 Ultrasound waves are longitudinal with high frequencies ( ≈ > 20,000 Hz, though medical Ultrasound is between 1 to 15 MHz.)  When an ultrasound reaches a boundary, some of it is reflected, and some passes through the material.  Those that pass through will undergo refraction if the angle of incidence is not 90°.  Reflected waves are detected by an Ultrasound scanner and are used to generate an image.

 This is the effect relied upon to create Ultrasound.  This works on the basis of certain crystals contracting upon putting a potential difference across them.  An example of this kind of crystal would be Lead Zirconate Titrate.  When a high frequency alternating p.d. is applied the crystals deform/oscillate at the frequency of the signal and send out Ultra sound waves.

 Because the process can work in reverse, the same crystal can also act as a receiver of Ultrasound.  They will convert sound-waves into alternating p.d’s. Lead Zirconate Titanate The thickness of the crystal is half the wavelength of the ultrasound it produces. Ultrasound of this frequency will make the crystal resonate and produce a large signal. This is heavily damped to produce short pulses and increase the resolution of the device.

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 This acts as both a transmitter and receiver of ultrasound.  It contains  Faceplate  Piezoelectric Crystal  Backing Material  Tuning Device  Cable  The faceplate is curved. This shapes the ultrasound into a narrow beam.  The tuning device controls the frequency of the ultrasound waves.  To ensure the sound enters the body, a gel is applied between the transducer and the skin.

 Ultrasound is reflected from surfaces rather than going right through a body.  Echoes are used.  A boundary between tissue and liquid, or tissue and bone, or air and skin, reflects the waves.  Ultrasound sent into the body must be pulsed.  One pulse is sent out, and there is a pause until reflected echoes come back to be detected.

 Speed of Ultrasound waves in muscle : 1600 m/s.  Speed of ultrasound in air: 340 m/s.  Frequency of Ultrasound: 1 MHz.  Calculate the time taken for the ultrasound to travel through 20 cm of muscle.

 This states that the transmission of pulses cannot be at a frequency that exceeds the maximum time allowance for a reflection.  Eg: If a minimum time of 1 ms is allowed for a reflection to be received, frequency must not exceed 1000 Hz.

A Scan – Range Measurement The Amplitude scan sends a short pulse of ultrasound into the body simultaneously with an electron beam sweeping across the Cathode Ray Oscilloscope (CRO) screen. The scanner receives reflected ultrasound pulses that appear as vertical deflections on the CRO screen. Weaker pulses are amplified more to avoid loss of valuable data – Time Gain Compensation Horizontal positions of the reflected pulses indicate the time the echo took to return, and are used to work out distances. A stream of pulses can produce a steady image on the screen due to persistence of vision.

B Scan – The Brightness value In a brightness scan, the electron beam sweeps down the screen rather than across. The amplitude of the reflected pulses is displayed as the brightness of the spot. You can use a linear array of transducers to produce a 2D image. This array of transducers, as well as a fanning out of US beam across the body, gives the B Scan. Many returning echoes are recorded and sued to build up an image on screen.

 If at the first boundary an ultrasound is completely reflected then there will be none left to be reflected at a further boundary.  To get multiple reflections from different boundaries depends on the fraction of intensity of the US reflected as transmitted.  Acoustic impedance, Z, is used in determining the fraction of the intensity that is refracted at a boundary between two materials of different acoustic impedance.  This is defined by the equation Z = ρ c

Materialc (m/s) ρ (kg m -3 ) Z (kg m -2 s -1 ) Air Bone x10 6 Muscle x10 6 Soft Tissue x10 6 Fat x10 6 Blood x10 6

 You need a coupling medium between the transducer and the body.  Soft tissue has a different a.i from air so almost all the US energy is reflected from the body.  The coupling medium displaces the air and has an impedance closer to that of soft tissue.  This is an example of impedance matching.  This coupling medium is usually an oil or a gel.