18.2 Major Structural Anomalies by System

ARRT Sonography Primary

Track your progress as you read

Finished sections get a check mark.

1 Introduction & Exam Overview

1.1 Current Exam Facts & Logistics 1.2 Scoring, Section Minimums & the August 2026 Cut-Score Change 1.3 Official Blueprint & Study Strategy

2 Patient Care: Ethics, Legal Practice & Communication

2.1 Ethical & Legal Aspects of Patient Care 2.2 Interpersonal Communication & Patient Education 2.3 Physical Assistance, Monitoring & Sonographer Ergonomics

3 Patient Care: Safety, Emergencies & Infection Control

3.1 Medical Emergencies & Critical Findings 3.2 Infection Control & Hazardous Materials 3.3 Ultrasound Safety & Interventional Procedure Care

4 Image Production: Transducers & the Ultrasound Beam

4.1 Piezoelectric Effect & Transducer Construction 4.2 Beam Configuration: Near Field, Far Field & Focus 4.3 Transducer Types, Selection, Care & Malfunctions

5 Image Production: Pulse Characteristics & Resolution

5.1 Pulse Characteristics: PRF, PRP, SPL & Duty Factor 5.2 Resolution: Axial, Lateral, Temporal, Elevational & Contrast 5.3 Frequency, Bandwidth, Q Factor, Power, Pressure, Intensity & Amplitude

6 Image Production: Imaging Modes & Doppler Principles

6.1 B-Mode & M-Mode Imaging 6.2 Spectral Doppler: Pulsed Wave vs Continuous Wave 6.3 Color Doppler & Power Doppler

7 Image Production: Optimization Controls

7.1 Power, Gain, TGC, Depth & Focal Zone 7.2 Frame Rate, Line Density, Dynamic Range, Harmonics & Compounding 7.3 Doppler Optimization Controls

8 Image Production: Sound–Tissue Interactions

8.1 Speed of Sound & the Range Equation 8.2 Acoustic Impedance, Reflection, Refraction & Scattering 8.3 Attenuation, Absorption & Intensity

9 Image Production: Signal Processing, Display & Bioeffects

9.1 Receiver, Signal Processing & Analog-Digital Conversion 9.2 Postprocessing, Magnification, Panoramic & 3D/4D Imaging 9.3 Bioeffects & Output Display Measures

10 Image Production: Image Evaluation & Artifacts

10.1 Criteria for Diagnostic Quality & Annotation 10.2 Grayscale Artifacts 10.3 Doppler Artifacts

11 Image Production: Real-Time Interpretation & Informatics

11.1 Real-Time Imaging, Echogenicity & Echotexture 11.2 Color & Spectral Analysis: Measurements and Waveforms 11.3 Imaging Informatics: HIS, RIS, EMR, PACS & DICOM

12 Procedures: Abdominal & Transplant Vasculature

12.1 Aorta, Its Branches & the IVC 12.2 Portal Venous System & Hepatic Vasculature 12.3 Liver & Renal Transplant Sonography

13 Procedures: Liver

13.1 Liver Anatomy, Variants & Examination Technique 13.2 Diffuse Liver Disease 13.3 Focal Liver Lesions

14 Procedures: Biliary System, Pancreas & Spleen

14.1 Gallbladder & Bile Ducts 14.2 Pancreas 14.3 Spleen

15 Procedures: Urinary Tract, GI Tract & Other Abdominal Structures

15.1 Kidneys & Ureters 15.2 Bladder & Prostate 15.3 GI Tract, Adrenals, Lymph Nodes, Hernias & Peritoneal Cavity

16 Procedures: First-Trimester Obstetrics

16.1 Early Gestation: Normal Development & Structures 16.2 First-Trimester Measurements, Dating & Maternal Anatomy 16.3 First-Trimester Abnormalities

17 Procedures: Second/Third-Trimester Biometry & Fetal Survey

17.1 Standard Biometry: BPD, HC, AC, FL & Growth Assessment 17.2 Fetal Anatomic Survey: Head, CNS, Face & Spine 17.3 Fetal Anatomic Survey: Heart, Chest, GI, GU, Skeleton & Fetal Position

18 Procedures: Chromosomal, Genetic & Structural Abnormalities

18.1 Chromosomal & Genetic Abnormalities 18.2 Major Structural Anomalies by System 18.3 Fetal Infection, Neoplasms & Hydrops

