NAMU CMU cheat sheet

Frequency and wavelength are inversely related (λ = c/f); in soft tissue c ≈ 1540 m/s, so 5 MHz gives λ ≈ 0.31 mm.

Axial resolution equals spatial pulse length divided by 2 (SPL/2); higher frequency and fewer cycles per pulse improve it.

Lateral resolution is best at the focal zone and is approximated by beam width there; it is always worse than axial resolution in practice.

Attenuation in soft tissue is about 0.5 dB/cm per MHz (0.5 × f × path length); round-trip attenuation doubles the one-way value.

Half-value layer thickness (cm) ≈ 6/f(MHz) for soft tissue: the depth at which intensity falls by 3 dB.

Doppler shift f_d = 2·f₀·v·cosθ/c; at 60° the measured velocity underestimates true velocity by half (cos 60° = 0.5).

Nyquist limit equals PRF/2; aliasing occurs when the Doppler shift exceeds it and is corrected by raising PRF, lowering frequency, or shifting the baseline.

The ALARA principle governs output: use the lowest mechanical index (MI) and thermal index (TI) consistent with diagnostic need; MI > 1.9 raises cavitation concern.

Normal adult common bile duct measures up to 6 mm, increasing roughly 1 mm per decade after age 60; 10 mm after cholecystectomy is accepted.

Gallbladder wall thickness greater than 3 mm, pericholecystic fluid, and a sonographic Murphy sign support acute cholecystitis.

Simple hepatic cyst: anechoic, thin-walled, posterior acoustic enhancement; internal septations or solid components require further evaluation.

Portal vein diameter up to 13 mm with hepatopetal flow; reversal of flow suggests portal hypertension.

Renal length 9–12 cm in adults; hydronephrosis appears as anechoic separation of the central sinus, graded mild to severe by calyceal blunting.

Pancreas is best seen with the patient upright or with water ingested to use the stomach as an acoustic window; normal duct up to 3 mm.

Fatty liver shows increased echogenicity relative to the right kidney with poor visualization of deep structures and diaphragm.

Spleen normal length up to 12–13 cm; splenomegaly is suggested when it extends below the costal margin or exceeds 13 cm.

Crown-rump length is the most accurate dating parameter in the first trimester; gestational age (days) ≈ CRL (mm) + 42 for CRL 5–65 mm.

Mean sac diameter grows about 1 mm/day; an MSD ≥ 25 mm without an embryo or a CRL ≥ 7 mm without cardiac activity is diagnostic of pregnancy failure.

Nuchal translucency is measured at 11–13+6 weeks with CRL 45–84 mm; increased NT raises aneuploidy and cardiac risk.

Estimated fetal weight uses BPD, HC, AC, and FL; AC is the single best predictor of fetal weight and growth restriction.

Amniotic fluid index 5–24 cm is normal; oligohydramnios < 5 cm, polyhydramnios > 24 cm, or deepest vertical pocket < 2 cm / > 8 cm.

Placenta previa is diagnosed when the placental edge covers or lies within 2 cm of the internal os after 20 weeks.

Ovarian simple cyst: anechoic, thin smooth wall, posterior enhancement; in postmenopausal women a simple cyst up to 3 cm needs no follow-up.

Endometrial thickness: ≤ 4 mm supports atrophy; > 4 mm in a postmenopausal woman with bleeding warrants tissue sampling.

Internal carotid artery shows low-resistance flow (broad systolic, continuous diastolic); external carotid shows high resistance with a sharp upstroke.

ICA stenosis thresholds by peak systolic velocity: < 125 cm/s normal, 125–230 cm/s indicates ≥ 50% stenosis, > 230 cm/s suggests ≥ 70%.

A ratio of ICA PSV to common carotid PSV > 4 supports ≥ 70% stenosis; near-occlusion may show paradoxically low velocities.

Normal lower-extremity venous exam: phasic flow with respiration, augmentation with distal compression, and complete collapse on compression.

DVT diagnosis requires noncompressibility of the vein; acute thrombus is hypoechoic and distends the vein.

Ankle-brachial index ≤ 0.9 indicates peripheral arterial disease; ≤ 0.4 suggests critical limb ischemia; > 1.4 suggests noncompressible vessels.

Tardus-parvus waveform (damped, delayed systolic upstroke) indicates significant proximal stenosis.

Pseudoaneurysm shows a yin-yang color pattern and a neck with to-and-fro spectral Doppler at the neck.

Ejection fraction by Simpson biplane uses end-diastolic and end-systolic volumes: EF = (EDV − ESV)/EDV; normal ≥ 52% in men, ≥ 54% in women.

Mitral inflow E/A ratio normally > 1; E/A reversal with prolonged deceleration indicates impaired relaxation (grade I diastolic dysfunction).

