1.1 Nature of sound wave:.Soundwaves is a result of mechanical potential that passesthrough medium as a wave gives alternating pressure and rarefaction. And soundcan travel through a liquid , gas and solid, A bell is a common example ofsound that generates vibrations to be heard. 1.
2 Range of sound wave:Sound which humans can heard start from 20 HZ to 20 KHZ,while the ultrasound range used in somemedical application ranges from 2 MHZ to 15 MHZ. 1.3 basic units of measurement for soundwaves. a- Time period (T): the required time to complete one cycle(second), T = 1/f.b- Frequency (f): isthe number of full cycle in a certain time (HZ), f = 1/T.
c- Wavelength (?): is the distance between trough or crestof wave (meter). Fig (1,2)The figure (1,2) explain the meaning of each parameter inthe wave motion.1.3a Sound speed equation: c=f?C is represent the speed of sound. 1.4 Types of sound propagation. Transverse wave: wave that the particles is vertical ondirection of wave. Fig (3)Longitudinal wave: the particles move with same wavedirection.
Fig (4)1.5 Acoustic Impedance (Z): is the result of density (?) ofthe medium which the sound pass through it and the speed of sound (C).Z = ?cAnd the difference in acoustic impedance determine theamount of reflection, with large difference acoustic impedance almost ofincident energy will be reflected, while small difference in acoustic impedancewil reflect apart of incident energy and the remainingwill go forward.
( Tissueproperties determine the acoustic impedance).1.6 Reflection: a sound wave strikes an interface betweentwo media with perpendicular, incidencesound can be reflected . And the amount of reflected sound is expressed as (R)reflection coefficient.
1.6a Reflection Cases: a. Specular reflection: have also another name that explainit more (mirrors for sound), If the interface is large and slick (smooth) andincident sound energy is approximately in right angle the sound energy willreflected to source (Transducer) as a mirror reflects light. b. Diffuse reflection: if the interface is small and roughit will scatter the the sound energy that falling on it in randomly directionand only one part will reflect to the sound energy source (Transducer). Fig (5)This figure (5) explain the incident sound energy and (A)expressed specular case (White arrow), While (B) expressed Diffuse case, and in(A) the yellow arrow Illustrates if the incident sound energy not in rightangle the reflected energy will not come back to sound source. 1.
6b Reflection coefficient (R): If the specular reflected is vertical to incident energy wecan calculate reflected energy by this equation:R=(Z2?Z1)2/(Z2+Z1)2Z1 is expressed as a acoustic impedance for first medium.Z2 is expressed as a acoustic impedance for second medium. 1.7 Refraction: Can defines it if the sound energy go from onemedium in a certain speed and specific direction and pass to another medium andhave a change in speed either lower or higher and the direction will bedifferent, ratio of changing direction is proportional to velocity of soundenergy, have more explain in Snell law. 1.7a Snell law: sin?1/sin?2=c1/c2c1 is sound speed in medium 1.c2 is sound speed in medium 2.
?1 is incident angle.?2 is refracted angle.If sound speed in medium 1 is lower than speed in medium 2the direction of medium 1 will be smaller than medium 2. Fig(6)The figure (6) explain Snell law ( T1,T2,T3 is the refractedangle and I is incident angle, 1 and 2 is the sound speed). 1.8 Interference of wave: is the change of when two waves interact with each other,and it has two types.
a. Destructive interference: when the waves are 180o out of phase and that occur a nowave (removed each other). Fig (7)b. Constructive interference: waves that are in Same phase that occur a single wave buthave a higher amplitude. Fig (8)1.9 Transmitter:In an ultrasound imaging, transmitter control the number ofsound wave pulses that released from transducer (that have a name pulse repetitionfrequency “PRF”), PRF which is specify the time between pulses of sound wavethat is important to ensure that each pulse back to the transducer beforerelease a new pulse.
