Three-dimensional ultrasound assessment of endometrial receptivity: a review

Reproductive Biology and Endocrinology Three-dimensional ultrasound assessment of endometrial receptivity: a review Juan Luis Alczar* 0 Address: Department of Obstetrics and Gynecology, Clinica Universitaria de Navarra, University of Navarra , Pamplona , Spain Three-dimensional ultrasound (3D US) is a new imaging modality, which is being introduced into clinical practice. Although this technique will not probably replace two-dimensional ultrasound, it is being increasingly used. It has been reported that 3D US is a very high reproducible technique. The endometrium has been paid special attention when using this technique. The aim of this paper is to address some technical aspects of 3D US and to review critically its current status in evaluating endometrial function with special focus in its role in predicting pregnancy in assisted reproductive techniques. In spontaneous cycles endometrial volume grows during follicular phase remaining constant through the luteal phase. Endometrial vascularization increases during follicular phase peaking 2-3 days before ovulation, decreasing thereafter and increasing again during mid and late luteal phase. Data from studies analysing the role of 3D US for predicting IVF outcome are controversial. An explanation for these controversial findings might be different design of reported studies, specially the timing of ultrasound evaluation. - Background Endometrial receptivity is a crucial fact in human reproduction. Endometrial assessment has been performed usually by endometrial biopsy [1]. However, such as invasive method is not acceptable when evaluating endometrial receptivity in order not to damage the endometrium. Therefore, endometrial receptivity should be ideally evaluated before implantation by a non-invasive method. Transvaginal ultrasonography may represent, theoretically, such an ideal non-invasive technique. Several parameters have been proposed for assessing endometrial receptivity, including endometrial thickness, endometrial pattern and endometrial and subendometrial blood flow [2-7]. These parameters may identify patients with low implantation potential. However, their positive predictive value is low [8,9]. Recently, three-dimensional ultrasound (3D US) has become available [10-13]. With this technology any desired plane through an organ can be obtained. With 3D US a volume of a region of interest (ROI) can be acquired and stored. This volume can be further analysed in several ways, such as navigation, multiplanar display, and surface rendering or volume calculation. This technique also allows a whole assessment of the endometrial and subendometrial vascularization [14,15] In this review I shall address current state-of-the-art of 3D US in assessing the endometrium throughout the menstrual cycle and its possible role in predicting endometrial receptivity in assisted reproductive techniques (ARTs). A Medline search (19952006) was performed using the following key words: "three-dimensional ultrasound", "angiography", "power Doppler", "endometrium", "endometrial", "receptivity". A total of 27 articles were identified. Twenty-three were clinical studies and were selected for review, whereas 4 papers were reviews and were excluded. Technical aspects Several published papers deal in detail the technical aspects of 3D US and an extensive description of these technical aspects is beyond the scope of this review [1619]. Notwithstanding, I shall explain briefly some basic considerations. 3D US images can be obtained by two methods: freehand and automated. The freehand method requires manual movement of the transducer through the ROI. The automated method acquires the images using dedicated 3D transducers. When these probes are activated, the transducer elements automatically sweep through the ROI selected by the operator (the so-called "volume box") while the probe is held stationary. This provides more accuracy to this method as compared with the freehand systems, in which speed of sweep is more difficult to maintain constant manually by the operator. The digitally stored volume data can be manipulated and presented in various displays: multiplanar display, "niche" mode or surface rendering mode. Probably, the most used and useful display is multiplanar display, which simultaneously shows three perpendicular planes (axial, sagital and coronal), allowing navigation through these three planes with the possibility of switch over any desired plane (Figure 1). TFhigrueere-d1imensional ultrasound depicting multiplanar display of the uterus Three-dimensional ultrasound depicting multiplanar display of the uterus. All three orthogonal planes can be displayed using this technique. Another important ability of 3D US is volume calculation, even in irregularly shaped structures, using the Virtual Organ Computer-aided AnaLysis (VOCAL) (Figure 2). This is a rotational method, based on rotations in given steps (6, 9, 15, 30) on a given orthogonal plane (A, B or C). This method has been demonstrated to be more accurate than 2D-volume estimation, with an error estimation of 7% for 3D US as compared of 22% for 2D US [17]. Vascularization of tissues within the ROI can be also assessed using 3D Power-Doppler ultrasound (3D-PDA) and the VOCAL program [18]. Using this method, three vascular indexes can be calculated: the Vascularization Index (VI), expressed as percentage, measures the number of colour voxels in the studied volume, representing the blood vessels within the tissue. The Flow Index (FI) is the average colour value of all colour voxels, representing average colour intensity. And the Vascular-Flow Index (VFI) is the average colour value of all grey and colour voxels, which represents both blood flow and vascularization (Figure 3). Using the "shell" function it is possible to calculate a volume at different thickness around the predetermined endometrium and estimate the vascularization in this "shell". This allows the assessment of the socalled "subendometrial region" (Figures 4 and 5) 3D US has a very low inter-observer and intra-observer variability for calculating endometrial volume, with intraclass correlation coefficients 0.97 [14,20-22]. However, this depends on the technique used, being the VOCAL method the most reproducible [23]. This technique has been also found to be highly reproducible for estimating ovarian and endometrial vascularization using 3D PD with intraclass correlation coefficients 0.99 for all indexes [15,24,25]. EFnigduormee2trial volume calculation by using the VOCAL software after three-dimensional ultrasound Endometrial volume calculation by using the VOCAL software after three-dimensional ultrasound. 3FDig-uProew3er Doppler indexes for assessing endometrial vascularization by means of the three-dimensional ultrasound 3D-Power Doppler indexes for assessing endometrial vascularization by means of the three-dimensional ultrasound. Correlation of endometrial ultrasound and histology Several studies h (...truncated)