Ultrasound Theory Lecture Notes PDF

ULTRASOUND THEORY Dr Anastasia Hadjiconstanti Prof Vered Aharonson Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 28. DESCRIBE THE PRODUCTION OF AN ULTRASOUND BEAM. 30. DESCRIBE THE INTERACTION OF ULTRASOUND WITH MATERIAL. Familiar waves and where to find them MHz GHz And sound… MHz GHz ?...

ULTRASOUND THEORY Dr Anastasia Hadjiconstanti Prof Vered Aharonson Acknowledgements: Dr. Constantinos Zervides LECTURE LOB’S 28. DESCRIBE THE PRODUCTION OF AN ULTRASOUND BEAM. 30. DESCRIBE THE INTERACTION OF ULTRASOUND WITH MATERIAL. Familiar waves and where to find them MHz GHz And sound… MHz GHz ? v = 340 m/s WHAT IS ULTRASOUND I Ultrasound allows the visualization and examination of different anatomical parts using high frequency sound waves. These wave are emitted from a probe and directed into the body. All the different diagnostic ultrasound techniques involve the detection and display of acoustic energy reflected off different tissues within the body. Different body structures have different properties that scatter and reflect sound energy in predictable ways. This makes it possible to recognize structures. WHAT IS ULTRASOUND II Diagnostic ultrasound is the only imaging technology favoured for routine use in obstetrics. Figure: Fetal ultrasound image: cross-sectional view of the head of a fetus at 18 week’s gestation WHAT IS ULTRASOUND III Ultrasound techniques can also be used to detect and display flow parameters. This makes ultrasounds very useful in vascular imaging. Of all the different imaging techniques, ULTRASOUND IS THE MOST INFLUENCED BY THE SKILL AND EXPERIENCE OF THE OPERATOR. WHAT IS ULTRASOUND IV Ultrasound Examination – What is this experience like? Outpatient procedure Exams are usually administered in the hospital’s radiology or imaging department in a small, specially equipped room, or, sometimes, a doctor’s office. During the exam, you recline on an examination table. The physician or medical technician (called a sonographer) applies a gelcoated transducer. The gel makes good contact with probe head and excluded all the air between the body and the probe. The exam is generally painless, the only sensation being due to the gel and the smooth head of the transducer pressed against the skin. WHAT IS ULTRASOUND V https://www.informedhealth.org/how-do-ultrasound-examinations-work.html https://www.informedhealth.org/how-doultrasound-examinations-work.html WHAT IS ULTRASOUND VI The images formed by an ultrasound scanner are displayed as white-on-black images on a monitor. They can be stored on disc and printed for later review. https://qcg.com.au/patients/anatomy-heart https://en.wikipedia.org/wiki/Medical_ultrasound WHAT IS ULTRASOUND VII Doppler Ultrasound Doppler, or colour flow ultrasound, allows the ultrasound imaging device to measure the speed with which blood flows in the body. Colour flow ultrasound allows cardiologists to visualize the beating heart along with a color-coded map of the flow of the blood within its chambers. https://www.youtube.com/watch?v=61LUoCB6MLM PROS & CONS I Diagnostic ultrasound offers several advantages over other imaging modalities. DOES NOT USE IONIZING RADIATION. This makes it THE SAFEST MODALITY and is why it is so important in obstetrical imaging. Besides its safety, it has the ability to image in real time. The cost of an ultrasound scanner is significantly less than other imaging modalities. Unfortunately, sometimes diagnostic images cannot be obtained due to a variety of reasons. In such cases, cross sectional imaging with computed tomography or MRI may be used instead. PROS & CONS II ADVANTAGES OF ULTRASONOGRAPHY Uses no ionizing radiation Non-invasive Painless Minimal preparation of patients Portable Inexpensive Has no known side effects Safe in pregnancy Gives direct vision for biopsies DESCRIBING SOUND WAVES I Sound waves and their