Table of Contents Ultrasound basics- ............................................................................................................ 2 Fundamentals .......................................................................................................................... 2 Ultrasound Wave ..................................................................................................................... 2 Speed of ultrasound ................................................................................................................. 3 Piezoelectric materials ............................................................................................................. 4 Piezoelectric effect ................................................................................................................... 4 Generation of ultrasound waves ............................................................................................... 5 Frequency Content of pulses .................................................................................................... 6 Interactions between ultrasound and surfaces .......................................................................... 7 Loss of ultrasound energy in tissue ........................................................................................... 9 Producing an ultrasound image ................................................................................................ 9 Amplification of Received Echos ............................................................................................. 10 Transducer Designs and Beam Forming ................................................................................... 10 Phased Array Transducer ........................................................................................................ 11 Focusing the Beam ................................................................................................................. 11 Image Resolution ................................................................................................................... 11 Tissue harmonic Imaging ........................................................................................................ 11 Doppler Ultrasound ........................................................................................................ 12 Ultrasound basicsFundamentals Ultrasound is a sound wave that is above the audible limit for humans It is produced at >20kHz which is above the audible spectrum of sound Ultrasound are mechanical pressure waves that propagate through a medium with a high frequency. Sounds needs a medium to travel through (unlike light) Ultrasound Wave Ultrasound is a wave In physics, there are two principals of a wave: A cycle and a period A cycle is the length of one positive and one negative deflection of the wave A period is the length of time to complete one cycle (represented by the symbol T) Frequency (represented by the symbol ) is the number of cycles per second (also known as Hertz, Hz). 1 Hertz is one complete cycle per second. Frequency is calculated by 1/period (= 1/T) as it is the number of periods passing through a point in 1 second. Amplitude is the peak (power) of the wave. Wavelength is the distance between two points on the curve. This is NOT period. Period is the time between the two points on the curve. Wavelength is represented by lambda (λ). There is a large range of frequencies of sounds. Humans can only hear from 20Hzz to 2kHz sounds, however clinical ultrasound uses frequencies in the range oef 2MHz to 15Mhz (higher than we can hear) A high frequency (more cycles per second) gives us a high resolution, however we are unable to penetrate as deep A low frequency (less cycles per second) gives us less resolution but allows for deeper images. Speed of ultrasound Sound travels through various media at different speeds (ie- sound travels through water quicker than through air). The speed of sound is noted by ‘c’ The speed of sound = frequency x wavelength (c= λ), it is measured in meters per second (m/s) The speed of sound depends on the density and compressibility of the material. The more dense and more compressible, the slower the wave will travel. The less dense and less compressible, the faster it will travel. The speed of sound is different from different tissues in the body. Knowledge of the speed of sound is needed to determine how far an ultrasound wave has travelled. In ultrasound machine, the assumption is made that the speed of sound is the same in all tissues= 1540m/s Piezoelectric materials Piezoelectric materials A crystal is any solid with atoms or molecules that are arranged in a very orderly way based on repetitions of the same basic atomic building block (the unit cell). In most crystals (such as in metals), the unit cell is symmetrical. In Piezoelectric materials, they are not symmetrically ordered. The atom arrangement may not be symmetrical, but the electrical charges are perfectly balanced: a positive charge in one place cancels out a negative charge nearby Stretching or squeezing a piezoelectric crystal deforms the structure, pushing some of the atoms closer together or further apart. This upsets the balance of positive and negative, and causes net electrical charges to appear. The most common piezoelectric material is quartz. Lead zirconate titanate is usually used in ultrasounds. Piezoelectric effect The piezoelectric effect refers to a change in electric polarization that is produced in certain materials when they are subjected to mechanical stresses. The piezoelectric effect occurs when a piezoelectric material produces electricity. It occurs when there is a conversion of kinetic or mechanical energy due to crystal deformation, which turns into electrical energy. Piezoelectric materials are those that can produce electricity due to mechanical stress. When materials are placed under mechanical stress, there is a shift in the positive and negative charge centers of the materials. This rapid mechanical stress causes an external electric field. The inverse piezoelectric effect is when an external electrical current causes a physical deformation in the material. In 1880 Jacques and Pierre Curie discovered that pressure generates electrical charges in certain types of crystals such as quartz and tourmaline. They called this phenomenon the "piezoelectric effect". The word "piezo" is derived from the Greek word ‘piezein’, which means to squeeze or press. If we think about a hexagon of a quarts crystal, there are positively charged silicon and negatively