Ultrasound Transducer Properties Lecture 3, RAD 354, King Saud University PDF

RAD 354 Lecture 3 Properties of ultrasound transducer Overview 1. Transducer definition 2. Components of a transducer 3. Spatial resolution Axial resolution (along the axis) Lateral resolution (perpendicular)  Transducers ⚫In general, transduc...

RAD 354 Lecture 3 Properties of ultrasound transducer Overview 1. Transducer definition 2. Components of a transducer 3. Spatial resolution Axial resolution (along the axis) Lateral resolution (perpendicular)  Transducers ⚫In general, transducers are devices that convert signals or energy from one form to another. Ultrasound transducers: Convert electrical signals to acoustic signals and convert acoustic signals back into electrical signals, acting both as a transmitter and receiver of ultrasound waves.  Transducer Basic components of a transducer: § Piezoelectric elements § Electrical connections § Backing material § Acoustic lens § Matching layer § Physical housing assembly What are the important properties of an U/S Transducer system? Construction of transducer 1) Piezoelectric plates or crystals ⚫ The primary component of the transducer is the Piezoelectric material (crystals). These crystals convert electrical energy into mechanical energy and vice versa. ⚫ They expand or contract when a positive or negative voltage is applied across them and conversely, generate positive or negative voltage when compressed or stretched by external force. ⚫ Some piezoelectric materials occur naturally such as Quartz. However, the piezoelectric material used in medical ultrasound are synthetic ceramic materials: Lead zirconate titanate (PZT).  PZT ⚫A mixture of piezoelectric ceramic and other material (ex.. epoxy) are now being used in transducers. ⚫Advantages: ⚫ Lower acoustic impedanceà easier to match tissue impedanceà effective transmission of produced waves into tissue ⚫ The same transducer can operate at various frequencies (Wide frequency bandwidths) ⚫ More sensitive  PZT ⚫Quartz has been replaced by PZT due to several reasons: 1. More efficient 2.Better sensitivity 3. Easily shaped ⚫To act as transmitters and receivers, PZT must first be polarized. This is done by heating the material just above the Curie temperature( 365° C) allowing them to move. ⚫High voltage is then applied across the element to produce polarization of the crystals. The element is then cooled still applying voltage to retain polarization. ⚫The PZT element can loose its polarization if heated above the Curie temp.  PZT ⚫ The PZT plate is coated on both sides with conductive paint, forming electrodes to which electrical leads are glued. The required voltage is applied across the plate via the leads making it expand and contract at this frequency. ⚫ The pressure vibrations from the returning echoes cause the plate to contract and expand, generating voltage variations across the plate. These voltage variations form the electrical signal.  Resonance frequency ⚫ The transducer has a resonance frequency at which it functions efficiently in converting U/S to electricity and vice versa ⚫ Resonance freq. is determined by piezoelectric element ⚫ It occurs when the crystal thickness equals half a wave length ⚫Wave length and frequency are inversely related thus: Thin element àSmall wave lengthà high resonance frequencies ⚫The thickness of the PZT plate determines (or govern) the frequency of the transducer. ⚫ Crystal Thickness=λ/2 ⚫Thin element àSmall wave lengthà high resonance frequencies ⚫ A transducer operating at a resonance frequency of 2 MHz would have a thickness of 1mm and a transducer operating at 7.5 MHz would typically have a thickness of 0.3 mm. ⚫Size and shape of Piezoelectic element will govern the size and shape of ultrasound beam 2)Matching layer: ØIn general, If the PZT where to be placed directly in contact with the patient. only 20% of the waves power would be transmitted through the front of the PZT-patient interface and most of the waves would be reflected (80% reflected) due to the acoustic impedance mismatch. So, Why Matching layer required? ØTo overcome this problem, a matching layer is bonded to the front of the PZT crystal. Properties of a Matching layer Ø A single matching layer can improve transmission up to 100% provided that two conditions are met: a) The thickness of the matching layer should be quarter (¼) of the wave length. b) It should have an impedance equal to geometric mean of the √(ZPZT. ZT), where Z PZT is the acoustic impedance of the PZT crystal and Z T is the acoustic impedance of the tissue.  