ARRT CT cheat sheet

Informed consent for iodinated IV contrast documents allergy history, renal function (eGFR), prior contrast reactions, and the risk/benefit discussion before injection.

An eGFR of 30 mL/min/1.73 m² or higher is generally acceptable for iodinated contrast; values below 30 require radiologist risk/benefit review.

Metformin is withheld for 48 hours after iodinated contrast only when acute kidney injury or eGFR below 30 mL/min/1.73 m² is present, then resumed after renal function is rechecked.

Mild reactions (nausea, limited urticaria) are managed with observation and symptomatic treatment; anaphylactoid reactions (bronchospasm, laryngeal edema, hypotension) require IM epinephrine 1:1000, 0.3–0.5 mg in adults, oxygen, and IV fluids.

Extravasation response: stop the injection, remove the cannula, elevate the limb, apply a cold compress, document volume and site, and give follow-up instructions.

Breath-hold coaching: full inspiration for chest and abdomen; hyperventilating the patient before the hold lengthens achievable breath-hold time and reduces motion artifact.

Pregnancy is not an absolute contraindication to CT; the ACR position is that the study is performed when clinically indicated, with documented justification and shielding when feasible.

Warm flush, metallic taste, and transient nausea are expected during injection; the patient must report pain, burning, or swelling at the IV site immediately.

Pediatric care: lowest kVp and mAs consistent with diagnosis, weight-based contrast near 1–2 mL/kg, and immobilization preferred over sedation.

Portal venous phase abdomen: approximately 60–70 seconds after start of injection at 3–5 mL/s; late arterial phase is approximately 35–40 seconds.

Bolus tracking: place the ROI in the target vessel, set a trigger near 100–150 HU, and begin the scan after the threshold plus a short post-trigger delay.

CT pulmonary angiogram: ROI in the pulmonary trunk with a threshold near 100 HU and a short post-threshold delay, or a timing test bolus.

CT urography excretory phase is acquired roughly 7–10 minutes after injection to opacify collecting systems and ureters.

Cardiac CT: target heart rate 60 bpm or below; prospective ECG-triggered step-and-shoot at 70–80% of the R-R interval lowers dose versus retrospective helical acquisition.

CT colonography requires bowel cleansing, CO2 insufflation to patient tolerance, and imaging supine and prone so stool and fluid redistribute.

Standard window settings: soft tissue W 350–400 / L 40–50; lung W ~1500 / L ~−600; bone W ~2000 / L 300–500; brain W ~80 / L ~40.

Restricting the scanned z-axis to the anatomy of interest and using automatic tube current modulation are the most reliable dose-reduction steps in routine protocols.

HU = (μ_tissue − μ_water) ÷ μ_water × 1000; water is 0, air about −1000, and dense cortical bone 1000 or greater.

Helical pitch = table travel per gantry rotation ÷ total detector beam width; pitch above 1 lowers dose and raises noise, pitch below 1 raises dose.

CTDIvol = CTDIw ÷ pitch for helical scans; DLP = CTDIvol × scan length in cm; effective dose ≈ DLP × body-region k factor (mSv/mGy·cm).

Noise is inversely proportional to the square root of mAs: quadrupling mAs halves noise, and doubling mAs reduces it by roughly 30–40%.

Lowering kVp moves the beam nearer iodine's 33 keV k-edge, increasing iodine attenuation and contrast while raising noise, so mAs must rise to compensate.

The bowtie filter shapes the beam so fewer photons traverse the thin patient periphery, reducing surface dose and improving dose efficiency.

Spatial resolution improves with a small focal spot, thin slices, sharp high-frequency kernels, and small reconstructed pixel size; it is limited by detector aperture.

Beam hardening produces cupping and dark streaks between dense structures and is reduced by correction software, higher kVp, and thinner collimation.

Detector configuration (e.g., 64 × 0.625 mm) sets the minimum slice thickness and the total beam width used in the pitch calculation.