US

Obstetric Ultrasound Study Notes

Ultrasound Transducers

• Linear Array
    - Piezoelectric crystals set in a line
    - Produces a rectangular image
    - Wide near field but limited depth
    - Common uses: Vessels, Musculoskeletal (MSK), Breast

• Curved Array
    - Crystals arranged in a curve
    - Produces a wide near field and a wide far field (fan shape)
    - Common uses: Abdominal, Obstetrics, Gynecology (Gynae)

• Phased Array
    - Crystals are fired in phases, producing a "pizza" shaped image
    - Common uses: Cardiac imaging, neonatal head, abdomen

Ultrasound Controls

• Power
    - Output from the transducer; less power required for patients with lower BMI
    - Excessive power can lead to noise and reverberation, obscuring details
    - Bioeffects such as overheating, cavitation, and radiation forces are monitored by:
      - Mechanical Index (MI)
      - Thermal Index (TI)

• Focus
    - Allows adjustment of the ultrasound beam focus onto different structures during scanning

• Gain
    - Controls the brightness of the image by amplifying received signals
    - Increasing gain may increase image noise, artifacts, and reduce contrast

• Depth
    - Adjusts the scanning depth; increases/decreases with impact on resolution

• Sector Width
    - Determines the scan area by widening or narrowing the sector angle to maximize the Region of Interest

Reasons for Scanning During Pregnancy

• Assess the location of pregnancy

• Confirm viability

• Determine gestational age

• Identify single or multiple pregnancies

• Assess fetal normality

• Evaluate fetal growth and wellbeing

1st Trimester Scan - Objectives

• Location of pregnancy confirmation

• Verify viability

• Measure gestational age using Crown Rump Length (CRL)

• Identify single or multiple pregnancies

• Screening for fetal normality, specifically:
    - Nuchal Translucency measurement
      - Part of the first trimester Down Syndrome Screening Programme (optional)

Key Measurements in 1st Trimester

• Crown Rump Length (CRL)

• Nuchal Translucency (NT)

• Head Circumference (HC)

• Abdominal Circumference (AC)

• Femur Length (FL)

1st Trimester Anatomy Survey

• Head
    - Assess shape and cranial vault development
    - Ensure symmetry and exclude gross hydrocephalus

• Thorax
    - Size and situs of the heart (four-chamber view may be seen)

• Abdomen
    - Inspect cord insertion to exclude abdominal wall defects
    - The stomach and bladder are visible in most fetuses

• Limbs
    - Examination of limb structure and abnormalities

2nd Trimester Scan - Fetal Anomalies

• Common anomalies detected include:
    - Open Spina Bifida
    - Anencephaly
    - Cleft lip
    - Diaphragmatic hernia
    - Gastroschisis
    - Exomphalos
    - Serious cardiac anomalies
    - Bilateral renal agenesis
    - Lethal skeletal dysplasia
    - Edward’s Syndrome (T18)
    - Patau’s Syndrome

3rd Trimester (Growth) Scan

• Not routine but recommended for women with major risk factors

• Referrals for ultrasound measurement of fetal size and wellbeing are necessary for:
    - Underweight or overweight patients
    - Advanced maternal age
    - Hypertension
    - Diabetes
    - History of previous stillbirth

• Abdominal Circumference (AC)
    - Length of the outer perimeter of the fetal abdomen
    - Most important measurement in assessing fetal size and growth

Role of Sonographer in Obstetrics

• Invaluable in diagnosing and managing pregnancy-related issues

• Requires proficiency in operating advanced technology

• Must possess advanced knowledge of physiological, sociological, and cultural factors relevant to patient management

• Effective communication is essential:
    - Non-Directional
    - Informative tailored to understanding levels
    - Honest & Empathetic communication is critical

Consent in Obstetric Ultrasound

• Consent requires sufficient information and time for informed decision-making

• Information can be delivered in both written and verbal forms

• Screening programs must be opt-in or opt-out

• Informed consent is a professional obligation

Common Ultrasound Terms

• Anechoic
    - Area appearing dark or black on the image, indicating no internal echoes

• Isoechoic
    - Areas showing reflected echoes similar to surrounding tissue

• Hyperechoic
    - Areas with more reflected echoes (brighter) than surrounding tissue

• Hypoechoic
    - Areas with fewer reflected echoes (darker) compared to surrounding tissue

Common Pathologies Diagnosed by Obstetric Ultrasound

• Spina Bifida
    - A type of neural tube defect/spinal dysraphism with varying severity; most common congenital CNS malformation
    - Result of abnormal neural tube development; multifactorial etiology with genetic and environmental components
    - Adequate folic acid intake offers protective benefits
    - High detection rate via ultrasound; may show splayed dorsal ossification centers or lateral pedicles
  

• Hydrocephaly
    - Refers to significant ventricular dilatation; often an extension of fetal ventriculomegaly
    - Can be obstructive or non-obstructive from various aetiologies
    - Ultrasound detects enlarged ventricles and may display choroid as dangling choroid sign
  

• Cleft Palate
    - Common neonatal facial anomaly, representing an opening in the roof of the mouth due to incomplete tissue fusion
    - May include cleft lip; detected via vertical hypoechoic region through the fetal upper lip on ultrasound
  

• Diaphragmatic Hernia
    - Occurs from failure of fusion of pleuroperitoneal canals around 8 weeks gestation; can involve stomach, intestines, liver, or spleen
    - Common non-cardiac fetal intrathoracic anomaly; can be left-sided (most common), right-sided, or bilateral
    - Two types: Bochdalek and Morgagni
    - Ultrasound may show absent bowel loops in the abdomen, liver herniation into the chest, bowel movements in the chest, and reduced abdominal circumference