properties are the basis for understanding ultrasound imaging. Sound is a mechanical wave in which particles in the medium move. Sound must travel through a medium, where molecules are alternately compressed and rarefied. Sound travels in a straight line and sound waves are longitudinal waves. DESCRIBING SOUND WAVES II The values of seven parameters are required to completely characterize a sound wave. 1. Period 2. Frequency 3. Amplitude 4. Power 5. Intensity BIGNESS PARAMETERS (Describe size, magnitude & strength of sound wave) 6. Wavelength 7. Propagation speed DESCRIBING SOUND WAVES III Period is the time it takes a wave to vibrate a single cycle, i.e., the period is the amount of time it takes for a wave to travel one wavelength. It is reported in units of time. Period is determined by the sound source only and can’t be changed by the sonographer while using a basic US system with a specific transducer. Frequency is the number of cycles that occur in one second. Frequency is reported in units of per second or hertz (Hz). In clinical imaging frequency ranges from approximately 2 MHz to 15 MHz. Frequency of a sound wave is determined by the sound source only and can’t be changed by the sonographer while using a basic US system with a specific transducer. DESCRIBING SOUND WAVES IV Amplitude is the difference between the maximum (or minimum) and average value of an acoustic variable. It can have units of any acoustic variable. Initially amplitude is determined only by the source, i.e., the US system. However, amplitude decreases as sound propagates through the body at a rate depending on the medium and sound characteristics. Amplitude can be changed by the sonographer. DESCRIBING SOUND WAVES V Power is the rate of energy transfer or the rate at which work is performed. Power has units of watts. Initially power is determined only by the source, i.e., the US system. Power decreases as sound propagates through the body at a rate depending on the medium and sound characteristics. Power can be changed by the sonographer. Mathematically, power is proportional to the wave’s amplitude squared: 𝑷𝒐𝒘𝒆𝒓 ∝ 𝑨𝒎𝒑𝒍𝒊𝒕𝒖𝒅𝒆𝟐 DESCRIBING SOUND WAVES VI Intensity if the concentration of energy in a sound beam and relates how the power in a wave spreads or is distributed in space. Intensity has units of watts/cm2 ( 𝑷𝒐𝒘𝒆𝒓 (𝑾) ). 𝑨𝒓𝒆𝒂 (𝒄𝒎𝟐 ) Initially intensity is determined only by the source, i.e., the US system. Intensity decreases as sound propagates through the body at a rate depending on the medium and sound characteristics. Intensity can be changed by the sonographer. Intensity is proportional to power and proportional to amplitude squared: 𝐈𝐧𝐭𝐞𝐧𝐬𝐢𝐭𝐲 ∝ 𝐏𝐨𝐰𝐞𝐫 𝐈𝐧𝐭𝐞𝐧𝐬𝐢𝐭𝐲 ∝ 𝐀𝐦𝐩𝐥𝐢𝐭𝐮𝐝𝐞𝟐 DESCRIBING SOUND WAVES VII Wavelength is the distance or length of one complete cycle. It is measured in units of length (i.e., mm). Wavelength is the only parameter determined by both the source and the medium. If a wave remains in one medium, wavelength and frequency are inversely related. To find the wavelength of a sound wave in soft tissue use: 𝑾𝒂𝒗𝒆𝒍𝒆𝒏𝒈𝒉𝒕 𝒎𝒎 = Wavelength can’t be changed by the sonographer. 𝟏.𝟓𝟒 𝒎𝒎/𝝁𝒔 𝑭𝒓𝒆𝒒𝒖𝒆𝒏𝒄𝒚 (𝑴𝑯𝒛) DESCRIBING SOUND WAVES VIII Frequency and wavelength of sound waves in soft tissue Frequency (Hz) Wavelength 100 15.4 m 1000 1.54 m 10,000 15.4 cm 100,000 1.54 cm 1,000,000 = 1 MHz 1.54 mm 10,000,000 = 10 MHz 0.154 mm DESCRIBING SOUND WAVES IX What is the speed of sound in soft tissue? 1540 m/s. What characteristics of a medium determine the speed of sound in that medium? STIFFNESS & DENSITY Describes object ability to resist compression. Describes relative weight of material. Material Lung Fat Soft Tissue Liver Blood Muscle Tendon Bone Velocity (m/s) 500 1450 1540 1560 1560 1600 1700 3500 SBA 1 SBA 1 - Solution 𝑣= 1540 𝑚/𝑠 ?