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Obstetric Ultrasound Scanners

21 December, 2010 | Ultrasound Scanners

Ultrasound ScannerObstetric ultrasound is the use of ultrasound scanning in pregnancy. Since its introduction in the 1950′s, ultrasonography has become a very useful diagnostic tool in obstetrics. Traditional obstetric sonograms are done by placing a transducer on the abdomen of the pregnant woman. One variant is a transvaginal sonography which is performed with a probe placed in the women’s vagina. Transvaginal scans usually provide clearer pictures during early pregnancy and in obese women. Also used is Doppler sonography which detects the heart beat of the fetus. Doppler sonography can be used to evaluate the pulsation in the fetal heart and bloods vessels for signs of abnormalities.

Obstetric ultrasound scans are used to identify and diagnose a range of features and pathologies, from fetal position and orientation, to subtle abnormalities and small measurements. Therefore, it is important that a particular ultrasound scanner has sufficient resolution, dynamic range and contrast to adequately visualize the desired features. This will depend on the applications for which the scanner is used, with substantial differences in the required performance.

There are several different ultrasonic modes used in obstetric ultrasound imaging. Standard grey-scale ultrasound images are captured using B–mode. However, there are a range of other ultrasound modes available on many systems. Some of these modes present distinctly different information to B–mode (such as the Doppler modes) while others alter the B–mode image. Doppler is not recommended for use in routine screening scans, but may be used as part of a local protocol or for more complex cases. Doppler scans are also useful for examining the uterine and umbilical blood vessels.

Power Doppler shows how well perfused tissue is through a color–coded image. This mode is quite sensitive to flow, but does not contain velocity information.

Color Doppler mode shows the speed and direction of fluid flow through a color–coded image, typically used to look at blood flow.

M-Mode shows the position of objects relative to the transducer, along a single image line over time. This scan, for example, can be used to observe the behavior of artery walls.

Tissue harmonic imaging mode makes use of the non–linear propagation of ultrasound in tissue (the frequency of the beam changes as it travels through the tissue). This mode can improve the signal-to-noise ratio in the image and reduces the impact of side lobes and scattered waves.

Compound imaging mode uses some form of averaging of B–mode images to reduce the impact of noise and speckle. Spatial compounding uses imaging beams of different angles, while frequency compounding uses images produced with different frequencies. These different images have noise and speckle characteristics and are averaged to reduce the impact of noise and speckle. This mode may provide benefits to imaging of the fetal organs by improving contrast, but depends on the individual scanner.

3D and 4D volume imaging, multi–slice imaging and multiple-plane imaging are all types of 3D or 4D ultrasound modes. 3D data may be collected with a hand-scanned, mechanically scanned or electronically scanned ultrasound beam. Surface rendering can often be used in conjunction with the 3D dataset to produce a rendered image of the fetus. Many systems require special probes to use 3D or 4D features. 3D imaging and surface rendering may assist in acquiring satisfactory images of the fetal anatomy for anomaly scans, but this depends on the individual scanner.

Different obstetric applications require different types of ultrasound probes. If a system has fixed probes, rather than interchangeable ones, then it may be appropriate for a limited subset of applications. For this reason, it is common for ultrasound systems to have interchangeable probes, and they often have more than one probe connection socket.

Scans conducted during the early phases of pregnancy will often require the use of a high-frequency transvaginal probe to obtain adequate images. Later scans can be conducted with lower frequency transabdominal probe. Additional consideration should be given to the appropriateness of the probes available for scanning women with a high body mass index, as obtaining images on these women can be more challenging.

The design of the ultrasound scanner can have a major impact on the health and wellbeing of the operator, due to the repetitive work and the way scans are performed. Ultrasound operators are vulnerable to repetitive stress injuries. The risk of this can be mitigated by using a well-designed scanner with an adjustable control panel and display monitor.

Ultrasound images are viewed live, with the operator acquiring the image, making measurements and identifying features. As a result, it is extremely important that the display monitor on the ultrasound system is of an acceptable quality. It is recommended for fetal anomaly screening that the display is large enough to provide an image are of 12 x 15cm.

As fetal anomaly scans involve making small and accurate (few millimeter, eg nuchal translucency, with sub–millimeter precision) measurements it is important that the ultrasound scanner calipers can be accurately and precisely positioned to minimize the uncertainty in measurements. Operators may need to print ultrasound images and will certainly need to store images in some form. Typically, if images are printed, a thermal printer is used. Image storage can be local (on the scanner), on removable media (eg CD, DVD or USB storage) or via a PACS.