The use of ultrasound to identify milk ejection in women – tips and pitfalls
© Geddes. 2009
Received: 24 February 2009
Accepted: 01 June 2009
Published: 01 June 2009
Diagnostic ultrasound imaging of the breast has been limited principally to the abnormal, non-lactating breast. Due to the rapid improvement of imaging technology, high-resolution ultrasound images can now be obtained of the lactating breast. Ultrasound scanning techniques, however, require modifications to accommodate the breast changes that occur in lactation. Furthermore, the function of the breast with regard to milk ejection can be assessed with ultrasound by identification of milk duct dilation and milk flow. At milk ejection, the echogenic duct walls expand as milk flows forward towards the nipple. Milk flow appears as echogenic foci rapidly moving within the milk duct. This paper provides a detailed description of the ultrasound technique used for the detection and reviews nuances associated with the procedure.
Although milk ejection is integral to lactation and thus the survival of the species, the lack of knowledge regarding milk ejection in women, in comparison to other lactating and dairy animals, is surprising. Oxytocin is a vital hormone for the maintenance of lactation, yet there are no studies to determine if women with lactation problems, such as low milk production, have normal milk ejections/oxytocin release. Since both milk synthesis and milk ejection must occur to ensure successful lactation, methods are required to assess both processes. Currently, milk production can be estimated in a relatively non-invasive way by the test weigh method , whereas milk ejection can be assessed using ultrasound imaging . Recent improvements in ultrasound equipment have allowed high resolution imaging of the lactating breast . Ultrasound monitoring offers a safe, non-invasive alternative to both serial blood sampling (to detect oxytocin levels) and intra-ductal pressure measurements (cannulation of a duct through the nipple pore). Furthermore, ultrasound duct dilation has been correlated with milk flow rates during pumping [4, 5]. This paper will describe the ultrasound technique and its analysis in detail and also provide visual examples of milk ejection to aid the clinician or researcher to apply this method.
Ultrasound imaging of the lactating breast requires equipment capable of resolving the ductal structures of the breast. The diameter of the main milk ducts (beneath the areola) in the lactating breast range from less than 1 millimetre up to 10 millimetres [2, 16]. The near field resolution of the ultrasound system (subcutaneous portion of the breast) should be as high as possible, as the ducts are very superficial in the areola region. In some mothers there is no visible intervening tissue between the duct and the overlying skin. Depending on the resolution of the system, a standoff may be necessary to improve focusing of the transducer superficially. An electronically focused linear array transducer with a frequency of 7–12 MHz and multiple focal zones is appropriate for breast imaging and, in particular, for the identification of milk ejection .
The time compensation curve (compensates for the normal attenuation of the sound waves in the tissue) is generally a gentle slope. The gain setting compensates for attenuation of the ultrasound beam without discriminating for depth, thus amplifying all of the returning echoes . Too high a gain setting will eliminate visualisation of the ductal walls, whereas too low a gain setting may eliminate the visualisation of milk flow at milk ejection. A compromise may be necessary in some women, particularly those where duct dilation is minimal and one must rely more on milk flow for identification of milk ejection. One focal zone focused at the level of the monitored duct is normally sufficient, however two focal zones may be considered in women with larger ducts (>5 mm). The depth setting should be optimized to show the main ducts that are superficial in the breast (20 mm). Too great a depth setting will result in the ducts appearing smaller and increase the possibility of not detecting a very small increase in duct diameter at milk ejection. The power setting should be high enough to ensure adequate visualisation, but can often be reduced due to the limited depth of insonation. The dynamic range is approximately 60 to 70 dB, depending on the particular machine.
Ultrasound technique for detection of milk ejection in the lactating mother
Normal ultrasonic appearances
Ultrasonic appearances of the lactating breast
The ultrasonic appearances of the structures of the lactating breast (adapted from Geddes )
Structures of the breast
Ultrasonic appearance of the lactating breast
Hypoechoic, can contain echogenic flecks representing milk fat globules
Echogenic walls may be visible when insonated at 90 degrees
Distend at milk ejection
Resting state 2 mm (range;1–10 mm)
Increased thickness in the areola region
Stromal fibrous tissue
Predominantly hyperechoic – tends to be more echogenic with more milk in the breast
Hypoechoic, variable amounts
Large breasts often contain a large proportion of adipose tissue
Arteries and veins
Hypoechoic, demonstrate blood flow on colour Doppler imaging
Ultrasound appearances of milk ejection
Summary of ultrasonic features particular to milk ejection in the lactating breast
Ultrasonic change at milk ejection
Milk duct diameter increases
Milk flow (echogenic flecks) towards the nipple
Sensation of milk ejection felt
Pumping – visualisation of jets of milk, rapid increase in milk flow
Breastfeeding – change in sucking to slower more rhythmical pattern
If flow is very fast the infant may pull off the breast
Milk duct diameter decreases
Milk flow reverses back into the breast
Sensation of milk ejection
Pumping – visualisation of milk jets, slowing of milk flow
Breastfeeding – slow, more rhythmical pattern
Infant may discontinue feeding during milk ejection if appetite met
Analysis of ultrasound for milk ejection
Clinical relevance of monitoring for milk ejection
The milk ejection process is critical to successful lactation, yet there is currently a lack of methods to assess whether or not milk ejection has occurred. This is particularly relevant for the proportion of women who do not sense milk ejection. Milk flow rate during pumping has been associated with milk ejection imaged as an increase in duct diameter, therefore measurement of flow rate may be useful to confirm milk ejection in women who are pump dependent, such as mothers of preterm infants and those women who can pump successfully. Unfortunately, a proportion of women are unable to express substantial amounts of milk with a breast pump, and for these women ultrasound imaging would provide a means of verifying a normal milk ejection reflex.
It is important that ultrasound technicians performing a diagnostic breast scan be familiar with both the ultrasonic and clinical signs of milk ejection. A woman who volunteered for a research study had a history of a prior ruptured breast abscess. As a result she had a soft lump that increased transiently in size during feeding. Scanning of the lump during breastfeeding showed that the lump contained functional milk ducts that expanded at milk ejection (Additional file 5) and was not a fluid filled cavity requiring aspiration. Spontaneous milk ejection may occur during scanning, particularly if either the breast is full of milk or the transducer stimulates the nipple initiating an oxytocin release. Furthermore, awareness of the symptoms of milk ejection may be helpful when scanning women who experience pain in their breasts. This would facilitate efforts to determine if the sensation of pain coincides with milk ejection.
Ultrasound imaging is a reliable, non-invasive means of identifying duct dilation that occurs during milk ejection during either breastfeeding or breast expression. The examination requires high-resolution ultrasound equipment and specific attention to anatomical appearances specific to the lactating breast. This technique has many potential applications for both the research and clinical lactation field.
I would like to thank the mothers and infants who participated in the studies that contributed to this manuscript and Professor Peter Hartmann for his critical revision of the intellectual content of this manuscript. The support of the Women and Infants Foundation for the use of facilities is also gratefully acknowledged.
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