Lecture 19 Ultrasound Imaging
Contents 1. Introduction 2. Ultrasound and its generation 3. Wave propagation in the matter 4. Data acquisition (A, B, M and Doppler model) 5. Imaging reconstruction (5 steps) 6. Equipment & clinic use
1. Introduction n For more than half a century. n Noninvasive, relatively inexpensive, portable, and has an excellent temporal resolution. n Beyond the medical imaging, crack detection, fish, seabed, submarine, gas field.
The basic principle of ultrasound imaging is simple. A propagating wave partially reflects at the interface between different tissues. If these reflections are measured as a function of time, information is obtained on the position of the tissue if the velocity of the wave in the medium is known.
The principle of velocity imaging was originally based on the Doppler effect and is therefore often referred to as Doppler imaging. The sudden pitch change of a whistling train when passing a static observer
n World War I in the quest for detecting submarines. n The Theory of Sound by Lord Rayleigh in 1877 and the discovery of the piezoelectric effect by Pierre Curie in 1880. n The first 2D gray scale image was produced in real time in 1965 by a scanner developed by Siemens.
2. Ultrasound & its gen. n longitudinal compression waves. n Mechanical wave which is quite different with EM wave. n 20 to 20 000 Hz (20 khz) audible sound < subsonic wave > ultrasonic wave, 2-10 MHz for imaging
Generation of ultrasound wave Push Pull n Piezoelectric crystal n when an alternating voltage is applied over the crystal, a compression wave with the same frequency is generated. n Transducer
3. Wave propagation in homogeneous media n It is characterized by its specific acoustic impedance Z. n Z, force (acoustic pressure p) to the particle velocity response (v) n Interference n Attenuation
Wave propagation in inhomogeneous media n Reflection & refraction Huygens principle
n Reflection & refraction T = 4Z 1Z 2 cosθ i cosθ t (Z 2 cosθ i + Z 1 cosθ t ) 2 R = (Z 2 cosθ i + Z 1 cosθ t ) 2 (Z 2 cosθ i + Z 1 cosθ t ) 2
Scattering Reflected signal as a function of time in a homogeneous object in water Reflected signal as a function of time in an inhomogeneous object in water
" A mode - amplitude " M mode - Motion 4. Data acquisition Amplitude Voltage Time (depth) Line number (time) Depth
" B mode - brightness B mode image acquisition can be done by either translating or tilting the transducer.
Wave propagation and motion: the Doppler effect Frequency shift f D Note that for θ = 90 the Doppler frequency is zero.
n Doppler effect l 连续多普勒超声诊断仪 l 脉冲多普勒诊断仪 l 彩色多普勒诊断仪
Imaging reconstruction 1. Filtering 2. Envelope detection 3. Attenuation correction 4. Log-compression 5. Scan conversion. 1. Filtering to remove the high frequency noise. Fundamental imaging mode Second harmonic mode
2. Envelope detection By means of a quadrature filter or a Hilbert transform.
3. Attenuation compensation Time gain compensation
4. Log-compression There is the large difference in amplitude between the specular and the scatter reflections, yielding a large dynamic range. A suitable gray level transformation can be applied, i.e., log-compression.
5. Scan conversion If the image is acquired by tilting the transducer instead of translating it, samples on a polar grid are obtained. Converting the polar into a rectangular grid needs interpolation.
6. Equipment 模拟发射 / 模拟接收 脉冲多普勒处理 显示控制 彩色血流处理 声束合成处理 解调 B 模式处理 扫描转换与其他后处理 前端 中端 后端 控制单元 电源
Transducer (a) 线阵换能器 (b) 相控阵换能器 (c) 凸面阵换能器
Transducers for 3D imaging
The shape of the transducer is adapted to the application
Special purpose transducers This transducer is swallowed by the patient and makes ultrasound images of the heart from within the esophagus
Important parameters for US imaging Dynamic range describes the amplitude change of the acquired signal. Transmitted frequency High frequency -> High resolution -> Large attenuation
Important parameters for US imaging FPS (Frame per second) - F ve c: velocity of sound; N: number of scanning line; R max : scanning depth; Resolution Lateral resolution is related to the width of US beam. Axial resolution is related to transmitted frequency.
Acquisition and reconstruction time Q1: If one want to scan a FOV with a depth of 20 cm and 120 scanning lines using ultrasound imaging, what is the maximum temporal resolution (? Hz)? c=1540 m/s n A depth of 20 cm, 40 cm, the acquisition 0f each line takes 260 µs. n 120 lines, 30 ms, so that 30 Hz n Current clinical scanners are able to acquire multiple scan lines simultaneously with little influence on the spatial resolution. 70-80 Hz
Assignment If the velocity of a moving object is 0.5m/s away from the transducer and the pulse frequency is 2.5 MHz, what is the Doppler shift? -1.6 khz
New US technology (a) Intravascular ultrasound (b) US angiography micro bubble (c) Photoacoustic imaging (d) Multi-modality (RF, photon, thermal, elastic)