3) Backing layer: §The vibrations come mainly from the front and back of the PZT plate, as we are only interested in the vibration that come from the front of the PZT plate, we should try to eliminate the vibrations from the back of the PZT plate and also to control the length of the vibration coming from the front. §This is done by a backing layer located behind the PZT plate, which damps the vibration of the transducer after each excitation. Usually consists of tungsten powder and plastic. §It stops (reduce)the ringing of the transducer after excitation to give a shorter pulse. Backing layer: §The backing material should have two properties: 1) Its acoustic impedance should be comparable to the PZT. 2)It must absorb the sound waves transmitted into it. § Once in the backing layer, the sound waves are absorbed and converted into heat. This somehow reduces the sensitivity of the PZT crystal. §For most applications, pulses should be kept as short aspossible to optimise the axial resolution ⚫Advantages: Reduction of transducer backing reflection; Decrease pulse duration, shorter pulse duration, higher frequency performance ⚫Downside: decrease acoustic signal strength 4) Acoustic lens: The lens is usually placed in front of the matching layer, its purpose is to improve the resolution by reducing the beam width of the transducer (i.e. Focus the beam). The width of the beam determines the lateral resolution.  SO! What are important properties of an U/S system? Thickness of PZT /crystals (Thus controlling freq) Matching layer (better transmission of ……) Backing layer (Thus better…… Acoustic lens (Thus, better…… resolution) Why is a short pulse desirable in U/S imaging? Bandwidth Band width is defined as the range of frequencies used in one transducer Shorter pulse length = wider frequency band width Broad bandwidth technology produces medical transducers that contain more than one operating frequency , for example: 2.5 – 3.5 MHz for general abdominal imaging 5.0 – 7.5 MHz for superficial imaging Transducers and spatial resolution  Spatial resolution: The ability to identify two structures as separate when they are closely positioned on an image. When the structures are displayed as separate elements, we say they are resolved It is closely related to the wave length and frequency of the transducer. Ex. The wave length of a 5 MHz beam is approx. 0.3 mm, so it wouldn`t be able to resolve objects less than 0.3 mm apart. Lateral Spatial resolution is divided into two components: 1) Axial resolution Axial 2) Lateral resolution  1) Axial resolution: The capability of the ultrasound system to represent separately objects which are arranged along the axis of the beam  Axial resolution is determined by the pulse duration, the shorter the pulse duration the better the axial resolution. The best resolution that can be achieved is half the pulse length. Ex. If the pulse length is 1 mm, then structures < 0.5 mm will not be resolved. The pulse duration is the time required for the transducer “ringing” to decrease to a negligible level after an excitation. It is equal to the number of cycles in the pulse multiplied by the wave period: PD = Nc χ T Where: Nc is the number of cycles and T is the period  2) Lateral resolution: It is the ability to distinguish two structures situated at the same depth perpendicular to the ultrasound beam Lateral resolution is closely related to beam width. Elevation resolution (slice thickness) Determines how close 2 objects are along a line perpendicular to the imaging plane ( thickness of the section of tissue that contribute to the image) Depends on the beam size in the plane perpendicular to the imaging plane  Temporal resolution It also known as frame rate. The ability of the system to display events occurring at different times as separate images. OR: the ability to distinguish very rabid events in sequence as separate once. High temporal resolution is important when imaging rapidly moving structures such as the heart  Constructive and destructive waves Constructive waves: Are waves occur at the same time and are in the same phase; therefore, the waves reinforce each other and the sound becomes louder Destructive waves : Are waves of equal frequency and opposite phase, resulting in their cancellation where the negative displacement of one always coincides with the positive displacement of the other.  Wave interference in ultrasound - There are two types of interference: Ø Constructive interference. Ø Destructive interference Focusing Focusing narrows the beam and locally improves lateral resolution Summary Transducer definition 4. Acoustic lens Components of a transducer Improve the resolution 1. Piezoelectric elements Spatial resolution Lead zirconate titanate (PZT) Definition Resonance frequency Axial resolution (along the axis) Thick element low R freq Lateral resolution (perpendicular) Thin element high R freq beam width 2. Backing layer Beam Characteristics (NF, FZ, FF) Stop reverberating Focusing 3. Matching layer Improve transmission Questions