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The principle of ultrasound: Difference between revisions
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Lateral resolution is the minimum distance that can be imaged between two objects that are located side to side or perpendicular to the beam axis. Again, the smaller the number the more accurate is the image. Since the beam diameter varies with depth, the lateral resolution will vary with depth as well. The lateral resolution is best at the beam focus (near zone length) as will discuss later when will talk about the transducers. Lateral resolution is usually worse than axial resolution because the pulse length is usually smaller compared to the pulse width. Temporal resolution implies how fast the frame rate is. FR = 77000/(# cycles/sector x depth). Thus frame rate is limited by the frequency of ultrasound and the imaging depth. The larger the depth, the slower the FR is and worse temporal resolution. The higher the frequency is, the higher is the FR and the temporal resolution improves. Sonographer can do several things to improve the temporal resolution: images at shallow depth, decrease the #cycles by using multifocusing, decrease the sector size, lower the line density. However one can realize quickly that some of these manipulations will degrade image quality. And this is in fact correct: improving temporal resolution often degrades image quality. M-mode is still the highest temporal resolution modality within ultrasound imaging to date. | Lateral resolution is the minimum distance that can be imaged between two objects that are located side to side or perpendicular to the beam axis. Again, the smaller the number the more accurate is the image. Since the beam diameter varies with depth, the lateral resolution will vary with depth as well. The lateral resolution is best at the beam focus (near zone length) as will discuss later when will talk about the transducers. Lateral resolution is usually worse than axial resolution because the pulse length is usually smaller compared to the pulse width. | ||
[[File:PhysicsUltrasound_Fig21.svg|thumb|left|300px| Fig. 21]] | |||
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Temporal resolution implies how fast the frame rate is. FR = 77000/(# cycles/sector x depth). Thus frame rate is limited by the frequency of ultrasound and the imaging depth. The larger the depth, the slower the FR is and worse temporal resolution. The higher the frequency is, the higher is the FR and the temporal resolution improves. Sonographer can do several things to improve the temporal resolution: images at shallow depth, decrease the #cycles by using multifocusing, decrease the sector size, lower the line density. However one can realize quickly that some of these manipulations will degrade image quality. And this is in fact correct: improving temporal resolution often degrades image quality. M-mode is still the highest temporal resolution modality within ultrasound imaging to date. | |||
Before we talk about '''Doppler Effect''', let us discuss the ultrasound transducer architecture and function. The current transducers became available after the discovery that some materials can change shape very quickly or vibrate with the application of direct current. As important is the fact that these materials can in turn produce electricity as they change shape from an external energy input (i.e., from the reflected ultrasound beam). This effect of vibration form an application of alternative current is called a piezoelectric effect (PZT). 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. 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. | Before we talk about '''Doppler Effect''', let us discuss the ultrasound transducer architecture and function. The current transducers became available after the discovery that some materials can change shape very quickly or vibrate with the application of direct current. As important is the fact that these materials can in turn produce electricity as they change shape from an external energy input (i.e., from the reflected ultrasound beam). This effect of vibration form an application of alternative current is called a piezoelectric effect (PZT). 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. 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. | ||