19 Procedures: Placenta, Amniotic Fluid, Cord & Cervix

19.1 Placenta: Normal, Previa, Abruption & Accreta 19.2 Amniotic Fluid Assessment & Growth Disorders 19.3 Umbilical Cord & Cervical Assessment

20 Procedures: Fetal Assessment, Doppler & Multiple Gestation

20.1 Biophysical Profile & Fetal Surveillance 20.2 Fetal & Uteroplacental Doppler 20.3 Multiple Gestations, Assisted Reproduction & Maternal Disease

21 Procedures: Gynecology — Uterus

21.1 Uterine Anatomy, Myometrium & Fibroids 21.2 Endometrium: Cyclic Changes, Hyperplasia & Cancer 21.3 Cervix & Congenital Uterine Anomalies

22 Procedures: Gynecology — Adnexa & Pelvis

22.1 Ovaries: Normal, Functional Cysts & Benign Masses 22.2 Ovarian Malignancy, Torsion & Fallopian Tubes 22.3 Cul-de-Sac, Vagina & Pelvic Floor

23 Procedures: Neck & Scrotum

23.1 Thyroid & Parathyroid 23.2 Salivary Glands & Neck Lymph Nodes 23.3 Scrotum & Testes

24 Procedures: Breast & Peripheral Vascular

24.1 Breast & Axilla 24.2 Venous Extremity Doppler 24.3 Carotid Doppler & Post-Catheterization Complications

25 Procedures: Pediatric, Interventional & Miscellaneous

25.1 Neonatal Head, Spine & Hip Sonography 25.2 Pediatric GI & Noncardiac Chest 25.3 Ultrasound-Guided Interventions, MSK & Superficial Masses

18.2 Major Structural Anomalies by System

Key Takeaways Explain This Topic AI

Anencephaly shows absence of the calvarium and cerebral hemispheres above the orbits (frog-eye appearance) with elevated maternal serum alpha-fetoprotein and polyhydramnios

Spina bifida is flagged by the intracranial lemon and banana signs — frontal bone scalloping and an anteriorly curved cerebellum with an obliterated cisterna magna — before the spinal defect itself is seen

Ventricular septal defect is the most common congenital heart defect, and tetralogy of Fallot combines a VSD, overriding aorta, pulmonary stenosis, and right ventricular hypertrophy

Gastroschisis is a right paraumbilical defect with free-floating, uncovered bowel and no significant aneuploidy association, whereas omphalocele is a membrane-covered midline cord-insertion defect strongly associated with trisomies 18 and 13

Posterior urethral valves produce the keyhole sign — a dilated bladder continuous with a dilated posterior urethra — with bilateral hydronephrosis and oligohydramnios in a male fetus

Last updated: July 2026

Central Nervous System Anomalies

Anencephaly is failure of the rostral neuropore to close, producing absence of the calvarium (skull vault) and cerebral hemispheres above the orbits. The prominent orbits with exposed disorganized tissue give the classic frog-eye appearance. Because the defect is open, maternal serum alpha-fetoprotein (MSAFP) is markedly elevated, and impaired swallowing causes polyhydramnios . Anencephaly is lethal and is considered the end stage of the acrania-exencephaly-anencephaly sequence.

Encephalocele is herniation of brain and meninges through a skull defect, most often occipital . It appears as a sac protruding from the cranium; when completely skin-covered, MSAFP may be normal.

Spina bifida (open neural tube defect of the spine) is usually detected first through two intracranial signs rather than the spine itself:

Lemon sign — bilateral scalloping of the frontal bones, so the skull contour resembles a lemon

Banana sign — the cerebellum is pulled down and curved around the brainstem (Arnold-Chiari II malformation), obliterating the cisterna magna

In the spine, look for splaying of the posterior ossification centers and a saccular myelomeningocele. Ventriculomegaly (atrial width > 10 mm) commonly coexists.

