= Medical physicist
{tag=Physicist}
A \b[medical physicist] applies <physics> to measurement, modeling and instrumentation in medicine. Typical scientific work concerns <radiation therapy>, <medical imaging> and <nuclear medicine>, together with research and teaching. A <medical physicist> connects an instrument's readout with a defined physical quantity, tests the assumptions behind that conversion and evaluates its <measurement uncertainty>. The professional role includes investigating equipment performance and consulting on the physical basis of diagnosis and treatment.<What do Medical Physicists Do? by the American Association of Physicists in Medicine>[Clinical Service & Consultation, Research & Development and Teaching]{ref}
An <ionization chamber> illustrates the reasoning. <Ionizing radiation> generates mobile charges in a chamber; the collected <electric charge> is an instrument reading, not already an <absorbed dose>. In a stipulated reference setup, let $Q$ be the measured charge, $Q_0$ a matched background reading and $N$ a calibrated conversion coefficient. A simple measurement model is
$$
D=N(Q-Q_0).
$$
The coefficient is valid only for the conditions covered by its <measurement calibration>. The IAEA describes the documented chain linking reference laboratories, calibrated instruments and their users; moving a coefficient into a different beam or geometry requires an appropriate justification.<Medical Physics and Dosimetry by the IAEA>[One-page overview: absorbed-dose measurement and calibration chain]{ref}
For an original arithmetic example choose $Q=101\,\mathrm{nC}$, $Q_0=1\,\mathrm{nC}$ and $N=0.050\,\mathrm{Gy/nC}$. The model gives $D=5.00\,\mathrm{Gy}$. Ignoring that background gives $5.05\,\mathrm{Gy}$. These stipulated detector values illustrate <background subtraction> and dimensional conversion; they do not specify a treatment or a clinical beam-calibration protocol.
Suppose the corrected charge $q=Q-Q_0$ has <standard uncertainty> $u(q)=1\,\mathrm{nC}$, and independently $u(N)=0.0005\,\mathrm{Gy/nC}$. First-order <uncertainty propagation> gives
$$
u^2(D)=N^2u^2(q)+q^2u^2(N),\qquad u(D)=0.071\,\mathrm{Gy}.
$$
A covariance term must be included if the input estimates are correlated. The <medical physicist>'s task includes establishing whether that model describes the actual readout, rather than just multiplying a displayed number by a remembered constant.
A <water phantom> provides a reproducible medium for measurement comparisons; measurements at specified positions can test a beam model without equating the test object with every patient's anatomy. In <medical imaging>, analogous comparisons use known geometry or material properties to assess image formation and instrument response. Repeated equipment checks test constancy, while comparison against calibrated references tests accuracy. Research, reference measurements and clinical implementation therefore involve connected scientific questions with distinct evidence.
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