(That the pulse in medical imaging composed of 2 to 4cycles). 1.10 Transducer: Transducer is an instrument which can change signal orenergy from one type or form to anotherone, these transducers can change multiple forms of energy such as mechanical,and electrical energy. And there are many types of transducers such as potentiometerwhich converts the change in a length of wire to a resistance for the wire, andstrain gage transducer which converts pressure energy into electrical signal,and Piezo electric transducer convert mechanical energy to electrical energyand vice versa, the meaning of piezoelectric is the pressure that occurs due toelectricity, these types of transducers are used for ultrasound imaging.
Piezoelectric transducer (PZT) content: it has a crystals (such as quartz) andceramic materials that is the functional component of transducer which is havea role to release mechanical and electrical energy. Piezoelectric transducer(PZT) work: electricity occur when the crystals be as a battery and havepositive in one face and negative in reverse face and it connected with eachother for current flow to create the circuit, and for opposite operation thecrystals have a mechanical pressure (vibrating) that resulting from voltagepass through the reverse faces, the crystals vibrate to generate high frequency(frequency increases with vibrate increasing). Important of PZT: it has highfrequency response and self generating and it affected from temperaturechanging. Fig (9)This figure (9) explain the parts of piezoelectric transducer. 1.11 Receiver: when the medium (tissue) have a large thickness the soundwill be attenuated, and also when wave come back from medium (such as tissue)to transducer it will loss an energy of signal, and this problem it can solveit by controlling time gain compensation (TGC) that placed in receiver, and TGCallow to have more brightness and higher resolution for image in deeper partsof the tissue. Fig (10) This figure (10) explain that the gain when it increase itwill have a good resolution and brightness for deeper distance and when it notused it will be a dark and low resolution for deeper distance.
1.12 Attenuation:When the sound waves travels intensity, power and amplitudewill decrease as it does. Its Units are dB, decibels. Attenuation has ThreeComponents which are Absorption, Scattering and Reflection.
Attenuation relatedto frequency, in low frequency will attenuated slower than high frequency. 1.12a Attenuation coefficient of sound (dB/cm/Hz) eq.
?water =ln(Vout/Vin)/x x represent thedistance from one transducer to the other one (between them), Vin represent theelectrical signal which is transmitted, ?water is the attenuation coefficientand Vout represent the electrical signal which is received. 1.12b factors affecting attenuation:The factors that have an effect on attenuation frequency(when it increase the attenuation will increase and vice versa), the traveldistance and tissue nature Fig (11)This figure (11) illustrates the attenuation amount in somemedium.
1.13 Arrays: Basically array produced by slicing transducer element(crystal) into a many smaller element and these element put in separate placesto prevent any interference (electrical or acoustic) to have a signal withoutany problem. And there are types of transducer arrays. a. Linear array: It works by firing groups or individual elements in asequence and it the beams will be perpendicular to transducer and parallel toeach other, and It is straight and has a rectangular image shape therefore itis used for small parts such as blood vessels.b. Curved array:Is a linear array but with a convex beams to create largefield of view, it is used for larger parts such as pelvic.
c. Phased Array:in phased array, the transducer elements produced sectorfield of view when the elements firing sequentially and controlling itelectronically , and from these the ultrasound beams will be can to go to anydirection and different depths, these arrays is used for neonatal headultrasound. Fig (12)The figure (12) explain each type of array ( A is linear, Bis curved, C is phased). 1.14 Properties of ultrasound beam: The beam of ultrasound travels in a longitudinal wave fromtransducer to a specified medium, there are two patterns of beam: a. Near field (Fresnel zone):Is a converging beam that have a distance determined bydiameter of transducer and wavelength of transmitted ultrasound wave, and theshape of beam is formed by transducer curvature, then the pressure amplitude isquite complex and varies greatly. To calculate the length of near field we havean equation:Near field length = d2 / 4?d is the diameter of transducer. ? is wavelength oftransmitted wave.
b. Far Field (Fraunhofer zone):Is a diverging beam, and get a less divergence beam in highfrequency and large transducer diameter, the ultrasound beam diverge when thedistance to transducer increases with pressure amplitude decreases. To get thevalue of ultrasound beam divergence angle we use this equation:sin ? = 1.22 ? / dWhere d is diameter of transducer? the wavelength.
? is the ultrasound beam divergence angle. Fig (13)The figure (13) explain briefly the difference between nearand far field and how get the angle of divergence beam.