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The principle of ultrasound: Difference between revisions
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Many materials exist in nature that exhibit piezoelectric effect. Ccommercial transducers employ ceramics like barium titanate or lead zirconate titanate. The transducer usually consists of many PZT crystals that are arranged next to each other and are connected electronically. The frequency of the transducer depends on the thickness of these crystals, in medical imaging it ranges 2-8 MHz. An ultrasound pulse is created by applying alternative current to these crystals for a short time period. Afterwards, the system “listens” and generates voltage from the crystal vibrations that come from the returning ultrasound. An important part of the transducer is the backing material that is placed behind the PZT, it is designed to maximally shorten the time the PZT crystal vibrates after the current input is gone also known as ringing response. By decreasing the ringdown time, one decreases the pulse length and improves the axial resolution. In addition, the backing material decreases the amount of ultrasound energy that is directed backwards and laterally. | Many materials exist in nature that exhibit piezoelectric effect. Ccommercial transducers employ ceramics like barium titanate or lead zirconate titanate. The transducer usually consists of many PZT crystals that are arranged next to each other and are connected electronically. The frequency of the transducer depends on the thickness of these crystals, in medical imaging it ranges 2-8 MHz. An ultrasound pulse is created by applying alternative current to these crystals for a short time period. Afterwards, the system “listens” and generates voltage from the crystal vibrations that come from the returning ultrasound. An important part of the transducer is the backing material that is placed behind the PZT, it is designed to maximally shorten the time the PZT crystal vibrates after the current input is gone also known as ringing response. By decreasing the ringdown time, one decreases the pulse length and improves the axial resolution. In addition, the backing material decreases the amount of ultrasound energy that is directed backwards and laterally. | ||
[[File:PhysicsUltrasound_Fig23.svg|thumb|left|350px| Fig. 23]] | |||
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In front of the PZT, several matching layers are placed to decrease the difference in the impedance between the PZT and the patient’s skin. This increases in efficiency of ultrasound transfer and decrease the amount of energy that is reflected from the patient. | |||
Let us talk about the shape of the ultrasound beam. Since there are many PZT crystals that are connected electronically, the beam shape can be adjusted to optimize image resolution. The beam is cylindrical in shape as it exits the transducer, eventually it diverges and becomes more conical. The cylindrical (or proximal) part of the beam is referred to as near filed or Freznel zone. The image quality and resolution is best at the focal depth that can be determined by Focal depth = (Transducer Diameter)^2 x frequency /4. When the ultrasound beam diverges, it is called the far field. One would state that the best images are acquired using a large diameter transducer with high frequency. However, as we have learned, high frequency transducers have significant attenuation issues. In addition, larger diameter transducers are impractical to use because the imaging windows are small. The way around these problems is electronic focusing with either an acoustic lens or by arranging the PZT crystals in a concave shape. In clinical imaging, the ultrasound beam is electronically focused as well as it is steered. This became possible after phased array technology was invented. By applying electrical current in a differential manner and adjusting the timing of individual PZT excitation, the beam can travel in an arch producing a two-dimensional image. If one applies electricity in a differential manner from outside inward to the center of the transducer, differential focusing can be produced resulting in a dynamic transmit focusing process. | Let us talk about the shape of the ultrasound beam. Since there are many PZT crystals that are connected electronically, the beam shape can be adjusted to optimize image resolution. The beam is cylindrical in shape as it exits the transducer, eventually it diverges and becomes more conical. The cylindrical (or proximal) part of the beam is referred to as near filed or Freznel zone. The image quality and resolution is best at the focal depth that can be determined by Focal depth = (Transducer Diameter)^2 x frequency /4. When the ultrasound beam diverges, it is called the far field. One would state that the best images are acquired using a large diameter transducer with high frequency. However, as we have learned, high frequency transducers have significant attenuation issues. In addition, larger diameter transducers are impractical to use because the imaging windows are small. The way around these problems is electronic focusing with either an acoustic lens or by arranging the PZT crystals in a concave shape. In clinical imaging, the ultrasound beam is electronically focused as well as it is steered. This became possible after phased array technology was invented. By applying electrical current in a differential manner and adjusting the timing of individual PZT excitation, the beam can travel in an arch producing a two-dimensional image. If one applies electricity in a differential manner from outside inward to the center of the transducer, differential focusing can be produced resulting in a dynamic transmit focusing process. | ||