How an ultrasound scan works
Posted on December 16, 2011
Diagnostic ultrasonography is a crucial tool in medical imaging, as it is non-invasive, has no known harmful side effects, and produces real time images which can yield a great deal of information when used by a skilled operator. These properties ensure its prominence in the field of obstetrics, although it does have many more medical applications such as in cardiology and ophthalmology. Ultrasound is defined as sound with a frequency above the upper range of human hearing, typically around 20,000 Hz, but generally ultrasound with a frequency of upwards of 2 MHz is employed for the purpose of medical imaging.
Ultrasound is typically produced by a piezoelectric crystal transducer, producing an arc of sound waves from its surface. The sound wave penetrate the body and are reflected back by the interfaces between tissue types of different densities to the transducer, which at this point acts in reverse as a microphone to receive sound and turn it into an electronic signal. Taking information from the strength of the reflected signal and the time it takes to return to the transducer, a computer is then able to build up an image of the interior tissues. In the field of pregnancy scans, there is a growing trend for more sophisticated computer programs that are able to build images using data from several different angles, creating a 3D image which is much more easily understood by someone without training in obstetrics. The most elaborate examples of these are able to show the foetus moving in real time, the so called 4D scan. These use a phased array of traditional 2D transducers. Despite the obvious attraction of this procedure for parents, it does not offer any great advantage over traditional 2D scanning for trained medical practitioners, and does not provide any enhanced detail.
The frequency used in an ultrasound scan always needs to be carefully considered. Although lower frequencies of around 2 MHz are more penetrative, higher frequencies provide a greater resolution and can image smaller structures in the body in more detail. For example, an early pregnancy scan or a detailed scan of the foetal spine to check for spina bifida require very high frequencies in order to achieve the correct resolution. When imaging is required to be both penetrating and detailed, one solution is to use the transducer is such a way as to produce a fundamental of a lower, penetrative frequency which will reflect back harmonics, providing the detail required for an improved image quality.
4D pictures of our baby gave us a glimpse of our precious little boy, something we will always cherish.Kim (Mansfield)