ultrasound_image How to understand ultrasound diagnostics? Vladan Bernard Department of Biophysics 2020 Lecture outline •introduction to ultrasound •physical properties of ultrasound and medium •classification of methods • mode A, B and 3D, 4D imaging doppler methods contrast media •safety, risks splash-water-waves-4554 listen to the comment, others are in better quality •Ultrasound (US) is mechanical oscillations with frequency above 20 kHz which propagate through an elastic medium. • •Ultrasound is similar physical phenomenon as the sound; the sound is described by characteristic frequencies from 16 Hz to 16 kHz • •In liquids and gases, US propagates as longitudinal waves. •In solids, US propagates also as transversal waves. Ultrasound direction of propagation l t [USEMAP] Physical properties •Physical properties of ultrasound • •wavelength: λ is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. • •frequency:f is the number of occurrences of a repeating event per unit time • •intensity: intensity I of beam US at a point is the amount of energy passing through unit cross-sectional area perpendicularly to the beam per unit time at that point. Intensity is expresed in Joule/second/square centimetre.Intensity can be specified as Watts/square centimetre (W=J/s) • •Amplitude may be in the case of ultrasound wave define as a sound pressure level at a given point, for example ( or as displacement of mass point in space). Physical properties wavelength_sm time pressure http://www.sprawls.org/ppmi2/USPRO/25USPROD03.png Physical properties velocity Approximate Velocity of Sound in Various Materials Material Velocity (m/sec) Fat 1450 Water 1480 Soft tissue (average) 1540 Bone 4100 Speed of US c depends on elasticity and density ρ of the medium: K - modulus of compression, in water and soft tissues c = 1500 - 1600 m.s-1, in bone about 3600 m.s-1 [USEMAP] •Physical properties of medium • •Interaction of US with medium – reflection and back-scattering, refraction, attenuation (scattering and absorption) Physical properties interactions [USEMAP] •Attenuation of US expresses decrease of wave amplitude along its trajectory. It depends on frequency • • Ix = Io e-2αx a = a´.f2 • •Ix – final intensity, Io – initial intensity, 2x – medium layer thickness (reflected wave travels „to and from“), a - linear attenuation coefficient (increases with frequency). •Since •a = log10(I0/IX)/2x • •we can express a in units dB/cm. At 1 MHz: muscle 1.2, liver 0.5, brain 0.9, connective tissue 2.5, bone 8.0 • Physical properties Physical properties of medium [USEMAP] Physical properties •Physical properties of medium Attenuation of ultrasound When expressing intensity of ultrasound in decibels, i.e. as a logarithm of Ix/I0, we can see the amplitudes of echoes to decrease linearly. depth [cm] I or P [dB] attenuation •US reflection and transmission on interfaces Physical properties We suppose perpendicular incidence of US on an interface between two media with different Z (Acoustic impedance – following page) - a portion of waves will pass through and a portion will be reflected (the larger the difference in Z, the higher reflection). P1 Z 2 - Z 1 R = ------- = --------------- P Z2 + Z1 P2 2 Z 1 D = ------- = --------------- P Z2 + Z1 Coefficient of reflection R – ratio of acoustic pressures of reflected and incident waves Coefficient of transmission D – ratio of acoustic pressures of transmitted and incident waves P – acoustic pressure [USEMAP] •Acoustic impedance: product of US speed c and medium density r • • Z = r . c (Pa. m/s) • •Acoustic impedance Z: muscles 1.7, liver 1.65 brain 1.56, bone 6.1, water 1.48 (x 10-6) • •Physical quantity describing the „willingness“ of tissue to transmit ultrasound. How to easy propagate ... Acoustic parameters of medium Physical properties [USEMAP] •As an ultrasound pulse passes through matter, such as human tissue, it interacts in several different ways. Some of these interactions are necessary to form an ultrasound image, whereas others absorb much of the ultrasound energy or produce artifacts and are generally undesirable in diagnostic examinations. The ability to conduct and interpret the results of an ultrasound examination depends on a thorough understanding of these ultrasound interactions. Types of Ultrasound Pulse Interactions interactions [USEMAP] • As the ultrasound pulse moves through matter, it continuously loses energy. This is generally referred to as attenuation. Several factors contribute to this reduction in energy. One of the most significant is the absorption of the ultrasound energy by the material and its conversion into heat. Ultrasound pulses lose energy continuously as they move through matter. This is unlike x-ray photons, which lose energy in "one-shot" photoelectric or Compton interactions. Scattering and refraction interactions also remove some of the energy from the pulse and contribute to its overall attenuation, but absorption is the most significant. Absorption absorption [USEMAP] •At most interfaces within the body, only a portion of the ultrasound pulse is reflected. The pulse is divided into two pulses, and one pulse, the echo, is reflected back toward the transducer and the other penetrates into the other material, as shown in the figure. The brightness of a structure in an ultrasound image depends on the strength of the reflection, or echo. This in turn depends on how much the two materials differ in terms of acoustic impedance Z. Reflection The Production of an Echo and