Normal measurements
58 adult reference values across every system — the numbers you reach for at the workstation: when a structure is normal, and what crosses the threshold into abnormal. Search by structure or scan by system.
58 values
🧠
Neuro / Head & Neck
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Evans index (frontal horn / max inner skull width) Ratio of maximal frontal horn width to maximal inner skull diameter on the same axial slice. | CT / MRI | < 0.30 | ≥ 0.31 suggests ventriculomegaly / hydrocephalus |
Optic nerve sheath diameter Ultrasound cutoff commonly quoted as > 5–5.7 mm; measure 3 mm posterior to the globe. | CT / US / MRI | ≤ 5–6 mm | > 6 mm (measured 3 mm behind globe) suggests raised intracranial pressure |
Pituitary gland height Physiological enlargement up to 10–12 mm in pregnancy and post-partum; upper convex margin can be normal in young women. | MRI | ≤ 6 mm (male / child), ≤ 9 mm (female) | Enlargement above sex-specific limit suggests hyperplasia or adenoma |
Third ventricle width Age-dependent; mild widening common in the elderly. | CT / MRI | ≤ 7 mm (adult), up to ~10 mm with age | > 10 mm suggests ventricular dilatation |
Temporal horn of lateral ventricle | CT / MRI | ≤ 2 mm | > 2 mm is an early/sensitive sign of hydrocephalus |
Midline shift Measured at the septum pellucidum relative to the ideal midline (falx anterior–posterior). | CT | 0 mm | > 5 mm often clinically significant / may prompt intervention |
Thyroid lobe (AP / transverse) Isthmus normally ≤ 2–3 mm thick. | US | ≤ 2 cm AP; craniocaudal ~4–6 cm | AP > 2 cm suggests goitre / enlargement |
Cervical lymph node — short axis Rounded shape, loss of fatty hilum, central necrosis and clustering raise suspicion independent of size. | CT / MRI / US | ≤ 10 mm (≤ 11 mm jugulodigastric) | Short axis > 10 mm (> 11 mm level II) suspicious; also assess shape, hilum, necrosis |
🫁
Chest / Cardiac
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Tracheal diameter (coronal) Sagittal ≤ 27 mm (M) / ≤ 23 mm (F). | CT / CXR | ≤ 25 mm (M), ≤ 21 mm (F) | > 25 mm (M) / > 21 mm (F) suggests tracheomegaly (e.g. Mounier-Kuhn) |
Ascending thoracic aorta Measure perpendicular to flow at the pulmonary artery level; adjust for body surface area and valve pathology. | CT / MRI | ≤ 3.5–4.0 cm | > 4 cm dilated; ≥ 4.5–5.5 cm surgical thresholds (aetiology-dependent) |
Descending thoracic aorta | CT / MRI | ≤ 2.5–3.0 cm | Aneurysmal when > 1.5× expected normal diameter |
Main pulmonary artery Measured at bifurcation level, perpendicular to long axis. | CT | ≤ 29 mm | > 29 mm, or PA:ascending aorta ratio > 1, suggests pulmonary hypertension |
Cardiothoracic ratio Only valid on a well-inspired erect PA film; unreliable on AP / supine / paediatric films. | CXR (PA erect) | ≤ 0.5 | > 0.5 suggests cardiomegaly |
Azygos vein diameter | CXR / CT | ≤ 10 mm (upright CXR) | > 10 mm suggests raised central venous pressure / volume overload |
Pleural effusion (layering) Ultrasound is far more sensitive than radiography for small effusions. | US / CXR (decubitus) | No effusion | ~1 cm layering usually tappable; ~50 mL detectable on lateral, ~200 mL blunts the frontal costophrenic angle |
Oesophageal wall thickness Non-distended oesophagus can appear falsely thickened. | CT | ≤ 3 mm (distended) | > 5 mm abnormal when adequately distended |
🩻
Abdomen / Hepatobiliary
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Common bile duct (CBD) Up to ≤ 10 mm may be normal post-cholecystectomy. Measure internal diameter at the porta. | US / CT / MRCP | ≤ 6 mm | > 6 mm suggests obstruction; add ~1 mm per decade over 60 y |