charged oxygen. If a force is applied onto the crystal, one of the crystal becomes more positive and the other becomes more negative If a force is applied in a different direction, the positive and negative ends invert. If this happens in rapid succession, you create an alternating electric field and therefore alternating current. When SOUND WAVES hit the crystal, it causes it to vibrate and move back and forth thereby creating an alternating current. This is how mechanical energy is transformed into electrical energy. Alternatively, if we apply an alternating current to the crystal, it will cause the lattice to move back and forth, thereby creating mechanical energy (and hence sound waves). In ultrasound, application of an electric field to the crystals in the transducer will create sound waves in extremely high frequency. In order to ‘mechanically’ change the crystal or have a change in mechanical structure to create an electrical wave, it is hit with sounds waves. These sound waves distort the crysal, create polarity in the crystal and thereby an alternating force. Good Youtube video: https://www.youtube.com/watch?v=v1enr8PIMOw Generation of ultrasound waves A transducer is a device that converts one form of energy into another. In an ultrasound, conversion is from electrical energy into mechanical vibration and vice versa. The piezoelectric effect is the methods by which most ultrasound is created (see above) The piezoelectric effect is the ability of certain materials to generate an electric change in response to applied mechanical stress They are used to create sound waves of extremely high frequencies (in the millions of Hz). The frequency of the voltage applied will affect the frequency with which the piezoelectric material vibrates. The thickness of the piezoelectric element will determine the frequency at which the element will vibrate most efficiecntly. A voltage is applied to the piezoelectric material at a specific frequency. When gel is used the vibrations are transmitted into a surrounding medium (such as the body). The returning ultrasound vibrations are converted into electrical signals which are then amplified, analysed and displayed to provide anatomical images. Electrical pulses Sound Waves (mechanical energy) Electrical Energy Modern transducers operate over a range (eg 3-9 MHz). A broad-band and narrow band transducer wil individually be useful. A broad band transducer is more efficient over a wider rrange of frequencies, a narrow band transducer is not. The active element, which is often referred to informally as the crystal, is protected from damage by a wearplate or acoustic lens, and backed by a block of damping material that quiets the transducer after the sound pulse has been generated. This ultrasonic subassembly is mounted in a case with appropriate electrical connections. THERE ARE THREE TYPES OF ULTRASOUND: B-MODE, COLOUR FLOW, PULSED WAVE DOPPLER Pulsed ultrasound - All ultrasound is pulsed doppler. - This is because it is was continuous, the ultrasound would be continuously transmitted along a path and the energy (sound waves) will be continuous reflected back from the boundary in the path of the beam it is impossible to see where the returning echos have come from - When we pulse a eave, we are able to predict where is has come from due to this formula: d=tc/2 - d= tc/2 - d= distance t= time between transmission and reception of the pulse c= velocity of the ultrasound along the path - The division of 2 is due to the pulsed wave travelling along the path twice. From this equation, you can determine the distance and therefore what it is reflecting. Frequency Content of pulses - The pulses used in imaging ultrasound are very short and only contain 1-3 cycles (a positive and negative reflection is a cycle). This is so that reflection from boundaries of different tissues that are close together can be separated and visualised more easily. Pulsed doppler signals are longer and contain several cycles with a RANGE OF FREQUENCIES of different AMPLITUDE. Different shaped pulses will have different frequency contents (it is made up of multiple different frequencies). Beam Shape - The shape of the ultrasound beam produced by a transduceder will depend of the 1. shape of the element 2. if the beam is focussed. Interactions between ultrasound and surfaces - The creation of an ultrasound image depends on how it interacts with the tissue as it passes through the body. When the ultrasound meets the interface (part) between two different media (or tissues in the body) some of the energy is reflected back. The proportion of the energy reflected and transmitted depends on the change in the acoustic impedence of a medium. The great the impedance of a medium, the more of the ultrasounds that are reflected back Eg- there is a large difference between soft tissue and bone with regards to acoustic impedence. Therefore, there will be a lot of reflection of the waves back from the bone. For this reason, if is impossible to image below bones too much of the ultrasound waves are reflected back and you can’t see what is beneath. - Sound waves can’t be transmitted through gas, therefore there is acoustic shadowing beyond on images. - The path back to transducer will also affect the amplitude of the signal. If the transducer is perpendicular to the thing it is imaging, more rays will bounce directly back and there will be more If the probe is at 60 degrees, less will be reflected back, but off away from the probe. This is why the best image of an interface is what the inferface is at right angels to the beam. The poorest images are when interface is parallel to the beam. When an artery is imaged in transverse section, the anterior and posterior walls - Loss of ultrasound energy in tissue - - Attenuation of the loss of energy from the ultrasound beam as it passes through tissue. The more the ultrasound energy is attenuated by the tissue, the less energy will be available to return to the probe Attenuation is caused by. Several different processes: absorption, scattering, reflection, and beam divergence. Absorption causes ultrasound energy to be converted into heat. The rate of absorption varies in different tissue types Ultrasound energy can also be lost by scattering from small structures within the tissue or reflection from large boundaries that are not perpendicular to the beam, preventing the ultrasound from returning to the transducer The rate of attenuation depends on the frequency of the ultrasound The higher the frequency, the more quickly they are attenuated when comparted to lower frequencies. This is