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Trick of the Trade: Linear Transducers for Point of Care Ultrasound in Intrauterine Pregnancy Evaluation

2017-07-19T00:05:47+00:00

Ultrasound in Intrauterine PregnancyThe volume of women presenting to the emergency department (ED) with newly diagnosed first-trimester pregnancies and suspected ectopic pregnancies sometimes seems like an infinitely growing number. As ED physicians, proper identification of an intrauterine pregnancy (IUP) in these patients is of paramount importance and the initial imaging test of choice for many has become bedside point-of-care ultrasound (POCUS).

Value of POCUS ultrasonography 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 a 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 IUP

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.

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1.
Tabbut M, Harper D, Gramer D, Jones R. High-frequency linear transducer improves detection of an intrauterine pregnancy in first-trimester ultrasonography. The American Journal of Emergency Medicine. 2016;34(2):288-291. doi: 10.1016/j.ajem.2015.11.001
2.
Wilson S. Point-of-care ultrasound versus radiology department pelvic ultrasound on emergency department length of stay. World J Emerg Med. 2016;7(3):178. doi: 10.5847/wjem.j.1920-8642.2016.03.003
3.
Tayal VS. Outcome of Patients with an Indeterminate Emergency Department First-trimester Pelvic Ultrasound to Rule Out Ectopic Pregnancy. Academic Emergency Medicine. 2004;11(9):912-917. doi: 10.1197/j.aem.2004.03.017

Ashley Sullivan, MD Expanded Review

Comment #1: In this prospective observational cohort study, of the 1,490 patients who met inclusion criteria, 300 patients (20%) had an ‘indeterminate’ pelvic ultrasound. Of those 300 patients classified as ‘indeterminate,’ 53% were due to embryonic demise, 29% with an IUP, 15% with an ectopic, and 3% were unknown. While this study does highlight the limitation of a curvilinear probe, the cited study numbers may be somewhat misleading. Of the 20% of indeterminate scans, only 88/300 patients actually went on to have an IUP on transvaginal ultrasound. It raises the question: Is there a 20% failure rate of a curvilinear probe to detect an IUP? Or is there a 20% failure rate of a curvilinear probe to diagnose the etiology of a symptomatic first trimester patient.

Ashley Sullivan, MD
St. John Hospital and Medical Center; Associate Director Emergency Ultrasound Fellowship, Wayne State University School of Medicine

Arun Nagdev, MD Expanded Review

Comment 1: This was a convenience sample of patients who presented to the ED with positive pregnancy test. There could have been clear selection bias based on body habitus, suspected weeks of gestation, etc. It would have been nice to know how many women came to the ED during this period with a positive pregnancy test, why they were excluded, and how this compared to the sample studied.

Comment 2: In the methods section, the scanning technique does not indicate if depth or zoom adjustments were made. These two adjustments often are key in identifying an IUP with a standard curvilinear transducer. This may have reduced the number of patient who had a negative curvilinear study and a positive linear study. Also, it is unclear if the bladder was full or empty when performing these scans. When using a curvilinear/low-frequency transducer, a full bladder will improve imaging.

Comment 3: Even though there are clear biases in the study, and a repeat study with better methods may not demonstrate such strong positive numbers, this seems like an easy addition to standard practice. A repeat study with better methodology may not show similar results.

Comment 4: If the patient\'s bladder is full, a high frequency linear transducer will not be ideal. The uterus will be too deep and the operator will not be able to image the uterus. The patient should empty their bladder if you plan to use a linear transducer.

Arun Nagdev, MD
Director of Emergency Ultrasound, Ultrasound Fellowship Director, Highland General Hospital
Mark Favot, MD

Mark Favot, MD

Assistant Professor of Emergency Medicine
Wayne State University School of Medicine
Co-Director, Emergency Ultrasound Fellowship
Detroit Medical Center/Wayne State University