Piezoelectric Effect  Sound waves striking a PZ material produce an electrical signal  Can be used to detect sound (and echoes)!

Piezoelectric Effect  Sound waves striking a PZ material produce an electrical signal  Can be used to detect sound (and echoes)!

Reverse Piezoelectric Effect  Applying an electrical signal causes the PZ element to vibrate  Produces a sound wave

Transducer  Define?  Many types of transducers exist –Pressure transducers –Air flow transducers, etc.  What is function of transducer? convert electrical signals to sound waves, and vice versa.

Ultrasound Transducer Materials  Quartz (naturally piezoelectric) –First used as a stable resonator in time measurement devices –Used in some laboratory ultrasound applications  Most current applications use piezoelectric ceramics (ie, lead zirconate titanate; barium titanate) –Lower “Q” (good for short pulses) –Good sensitivity –Many shapes are possible Miniature quartz tuning fork; 32,768 Hz.

Polarizing a Piezoelectric Element  Most ultrasound transducer materials are not ‘naturally’ piezoelectric –Lead zirconate titanate –Microscopic crystals, randomly oriented  Must be polarized –Heat to ~350 o C (Curie Temperature) –Apply strong voltage across crystal –Cool while voltage is still applied

Polarization

Single Element Transducers  Uses –Simple A-mode machines –Mechanical scanning transducers  The design serves as a useful example of general construction methods

Single element transducer construction

Ultrasound Transducers Piezoelectric (PZT) ceramic elements Matching layers, lens Backing layer

½ wavelength resonance  Resonance frequency corresponds to the thickness = ½ wavelength  Speed of sound in Piezoelectric material ~ 4,620 m/s  What thickness is required for a 3 MHz frequency transducer? d

½ wavelength resonance  Resonance frequency corresponds to the thickness = ½ wavelength  Speed of sound in Piezoelectric material ~ 4,620 m/s  What thickness is required for a 3 MHz frequency transducer? d

½ wavelength resonance  Resonance frequency corresponds to the thickness = ½ wavelength  Speed of sound in Piezoelectric material ~ 4,620 m/s  What thickness is required for a 5 MHz frequency transducer? d

Resonance Frequency

Backing (Damping) Layer  Need short duration pulses for decent axial resolution (we will discuss this later)  Backing layer helps to reduce vibrations of the element following excitation –Like placing your hand on a bell to stop the ringing!

Matching Layers  Thin layer of material –¼ wavelength thick –Impedance is between that of the element (quite high) and that of tissue  Provides better sound transmission from the transducer-patient-transducer  Improves sensitivity

Focusing, Methods  Focusing reduces the beam width in the focal zone  Methods –Lens –Curved element –Electronic

2.5 MHz 20 mm 5.0 MHz In Most Applications, Beams Are Focused - curved element - lens - electronic (arrays) Improves lateral resolution near the focal distance Higher frequencies produce narrower beams Dr.Awad Elkhadir

2.5 MHz 10 mm 20 mm 5.0 MHz - Previous diagrams exhibit sidelobes - Must be eliminated for good image quality - Pulsing reduces (or even eliminates) side lobes 5.0 MHz CW Short pulse (50% bw)

Array Transducer  “Scanhead” containing many small PZT elements  Element, along with a transmit-receive circuit in the machine is a channel.  128 channels are common.

Beam Forming (Transmit) Group also permits electronic beam steering and electronic focusing.

Curvilinear

Phased Array

Linear-Phased ( “Virtual Convex”)  Linear array –Rectangular FOV, defined by transducer footprint  VC adds beam steering to expand imaged region at edges

Annular