Chapter 37: Intrapartum Imaging

ULTRASOUND

In the diagnosis of obstetric and fetal conditions, ultrasound is a valuable and frequently used tool. Traditionally, labor and delivery units have managed with “hand-me-down” scanners, retired from the hospitals' main ultrasound department. More recently, numerous feature-packed and extremely portable ultrasound machines have appeared that are designed for bedside imaging in the acute setting. This has allowed advances in prenatal ultrasound to be transferred to the intrapartum setting and enabled novel approaches to age-old assessments. Changing patient demographics, including increasing maternal obesity, are further drivers for change.

The chapter focuses on imaging techniques directly applicable to the process of labor and birth, the focus of the book. Labor units often function as an out-of-hours walk-in clinic or emergency room, triaging a variety of obstetric complaints, such as reduced fetal movement or nonspecific pain. Imaging undertaken in these situations is more related to the operational hours of the obstetric ultrasound unit and is not covered. Some other diagnostic uses of ultrasound, including preterm labor and antepartum hemorrhage, are covered in other chapters.

Safety of Ultrasonography

The American Institute of Ultrasound in Medicine Bioeffects Committee has noted in its “2008 Statement on Mammalian In Vivo Ultrasonic Biological Effects” that “information from experiments using laboratory mammals has contributed significantly to our understanding of ultrasonically induced biological effects and the mechanisms that are most likely responsible. The following statement summarizes observations relative to specific diagnostic ultrasound parameters and indices.

In the low-megahertz frequency range there have been no independently confirmed adverse biological effects in mammalian tissues exposed in vivo under experimental ultrasound conditions, as follows:

1. Thermal Mechanisms

   1. No effects have been observed for an unfocused beam having free-field spatial-peak temporal-average (SPTA) intensities below 100 mW/cm², or a focused beam having intensities below 1 W/cm², or thermal index values of less than 2

   2. For fetal exposures, no effects have been reported for a temperature increase above the normal physiologic temperature, ΔT, when ΔT < 4.5 – (log₁₀t/0.6), where t is exposure time ranging from 1 to 250 minutes, including off time for pulsed exposure”

Basic Intrapartum Imaging

The following ultrasound assessments should be within the skill set of most obstetricians practicing on labor and delivery. A key practical point is to always ensure the probe orientation matches the screen orientation. This ensures “left” and “cranial” are always depicted correctly on the screen. Gently moving your finger on one corner of the probe and observing the screen will confirm this before you start scanning.

Because ultrasound is transmitted poorly through air, the face of the transducer must be coupled to the patient's skin by a fluid medium, such as gel, so that the sound waves may penetrate the skin surface–air interface. Higher ultrasound frequencies are used on vaginal probes, ...