CAMRT RTR Exam key terms

A radiation protection principle meaning radiation exposure should be kept as low as reasonably achievable, considering economic and social factors. It guides shielding, technique selection, and workflow in every imaging modality.

The product of x-ray tube current in milliamperes and exposure time in seconds. It primarily controls the number of x-rays produced and therefore affects image noise and patient dose.

The peak kilovoltage applied across an x-ray tube. It determines the maximum photon energy and influences beam penetration, subject contrast, and the proportion of photoelectric versus Compton interactions.

Restricting the x-ray beam to the area of clinical interest using adjustable shutters or cones. It reduces patient dose, scatter radiation, and unnecessary irradiation of surrounding tissue.

A device placed between the patient and image receptor containing alternating radiopaque and radiolucent strips. It absorbs scattered radiation before it reaches the receptor, improving contrast at the cost of increased dose.

The ability of an imaging system to distinguish small objects that are close together. It is often described by the smallest resolvable line pair or pixel size and depends on detector, focal spot, and motion.

The ability to distinguish tissues with similar attenuation or signal intensity. It depends on factors such as dose, detector efficiency, windowing, and the intrinsic difference between adjacent structures.

Reduction in the intensity of a radiation beam as it passes through matter due to absorption and scattering. The amount depends on tissue density, atomic number, and photon energy.

Digital Imaging and Communications in Medicine: a standard format and communication protocol for storing, transferring, and displaying medical images and associated information across imaging systems.

Picture Archiving and Communication System: a networked system for storing, retrieving, and displaying medical images. It integrates with modalities and workstations using standards such as DICOM.

The process by which longitudinal magnetization recovers toward its equilibrium alignment after a radiofrequency pulse. It is characterized by the time constant T1, also called spin-lattice relaxation.

The decay of transverse magnetization after a radiofrequency pulse due to dephasing of spins. It is characterized by the time constant T2, also called spin-spin relaxation.

Repetition time is the time between successive radiofrequency excitation pulses in a pulse sequence. It strongly influences T1 weighting, signal-to-noise ratio, and scan time.

Echo time is the time from the center of the radiofrequency excitation pulse to the center of the acquired echo. It influences T2 weighting and signal-to-noise ratio.

The precessional frequency of a nuclear spin in a magnetic field, calculated as the gyromagnetic ratio multiplied by the magnetic field strength. It determines the resonant radiofrequency for MR imaging.

The wobbling motion of a nuclear magnetic moment around the direction of an applied magnetic field. In MRI, protons precess at the Larmor frequency and generate the measurable MR signal.

Applying a magnetic field gradient for a short time to make the MR signal phase depend on spatial position. It provides one dimension of spatial localization and is repeated for each image line.

Applying a magnetic field gradient during signal readout so that spins at different positions resonate at different frequencies. It provides one dimension of spatial localization in MR imaging.

Specific absorption rate is the rate at which radiofrequency energy is absorbed per unit mass of tissue, usually expressed in watts per kilogram. It is monitored to limit tissue heating during MRI.

A paramagnetic contrast agent used in MRI to shorten T1 relaxation times and increase signal on T1-weighted images. It is typically chelated to reduce toxicity and requires renal function screening.

A radioactive compound administered for diagnosis or therapy. It consists of a radionuclide attached to or combined with a carrier molecule that determines its biodistribution and target.

The time required for a radioactive substance to decay to half of its initial activity. It is a fixed property of the radionuclide and differs from biological and effective half-life.

An imaging device that detects gamma rays emitted from a radiopharmaceutical and produces planar nuclear medicine images. It uses a collimator, scintillation crystal, photomultiplier tubes, and image processing electronics.

A vacuum tube that converts light photons from a scintillation crystal into an electrical signal and amplifies it. An array of tubes provides position and energy information in gamma cameras and PET detectors.

Positron emission tomography is an imaging technique that detects pairs of annihilation photons produced by positron-emitting radionuclides. It provides functional and metabolic information with high sensitivity and quantitative potential.

Standardized uptake value is a semiquantitative measure of radiotracer accumulation in a lesion normalized to injected dose and patient size. It is commonly used in PET imaging to assess metabolic activity.

A device that accelerates electrons to high energy and directs them onto a target to produce x-rays, or uses the electrons directly for treatment. It delivers external beam radiation therapy.

The point in space about which a radiation therapy treatment unit rotates. Patient positioning and treatment planning aim to align the target with this point for accurate dose delivery.

Gross tumor volume is the visible or palpable extent of the tumor demonstrated by imaging or clinical examination. It represents the primary target that must receive a therapeutic radiation dose.

Clinical target volume is the gross tumor volume plus surrounding tissue that may contain microscopic tumor spread. It is defined by the radiation oncologist to guide treatment planning.

Planning target volume is the clinical target volume plus a margin for setup error, organ motion, and other uncertainties. It ensures the prescribed dose reaches the intended target despite daily variations.

Dividing a prescribed radiation dose into multiple smaller treatments, usually delivered once daily. It allows normal tissues to recover between fractions while improving tumor cell kill.

A radiation therapy technique in which a radioactive source is placed inside or very close to the tumor. It delivers a high local dose while limiting dose to surrounding normal tissues.

Intensity-modulated radiation therapy is an advanced external beam technique that varies radiation beam intensity across the field. It improves conformity of dose to the target and spares nearby organs at risk.

A tissue-equivalent material placed on the patient's skin to modify the radiation dose distribution. It is used to build up dose in superficial tissues or compensate for irregular body contours.

Dose-volume histogram is a graph or numerical summary showing the volume of a structure receiving a given radiation dose. It is used to evaluate treatment plan coverage and organs at risk.