CT cheat sheet

IV contrast extravasation: stop injection immediately, remove cannula, elevate the limb, apply a cold compress for most agents (warm compress for hyaluronidase-assisted protocols), document volume and site, and monitor for compartment syndrome signs.

Metformin: hold for 48 hours after iodinated contrast in patients with eGFR <30 or AKI risk; resume only after renal function is confirmed stable.

Pregnancy: CT is justified when the study is medically necessary; risk of fetal harm from a diagnostic scan is far below the threshold for deterministic effects, but document justification and use dose reduction.

Pediatric dose reduction: use weight- or size-based kV/mAs protocols, limit scan length, avoid multiphase scanning, and use iterative reconstruction to lower dose while preserving diagnostic quality.

Contrast-induced nephropathy risk is highest with eGFR <30; use lowest contrast volume, isotonic saline hydration, and avoid repeat contrast within 24-48 hours when possible.

Radiation dose quantities: CTDIvol reflects dose per slice; DLP = CTDIvol x scan length; effective dose (mSv) = DLP x k-factor (body ~0.015, head ~0.0021).

Patient communication: explain breath-hold instructions, warmth or flushing from contrast, and the need to remain still; use a calm, clear voice and confirm understanding before the scout.

Anaphylactoid reaction management: stop contrast, call for help, maintain airway, give oxygen, and administer epinephrine per protocol for severe reactions; monitor vitals and document timing.

CT numbers (Hounsfield units): water = 0 HU, air = -1000 HU, bone = +1000 HU or higher; window width and level are set to display the tissue of interest.

Window settings: WL centers the display range; WW controls contrast. Narrow WW increases contrast but may obscure soft tissue; wide WW shows both bone and soft tissue.

Helical pitch = table feed per rotation / beam collimation; pitch >1 speeds coverage and lowers dose but may increase noise; pitch <1 increases dose and overlap.

Multidetector row CT: more detector rows allow thinner slices, faster coverage, and improved z-axis resolution; use isotropic voxels for multiplanar reformats.

Protocol selection: choose non-contrast, arterial, venous, or delayed phases based on the clinical question; timing is critical for vascular and oncologic studies.

Reconstruction: filtered back projection is fast but noisier; iterative reconstruction reduces noise and dose but can alter texture; use appropriate kernel for the anatomy.

Patient positioning: center the patient in the gantry, use isocenter alignment, and avoid arms in the scan field to reduce artifacts and dose.

Artifacts: beam hardening from dense bone or metal, motion from patient movement or breathing, and streak artifacts from high-attenuation objects; recognize and mitigate with positioning or protocol changes.

X-ray production: electrons accelerated by high voltage strike a tungsten target; bremsstrahlung and characteristic radiation are produced; kVp controls beam energy and penetration.

Attenuation follows exponential law: I = I0 e^(-mu x); mu depends on tissue density, atomic number, and photon energy; higher energy reduces photoelectric absorption.

Hounsfield unit formula: HU = 1000 x (mu_tissue - mu_water) / mu_water; calibrate and recognize that HU values vary with beam energy and scanner.

Detector technology: scintillation detectors convert x-rays to light then to electrical signal; solid-state and gas detectors differ in efficiency and stability.

Image reconstruction: back projection, filtered back projection, and iterative methods; spatial resolution, contrast resolution, and noise trade off with dose and reconstruction kernel.

CT dose index: CTDI100 measured in a phantom; CTDIvol accounts for pitch; DLP = CTDIvol x scan length; effective dose estimates stochastic risk.

Pitch and collimation: collimation determines slice thickness; pitch affects dose and coverage; overlapping reconstruction improves z-axis resolution.

Image quality metrics: spatial resolution (line pairs), contrast-to-noise ratio, and noise; optimize kV, mAs, and reconstruction to balance dose and diagnostic quality.

Image quality: assess noise, spatial resolution, contrast resolution, and artifacts; use uniform phantoms and consistent protocols for quality control.

Reconstruction kernels: soft-tissue kernels reduce noise but blur edges; bone kernels sharpen edges but increase noise; choose based on anatomy and clinical question.

Slice thickness and interval: thinner slices improve z-axis resolution but increase noise and data volume; use overlapping intervals for multiplanar reformats.

Windowing: adjust WW and WL to display pathology; document settings and ensure consistent display across studies for comparison.

Artifact recognition: motion, beam hardening, metal streak, partial volume, and ring artifacts; identify cause and correct with positioning, protocol, or reconstruction.

Quality control: daily scanner warm-up, phantom scans for CT number accuracy and noise, and preventive maintenance; document results and address deviations.

Dose monitoring: track CTDIvol and DLP per study, compare to diagnostic reference levels, and optimize protocols when values exceed local benchmarks.

Image display: use appropriate monitor calibration, ambient lighting, and hanging protocols; verify patient identification and study completeness before interpretation.