ARRT CT flashcards

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

Informed consent for contrast

Obtained before iodinated IV contrast to document allergy history, renal function, prior reactions, and the risk/benefit discussion, allowing the patient to accept or refuse the injection.

Metformin and iodinated contrast

Hold metformin for 48 hours after contrast only when acute kidney injury or eGFR below 30 mL/min/1.73 m² is present; restart after renal function is rechecked.

Extravasation first response

Stop the injection immediately, remove the cannula, elevate the limb, apply a cold compress, and document the volume, site, and patient symptoms with follow-up instructions.

Anaphylactoid reaction treatment

Bronchospasm, laryngeal edema, or hypotension after contrast requires IM epinephrine 1:1000 at 0.3–0.5 mg in adults, plus oxygen, IV fluids, and emergency support.

Breath-hold coaching

Instruct the patient to take a full inspiration and hold still; having the patient hyperventilate beforehand lengthens the achievable breath-hold and reduces motion artifact.

Pediatric CT dose principle

Use the lowest kVp and mAs that produce a diagnostic image, weight-based contrast volumes near 1–2 mL/kg, and immobilization instead of sedation when possible.

eGFR threshold for contrast

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

Expected contrast sensations

A warm flush, metallic taste, and transient nausea are normal during injection; the patient should report pain, burning, or swelling at the IV site immediately.

Pregnancy and CT

CT is performed when clinically indicated regardless of pregnancy; justification is documented, shielding is used when feasible, and no single dose threshold defines fetal injury.

IV cannula size for CTA

Use a 20-gauge or larger cannula in an antecubital vein for flow rates of 4–5 mL/s; smaller or distal veins limit flow and degrade arterial opacification.

Saline flush purpose

A saline chaser after contrast pushes the column out of the tubing, reduces streak artifact from dense contrast in the right heart, and lowers total contrast volume needed.

Portal venous phase timing

Approximately 60–70 seconds after the start of IV contrast injection at 3–5 mL/s; hepatic and portal veins are opacified and liver parenchyma is enhanced.

Late arterial phase

About 35–40 seconds after injection, opacifying the hepatic artery and hypervascular tumors while portal vein enhancement remains incomplete.

Bolus tracking technique

Place a region of interest in a target vessel, set a trigger threshold near 100–150 HU, and start the scan after the threshold plus a short post-trigger delay.

Excretory phase CT urography

Imaging roughly 7–10 minutes after injection opacifies the renal collecting systems and ureters for evaluation of urothelial lesions or obstruction.

Prospective cardiac gating

Step-and-shoot acquisition triggered at 70–80% of the R-R interval lowers dose compared with retrospective helical scanning but requires a low, regular heart rate.

CT colonography prerequisites

Requires bowel cleansing, gas insufflation with CO2 to patient tolerance, and imaging in both supine and prone positions to redistribute stool and fluid.

Lung window settings

Window width about 1500 HU and window level about −600 HU display lung parenchyma; soft tissue windows would obscure fine interstitial detail.

Brain window settings

Window width about 80 HU and level about 40 HU; the narrow width maximizes gray-white differentiation in noncontrast head CT.

Scan range reduction

Restricting the scanned z-axis to the anatomy of interest is one of the most effective dose-reduction steps and does not degrade diagnostic image quality.

Test bolus versus tracking

A small test bolus measures time to peak enhancement before the main scan, while bolus tracking monitors enhancement in real time and triggers the scan automatically.

Hounsfield unit formula

HU = (μ_tissue − μ_water) ÷ μ_water × 1000, giving water a value of 0, air about −1000, and dense cortical bone 1000 or higher.

Pitch definition

Table travel per gantry rotation divided by the total detector beam width; pitch greater than 1 reduces dose but increases noise and slice broadening.

CTDIvol and DLP

CTDIvol equals CTDIw divided by pitch; DLP equals CTDIvol multiplied by scan length in centimeters, and effective dose is DLP times a body-region k factor.

Noise versus mAs

Image noise is inversely proportional to the square root of mAs, so quadrupling mAs halves noise while doubling mAs reduces it by about 30–40%.

kVp and iodine contrast

Lowering kVp moves the beam closer to iodine's 33 keV k-edge, increasing iodine attenuation and contrast while raising noise and requiring higher mAs.

Bowtie filter function

Shaping the beam so fewer photons pass through the thin patient periphery reduces surface dose and improves dose efficiency across the field of view.

In-plane spatial resolution

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

Beam hardening artifact

Appears as cupping or dark streaks between dense structures; it is reduced by beam-hardening correction software, higher kVp, and thinner collimation.

Detector configuration and pitch

A configuration such as 64 × 0.625 mm sets the minimum slice thickness and the total beam width that enters the pitch calculation.

Photon starvation artifact

Occurs when very few photons reach the detectors through thick body parts, producing horizontal streaks; tube current modulation and thicker slices reduce it.

Ring artifact cause

Caused by a miscalibrated or failed detector element that produces a circular ring centered on the rotation axis in every reconstructed slice.