The sound field of a focused sound wave has three components:

The near field (Fresnel region): The Fresnel zone is adjacent to the transducer surface and has a converging sound beam profile. The image quality in the near field can be disturbed by noise from the transducer crystals. The length of the near field is determined by sound frequency and transducer diameter.

The mid field: the sound beam is narrowest in the midfield and has the highest resolution. The midfield is the boundary between the near field and the far field.

The far field (Fraunhofer region): the lateral resolution becomes increasingly reduced in the far field due to divergence of the sound waves and continuous loss of ultrasound intensity with distance to the probe.

In B-mode the amplitude (energy) of the reflected sound wave is represented by a dot on the screen with a proportional intensity displayed by a specific greyscale value.

High-intensity echoes are displayed as bright, low intensity as dark. An area in the body that returns no echoes to the transducer is represented as anechoic (black).

In B-mode a 2D cross-sectional image of a 3D tissue volume is produced based on real-time generation of 20-50 images per second. 2D mode can be combined with any other mode.

Every dot is defined by:

Colour Doppler (CD) displays a real-time 2D cross-section of blood flow. The different colours represent different velocities.

Positive velocity (i.e. the blood moves towards the transducer) is indicated by red color. Negative velocity (i.e. the blood moves away from the transducer) is indicated by blue color.

When the ultrasound beam is reflected by a moving target (e.g. blood cells), the frequency changes due to the Doppler effect. When the echo returns to the transducer it can detect subtle changes in frequency and display that visually. As mentioned above it can detect direction of flow, but it can also display the velocity of flow as a variation in the intensity of the color.

CD requires a minimum angle between the emitted ultrasound beam and the direction of the flow. I.e. no flow is detected with colour Doppler with slow flow or when the angle of the beam is perpendicular to the direction of the flow.

Static or slow motion tissue cannot be seen with colour Doppler. However, a greyscale B-mode image is usually combined with the Doppler image.

Pulsed Wave (PW) Doppler Mode represents blood flow in real-time as velocity (or frequency) as the vertical axis and time along the horizontal axis. The velocity is expressed as different greyscale values.

The ultrasound data are sampled from a single area – the sampling volume – on a stationary beam. PW allows estimation of blood flow velocity in this particular location.

Typically, PW is combined with 2D imaging in order to obtain spatial reference and a D-line superimposed on the 2D image to indicate the location of the sampling.

The position and size of the sampling volume is indicated on the D-line.

Colour Power Doppler (CPD) is a special type of Colour Doppler used to visualise the presence of blood flow that is difficult to detect either due to slow flow or a scanning angle that is almost perpendicular to the direction of the flow. The detection is based on the amplitude of the ultrasound echoes returned to the transducer after being reflected from moving cells.

Colour Doppler was based on detection of velocity of the moving cells in the sample volume and the velocity was derived from the frequency shift. The advantage of CPD is that it is very sensitive to slow blood flow. However, it does not provide information about velocity or direction of flow.

CPD is not as angle dependent as traditional Colour Doppler.

M-mode was the first ultrasound modality used to display dynamic real-time imaging of echoes from the beating heart. Originally M-mode was not displayed as 2D images.

A single crystal emits sound pulses in one stationary direction. The echoes are received and recorded along the axis of sound pulse propagation and displayed along a time axis.

M-mode is only interesting in examination of moving structures. It is used mainly in echocardiography and lung ultrasound.

Continuous Wave (CW) Doppler Mode measures blood velocity along a cursor line. The probe is continuously emitting ultrasound pulses as well as receiving reflected frequency shifts. Because of this continuous sending and receiving mode, the system can detect flow with very high velocity.

The downside of the method is that it is not possible to detect the flow velocity in a particular position along the probing line.

Typically, CW is combined with 2D imaging in order to obtain spatial reference and a D-line superimposed on the 2D image to indicate the direction of the sampling.

The digital image is displayed on a video monitor. The synchronised video signals sent to the monitor control the brightness of every pixel on the screen by means of an electron beam sweeping the screen repetitively in a fixed horizontal raster pattern. The 525 lines of one image frame are read in 1/30 second. This allows a frame rate of 30 per second (i.e. number of images per second).

The ultrasound image is composed by a matrix of pixels. Each pixel is a grey scale dot. The brightness is determined by the intensity of the reflected ultrasound wave from the particular tissue interface from which the echo is produced.

The operating modes of the ultrasound system are:

B-mode (= 2D mode)

M-mode (= time-motion mode)

Colour Doppler (CD)

Colour Power Doppler (CPD)

Continuous Wave Doppler (CWD)

Pulsed Wave Doppler (PWD)

The master synchroniser contains the central processing unit (CPU) and keeps track of and synchronises all the functions of the ultrasound system.

The CPU is the brain of the ultrasound system. It organizes the data and tells the rest of the system what to do with the data.