Penetrating Pulse at a Tissue Interface [USEMAP] •Ultrasonography – diagnostic method, used reflection of ultrasound • •Characteristic: •Passive US – low intensity waves which cannot cause substantial changes of medium. •In US diagnostics (ultrasonography = sonography = echography) - used frequencies are 2 - 40 MHz with (temporal average, spatial peak) intensity of about 1 kW/m2 •Impulse reflection method: a probe with one transducer which is source as well as detector of US impulses. A portion of emitted US energy is reflected on the acoustic interfaces and the same probe then receives reflected signal. After processing, the signal is displayed on a screen. • Ultrasonography X Ultrasound Ultrasonography US probe -source US wave object reflection reflection detection (time, intensity) detection (time, intensity) intensity time US probe -detector [USEMAP] Ultrasonography US probe US wave object reflection detection (time, intensity) detection (time, intensity) intensity time US probe -detector physimage1 M200129P01WL Supplement - piezo element Material that changes its conformation when an electric pulse is present. Attached AC causes vibration of the material and following mechanic vibration of tissue. [USEMAP] Supplement – ultrasound probe 1 main_pic The piezoelectric element is an essential part of the probe to generate ultrasonic waves. On both sides of the piezoelectric element electrodes are affixed and a voltage is applied. The element then oscillates by repeatedly expanding and contracting, generating a sound wave. When the element is externally applied with viblation (or an ultrasonic wave) in turn, it generates a voltage. Among the several types of piezoelectric elements, piezoelectric ceramic (PZT: lead zirconate titanate) is most commonly used because of its high conversion efficiency. http://www.ndk.com/en/sensor/ultrasonic/images/basic02/pic_01.gif http://www.ndk.com Supplement – ultrasound probe 2 http://www.ndk.com The backing material is located behind the piezoelectric element to prevent excessive vibration. Reducing excessive vibration will cause the element to generate ultrasonic waves with a shorter pulse length, improving axial resolution in images. Function of the backing material The Basic Ultrasound Imaging Process http://www.sprawls.org/ppmi2/USPRO/usimage2.JPG The transducer is the component of the ultrasound imaging equipment that is placed in direct contact with the patient's body. It performs several functions. It's first function is to produce the ultrasound pulses when electrical pulses are applied to it. A short time later, when echo pulses return to the body surface they are picked up by the transducer and converted back into electrical pulses that are then processed by the system and formed into an image. [USEMAP] Principles of generation of ultrasound pulse http://www.sprawls.org/ppmi2/USPRO/25USPROD02.png The source of sound is a vibrating object, the piezoelectric transducer element. Since the vibrating source is in contact with the tissue, it is caused to vibrate. The vibrations in the region of tissue next to the transducer are passed on to the adjacent tissue. This process continues, and the vibrations, or sound, is passed along from one region of tissue to another. The rate at which the tissue structures vibrate back and forth is the frequency of the sound. The rate at which the vibrations move through the tissue is the velocity of the sound. [USEMAP] •Transducers can be designed to produce either a focused or non-focused beam, as shown in the following figure. A focused beam is desirable for most imaging applications because it produces pulses with a small diameter which in turn gives better visibility of detail in the image. The best detail will be obtained for structures within the focal zone. The distance between the transducer and the focal zone is the focal depth. Transducer Focusing Beam Width and Pulse Diameter Characteristics of Both Unfocused and Focused Transducers A-mode – one-dimensional • Distances between reflecting interfaces and the probe are shown. • Reflections from individual interfaces (boundaries of media with different acoustic impedances) are represented by vertical deflections of base line, i.e. the echoes. • Echo amplitude is proportional to the intensity of reflected waves (Amplitude modulation) • Distance between echoes shown on the screen is approx. proportional to real distance between tissue interfaces. • Today used mainly in ophthalmology. • • A-mode shows reflections of ultrasound in the axis of US wave time (s) - distance (m) intensity [USEMAP] echofig3 A-scan echofig1 A-mode – one-dimensional [USEMAP] B-mode – two-dimensional •A tomogram is depicted. •Brightness of points on the screen represents intensity of reflected US waves (Brightness modulation). • •Static B-scan: a cross-section image of examined area in the plane given by the beam axis and direction of manual movement of the probe on body surface. The method was used in 50‘ and 60‘ of 20th century • • [USEMAP] •One-dimensional static B-scan shows movement of reflecting tissues. The second dimension is time in this method. • •Static probe detects reflections from moving structures. The bright points move vertically on the screen, horizontal shifting of the record is given by slow time-base. • •Displayed curves represent movement of tissue structures • M-mode plice [USEMAP] Comparison of A-, B- and M-mode principle zobrABM B-mode - dynamic •Repetitive formation of B-mode images of examined