CT flashcards

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

Hounsfield unit formula

HU = 1000 x (mu_tissue - mu_water) / mu_water. Water is 0 HU, air is -1000 HU, and dense bone is typically +1000 HU or higher. Values depend on beam energy and scanner calibration.

CTDIvol

Volume CT dose index estimates average dose per slice in a phantom, accounting for pitch. It is reported by the scanner and used with scan length to calculate DLP and effective dose.

DLP calculation

Dose-length product equals CTDIvol multiplied by scan length in centimeters. It reflects total radiation output for a series and is used to estimate effective dose with a body-region k-factor.

Pitch definition

Pitch equals table feed per gantry rotation divided by total beam collimation. Pitch greater than 1 reduces dose and scan time but may increase noise; pitch less than 1 increases dose.

Window width and level

Window level sets the center HU of the display range; window width sets the range of HU values shown. Narrow width increases contrast; wide width displays bone and soft tissue together.

Metformin and contrast

For patients with eGFR below 30 or acute kidney injury risk, hold metformin for 48 hours after iodinated contrast and resume only after renal function is confirmed stable.

Contrast extravasation response

Stop the injection, remove the cannula, elevate the limb, and apply a cold compress unless a warm-compress or hyaluronidase protocol is specified. Document volume and monitor for compartment syndrome.

Anaphylactoid reaction steps

Stop contrast administration, call for help, maintain the airway, give oxygen, and administer epinephrine per protocol for severe reactions. Monitor vital signs and document timing of events.

Pediatric dose reduction

Use weight- or size-based kV and mAs, limit scan length, avoid multiphase scanning, and apply iterative reconstruction. These measures lower dose while preserving diagnostic image quality in children.

Effective dose formula

Effective dose in mSv equals DLP multiplied by a body-region k-factor, approximately 0.015 for body and 0.0021 for head. It estimates stochastic risk from a CT study.

Helical pitch effect

Increasing pitch speeds table coverage and lowers dose but may raise image noise. Decreasing pitch increases overlap and dose while improving z-axis sampling and reformat quality.

Multidetector row CT

Multiple detector rows allow thinner slices, faster coverage, and improved z-axis resolution. Isotropic voxels enable high-quality multiplanar reformats and three-dimensional reconstructions.

Contrast phase timing

Non-contrast, arterial, venous, and delayed phases answer different clinical questions. Timing depends on circulation and injection rate; incorrect timing can miss vascular or tumor enhancement.

Iterative reconstruction

Iterative reconstruction reduces image noise and allows lower dose compared with filtered back projection. It can alter image texture, so protocols and kernels must be validated for each anatomy.

Beam hardening artifact

Dense bone or metal absorbs low-energy photons, causing streaks and dark bands. Centering the patient, using appropriate kV, and applying metal artifact reduction can reduce the effect.

Motion artifact

Patient movement or breathing during acquisition causes blurring and streaks. Clear breath-hold instructions, immobilization, and faster scan times reduce motion artifacts in CT.

Isocenter positioning

Centering the patient at gantry isocenter improves dose distribution and image quality. Off-center positioning can increase dose and produce shading or streak artifacts.

X-ray production

Electrons accelerated by high voltage strike a tungsten target, producing bremsstrahlung and characteristic radiation. kVp controls beam energy and penetration; mAs controls photon quantity.

Exponential attenuation

Intensity after attenuation follows I = I0 e^(-mu x). The linear attenuation coefficient depends on tissue density, atomic number, and photon energy; higher energy reduces photoelectric absorption.

Photoelectric effect

The photoelectric effect dominates at low photon energies and in high-atomic-number tissues. It increases patient dose and image contrast; higher kVp reduces its contribution.

Compton scatter

Compton scatter occurs when photons interact with outer-shell electrons, changing direction and contributing to image noise. Higher energy increases Compton relative to photoelectric interactions.

Detector types

Scintillation detectors convert x-rays to light and then to electrical signal. Solid-state and gas detectors differ in efficiency, stability, and afterglow, affecting image quality and calibration.

Spatial resolution

Spatial resolution is the ability to distinguish small high-contrast objects, measured in line pairs per centimeter. It depends on detector size, focal spot, slice thickness, and reconstruction.

Contrast resolution

Contrast resolution is the ability to distinguish tissues with small attenuation differences. It improves with higher dose, lower noise, and appropriate window settings and reconstruction kernels.

Reconstruction kernels

Soft-tissue kernels reduce noise but blur edges; bone kernels sharpen edges but increase noise. Kernel choice should match the anatomy and clinical question being evaluated.

Slice thickness tradeoff

Thinner slices improve z-axis resolution but increase noise and data volume. Overlapping reconstruction intervals improve multiplanar reformats without additional patient dose.

Partial volume artifact

Partial volume occurs when a voxel contains tissues of different attenuation, averaging their values. Thin slices and appropriate reconstruction reduce partial volume effects in small structures.

Ring artifact

Ring artifacts appear as concentric circles from detector calibration errors or a faulty channel. Phantom quality control scans can detect and localize the defective detector element.

Quality control phantom

A uniform phantom checks CT number accuracy, noise, and uniformity. Daily or periodic scans detect calibration drift and artifacts before they affect clinical images.

Diagnostic reference levels

Diagnostic reference levels are local or national dose benchmarks for common studies. Protocols exceeding them should be reviewed and optimized to keep dose as low as reasonably achievable.

Pregnancy and CT

CT is justified when medically necessary during pregnancy; diagnostic fetal risk is far below deterministic thresholds. Document justification, limit scan length, and use dose-reduction techniques.

Patient breath-hold instruction

Clear breath-hold instructions and confirmation of understanding reduce motion artifact. Explain warmth or flushing from contrast and the need to remain still before the scout image.