Ultrasound Learning Objectives: Describe the properties of ultrasound; Describe the piezoelectric effect; Explain how ultrasound transducers emit and receive high-frequency sound; Describe the principles of ultrasound scanning; Describe the difference between A-scan and B-scan; Calculate the acoustic impedance using the equation Z = ρc; Calculate the fraction of reflected intensity Describe the importance of impedance matching; Explain why a gel is required for effective ultrasound imaging techniques. Explain what is meant by the Doppler effect; Explain qualitatively how the Doppler effect can be used to determine the speed of blood.

What is Ultrasound? Ultrasound is defined as any sound wave above 20000Hz. Sound waves of this frequency are above the human audible range and therefore cannot be heard by humans. All sound waves, including ultrasound are longitudinal waves. Medical ultrasounds are usually of the order of MEGAHERTZ (1-15MHz). Ultrasound as all sound waves are caused by vibrations and therefore cause no ionisation and are safe to use on pregnant women. Ultrasound is also able to distinguish between muscle and blood and therefore show blood movement. When an ultrasound wave meets a boundary between two different materials some of it is refracted and some is reflected. The reflected wave is detected by the ultrasound scanner and forms the image.

Uses of Ultrasound Obstetrics and Gynecology Cardiology Urology The development and monitoring of a developing foetus http://www.createhealth.org/ http://archos.kewego.co.uk/video/iLyROoaft3bH.html Cardiology Seeing the inside of the heart to identify abnormal structures or functions and measuring blood flow through the heart and major blood vessels Urology measuring blood flow through the kidney seeing kidney stones detecting prostate cancer early

The Piezoelectric Effect piezoelectric means pressure electricity Ultrasound waves are produced using the piezoelectric effect. When a potential difference is applied across certain crystals (piezoelectric) the crystals themselves deform and contract a little. If the p.d. applied is alternating then the crystal vibrates at the same frequency and sends out ultrasonic waves. For ultrasound - lead zirconate titanate (PZT) crystals are used. This process also works in reverse. The piezoelectric crystal acts a receiver of ultrasound by converting sound waves to alternating voltages and as a transmitter by converting alternating voltages to sound waves Discovered by Pierre and Jacques Curie in 1880.

The Transducer The transducer probe is the main part of the ultrasound machine. The transducer probe transmits and receives the ultrasound. The curved faceplate shapes the ultrasound waves into a narrow beam. Transducer probes come in many shapes and sizes. The shape of the probe determines its field of view, and the frequency of emitted sound waves (controlled by the tuning device) determines how deep the sound waves penetrate and the resolution of the image. The ultrasound is pulsed. There must be a pause to allow the reflected wave to be detected.

Transducer pulse controls Ultrasound Equipment Screen/Display Transducer pulse controls Computer Various Transducers Portable

Why Ultrasound? ADVANTAGES No known hazards – non ionizing for patient and sonographer. Good for imaging soft tissue. Relatively cheap and portable. DISADVANTAGES Cannot pass through bone Cannot pass through air spaces. Poor resolution.

Exam Style Question Explain what an Ultrasound wave is. (2 marks) Describe how ultrasound images are carried out (4 marks) c) Discuss the uses, advantages and disadvantages of Ultrasound in medicine.

TASK: What are the units of Z? Acoustic Impedance, Z As stated earlier, when an ultrasound wave meets a boundary between two different materials some of it is refracted and some is reflected. The reflected wave is detected by the ultrasound scanner and forms the image. The proportion of the incident wave that is reflected depends on the change in the acoustic impedance, Z. Acoustic Impedance, Z of a medium is defined as: Z = c Where  = the density of the material, kgm-3 c = speed of sound in that material, ms-1 TASK: What are the units of Z? See page 201/203 of textbook for typical values

Exam Style Question The acoustic impedance of a certain soft tissue is 1.63 x 106 kgm-2s-1 and its density is 1.09 x 103 kgm-3. What is the speed of the ultrasound in this medium? (2 marks) 2) The time base on a CRO was set to 50μscm-1. Reflected pulses from either side of a fetal head are 2.4cm apart on the screen. If the ultrasound travels at 1.5kms-1, calculate the diameter of the fetal head. (4 marks)

Intensity reflection coefficient,  At a boundary between mediums, the ratio of the intensity reflected, Ir to the intensity incident, I0 is known as the intensity reflection coefficient, .  = Ir I0 The intensity of both the reflected and incident ultrasound waves depend on the acoustic impedance, Z of the two mediums. Therefore the fraction of the wave intensity reflected can be calculated for an ultrasound wave travelling from medium 1, (acoustic impedance Z1) to medium 2 (acoustic impedance Z2).  = Ir = Z2 - Z1 I0 Z2 + Z1 If 2 mediums have a large difference in impedance, then most of the wave is reflected. If they have a similar impedance then none is reflected. 2

Impedance Matching / Gel When ultrasound passes through two very different materials the majority of it is reflected. This happens between air and the body, meaning that most ultrasound waves never enter the body. To prevent this large difference in impedance a coupling medium (gel) is used between the air and the skin. The need to match up similar impedances to ensure the waves pass through the body is known as impedance matching.

A-Scan A-Scan (Amplitude scan) Gives no photo image Pulses of ultrasound sent into the body, reflected ultrasound is detected and appear as vertical spikes on a CRO screen. The horizontal positions of the ‘spikes’ indicate the time it took for the wave to be reflected. Commonly used to measure size of foetal head.

B-Scan B-Scan (Brightness scan) An array of transducers are used and the ultrasound beam is spread out across the body. Returning waves are detected and appear as spots of varying brightness. These spots of brightness are used to build up a picture.

The Doppler Effect The apparent frequency of a wave increases when the source of a wave is moving towards you. http://paws.kettering.edu/~drussell/Demos/doppler/carhorn.wav The Doppler effect can be used to measure blood flow in adults, children and developing babies. Both the time for the reflected ultrasound wave and the ‘new’ frequency of the reflected wave are measured. This enables the speed of blood flow to be calculated. The greater the difference between the original frequency and the reflected frequency, the greater the speed. Computers then display this info as ‘moving images’ by updating data several times per second.

Cardiac B-scan images with Doppler scans overlaid Doppler Images Red represents blood moving towards the transducer (increased frequency). Blue represents blood moving away from the transducer (decreased frequency). Cardiac B-scan images with Doppler scans overlaid