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Ultrasonography (USG) - A Comprehensive Basics Guide
1. What Is Ultrasound?
Ultrasound (USG) uses high-frequency sound waves (above 20,000 Hz; clinically 1-20 MHz) that are beyond the human hearing range. When these waves are transmitted into body tissue, they reflect back from acoustic interfaces between structures of different densities. The returning echoes are detected by the transducer, processed, and displayed as a real-time grayscale image on the monitor.
Key physical quantities:
| Term | Definition |
|---|---|
| Frequency | Cycles per second (Hz). Clinical USG uses 1-20 MHz |
| Wavelength | Distance per cycle. Higher frequency = shorter wavelength = better resolution |
| Propagation speed | ~1540 m/s in soft tissue (varies by medium) |
| Impedance | Resistance of a medium to sound propagation (density × velocity) |
| Attenuation | Loss of sound energy as waves travel deeper into tissue |
Resolution vs. Penetration trade-off:
- High frequency (7-15 MHz) = excellent resolution, but shallow penetration (superficial structures)
- Low frequency (2-5 MHz) = deeper penetration, lower resolution (deep structures: liver, kidney)
2. The Piezoelectric Effect - How Images Are Made
The transducer's heart is the piezoelectric crystal (quartz or ceramic). When an alternating electric current is applied, the crystal alternately compresses and expands - generating ultrasound waves. Conversely, when returning echoes strike the crystal, it generates an electric signal. This dual role makes one device both sender and receiver.
The cycle is: Transmit brief pulse → Switch to receive mode → Detect returning echoes → Calculate depth from time of flight → Build image pixel by pixel
Image depth is calculated from: Depth = (Speed of sound × Time) / 2
3. Internal Structure of the Transducer
Textbook of Clinical Echocardiography - Fig. 1.5
Components of the transducer:
- Piezoelectric crystal - generates and receives sound waves
- Damping material - shortens pulse length, improves axial resolution
- Acoustic lens - focuses the beam
- Impedance matching layer - reduces reflection at skin interface; coupling gel also helps here
4. Types of Transducers (Probes)
| Probe Type | Frequency | Footprint | Best For |
|---|---|---|---|
| Linear (High-frequency) | 5-15 MHz | Wide, flat | Superficial: thyroid, vessels, breast, tendons, nerves, vascular access |
| Curvilinear (Convex) | 2-5 MHz | Curved, wide | Deep abdominal/pelvic organs: liver, kidney, OB |
| Phased Array (Sector) | 2-5 MHz | Small footprint | Cardiac (echocardiography), intercostal windows |
| Endocavity | 5-10 MHz | Inserted internally | Transvaginal, transrectal |
| Microconvex | 4-8 MHz | Small curved | Neonatal head, emergency point-of-care |
(Comprehensive Clinical Nephrology, 7th Edition)
5. Imaging Modes
A-Mode (Amplitude Mode)
A one-dimensional display showing echo amplitude vs. depth along a single line. Rarely used clinically today (mainly in ophthalmology for axial length measurement).
B-Mode (Brightness Mode) - The Standard
The universal 2D grayscale display. Echo amplitudes are represented as dots of varying brightness - bright (white) for strong reflectors, dark (black) for none. This is what you see in nearly all clinical scans.
M-Mode (Motion Mode)
Displays movement of a single scan line over time. Used to:
- Measure IVC diameter and respiratory variation (volume status)
- Assess cardiac valve excursion
- Detect pleural sliding in lung USG
Doppler Modes
Doppler effect: when a sound source or target moves, the reflected frequency shifts. This frequency shift is proportional to the velocity of the moving target (red blood cells).
Doppler Shift Formula:
$$f_d = \frac{2 \times v \times f_0 \times \cos\theta}{c}$$
Where: fd = Doppler shift, v = blood velocity, f₀ = transmitted frequency, θ = angle of insonation, c = speed of sound
Key point: When the beam is perpendicular to flow (θ = 90°), cosine = 0, so no Doppler signal is detected. The optimal angle is < 60°.
