Diagnostic Radiology Physics Flashcards

A set of vocabulary flashcards covering the core physical concepts, terminology, and mathematical definitions used in Diagnostic Radiology as presented in the lecture transcript.

Kerma

An acronym for Kinetic Energy Released per unit Mass, defined as $K = \frac{d\epsilon_{tr}}{dm}$, representing the liberation of energy in the form of kinetic energy in the medium.

Absorbed Dose

The mean energy imparted into a medium per unit mass, defined as $D = \frac{d\epsilon}{dm}$. The unit is the Gray ($J\,kg^{-1}$).

Ionization in air

A measurement technique for radiation sensitive to energy deposition ($\sim34\,eV$ per ion pair) with an effective atomic number ($effZ \approx 7.6$) similar to biological soft tissue ($effZ \approx 7.4$).

Fluence

The number of photons $dN$ cross-sectional to an area $da$, expressed as $\Phi = \frac{dN}{da}$, with units of $m^{-2}$.

Energy Fluence

The radiant energy $dR$ (where $dR = E \times dN$) per unit area $da$, expressed as $\Psi = \frac{dR}{da}$, with units of $J\,m^{-2}$.

Mass Attenuation Coefficient

The linear attenuation coefficient ($\mu$) normalized to density ($\rho$), expressed as $\mu/\rho$ with dimensions of $cm^2/g$.

Radiation Exposure (X)

The sum of all electrical charges of one sign produced in air per unit mass, defined as $X = \frac{\Delta Q}{\Delta m}$ and measured in $C\,kg^{-1}$.

Bremsstrahlung

Meaning 'breaking radiation', it is produced when a charged particle interacts with the nucleus via the Coulomb force, emitting a photon with energy equivalent to the change in the electron's kinetic energy.

Characteristic X-ray

A photon produced when an electron drops from a higher shell to fill a vacancy in a lower shell, with energy equal to the difference in binding energies unique to the element.

Heel Effect

An effect where X-ray flux is higher on the cathode side than the anode side because photons produced deeper in the target or at certain angles undergo more self-attenuation within the anode.

Geometric Unsharpness ($U_g$)

Blurring caused by the finite size of the focal spot, calculated as $U_g = \text{effective focal spot size} \times \frac{ODD}{FOD}$.

Modulation Transfer Function (MTF)

The ability of an imaging system to transfer object contrast to the image across various spatial frequencies; it is the magnitude response of a system to sinusoids.

Detective Quantum Efficiency (DQE)

A measure of the fidelity of a system in transferring incident Signal-to-Noise Ratio (SNR) to the image SNR, defined as $DQE = \frac{SNR_{out}^2}{SNR_{in}^2}$.

Hounsfield Unit (HU)

A normalized value of the linear attenuation coefficient in CT, calculated as $HU = 1000 \times \frac{\mu_{med} - \mu_{water}}{\mu_{water}}$, where water is fixed at $0$ and air at $-1000$.

Pitch (CT)

In helical CT, the ratio of couch movement per rotation to the physical X-ray beam width.

Mean Glandular Dose (MGD)

The primary dosimetric quantity in mammography, calculated as $MGD = K \times g \times c \times s$, where $K$ is the incident air kerma and $g, c, s$ are correction factors.

Contrast to Noise Ratio (CNR)

A metric used to quantify image quality, calculated as $\text{CNR} = \frac{\text{MPV}<em>{background} - \text{MPV}</em>{detail}}{\sigma_{background}}$, where MPV is Mean Pixel Value and $\sigma$ is standard deviation.

Dose Detector Index (DDI)

A manufacturer-specific measure (also known as Exposure Index) based on the median kerma ($K_{IND}$) at the detector within a segmented anatomical region.

Deviation Index (DI)

A universal indicator of how much the actual exposure deviates from the target exposure, defined as $DI = 10 \times \log_{10}\left(\frac{K_{IND}}{K_{TGT}}\right)$.

Fano Theorem

A theorem stating that in a medium of given composition exposed to a uniform field of primary radiation, the secondary radiation field is uniform and independent of density variations.

Bragg-Gray Cavity Theory

A theory for small detectors where the absorbed dose in the medium is related to the dose in the cavity by the ratio of their mass stopping powers, assuming the cavity does not perturb the charged particle fluence.

Flat-Panel Detector (FPD)

A digital X-ray detector consisting of a matrix of pixels, either using a scintillator (indirect) or a photoconductor like amorphous Selenium (direct) to convert X-rays into electrical charge.