GENERATING AND DETECTING OF ULTRASOUND

The Piezoelectric Effect

*Certain substances change their dimensions when an electric field is applied across them (electrical E to mechanical E)

*they develop electrical charges on their surfaces when are subjected to a mechanical force (mechanical E. to electrical E)

The Piezoelectric Effect

Mechanical E. to Electrical E .1 -When a pressure is applied on both

sides of the crystal, an electrical voltage develops between the two

opposite faces .2 -when the pressure is reversed in

direction, the voltage is also reversed in polarity .

The voltage developed is directly proportional to the pressure

The Piezoelectric Effect

Electrical E. to Mechanical E .1 -If a voltage from an external

source is connected across the crystal, it causes the thickness of the crystal to change (crystal deformed or strained)

2 -When the voltage is reversed in polarity the strain is also reversed m

polarity .The magnitude of the strain is directly proportional to the voltage applied

Generate of Ultrasound

*If the applied alternating voltage has a frequency 1 MHz, then the crystal vibrates at 1 MHz, and ultrasound of frequency 1 MHz is generated

Detection of Ultrasound*If such a crystal is placed in an ultrasound wave,

an alternating voltage of the same frequency is created across the crystal disc .

*This alternating voltage can be recorded and the ultrasound wave can be detected.

Piezoelectric Materials

*naturally occurring crystalline materials: Single crystals of quarts, Rochelle salts, and Tourmaline (most widely used)

*certain artificial materials can be also used:Barium Titanate, Lead Zirconate Titanate (PZT), and Lead Titanate.

Transducer Design

The two opposite faces of the crystal are connected with two silver electrodes for the electrical connections. The tube that contains the crystal is metallicReason (S):

-to ensure that no external electromagnetic fields induce interference voltages in the crystal or its connecting wires.

-it minimizes the emission of such radiation from the crystal and the wires to the surrounding medium.

Transducer Design

The front face of the crystal is coated with a thickness of a polymeric or plastic substance. Reason (S):

1 .To mechanically protect the crystal from scratching and impact. Scratching could remove the electrode layer and impact could damage the crystal.

2.To provide acoustic coupling between the crystal and the body tissue into which ultrasound is to be transmitted

Good Matching Condition *if the crystal was placed directly on the skin,

Strong reflection prevents good coupling (matching) .

Reason (S):This is because the acoustic impedance of the transducer crystal material is much greater than that of the skin. For optimum coupling between the two acoustic impences of the crystal material and the skin

Z = √ Zc x Zs

Resonance FrequencyWhen the crystal is excited by an electric pulse, it will change shape and vibrate

The number of vibrations per unit time determines the frequency of the crystal

For medical applications, the most important factor in determining the frequency of the transducer is the thickness of

the crystal .the transducer crystal to emit more than one single frequency Reason :

Due to imperfections in the manufacturing process of the crystal, effects from the backing material, and effects from the exciting electric pulse itself

Resonance FrequencyA transducer that emits many frequencies on either side of the main

frequency has a wide-bandwidth ,

while a transducer that emits only a few frequencies on either side of the main frequency is called a narrow-

bandwidth transducer .

Resonance frequency, which produces greatest intensity, is determined by the thickness of the crystal f = c / 2d

c is the speed of ultrasound in the crystal material

Ultrasound Beam

When a transducer crystal is caused to vibrate by an appropriate electrical voltage, it creates an ultrasound wave disturbance in the medium (ultrasound beam)

Ultrasound Beam-When a transducer crystal is caused to vibrate by an

appropriate electrical voltage, it creates an ultrasound wave disturbance in the medium (ultrasound beam)

-The beam remains cylindrical along the direction of the beam axis .

-After some distance, however, the beam starts to diverge

-near fields or Fresnel: is the area Before the point of divergence of the beam

-far field, or Fraunhofer Zone : is the diverging part of the beam

Factors affecting the length of near field Both the length of near field and the angle of divergence of far field are dependent on:

1 -ultrasound frequency2 -transducer crystal radius (r)

Zn.f. α (r2 / λ)Sin () = (0.61 λ / r)

*For medical applications of ultrasound beams with long Fresnel Zone are preferred (high (r/ λ) ratio, high frequency)

*For medical applications of ultrasound beams with long Fresnel Zone are preferred (high (r/ λ) ratio, high frequency)

Focused ultrasound beamTo have narrow the ultrasound beam width in the near field. the wavefronts of the ultrasound waves into a concave shape so that they converge to a focus

Focused ultrasound beam*One method of focusing is

to form the transducer into part of the surface of a sphere

*the degree of focusingdepends on:

Radius of curvature of theTransducer

Focused ultrasound beam

*increasing the strength of focusing by reducing the radius of curvature of the transducer will give a narrower beam at the focus but the useful length of narrowed beam (length of focal zone) will decrease

*Focusing: decreases the width of the ultrasound beam and increases the intensity of the beam at the focus as a result of concentrating the power of the beam within a smaller area

Different methods of focusing