ABR DR Core key terms

Interaction in which an incident photon transfers its entire energy to an inner-shell electron, ejecting it; probability rises sharply with atomic number and falls rapidly as photon energy increases, producing high subject contrast.

Interaction in which a photon transfers part of its energy to an outer-shell electron and is deflected; probability depends on electron density and is nearly independent of atomic number, degrading contrast and creating occupational dose.

Sudden increase in attenuation at the photon energy just above the binding energy of a shell's electrons, named for the K shell; exploited by iodine and barium to boost contrast at diagnostic energies.

Probability per unit path length that a photon interacts while traveling through material; depends on photon energy and material density, and is the basis for exponential attenuation of a beam.

Thickness of a specified material that reduces the intensity of a beam to half its original value; used to characterize beam quality and required filtration in radiography.

Progressive shift of a polyenergetic x-ray beam toward higher average energy as low-energy photons are preferentially absorbed in tissue; causes cupping and streak artifacts in CT.

Scaled CT attenuation value defined so that water is 0 and air is approximately -1000; each unit represents about 0.1 percent difference in attenuation from water.

Display settings that map the CT attenuation range to visible gray shades; level sets the center of the range and width sets its span, controlling contrast and brightness on the monitor.

Curve describing how well an imaging system preserves the contrast of structures at each spatial frequency; it quantifies spatial resolution and falls toward zero at the limiting frequency.

Ratio of the squared signal-to-noise ratio at the detector output to that at its input, expressed as a percentage; a combined measure of a detector's dose efficiency and image quality.

Speckled noise in an image caused by statistical fluctuation in the number of detected photons; becomes prominent when too few photons reach the detector and is reduced by increasing dose or pixel area.

Artifact in which spatial frequencies above the sampling limit are misrepresented as lower frequencies; prevented by adequate sampling or by filtering before digitization.

Time constant describing recovery of longitudinal magnetization after an radiofrequency pulse; differs among tissues and is the primary contrast source in spin-echo and inversion-recovery imaging.

Time constant describing decay of transverse magnetization from spin-spin dephasing after an radiofrequency pulse; short in solids and long in fluids such as cerebrospinal fluid.

Pulse sequence that refocuses transverse magnetization by reversing a magnetic field gradient rather than using a 180-degree pulse; allows short repetition times and is sensitive to field inhomogeneity, producing T2-star weighting.

Radiofrequency energy deposited per unit mass of tissue per unit time, usually in watts per kilogram; regulatory limits constrain SAR to prevent heating during MRI.

Interval between the radiofrequency excitation pulse and the center of the acquired echo; controls the degree of T1 or T2 weighting in MRI.

Interval between successive radiofrequency excitation pulses in a pulse sequence; determines how much longitudinal magnetization recovers before the next pulse and influences T1 weighting and scan time.

Average energy deposited per unit path length by ionizing radiation; high-LET radiations such as alpha particles and neutrons cause dense ionization and greater biological damage per unit dose.

Ratio of a reference radiation dose to the test radiation dose producing the same biological effect; increases with linear energy transfer and is used to derive equivalent dose.

Ratio of dose required to produce a given effect under hypoxic versus well-oxygenated conditions; typically about three for low-LET radiation and near one for high-LET radiation.

Dose quantity obtained by multiplying absorbed dose by a radiation weighting factor to account for biological effectiveness; expressed in sieverts and used for radiation protection purposes.

Sum of tissue-weighted equivalent doses across organs, reflecting overall stochastic risk from a nonuniform exposure; expressed in sieverts and used to compare imaging procedures.

Dose at which linear and quadratic components of cell killing are equal in the linear-quadratic model; low values indicate strong fractionation sensitivity, typical of late-responding tissues and prostate cancer.

Dose metric derived from the linear-quadratic model that accounts for fraction size and number, allowing comparison of regimens with different fractionation; expressed in gray equivalents.

Mathematical model in which cell survival after radiation is described by a linear and a quadratic term in dose; predicts survival for different fractionation schemes and underlies biologically effective dose calculations.

Principle stating that cells are most radiosensitive when they are rapidly dividing, undifferentiated, and have a long proliferative future; explains sensitivity patterns of tissues and tumors.

Probability that a tumor is eradicated by a given radiation dose; rises sigmoidally with dose and depends on tumor cell number and intrinsic radiosensitivity.

Modeled probability that a specified normal tissue develops a defined complication after a given dose distribution; used to compare treatment plans and set dose constraints.

Sum of effective doses to all exposed individuals in a population, expressed in person-sieverts; used to estimate population-level stochastic risk from a practice or event.

Principle of keeping radiation doses as low as reasonably achievable, considering economic and social factors; applied through time, distance, and shielding in occupational and patient protection.

Core radiation protection principle requiring that a proposed exposure produce more benefit than harm; for medical imaging, that the examination is appropriate for the clinical question.

Benchmark dose or activity for a given examination and patient group, usually set near the upper quartile of observed values; used to identify practices with unusually high exposure.

Process of disclosing material risks, benefits, and alternatives to a patient and confirming voluntary agreement before a procedure; required for contrast administration and invasive interventions per local policy.

Acute kidney injury occurring after intravascular iodinated contrast administration, attributed to the contrast after other causes are excluded; risk is highest with preexisting renal impairment, diabetes, and dehydration.

Retention of small amounts of gadolinium in tissues such as brain and bone after administration of linear agents; motivates preferential use of macrocyclic agents and avoidance of unnecessary scans.

Rare fibrosing disorder of skin and organs associated with gadolinium exposure in patients with severe renal impairment; risk is minimized by screening renal function and using macrocyclic agents.

Standardized reporting system for prostate multiparametric MRI that assigns a category from 1 to 5 reflecting the likelihood that a lesion is clinically significant cancer, guiding biopsy decisions.

Standardized system for interpreting liver imaging in patients at risk for hepatocellular carcinoma, categorizing observations from definitely benign to definitely malignant and guiding management.

Standardized lexicon and final assessment category for breast imaging, ranging from 0 for incomplete to 6 for known biopsy-proven malignancy, with defined management recommendations for each category.

Fluoroscopic examination of the colon using high-density barium and insufflated air to coat the mucosa; largely replaced by colonoscopy and CT colonography but still used selectively.

Substance administered to alter tissue attenuation or signal and improve conspicuity of structures; examples include iodinated compounds for radiography and CT and gadolinium chelates for MRI.

Ratio of the strength of the desired signal to the level of background noise in an image; higher values improve lesion detectability and can be traded against resolution or scan time.