ULTRASOUND PHYSICS.........

Ultrasound is sound with frequency greater than 20,000 cycles per second or 20kHz. Audible sound sensed by the human ear are in the range of 20Hz to 20kHz.

Advantages

Ultrasound can be directed as a beam.
Ultrasound obeys the laws of reflection and refraction.
Ultrasound is reflected by objects of small size.

Disadvantages

Ultrasound propagates poorly through a gaseous medium.
The amount of ultrasound reflected depends on the acoustic mismatch.

Creating an ultrasound image is done in three steps - producing a sound wave, receiving echoes, and interpreting those echoes.

Producing a sound wave

• Ultrasound waves are produced by a transducer. A transducer is a device that takes power from one source and converts the energy into another form eg electricity into sound waves. The sound waves begin with the mechanical movement (oscillations) of a crystal that has been excited by electrical pulses, this is called the piezoelectric effect. 
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• The sound waves are emitted from the crystal similar to sound waves being emitted from a loud speaker. The frequencies emitted are in the range of (2- 15MHz) and are unable to be heard by the human ear. Several crystals are arranged together to form a transducer. It is from the transducer that sound waves propagate through tissue to be reflected and returned as echoes back to the transducer.
• Sound is produced using Piezoelectricity which is the ability of some materials (notably crystals and certain ceramics) to generate an electric charge in response to applied mechanical stress, the reverse applies when
• The word is derived from the piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect converse piezoelectric effect (the production of stress and/or crystals will exhibit a maximum shape change of about 0.1% of the original dimension.  
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• Precise electrical pulses from the ultrasound machine make the transducer create sound waves at the desired frequency. The sound is focused either by the shape of the transducer (Curved, Linear, Sector) or a set of control pulses from the ultrasound machine. This focusing produces the desired shaped sound wave from the face of the transducer. The wave travels into the body and comes into focus at a desired depth.

• On the face of the transducer a rubber material enables the sound to be transmitted efficiently into the body. This rubber coating is required for impedance matching and allows good energy transfer from transducer to patient a vice verse. To help with the transmission of sound waves a water based gel is placed between the patient's skin and the probe.The gel establishes good acoustic contact with the body, since air is a very good acoustic reflector.

Receiving the echoes

• The image is formed by the reverse of the process used to create the sound waves. The returning echoes to the transducer are converted by the crystals into electrical signals and are then processed to form the image.

Forming the image

• To form the image ultrasound machine needs to determine the direction of the echo, how strong the echo was and how long it took the echo to be received from when the sound was transmitted. Once the ultrasound scanner determines these three things, it can locate which pixel in the image to light up and to what intensity.

Sound in the body

• When a sound wave encounters a material with a different density (acoustic impedance), part of the sound wave is reflected back to the transducer and is detected as an echo. The time it takes for the echo to travel back to the transducer is measured and used to calculate the depth of the tissue interface causing the echo. The greater the difference between acoustic impedance, the larger the echo is.

• Highly reflective interfaces give rise to a strong echo which is represented on the screen as a bright spot, whilst the opposite is true of weak reflective interfaces. Areas without acoustic interfaces such as the lumen of vessels and other cavities containing liquid (blood, bile, ascites or urine) give no reflection and no spot on the screen ie a black space on the monitor. If the waves hits gases or solids the density difference is so great that most of the acoustic energy is reflected and it becomes impossible to see deeper.

• The speed of sound is different in different materials, and is dependent on the acoustical impedance of the material. However, the ultrasound scanner assumes that the acoustic velocity is constant at 1540 m/s. An effect of this assumption is that in a real body with non-uniform tissues, the beam becomes de-focused and image resolution is reduced.

• The formula for the velocity of sound is (velocity = frequency x wavelength). The frequencies used for medical imaging are generally in the range of 2 to 15 MHz. Higher frequencies have a smaller wavelength (as can be seen from the formula for velocity of sound), and can be used to make images with smaller details. However, the attenuation of the sound wave is increased at higher frequencies, so in order to have better penetration of deeper tissues, a lower frequency 3-5 MHz is used. Seeing deep structures in the body with ultrasound is very difficult as some acoustic energy is lost every time an echo is formed, but most of it is lost from acoustic absorption.