Calculators

Surveillance interval for an infrarenal abdominal aortic aneurysm by maximum outer-to-outer diameter.

Source: SVS practice guidelines on AAA — Chaikof EL et al. J Vasc Surg (2018)

Point-based ultrasound risk stratification of a thyroid nodule.

Source: ACR TI-RADS white paper — Tessler FN et al. J Am Coll Radiol (2017); ACR TI-RADS — American College of Radiology (2017)

Signal intensity index for a lipid-rich adenoma on in-/opposed-phase MRI.

Signal intensity index = (in-phase − opposed-phase) / in-phase × 100. A drop ≥ 16.5% at 1.5 T indicates a lipid-rich adenoma. Note: clear-cell RCC and HCC metastases can also drop signal; the spleen is an unreliable internal reference in iron overload (use kidney).

Source: Chemical-shift adrenal MRI — Fujiyoshi F et al. AJR (2003)

Absolute and relative washout to characterize a homogeneous adrenal mass.

Absolute washout (APW) = (enhanced − delayed) / (enhanced − unenhanced) × 100; ≥60% = adenoma and is the primary metric when an unenhanced phase is available. Relative washout (RPW) = (enhanced − delayed) / enhanced × 100; ≥40% is a supportive criterion, most useful when no unenhanced phase was obtained. Unenhanced ≤10 HU alone indicates a lipid-rich adenoma (washout unnecessary). Apply only to homogeneous masses; do not use when pheochromocytoma, hypervascular metastasis, or adrenocortical carcinoma is suspected.

Source: Adrenal washout CT — Caoili EM et al. Radiology (2002); Incidental adrenal mass — Mayo-Smith WW et al. J Am Coll Radiol (2017)

10-point topographic score for early ischemic change in the MCA territory on non-contrast CT. Start at 10 and subtract 1 for each involved region.

Source: ASPECTS — Barber PA et al. Lancet (2000)

Classification of a cystic renal mass on contrast-enhanced CT or MRI.

Source: Bosniak Classification version 2019 — Silverman SG et al. Radiology (2019)

Coronary artery disease category from coronary CT angiography, by the highest-grade luminal stenosis.

Source: CAD-RADS 2.0 — Cury RC et al. J Cardiovasc Comput Tomogr / Radiology: Cardiothoracic Imaging (2022)

Doppler estimate of ICA stenosis from peak systolic velocity.

Source: Carotid US consensus — Grant EG et al. Radiology (SRU) (2003)

Liver fat estimate from unenhanced CT attenuation.

Applies to UNENHANCED CT only. Liver − spleen ≤ −10 HU, or absolute liver attenuation < 40 HU, indicates moderate–severe steatosis. Contrast-enhanced attenuation is unreliable.

Source: CT quantification of liver fat — Kodama Y et al. AJR (2007)

Resistive index from peak systolic and end-diastolic velocities.

RI = (PSV − EDV) / PSV. Typical adult renal/parenchymal RI ≈ 0.5–0.7.

Source: Doppler resistive index — Tublin ME et al. AJR (2003)

Volume of an ellipsoid lesion or organ from three orthogonal diameters.

Ellipsoid formula V = (π/6) × L × W × H. 1 cm³ = 1 mL.

General prolate-ellipsoid volume formula; no single clinical source.

Incidental pulmonary nodule follow-up. Applies to patients ≥35 years; not for lung-cancer screening, immunocompromised patients, or those with known primary cancer.

Source: Fleischner Society 2017 guidelines — MacMahon H et al. Radiology (2017)

Management of an incidentally detected gallbladder polyp on ultrasound, stratified by morphology and size. Does not apply to primary sclerosing cholangitis (refer to GI guidelines).

Source: SRU gallbladder polyp consensus — Kamaya A et al. Radiology (2022)

Gestational age from crown–rump length (Robinson–Fleming).

Robinson–Fleming: GA (days) = 8.052 × √(1.037 × CRL) + 23.73, with CRL in mm. Most accurate in the first trimester (CRL ≈ 7–84 mm); CRL dating is preferred over LMP when they differ by more than the accepted margin.

