Three-dimensional ultrasound in gynaecology

Juan Luis Alcázar
MD
Chief of Ultrasound Unit
Department of Obstetrics and Gynaecology
Clínica Universitaria de Navarra
University of Navarra
Pamplona
Spain

Two-dimensional transvaginal ultrasound (2D US) is widely used in gynaecological practice and the reliable diagnostic value of this technique is well established. Although an experienced examiner may develop a three-dimensional (3D) image in her/his mind by “mental processing” of a sequence of 2D images, the ability to obtain certain planes of the pelvic organs is limited.

Three-dimensional ultrasound (3D US) has been introduced into clinical practice in the last 15 years. 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. Although this technique has been more extensively used in obstetrics,(1) clinical applications in gynaecology have been and are currently being explored.(2)

Technical 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 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 remains stationary. The operator can select a constant speed of sweep through the ROI using different settings.

The main drawback of all freehand systems is that measurements are not as accurate as with automated 3D transducers and image quality is low. Acquired images may be processed within the ultrasound machine or in an offline workstation. The digitally stored volume data can be manipulated and presented in various displays: multiplanar displays, “niche” mode, surface-rendering mode, inversion mode or TUI (tomographic ultrasound image). The possibility for assessing 3D volumes offline has been shown to be more efficient than 2D real-time ultrasound.(3)

Another important ability of 3D US is volume calculation using the Virtual Organ Computer-aided AnaLysis (VOCAL), even in irregularly shaped structures. This method has been demonstrated to be more accurate than 2D volume estimation.(4) Vascularisation of tissues within the ROI can be also assessed using 3D power-Doppler ultrasound (3D PD) and the VOCAL program.(5)

Artifacts may be present when using 3D US. Some of them, such as acoustic shadowing, reverberation and motion artefacts, are the same than in 2D US. Others are unique to 3D US and may be derived from acquisition, rendering or editing.(6)

One of the main problems associated with 2D US is lack of reproducibility, both for volume and Doppler measurements. 3D US has been demonstrated to be a very reproducible technique.(7)

Current clinical applications
Probably one of the most interesting application of 3D US is the evaluation of congenital uterine anomalies. Although conventional 2D US can discriminate among different types of uterine anomalies, this is highly dependent on the examiner’s expertise; in addition, the technique limited in its ability to obtain the coronal plane of the uterus in most cases. Some studies have demonstrated the advantages of 3D US in this case.(8)

Saline infusion sonohysterography (SIS) has been shown to be a very useful tool for diagnosing intrauterine abnormalities. However, the possibility to assess the uterine cavity by 3D US has raised a lot of interest among clinicians.

Several authors have tried to answer the question of whether 3D SIS would add useful information to 2D SIS. All these studies show that 3D SIS is not superior to 2D SIS but could offer additional information in some cases.(9) Another use for this technique is for location of missing intrauterine devices. The differential diagnosis of adnexal masses still represents a challenge despite the tremendous efforts that have been made to improve ultrasound-based diagnosis. This diagnosis is based on ­morphological ultrasound assessment. The Doppler technique has been also proposed, although its usefulness remains controversial. Some studies have evaluated whether 3D US might add information to conventional 2D US when results are controversial.(10,11)

Future perspectives
One of the most promising applications of 3D US in gynaecology is the assessment of organ vascularisation by means of 3D PD using the VOCAL program. The technique has been proven to be highly reproducible, thus overcoming one of the main problems of conventional colour and pulsed Doppler. In addition, this technique allows for the assessment of the global vascularisation of a given ROI, which could be interpreted as a “vascular biopsy” of the ROI.

Recently, we have explored the potential role of 3D PD as a means to assess tumour vascularisation in endometrial hyperplasia and cancer, as well as and correlation with histoprognostic factors. We established that endometrial cancers tended to be more vascularised than endometrial hyperplasia, and that deeply infiltrating, poorly differentiated, advanced-stage endometrial cancers were more vascularised than their healthy counterparts.(12) This is also useful for predicting ovarian cancer in vascularised complex adnexal masses on conventional colour Doppler ultrasonography, because vascularisation is higher in malignant tumours than in benign lesions.(13)

All these studies point out to the possibility of using this technique for the “vascular biopsy” of a given organ or tumour. This opens up new routes for in-vivo and noninvasive assessment of vascularisation, with a wide spectrum of potential applications, not only in gynaecology but also in other clinical areas. Another potential application is in urogynaecology.(14) 3D US may provide excellent images of the pelvic floor, making the assessment of structures and measurements more accurate.

Conclusion
Three-dimensional sonography is the most recent major development in ultrasound imaging. It is ­progressively gaining acceptance in clinical practice. It will not replace conventional 2D ultrasound, but it is becoming more popular because of its use in gynaecology in certain specific clinical circumstances.This new technique has become a useful research tool in many gynaecological applications, and its potential is likely to be further expanded to other clinical areas.

References

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12. Alcázar JL, et al. Ultrasound Obstet Gynecol 2004;24 S1:246.
13. Alcázar JL, et al. J Ultrasound Med 2005;24:689-96.
14. Dietz HP. Ultrasound Obstet Gynecol 2004;23:615-25.