Holoprosencephaly is failed cleavage of the prosencephalon. The alobar form — the most severe — shows a single horseshoe-shaped ventricle, fused thalami, absent falx and cavum septi pellucidi, and midline facial defects; it is strongly associated with trisomy 13 . Dandy-Walker malformation combines cystic dilation of the fourth ventricle with vermian agenesis and an elevated tentorium; milder posterior fossa variants include inferior vermian hypoplasia and mega cisterna magna (> 10 mm). Agenesis of the corpus callosum produces colpocephaly — teardrop-shaped ventricles with dilated occipital horns — an absent cavum septi pellucidi, and a high-riding third ventricle.

Cardiac and Thoracic Anomalies

The ventricular septal defect (VSD) is the most common congenital heart defect and the easiest to miss on a four-chamber view; color Doppler across the septum improves detection. Hypoplastic left heart syndrome shows a diminutive or absent left ventricle with mitral and/or aortic atresia. Tetralogy of Fallot has four components: a VSD, an overriding aorta , pulmonary stenosis, and right ventricular hypertrophy; sonographically the overriding aorta is the key clue on the outflow views. An echogenic intracardiac focus is not a structural defect at all but a soft aneuploidy marker.

In the chest, congenital diaphragmatic hernia — usually left-sided through the foramen of Bochdalek — lets abdominal contents into the thorax. The signature finding is the stomach bubble in the chest beside the heart, with mediastinal shift to the right and polyhydramnios; lung compression causes pulmonary hypoplasia.

• CNS anomaly Signature finding
• Anencephaly Absent calvarium above the orbits, elevated MSAFP
• Spina bifida Lemon and banana signs, splayed posterior elements
• Alobar holoprosencephaly Single ventricle, fused thalami, absent falx
• Dandy-Walker Fourth-ventricle cyst, vermian agenesis
• Agenesis of corpus callosum Colpocephaly, absent cavum septi pellucidi

Gastrointestinal Anomalies

Esophageal atresia , usually with a tracheoesophageal fistula, prevents the fetus from routing swallowed fluid to the stomach, so the stomach bubble is absent or persistently small and polyhydramnios develops. The combination of an absent stomach bubble plus polyhydramnios should trigger a careful search for this diagnosis.

Duodenal atresia produces the double-bubble sign : two fluid-filled structures in the upper abdomen representing the dilated stomach and the dilated proximal duodenum, with polyhydramnios. Its importance is the association — roughly 30% of fetuses with duodenal atresia have trisomy 21 .

The two anterior abdominal wall defects are a favorite exam contrast:

• Feature Gastroschisis Omphalocele
• Location Right paraumbilical, beside the cord Midline, at the cord insertion
• Covering membrane None — bowel floats freely Yes — peritoneum/amnion sac
• Contents Bowel only (usually) Bowel and often liver
• Aneuploidy association Not associated Strongly associated (trisomy 18, 13)
• MSAFP Elevated Elevated (less so if covered)
• Other associations Young maternal age, bowel damage Beckwith-Wiedemann, cardiac defects
• Genitourinary Anomalies

Bilateral renal agenesis means no kidneys form; the renal fossae are empty and the flattened adrenal glands fill the space (the lying-down adrenal sign). Without fetal urine, severe oligohydramnios develops after about 16 weeks, producing the Potter sequence — pulmonary hypoplasia, limb contractures, and compressed facies — which is lethal.

Multicystic dysplastic kidney (MCDK) shows multiple non-communicating cysts of varying sizes replacing the kidney, with no normal parenchyma; it is usually unilateral, often involutes spontaneously, and is compatible with normal outcome if the other kidney is normal. Infantile (autosomal recessive) polycystic kidney disease instead causes bilaterally enlarged, diffusely echogenic kidneys with oligohydramnios.

Posterior urethral valves , seen in male fetuses, obstruct the outlet: the bladder is massively dilated (megacystis) and continuous with a dilated posterior urethra, forming the keyhole sign , accompanied by bilateral hydronephrosis and oligohydramnios.

Skeletal and Growth Anomalies

Macrocephaly — head circumference above the 95th percentile — prompts evaluation for hydrocephalus, hydranencephaly, intracranial masses, and overgrowth syndromes. Limb reduction defects range from absence of a single digit to amelia (complete absence of a limb) and phocomelia (hands or feet attached close to the trunk, historically linked to thalidomide). Club foot (talipes equinovarus) shows the foot fixed at a right angle to the lower leg in the same plane as the tibia and fibula; when isolated it is usually benign, but with other anomalies it raises the risk of aneuploidy.