Source: CRL dating — Robinson HP, Fleming JEE. Br J Obstet Gynaecol (1975)

Gestational age from mean sac diameter (before an embryo is visible).

GA (days) ≈ mean sac diameter (mm) + 30. Use only before a measurable embryo appears; once a CRL is available, date by CRL. MSD alone should not be used to diagnose pregnancy failure.

Source: Gestational sac dating — Hellman LM et al. / ISUOG guidance (1969)

Diagnostic category for an untreated liver observation in a patient at high risk for HCC (cirrhosis, chronic hepatitis B, or prior HCC).

Source: CT/MRI LI-RADS v2018 — American College of Radiology (2018)

Category and management for a screen-detected pulmonary nodule on low-dose CT lung cancer screening.

Source: ACR Lung-RADS v2022 — American College of Radiology / Christensen J et al. (2022)

Severity of acute pancreatitis on contrast-enhanced CT.

Source: Modified CT severity index — Mortele KJ et al. AJR (2004)

Estimated hepatic fat fraction from in-phase and opposed-phase signal.

Two-point Dixon estimate: FF = (IP − OP) / (2 × IP) × 100. Confounded by iron and valid only up to ~50% fat. Dedicated PDFF sequences are preferred; thresholds are approximate.

Source: Quantitative MRI of hepatic fat (PDFF) — Reeder SB et al. J Magn Reson Imaging (2011)

Percent internal carotid stenosis by the NASCET method (from CTA/MRA/DSA).

NASCET % = (1 − narrowest ICA lumen / normal distal ICA lumen) × 100. In near-occlusion the distal ICA collapses, invalidating the ratio — report as 'near-occlusion' rather than a percentage.

Source: NASCET — North American Symptomatic Carotid Endarterectomy Trial. NEJM (1991)

Risk stratification of an adnexal lesion on MRI (ADNEX MR / O-RADS MRI). Solid-tissue lesions are scored by the dynamic contrast-enhanced time-intensity curve relative to the outer myometrium.

Source: O-RADS MRI risk stratification system — Sadowski EA, Thomassin-Naggara I et al. Radiology (2022); Reporting adnexal lesions with O-RADS MRI — Sadowski EA et al. RadioGraphics (2022)

Risk stratification of an adnexal lesion on ultrasound.

Source: O-RADS US v2022 update — Strachowski LM et al. Radiology (2023)

Percentage change between a prior and current measurement.

Percentage change = (current − prior) / prior × 100. Use consistent units (diameter or volume).

General arithmetic (percentage change); no clinical source.

Prostate volume from three orthogonal diameters (ellipsoid method).

Ellipsoid formula V = (π/6) × L × W × H. 1 mL ≈ 1 g of prostate tissue.

General prolate-ellipsoid volume formula; no single clinical source.

Serum PSA divided by prostate volume.

PSA density = serum PSA ÷ prostate volume. A threshold of ≥0.15 ng/mL/cc is commonly used but is not universal.

Source: PSA density — Benson MC et al. J Urol (1992)

Anatomic complexity of a solid renal mass for surgical planning.

Source: RENAL nephrometry score — Kutikov A, Uzzo RG. J Urol (2009)

Splenic index and estimated volume from three orthogonal dimensions.

Splenic index = length × width × thickness (cm); normal ≈ 120–480 cm³. Estimated volume = 0.524 × L × W × Th. A craniocaudal length > 13 cm on ultrasound also indicates splenomegaly. Use pediatric-specific references under 18 years.

Source: Splenic index — Prassopoulos P et al. Eur Radiol (1997)

Total thyroid volume from per-lobe dimensions (ellipsoid method).

Lobe volume = 0.529 × length × width × depth (isthmus excluded). Normal upper limit ≈18 mL (women), ≈25 mL (men).

Source: Thyroid volume by US — Shabana W et al. AJR (2006)