Presentation on theme: "Ultrasound Transducers"— Presentation transcript:
1 Ultrasound Transducers Saudi Board of Radiology: Physics Refresher CourseUltrasound TransducersKostas Chantziantoniou, MSc2, DABRHead, Imaging Physics SectionKing Faisal Specialist Hospital & Research CentreBiomedical Physics DepartmentRiyadh, Kingdom of Saudi Arabia
2 Ultrasound Pulse Production and Reception A transducer is a device that can convert one form of energy into another. Ultrasoundtransducers are used to convert an electrical signal into ultrasonic energy that can betransmitted into tissue, and to convert ultrasonic energy reflected back from the tissueinto an electrical signal.The general composition of an ultrasound transducer is shown below:the most important component is a thinpiezoelectric (crystal) element located near theface of the transducerthe front and back face of the element is coatedwith a thin conducting film to ensure goodcontact with the two electrodesthe outside electrode is grounded to protect thepatient from electrical shockan insulated cover is used to make the devicewatertightan acoustic insulator made of cork or rubber isused to prevent the passing of sound into thehousing (i.e.: reduces transducer vibrations)the inside electrode is against a thick backing block that absorbs sound wavestransmitted back into the transducer
3 Matching LayerA matching layer of material is placed on the front surface of the transducer to improvethe efficiency of energy transmission into the patient. The material used has animpedance in between that of the transducer and tissue; and it has a thickness one forththe wavelength of sound in the transducer crystal material (quarter wave matching).Piezoelectric CrystalCertain material (or crystals) are such that the application of an electrical field causes a change in their physical dimensions. The reverse effect, where an external pressure causes a change in the crystal’s physical dimensions and thus induces a voltage between electrodes, is called the piezoelectric effect. Piezoelectric means pressureelectricity.some naturally occurring materials posses piezoelectric properties (eg: quartz) butmost crystals used in diagnostic ultrasound are man-made ceramics like leadzirconate titanate (PZT)the advantage is using ceramics is that they can be formed into different shapespiezoelectric crystals can be designed to vibrate in either the thickness or radial mode,but in medical imaging it is the thickness mode that is usedthe piezoelectric effect of a transducer element is destroyed if heated above its Curietemperature limit (328C for PZT and 573C for quartz)transducer crystals do not conduct electricity
4 Creating a sound wave from an electrical pulse When a positive voltage (A) is applied across the surface of the crystal, it creates an electric field across the crystal surface which cause the molecules (dipoles) in the crystal to realign and thus changing the shape (width) of the crystal.ABCVoltage PulsePositiveTimeNegativeWhen the voltage polarity is changed from positive to negative, there is a point in timewhen the electric field across the crystal is zero (at voltage equal to zero) and the crystalrelaxes (B). When the voltage polarity is reversed (i.e.: negative) the crystal realignsonce again and changes its width once again (C).
5 The net effect the alternating voltage pulse has on the crystal is to make it oscillate back and forth about its width. This change in shape of the crystal increases and decreasesthe pressure in front of the transducer, thus producing ultrasound waves.Ultrasound wave directionRarefaction region created when crystalsurface is contracting (less pressure on surface)wavefront diagramCompression region created when crystalsurface is expanding (more pressure on surface)Ultrasound wave direction
6 Creating an electrical signal from a sound wave When the compression region (A) of the ultrasound wave is incident on the front surface of the crystal, it induces a high pressure region on the surface which in turn compresses the crystal. This cause the molecules in the crystal to re-align and induce an electric field across the crystal which generates an electrical voltage signal that is proportional to the intensity of the compression region.ABRarefaction region relaxes crystal surface(less pressure on surface)wavefront diagramCompression region compresses crystalsurface (more pressure on surface)When the rarefaction region (B) of the ultrasound wave is incident on the frontsurface of the crystal, it induces a low pressure region on the surface which in turnrelaxes the crystal.
