Obstetric imaging Introduction
The imaging technologies that may be used in obstetric practice are: Ultrasound (US)
Transvaginal ultrasound (TVUS)
Colour Flow Imaging (CFI)
Conventional Radiography (X-ray)
Computed Digital Radiography (CDR)
Computed Tomography (CT)
Magnetic Resonance Imaging (MRI)
Sometimes termed conventional, or transabdominal, ultrasound is the most widely available and most used imaging technique in obstetrics. It is safe, provides images and measurements essential to obstetric practice and generally only requires the inconvenience of a full bladder technique for adequate visualisation of the lower uterus.
Also known as endovaginal or endolumenal US, it by-passes the need for a full bladder. Being closer to the uterus and ovaries enables higher frequency transducers and therefore higher resolution images to be obtained.
It is indispensible in the first trimester and its problems, and in assisted reproduction (IVF and GIFT). In this field it plays an important role in gynaecological interventional imaging, particularly ovum pickup, and the relatively new technique of coelomocentesis which may replace chorionic villous biopsy. In the third trimester it can be useful in difficult cases of suspected placenta praevia.
Virtually, all manufacturers supply an appropriately constructed probe that works from the standard transabdominal US units. The use of a protective condom on the probe is mandatory. The probe can be introduced by the sonographer or the patient herself, the latter allaying anxiety and embarrassment.
The patient's position can either be in a supine frog-leg on a flat bed, or lithothomy, or even decubitus depending on the preferences of the operator.
Colour Flow Imaging (CFI)
CFI is based most commonly on the colour Doppler imaging principle (CDI) or colour velocity imaging principle (CVI).
Colour Doppler has given rise to anxieties regarding insonation power levels and possible hazard to the developing fetus. Most manufacturers have built - in safety features that will set the Doppler to lower levels when obstetric examinations are activated. No harm has even been demonstrated in actual practice. Colour velocity imaging does not generate the same anxieties and therefore has a theoretical safety advantage.
Computed digital radiography (CDR)
CDR uses the enhancing qualities of digitized images and computers and substantially reduces the dose of ionizing radiation to mother and fetus. It would therefore be ideal for chest x-ray in pregnancy and for pelvimetry, when MRI is unavailable.
Computed tomography (CT)
Despite the generally high radiation doses produced by CT a significant dose reduction can be obtained in CT pelvimetry using the CT scout film function which is essentially a digitally enhanced computed radiograph. CT is more widely available than CDR apparatus.
Magnetic Resonance Imaging (MRI)
MRI in obstetrics is still in its very early phase. There are three major factors to be considered: availability and cost, unsureness of obstetricians and anxiety regarding potential hazards.
Although MRI does not involve ionizing radiation, potential hazards may arise from the use of such unnaturally powerful static and time variable magnetic fields required in MRI. An increased rate of ocular abnormalities in a susceptible mouse strain has been demonstrated. Therefore, in early pregnancy the use of MRI must be carefully considered.
In later pregnancy, hazard may still arise from the excessive temperature rise caused by the heat deposition as a result of the changing radiofrequency fields.
The Department of Health in the United Kingdom has laid down guidelines regarding patient exposure. They suggest that there should be a lower level below which exposure may be considered to be safe and an upper level which present knowledge suggests should not be exceeded.
Their recommendations suggest that the first level is no greater than 2 W/kg averaged over 15 minutes. This should ensure a rise of less than O.5°C for the mother and a temperature no greater than 38°C for the fetus. The ambient temperature should be kept below 22°C and adequate air flow for cooling is required. In exceptional circumstances these guidelines may need to be disregarded.
Different sequences used in MRI not only produce images with different appearances and ease of identification of structures but also different levels of heat deposition. Furthermore, equipment design may affect heat deposition for the same apparent sequence. Therefore, each unit should be assessed for its own heat deposition as the same assumptions cannot be made for different designs.
The role of MRI in obstetrics has so far been in examining disorders in the maternal brain, liver and adrenals in conditions such as: eclampsia, acute fatty liver of pregnancy and phaeochromocytoma. It can replace radiography, CDR and CT in pelvimetry. It is finding increasing use in diagnosis of fetal malformation but at present only as a technique supplementary to prior us.
Con Metreweli