6. Doppler Types
Color Doppler
Velocity data is color-coded and overlaid on B-mode image. The mnemonic BART helps:
- Blue Away, Red Towards (relative to probe)
Clinical renal color Doppler:
Pulsed Wave (PW) Doppler
Emits pulses at a specific depth (sample volume). Allows measurement of flow velocity at a precise location. Limited by the Nyquist limit - cannot accurately measure very high velocities (aliasing occurs).
Continuous Wave (CW) Doppler
Continuously transmits and receives. Can measure high velocities (no Nyquist limit) but cannot localize the signal to a specific depth - records all velocities along the entire beam path. Used in echocardiography for stenotic valve gradients.
Power Doppler
Shows the presence of flow (not direction). More sensitive than color Doppler for slow flow. Used for vascularity assessment of nodules, inflammation.
7. Echogenicity - How Tissues Appear
Echogenicity describes how strongly a tissue reflects ultrasound and therefore how bright it appears on screen.
| Term | Appearance | Examples |
|---|---|---|
| Anechoic | Black (no echoes) | Fluid: urine in bladder, bile, blood in vessels, cysts, effusions |
| Hypoechoic | Darker gray than reference | Lymph nodes, muscle, thyroid adenoma, most solid tumors |
| Isoechoic | Same brightness as reference tissue | Normal thyroid vs. some nodules |
| Hyperechoic | Brighter/whiter than reference | Fat, renal sinus, gallstone, bone, bowel gas |
| Echogenic | Bright reflector (general term) | Air, calcium, foreign bodies |
Reference tissue for the abdomen is typically the liver parenchyma.
8. Common USG Artifacts
Artifacts arise from the way sound behaves at tissue interfaces. Some are helpful diagnostically; some are misleading.
| Artifact | Appearance | Cause | Clinical Use |
|---|---|---|---|
| Acoustic shadowing | Dark stripe posterior to a bright structure | Strong reflector/attenuator (stone, bone, gas) blocks deeper transmission | Identifies gallstones, renal calculi, calcifications |
| Posterior acoustic enhancement | Brighter area deep to a fluid collection | Fluid attenuates sound less than surrounding tissue | Confirms cystic vs. solid nature |
| Reverberation | Equally spaced parallel lines | Sound bouncing between two strong reflectors | Seen with metallic objects, near-field artifacts |
| A-lines (lung) | Horizontal lines parallel to pleural line | Reverberation at air-tissue interface | Normal lung, pneumothorax |
| B-lines (comet-tail lung) | Vertical hyperechoic lines from pleural line to screen edge | Fluid-thickened subpleural interlobular septa | Pulmonary edema, interstitial lung disease |
| Mirror artifact | Duplicate image on other side of strong reflector | Reflection at diaphragm | Pseudomasses below/above diaphragm |
| Aliasing | Mosaic color wrap-around in Doppler | Velocity exceeds Nyquist limit in PW Doppler | Sign of high-velocity flow |
9. Image Optimization Controls
| Control | Function |
|---|---|
| Gain | Overall brightness of the image. Too much = noise; too little = dark |
| TGC (Time Gain Compensation) | Depth-selective gain to compensate for attenuation |
| Depth | Adjusts how deep the image shows |
| Focus | Sets the focal zone for best lateral resolution |
| Frequency | On broadband probes, switch to higher frequency for better superficial detail |
| Harmonic imaging | Uses tissue harmonics at double the transmitted frequency - reduces noise/clutter |
10. Safety: ALARA Principle
USG is considered very safe - no ionizing radiation. However, high-intensity ultrasound can cause:
- Thermal effects - heating of tissue (especially bone)
- Mechanical effects (cavitation) - gas bubble formation
The ALARA principle applies: As Low As Reasonably Achievable. Use minimum power and exposure time necessary for diagnostic quality. The machine displays:
- Thermal Index (TI) - estimated temperature rise
- Mechanical Index (MI) - risk of cavitation
11. Clinical Applications at a Glance
| System | What USG Shows |
|---|---|
| Abdomen | Liver, gallbladder stones/wall, bile ducts, pancreas, spleen, ascites |
