Shear wave elastography of the maternal cervix: A transabdominal technique

Abstract

1Objectives This research aimed to identify biological and technical confounders in the non-2 pregnant cervix when applying shear wave elastography with an endovaginal transducer.3 Methods Cervical speed measurements were obtained at the internal and external os, anterior 4 and posterior portions of the cervix using a transvaginal approach in 69 non-gravid patients. 5Results Reliable measurements were obtained at the external os and internal os, anteriorly 6 and posteriorly, in 63, 55, 55 and 26 patients respectively. The mean speed obtained at the 7 external os, anteriorly and posteriorly, was 2.52 ± 0.49m/s and 2.87 ± 0.63m/s respectively, 8 and at the internal os anteriorly and posteriorly, 3.29 ± 0.79m/s and 4.10 ± 1.11m/s 9 respectively. The difference in speed between all regions was statistically significant 10 (p<0.05). 11 Conclusion Ultrasound -induced artifacts appear to affect the transmission of the 12 elastographic main pulse, with cervical position contributing to suboptimal shear wave 13 production in the posterior cervix. Reliable shear wave propagation can be achieved in the 14 anterior cervix in most patients. 15 16Retention of a pregnancy requires the cervix to maintain strength to withstand multiple forces 22 from the uterus, from the weight of the growing fetus and amniotic sac, and also passive 23 pressure from the uterine wall. The cervix softens and shortens throughout pregnancy and 24 finally dilates for the fetus to be delivered vaginally. Premature softening of the cervix is 25 associated with early shortening of cervical length and subsequent spontaneous preterm birth 26 (SPTB). 1 The cervix can be described as soft, firm or medium based on a digital 27 examination, but this method is subjective to the clinician, and creates difficulties due to the 28 expectation that the cervix will soften initially at the proximal portion. 1 29Currently, the length of the cervix assessed with transvaginal ultrasound (TVU) is the feature 30 that is assessed to indicate cervical strength and premature softening. 2 A short cervix has 31 been shown to be a significant risk factor for subsequent SPTB. 3 In women with a high risk 32 of SPTB due to medical history, a shortened cervical length on TVU has a sensitivity of over 33 50% for subsequent SPTB. However, in low risk women the sensitivity is reduced to 37%, 4,5 34 and the appropriate method for screening for SPTB in these women is yet to be established. 35With preterm birth affecting 13 million babies every year and the implications for neonatal 36 mortality and morbity, 6 there is a need for a non-invasive technique to assess cervical 37 strength with greater sensitivity than length alone. 38Ultrasound elastography assesses mechanical properties of tissues in the region being 39 examined. The basis for this technique is that soft tissue deforms differently from firm tissue 40 and the elastographic images reflect this difference. 7 Utilizing strain elastography, it has 41 been proposed it may be possible to identify women in the historically low r...

Shear Wave Elastography of the Maternal Cervix

“…The elastographic opacity was set to 0.6. The ROI was set to a 5 mm circle (equating to a 5 mm diameter) to obtain the mean SWS and standard deviation 14 …”

“…The maternal bladder was partially filled to provide a B‐mode window for visualization of the cervix and through‐transmission of the shear wave ARFI pulses to the cervix. The total bladder volume was varied among patients, with bladder filling adequate to allow the superior‐to‐inferior margins of the bladder to cover the length of the cervical canal 14 . With the patient in a supine position, the transducer was placed longitudinally just cephalad to the symphysis pubis in the midline, providing a sagittal view of the cervix.…”

“…The transducer aperture was aligned to the length of the cervical canal and tilted to as close to a perpendicular approach to the canal as technically possible. Transducer pressure was reduced to a minimum while still providing good visualization of the cervix on the B‐mode image before the application of the shear wave 14 …”

“…This allowed visualization of the internal os and external os on the same image. Transducer pressure was reduced to a minimum while still providing good visualization of the cervix on the B‐mode image before the application of the shear wave 14 …”

“…Some ultrasound vendors provide shear wave technology on a curvilinear transabdominal (TA) transducer and also on intracavitary TV transducers. It has been shown that it is possible to assess the maternal cervix with shear wave elastography in both the TV and TA ultrasound approaches, 9,14 although it is expected that the different excitation frequencies of the ARFI pulses used for each transducer may produce different SWS values in viscoelastic tissues, 15 as well as different mechanical stresses on the cervix due to the different body posture during scanning. It is not clear whether SWSs obtained in the maternal cervix would be concordant between the TA and TV ultrasound approaches, and this could be of importance for future clinical use.…”