Single-element transducers properties Chapter 4 Single-element transducers properties

Key terms Bandwidth (DF) Duty factor (DF) Fractional bandwidth Matching layer Piezoelectric effect Pulse duration (PD) Pulse repetition frequency (PRE) Pulse repetition period (PRP) Mechanical coefficient (Q – value) Sensitivity Spatial pulse Length (SPL)

FREQUENCY DETERMINATION The first requirement of an ultrasonic scanner is that a sound wave at ultrasonic frequencies (greater than 20 KHZ) be generated.

Wavelength of sound sources the human ear can detect sound waves from approximately 20 HZ to 20 kHZ. For example in tissue:

Frequency Requirement If one wavelength is a good approximation of the smallest detectable object, what frequency is required for a resolution of 1 mm in tissue? For medical diagnostic ultrasound it is evident that megahertz frequencies are necessary.

Piezoelectric effect (pressure electric) This effect is commonly found in crystalline materials that have dipoles (regions of positive and negative charge) on each molecule. Dipolar molecules are positive at one end and negative at the other. To have maximum output the crystal should have thickness of nλ/2 Some piezoelectric materials are quartz, lead-zirconate-titanate (PZT) etc

Transducer components - crystal with coated electrodes on each side Backing material Matching layer Housing and insulator Ground electrode Connector

Pulse wave output Output pulse shape depends on type of transducer and excitation waveform

PULSE REPETITION FREQUENCY The number of times the crystal is pulsed or electrically stimulated per second is called the pulse repetition frequency (PRF).

PULSE REPEITION PERIOD The time required to transmit a pulsed ultrasound wave plus the time devoted to listening for the returning echoes from that wave is called pulse repetition period (PRP).

SPATIAL PULSE LENGTH The operating frequency is also called the center frequency, which represents the midpoint of the frequency distribution. The length of this short- duration pulse can be estimated and is called the spatial pulse length (SPL).

PD= n T T= the period of the wave PULSE DURATION the pulse duration (PD), or temporal pulse length is the time interval for one complete pulse. It describes the actual time that the transducer is generating the ultrasonic pulse. A more formal definition is the elapsed time from initiation of the pulse to the point 2odB below the maximum P-P pressure amplitude or 25% less than the P-P PD= n T T= the period of the wave

DUTY FACTOR The duty factor (DF) is the fraction of time the unit is active and is calculated by obtaining the ratio of the pulse duration and the pulse repetition period:

Transducer factors and function

SENSTIVITY Sensitivity describes the ability of an ultrasound system to distinguish low reflectivity objects with nearly the same acoustic properties at specific locations in the medium, All the transducer factors mentioned previously influence it.

Sensitivity factors Conversion efficiencies Circuit Impedance Backing material. Matching layers. Multiple matching layers.

Optimizing matching layer To have a best axial resolution (shortest pulse) with maximum energy transmission, the acoustic impedance of the matching layer should be: Zml=Zc1/3Zt2/3 Multiple matching layers For pulsed ultrasound with a band of frequency using a multiple matching layer can improve transmission to the tissue

Focusing and Q-value Focusing can be done to reduce diffraction and increase resolution Q–value assesses an essential characteristic of the pulsed ultrasound beam- pulse duration and bandwidth. The Q-Value can thus be thought of as having two separate definitions relating to the two affected beam characteristics.

Pulse Duration and Bandwidth A low–Q: transducer has a short pulse and a broad bandwidth. A high–Q: transducer has a long pulse length and a narrow bandwidth.

FWHM This process of determining the width of the frequency distribution at 50% maximum output is called full width at half maximum (FWHM).

Fractional Frequency=Δf/fc Fractional bandwidth PD and BW are inversely related: Δf=1/PD(µs) BW is often expressed as a fraction of center frequency Fractional Frequency=Δf/fc FB can be estimated from the number of cycle in the pulse Δf/fc=fc/nfc=1/n Matching layer effect on bandwidth: ML enhances the transmission of energy into the patient and shortens the pulse duration and hence increase BW (frequency distortion occurs)

Frequency shift in tissue As the beam transmitted though tissue, higher frequency absorb faster and hence shift in frequency BW occurs Imaging Characteristics of medical transducers