Technology

Ultrasound imaging is one of the most popular and accessible imaging modalities in medicine. This is for many reasons. Images can be taken without intervention, the equipment is simple and portable compared to X-ray, computer tomography (CT), and magnetic resonance imaging (MRI), and the method has few dangerous side effects. Ultrasound imaging can be used within many specialties in medicine, especially cardiology (cardiac imaging), abdominal imaging (internal organs such as the liver and kidneys), obstetrics (foster imaging), and gynecology as well as vascular imaging (blood vessels in the throat, legs, and arms). 

In Norway, there is a big atmosphere of technical and clinical development of medical ultrasound imaging. The GE Vingmed Ultrasound firm in Horten develops, produces, and sells ultrasound instruments to the whole world and is the "Center of Excellence" for ultrasound in cardiology in GE Healthcare. There are 130-140 employees in Norway and many outside of Norway. Figure 1 shows the new generation of ultrasound scanners. Vingmed was started by technical-medial researchers in Trondheim. It was the development of Doppler-based techniques in the 70s and 80s which gave the basis for Vingmed’s success. This was followed by the development of digital scanners with phase-controlled arrays in the 90s and new modes such as elasticity and real-time 3D in later years. Other firms include Medistim, which produces ultrasound equipment for quality control and documentation under cardiac surgery and Sonowand which produces 3D ultrasound-based equipment for navigation during operations in the head.

Figure 1: 3D ultrasound scanner for medicine, GE Vivid 7 Dimension (Used with permission by GE Healthcare).

The use of ultrasound, as well as research on clinical aspects of ultrasound, takes place at many hospitals in Norway. Research on the physical and technical sides takes place at several universities. Furthermore, GE Healthcare (earlier Nycomed Amersham Imaging) had a group in Oslo which worked with the development of ultrasound contrast fluid, however this work was stopped in 2003.

There is also an active atmosphere of ultrasound research in Denmark, especially at The Technical University of Denmark. They work closely with B-K Medical, which produces imaging systems specifically for urology and surgery, and which has almost 300 employees across the world. 

A typical ultrasound heart image is shown in Figure 2. This is a 2-dimensional spacious image which shows a slice through the heart. In this image, the brightness has been increased with echo strength. The figure shows the heart as seen from the side (see Figure 3), and you can see the left chambers to the left (depth 3.5-7.5 cm), the closed aortic valve and the aorta at the top right (depth 4-7 cm), the open mitral valve between the left atrium and ventricle in the middle of the image, and the left atrium at the bottom right (depth 8-11 cm). The walls of the heart are white in the image, as they give off a stronger echo than the blood in the chambers.

Figure 2 (left): 2-dimensional B-mode image from a person with good image quality, taken cross-sectionally from the heart (parasternal long axis view)
Figure 3 (right): Anatomical image of the heart. The ultrasound image in Fig. 2 is taken from the cross-section which is marked. 

These images are, as a rule, simpler to understand when they are shown as a time sequence rather than still images. One can immediately note that the distinction between the walls of the heart and the blood in the chambers is not clear, which can make some processes more challenging (for example, automatic edge detection). This illustrates some of the challenges one faces when using signal and image processing with ultrasound instruments.

The most important physical principles for ultrasound will be covered on this website. Furthermore, the most important ways in which these are utilized in scanners will also be presented. The article is written for both medical students who wish to gain a better understanding of ultrasound imaging by learning background information, and for physicians who wish to see how principles in acoustics are utilized in imaging. Formulas are collected in their own fact boxes so that those interested can study them as an extension of the main text.

The article starts with an overview of the most important physics principles which determine the resolution, and the ability to create an image in depth. Then, the Doppler effect and effects of non-linear acoustics are discussed. The different methods of operation and types of images which are used for imaging of tissues and measurement of the speed of blood flow is then covered. Next, we discuss the most important parts of an ultrasound machine. Many types of probes and image formats have been developed, and we go through how to adapt to limitations in size and accessibility presented by different parts of the body. Finally, we discuss safety with ultrasound imaging. 

Professor Sverre Holm, Department of Informatics