Ultrasound

Describe the properties of ultrasound Describe the piezoelectric effect Explain how inducers emit and receive big frequency sound Describe the principles of ultrasound scanning Describe the difference between A-scan and B-scan Be able to calculate acoustic impedance Calculate the fraction of reflected intensity Describe impedance matching Explain the use of gel in ultrasound scans

Ultrasound These are sound waves that are above the audible sound frequency range (20 Hz to 20,000 Hz) In medical ultrasounds the frequencies are in the 2 to 10 megahertz range. These cause no ionisation. Ultrasound can show both muscle and blood. A low frequency is used as often as possible as high frequency ultrasounds can be destructive to tissues.

Ultrasound Speed [ms-1] Air 330 Bone 2700-4100 Muscle 1545-1630 Soft Tissue 1460-1615 Fat 1450 Blood 1570

Recap the Doppler Effect If you move towards a stationary source of sound you will not hear the same frequency as when you were stationary. Extra waves will have passed into your ears. This change in frequency due to the persons movement is called the doppler effect.

Numerical Example Stationary source 200 Hz c = 340 m/s Wavefronts = 1.7 m apart If person is stationary, wavefronts go past at 200/s Imagine you are travelling towards the source at 30 m/s. You will have passed an extra (30/1.7) = (17.6 wavefronts. Altogether, 217.6 wavefronts will have passed. This means the frequency will be 217.6 Hz. You will hear a higher pitch.

Source movement When the source of the sound is moving, it creates a different effect. Wavelengths behind the source are larger. Wavelengths in front are shorter. This is the principle of ultrasound.

 

Question Calculate the frequency heard when a train, travelling towards you with a speed of 60 m/s, sounds its whistle of frequency 400 Hz. Velocity of sound is 340 m/s.

Doppler Ultrasound to Measure Bloodflow https://www.youtube.com/watch?v=1Wa1qJsM7D0 Not only does the time at which the pulse is reflected from the blood needs to be recorded but also the frequency of the return wave. This can be used to determine the speed of the blood flow (and direction) The frequency is increased to 8 MHz to produce a readable image.

This is the process by which ultrasound is produced Piezoelectric Effect This is the process by which ultrasound is produced 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.

PZT Crystal Stretched Unstressed Compressed

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.

The Ultrasound Transducer 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.

a) Describe the properties of Ultrasound 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.

Principles of Ultrasound Scanning. 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.

Pulse Repetition Frequency 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.

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e) Describe the difference between A scan and B scan. 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.

e) Describe the difference between A scan and B scan. 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 used to build up an image on screen.

(f) calculate the acoustic impedance using the equation Z = ρc 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 = ρc

(f) calculate the acoustic impedance using the equation Z = ρc  

Material c (m/s) ρ (kg m-3) Z (kg m-2 s-1) Air 340 1.3 440 Bone 3500 1600 5.6 x106 Muscle 1000 1.6x106 Soft Tissue 1500 1.5x106 Fat 1400 1.4x106 Blood  

Impedance Matching 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.