Computed Tomography (CT): Physics, Instrumentation, Patient Care, and Protocols

Comprehensive vocabulary flashcards covering the core principles of computed tomography (CT), including scanner physics, instrumentation, image reconstruction, quality assurance, patient care, contrast kinetics, and specialized clinical imaging protocols.

Pitch

The travel distance of the CT scan table per $360^\circ$ rotation of the x-ray tube, divided by the x-ray beam collimation width.

Attenuation

The degree to which an x-ray beam is reduced in intensity by an object through absorption and scatter.

Linear Attenuation Coefficient ($\mu$)

The fraction of an x-ray beam that is scattered or absorbed per unit thickness of the absorber; it decreases with increasing photon energy and increases with increasing atomic number and physical density.

Spatial Resolution

The ability of a CT imaging system to resolve and define small, high-contrast objects as separate and distinct structures.

Contrast Resolution

The ability of a system to differentiate between objects with very similar densities or minimal density differences relative to their background.

Temporal Resolution

The speed with which data are acquired, primarily determined by gantry rotation speed, detector channel count, and signal recording capacity.

Volume Averaging

The process in CT whereby different tissue attenuation values within a voxel are averaged together to produce a single, less accurate pixel reading.

Slip Rings

Electromechanical devices consisting of brushes and circular conductive tracks that allow continuous rotation of the gantry frame, enabling helical CT scanning modes.

Data-Acquisition System (DAS)

The sub-system that measures the number of transmitted photons striking the detector array, converts electrical analog signals into digital signals, and transmits them to the computer.

Interpolation

A mathematical method of estimating unknown or missing data values between known data points.

Raw Data

All unmanipulated measurement data acquired directly by the detector array before computer processing into an image.

Image Data

The processed mathematical data that result after the computer reconstructs raw attenuation data into pixel values forming a viewable image.

Back Projection

A rudimentary reconstruction method (also known as the summation method) that projects ray sums back across the matrix, producing a characteristic star pattern artifact.

Filtered Back Projection

A mathematical reconstruction technique employing an analytic convolution filter to remove star pattern artifacts generated by simple back projection.

Iterative Reconstruction

An advanced reconstruction method that repeatedly corrects image estimates to reduce image noise, capable of lowering patient radiation dose by up to $50\%$.

Scan Field of View (SFOV)

The circular area within the gantry opening from which raw projection data are acquired during a scan.

Display Field of View (DFOV)

The specific region of raw projection data selected and magnified for reconstruction and visual display on the monitor.

Analog-to-Digital Converter (ADC)

A hardware device that converts continuous time-varying electrical analog signals from CT detectors into discrete numeric digital units.

Digital-to-Analog Converter (DAC)

A device that converts processed digital pixel numbers from computer memory into an analog format suitable for display on a video monitor.

Window Width (WW)

The range of Hounsfield units assigned to the display gray scale; widening the window width increases the scale of contrast (more shades of gray).

Window Level (WL)

The midpoint or central CT number of the displayed window width; increasing the window level increases the overall image brightness.

Reconstruction vs. Reformation

Reconstruction refers to processing raw projection data into image pixels; reformation refers to assembling existing image data into alternative planes or 3D displays.

Surface-Shaded Display (SSD)

A 3D rendering technique that models the outer shell of an anatomical structure using surface voxels while omitting underlying internal voxels.

Volume Rendering (VR)

A 3D post-processing technique in which all voxels within the volume contribute to the final image, allowing simultaneous visualization of multiple tissue types and spatial relationships.

Maximum Intensity Projection (MIP)

A post-processing rendering technique that projects rays through a volume of data, displaying only the voxel with the highest attenuation value along each projection path.

Minimum Intensity Projection (MinIP)

A post-processing rendering technique that selects and displays only the voxel with the lowest attenuation value along a ray path, optimal for evaluating the bronchial tree.

CTDIvol

Volume Computed Tomography Dose Index; the standardized dose descriptor used as the preferred metric for radiation dose estimation in CT dosimetry.

Bismuth Shield

A radioprotective superficial shield placed over radiosensitive organs such as the eyes, thyroid, and breasts to attenuate entrance radiation dose during medical CT examinations.

Beam-Hardening Artifact

An artifact appearing as broad dark streaks, cupping, or vague low-density areas caused by the preferential absorption of low-energy photons from a polychromatic x-ray beam.

Aliasing Artifact

An artifact characterized by fine striping lines across the image resulting from an insufficient number of angular or transmission samples.

Edge Gradient Effect

Streak artifacts radiating from high-contrast interfaces where extreme attenuation differences meet, such as dense barium adjacent to bowel gas.

Ring Artifact

A circular artifact consisting of one or more concentric rings centered on the axis of rotation, caused by a miscalibrated or defective detector element in third-generation scanners.

