Value of ultrasound (POCUS) in first trimester pregnancy
The selection of POCUS as the initial imaging test compared to a radiology pelvic ultrasound (US) led to a 120-minute reduction in average ED length of stay in one single center randomized study.1 Typically these POCUS exams are performed using a standard low-frequency, transabdominal curvilinear probe. Resolution of the low-frequency curvilinear transducer limits identification of IUP if <6-7 weeks gestation, with a majority of literature estimating approximately 20% failure rate.2
Trick of the Trade: Use an ultrasound linear transducer
A recent prospective study by Tabbut et al. has shown a clinically significant decrease in the need for radiology performed transvaginal US when the high-frequency linear probe is used for detection of early IUP not seen with the curvilinear transducer.3 Briefly, here is what they found in the 81 patients studied:
- 27/81 patients did NOT have an IUP visualized with the curvilinear probe using POCUS.
- 9/27 had an IUP visualized with the high-frequency linear probe on POCUS.
- 18/27 did not visualize an IUP.
- 3/18 had an IUP seen on a radiology department performed transvaginal US study. None of the patients had an ectopic pregnancy.
Although this was a small sample size study, the authors demonstrated approximately a 10% decrease in failure rate when using high-frequency linear transducer (Figure 1).
Figure 1: Comparison of Image quality obtained with low-frequency curvilinear and linear transducer
Limitations of the linear transducer: The linear transducer is limited by beam penetration, with an average depth to proper identification of IUP being approximately 4 cm in the Tabbut study. Therefore image acquisition will likely be inadequate for deeper structures, a retroverted uterus, or patients using this transducer.
Technique: Identifying a first-trimester intrauterine pregnancy
Transabdominal curvilinear transducer
Scanning in the usual fashion using the curvilinear transducer, images of the uterus should be obtained in the sagittal and transverse planes (Figure 2 & 3).
Figure 2: Sagittal view of the uterus using a curvilinear transducer
Figure 3: Transverse view of the uterus using a curvilinear transducer
High frequency linear transducer
If no IUP is identified (defined as the presence of yolk sac and/or fetal pole within a gestational sac in the uterus), a transition to a linear transducer can be made. The increased frequency of the linear transducer provides improved spatial resolution up to a depth of approximately 5-6 cm depending on your transducer (Figure 4).
Figure 4: Improved resolution view of the uterus using a linear transducer
Take Home Points
The next time you may be imagining a yolk sac within a gestational sac with the curvilinear probe on POCUS, remember the magnifying glass in your back pocket – a high-frequency linear transducer.
References
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