Sarah Gillies Sarah.gillies@ekht.nhs.uk Ultrasound Sarah Gillies Sarah.gillies@ekht.nhs.uk

Sound: What is it? - A longitudinal, ‘pressure’ wave Wavelength,  – length of one complete cycle of a wave (m) Period, T – the time required to complete a full cycle Frequency, f – the number of cycles per second = 1/T Amplitude, A – maximum pressure displacement Speed, c – distance travelled by a given point on the wave in a given interval of time (m/s)

Ultrasound Ultrasound is acoustic (sound) energy in the form of waves which have a frequency above the human hearing range. Human hearing frequency range: The highest frequency that the human ear can detect is approximately 20 thousand cycles per second - 20,000 Hz. Diagnostic ultrasound frequency range: Typical diagnostic ultrasound scanners operate in the frequency range of 2 to 18 megahertz (MHz)

Speed, c = distance = λ = f λ A denser medium → slower speed Speed of Sound Speed, c = distance = λ = f λ Wave speed, c, depends on the density and compressibility of the medium: A denser medium → slower speed Harder materials are more difficult to compress, this means the material impedes the formation of compressions and rarefactions. Vs

Speed of sound in the body. Medium Speed (m/s) Air 330 Water 1480 Fat 1460 Liver 1570 Muscle 1600 Soft tissues (average) 1540 The speed of sound in a given medium remains fixed. Therefore…

if  decreases …. f increases (and vice versa) Frequency if  decreases …. f increases (and vice versa) High Frequency: Low Frequency: Double the frequency half the wavelength

Gel ! Sound Sound needs a medium to travel through In diagnostic ultrasound, this medium is provided by using……? Gel !

Using sound-waves to create an image. Transducer placed onto gel applied to skin. Short bursts of ultrasound are sent into the patient. As the pulses travel into the body they are reflected and scattered, generating echoes. Some echoes travel back to the transducer where they are detected-these echoes are used to form a B-Mode image.

Echo Ranging To display each echo in a position that corresponds correctly to the target detected, the ultrasound system needs to know: The distance of the target from the transducer The direction of the target from the transducer The range of the target from the transducer is measured using the pulse echo principle.

Pulse Echo Principle The same principle is used in echo sounding equipment in boats to measure the depth of water: Transducer transmits pulse of ultrasound, it travels through water to bottom of seabed, a reflection/echo is produced and is detected on its return.

Pulse Echo Principle t = 0 t = d/c t = 2d/c To measure send & return time a clock is started as the pulse is sent (t = 0) If the speed of sound in water is c and the depth d, then the pulse reaches the seabed at time t = d/c The returning echo also travels at speed c and takes a further time d/c to reach the transducer where it is detected. Therefore the echo arrives back at a total of t = 2d/c d = ct/2 Therefore system calculates distance by measuring arrival time t of an echo assuming a fixed value for the speed of sound Depth is calculated from the time of transmission of pulse to reception of echo, taking into account the speed of sound.

A sonographic study of valves in a patient's heart The B-Mode Image A sonographic study of valves in a patient's heart Cyst in breast tissue

Creation of an image The B-mode image is formed from a a large number of B-mode scan lines where each line is produced by a pulse-echo sequence. In a typical linear array transducer, the beam is stepped across the transducer array producing an image line of echoes which are displayed on-screen as bright spots.

How do we create the sound waves for diagnostic ultrasound? Sound generally comes from a vibrating device. The source of this vibration for diagnostic ultrasound comes in the form of a small wafer of piezoelectric material which vibrates a millions of times per second. The piezoelectric material (approx 128 individual piezoelectric elements in a transducer) is the main component within the ultrasound transducer.

It does this by physical deformation of the crystal/ceramic structure. Piezoelectrics Convert electrical energy into mechanical energy (sound) to produce ultrasound. Mechanical energy into electrical energy for ultrasound detection. It does this by physical deformation of the crystal/ceramic structure.

Transducers Linear array: linear probe Linear array: Phased array curved probe Phased array Intra-cavity probe

B-mode images Gall Stones Breast cysts Twin Pregnancy at 10 weeks

The Doppler Effect The doppler effect is the change in the observed frequency of the sound wave (fr) compared to the emitted frequency (ft) which occurs due to the relative motion between the observer and the source.

Doppler example The sound the driver hears will remain the same. The observer located in front of the car will hear a higher-pitched noise. Why? Because the sound waves compress as the vehicle approaches the observer located in front. This increases the frequency of the wave, and the pitch of the vroom rises. The observer located behind the car will hear a lower-pitched noise because the sound waves stretch out as the car recedes. This decreases the frequency of the wave, and the pitch of the vroom falls.

Using doppler in medical ultrasound In a Doppler ultrasound examination, sound waves of a certain frequency are transmitted into i.e. the heart. The sound waves bounce off blood cells moving through the heart and blood vessels. The movement of these cells, either toward or away from the transmitted waves, result in a frequency shift that can be measured.

The diagram shows a Doppler transducer placed on the skin and aimed at an angle, θ, towards a blood vessel, which contains blood flowing with a velocity of u m/s, at any instant. The transducer emits ultrasound waves of frequency, fo, and echoes generated by moving reflectors in the blood, e.g. red blood cells, have a frequency, fr. The difference between these two frequencies, Δf, is related to the velocity of the flowing reflectors through the following equation:

Effect of the Doppler angle in the sonogram. A higher-frequency Doppler signal is obtained if the beam is aligned more to the direction of flow. In the diagram, beam (A) is more aligned than (B) and produces higher-frequency Doppler signals. The beam/flow angle at (C) is almost 90° and there is a very poor Doppler signal. The flow at (D) is away from the beam and there is a negative signal. Effect of the Doppler angle in the sonogram.

The Doppler Effect

Technological Advances 3D imaging PACS Picture Archiving & Communications System Portable scanners