The Transducer

Diagnostic transducers are made from piezoelectric materials (see chapter on Transducers) and are able to convert electrical energy into ultrasound energy and vice versa. As a consequence of this they can act as a transmitter and receiver of ultrasound. They are able to produce beams which can be directed in various ways which are controlled by the ultrasound machine to improve image quality.

The Pulser

The pulser produces the electric voltage that drives the transducer. This driving voltage governs the output power of the ultrasound machine and can be adjusted by the operator through the power or output control. Changes in the applied voltage to the transducer changes the strength and intensity of the ultrasound beam, and affects the overall brightness of the B-mode image. The greater the applied voltage, the stronger the ultrasound pulse and the higher the pulse intensity. Figure 10.2 illustrates the effect that increasing the applied voltage has on the B-mode image. Increasing the applied voltage to the transducer increases the ultrasound pulse intensity, resulting in an increase in the overall brightness of the image.

Fig. 10.2 Increasing the applied voltage to the transducer increases the pulse intensity and output power, resulting in brighter images

Beam Former

The beam former controls the shape and direction of the ultrasound beam and the scanning patterns used to form the images that we see. This enables the operator to have indirect control of:

Receiver

The job of the receiver is to combat attenuation, i.e. the energy lost from the beam as it propagates through soft tissue. As a consequence of attenuation the returning echo amplitudes and intensities are decreased. Most of the energy is lost from the beam through absorption which is mainly converted into heat. The receiver applies amplification to the returning echoes to make them stronger and to enable them to be visualized. The ultrasound machine can compensate for the effects of attenuation by amplifying the received signals in two ways, using the overall gain and the time-gain compensation (TGC) controls.

Processor

The processor can be divided into two individual parts, each having very different tasks to fulfil, and consists of:

A simple block diagram of the signal processor is illustrated in Figure 10.3. The signal processor picks up the amplified signals from the receiver and carries out operations such as filtering, amplitude detection, and signal compression.

Fig. 10.3 Block diagram of the signal processor outlining the key functions of each component

Filtering cleans up the signal, removing unwanted noise, and also controls the signal bandwidth (see chapter on Transducers). Amplitude detection performs a process of demodulation which means it converts the received signal voltages into video form, retaining the amplitude information required for B-mode imaging. Compression controls the dynamic range of the B-mode image (the number of shades of gray displayed in the image).

Once the signals have passed through the signal processor they are fed into the image processor. A block diagram of the image processor is illustrated in Figure 10.4. The image processor performs operations such as scan conversion which reformats the scan line data into two-dimensional images, i.e. either linear or sector form. The ability of the scan converter to rapidly process the thousands of scan line data received every second from the transducer enables real-time dynamic ultrasound imaging to be performed.

Fig. 10.4 A block diagram of the image processor outlining the key functions of each component

There is a range of processing performed on the image data prior to and following storage into memory before they are finally sent to the display. Functions such as edge enhancement are known as pre-processing functions because they are performed before being stored.

Image memory stores a number of sequentially acquired and processed static image frames every second which are rapidly sent to the display to provide dynamic real-time ultrasound. This process can be interrupted by activating the freeze control which stops any further acquisition and processing of data from the transducer. In freeze mode only one static image in the memory is displayed. The numerous frames of image data held and stored can be reviewed in turn using the machine’s cineloop control function.

The Display

Once the images have been processed they can either be displayed on a traditional cathode ray tube which is used in conventional televisions or be presented on a computer monitor or a flat panel screen. There are only two controls that can be adjusted on most displays, which are brightness and contrast. These can be set by the operator according to individual preference or ambient room lighting conditions.

Hard Copy and Soft Copy Storage

The images displayed on screen can be sent out to an external hard copy device such as radiographic film or thermal paper printers. Alternatively, the images, which can also consist of short cineloop review video clips, can be digitally stored on the machine’s hard disc or alternatively burnt to CD or DVD. Many ultrasound departments are linked into a picture archiving and communications system, more commonly known as PACS. This enables images such as X-rays and ultrasound scans to be stored electronically and viewed on any PACS display monitor around the hospital at the touch of a button.