Presentation on theme: "Saudi Board of Radiology: Physics Refresher Course Kostas Chantziantoniou, MSc 2, DABR Head, Imaging Physics Section King Faisal Specialist Hospital &"— Presentation transcript:
Saudi Board of Radiology: Physics Refresher Course Kostas Chantziantoniou, MSc 2, DABR Head, Imaging Physics Section King Faisal Specialist Hospital & Research Centre Biomedical Physics Department Riyadh, Kingdom of Saudi Arabia Ultrasound Transducers
Ultrasound Pulse Production and Reception A transducer is a device that can convert one form of energy into another. Ultrasound transducers are used to convert an electrical signal into ultrasonic energy that can be transmitted into tissue, and to convert ultrasonic energy reflected back from the tissue into an electrical signal. The general composition of an ultrasound transducer is shown below: the most important component is a thin piezoelectric (crystal) element located near the face of the transducer the front and back face of the element is coated with a thin conducting film to ensure good contact with the two electrodes the outside electrode is grounded to protect the patient from electrical shock an insulated cover is used to make the device watertight an acoustic insulator made of cork or rubber is used to prevent the passing of sound into the housing (i.e.: reduces transducer vibrations) the inside electrode is against a thick backing block that absorbs sound waves transmitted back into the transducer
Piezoelectric Crystal Certain 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 pressure electricity. some naturally occurring materials posses piezoelectric properties (eg: quartz) but most crystals used in diagnostic ultrasound are man-made ceramics like lead zirconate titanate (PZT) the advantage is using ceramics is that they can be formed into different shapes piezoelectric crystals can be designed to vibrate in either the thickness or radial mode, but in medical imaging it is the thickness mode that is used the piezoelectric effect of a transducer element is destroyed if heated above its Curie temperature limit (328 C for PZT and 573 C for quartz) transducer crystals do not conduct electricity Matching Layer A matching layer of material is placed on the front surface of the transducer to improve the efficiency of energy transmission into the patient. The material used has an impedance in between that of the transducer and tissue; and it has a thickness one forth the wavelength of sound in the transducer crystal material (quarter wave matching).
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. When the voltage polarity is changed from positive to negative, there is a point in time when the electric field across the crystal is zero (at voltage equal to zero) and the crystal relaxes (B). When the voltage polarity is reversed (i.e.: negative) the crystal realigns once again and changes its width once again (C). A B C Positive Negative Voltage Pulse Time
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 decreases the pressure in front of the transducer, thus producing ultrasound waves. Ultrasound wave direction Compression region created when crystal surface is expanding (more pressure on surface) Rarefaction region created when crystal surface is contracting (less pressure on surface) wavefront diagram
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. A B When the rarefaction region (B) of the ultrasound wave is incident on the front surface of the crystal, it induces a low pressure region on the surface which in turn relaxes the crystal. Compression region compresses crystal surface (more pressure on surface) Rarefaction region relaxes crystal surface (less pressure on surface) wavefront diagram
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 signal that also varies in time and in amplitude. NOTE A transducer can function both as a transmitter and a receiver of ultrasound energy, but it can not transmit and receive at the same time. Transmitter Mode Receiver Mode
Transducer Characteristics Transducer Thickness A transducer can be made to emit sound of any frequency by driving it (in continuous mode) with an alternating voltage of that frequency. However, a transducer vibrates most violently and produces the largest output (pressure amplitude) of sound when = 2 t where the is wavelength of sound and t is the thickness of the piezoelectric crystal. The frequency of the emitted sound waves is then given by frequency = v = v 2 t where v is the speed of sound in the piezoelectric crystal. operating frequency crystal thickness
Why should the transducer thickness be equal to 1/2 of the desired wavelength? When the piezoelectric element is driven by a alternating voltage the crystal vibrates (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. Front surfaceBack surface Thickness (t) A B C D wave front (A) will get absorbed by the transducer’s backing material wave front (D) will enter into the patient the wave front (C) is reflected at the back face of the disk, and by the time it joins wave front (D), it has traveled an extra distance 2t. If this distance equals a wavelength the wave fronts (D) and (C) reinforce for they are in phase, and constructive interference or resonance occurs. if wave fronts (D) and (C) are not in phase, then there will be some destructive interference same reasoning applies to wave front (B) Patient Backing Block
Constructive Interference (waves A & B add to form a new wave of amplitude A + B) Destructive Interference (waves A & B add to form a new wave 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.
