The following is an excerpt from a paper entitled: Volume scanning in the evaluation of fetal malformations: a new dimension in prenatal diagnosis by E. Merz, F. Bahlmann and G. Weber from the Gutenberg University at Mainz, Germany, appearing in the journal " Ultrasound in Obstetrics and Gynecology " Volume 5 pp. 222-227, 1995 :

The abdominal Voluson sector transducer is a 90 mechanical annular array transducer with a relatively large coupling area. Its fast scan sector is swept automatically in a direction perpendicular to the fast scan plane.

The three-dimensional investigation is performed in a similar manner to the conventional two-dimensional ultrasound examination. After the field of interest is targeted with a volume box in the normal two-dimensional scan, a volume scan can be activated, causing the transducer inside the transducer housing automatically to sweep 40 with 4 s in the normal velocity mode. By this volume scan, the data set from a pyramid-shaped tissue volume is acquired. The complete data set is stored in the Combison random-access memory (RAM). In the stored volume, a set of precisely equally spaced sector scans can be reviewed simultaneously in the three dimensions, producing tomographic images.

Depending on the volume size and data acquisition time, different numbers of scans are available. In the longitudinal section, a maximum of 256 scans are accessible and can be displayed. In the transverse section, and in the frontal or coronal section (parallel to the abdominal wall), a maximum of 1024 scans can be displayed.

For surface reconstruction, the volume can be rotated in a dimension such that the fetus or the organ of interest is directly facing the examiner . Disturbing factors such as the placenta, the arm or leg of the fetus or the umbilical cord can be eliminated by so-called Cartesian storing, where only the most interesting part of the volume is stored. After the selection of such a volume of interest, unwanted small echo signals can be suppressed with the lower threshold. The view angle and the number of the three-dimensional reconstructions are then defined. A Three-dimensional image in the surface mode is available for review after a data processing time of 8-20 s, depending on the volume size. The reconstruction of a translucency view takes the same time, but there is no necessity to reduce the small echo signals.

After reconstruction of several views from various angles, the object can be rotated on the screen to provide a better representation of spatial geometry.

Archival storage of data is accomplished with removable cartridge hard disks (SyQuest) with 88 Mbyte capacity.

In our experience, the use of the three-dimensional ultrasound technique is superior to the conventional two-dimensional technique in several ways:

(1) The examiner has a complete overview of the field of interest, as all three orthogonal planes (sagittal, transverse and frontal) are seen simultaneously on the screen, This allows an exact identification of anatomical planes for biometry as well as precise volume measurements. Frontal planes parallel to the abdominal wall are also visible. These views are not available with conventional ultrasound. For slice orientation relative to the patientís anatomy, a framework is displayed simultaneously with the orthogonal planes.

(2) Because of the large memory, the stored volume can be dissected millimeter by millimeter in all three dimensions, allowing post- scan tomographic examination.

(3) Surface and transparent images can be calculated and displayed directly by the ultrasound unit within a few seconds. The surface image gives a sculpture-like impression of the fetus; the transparent view shows clear skeletal details similar to a postnatal X-ray.

(4) The patient has the advantage of a reduced scanning time, because evaluation of the stored volume is carried out off-line.

(5) Transfer of data onto hard disks allows degradation-free storage of the complete data sets.

(6) Any stored volume can be reviewed at any time by the same examiner or by an another person. Any plane can be re-analyzed and any distance can be re-measured.

Despite these advantages, there are still some problems in the three-dimensional ultrasound technique that must be taken into account:

(1) The examiner must adjust to the freedom of moving through a stored volume. Common means of orientation-left, right, cranial and caudal-are not any longer strictly related to the position on the screen as by convention, but must be directly correlated with the image of the fetus, as the volume can be rotated in all directions.

(2) Storage of a volume takes about 4 s. During this period, the probe as well as the fetus should not move, otherwise there will be movement artifacts.

(3) The quality of surface reconstruction depends very much on the quality of the original data and the knowledge of the examiner of how to rotate the volume and how to remove structure that disturb the surface which is to be examined: an interposed umbilical cord or an interposed fetal leg of arm can lead to low image quality of the fetal surface. Furthermore, if the fetus is in contact with the placenta of the uterine wall, it is difficult to provide an adequate surface reconstruction. In oligohydramnios, surface reconstruction is almost impossible at the present time.

(4) Surface and transparent imaging need a computer calculation time of 20 s for each plane.

(5) At present, three-dimensional imaging gives no time benefit for the skilled examiner, especially when there is an intention to reconstruct a series of images in the surface mode.

(6) Documentation of three-dimensional images requires a relatively high storage capacity )one volume requires approximately 5-10 Mbyte). Therefore, only the most interesting parts of the fetus can be stored within one or two volumes.

Nevertheless for the diagnostic evaluation of fetal malformations, three-dimensional ultrasound provides new diagnostic pathways for enhancing the visualization of suspicious areas, which can be studied with more defined scrutiny under standardized conditions.

Especially when there are lesions of the fetal surface such as facial dysplasia, cleft lip/palate, dysplastic low-set ears, spina bifida and omphalocele, surface rendering can give a vivid and clear demonstration of the defect. In intrafetal anomalies such as hydrocephalus, holoprosencephaly or intestinal atresia, the surface mode can be helpful in depicting the width and depth of the defect. It also allows evaluation of the inner wall of the cystic structure. In the deformed fetus with a distortion of the normal anatomical axis, as in club foot, the surface mode is advantageous in demonstration the deviation.

In the transparent mode, bony structures are clearly visible, as in an X-ray. This opens up a new field in demonstrating skeletal deformities.