Ultrasound Transducers Ravi Managuli Last Week Ultrasound Wave Parameters Frequency, Wavelength, Period Intensity, Power Pulse length, Pulse duration, Axial resolution Ultrasound wave propagation Reflection, Scattering, Absorption Ultrasound interaction with the media Reflection, Transmission Refraction Spatial pulse length Spatial pulse duration System Beamformer Media (organ system) Transducer ADC Digital Delay Summer Signal/Image Processor Display Clinical Transducer : Many types Responsible for axial, lateral and elevation resolution Signal strength Quality of the reflected signal Types of transducers Curvilinear array Phased array Linear array 3D array Mechanical or fully sampled Transducer A device that converts one form of energy into another form of energy US transducers: Converts electrical energy into acoustic energy (sound waes) during transmission Coverts acoustic energy to electrical energy during reception Conversion is accomplished through the piezoelectric effect Ultrasound Transducers Major parts of a transducer Piezoelectric (PZT) elements – functional component of the transducer : Frequency Matching layer – reduces acoustic impedance b/w PZT & tissue Backing (damping) block – absorbs backwards directed (stray) U/S from housing Bandwidth (pulse length) c.f Bushberg, et al The Essential Physics of Medical Imaging, 2nd ed., p 484 Ultrasound Transducer Elevation direction (y) Lateral direction (x) Axial direction (z) Axial Resolution Cannot resolve : Reflections overlap Resolve Spatial pulse length Axial resolution: Half of spatial pulse length • Inversely proportion to the frequency • Axial resolution of 12 MHz probe is better than MHz Spatialpulselength l * N c ´ N AR = = = 2 2f Bandwidth Bandwidth is the useful range of contiguous frequencies over which transducer can operate Ultrasound transducer typically have multiple frequencies For example C5-1 transducer means : It has frequencies ranging from to MHz L12-5 means frequencies ranging from to 12 MHz Bandwidth Bandwidth : Inversely proportional to number of cycles present in a pulse (little dampening) Low BW (heavy dampening) High BW Bandwidth is related to spatial pulse length Large bandwidth Small pulse length Better axial resolution BW-mode Narrow bandwidth Long pulse length Worse axial resolution Color-Doppler Target Delays Electornic Focusing Multiple-focus? Apply different delays for different location Electronic Focusing Multiple-focus : Different delays Receive Focusing Similar to transmit focusing Target td n Time delay adjustment summation Receive Focusing Multiple Focus Target td n Time delay adjustment summation Update Steering For Different Lateral Resn Normal connecting the majority delays indicate the beam direction Ultrasound Image Quality Side lobes – off-axis energy emission directed away from the main beam Unavoidable; remapped along the main beam in receive mode Wide broad bandwidth– reduces emission of side lobe energy Grating lobes – energy emitted far off-axis (large angles) from the main beam by multi-element arrays Consequence of a non-continuous transducer surface Low amplitude; appears as highly reflective, off-axis objects in the main beam © UW and Renée Dickinson, MS Beamformer Two techniques are employed: Apodization Weight across channel to reduce side-lobe Grating lobes PZT sub-dicing Weights Transducer Types of Transducers Types of transducers and their focusing technique Curved/Curvilinear/convex array Linear Phased array Linear sequential array Linear phased array Annular phase array Vector array/Linear sequential phased array Linear Array Transducer High frequency Group of elements are fired in sequence Electronically focused Width of each element = wavelength Wide near filed of view Excellent superficial imaging 10,, Disadvantages Difficult to use in small acoustic windows Limited far field of view Rectangular field of view Convex Array Transducer Convex/Curved Array Transducer 96 to 128 elements Width of each element = wavelength Elements are arranged along arc Only few elements are used for steering Wide far field of view Wider near-filed than phased array Electronic focusing Disadvantages Difficult to use in small acoustic windows Degradation of resolution in the far field Blunted sector shape Ultrasound Transducer Microconvex or transviginal Array Transducer High frequency transducer Wide near and far field Phased Array Transducer Transducer 128 elements : All elements are fired simultaneously Width of each element ¼ to ½ of wavelength Electronic beam steering Advantages Small footprint for tight acoustic windows Rapid frame rates Wide far filed of view Disadvantages Complex and expensive to manufacture Very small near filed of view Fan/sector shaped Ultrasound Transducer Array Transducer for 3D imaging Mechanically steering 1D array transducer Voluson 730 (GE) / SonoAce 9900 (Medison) Realtime 3D (or 4D) About frames / sec Ultrasound Transducer Array Transducer for 3D imaging Fully 2D Array Transducer About 15 frames/sec, Limited FOV Cardiac application