changing the thickness of the crystal changes the frequency but not the ultrasound amplitude (determined by applied voltage waveform) or speed (determined by piezoelectric crystal) high frequency transducers are thin and low frequency transducers are thicker to change the frequency one has to change the transducer
Resonant Frequency The frequency at which the transducer is the most efficient as a transmitter of sound is also the frequency at which it is most sensitive as a receiver of sound. This frequency 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 piezoelectric crystal determines the resonant frequency of the transducer transducers crystals are normally manufactured so that their thickness (t) is equal to one-half of the wavelength ( ) of the ultrasound produced by the transducer Bandwidth The 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 Frequency bandwidth SPL
Continuous voltage waveform Pulsed voltage waveform Continuous waveform can be represented by a single sine wave (one frequency), thus frequency distribution is very narrow Pulsed waveform can be represented by the sum of many sine waves each of different frequency, thus frequency distribution is wide Frequency distribution of emitted ultrasound wave
Q-factor The Q-factor of a transducer system describes the shape of the frequency distribution (response curve) and is defined as where f 0 is the resonance frequency, f 1 is the frequency below resonance at which intensity is reduced by half and f 2 is the frequency above resonance at which intensity is reduced by half Q-factor = f 0 (f 2 - f 1 ) Bandwidth = (f 2 - f 1 ) high Q transducers produce relatively pure frequency spectrums and low Q transducers produce a wider range of frequencies short pulses correspond to reduce Q values and vice versa bandwidth Q-factor
Pulse Ultrasound Mode Because 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 transducer NOTE most transducers are designed to have short pulses (improved resolution) with low Q values (broad bandwidth - desirable in order to receive echoes of many different frequencies)
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 damping if damping is heavy the transducer has a short ring down time and is said to have a low Q value a transducers with lighter damping is said to have a high Q value
Pulse Repetition Frequency (PRF) Pulse Repetition Period (PRP) PRF is the number of pulses occurring in 1 second PRP is the time from the beginning of one pulse to the beginning of the next pulse
Spatial Pulse Length (SPL) SPL is the length of space over which a single pulse occurs, and is defined as SPL = n where n is the number of cycles in the pulse and is the wavelength. NOTE An important parameter when considering axial resolution
Pulse Duration (PD) PD is the time it takes for a single pulse to occur and is defined as PD = n T where 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, and is defined as: DF = PD PRP
Ultrasound Beam Characteristics In order to understand the beam characteristics of ultrasound we have to revisit our view of wave front (compression region) generation. A piezoelectric crystal surface actually behaves more like a series of vibrating points and not as the piston-like surface that we have implied previously. simplified model more accurate model the compression waves are not uniform (at least not close to the crystal surface) each vibrating point produces multiple concentric rings or waves that eventually form a continuous front as they reinforce each other along a line parallel to the surface of the crystal the 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
Fresnel Zone (Near Field) The length of the Fresnel zone is given by: d 2 4 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 in which resolution is poor beam intensity falls off because of attenuation
Fraunhofer Zone (Far Field) The angle of divergence of the Fraunhofer zone is given by: sin( = 1.22 d where d is the diameter of the transducer and is the wavelength. beam intensity falls of due to attenuation and beam divergence angle of divergence increases with decreasing transducer diameter and frequency no useful imaging can be made in this region
Side Lobes Side lobes are small beams of greatly reduced intensity that are emitted at angles to the primary beam and they often cause image artifacts. the origin of these lobes are due from radial vibrations from the edges of the transducer
Grating Lobes Grating lobes result when ultrasound energy is emitted far off-axis by multi-element arrays, 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 of highly, off-axis objects in the main beam
Axial (Linear, Range, Longitudinal, Depth) Resolution Axial resolution is the ability to separate two objects lying along the axis of the beam and is determined by the spatial pulse length (SPL). Case: SPL < X Objects a and b separate by distance X Objects resolvableObjects not resolvable Case: SPL = 2X limiting resolution = SPL = n where n is the number of cycles and 2 2 is the wavelength
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 resolution is approximately 0.5 mm axial resolution deteriorates with increasing pulse length, decreasing frequency and increasing wavelength the use of damping transducers (low Q) produces short pulses that improves axial resolution frequency axial resolution (improved)cycle/pulse axial resolution SPL axial resolution (worsened)
Lateral (Azimuthal) Resolution Lateral resolution is the ability to separate two adjacent objects and is determined by the width of the beam and line density. lateral resolution is equal to beam width in the scan plane lateral resolution is best in the Fresnel zone where ultrasound waves are parallel lateral resolution is generally a few millimeters Object separation wider than beam widthObject separation narrower than beam width beam width lateral resolution (improved)
Elevation Resolution (Slice Thickness) Elevation resolution is the dimensions of the ultrasound beam perpendicular to the image plane and depends on the transducer element height in much the same way that the lateral resolution is dependent on the transducer element width.
Focused Transducers High frequency beams have two advantages over low-frequency beams: (1) axial resolution is superior; and (2) the Fresnel zone is longer It would seem logical to use high frequencies for all imaging. High frequencies however, have a major drawback related to penetration. Tissue absorption increases with 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. NOTE focused transducers reduce beam width which improves lateral resolution they also concentrate beam intensity thereby increasing penetration and echo intensity thus improving image quality
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 zone the depth of focus is the distance over which the beam is in a reasonable focus a small diameter transducer has a shorter focal zone and spreads more rapidly in the far zone most diagnostic transducers are focused, which is achieved using a either a curved piezoelectric crystal, an acoustic lens or electronics (phased arrays)
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.
In reality we have: It is important to realize, that even for flat, unfocused transducer elements, there is some beam narrowing or “focusing”.