| Kidney/Urinary | Hydronephrosis, stones, cysts, masses, cortical thickness |
| Cardiac (Echo) | Chambers, valves, pericardial effusion, wall motion |
| Obstetric | Fetal biometry, placenta, amniotic fluid, fetal heart |
| Vascular | DVT, carotid stenosis, AAA, portal hypertension |
| Thyroid/Neck | Nodules (TIRADS), lymph nodes, parathyroid |
| Lung (POCUS) | Pneumothorax (A-lines only), pulmonary edema (B-lines), effusion |
| MSK | Tendons (rotator cuff tears), joints (effusion), nerves |
| POCUS/EM | FAST exam (trauma), volume status (IVC), cardiac arrest |
12. Advantages and Limitations
Advantages:
- No ionizing radiation (safe in pregnancy, pediatrics)
- Real-time imaging
- Portable, bedside use (POCUS)
- Doppler for vascular assessment
- Guides procedures (biopsy, drainage, vascular access) - significantly lowers complication rates
- Relatively inexpensive
Limitations:
- Operator-dependent (significant skill required)
- Poor through gas (bowel, lungs under normal conditions) and bone
- Limited by body habitus (obesity reduces penetration)
- Limited field of view compared to CT/MRI
- Cannot penetrate deep calcifications
Sources: Textbook of Clinical Echocardiography (9780323882088); Comprehensive Clinical Nephrology 7th Ed (9780323825924); Roberts and Hedges' Clinical Procedures in Emergency Medicine (9780323354783); POCUS 101 (pocus101.com)
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Obstetric Ultrasound (USG) in Pregnancy - Complete Basics Guide
Why USG in Pregnancy?
Ultrasound is the cornerstone of antenatal care. It is safe (no ionizing radiation), real-time, and provides critical information at every stage of pregnancy - from confirming intrauterine location at 5 weeks to assessing fetal well-being in the third trimester. The ALARA principle always applies.
Probe used: Curvilinear probe (3-5 MHz) for transabdominal scanning; endocavity/transvaginal probe (5-10 MHz) for early pregnancy, cervical assessment, and when transabdominal views are suboptimal.
Routine Scans in Pregnancy - Overview
| Trimester | Weeks | Scan Name | Primary Purpose |
|---|---|---|---|
| First | 6-10 | Early pregnancy / viability scan | Confirm IUP, cardiac activity, number |
| First | 11-14 | Dating + NT scan | Gestational age, NT, early anomaly |
| Second | 18-20 | Anomaly scan / TIFFA | Fetal anatomy, biometry, placenta |
| Third | 28-32 | Growth scan | Growth, AFI, presentation, Doppler |
| Third | 36-40 | Term scan | Presentation, EFW, AFI, placenta |
FIRST TRIMESTER SCAN (6-14 Weeks)
Transabdominal vs. Transvaginal
Transvaginal (TVS) picks up early structures ~1 week earlier than transabdominal:
- Gestational sac: 4.5 weeks (TVS) / ~5.5 weeks (TAS)
- Yolk sac: 5.5 weeks (TVS)
- Fetal pole: 6.0 weeks (TVS)
- Cardiac activity: detected when CRL ≥ 5 mm on TVS
Normal Early Pregnancy Progression
| Gestational Age | Sonographic Finding |
|---|---|
| 4-5 weeks | Gestational sac (anechoic ring in uterine cavity) |
| 5-5.5 weeks | Yolk sac appears within sac |
| 6 weeks | Fetal pole visible, cardiac activity seen |
| 7-8 weeks | Embryonic movement, limb buds |
| 10-13 weeks | Fetal anatomy begins to be visible |
What to Report in First Trimester
- Intrauterine pregnancy confirmed? (location - rule out ectopic)
- Number of gestational sacs / embryos
- Cardiac activity - present or absent
- Gestational age (CRL most accurate)
- Uterus, adnexae (any masses/cysts)
- NT measurement at 11-14 weeks
GESTATIONAL AGE DATING - Key Measurements
Dating Parameters by Gestational Age
(Pfenninger and Fowler's Procedures for Primary Care)
| Weeks | Best Parameter | Comment |
|---|---|---|
| 7-10 wks | GS (gestational sac) + CRL average | |
| 11-14 wks | CRL (most accurate) | ± 5 days accuracy |
| 15-28 wks | BPD most accurate; + HC, FL, AC | ± 1 week |
| After 28 wks | Average of BPD, HC, FL, AC | ± 2-3 weeks accuracy |
CRL - Crown Rump Length
- Measured from top of head to bottom of rump (NOT legs)
- Most accurate dating parameter in the first trimester
- Performed at 11-14 weeks
BPD - Biparietal Diameter
(Creasy & Resnik's Maternal-Fetal Medicine)
