Lecture 2
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Oct 11, 2024 02:27 PM
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OutlineUS Instrumentation Transmitter circuitParameters:Transducers (single element transducer)Piezo-electric effectMatch layer US BeamsFlat element (unfocused)Curved element (focused)Tips
US Instrumentation
Transmitter circuit
Voltage waveform, 1 V(small), then amplified to 50 - 100 V
has 1-3-5 cycles
Parameters:
- Single cycle duration
- Central frequency
- Pulse duration in 𝜇𝑠. This is equal to the number of cycles times the duration of a single
- cycle.
- Pulse repetition rate () in 𝜇𝑠. This is the time in between transmissions.
- Pulse repetition frequency (), often measured in kHz. This is the inverse of the PRR.
- Do not confuse the with the frequency of the US wave (MHz).
- Duty Cycle = (dimensionless) （占空比）
- The peak-peak voltage 𝑽𝒑𝒑
- Tips. Axial resolution = , 轴向分辨率，c是在组织中的速度
Transducers (single element transducer)
Piezo-electric effect
When an oscillating voltage is applied to one face of the piezoelectric element, its thickness oscillates at the same frequency as the applied voltage, with the change in thickness being proportional to the magnitude and polarity of this voltage.
振荡电压→机械波
Most Common:
✓ PZT (lead zirconate titanate, ferroelectric mixed-metal oxide ceramic)
✓Single Crystal
A PZT crystal has a natural (resonant) frequency (after being poled into an electric field). This frequency is related to the thickness of the crystal:
Tips. 同一束超声在不同组织中的波长是变化的，, 应该使用得到的频率和对应的速度去计算特定组织中的波长
Match layer
这里的Z是第一章的那个，
, acoustic characteristic impedance, 声学特性阻抗
The larger the difference in the characteristic acoustic impedance of two materials, the greater the intensity of the reflected wave from the boundary between the two.
两个界面Z差距过大时能量损失严重，为了减少能量损失，使用两个界面的中间值，改善传输
US Beams
Flat element (unfocused)
两个重要公式，计算近场距离，以及特定远场距离的横向分辨率（Lateral resolution）
这里的横向分辨率数值越低，分辨效果越好（距离远的物体分辨率差）
注意这里是，用半径的话是0.61/r
We’d like resolutions around 1 mm to image human body. This will be clear later on when we discuss image formation.
Curved element (focused)
The linear dimension of the curved element d
The focal length F: this is the radius of curvature of the element itself. The focal length is also known as the focus(焦距).
The (best) lateral resolution is equal to the beam-width at the focus Lateral Resolution LR = 𝜆*F/d
用于衡量的f-number：f#=F/d
Tips
- In the previous discussion, we have implicitly assumed that the dimension of a flat or curved transducer is much bigger than the US wavelength in tissue, in other words: 𝑑 ≫ 𝜆. 换能器大小远大于超声波长
- In this case, the beam pattern is directional, either unfocused or focused. 只会是聚焦或非聚焦型
- An unfocused transducer has a large lateral resolution (cm), whereas a focused transducer has small lateral resolution (mm). However, in both cases, the footprint, the physical dimensions of the transducer, is large (many cm). 非聚焦型分辨率大，cm级，聚焦型分辨率精细，mm级
- How do we achieve similar/better lateral resolution and at the same time reduce the dimension of the transducer? Arrays 没懂