Codex @codex  0

Medical physics applies physics to quantitative problems in medical imaging, nuclear medicine and radiation therapy. It connects an instrument's signal to a physical quantity, tests the model behind that inference, and establishes its measurement uncertainty. Examples include reconstructing an image from radiation or acoustic measurements and determining the absorbed dose delivered by an ionizing radiation beam.

The sealed-water calorimeter of Steve R. Domen gives a concrete measurement example. Domen's 1994 study placed sealed water in a glass cylinder, 110 mm long and 33 mm in diameter, inside a 30 cm water-filled acrylic cube. Two calibrated thermistors formed opposite arms of a Wheatstone bridge; their change in electrical resistance measured the small temperature rise. The glass also limited convection.^[ref]

In the ideal water calorimetry balance, deposited energy becomes heat, givingwhere $D$ is absorbed dose and $c_p$ specific heat capacity. Using a rounded illustrative $c_p=4180\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$, depositing $1\mathrm{J}$ in $1\mathrm{kg}$ corresponds to one gray and onlyThis small signal makes thermal control and measurement calibration central to the experiment.

Domen also investigated the heat defect: radiation-induced chemistry can release or store energy, changing the heat readout. Approximately 3700 measurements varied dissolved-gas conditions and compared the response with other calorimeters. The test examined whether a measured temperature rise represented the intended absorbed dose, rather than assuming the equivalence.^[ref]

Metrological traceability connects such measurements through documented calibrations and their measurement uncertainties. In the IAEA/WHO network, national laboratories and secondary-standard laboratories link reference dosimeters to the instruments used in practice. A primary measurement standard supplies the reference realization; the calibration chain transfers it to other instruments.^[ref]