Tissue Doppler e′ < 7 cm/s (septal) or < 10 cm/s (lateral) indicates impaired relaxation; E/e′ > 14 suggests elevated filling pressures.

Aortic stenosis severity: peak velocity > 4 m/s, mean gradient > 40 mmHg, or valve area < 1.0 cm² indicates severe disease.

Pericardial effusion appears as an anechoic space; right atrial systolic collapse and right ventricular diastolic collapse suggest tamponade.

Tricuspid regurgitation velocity > 2.8 m/s estimates pulmonary systolic pressure > 35 mmHg plus right atrial pressure.

Parasternal long axis is the standard view for measuring LV dimensions, aortic root, and ascending aorta.

Normal LV wall thickness ≤ 1.1 cm; ≥ 1.5 cm suggests hypertrophic cardiomyopathy when other causes are excluded.

Thyroid nodules are stratified by TIRADS: solid hypoechoic composition, irregular margins, microcalcifications, and taller-than-wide shape increase malignancy risk.

Normal thyroid lobe dimensions: length 4–6 cm, AP and transverse up to 2 cm; isthmus up to 0.5 cm.

A simple thyroid cyst is anechoic with posterior enhancement; spongiform nodules are almost always benign.

Breast BI-RADS categories: 1 negative, 2 benign, 3 probably benign (≤ 2% malignancy), 4 suspicious (2–95%), 5 highly suggestive (≥ 95%).

Normal lymph node: reniform shape, thin hypoechoic cortex, echogenic fatty hilum with hilar vascularity; loss of hilum and rounded shape are suspicious.

Testicular torsion shows absent or reduced intratesticular flow with a high-resistance arterial waveform; the bell-clapper deformity predisposes.

Parathyroid adenoma appears as a hypoechoic solid nodule posterior to the thyroid, often with an echogenic feeding vessel.

Sialolithiasis: hyperechoic focus with posterior shadowing within the salivary gland duct or parenchyma.

Tendinopathy appears as tendon thickening, hypoechogenicity, and loss of normal fibrillar pattern without discontinuity.

Full-thickness tendon tear shows a gap with retracted ends; dynamic imaging with passive motion improves detection.

Normal tendon is hyperechoic with a fibrillar echotexture; anisotropy causes artifactual hypoechogenicity when the beam is not perpendicular.

Rotator cuff: supraspinatus is imaged in the subacromial bursa; dynamic subacromial impingement shows bursal bunching.

Joint effusion is anechoic fluid displacing the capsule; synovial hypertrophy is echogenic and does not compress.

Nerve appears as a honeycomb (fascicular) pattern in long axis and a cluster-of-grapes pattern in short axis; normal nerves are hypoechoic with echogenic fascicles.

Muscle tear or strain shows disrupted normal pennate architecture with fluid and possible hematoma.

Gouty tophus appears as a hyperechoic, heterogeneous deposit with the double-contour sign on cartilage.

Central venous access: use the short-axis out-of-plane approach with continuous needle-tip visualization; confirm the tip in the vein and exclude pneumothorax.

Needle bevel and angle determine visibility; a steeper angle increases specular reflection and improves tip visualization.

For thoracentesis, mark a site above the rib to avoid the intercostal neurovascular bundle, and verify safe depth with the patient breathing.

FAST exam evaluates pericardial, perihepatic, perisplenic, and pelvic windows; a positive pericardial window requires immediate attention.

Lung ultrasound: B-lines indicate interstitial edema; lung sliding absence with a lung point suggests pneumothorax.

Focused cardiac exam uses parasternal, apical, and subcostal views; subcostal view is best during cardiac arrest.

Ultrasound-guided biopsy should keep the needle shaft aligned with the transducer plane; confirm needle tip before firing.

Post-procedure pneumothorax is excluded by lung sliding and absence of a lung point on the ipsilateral side.

Thermal index (TI) estimates temperature rise; keep TI < 1.0 for obstetric scanning and < 2.0 for general adult scanning where feasible.

Mechanical index (MI) relates to cavitation risk; avoid MI > 0.4 with gas bodies (lung, bowel) and MI > 1.9 in general.

The output display standard requires the operator to monitor MI and TI during every exam under ALARA.

Diagnostic ultrasound is generally considered safe, but non-medical or keepsake fetal imaging without medical indication is discouraged.

Patient confidentiality, informed consent for invasive procedures, and accurate documentation are core ethical obligations.

Scope of practice: sonographers perform exams and report findings; interpretation and diagnosis remain the physician's responsibility.

Credentialing bodies (e.g., ARDMS) require continuing medical education for maintenance of certification.

Ergonomics: adjust bed height, use neutral wrist posture, and take micro-breaks to reduce work-related musculoskeletal disorders.