The History of Ultrasound

By the early twentieth century, the existence of inaudible high-frequency sound waves had been established, and in 1916 the first working version of a marine sonar (sound navigation and ranging) system was used during the First World War to detect enemy submarines. From this, a new technique was developed for industry using high-frequency pulse-echo techniques which were applied to the detection of flaws in metallic structures.

Research progressed into using the technique for biological therapeutic and diagnostic applications, but it was not until the early 1950s that the first clinical images were published. Because air does not transmit ultrasound waves efficiently, the air interface between the transducer and the patient’s skin was initially a problem. Early scanning techniques required the patient to be immersed in a bath of water in order to provide good transmission of sound waves into the body (see Fig. 1.3).

Fig. 1.3 A patient being scanned in a waterbath in 1954

(Reproduced from Shirley et al 1978.)

However, as this was an inconvenient technique for routine clinical use, the first contact compound B-scanner (using olive oil as a lubricant) was developed in the late 1950s (see Fig. 1.4). This equipment used a single crystal, mounted on the end of an articulating arm (see Fig 1.5), to produce static images. Each scan was time consuming to complete, as it required the operator to move the crystal across the area of interest in a linear sweep, in order to provide sufficient cross-sectional information to make a diagnosis.

Fig. 1.4 A contact compound B-scanning machine

(Reproduced from Shirley et al 1978.)

Fig. 1.5 A single crystal transducer mounted on an articulating arm

(Reproduced from Shirley et al 1978.)

Early ultrasound equipment visual displays used storage oscilloscopes which produced bi-stable (black and white) images (see Fig. 1.6). It was only in the 1970s that gray scale imaging was introduced, enabling the display of a wide range of echo amplitudes. More subtle variations in tissue density were provided by these images, enabling the diagnosis of a wider range of disease processes.

Fig. 1.6 A bi-stable image of a renal cyst

(Reproduced from Shirley et al 1978.)

Real-time scanning systems were introduced in the mid 1970s, which not only enabled the visualization of moving structures, but also helped to speed up the examination time. Dynamic sonographic information was available for the first time, greatly enhancing the use of ultrasound as a diagnostic tool.

During these developments, research into the application of Doppler techniques was ongoing for many years, and the technique began to be used routinely for the detection and measurement of blood flow during the 1980s. Ongoing research into this, together with the use of micro-bubbles as a contrast medium, continues to expand the diagnostic applications of sonography.

Safety of Ultrasound

The ultrasound beam is potentially hazardous to the patient. As it travels through the body the beam interacts with tissue, and if exposure is sustained and of sufficient intensity, it has the potential to cause a lasting biological effect. The implications of many of the processes that occur are not yet fully understood, however two of the effects that have been observed are caused by thermal and mechanical processes. The thermal effects arise because some of the ultrasound energy is converted into heat, causing an increase in tissue temperature. The mechanical processes are a result of the presence of air in soft tissues (such as the lungs or intestines) which can cause damage to cells as the air bubbles expand under the influence of the ultrasound beam (see Chapter 12 on Ultrasound safety).

The degree of damage caused by these effects depends on a number of factors, including the length of the scan, the type of tissue being scanned and the power of the ultrasound beam. Anyone performing an ultrasound scan therefore needs to be aware of the possible hazards involved, in order to take the necessary precautions to limit possible damage to the patient.

Ultrasound Equipment

The device which transmits the sound into the patient and detects the returning echoes is called the transducer. The term transducer means a device that converts one form of energy into another. The primary component of the transducer is the piezo-electric crystal which enables the conversion of electrical pulses into mechanical energy (sound), and will also convert mechanical energy into electrical signals. Most ultrasound equipment now uses pulsed sound, which means that a voltage is applied to the crystal for a fraction of a second. This distorts the crystal and a pulse of ultrasound is therefore produced. The crystal will then relax while it is waiting to receive a returning echo. Only 1 per cent of the operating time is spent by the transducer transmitting sound, whereas 99 per cent of the time is spent listening for echoes.

Echoes detected by the transducer give rise to very small voltage signals in the piezoelectric crystal. By measuring the time it takes for an echo to arrive back at the transducer, the depth of the echo can be calculated by the equipment. These signals are then amplified, converted into a digital format and stored as digital numbers in a computer memory known as a scan converter. Each digital number will be assigned to an individual pixel according to how large the returning echo is, and the digital number for each pixel determines the shade of gray used to represent it on the display. Considerable processing and manipulation of this echo data has to occur before it can be meaningfully displayed. Processing takes place either before storage (pre-processing) or after (post-processing). Many processing functions can be manipulated by the operator (see Chapter 10 on Instrumentation and controls) and have a considerable effect on the quality of the image. Incorrect use of these controls can lead to the production of artifacts and misinterpretation of the image (see Chapter 9 on Artifacts).

The Image

After the returning echo signals have been electronically processed and amplified they are displayed as shades of gray, ranging from black to white. Most systems have the capability to display up to 64 shades of gray on a monitor. Stronger reflectors will be displayed as white on an image, weaker echoes will be less bright, and areas with no echoes will be portrayed as black.

All ultrasound images are composed of many lines of echo data, placed closely together so that the image appears continuous. The area of the patient displayed on any image is limited by the section through which the beam will sweep. The width is determined by the type of the transducer, and the depth by the length of time for which the echoes are recorded following transmission of a pulse. The depth is variable according to operator requirements, but usually has a maximum of 25 cm.

The plane that an ultrasound image represents is the section through which the ultrasound beam sweeps, and this is determined by the position and orientation of the transducer. During scanning, the operator is required to move the transducer in order to view images in at least two planes, and to use this information to build up a mental three-dimensional image of the area being surveyed.

In order to understand the basic principles of diagnostic ultrasound it is important to have a working knowledge of the basic physics of sound, and the way in which high-frequency sound interacts with biological tissue. The following chapters will explore in more detail some of these principles.