prenatal us the basics - pedrad
TRANSCRIPT
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Prenatal US – The Basics
Dorothy Bulas, M.D. Professor of Pediatrics and Radiology
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Objectives
• Review guidelines for prenatal scanning
• Describe standard anatomic measurements
• Discuss normal anatomic structures identified on routine prenatal US
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Overview: Normal Maternal Anatomy
• Uterus and adnexa
• Placenta
• Cervix
• Amniotic fluid
Fetal Anatomy
• Sonographic predictors of gestational age
• Head, face, and neck
• Chest
• Abdomen
• Spine
• Extremities
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ACR Practice Guidelines & Technical Standards
Imaging Parameters for a Standard Fetal Examination
Minimum Elements of a Fetal Anatomic Survey
Benacerraf BR, et al. AIUM practice guideline for the
performance of obstetric ultrasound examinations. J Ultrasound
Med. 2010 Jan;29(1):157-66.
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Guidelines: 1st Trimester
• Location of pregnancy: (sac, embryo)
• Gestational Age: CRL, Mean gestational Sac
• Cardiac Activity (>5mm)
• Number: Chorionicity, amnionicity
• Nuchal region
• Uterus, cervix, adnexa
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Basic 2nd Trimester – WHO? WHEN?
• All pregnancies
• 18-22 weeks
• Well trained professionals
•Up to date equipment
–Real time
–3-5 MHz abdominal transducer
–Higher MHz linear transducer when possible
–≥5-10 MHz transvaginal transducer
–ALARA (power exposure)
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Basic 2nd Trimester – HOW?
6 BASIC STEPS
1. Maternal Anatomy
2. Location /Viability
3. Number
4. Biometry
5. Basic Anatomy
6. Report documentation
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1. Maternal Anatomy
Guidelines
• Uterus and adnexa
• Placenta
• Cervix
• Amniotic fluid
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Placenta • Discoid • Thickness = GA in weeks
(+/- 10 mm). max ~ 4 cm3
• Asses shape, location, distance from internal cervical os, retroplacental complex
Abnormal shape: circumvallate, succenturiate lobe (5%), masses Abnormal distance from os: > 2 cm from internal os; placenta previa, vasa previa Retroplacental complex: < 2 cm , hemorrhage, contraction, abruption
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Placenta Differential
Placental penetration
• Placenta accreta
• Placenta increta
• Placenta percreta
Types of placenta previa
– Low-lying
– Marginal previa
– Complete previa
– Central previa
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Placenta Previa
abnormal implantation of placenta over lower uterine segment or os
• Subtypes
• Low-lying placenta – within 2 cm of internal os
• Marginal previa –to edge but doesn’t cover
• Complete previa – covers internal os
• Central previa – placenta centered over internal os
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Cervix
• 3-4 cm in length. Incompetent <2.5 cm
• < 2 cm increased risk of preterm birth
• If abnormal, image post-void and use translabial or transvaginal sonography
• funneling
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Amniotic Fluid
• Subjective vs. AFI
• 6-24 cm normal AFI
• maternal
• PROM, Dm
• fetal
• obstruction, CNS
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2. Location/Viability
• Heart beating
–120-160bpm
• Fetal motion
• Fetal lie
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3. Number
• 1st trimester best time
• Chorionicity/amnionicity
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4. Biometry
• Gestational age
• Growth
•BPD/HC/AC/FL
• Rigorous technique
•Repeated measurements
•Report percentiles, mean for GA, curves
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Biparietal Diameter (BPD)
•Axial - Third ventricle and thalami
• perpendicular to falx
Symmetric hemispheres
•Measure outer skull margin to opposite inner skull margin
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Head Circumference
•Plane of third ventricle and thalami
•Cavum septum pellucidum anteriorly
•Outer to outer margin of skull
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Abdominal Circumference Axial – round
• Measure skin line on transverse view
• level of the junction of the umbilical vein, portal sinus, and fetal stomach
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Femur Length • Measured long axis of diaphysis –
• cursors at junctions of bone with cartilage
• avoid obliquity / exclude cartilaginous epiphyses
• Mild curved appearance normal
• <60 degree insonation
FL mild shortening
• ?fetal chromosomal abnormality or syndrome
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5. Basic Fetal Anatomy
• Be SYSTEMATIC
• Only way to not miss major anomalies
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SITUS
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Fetal Head
• Skull
• Hemispheres – ventricles, choroid
• Midline- cavum, falx
