Physics of Ultrasound Waves

In soft tissues, the propagation speed averages 1540 m/s (with a range of 1400 to 1640 m/s). However, bone or lung tissues are impermeable to ultrasound. The degree of difficulty a sound wave exhibits in propagating through a medium is known as acoustic impedance (z), which is determined by multiplying density by acoustic velocity (z = c). If the medium's density or propagation speed rises, it gets bigger. The parameter known as the attenuation coefficient is used to calculate the reduction in ultrasonic amplitude in specific media as a function of ultrasound frequency. The practical result of attenuation is that the penetration reduces as frequency increases since the attenuation coefficient grows with frequency.

The self-focusing effect of ultrasound waves refers to the ultrasound beam's natural narrowing at a specific travel distance in the ultrasonic field. It is a level in between the close field and the far field. Half of the transducer's diameter corresponds to the beam width at the transition level. The beam width reaches the transducer diameter at a distance that is two times the near-field length away. By raising acoustic pressure, the self-focusing effect enhances ultrasound waves.

Axial and lateral spatial resolution are both present in ultrasonic imaging. The smallest distance between above-below planes along the beam axis is known as axial resolution. It is determined by spatial pulse length, the sum of a pulse's wavelength and cycle count. It can be expressed using the formula below:

Axial resolution = Wavelength λ × Number of cycles per pulse n ÷ 2

The damping properties of the transducer control how many cycles make up a pulse. The ultrasound equipment manufacturer typically sets the number of cycles within a pulse between 2 and 4. A 21-gauge needle, for instance, would be impossible to see using a 2-MHz ultrasonic transducer since the axial resolution would be between 0.8 and 1.6 mm. Higher-frequency ultrasound can identify tiny objects and produce an image with better resolution for constant sonic velocity. Current ultrasound devices have an axial resolution of 0.05 to 0.5 mm.

Another measure of sharpness is lateral resolution, which describes the smallest space between two objects when they are side by side. Beam width and ultrasonic frequency both play a role in determining it. The higher frequencies provide a smaller focus and enhanced axial and lateral resolution. By altering focus to narrow the beam, lateral resolution can also be increased.

Temporal resolution is equally crucial for studying a moving item, such as the heart and blood arteries. An ultrasonic image must be updated at a pace of at least 25 times per second for the human eye to perceive it as continuous, just like in a movie or cartoon. However, raising the frame rate will reduce the image resolution. The resolution-to-frame rate ratio must be optimised to deliver the most excellent image possible.

History of Ultrasound

1880

Pierre and Jacques Curie discovered the piezoelectric effect in crystals.

1915

Ultrasound was used by the navy for detecting submarines.

1920s

Paul Langevin discovered that high-power ultrasound can generate heat in osseous tissues and disrupt animal tissues.

1942

The Dussik brothers described ultrasound use as a diagnostic tool.

1950s

Ultrasound was used to treat patients with Ménière disease, Parkinson disease, and rheumatic arthritis.

1965

The real-time B-scan was developed and was introduced in obstetrics.

1978

La Grange published the first case series of ultrasound application for placement of needles for nerve blocks.

1989

Ting and Sivagnanaratnam used ultrasonography to demonstrate the anatomy of the axilla and to observe the spread of local anaesthetics during an axillary block.

1994

Steven Kapral and colleagues explored brachial plexus block using B-mode ultrasound.

Piezoelectric Effect

Materials with a piezoelectric property can produce ultrasound waves. The creation of an electric charge in response to the application of a mechanical force (such as a squeeze or stretch) to a particular material is known as the piezoelectric effect.

On the other hand, the piezoelectric effect, which applies an electric field to a material, can result in mechanical deformation. Materials manufactured by humans and by nature alike, such as ceramics and quartz crystals, can exhibit piezoelectric qualities. Lead zirconate titanate has recently been utilised as a piezoelectric material for imaging in medicine. Additionally being developed are piezoelectric materials without lead.

Each piezoelectric substance produces a tiny amount of energy. However, a transducer can transform electrical energy into mechanical oscillations by layering piezoelectric components.

Ultrasound Terminology

Period is the time for a sound wave to complete one cycle; the period unit of measure is the microsecond (µs).
Wavelength is the length of space over which one cycle occurs; it is equal to the travel distance from the beginning to the end of one cycle.
Frequency is the number of cycles repeated per second and measured in hertz (Hz).
Acoustic velocity is the speed at which a sound wave travels through a medium. It is equal to the frequency times the wavelength. Speed c is determined by the density ρ and stiffness κ of the medium (c = (κ/ρ)1/2).
Density is the concentration of a medium.
Stiffness is the resistance of a material to compression. Propagation speed increases if the stiffness is increased or the density is decreased.