Ultrasound transmission

Ultrasound Transmission

• Ultrasound waves travel in a longitudinal plane, the displacement of the medium is parallel to the propagation (direction) of the wave (unlike ripples on water or light which are transverse waves travelling perpendicular to the direction they direction of propagation i.e., their motion is up and down as wave travels)

• Think of a wave travelling along the length of a spring – the distance between the coils of the spring increases and decreases as it moves along. In the same way, as the sound wave is transmitted through a medium the molecules in the medium are pushed together (compression) and pulled apart (rarefaction)

• Each medium/tissue type/organ varies in its ability to allow the transmission of ultrasound through it. This resistance to the sound wave passing is called the acoustic impedance, which is determined by the medium’s density and its stiffness

• A substance or tissue type (medium) composed of densely packed molecules or particles, such as bone, will have a high acoustic impedance (the sound wave cannot be transmitted because the molecules are too tightly packed to allow them to go through the compression and rarefaction required)

• A substance or tissue type (medium) composed of loosely packed molecules or particles, such as urine, will have a low acoustic impedance (the molecules are spaced apart so are easy to push together and pull apart

• The difference in ability of different structures to allow the sound wave to pass through is called the acoustic impedance mismatch and it is this difference which is responsible for the quantity of ultrasound energy returning (reflected) to the probe and creating the on screen image

• Air and bone have a large acoustic impedance mismatch to adjacent tissue types which means that a large proportion of the energy is reflected to the transducer giving a strong echo and therefore a bright image on the display screen (hyperechoic or white pixel representation)

• The reason there is no useful image produced when scanning anatomy containing gas or bone is because of the acoustic impedance mismatch – only a small amount of sound is transmitted through both these tissue types. Subsequently, very little sound returns to the transducer from any structures which lie beyond these specific tissue interfaces. This principle explains why there is acoustic shadowing for example behind gallstones or areas of mineralisation

• Soft tissues within the body (fat, blood, water, muscle, liver, kidney etc) have similar acoustic impedances, and will be displayed on the screen as a range of grey shades attributed to the pixels representing their reflections

• In order for the ultrasound system to form an image, the speed of sound in ALL tissues is assumed to be 1540m/s i.e. the speed of sound in soft tissue is therefore a ‘constant’ and does not change during a scan.

Image formation; how does it happen?

• The ultrasound system basically consists of the probe or transducer (which both transmits and receives the sound wave) and a computer (which converts the received sound into a series of signals displayed in a pixel format on the screen to formulate the image)

• The elements of the transducer face are not ‘fired’ individually or all at the same time. A group of elements are excited, producing a narrow beam which forms a scan line. As soon as the returning echoes are received back at the transducer, the next group of elements is fired and so on. This rapid sequential sweep is what produces the cross-sectional image on our screens and constitutes a single frame

• The ultrasound system is able to attribute pixel signals (which appear as dots) of varying brightness on the image screen which are proportional to the size of the force applied to the PZ crystals by the reflected/returning sound. Highly reflective tissue interfaces will produce a strong force and subsequently a bright signal and vice versa.

• Which screen pixel is assigned a specific level of brightness will be determined by the depth of a reflection occurring within the tissue. The ultrasound system memory knows how long the pulse took to ‘go and return’. It’s basic physics really, how fast we were going is worked out by dividing the distance travelled with how long it took us

• The system assumes that the speed of ultrasound through tissue is a constant 1540m/s, and it has determined the time taken for each individual pulse ‘journey’, so calculating the depth (and therefore where in the image to place the signal or ‘dot of light’ on the pixel matrix is simple:

Speed (1540m/s) = Distance Travelled/Time Taken

• A pulse which takes a long time to go and return, will appear in the far field of the image (i.e. the bottom of the image ) on the screen and one from a superficial reflection which will appear in the near field (top of the screen)

• Remember, the strength or amplitude of the returning signal depends on the type of tissue from which it has been reflected and this will determine the level of brightness on the screen – a strong reflection e.g. from bone, appears bright white on the screen, weaker reflections will be different shades of grey and areas with no reflections (e.g. urine) will be black. This is what creates our greyscale

• Basically, as each returning reflection is processed and amplified, it is plotted, rather like a map on the screen, a 2-dimensional image is formed. Because the ultrasound images are formed of many lines of returning echo data placed closely together the image appears continuous – known as a ‘real time’ image.

• Adjusting certain ultrasound system controls will impact on the speed at which each real time image or ‘frame’ is built. For example, asking the sound pulses to travel to a greater depth in the body will mean the go and return time is longer, it therefore takes longer to build each individual frame of the image.

• Having too slow a frame rate will create a ‘lag’ or slight delay on the image which can be distracting. On the other hand, a higher frame rate is actively selected when assessing fast-moving structures such as the heart.