Why it Is Important to Use Gel in Ultrasound
Why we use ultrasound gel
When using ultrasound, it is commonplace to apply a gel, which consists mostly of water, as a contact medium between the tissue (i.e. the skin) and the ultrasound head. The purpose of the gel is to convey the acoustic energy (soundwaves) from the ultrasound head to the tissue without crossing through the air at any point. The reason this is important is that both reflection and refraction are markedly less when crossing from a gel into body tissues than they are when crossing from air into body tissues. Before we can understand reflection and refraction, however, we must first learn about acoustic impedance.
Acoustic impedance, simply put, is the resistance a substance’s molecules have to a change in their state of mechanical vibration. A material with a high acoustic impedance, such as bone, will be much more resistant to a change in vibration than a material with a low acoustic impedance, such as air or water. Every material has a unique acoustic impedance, which increases in proportion to that material’s density and the speed of the ultrasound waves travelling through it. Acoustic impedance is therefore a product of material density (g/cm3) and acoustic wave velocity (m/s). For this analysis of ultrasound gel, the acoustic impedances we are concerned with are;
- Air – 0.0004
- Water – 1.48
- Soft Tissue – 1.63
When an ultrasound beam encounters an acoustic boundary - a point at which two materials with different acoustic impedances are in contact, such as water and soft tissue – some of the acoustic energy crosses the boundary, while some is reflected, meaning it is redirected away from the acoustic boundary at an angle equal to the angle at which it approached, only mirrored around the perpendicular line to the ultrasound head. These angles of incidence and reflection shall be ignored for now as we are assuming that all energy is being directed at the tissues at an angle of 900. This is a relatively safe assumption, as it is common practice to hold the ultraound head perpendicular to the tissue.
How gel reduces reflection
The reflected energy is called an echo, and as our angle of incidence is 00, the intensity of the echo is entirely dependent on the difference in acoustic impedance values of the two materials forming the acoustic boundary.
In this equation, Z1 and Z2 are the acoustic impedances of the first and second materials. Using the acoustic impedance values for air, water and soft tissue above, the energy reflected by an air/tissue acoustic barrier and a water/tissue acoustic barrier can be found as follows;
The percentage of energy reflected is almost 500 times smaller for a water/tissue acoustic barrier than it is for an air/tissue barrier. This means that the efficacy of any treatment of diagnostic imaging will be increased five-hundredfold if gel is used. The magnitude of this difference illustrates the importance of using a gel when applying ultrasound.
How to reduce refraction
It is worth briefly noting the consequences of applying acoustic energy at an angle to an acoustic barrier. Firstly, this will increase the percentage of the original intensity reflected. As the angle of incidence increases, so too will the amount of energy reflected. Secondly, increasing the angle of incidence will increase the amount of refraction. Refraction is where the direction of a sonic beam changes as it crosses an acoustic boundary. You can observe a similar effect by looking at the effect of water on beams of light. Additionally, the amount of refraction will increase as the difference in acoustic impedance across an acoustic border increases. For this reason, it is instructive to keep the ultrasound head perpendicular to the tissues being treated.