- Plane: Transverse cross-section at level of thalami
- Midline falx cerebri interrupted by cavum septi pellucidi
- Measurement: Outer table of proximal parietal bone to inner table of distal parietal bone
- No cerebellum visible in this plane
HC - Head Circumference
- Same plane as BPD
- Trace outer edges of cranial bones with ellipse
- Formula: HC = 1.62 × (BPD + OFD)
- More reliable than BPD alone in dolichocephaly/brachycephaly
AC - Abdominal Circumference
- Plane: Transverse at level of portal sinus
- Must show: Stomach bubble + umbilical vein (at portal sinus) - no kidneys visible
- Formula: AC = 1.57 × (LAD + TAD)
- Most sensitive for detecting IUGR (asymmetric type)
- Least accurate for dating but best for growth assessment
FL - Femur Length
- Ossified femoral diaphysis length (exclude distal epiphysis)
- Femur seen at ~45° angle from fetal spine
- FL/BPD ratio normal = 79% ± 8% (from 22-40 weeks)
NUCHAL TRANSLUCENCY (NT)
- When: 11 weeks 0 days to 13 weeks 6 days (CRL 45-84 mm)
- What: Fluid-filled translucent space at the back of fetal neck
- Normal: < 3.0 mm (or < 95th percentile for CRL)
- Increased NT (≥ 3.0 mm) associated with:
- Trisomy 21 (Down syndrome) - most common
- Trisomy 18 (Edwards), Trisomy 13 (Patau)
- Turner syndrome
- Cardiac defects
- Other structural anomalies
Combined first trimester screening = NT + maternal serum free beta-hCG + PAPP-A + maternal age
- Detection rate for Down syndrome: ~85-90%
(Langman's Medical Embryology)
SECOND TRIMESTER SCAN - TIFFA / Anomaly Scan (18-20 Weeks)
The Targeted Imaging for Fetal Anomalies (TIFFA) scan at 18-20 weeks is the most important routine obstetric scan. It systematically evaluates all major fetal systems.
Fetal Anatomy Checklist (Required by AIUM Standards)
(Pfenninger and Fowler's Procedures for Primary Care; Creasy & Resnik's)
HEAD & BRAIN:
- Midline falx cerebri
- Lateral cerebral ventricles (atrium ≤ 10 mm = normal)
- Cavum septum pellucidum (CSP)
- Choroid plexus (normally hyperechoic, fills ventricles early)
- Cerebellum (dumbbell shape, measure transcerebellar diameter)
- Cisterna magna (2-10 mm)
- Nuchal fold (NF) 15-20 weeks: < 5 mm normal; ≥ 6 mm = risk for trisomy 21
FACE:
- Lips (cleft lip/palate)
- Nose, profile
SPINE:
- Cervical, thoracic, lumbar, sacral - longitudinal and transverse
- Screen for spina bifida: "lemon sign" (frontal bone indentation) + "banana sign" (cerebellar herniation)
HEART - Four Chamber View:
- Right and left ventricles roughly equal size
- Interventricular septum intact
- Atrioventricular valves visible and functioning
- Also obtain: LVOT, RVOT views
ABDOMEN:
- Stomach bubble visible (if absent - check for esophageal atresia or swallowing issues)
- Kidneys (echogenic initially, renal pelvis AP diameter ≤ 7 mm)
- Urinary bladder visible
- Umbilical cord insertion site
- Anterior abdominal wall intact (rule out omphalocele/gastroschisis)
EXTREMITIES:
- Count bones and digits (both femora, tibiae, humeri, radii-ulnae)
PLACENTA
Normal Placenta Appearance
- Homogeneous, moderately echogenic structure on the uterine wall
- Thickness: ~1 mm per week gestation (up to ~4 cm at term)
- Posterior position most common; anterior, fundal also normal
Placental Grading (Grannum Classification)
| Grade | Timing | Features |
|---|---|---|
| 0 | Early | Homogeneous, smooth chorionic plate |
| I | Mid pregnancy | Subtle calcifications |
| II | ~36 weeks | Comma-shaped calcifications, indentations |
| III | Term | Complete septations, echogenic cotyledons |
Grade III before 34 weeks = premature placental maturation (risk of IUGR, preeclampsia)
Placenta Previa
- Low-lying placenta: Placental edge < 20 mm from internal cervical os
- Placenta previa: Placental edge covers the internal cervical os
- Diagnosis confirmed by transvaginal scan (transabdominal overestimates)
- Follow-up at 32 weeks for both low-lying and previa
- (Creasy & Resnik's Maternal-Fetal Medicine)
AMNIOTIC FLUID ASSESSMENT
Amniotic Fluid Index (AFI)
The uterus is divided into 4 quadrants. The largest vertical pocket in each quadrant is measured. Values are summed.