@ 𝑚 = 0.77 𝑚𝑚 𝜆= = = 7.7 ×10 𝑓 2.0 × 10> 𝐻𝑧 SBA 2 SBA 2 - Solution 𝑣= 3500 𝑚/𝑠 ?A 𝑚 = 1.75 𝑚𝑚 𝜆= = = 1.75 ×10 𝑓 2.0 × 10> 𝐻𝑧 SBA 3 SBA 3 - Solution 𝑣= 1540 𝑚/𝑠 𝜆= = = 7.7 𝑚 𝑓 200 𝐻𝑧 DESCRIBING PULSED WAVES I In diagnostic ultrasound, continuous wave sound cannot create anatomic images. Imaging systems produce short bursts or pulses of acoustic energy to create every picture of anatomy. A pulse of ultrasound is a collection of cycles that travel together. Pulsed ultrasound has two components: 1. Transmitting (“on” time). 2. Receiving (“off” time). DESCRIBING PULSED WAVES II Five parameters describe pulsed sound completely. 1. Pulse duration. 2. Spatial pulse length. 3. Pulse repetition period. 4. Pulse repetition frequency. 5. Duty factor 𝐏𝐮𝐥𝐬𝐞 𝒅𝒖𝒓𝒂𝒕𝒊𝒐𝒏 (𝐏𝐮𝐥𝐬𝐞 𝐫𝐞𝐩𝐞𝐭𝐢𝐭𝐢𝐨𝐧 𝒑𝒆𝒓𝒊𝒐𝒅 ×𝟏𝟎𝟎). DESCRIBING PULSED WAVES III What is depth of view? It describes the maximum distance into the body that an ultrasound system is imaging. Markers along the edge of an image indicated depth. Pulse repetition period and imaging depth are directly related. DESCRIBING PULSED WAVES IV SHALLOW IMAGING Less listening Shorter PRP. Higher PRF. Higher Duty factor. DEEP IMAGING More listening Longer PRP. Lower PRF. Lower Duty factor. PARAMETER ADJUSTABLE UNITS DETERMINED BY TYPICAL VALUES Pulse duration No μsec, Time Source 0.5 to 3.0 μs Pulse repetition period Yes msec, Time Source 0.1 to 1.0 ms Pulse repetition frequency Yes Hz, per sec Source 1 to 10 kHz Spatial pulse length No Mm, Distance Source and Medium 0.1 to 1.0 mm Duty factor Yes none Source 0.2 % to 0.5 % SBA 4 Which of the following describes line A? A. B. C. D. E. F. Frequency Period Duty Factor Pulse Repetition Period Pulse duration Amplitude SBA 4 Which of the following describes line A? A. B. C. D. E. F. Frequency Period Duty Factor Pulse Repetition Period Pulse duration Amplitude ULTRASOUND-TISSUE INTERACTIONS I A transducer for sound converts electrical energy into sound energy and vice versa. ULTRASOUND-TISSUE INTERACTIONS II Scatter refers to the propagation of incident sound waves in oblique directions.. ULTRASOUND-TISSUE INTERACTIONS III Reflection in ultrasound refers to the return of the sound wave energy back to the transducer. ULTRASOUND-TISSUE INTERACTIONS IV Refraction occurs when the incident sound wave contacts the boundary of tissues at an oblique angle. ULTRASOUND-TISSUE INTERACTIONS V ULTRASOUND-TISSUE INTERACTIONS: ATTENUATION I As sound waves travel through tissue, there is a progressive reduction in the intensity of the wave. This process is known as attenuation. This attenuation of sound energy is clinically important because it affects the depth of penetration into the tissues. It governs transducer selection and operator-controlled instrument settings. Attenuation is the result of the combined effects of: absorption, scattering, and reflection. ULTRASOUND-TISSUE INTERACTIONS: ATTENUATION II Attenuation is directly proportional to the distance that the ultrasound travels and to the ultrasound frequency. It is also affected by the characteristics of the medium encountered. ü Distance: Attenuation increases as the ultrasound propagates deeper into the body. ü Frequency: High frequency ultrasound attenuates more rapidly than low frequency ultrasound. ü Type of medium: Air and bone cause a higher degree of attenuation. Fluid causes a low degree of attenuation. ULTRASOUND-TISSUE INTERACTIONS: REFLECTION I Specular Reflection If the acoustic interface is large and smooth, it reflects sound much as a mirror reflects light. Such interfaces are called specular reflectors and examples of these in the body are: the diaphragm and the urine-filled bladder. Specular echoes originate from relatively large, strongly reflective, regularly shaped objects with smooth surfaces. Specular echoes are relatively intense and angle dependent. ULTRASOUND-TISSUE INTERACTIONS: REFLECTION II ULTRASOUND-TISSUE INTERACTIONS: REFLECTION III Diffuse Reflection Most interfaces in the body are not smooth and have