High-frequency linear transducer improves detection of an intrauterine pregnancy in first trimester ultrasonography
The Article: M. Tabbut, D. Harper, D. Gramer, R. Jones. “High-frequency linear transducer improves detection of an intrauterine pregnancy in first trimester ultrasonography.” American Journal of Emergency Medicine 2016: 34, pp. 288-291.
The Idea: Ectopic pregnancy is a dangerous early pregnancy complication with a 6-9% maternal mortality rate. Traditionally, if a transabdominal ultrasound with a curvilinear probe is negative or equivocal, a transvaginal ultrasound is required. Transvaginal ultrasounds are more invasive, more time consuming (often requiring radiology), require specific disinfection, and are not available at all centers. Anecdotally, the authors used the linear probe transabdominally to confirm IUPs instead of the transvaginal probe.
The Methods: This study is a single center, prospective, convenience, pilot sample of 88 women presenting to the ED in the first trimester who required ultrasonographic evaluation for IUP. Inclusion criteria were positive pregnancy test and no prior imaging during that pregnancy. Seven women were excluded based on incomplete data. Patients were initially scanned with the curvilinear probe transabdominally, then, if no IUP was seen, with the linear probe, also transabdominally. If neither exam showed positive IUP, transvaginal ultrasound was performed.
The Results: In this study, the curvilinear probe alone has a failure rate of 33.3% (9/27), but when combined with the linear transducer, the rate decreases to 22.2% (6/27). The combined rate is similar to the previously reported failure rate of 20% for the combination of the curvilinear and transvaginal probes. Additionally, using the linear probe, the researchers were able to reduce the number of women requiring transvaginal ultrasound by 33.3% (6 vs. 9). The 15 pregnancies that were not detected by any probe were likely too early based on hCG levels. The only limitation with the linear transducer is depth of the pregnancy and other structures since it sacrifices beam penetration for image quality. Thus, it may not be reliable in obese patients. Also, the adnexa were not explored via this method, so if a more comprehensive evaluation is required, the transvaginal probe is still preferred.
Takeaways: The linear probe is a comparable method to transvaginal ultrasound to evaluate for IUP in first trimester pregnancies confirmed by positive pregnancy test in the ED. It can decrease the failure rate of the curvilinear probe to almost the same as with the transvaginal probe (22.2% vs. 20%), thus providing an accessible, faster alternative to transvaginal ultrasound when confirming an IUP.
Post by Jani Swiatek, MS4