(Campbell Walsh Wein Urology; Pfenninger and Fowler's)
| AFI Value | Classification |
|---|---|
| 8-18 cm | Normal |
| < 5 cm | Oligohydramnios |
| 5-8 cm | Borderline |
| > 20-24 cm | Polyhydramnios |
Single Deepest Pocket (SDP)
Alternative method: single largest pocket measured vertically
- Normal: 2-8 cm
- < 2 cm = oligohydramnios
-
8 cm = polyhydramnios
Clinical Significance
| Finding | Causes |
|---|---|
| Oligohydramnios | Renal agenesis/dysplasia, IUGR, PROM, post-term pregnancy |
| Polyhydramnios | Fetal anomalies (esophageal atresia, anencephaly), maternal diabetes, twin-twin transfusion |
ECTOPIC PREGNANCY - USG Diagnosis
USG Findings in Ectopic Pregnancy
(Tintinalli's Emergency Medicine)
| Finding | Significance |
|---|---|
| Confirmed IUP in uterus | Effectively excludes ectopic (except heterotopic) |
| Empty uterus + adnexal mass | High suspicion for ectopic |
| Empty uterus + free fluid + echogenic adnexal mass | Near 100% risk of ectopic |
| Living embryo outside uterus | Diagnostic of ectopic (<10% cases) |
| "Pseudogestational sac" | Intrauterine fluid collection in ectopic - do not confuse with true GS |
TVS visualizes early signs:
- Gestational sac at 4.5 weeks
- Yolk sac at 5.5 weeks
- Fetal pole at 6.0 weeks (TAS approximately 1 week later)
Beta-hCG discriminatory zone: When beta-hCG > 1500-2000 IU/L, an IUP should be visible on TVS. Empty uterus at this level = ectopic until proven otherwise.
DOPPLER IN PREGNANCY
Umbilical Artery Doppler
The umbilical artery carries deoxygenated blood from fetus to placenta. As normal pregnancy progresses, placental vascular resistance decreases - so diastolic flow increases.
Key indices:
- S/D ratio (Systolic/Diastolic)
- Resistive Index (RI) = (S-D)/S
- Pulsatility Index (PI) = (S-D)/mean
| Umbilical Artery Finding | Significance |
|---|---|
| Normal diastolic flow | Normal placental resistance |
| Elevated S/D ratio | Increased placental resistance - IUGR risk |
| Absent End-Diastolic Flow (AEDF) | Severe placental insufficiency - fetal compromise |
| Reversed End-Diastolic Flow (REDF) | Imminent fetal danger - consider immediate delivery |
Middle Cerebral Artery (MCA) Doppler
- Normally high resistance vessel
- In fetal hypoxia: "brain-sparing effect" - MCA resistance falls (increased diastolic flow)
- MCA Pulsatility Index decreases = fetal compromise
- Cerebro-placental ratio (CPR) = MCA PI / UA PI (normal >1)
Uterine Artery Doppler
- Done at 11-14 weeks or 20-24 weeks
- Elevated resistance + notching = risk of preeclampsia and IUGR
BIOPHYSICAL PROFILE (BPP)
Used to assess fetal well-being in the third trimester (after 28 weeks). Each parameter scores 0 or 2.
