Crystal Thickness. thicker crystal lower frequency. thinner crystal higher frequency crystal thickness = ½ λ for the frequency

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1 TRANSDUCERS

2 Piezoelectric materials are characterized by a well-defined molecular arrangement of electrical dipoles. Ultrasound transducers for medical imaging applications employ a synthetic piezoelectric ceramic, most often leadzirconate- titanate (PZT). Resonance transducers for pulse echo ultrasound imaging are manufactured to operate in a "resonance" mode, whereby a voltage (commonly 150 V) of very short duration (a voltage spike of ~1 f.lsec) is applied. The operating frequency is determined from the speed of sound in the medium and the thickness of, the piezoelectric material.

3 Crystal Thickness thicker crystal lower frequency thinner crystal higher frequency crystal thickness = ½ λ for the frequency Typical diagnostic pulsed ultrasound elements are.2 1 mm thick

4 Damping Block The damping block, layered on the back of the piezoelectric element, absorbs the backward directed ultrasound energy and attenuates stray ultrasound signals from the housing. Also dampens the transducer vibration to create an ultrasound pulse with a short spatial pulse length, which is necessary to preserve detail along the beam axis (axial resolution). Limits the crystal from ringing & absorbs any energy emitted in a backwards direction Limiting the amount of ringing of the crystal, increases the transducer s bandwidth Imaging transducers have wide bandwidth

5 Quality Factor (Q Factor or Mechanical Coefficient) High Quality Factor: Crystal rings for a long time (CW transducers), bandwidth is narrow & poor axial resolution Low Quality Factor: Crystal rings for a very short time (PW transducers), bandwidth is broad & good axial resolution We use low Q-factor with a value of 2 to 3 Q factor = Resonating Frequency (MHz) Bandwidth (MHz)

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7 Matching Layer (facing material) Thin layer of aluminum powder in epoxy resin in front (facing) of the crystal Decreases the impedance difference between the crystal & the skin It consists of layers of materials with acoustic impedances that are intermediate to those of soft tissue and the transducer material. Thickness of each layer = one fourth of the wavelength

8 Multifrequency Transducers or Multi Hertz transducers Excitation of the multifrequency transducer is accomplished with a short square wave burst of ~ 150 V with one to three cycles, unlike the voltage spike used for resonance transducers.

9 Transducer Arrays The majority of ultrasound systems employ transducers with many individual rectangular piezoelectric elements arranged in linear or curvilinear arrays. 128 to 512 individual rectangular elements compose the transducer assembly. Each element has a width typically less than half the wavelength and a length of several millimeters. Two modes of activation are used to produce a beam. These are the "linear" (sequential) and "phased" activation/receive modes.

10 Linear Array Linear array transducers typically contain 256 to 512 elements; Physically these are the largest transducer assemblies. For a curvilinear array, a trapezoidal field of view is produced. Phased Array A phased array transducer is usually composed of 64 to 128 individual elements in a smaller package than a linear array transducer. All transducer elements are activated nearly simultaneously to produce a single ultrasound beam. During ultrasound signal reception, all of the transducer elements detect the returning echoes from the beam path.

11 Beam Properties The near field, also known as the Fresnel zone, is adjacent to the transducer face and has a converging beam profile. The far field is also known as the Fraunhofer zone, and is where the beam diverges. Less beam divergence occurs with high frequency, large diameter transducers. A higher transducer frequency (shorter wavelength) & larger diameter element will result in a longer near field. For a l0cm diameter transducer, the near field extends 5.7 cm at 3.5 MHz and 16.2 cm at 10 MHz in soft tissue. For a 15 mm diameter transducer, the corresponding near field lengths are 12.8and 36.4 cm, respectively.

12 Characteristics of near & far field Lateral resolution is dependent on the beam diameter and is best at the end of the near field for a single element transducer. Pressure amplitude pattern is complex due to constructive and destructive interference. Peak ultrasound pressure occurs at the end of the near field, corresponding to the minimum beam diameter for a single element transducer. Pressures vary rapidly from peak compression to peak rarefaction several times during transit through the near field. Far Field pressure amplitude variation is less. Ultrasound intensity in the far field decreases with distance.

13 Focused Transducers Single element transducers are focused by using a curved piezoelectric element or a curved acoustic lens to reduce the beam profile. Single transducer or group of simultaneously fired elements in a linear array, the focal distance is a function of the transducer diameter. Phased array transducers and many linear array transducers specific timing delays between transducer elements that cause the beam to converge at a specified distance.

14 Receive focus The receive focus timing must be continuously adjusted to compensate for differences in arrival time across the array as a function of time (depth of the echo). Depicted are an early time of proximal echo arrival, and a later time of distal echo arrival. To achieve phase alignment of the echo responses by all elements, variable timing is implemented as a function of element position after the transmit pulse in the beam former. The output of all phase aligned echoes is summed.

15 Axial Resolution Spatial Resolution Axial resolution (also known as linear, range, longitudinal, or depth resolution) refers to the ability to discern two closely spaced objects in the direction of the beam. Achieving good axial resolution requires that the returning echoes be distinct without overlap. The SPL is the number of cycles emitted per pulse by the transducer multiplied by the wavelength.

16 Lateral Resolution Lateral resolution, also known as azimuthal resolution, refers to the ability to discern as separate two closely spaced objects perpendicular to the beam direction. For both single element transducers and multi element array transducers, the beam diameter determines the lateral resolution Elevational Resolution The elevational or slice thickness dimension of the ultrasound beam is perpendicular to the image plane. Elevational resolution is dependent on the transducer element height in muchthesamewaythatthelateral resolution is dependent on the transducer element width

17 Image Data Acquisition

18 Pulser The pulser (also known as the transmitter) provides the electrical voltage for exciting the piezoelectric transducer elements, and controls the output transmit power by adjustment of the applied voltage. Transmit/Receive Switch The transmit/receive switch, synchronized with the pulser, isolates the high voltage used for pulsing (~150 V) from the sensitive amplification stages during receive mode, with induced voltages ranging from 1 V to 2 V from the returning echoes. C the speed of sound, is expressed in cm/sec; D distance from the transducer to the reflector is expressed in cm; the constant 2 represents the round trip distance;

19 Beam Former The beam former is responsible for generating the electronic delays for individual transducer elements in an array to achieve transmit and receive focusing and, in phased arrays, beam steering. It controls: transmit/receive switches, digital to analog and analog to digital converters, and preamplification and time gain compensation circuitry for each of the transducer elements in the array Other Terminologies Pulse Repetition Period, Pulse Duration

20 Preamplification and A/D Conversion An initial preamplification increases the detected voltages to useful signal levels. This is combined with a fixed swept gain to compensate for the exponential attenuation occurring with distance travelled. ADC A typical sampling rate of 20 to 40 MHz with 8 to 12 bits of precision is used. Echo reception includes electronic delays to adjust for beam direction and Dynamic receive focusing to align the phases of detected echoes from the individual elements in the array as a function of echo depth. Following phase alignment, the pre processed signals from all of the active transducer elements are summed. The output signal represents the acoustic information gathered during the pulse repetition period along a single beam direction. This information is sent to the receiver for further processing before rendering into a 2D image

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