CT key terms

A standardized dose descriptor that estimates the average absorbed dose within a scanned volume for one rotation, accounting for pitch. It is displayed on the scanner console and used to compare protocols, but it does not represent patient size or the dose to a specific organ.

The product of CTDIvol and the scanned length in centimeters, expressed in mGy·cm. It reflects total radiation output over a whole examination and is converted to effective dose using a body-region conversion coefficient.

The principle of keeping radiation exposure as low as reasonably achievable while still obtaining diagnostic image quality. In CT it drives protocol optimization, limiting scan length, lowering tube current where possible, and avoiding unnecessary multiphase studies.

A benchmark dose value, usually set at the 75th percentile of a survey of facilities, used to flag protocols that deliver unusually high dose for a given examination. Exceeding a DRL prompts review rather than proving unsafe practice.

A system that modulates tube current during a scan, using a scout image or a localizer to estimate patient attenuation and adjust milliamperage per rotation. It reduces dose and improves noise uniformity across body regions of differing thickness.

For helical scanning, the table feed per gantry rotation divided by the total collimated beam width. A pitch below 1 overlaps data and increases dose; a pitch above 1 reduces dose but may increase noise and artifacts.

A acquisition mode in which the table moves continuously while the tube and detectors rotate, producing a volume of projection data. Images are reconstructed at any position along the volume, enabling overlapping slices and multiplanar reformats without gaps.

A mode in which the table remains stationary during each rotation and advances between rotations. It avoids the continuous tube output of helical scanning and is common in cardiac gating and low-dose screening protocols.

A scaled attenuation value in which water is 0 and air is approximately -1000, with dense bone reaching well above +1000. It allows tissue characterization and window setting independent of the display device.

Display settings that map the CT number range to visible gray shades. Level sets the center of the displayed range and width sets its span; narrow widths increase contrast but reduce the range of densities shown.

A measure of how much a material reduces X-ray intensity per unit path length, depending on photon energy, density, and atomic number. CT reconstruction converts measured attenuation into tissue values relative to water.

The two dominant X-ray interaction processes in the diagnostic energy range. Photoelectric absorption rises sharply with atomic number and low energy, giving bone and iodine high contrast; Compton scatter depends mainly on electron density and contributes to image noise.

The increase in mean photon energy as low-energy photons are preferentially absorbed while passing through tissue. It produces cupping and dark streaks between dense structures, corrected by filtration, calibration, and iterative or dual-energy methods.

Random variation in pixel values that gives images a grainy appearance, quantified as the standard deviation of CT numbers in a uniform region. Noise falls as tube current, slice thickness, or reconstruction kernel smoothing increases.

The ratio of the difference in CT number between a structure and its background to the image noise. Higher SNR indicates a structure is more reliably detectable, and it can be improved with more photons or thicker slices.

The difference in CT number between two tissues divided by the background noise. It expresses how visible a lesion is relative to surrounding tissue and is improved by contrast timing, photon count, and appropriate windowing.

A mathematical filter applied during image reconstruction that shapes spatial resolution and noise texture. Sharp or bone kernels enhance edges at the cost of noise, while smooth or soft-tissue kernels reduce noise and blur fine detail.

A reconstruction approach that repeatedly compares modeled projections with measured data to reduce noise while preserving detail. It permits lower tube current than filtered back projection but can alter noise texture and is vendor-specific.

The classical CT reconstruction method that back-projects filtered projection data to form an image. It is fast and well understood but amplifies noise, which newer iterative and model-based methods address.

The ability to distinguish two small, closely spaced objects as separate, often expressed as line pairs per centimeter or millimeters. In CT it is limited by detector aperture, focal spot size, slice thickness, and reconstruction kernel.

The nominal thickness of a reconstructed image and the physical width of the X-ray beam defined by collimators. Thinner slices improve z-axis resolution and reduce partial volume averaging but increase noise and dose per image.

Blending of attenuation values when a voxel contains more than one tissue type, causing a structure to take an intermediate CT number. It is reduced by thinner slices and is a common cause of missed small lesions.

The ability to resolve structures along the patient's long axis, determined mainly by slice thickness and detector configuration. It is critical for reformats, small-lesion detection, and accurate volume measurement.

Creation of coronal, sagittal, or oblique images from a stack of thin axial slices. Quality depends on thin slices and isotropic voxels, and MPR is routine in trauma, vascular, and orthopedic protocols.

A rendering technique that displays the brightest voxel along each line of sight, making vessels and high-density structures stand out. It is widely used for CT angiography and for highlighting lung nodules or bone.

A three-dimensional display method that assigns color and opacity to voxel values to depict surfaces and internal structures. It aids surgical planning and communication but is a visualization tool, not primary diagnostic data.

A drawn area in which mean CT number and standard deviation are calculated. It is used to characterize lesions, verify contrast enhancement, and measure image noise in quality assurance.

Intravascular or oral agents containing iodine that increase X-ray attenuation, used to opacify vessels and enhance tissue. Risks include contrast-induced kidney injury and allergic-like reactions, and patients require screening before administration.

An acute rise in serum creatinine after iodinated contrast administration, typically within days. Risk is higher with pre-existing renal impairment, diabetes, dehydration, and high contrast volume, so renal function is checked and volume minimized.

Leakage of contrast from a vein into surrounding soft tissue during injection, causing pain, swelling, and rarely compartment syndrome. Management includes stopping the injection, elevating the limb, cold compresses, and monitoring.

The process of explaining a procedure's purpose, risks, benefits, and alternatives so the patient can decide voluntarily. For CT it covers contrast risks, radiation, and alternatives, and is documented before the examination.

The practice of asking patients of childbearing age about possible pregnancy before scanning and weighing whether the examination is justified. If needed, the scan is modified to limit dose to the pelvis and fetus while preserving diagnostic value.

A whole-body equivalent dose that reflects the relative radiation risk of a partial-body exposure, expressed in millisieverts. It allows comparison across modalities but is a population estimate, not a patient-specific risk.

Bright and dark streaks radiating from dense implanted material such as hardware, dental fillings, or clips. It degrades adjacent tissue visibility and is reduced by thinner slices, higher tube output, and dedicated metal-reduction algorithms.

Blurring, ghosting, or misregistration caused by patient movement, respiration, or cardiac motion during acquisition. It is limited by breath-hold coaching, immobilization, faster gantry rotation, and ECG gating.

A cardiac CT technique in which the tube is triggered during a selected phase of the cardiac cycle, usually diastole. It lowers dose compared with retrospective gating but requires a stable, slow heart rate and regular rhythm.

A cardiac CT technique in which projection data are acquired continuously and images are reconstructed at chosen cardiac phases. It allows functional assessment and works with irregular rhythms but delivers a higher radiation dose.

A contrast-enhanced study timed so that arteries are opacified during acquisition, often followed by thin-slice reformats. It requires accurate bolus timing or bolus tracking and careful attention to renal function and radiation dose.

A method for timing contrast acquisition by monitoring a vessel's CT number after injection and triggering when a threshold is reached, or by using a small test injection. It helps ensure peak arterial enhancement during the scan.