4
Imaging in obstetrics and gynaecology
Introduction
Imaging plays an important role in both obstetrics and gynaecology. Ultrasound is extremely valuable at all stages of pregnancy: early in pregnancy for viability and establishing gestation, mid-pregnancy to look for fetal abnormality and later in pregnancy, for growth, well-being and presentation. Ultrasound is also very useful in gynaecology to assess both the uterus and the ovaries, and further information can be obtained with computed tomography (CT) and magnetic resonance imaging (MRI) if required. Other uses of imaging will also be discussed.
Obstetrics
Fetal assessment
Ultrasound is an excellent modality for pregnancy. In particular, it has a rapid acquisition time (useful when movement is present), is portable and convenient (can be used in labour ward if required), is almost certainly extremely safe and is socially acceptable. Its accessibility, however, occasionally means that there is a temptation for untrained practitioners to use it and, although good images can easily be obtained, interpretation is often quite subtle – even in simple terms of orientation.
When scanning in pregnancy, it is important to determine the number of fetuses, their presentation and position, the presence of fetal heart activity and the position of the placenta. The rest of the scan will then depend on gestation: gestation and viability in early pregnancy (Chapter 10) nuchal measurements between 11 and 14 weeks; fetal abnormality between 18–21 weeks (Chapter 33); and growth and well-being for the remainder of pregnancy (Chapter 35). Finally, cervical length (Chapter 37) can be used to predict the likelihood of pre-term delivery.
Growth measurements are made at specific anatomical sites to ensure reliability and reproducibility. The head, abdominal circumference and femur length are usually measured, and can be charted to quantify growth (Figs 4.1–4.3). It is important to recognize that, even in the most skilled hands, these measurements have a margin for error and that clinical decisions should not be based on small changes on the charts. Fetuses who are apparently very large may well deliver normally, and babies who appear small may simply be small-for-dates rather than growth-restricted (Chapter 35
