Three-dimensional ultrasound of uterine cervix in pregnancy: prospects

Abstracts

A medida do comprimento cervical por ultra-sonografia transvaginal é útil no rastreamento do parto prematuro, sendo o encurtamento do colo fator preditor do trabalho de parto pré-termo. Os métodos tradicionais para avaliar a cérvice na gestação são limitados e insatisfatórios. O exame de toque digital, considerado método padrão, demonstra variação entre diferentes examinadores, entretanto, a ultra-sonografia transvaginal é exame eficiente durante a gravidez. Recentemente, o exame ultra-sonográfico tridimensional tem sido utilizado na prática clínica, incluindo o estudo do colo. Grande volume de informações pode ser obtido e armazenado utilizando-se a tecnologia tridimensional. A informação armazenada permite ser manipulada e analisada por número infinito de planos. O exame ultra-sonográfico transvaginal tridimensional é o único capaz de obter o plano coronal pela visualização da imagem em organização multiplanar. Este método aparenta oferecer potencial diagnóstico no aumento da acurácia da ultra-sonografia cervical.

Colo uterino; Gravidez; Ultra-sonografia tridimensional

COMMUNICATION

Three-dimensional ultrasound of uterine cervix in pregnancy: prospects ^*

Rosieny Souza Brandão^I; Carlos Geraldo Viana Murta^II; Antonio Fernandes Moron^III; Rosiane Mattar^IV; Claudio Rodrigues Pires^V; Eduardo Almeida Guerzet^VI

^IMaster Degree by Universidade Federal de São Paulo-Escola Paulista de Medicina, MD, Ultrasonographist at CDI – Centro de Diagnóstico por Imagem, Vitória, ES

^IIDoctor Professor at Department of Gynecology and Obstetrics, Universidade Federal do Espírito Santo

^IIITitular Doctor Professor at Department of Obstetrics, Universidade Federal de São Paulo-Escola Paulista de Medicina

^IVAdjunct Doctor Professor at Department of Obstetrics, Coordinator of the Post-Graduation Program at Universidade Federal de São Paulo-Escola Paulista de Medicina

^VDoctor Professor, Universidade Federal de São Paulo-Escola Paulista de Medicina, MD, Director at Cetrus – Centro de Treinamento de Ultra-Sonografia

^VIAdjunct Doctor Professor at Department of Tocogynecology at School of Medicine, Santa Casa de Misericórdia de Vitória

^{Mailing address}

ABSTRACT

The measurement of cervical length utilizing transvaginal ultrasound is of help for pregnant women screening for premature labor, the shortening of the cervix being a predictor of higher risk of preterm delivery. Traditional methods for evaluating the uterine cervix during pregnancy are limited and unsatisfactory. Digital examination of cervix, considered as the standard method, demonstrates large variation among different examiners; on the other hand, transvaginal sonography is an effective method of examination during pregnancy. Recently, three-dimensional ultrasound has been used in the clinical practice including the study of uterine cervix. The three-dimensional technology allows acquisition and storage of a large volume of data. Such stored information can be reformatted and analyzed through an unlimited number of planes. The three-dimensional transvaginal ultrasound is the unique imaging method capable of obtaining a true coronal plane by direct correlation of the views in a multiplanar display. This method seems to offer a diagnostic potential because of the cervical ultrasonography increased accuracy.

Keywords: Cervix; Pregnancy; Three-dimensional ultrasonography.

INTRODUCTION

The development of the three-dimensional ultrasonography⁽¹⁾ (3D US) was proposed in 1950. The Voluson 350, the first commercially available 3D US equipment was introduced in the International Congress of Radiology held in Paris, in 1989⁽²⁾. However, the greatest progress of the 3D technology, especially in gynecology and obstetrics, occurred late in the eighties, after the development of computing systems, especially the graphic computing⁽³⁾.

Since the seventies, US has been utilized for assessing the uterine cervix during gestation, identifying possible structural alterations of the cervix in the course of pregnancy^(4,5). According to some researchers, the risk of premature labor is inversely proportional to the cervical length measured by means of endovaginal US^(6–11).

Aiming at improving the uterine cervix imaging study, Hoesli et al.⁽¹²⁾, in an original research performed in 1999, published results regarding application of 3D US for analysis of uterine cervix in pregnant women.

Afterwards, other authors have evaluated the measurement of the cervical length with three-dimensional technology, compared to the conventional 2-dimensional ultrasound (2D US), evidencing the superiority of the 3D US over the 2D US for studying the cervical morphology⁽¹³⁾ and the existence of differences between the measurements of cervical length by means of 2-dimensional and tree-dimensional studies⁽¹⁴⁾.

