Ultrasound – Physics & Advances

Modes of Vibration 2 TYPES: thickness mode most common Used in medical crystals radial mode

Resonance Transducers Non Resonance Transducers Resonant Frequency Natural frequency to which the transducer is sensitive Resonant frequency determined by thickness of crystal Thick crystal – low frequency sound Natural frequency – one that produces internal wavelengths that are twice the thickness of crystal Frequency corresponding to half the wavelength is- fundamental resonant frequency Transducers Resonance Transducers Non Resonance Transducers

Resonance Transducers They are manufactured to operate in a “resonance” mode, whereby a voItage (commonly 150 V) of very short duration (a voltage spike of 1 msec) is applied, causing the piezoelectric material to initially contract, and subsequently vibrate at a natural resonance frequency. The operating frequency is determined from the speed of sound in and the thickness of, the piezoelectric material.

Non-resonance (Broad-Bandwidth) “Multi-frequency” Transducers Modern transducer design coupled with digital signal processing enables “multifrequency or “multihertz” transducer operation, whereby the center frequency can be adjusted in the transmit mode. 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.

Unlike the resonance transducer design, the piezoelectric element is intricately machined into a large number of small “rods,” and then filled with an epoxy resin to create a smooth surface.

Characteristics of Ultrasound Beam A single vibrating point sets out waves in all directions Waves move away as concentric circles

When two sound waves interact , they cancel each other or reinforce each other

Beam Properties The ultrasound beam propagates as a longitudinal wave from the transducer surface into the propagation medium, and exhibits two distinct beam patterns: a slightly converging beam out to a distance specified by the geometry and frequency of the transducer (the near field), and a diverging beam beyond that point (the far field).

The Near Field The near field, also known as the Fresnel zone, is adjacent to the transducer face and has a converging beam profile. Beam convergence in the near field occurs because of multiple constructive and destructive interference patterns of the ultrasound waves from the transducer surface.

Near Field Length………. In single-element transducer The near field length for an unfocused, single-element transducer is dependant on transducer frequency and diameter For multiple transducer element, an “effective” transducer diameter is determined by the excitation of a group of’ transducer elements. Because of the interactions of each of the individual beams and the ability to focus and steer the overall beam, the formulas for a single-element, unfocused transducer are not directly applicable.

Lateral resolution (the ability of the system to resolve objects in a direction perpendicular to the beam direction) is dependent on the beam diameter and is best at the end of the near field for a single- element transducer. Lateral resolution is worst in areas close to and far from the transducer surface.

Pressure amplitude characteristics in the near field are very complex, caused by the constructive and destructive interference wave patterns of the ultrasound beam. 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. Only when the far field is reached do the ultrasound pressure variations decrease continuously.

Far Field The far field is also known as the Fraunhofer zone, and is where the beam diverges. Less beam divergence occurs with: High - frequency transducers Large - diameter transducers

Unlike the near field, where beam intensity varies from maximum to minimum to maximum in a converging beam, ultrasound intensity in the far field decreases monotonically with distance.

Solution: Focused Transducer High frequency beams – fresnel zone is longer depth resolution is superior Disadvantage : Tissue absorption is more, leading to deterioration of side to side resolution Solution: Focused Transducer

Factors influencing beam shape The shape of the ultrasound beam is affected by: the size and shape of the ultrasound source ,frequency & beam focusing: The size of the ultrasound: the effects of source size on beam shape are: a small source provides a narrow beam initially, is associated with a short Fresnel zone, and the beam diverges rapidly beyond the near field. (ii) a large source provides a broader beam initially, gives a longer Fresnel zone, and the beam diverges more gradually, thus providing better resolution of deeper structures.

Effect of beam frequency: the length of the Fresnel zone increases as the beam frequency is increased. Also, the angle of divergence beyond the near field diminishes with increasing frequency leading to increase resolution . In practice, however, some of this advantage is taken away by increased beam attenuation at higher frequencies

Focused Transducers Focusing of the ultrasound beam: The shape of the ultrasound beam can be influenced to varying extents by applying different focusing methods. Shape of the crystal element: The crystal element can be suitably shaped by concave curvature to focus the ultrasound beam. This is an internal focusing method, Acoustic lenses made from materials which propagate ultrasound at velocities different from that in soft tissue can be used to focus the beam by refraction. Acoustic mirrors A concave mirror can be used to focus ultrasound by reflection Electronic focusing: Electronic focusing is employed in multicrystal transducers N.B: the degree of focusing will depend on the radius of curvature. Acoustic lenses and mirrors provide external focusing.

Focal Distance The focal distance, the length from the transducer to the narrowest beam width, is shorter than the focal length of a non-focused transducer and is fixed.

Focal Zone The focal zone is defined as the region over which the width of the beam is less than two times the width at the focal distance; Thus, the transducer frequency and dimensions should be chosen to match the depth requirements of the clinical situation.

Classification of focusing The degree of focusing may be classified into three categories as follows: strong focusing (or short focusing) medium focusing weak focusing (or long focusing) In all cases, fixed focusing gives a focal point which is nearer to the transducer than the transition distance (length of the Fresnel zone). Strong focusing brings the focal point very close to the transducer, typically 2 - 4 cm. It achieves a high degree of beam narrowing, but the beam diverges rapidly beyond the focal distance. It can only be applied to transducers for high resolution examinations of small parts. Weak focusing gives a focal point further away from the transducer - typically more than 8 cm - and a gentle divergence of the beam beyond the focus. It is preferred in diagnostic applications because it provides an extended useful, narrow beam.