why higher frequencies penetrate tissue less effectively than lower freqeuecny ultrasound. The rate of attenuation per unit distance is called the attenuation coefficient (decibels/centimeter) It depends on both the medium at the sound frequency. Attenuation is high for muscle and skin, intermediate for larger organs such as liver and very low for fluids. Producing an ultrasound image - - There is an assumption tha the ultrasound waves travel through tissue at a constant speed. Based upon this, it is possible to predict the distance from a reflective boundary or scattering particle. When an ultrasound returns to the transducer, it will cause the transducer to vibrate and this will generate a voltage across the piezoelectric element. The amplitude of the returning pulse will depend on the proportion of the ultrasound reflected back or scatterd back to the transducer. The amplitude of the pulse received back at the transducer can be displayed against time. This delay is calibrated against time and the returning pulse represents the distance of the boundary from the transducerthereby showing the depth of the boundary in tissue. The varying amplitude of the signal can be displayed as a spot of varying brightness that travels across the display with time. This is called a B-mode scan (Brightness Scan). - If a second pulse is sent into the tissue along the same path the B scan generated by the second pulse is displayed enxt to the first. - This shows the time of travel of the pusles converted into distance. Amplification of Received Echos - - There are two ways of increasing the amplitude of a returning signal: increase the output power and increase the receiver gain. Increasing the voltage of the excitation pulse across the transducer will cause the transducer to transmit a larger amplitude ultrasound pulse. However, increasing the output power will mean the patient is exposed to more ultrasound energy. The alternative is to amplify the received signal- but there is a limit. The limit exists as the greater the amplitude of received signals, the greater the background noise at some point you will get more background noise and wrose images. There is a method called Time Gain Compensation (TGC)- this allowed for the varying gain over time. BY changing gain over time the retunring echos from the boundaries can be displayed at similar brightness. Transducer Designs and Beam Forming Linear Array Transducers - Most modern transducers are made up of 128 elements arranged in a row, around 4cm long. - When the wavelets are all excited simultaneously, the wavelets will interefere to produce a beam that is perependicular to the transducer face. - This produces a field of view which is the same at depath as it is lose to the transducer. Curvilinear transducer - A Curvilinear transducer is used for deeper imaging. As the paths diverge the image fans out and the scan lines run more cloely in the proprtio of the image closer to the transducer (and are more spread out at depth- allowing for a larger field of view compared with a linear array- hwoever the image does lose some of it’s quality at depth. Phased Array Transducer - - - Phased array transducers- these are used to manipulate the beam of the ultrasound based upon excitation of the elements at different times. If the elements used to form the beam are excited at the slightly different times, the wavefronts produced by the elements will interfere differently than they would if they were excited at all the same time. This way you can steer the beam through a range of angles. Phased transducers use a smaller array of elements and steer the beam in this way to produce a sector image. This produces a large field of view compared to the size of the transducer (used for things such as echo). Compound imaging is when the target is isonated several times with the beam steered at different angles and then these are combined to create a single image. Focusing the Beam - The focus of the beam is created via changing the focal length of the beam Need to do more Image Resolution Tissue harmonic Imaging Doppler Ultrasound Doppler effect - The Doppler effect is the change in the observed frequency due to the relative motion of the source and the observer. It is why sounds are different from an ambulance coming to and from you If you and an object giving off doppler signals are static then the sounds are hitting you at the same frequency However if you start to move towards the object, they are hitting you at a quicker pace (a higher frequency, therefore a higher pitch) If you move away from the object, the wave forms will hit you less often and therefore at a lower frequency (and therefore lower pitch) The change in observed frequency is the Doppler Effect It was first discovered in 1842 by Christian Doppler – an Austrian Physicist. The haemodynamic of Vascular Disease -POLAK For blood flow to occur between two points, there must be an energy difference between the two points. This difference is due to a blood pressure distance. The circulatory system generally consists of a high-pressure, high kinetic energy arterieal reservoir, a large venous pool with low pressure and low kinetic energy. As blood flows throughout the circulatory ystem, energy is continuously lost because of the friction between the layers of flwoing blood. Both pressure and kinetic energy decrease as the red cells transit from arterial to venous. The blood flow is created by continuously pumping via the heart. Blood flow to all body tissues is adjusted accoringt o tisseu’s need at a particular time. The adjustments is accomplished by local alteration in the level of arterial and venous constriction within a certain organ. Maintenance of arteries ensures distribution of blood floow. Potential and kinetic energy The physical factors that goven how blood once ejected from the ehart dissipates as it transits is due to four things: friction, resistance and the influence of laminar and turbulent flow. These things are sumamrized by Bernoulli equation, Poiseulli’s law and Poiseuliie;s equiation. Over straight blood vessels, the sum of kinetic (blood flow) na dpotential (blood pressure) energy is constant. This is summarised by Bernoulli’s equation. If the artery lumen increases, kinetic energy is converted back into pressure and the velocity decreases. However, if the lumen narrows, the potential energy is converted into kinetic energy. Energy differences related to different body parts. There are large variations in the potential energy of blood due to difference in posture. Eg- pressure in feet is proportional to height fo column.