Main portal vein Measured at the point it crosses the IVC; loss of respiratory variation also supports portal hypertension. | US / CT | ≤ 13 mm | > 13 mm suggests portal hypertension |
Gallbladder wall thickness Measure the anterior wall in a fasted, distended gallbladder. | US / CT | ≤ 3 mm | > 3 mm abnormal (cholecystitis, hepatitis, ascites, hypoalbuminaemia, post-prandial) |
Pancreatic duct (main) | US / CT / MRCP | ≤ 3 mm (head), tapering distally | > 3 mm dilated; consider obstructing lesion or chronic pancreatitis |
Liver span (mid-clavicular line) Considerable normal variation with body habitus; morphology also matters. | US / CT | ≤ 15–16 cm craniocaudal | > 15.5–16 cm suggests hepatomegaly |
Spleen length (craniocaudal) | US / CT | ≤ 12–13 cm | > 13 cm suggests splenomegaly; > 20 cm = massive |
Appendix diameter (outer wall) Size alone is imperfect; look for non-compressibility, hyperaemia, appendicolith and secondary signs. | US / CT | ≤ 6 mm | > 6 mm with wall thickening / fat stranding suggests appendicitis |
Bowel wall thickness Collapsed / underdistended loops appear falsely thick. | CT | ≤ 3 mm (well distended) | > 3–4 mm abnormal when adequately distended |
Small bowel calibre | CT / X-ray | ≤ 2.5–3 cm | > 3 cm suggests dilatation / obstruction |
Large bowel calibre | CT / X-ray | ≤ 6 cm (colon); caecum ≤ 9 cm | Colon > 6 cm or caecum > 9 cm suggests obstruction / toxic megacolon (perforation risk) |
🫘
Genitourinary
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Renal length (craniocaudal) Left kidney is typically slightly longer than the right. | US / CT | 9–12 cm | < 9 cm small kidney; > 12–13 cm enlarged; > 1.5 cm side-to-side asymmetry notable |
Renal cortical thickness | US / CT | ≥ 7–10 mm | < 7 mm suggests chronic parenchymal loss |
Renal pelvis AP diameter (hydronephrosis) Physiological dilatation is common in pregnancy, especially on the right. | US / CT | ≤ 10 mm (adult, non-pregnant) | 10–15 mm mild, 15–20 mm moderate, > 20 mm severe hydronephrosis |
Ureter diameter | CT / US | ≤ 3 mm (up to ~8 mm at physiological narrowings) | > 8 mm generally abnormal / obstructed |
Urinary bladder wall thickness | US / CT | ≤ 3 mm (distended), ≤ 5 mm (empty) | > 3 mm (full) / > 5 mm (empty) suggests outlet obstruction, cystitis or neurogenic bladder |
Prostate volume Volume ≈ length × width × height × 0.52 (ellipsoid formula). | US / MRI | ≤ 30 mL (≈ 20–25 g young adult) | > 30 mL enlarged (BPH); volume used to derive PSA density |
Adrenal limb thickness | CT | ≤ 10 mm (limb), body ≤ ~5 mm | Limb > 10 mm suggests hyperplasia; discrete nodule = adenoma vs other |
Endometrial thickness (double-layer) Measure the thickest anteroposterior double-layer in the sagittal plane; exclude intracavitary fluid. | US | Premenopausal: varies 4–16 mm by cycle; Postmenopausal: ≤ 4–5 mm | Postmenopausal > 4–5 mm (with bleeding) warrants evaluation; > 8–11 mm without bleeding debated |
Ovarian volume Volume ≈ length × width × height × 0.52. | US | Premenopausal ≤ ~10–18 mL; Postmenopausal ≤ ~8 mL | Enlargement above age-expected volume warrants characterisation |
🩸
Vascular
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Abdominal aorta (infrarenal) Measure outer-to-outer wall, perpendicular to the vessel axis. | US / CT | ≤ 3.0 cm | ≥ 3.0 cm defines AAA; ≥ 5.5 cm (M) / ≥ 5.0 cm (F) common repair thresholds |
Common iliac artery | CT / US | ≤ 1.2 cm | > 1.5 cm aneurysmal; ≥ 3–3.5 cm often considered for repair |