| Parameter | Normal (score 2) | Duration |
|---|---|---|
| Fetal breathing movements | ≥ 1 episode of ≥ 30 sec in 30 min | |
| Fetal movements | ≥ 3 body/limb movements in 30 min | |
| Fetal tone | ≥ 1 active extension/flexion (hand open/close, limb movement) | |
| Amniotic fluid | Single deepest pocket ≥ 2 cm | |
| Non-stress test (NST) | Reactive (2 accelerations ≥ 15 bpm × 15 sec in 20 min) |
Interpretation:
- 8-10/10: Normal - low risk of fetal asphyxia
- 6/10: Equivocal - repeat in 24 hours
- 4/10: Abnormal - consider delivery
- 0-2/10: Strongly abnormal - deliver
FETAL PRESENTATION & PRESENTATION TYPES
In the third trimester, report:
- Cephalic (vertex): Head down - normal
- Breech: Bottom/feet down (frank, complete, footling)
- Transverse: Horizontal lie
- Oblique: Diagonal lie
Presentation is clinically relevant from 34-36 weeks onward.
COMMON ABNORMALITIES DETECTED ON OBSTETRIC USG
| Condition | USG Finding |
|---|---|
| Anencephaly | Absent calvarium, "frog-eye" appearance |
| Spina bifida | Lemon sign + banana sign; defect in spine |
| Hydrocephalus | Lateral ventricles > 10 mm (ventriculomegaly) |
| Gastroschisis | Bowel loops floating freely outside abdomen (no membrane) |
| Omphalocele | Abdominal contents in membrane-covered sac at cord insertion |
| Diaphragmatic hernia | Stomach/bowel in thorax, mediastinal shift |
| Renal agenesis | Absent kidneys, oligohydramnios (Potter sequence) |
| IUGR | AC < 5th percentile, EFW < 10th percentile, abnormal Doppler |
| Placenta previa | Placenta covers cervical os |
| Vasa previa | Fetal vessels crossing internal os (color Doppler) |
| Hydrops fetalis | Skin edema + 2 or more: ascites, pleural/pericardial effusion, placental edema |
| Down syndrome markers | Increased NT, short femur, echogenic bowel, choroid plexus cysts, absent nasal bone, short humerus |
FETAL WEIGHT ESTIMATION (EFW)
Calculated from combinations of BPD, HC, AC, FL using formulas (Hadlock most widely used):
Hadlock formula (most common): Uses HC + AC + FL
- EFW < 10th percentile for gestational age = Small for Gestational Age (SGA) / suspect IUGR
- EFW > 90th percentile = Large for Gestational Age (LGA) / suspect macrosomia
IUGR classification:
- Symmetric IUGR: All measurements proportionally small (chromosomal, early insult)
- Asymmetric IUGR: AC most affected, HC spared (placental insufficiency, late onset) - AC is the most sensitive measurement
CERVICAL LENGTH - Preterm Labor Screening
- Measured by transvaginal scan (TVS) - more accurate than transabdominal
- Normal: > 25 mm at 16-24 weeks
- < 25 mm at 16-24 weeks = increased risk of preterm birth
- < 15 mm = high risk
- Funneling of internal os (beaking) is an additional sign of incompetence
TIPS FOR GOOD IMAGE ACQUISITION
| Tip | Rationale |
|---|---|
| Full bladder for TAS in early pregnancy | Acts as acoustic window |
| Empty bladder for TVS | Better visualization |
| Use curvilinear probe for abdomen | Best depth/FOV for obstetric scanning |
| Harmonic imaging | Reduces noise, improves margins |
| Patient in left lateral tilt (>20 wks) | Reduces aortocaval compression |
| Increase depth for late third trimester | Fetal head may be deep in pelvis |
Sources: Creasy & Resnik's Maternal-Fetal Medicine (9780323828499); Langman's Medical Embryology (9781496383907); Tintinalli's Emergency Medicine (9781260019933); Pfenninger and Fowler's Procedures for Primary Care (9780323052672); Campbell Walsh Wein Urology (9780323546423); ISUOG Practice Guidelines
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