Medical physics
= Medical physics
{wiki}
\b[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>.<A Sealed Water Calorimeter for Measuring Absorbed Dose by Steve R. Domen>[Sections1–2, pp.121–122: measurement principle, bridge and glass-container geometry]{ref}
In the ideal <water calorimetry> balance, deposited <energy> becomes <heat>, giving
$$
D=\frac{E}{m}=c_p\Delta T,
$$
where $D$ is <absorbed dose> and $c_p$ <specific heat capacity>. Using a rounded illustrative $c_p=4180\,\mathrm{J\,kg^{-1}\,K^{-1}}$, depositing $1\,\mathrm{J}$ in $1\,\mathrm{kg}$ corresponds to one <gray> and only
$$
\boxed{\Delta T=\frac{1}{4180}\,\mathrm K\simeq0.239\,\mathrm{mK}.}
$$
This 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.<A Sealed Water Calorimeter for Measuring Absorbed Dose by Steve R. Domen>[Abstract and Sections7–8: gas-dependent heat defect, comparisons and uncertainty assessment]{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.<Medical Physics and Dosimetry by the IAEA>[Main text: national laboratories, secondary standards and end-user dosimeter calibration]{ref}