Inferior vena cava (IVC) diameter Measured ~2 cm distal to the RA junction (subcostal view). | US | 1.5–2.5 cm with > 50% inspiratory collapse | > 2.1 cm with < 50% collapse suggests elevated right atrial pressure |
Carotid intima-media thickness (IMT) Measured on the far wall of the distal common carotid. | US | < 0.9 mm (age-dependent) | > 0.9 mm suggests subclinical atherosclerosis; ≥ 1.5 mm = plaque |
Internal carotid artery — peak systolic velocity Corroborate with ICA/CCA PSV ratio (> 4 for ≥ 70%) and end-diastolic velocity. | US Doppler | < 125 cm/s | 125–230 cm/s ≈ 50–69% stenosis; > 230 cm/s ≈ ≥ 70% stenosis (NASCET/SRU criteria) |
Aortic root (sinus of Valsalva) | Echo / CT / MRI | ≤ ~3.6–4.0 cm (BSA-indexed) | Enlargement risks dissection; thresholds vary with height/BSA and connective tissue disease |
Popliteal artery | US / CT | ≤ ~1.0 cm | > 1.5 cm aneurysmal; often bilateral and associated with AAA |
🦴
Musculoskeletal / Spine
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Cervical spinal canal AP diameter Torg-Pavlov ratio (canal/vertebral body) < 0.8 also suggests congenital narrowing. | CT / MRI / X-ray | ≥ 13 mm | ≤ 13 mm relative stenosis; ≤ 10 mm absolute stenosis |
Lumbar spinal canal AP diameter | CT / MRI | ≥ 12 mm | 10–12 mm relative stenosis; < 10 mm absolute central stenosis |
Intervertebral disc height | MRI / X-ray | Increases cranio-caudally; L4/5 typically greatest in lumbar spine | Focal loss of height (with signal change) suggests degeneration |
Spinal cord AP diameter (cervical) | MRI | ~6–8 mm | Focal enlargement (tumour/oedema) or thinning (atrophy/compression) abnormal |
Achilles tendon thickness (AP) Fusiform > 6 mm thickening without a tear typifies tendinosis. | US / MRI | ≤ 6 mm | > 6 mm suggests tendinopathy |
Supraspinatus tendon thickness | US / MRI | ~4–6 mm | Focal thinning / defect suggests partial or full-thickness tear |
Hip joint space | X-ray | ≥ 3–4 mm (superior) | < 2 mm suggests significant osteoarthritis |
Acromiohumeral interval | X-ray | 7–14 mm | < 7 mm suggests rotator cuff (supraspinatus) tear / superior migration |
🤰
Obstetric / Paediatric
| Structure | Modality | Normal | Abnormal / threshold |
|---|---|---|---|
Nuchal translucency (11–13⁺⁶ weeks) Measure in the mid-sagittal plane with CRL 45–84 mm; strict technique required. | US | ≤ 3.0 mm (≤ 3.5 mm often used as cutoff) | > 3.5 mm raises risk of aneuploidy / cardiac anomaly |
Gestational sac — mean sac diameter (MSD) Uses conservative 2011 SRU criteria to avoid false diagnosis of failed pregnancy. | US (transvaginal) | Yolk sac expected by MSD ≥ 8 mm; embryo by MSD ≥ 25 mm | MSD ≥ 25 mm without embryo suggests failed pregnancy |
Yolk sac diameter | US | ≤ 6 mm (first trimester) | > 6 mm associated with increased risk of pregnancy failure |
Cisterna magna (AP depth) | US / MRI | ≤ 10 mm | > 10 mm = mega cisterna magna / posterior fossa anomaly |
Lateral ventricle atrium width (fetal) Measured at the atrium/glomus of the choroid plexus. | US / MRI | ≤ 10 mm | 10–15 mm mild ventriculomegaly; > 15 mm severe |
Cervical length (second/third trimester) | US (transvaginal) | ≥ 25 mm | < 25 mm associated with increased risk of preterm birth |
Nuchal fold (15–20 weeks) Distinct from first-trimester nuchal translucency. | US | ≤ 6 mm | > 6 mm is a soft marker for trisomy 21 |
Amniotic fluid index (AFI) | US | ~8–24 cm (single deepest pocket 2–8 cm) | < 5 cm oligohydramnios; > 24 cm polyhydramnios |
Reference values are commonly-cited adult norms for decision support; thresholds vary by technique, age, body habitus and source. Always correlate clinically and confirm against current local guidelines.