area by fast deflection of US beam mechanically (in the past) or electronically „in real time“ today. •Electronic probes consist of many piezoelectric transducers which are gradually activated. sondaUZ Basic characteristics of US images •Degree of reflectivity – echogenity. The images of cystic (liquid-filled) and solid structures are different. According to the intensity of reflection in the tissue bulk we can distinguish structures: • • hyperechogenic, izoechogenic, hypoechogenic, anechogenic. • •Solid structures – acoustic shadow (caused by absorption and reflection of US) • •Air bubbles and other strongly reflecting interfaces cause repeating reflections (reverberation, „comet tail“). Nephrocalcinosis_Ultrasound [USEMAP] Acoustic shadow caused by absorption and reflection of US by a kidney stone (arrow) Hyperechogenic area below a cyst (low attenuation of US during passage through the cyst compared with the surrounding tissues – arrow) Three-dimensional (3D) imaging - The probe is linearly shifted, tilted or rotated. The data of reflected signals in individual planes are stored in memory of a powerful PC which consequently performs mathematical reconstruction of the image. Disadvantages of some 3D imaging systems: relatively long time is needed for mathematical processing, price. [USEMAP] Four-dimensional (4D) image Image of 4D Fetal Profile The fourth dimension is time (x,y,z,t) http://www.youtube.com/watch?v=NNHk3GJwN7o&feature=related Doppler flow measurement The Doppler effect (frequency shift of waves formed or reflected at a moving object) can be used for detection and measurement of blood flow, as well as, for detection and measurement of movements of some acoustical interfaces inside the body (foetal heart, blood vessel walls) Christian. A. Doppler (1803-1853), Austrian physicist and mathematician, formulated his theory in 1842 during his stay in Prague. DOPPLER [USEMAP] Principle of Doppler effect perceived (detected) frequency corresponds with frequency of source in rest perceived (sense) frequency is higher when source is in motion to the object perceived frequency is lower when source is moving away Doppler effect Principle of Doppler effect Application of Doppler effect in blood flow velocity measurement Moving reflector (back scatterer) = erythrocytes Doppler effect II [USEMAP] Doppler flow measurement 1)Calculation of Doppler frequency change fd 2)Calculation of „reflector“ (erythrocytes) velocity v 1) 2) fv - frequency of emitted US waves α - angle made by axis of emitted US beam and the velocity vector of the reflector c – US speed in the given medium (about 1540 m/s in blood) angle alpha [USEMAP] •is a combination of dynamic B-mode imaging (the morphology of examined area with blood vessels is depicted) and the PW Doppler system (measurement of velocity spectrum of blood flow). •It allows to examine blood flow inside heart or in deep blood vessels (flow velocity, direction and character) • DUPLEX doppler method Doppler beam Scheme: sector image with sampling volume Image of carotid with spectral analysis of blood flow velocity •The image consists of black-white and colour part. •The black-white part contains information about reflectivity and structure of tissues. •The colour part informs about movements in the examined section. (The colour is derived from average velocity of flow.) •The apparatus depicts distribution and direction of flowing blood as a two-dimensional image. •BART rule – blue away, red towards. The flow away from the probe is coded by blue colour, the flow towards the probe is coded by red colour. The brightness is proportional to the velocity, turbulences are depicted by green patterns. • Colour Doppler imaging Carotid bifurcation [USEMAP] •A combination of duplex method (B-mode imaging with PW Doppler) and colour flow mapping •Normal finding of blood flow in a. carotis communis TRIPLEX doppler method ultrasm Cartoid Artery Stenosis •Colour coding of information about velocity and direction of movements of tissues • •Velocities 1-10 mm/s •are depicted. • Tissue Doppler Imaging (TDI) TDI of a. carotis communis during systole image_gallery?img_id=%201492508&t=1259873571845 Ultrasound elastography Skin cancer is shown in an elastogram, with elasticity strain ratio on the left, and in an ultrasound image on the right. US elastography tracks tissue movement during compression by dynamic B-mode. Finding the deformation of objects – measurement of value of eleasticity deformation [USEMAP] •increase echogenity of streaming blood •Gas microbubbles (mainly air or volatile hydrocarbons) •- free •- enclosed in biopolymer envelope • • • • • • • • • • • • •A SEM micrograph of encapsulated echocontrast agent Echocontrast agents [USEMAP] •Enhanced demarcation of heart ventricle after application of the echocontrast agent Echocontrast agents Slide17 •Reducing Ultrasound ‘Doses’ Patient Safety US is non-ionising BUT since many bioeffects of ultrasound have not yet been studied fully, ‘prudent’ use is recommended ALARA – as low as reasonably achievable (exposure) In practice ‘prudent’ = justification + optimisation ronnie_colemans_wifes_ultrasound-151520 ultrasound cartoons, ultrasound cartoon, ultrasound picture, ultrasound pictures, ultrasound image, ultrasound images, ultrasound illustration, ultrasound illustrations http://www.cartoonstock.com/lowres/dre0714l.jpg •Sources: •Lecture of prof. Mornstein •Ultrasound Production and Interactions Perry Sprawls, Ph.D. • • • Thank you for your attention