CAMRT RTR Exam flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Milliampere-second (mAs)

The product of tube current and exposure time. It determines the total number of x-ray photons produced and therefore beam quantity and receptor dose without substantially changing beam penetrability.

Inverse square law

X-ray intensity from a point source falls in proportion to the square of the distance. Doubling the distance from the source reduces intensity to one quarter of its original value.

Grid ratio

The height of the lead strips divided by the distance between them. Higher ratios remove more scattered radiation and improve contrast, but require greater mAs and increase patient dose.

Photoelectric absorption

An incident photon is completely absorbed and ejects an inner-shell electron. Probability rises steeply with atomic number, roughly as Z cubed, and falls with increasing photon energy.

Compton scattering

A photon transfers part of its energy to an outer-shell electron and is deflected. Probability depends mainly on electron density, changes little with atomic number, and dominates at higher diagnostic energies.

Magnification

The ratio of image size to object size, equal to source-to-image distance divided by source-to-object distance, where source-to-object distance is SID minus object-to-image distance.

CT pitch

Table travel per gantry rotation divided by total beam collimation. Pitch below one means overlapping slices and higher dose; pitch above one means gaps and lower dose.

Window width and level

Window level sets the CT number displayed as mid-grey, controlling brightness. Window width sets the range of CT numbers mapped to grey shades, controlling contrast and displayed latitude.

Larmor equation

Precessional frequency equals the gyromagnetic ratio multiplied by magnetic field strength. For hydrogen-1 the ratio is about 42.58 MHz per tesla, giving 63.9 MHz at 1.5 T.

T1 relaxation time

The time constant for recovery of longitudinal magnetization after a radiofrequency pulse, with about 63% recovered after one T1. T1 lengthens as field strength increases.

T2 star decay

Observed transverse decay that combines true T2 dephasing with additional dephasing from static field inhomogeneities. Gradient-echo sequences show T2 star contrast; spin echoes refocus the inhomogeneity component.

Ernst angle

The flip angle producing maximum signal for a given repetition time and tissue T1 in gradient-echo imaging. It becomes smaller as repetition time shortens.

STIR fat suppression

Short-tau inversion recovery applies an inversion pulse timed so fat magnetization passes through zero at excitation. It suppresses fat uniformly despite field inhomogeneity but lowers signal-to-noise ratio.

Specific absorption rate

Radiofrequency power deposited per unit mass, measured in watts per kilogram. It increases with field strength squared, flip angle, duty cycle and slice number, and is capped by safety standards.

Gadolinium chelates

Paramagnetic contrast agents that shorten T1 and T2 relaxation times, brightening tissue on T1-weighted images. They are avoided in severe renal impairment because of nephrogenic systemic fibrosis risk.

K-space center

The central lines of k-space encode image contrast and overall signal-to-noise, while the outer lines encode edges and fine detail. Centre-first filling produces high contrast early in the acquisition.

Technetium-99m

A generator-produced radionuclide with a 6.02-hour half-life and a 140 keV gamma emission, well matched to gamma camera imaging. It is the most widely used tracer in nuclear medicine.

Effective half-life

The combined result of physical decay and biological clearance, calculated as the reciprocal sum of the physical and biological half-lives. It determines how long activity remains in the patient.

Positron annihilation

A positron emitted by a proton-rich radionuclide travels a short distance, combines with an electron, and produces two 511 keV photons emitted about 180 degrees apart, detected in coincidence by PET.

Photopeak energy window

A pulse-height analyser setting that accepts events within a narrow range, typically plus or minus 10%, around the photopeak energy to include primary photons while rejecting scattered events.

Random coincidence

Two unrelated photons from separate annihilations detected within the coincidence timing window. The random rate rises with the square of activity and is reduced by shorter timing windows.

Counting statistics

Radioactive decay follows Poisson statistics, so the standard deviation of a count equals its square root. Relative uncertainty therefore falls as the total number of counts increases.

Molybdenum-technetium generator

A column containing molybdenum-99, half-life 66 hours, which decays to technetium-99m eluted with saline. Elution efficiency and molybdenum breakthrough are checked by routine quality control.

Fluorodeoxyglucose

A glucose analogue labelled with fluorine-18, half-life 110 minutes. It accumulates where glucose metabolism is increased, so fasting and glucose control before scanning are essential.

Gray

The SI unit of absorbed dose, equal to one joule per kilogram. One gray equals 100 rad, and a typical curative external beam course delivers 60 to 70 gray.

Biologically effective dose

Calculated as n times d times (1 plus d divided by alpha-beta), where n is fraction number and d is dose per fraction. It compares schedules differing in fraction size.

Planning target volume

A geometric volume adding set-up and organ-motion margins around the clinical target volume, ensuring the prescribed dose covers the target despite daily positioning uncertainty.

Percentage depth dose

The absorbed dose at a given depth expressed as a percentage of the maximum dose on the central axis. It increases with beam energy, field size and source-to-surface distance.

Alpha-beta ratio

A radiosensitivity parameter describing cell survival curve shape. Values near 10 gray indicate tumours and early-responding tissues; near 3 gray indicate late-responding tissues.

Bolus

Tissue-equivalent material placed on the skin surface to build up dose in superficial layers, bringing the maximum dose to the skin when treating lesions with megavoltage beams.

Inverse planning

An optimisation method used in IMRT where dose objectives and constraints are specified first, and the computer calculates the fluence maps instead of the planner shaping beams manually.

Brachytherapy dose falloff

Dose around an implanted source follows approximately the inverse square of distance, giving very high dose close to the source and rapid sparing a few centimetres away.