Tube Arcing

Severe image mottling or irregular streak artifacts without a fixed geometry caused by sudden electrical discharges or surges within the x-ray tube envelope.

Cone Beam Effect

A windmill-like distortion artifact seen exclusively in multidetector CT (MDCT) systems due to divergence of the x-ray beam along the z-axis.

Blood Urea Nitrogen (BUN)

A serum clinical laboratory test assessing renal function, with an approximate normal reference range of $7\text{ to }25\,\text{mg/dL}$.

Serum Creatinine

A metabolic waste product measured in blood to evaluate renal function prior to intravascular contrast media administration, normally ranging from $0.6\text{ to }1.7\,\text{mg/dL}$.

Prothrombin Time (PT)

A coagulation screening test measuring the extrinsic clotting cascade, with a normal reference range of $11\text{ to }14\,\text{seconds}$.

Partial Thromboplastin Time (PTT)

A laboratory blood test evaluating the intrinsic coagulation pathway, with a normal reference range of $25\text{ to }35\,\text{seconds}$.

Osmolality

A physical property of contrast media reflecting the number of active particles per unit mass of solvent, compared directly with blood plasma.

Chemotoxic Contrast Reaction

An adverse reaction driven directly by the physicochemical properties of contrast media (hyperosmolality, viscosity), dose, and injection rate, such as contrast-induced nephropathy.

Idiosyncratic Contrast Reaction

An unpredictable, non-dose-dependent adverse reaction resembling an allergic or anaphylactoid response, typically occurring within 1 hour of contrast administration.

Contrast-Induced Nephropathy (CIN)

Acute impairment of renal function precipitated by intravascular iodinated contrast administration, mitigated by hydration and the use of LOCM or IOCM.

Barium Peritonitis

An inflammatory complication carrying a high mortality rate caused by the leakage of insoluble barium sulfate into the peritoneal cavity through a gastrointestinal perforation.

Supraorbital Meatal Line (SOML)

The cranial positioning line preferred over the orbitomeatal line for angling axial brain CT slices to reduce radiation exposure to the lenses of the eyes.

High-Resolution CT (HRCT)

A specialized chest CT imaging protocol utilizing narrow collimation and high spatial frequency reconstruction algorithms to examine lung parenchyma for diffuse interstitial lung disease.

Prospective ECG-Triggering

A cardiac CT scan technique where x-ray exposure is sequentially pulsed only during a predetermined quiescent cardiac phase (usually end-diastole), delivering a low radiation dose.

Retrospective ECG-Gating

A cardiac CT scanning mode that delivers continuous radiation throughout the cardiac cycle while simultaneously recording the ECG trace, permitting functional cardiac analysis at the cost of higher patient dose.

Fatty Infiltration of the Liver

A hepatic pathology confirmed on unenhanced CT when the attenuation of the liver measures at least $10\,\text{HU}$ lower than that of the spleen.

McBurney's Sign

A clinical indication of acute appendicitis manifested as localized tenderness or abdominal guarding upon palpation of the right lower quadrant.

Rovsing's Sign

A diagnostic sign of acute appendicitis wherein palpation of the left lower quadrant elicits referred pain in the right lower quadrant.

Dunphy's Sign

A physical finding of acute appendicitis characterized by sharp exacerbation of right lower quadrant abdominal pain provoked by coughing.

Corticomedullary Phase

The early renal enhancement phase occurring approximately $30\text{ to }45\,\text{seconds}$ ($30\text{ to }70\,\text{seconds}$ in multiphasic tables) after IV contrast injection, displaying marked differentiation between renal cortex and medulla.

Nephrographic Phase

The parenchymal phase of renal enhancement occurring approximately $80\text{ to }130\,\text{seconds}$ post-injection, representing the optimal window for evaluating renal masses and parenchymal disease.

Excretory Phase

The delayed phase of renal enhancement appearing at approximately $3\text{ to }15\,\text{minutes}$, when contrast fills the renal calyces, pelvis, and ureters as parenchymal enhancement fades.

Calcium Score

A standardized quantification of the extent of mineralized atherosclerotic calcification present within coronary artery walls on cardiac CT.

Hounsfield Units (HU)

The numeric scale used in CT to quantify linear attenuation relative to water ($0\,\text{HU}$) and air ($-1000\,\text{HU}$).

CT Scanner Generations

The classification of CT scanner designs based on tube and detector geometry, progressing from translate-rotate single pencil beams ($1^{\text{st}}$) to stationary continuous $360^\circ$ detector rings ($4^{\text{th}}$).

Digital Imaging System Workflow

The hardware architecture by which analog detector signals are converted via an ADC into digital data for computer processing, and later routed through a DAC for display.

Renal Enhancement Phases

The temporal sequence of renal opacification following IV contrast administration, comprising noncontrast, corticomedullary, nephrographic, and excretory phases.