroid of PMT signals). Pulse-height analysis accepts events within the energy window (commonly 20% centered on the photopeak). Uniformity corrections (flood) and linearity corrections (spatial distortion) are applied.
Traps
- Higher-resolution collimators have lower sensitivity - questions often ask the trade-off direction.
- Pinhole collimators invert the image and magnify; they are used for thyroid or small parts, not whole-body imaging.
- Confusing spatial resolution (FWHM in mm) with energy resolution.
- Forgetting that septal penetration worsens with thinner septa and higher-energy isotopes.
Rule of thumb
For parallel-hole collimators, resolution improves (smaller FWHM) as hole diameter decreases and hole length increases - but sensitivity falls.
Worked example: A low-energy high-resolution (LEHR) collimator gives better FWHM than a low-energy all-purpose (LEAP) collimator but lower sensitivity, so count-poor studies may favor LEAP.