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The principle of ultrasound: Difference between revisions
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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. | 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. | ||
[[File:PhysicsUltrasound_Fig24.svg|thumb|left|400px| Fig. 24]] | |||
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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. | |||
Briefly, I would like to touch upon '''real time 3D imaging'''. In order to accomplish this, the PZT elements need to be arranged in a 2D matrix. Each PZT element represents a scan line, by combining all the data, a 3D set is reconstructed. For example, if we have a matrix of 128 by 128 PZT elements, one can generate over 16 thousand scan lines. With careful timing for individual excitation, a pyramidal volumetric data set is created. When imaged several times per minute (>20), a real time image is achieved. | Briefly, I would like to touch upon '''real time 3D imaging'''. In order to accomplish this, the PZT elements need to be arranged in a 2D matrix. Each PZT element represents a scan line, by combining all the data, a 3D set is reconstructed. For example, if we have a matrix of 128 by 128 PZT elements, one can generate over 16 thousand scan lines. With careful timing for individual excitation, a pyramidal volumetric data set is created. When imaged several times per minute (>20), a real time image is achieved. | ||