The increasing concern over neonatal morbidity and mortality connected to prematurity, the understanding about cervical transformations in pregnancy, as well as the possibility of improving the sonographic evaluation of uterine cervix with the 3D technology, for predicting preterm labor, have motivated us to carry out this review, imagining prospects for employment of 3D US to improve the cervical evaluation.

3D US EVALUATION OF UTERINE CERVIX IN PREGNANCY

Late in the nineties, the 3D US started to be utilized for studying the uterine cervix during pregnancy aiming at patients screening for premature labor⁽¹²⁾. The first researches related the cervix volume measured through the vagina⁽¹²⁾ and/or abdomen^(15,16) to the cervical biometry in the screening for preterm labor.

The results of these studies have evidenced a good correlation between 2D US and 3D US, respectively in the measurement of the cervical length and volume^(12,15). However, the measure of the cervical length was larger than the cervical volume in the detection of women at risk of premature delivery⁽¹²⁾.

On the other hand, the positive predictive value (PPV) of the analysis of cervical volume measured by 3D US in pregnant women presenting short uterine cervix (< 26 mm) was higher than the PPV found in the measurement of the cervix length by 2D US and accurately identified women who ended up having a preterm delivery⁽¹⁷⁾.

In 2000, Bega et al.⁽¹⁸⁾ were the first researchers to apply the three-dimensional endovaginal study and 2D US for measuring the uterine cervix length during pregnancy, comparing both methods. Pregnant women between 11 and 32 weeks of gestation were evaluated (a total of 37 examinations), the cervical length and/or funneling being measured by 2D US in sagittal plane and by 3D multiplanar display in sagittal and coronal planes⁽¹⁸⁾. It is important to note that the multiplanar display peculiar to the 3D system enables simultaneous visualization of the three different imaging planes: sagittal, transverse and axial⁽¹⁸⁾.

According to Baga et al.⁽¹⁸⁾, the 2D US has been ineffective for defining the true sagittal plane of uterine cervix in a considerable number of studies, since differences of 5 mm to 15 mm in the cervical length measured by both methods were detected. However, comparing the mean measures, these researchers have evidenced statistically significant differences between the diameter of the cervical funnel measured by 2D US in sagittal plane and by 3D US in coronal plane, and between sagittal and coronal planes at 3D US. They have reported that 2D US has evidenced a technical limitation in the evaluation of the actual cervical length because of the difficulty in moving the probe inside the vaginal canal, resulting in a mid-oblique image of the cervix in the sagittal plane, especially when the cervix was bent or curved. As regards cervical funneling, 3D US has detected a higher number of cases than the conventional US, disclosing the fact that, sometimes, the dilatation of the internal orifice of the uterine cervix presented asymmetrical, flattened or fissure-like, its identification being possible through the coronal cut of the cervix depicted at 3D US⁽¹⁸⁾. It is important to note that the reproduction and evaluation of images in coronal cut can be made only through the 3D technology⁽¹⁸⁾. Bega et al. have concluded that the 3D US favors a more detailed study of cervical anatomy and biometry than 2D US.

Researching this same subject in 2003, Severi et al.⁽¹³⁾ carried out a study similar to that of Bega et al., examining 103 pregnant women among the general population, in the second and third quarter of gestation, aiming at improving the predictive value of the echographic evaluation in the screening for preterm labor. Severi et al. have mentioned in their results a difference ³ 5 mm between 2D and 3D cervical measurements in sagittal cut in 48.5% of cases, the cervical length being larger at 3D US than at 2D US⁽¹³⁾. With regard to the comparison between averages of cervical length measures, these researchers have reported a significant difference between 3D and 2D methods⁽¹³⁾.

As regards funneling, US 3D has diagnosed cervical internal orifice dilatation in a higher number of women than 2D US, attributing the 3D technology superiority to the possibility of visualizing the cervix in coronal cut⁽¹³⁾. According to Severi et al.⁽¹³⁾, the study of the uterine cervix utilizing the 3D technology was successful for the multiplanar display of images, highlighting the relevance of the coronal cut and digital manipulation of images, allowing a reliable identification of the endocervical canal and internal orifice, facilitating the cervical biometry.

