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A position-sensitive detector, usually using a collimator, scintillator and light sensors, that forms a projection image of a gamma-emitting tracer. Energy selection reduces scattered-photon contamination.

A homogeneous calorimeter uses its showering volume as the active medium, such as a scintillating crystal. It avoids absorber-versus-active sampling fluctuations, but still has signal statistics, intrinsic shower fluctuations, noise and calibration nonuniformity. Its constant and noise terms need not be smaller than those of a sampling calorimeter.

For independent contributions the parametrization meansThe stochastic coefficient $A$ includes fluctuations in the sampled shower fraction, the finite number of signal quanta, and intrinsic shower fluctuations. A number of detected quanta proportional to $E$ has fluctuations proportional to $\sqrt{E}$, giving relative resolution proportional to $E^{-1/2}$. The noise coefficient $B$ describes an approximately fixed energy-equivalent readout uncertainty, including electronic noise and, for specified running conditions, Pile-up contributions. The constant term $C$ represents residual fractional nonuniformity, calibration and response variations, imperfect correction for inactive material, and leakage effects over the fitted range. The simple three-term model is an approximation; leakage and Pile-up need not follow these powers exactly.

A homogeneous calorimeter uses the showering material itself as the active medium, for example a scintillating crystal. It removes the absorber-versus-active sampling fluctuation and can have a much smaller stochastic term, but it does not eliminate signal statistics or intrinsic shower fluctuations. Its noise and constant terms are not automatically smaller: they depend on light collection, calibration, temperature, radiation damage, electronics and geometry. Homogeneous detectors can offer excellent energy resolution and compact construction, but large crystal volumes can be costly and fine longitudinal segmentation difficult. Sampling detectors often provide economical large coverage and flexible segmentation with robust absorbers, at the cost of sampling fluctuations and inactive material. If energy is expressed in GeV, $A$ has units ${\mathrm{GeV}}^{1/2}$, $B$ has units GeV and $C$ is dimensionless.

After injection, the radiopharmaceutical circulates and accumulates according to its biological targeting and clearance. Technetium-99m undergoes an isomeric nuclear transition, producing approximately $140\mathrm{keV}$ gamma rays; its roughly six-hour half-life permits useful counting with limited persistence. Some photons undergo absorption or Compton scattering in the patient. A lead collimator in the gamma camera accepts a narrow range of directions so the accepted event retains positional information.

An accepted gamma ray deposits energy, commonly in a NaI(Tl) scintillator, by photoelectric effect or Compton scattering. The resulting visible particle scintillation light is collected by photomultipliers. Their relative pulse sizes determine an event position and their summed pulse estimates its energy. A pulse-height analyser accepts an energy window around the photopeak to reject many scattered events. Position and uniformity corrections are applied; accepted events increment a two-dimensional pixel histogram. After acquisition the computer displays this count map as a planar projection of tracer activity. Finite counting statistics, attenuation, scattering and the collimator limit the image's quantitative accuracy and spatial resolution.^[ref]