7 The net effect the ultrasonic wave has on the crystal is to make it oscillate back and forth about its width. This change in shape of the crystal induces a voltage signalthat also varies in time and in amplitude.NOTEA transducer can function both as a transmitter and a receiver of ultrasound energy, butit can not transmit and receive at the same time.Transmitter ModeReceiver Mode
8 Transducer Characteristics Transducer ThicknessA transducer can be made to emit sound of any frequency by driving it (in continuousmode) with an alternating voltage of that frequency. However, a transducer vibratesmost violently and produces the largest output (pressure amplitude) of sound when = • twhere the is wavelength of sound and t is the thickness of the piezoelectric crystal.The frequency of the emitted sound waves is then given byfrequency = v = v • twhere v is the speed of sound in the piezoelectric crystal.operating frequency crystal thickness
10 Why should the transducer thickness be equal to 1/2 of the desired wavelength? Back surfaceFront surfaceABCDBackingBlockPatientThickness (t)When the piezoelectric element is driven by a alternating voltage the crystalvibrates (i.e.: the width of the crystal moves back and forth). The front face of the crystal emits sound both in the forward and backward directions as does the back surface.wave front (A) will get absorbed by the transducer’s backing materialwave front (D) will enter into the patientthe wave front (C) is reflected at the back face of the disk, and by the time it joinswave front (D), it has traveled an extra distance 2t. If this distance equals awavelength the wave fronts (D) and (C) reinforce for they are in phase, andconstructive interference or resonance occurs.if wave fronts (D) and (C) are not in phase, then there will be some destructiveinterferencesame reasoning applies to wave front (B)
11 Constructive Interference (waves A & B add to form anew wave of amplitude A + B)Destructive Interference(waves A & B add to form a newwave of amplitude A + B = 0)If wave B is wave front (C) and wave F is wave front (D) then we see that when transducer thickness is one half the wavelength, both wave fronts are in phase and constructive interference (ie: their individual amplitudes add) occurs.
12 changing the thickness of the crystal changes the frequency but not the ultrasound amplitude (determined by applied voltage waveform) or speed (determined bypiezoelectric crystal)high frequency transducers are thin and low frequency transducers are thickerto change the frequency one has to change the transducer
13 Resonant FrequencyThe frequency at which the transducer is the most efficient as a transmitter of soundis also the frequency at which it is most sensitive as a receiver of sound. Thisfrequency is called the natural or resonant frequency of the transducer.the thickness and the material (i.e.: speed of sound in the crystal) of the piezoelectriccrystal determines the resonant frequency of the transducertransducers crystals are normally manufactured so that their thickness (t) is equal toone-half of the wavelength () of the ultrasound produced by the transducerBandwidthThe range of frequencies in the emitted ultrasound wave is called the bandwidth and is defined to be the full width of the frequency distribution at half maximum (FWHM).Resonant Frequencybandwidth SPL
14 Continuous voltage waveform Pulsed voltage waveform Frequency distribution of emitted ultrasound waveContinuous waveformcan be represented bya single sine wave (onefrequency), thus frequencydistribution is verynarrowPulsed waveformcan be represented bythe sum of many sinewaves each of differentfrequency, thus frequencydistribution is wide
15 Q-factorThe Q-factor of a transducer system describes the shape of the frequency distribution(response curve) and is defined asQ-factor = f0(f2 - f1)Bandwidth = (f2 - f1)where f0 is the resonance frequency, f1 is the frequency below resonance at whichintensity is reduced by half and f2 is the frequency above resonance at whichintensity is reduced by halfhigh Q transducers produce relativelypure frequency spectrums and low Qtransducers produce a wider range offrequenciesshort pulses correspond to reduce Qvalues and vice versabandwidth Q-factor
16 Pulse Ultrasound ModeBecause a transducer can be a transmitter and a receiver of ultrasonic energy, it clearly stands to reason that a continuous voltage waveform can not be used. If such a waveform was used, the transducer would always function as a transmitter. Since the internally generated sound waves are stronger than the returning echoes, the returning signal is lost in the noise of the system. To over come this problem, most transducers are used in a pulse mode where the voltage waveform consists of many pulses each separated by a fixed distance and time. The transducer functions as a transmitter during pulse excitation and as a receiver during the time interval between pulses.Voltage waveformUltrasound pulses produced by transducerNOTEmost transducers are designed to have short pulses (improved resolution) with lowQ values (broad bandwidth - desirable in order to receive echoes of many differentfrequencies)
17 Blocks of damping material, usually tungsten/rubber in a epoxy resin, are placed behind transducers to reduce (or dampen) the vibrations and to shorten pulses.the exponential decay of the pressure wave over time is called dampingif damping is heavy the transducer has a short ring down time and is said to have alow Q valuea transducers with lighter damping is said to have a high Q value
18 Pulse Repetition Frequency (PRF) PRF is the number of pulses occurring in 1 secondPulse Repetition Period (PRP)PRP is the time from the beginning of one pulse to the beginning of the next pulse