Towner et al.⁽¹⁴⁾, in a study published in 2004, reported larger uterine cervix length measured at 3D US than at 2D US. According to the authors, the average of cervical lengths measured at 2D US in sagittal cut was 35.7 mm and, at 3D US, was 43.7 mm; in 21 of the 34 patients evaluated the cervical length was > 5 mm at 3D US than at 2D US, demonstrating a significant difference resulting from the comparison between the averages in 3D and 2D methods⁽¹⁴⁾. These researchers⁽¹⁴⁾ reported that the difference between both imaging methods is directly proportional to the uterine cervix length measured at 3D US, that is to say, the larger the cervix, the higher the difference between the different techniques. According to these investigators, the cervical length in pregnant women presenting a large uterine cervix is underestimated by the 2D US. Towner et al. have concluded that the evaluation of the cervical biometry by means of 2D US is incomplete, justifying the low PPV attributed to this method. The 3D US for evaluating the uterine cervix in the screening for preterm labor may be useful; however, according to the authors, there is a need to study a higher number of cases in order to establish criteria for measurement of the cervical length by means of 3D US⁽¹⁴⁾.

THREE-DIMENSIONAL TRANSVAGINAL ULTRASOUND TECHNIQUE FOR CERVICAL EVALUATION

In the 3D US, information originates from 2D image consecutive cuts generated by the transducer movements⁽¹⁹⁾. The ultrasound device utilized presents both the 2-dimensional and three-dimensional modules, with a multifrequency vaginal probe (5.0–9.0 MHz)⁽¹⁹⁾.

There are two different ways to capture data with the ultrasound device operating in the 3D mode^(2,19). In the free-hand mode, data are captured by common probes utilized in 2-dimensional examination through manual movement of the transducer and transferred via video signals for elaboration and processing by a computer coupled to the US equipment⁽²⁾. In the internal mode, or automatic capture, a special probe is utilized and the three-dimensional system responsible for the data processing is an integral part of the ultrasound equipment⁽²⁾. Inside the transducer, there is an engine that moves the crystals in different planes, making the acquisition of 2D images sequences⁽²⁾. Especially in the free-hand mode, any abrupt movement changes the 3D reconstruction, affecting the image quality⁽¹⁹⁾. The development of a special probe with an automatic mechanical scanning device has increased four times the velocity of data acquisition, improving the 3D US and presently this mode is the most frequently employed⁽²⁾.

After the data acquisition that lasts about 0.5-5.0 seconds, based on the sagittal 2D image of the cervix and independently from the system employed, the information is transferred into the memory of the equipment for storage and processing and the images are simultaneously displayed on the monitor in an orthogonal plane (multiplanar display) in axial, transverse and coronal cuts, these images digital manipulation by the sonographist favoring a detailed study of the object and the later three-dimensional reconstruction^(19,20).

With the three-dimensional system, structures can be linearly measured in multiplanar display, or a volumetric calculation can be performed through manual outlining of the object-of-interest showed on the monitor in multiplanar display or by the VOCAL system (automatic)⁽¹⁹⁾.

At 3D US, cervical length can be measured in multiplanar display of sagittal and coronal cuts (Figure 2), as well as the funneling measures (diameter and length)^(12–14,18).

Figure 2 - click here to enlarge

It is important to note that the 3D method superiority over the 2D method in the study of the uterine cervix is due to the possibility of 3D images digital manipulation, especially in cervices presenting anatomical variants, allowing an accurate identification of cervical orifices with a clear visualization of the endocervical canal, improving the acquisition of the cervical length.

One of the great advantages of 3D US is its data storage capacity, allowing the reanalysis of the same study by a different observer or its evaluation in the absence of the patient in cases of doubtful diagnosis^(20,21).

CONCLUSIONS

This new technology opens prospects for an increase in the accuracy of the cervical US in pregnant women screening for preterm labor, reducing false-negative results of 2D US, since, through the 3D visualization, the uterine cervix can be more accurately explored in search of a funneling diagnosis in cases supposedly normal at 2D US, besides helping to achieve a precise identification of the endocervical canal course.

Considering the differences between the 2D and 3D methods of examination in the biometry measurement and easiness of analysis of the cervical morphology with the 3D technology, we think that the cervical 3D US may be added to the battery of examinations presently being utilized for cervical evaluation in the screening for preterm labor, aiming at improving the prematurity prognosis. However further researches with a higher number of cases are necessary for confirming these results.

REFERENCES

Received July 30, 2004.

Accepted after revision February 22, 2006.

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*

Study developed at Department of Obstetrics, Discipline of Fetal Medicine, Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, SP, and Centro de Ultra-Sonografia Capixaba (MedfetUS), Vitória, ES, Brazil.

Publication Dates

• Publication in this collection
  26 Sept 2006
• Date of issue
  Aug 2006

History

• Accepted
  22 Feb 2006
• Received
  30 July 2004