• Posterior fossa- Cerebellum, cisterna magna
• Nuchal thickness
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Ventricles
•transverse plane at distal atrium
•cursors inside/perpendicular to
ventricular walls at level of choroid
• N <10 mm •3rd ventricle should not exceed 3 mm in diameter
•4th ventricle triangular
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presence of CSP to exclude agenesis of the corpus callosum
Cavum Septum Pellucidum
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Corpus Callosum
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Cerebellum
•Bilobed shape
•Hyperechoic midline vermis
•Hypoechoic cerebellar hemispheres
•Growth proportional to GA
•Cisterna Magna
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Nuchal Fold
•Midline soft tissue posteriorly from outer surface of occipital bone
•Measured standard posterior fossa plane b/w 15 -21 wks
•> 6 mm abnormal
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Guidelines – orbits/mouth, lip, nose
Coronal plane - upper lip, philtrum, nose, brow, orbits exclude cleft, midface anomalies
Sagittal plane - micrognathia, frontal bossing
Lips/Face
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Nasal Bone Orbit
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Rule of 3’s
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Spine
• longitudinal and transverse - cervical, thoracic, lumbar, and sacral spine
• transverse images best
• Overlying Skin
• Ossification centers
• Head to sacrum
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Spine
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Spine - Axial • Vertebra 3 ossification centers
• Posterior 2 ossification centers (lamina) oriented inward or parallel
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Chest: Normal
Size, appearance, shape
Lung parenchyma- homogeneous
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4 Chamber Heart
Axial
Apex deviates leftward 45 degrees
Chamber closest to the chest wall is right ventricle
Chamber closest to the spine is left atrium
Evaluate atria, ventricles, septa, valves
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Right Ventricular Outflow Tract (RVOT)
•Short axis base view of the heart –
• perpendicular origins of RVOT and aorta
•Cephalad from standard 4 chamber view - axial plane towards base of heart
• exclude outflow tract anomalies (e.g. truncus arteriosus, transposition)
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Left Ventricular Outflow Tract (LVOT)
• Scanning left ventricle and then scanning toward the right shoulder of fetus - longitudinal axis view of LVOT
• Rotation in other direction obtain RVOT
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3 vessel view
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AIUM Guidelines
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Abdomen
•Stomach ( presence, position)
•Bowel (< 2mm, < bone, no free fluid)
•Cord insertion
•Genitalia ( when indicated)
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Cord Insertion
3 vessel cord inserting into midline anterior abdominal wall
•Omphalocele midline with cord inserting into defect; gastroschisis lateral to cord insertion
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3 Vessel Cord
2 umbilical arteries and 1 umbilical vein
• single uterine artery associated with 30-60% increased incidence of other anomalies
Umbilical arteries along bladder margin caudally
Umbilical vein courses cephalic into liver
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• 2 kidneys Length = GA
• Corticomedullary differentiation
• Minimal pyelectasis normal if < 4-5 mm
Kidneys
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Anechoic round, ovoid pelvic structure
Absent or distended bladder requires further evaluation
Bladder
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Extremities
• 4 limbs - Check presence of humerus, radius, ulna, femur, tibia, fibula
• Exclude club foot
• Assess hands
• Assess Motion
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Extremities
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Extremities
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Dopppler Normal umbilical artery /vein
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Umbilical artery: Reversal of flow
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MCA Doppler
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MCA: increased diastolic “brain sparing”
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Portal Vein Ductus Venosus
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Monophasic
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Report
• Date
• GA
• Limits of exam
• Anomalies
• Recommendations
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Summary
• Appropriate screening requires complete assessment of fetal anatomy
• Optimal time 18-22 weeks
– ? First trimester
– ? Third trimester
• Need to know anatomy, optimal positioning, and gestational age ranges to distinguish normal from abnormal