19 Spatial Pulse Length (SPL) SPL is the length of space over which a single pulse occurs, and is defined asSPL = n • where n is the number of cycles in the pulse and is the wavelength.NOTEAn important parameter when considering axial resolution
20 Pulse Duration (PD)PD is the time it takes for a single pulse to occur and is defined asPD = n • Twhere n is the number of cycles in the pulse and T is the period.Duty Factor (DF)DF is the fraction of time that ultrasound generation (in the form of pulses) is ON, andis defined as:DF = PDPRP
22 Ultrasound Beam Characteristics In order to understand the beam characteristics of ultrasound we have to revisit ourview of wave front (compression region) generation. A piezoelectric crystal surfaceactually behaves more like a series of vibrating points and not as the piston-likesurface that we have implied previously.simplifiedmodelmore accuratemodelthe compression waves are not uniform (at least not close to the crystal surface)each vibrating point produces multiple concentric rings or waves that eventually forma continuous front as they reinforce each other along a line parallel to the surface ofthe crystalthe distance at which the waves become synchronous depends on their wavelength,the shorter the wavelength the close the front forms to the surface of the transducer
25 Fresnel Zone (Near Field) The length of the Fresnel zone is given by:d24 • where d is the diameter of the transducer and is the wavelength.the Fresnel zone increases with transducer size and frequency (lower wavelength)ultrasound imaging normally uses the Fresnel zone but not the Fraunhofer zone inwhich resolution is poorbeam intensity falls off because of attenuation
27 Fraunhofer Zone (Far Field) qThe angle of divergence of the Fraunhofer zone is given by:sin(q) = dwhere d is the diameter of the transducer and is the wavelength.beam intensity falls of due to attenuation and beam divergenceangle of divergence increases with decreasing transducer diameter and frequencyno useful imaging can be made in this region
28 Side LobesSide lobes are small beams of greatly reduced intensity that are emitted at angles tothe primary beam and they often cause image artifacts.the origin of these lobes are due from radial vibrations from the edges of thetransducer
29 Grating LobesGrating lobes result when ultrasound energy is emitted far off-axis by multi-elementarrays, and are a consequence of the non-continuous transducer surface of the discrete elements.this misdirected energy of relatively low amplitude results in the appearance ofhighly, off-axis objects in the main beam
30 Axial (Linear, Range, Longitudinal, Depth) Resolution Axial resolution is the ability to separate two objects lying along the axis of the beamand is determined by the spatial pulse length (SPL).Case: SPL < XCase: SPL = 2XObjects a and b separate by distance XObjects resolvableObjects not resolvablelimiting resolution = SPL = n• where n is the number of cycles and is the wavelength
31 because axial resolution is depended to the SPL then it is also depended on pulse frequency and duration (PD)with a typical wavelength of 0.3 mm and three cycles per pulse, the axial resolutionis approximately 0.5 mmaxial resolution deteriorates with increasing pulse length, decreasing frequency andincreasing wavelengththe use of damping transducers (low Q) produces short pulses that improves axialresolutionfrequency axial resolution (improved)cycle/pulse axial resolution SPL axial resolution (worsened)
32 Lateral (Azimuthal) Resolution Lateral resolution is the ability to separate two adjacent objects and is determined bythe width of the beam and line density.Object separation narrower than beam widthObject separation wider than beam widthlateral resolution is equal to beam width in the scan planelateral resolution is best in the Fresnel zone where ultrasound waves are parallellateral resolution is generally a few millimetersbeam width lateral resolution (improved)
33 Elevation Resolution (Slice Thickness) Elevation resolution is the dimensions of the ultrasound beam perpendicular to theimage plane and depends on the transducer element height in much the same way that the lateral resolution is dependent on the transducer element width.
34 Focused TransducersHigh frequency beams have two advantages over low-frequency beams:(1) axial resolution is superior; and(2) the Fresnel zone is longerIt would seem logical to use high frequencies for all imaging. High frequencies however, have a major drawback related to penetration. Tissue absorption increaseswith increasing frequency, so a relatively low frequency beam is required to penetrate thick parts.It would then seem logical to use low frequency transducers and to increase the size of the transducer to keep the beam coherent for sufficient depth to reach the point of interest (longer Fresnel zone). Although larger transducers improve coherence they deteriorate lateral resolution. The dilemma is at least partially resolved with the use focused transducers.NOTEfocused transducers reduce beam width which improves lateral resolutionthey also concentrate beam intensity thereby increasing penetration and echointensity thus improving image quality
35 the focal zone is the region over which the beam is focused the focal length is the distance from the transducer to the centre of the focal zonethe depth of focus is the distance over which the beam is in a reasonable focusa small diameter transducer has a shorter focal zone and spreads more rapidly inthe far zonemost diagnostic transducers are focused, which is achieved using a either a curvedpiezoelectric crystal, an acoustic lens or electronics (phased arrays)
36 A focused transducer produces a narrower beam at the focal zone and, therefore, has better lateral resolution than an unfocused transducer of the same size.
37 In reality we have:It is important to realize, that even for flat, unfocused transducer elements, there issome beam narrowing or “focusing”.