irregularities. When a wave reflects off an irregular surface, it radiates in more than one direction. This form of reflection is called diffuse reflection, or backscatter. The advantage of diffuse reflections is that interfaces at suboptimal angles to the sound beam will still produce reflections which can be imaged. The disadvantage, is that diffuse reflections have lower signals than specular reflections. ACOUSTIC IMPEDANCE I Acoustic impedance refers to a tissue’s property that allows propagation of sound waves. The acoustic impedance, 𝑧, is the product of the medium’s density, 𝜌, and the speed of sound in the medium 𝑧 = 𝜌×𝑣= Higher acoustic impedance of the tissue results in less propagation of the sound wave. The amount of the sound energy reflected back to the transducer is directly proportional to the difference in acoustic impedance between tissues. ACOUSTIC IMPEDANCE II Hyperechoic: Bone appears hyperechoic (white) on the ultrasound image. Hypoechoic: Muscle and liver appear hypoechoic (grey) on the ultrasound image. Anechoic: Fluid (blood, ascites, pleural effusions) appear anechoic (black) on the ultrasound image. Terminology Appearance on ultrasound image Medium Density of structure Hyperechoic White Bone High Hypoechoic Grey Muscle, liver Medium Anechoic Black Fluid Low ULTRASOUND-TISSUE INTERACTIONS: REFLECTION V REMEMBER… 1. High-frequency sound waves help provide higher resolution images of relatively superficial structures but low-frequency sound waves have better penetration of deeper tissue. 2. Keep the incident sound waves as close to perpendicular to the tissue under evaluation as possible to allow most of the sound waves to return back to the transducer and therefore, produce best visualization. 3. Reflection of sound waves back from the tissues with the largest difference in impedance provides the most hyperechoic (brightest) signals. Bone has a very high impedance and appears hyperechoic on ultrasound. EXERCISE FOR HOME - SBA What is the wavelength of a wave with unknown frequency travelling in soft tissue? A. 0.51 μs B. 0.51 m/s C. 0.51 Pa D. 0.51 W E. 0.51 mm EXERCISE FOR HOME – SBA Which of the following characteristics will create the slowest speed of sound? A. High Density, High Stiffness B. Low Density, High Stiffness C. High Density, Low Stiffness D. Low Density, Low Stiffness SUMMARY I Ultrasound allows the visualization and examination of different anatomical parts using high frequency sound waves. All the different diagnostic ultrasound techniques involve the detection and display of acoustic energy reflected off different tissues within the body. Diagnostic ultrasound offers several advantages over other imaging modalities. Unfortunately, some times diagnostic images cannot be obtained due to a variety of reasons. When used to image the body, ultrasound techniques use brief pulses of acoustic energy. SUMMARY II The speed of the pressure wave varies according to : the physical properties of the different tissues, their resistance to compression which depends on density and stiffness of the medium. Ultrasounds can be attenuated, absorbed, reflected, scattered or diffracted when interacting with tissue. REFERENCES Authors Title Edition Publisher Year ISBN R.K.Hobbie and B.J.Roth Intermediate Physics for Medicine and Biology 5th Edition Springer 2015 9783319126814 M.A. Haidekker Medical Imaging Technology 1st Edition Springer 2013 9781461470724 A.B. Wolbarst, P. Capasso and A.R. Wyant Medical Imaging: Essentials for Physicians 1st Edition Wiley-Blackwell 2013 9780470505700 Sidney K. Edekman Understanding Ultrasound Physics 4th Edition E.S.P. Ultrasound 2012 9780962644450 Suzanne Amador Kane Introduction to Physics in Modern Medicine 2nd Edition CRC Press 2009 9781584889434 V. Gibbs, D. Cole, A. Sassano Ultrasound Physics and Technology. How, Why and When 1st Edition Churchill Livingstone 2009 9780702030413