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The Physics of Diagnostic Ultrasound FRCR Physics Lectures Session 3 & 4 Mark Wilson Clinical Scientist (Radiotherapy) [email protected] Hull and East Yorkshire Hospitals NHS Trust Session 3 Overview Session Aims: • Recap • Image Artefacts • Contrast Agents • Introduction to Doppler US Hull and East Yorkshire Hospitals NHS Trust Recap Hull and East Yorkshire Hospitals NHS Trust Recap • The term Ultrasound refers to high frequency sound waves. • Sounds waves are mechanical pressure waves which propagate through a medium causing the particles of the medium to oscillate backward and forward • The velocity and attenuation of the ultrasound wave is strongly dependent on the properties of the medium through which it is travelling =c/f c=k/ Hull and East Yorkshire Hospitals NHS Trust Recap • Diagnostic ultrasound utilises the pulse-echo principle D Source of sound ) ) ) Pulse Echo ) ) Sound reflected at boundary Distance = Speed x Time 2D = c x t • Each pulse-echo sequence produces one line of the image • Several pulse-echo sequences are needed to compose a full image frame. Hull and East Yorkshire Hospitals NHS Trust Recap Ultrasound waves undergo the following interactions: • Reflection • Scatter • Refraction • Attenuation and Absorption • Diffraction Hull and East Yorkshire Hospitals NHS Trust Recap Reflection z1 z2 pi , Ii pt , It pr , Ir Intensity Reflection Coefficient (R) R= Ir Ii = ( Z2 – Z1 Z1 + Z2 2 ) Acoustic Impedance z = k Acoustic Impedance z = c Hull and East Yorkshire Hospitals NHS Trust Recap Reflection • Strength of reflection depends on the difference between the Z values of the two materials • Ultrasound only possible when wave propagates through materials with similar acoustic impedances – only a small amount reflected and the rest transmitted • Therefore, ultrasound not possible where air or bone interfaces are present Hull and East Yorkshire Hospitals NHS Trust Recap Scatter • Reflection occurs at large interfaces such as those between organs where there is a change in acoustic impedance • Within most organs there are many small scale variations in acoustic properties which constitute small scale reflecting targets • Reflection from such small targets does not follow the laws of reflection for large interfaces and is termed scattering • Scattering redirects energy in all directions, but is a weak interaction compared to reflection at large interfaces Hull and East Yorkshire Hospitals NHS Trust Recap Refraction When an ultrasound wave crosses a tissue boundary at an angle (non-normal incidence), where there is a change in the speed of sound c, the path of the wave is deflected as it crosses the boundary c1 c2 (>c1) Snell’s Law i t sin (i) c1 = c sin (t) 2 Hull and East Yorkshire Hospitals NHS Trust Recap Attenuation • As an ultrasound wave propagates through a medium, the intensity reduces with distance travelled Intensity, I • Attenuation describes the reduction in intensity with distance and includes scattering, diffraction, and absorption • Attenuation increases linearly with frequency Low freq. High freq. • Limits frequency used – trade off between penetration depth and resolution I = Ioe- d Distance, d Where is the attenuation coefficient Hull and East Yorkshire Hospitals NHS Trust Recap Absorption • In soft tissue most energy loss (attenuation) is due to absorption • Absorption is the process by which ultrasound energy is converted to heat in the medium • Absorption is responsible for tissue heating Decibel Notation Attenuation and absorption is often expressed in terms of decibels Decibel, dB = 10 log10 (I2 / I1) Hull and East Yorkshire Hospitals NHS Trust Image Artefacts Hull and East Yorkshire Hospitals NHS Trust Artefacts Image Artefacts When forming a B-mode image, a number of assumptions are made about ultrasound propagation in tissue. These include: • Speed of sound is constant • Attenuation in tissue is constant • Ultrasound pulse travels only to targets that are on the beam axis and back to the transducer Significant variations from these conditions in the target tissues are likely to give rise to visible image artefacts Hull and East Yorkshire Hospitals NHS Trust Artefacts Range Errors • The distance, d, to the target is derived from the time elapsed between transmission of the pulse and receipt of the echo from the target, t • In making this calculation the system assumes that t = 2d / c, where the speed of sound is constant at 1540 m/s • If the speed of sound in the medium between the transducer and target is greater (or less) than 1540 m/s, the echo will arrive back at the transducer earlier (or later) than expected for a target of that range Fat c = 1420 m/s Tissue c = 1540 m/s Target Displayed at Hull and East Yorkshire Hospitals NHS Trust Artefacts Refraction Refraction of the ultrasound beam as it passes between tissues with a different speed of sound can result in objects appearing at an incorrect position in the image Target Displayed at Medium 1 c1 Medium 2 c2 > c1 Hull and East Yorkshire Hospitals NHS Trust Artefacts Attenuation Artefacts • During imaging the outgoing pulse and returning echoes are attenuated as they propagate through tissue, so that echoes from deeper targets are weaker than those from similar superficial targets • Time Gain Compensation (TGC) is applied to correct for such changes in echo amplitude with target depth • Most systems apply a constant rate of compensation designed to correct for attenuation in typical uniform tissue • The operator can also make additional adjustments to compensate via slide controls that adjust the gain applied specific depths in the image • TGC artefacts may appear in the image when the applied compensation does not match that actual attenuation rate in the target tissue Hull and East Yorkshire Hospitals NHS Trust Artefacts Acoustic Enhancement • Occurs when ultrasound passes through a tissue with low attenuation Image of renal cyst • Echoes from deeper lying tissues are enhanced due to the relatively low attenuation in the overlying tissue • This occurs because the TGC is set to compensate for the greater attenuation in the adjacent tissues Low attenuation Hull and East Yorkshire Hospitals NHS Trust Artefacts Acoustic Shadowing • Occurs when ultrasound wave encounter a very echo dense (highly attenuating) structure Image of Gallstone • Nearly all of the sound is reflected, resulting in an acoustic shadow • This occurs because the TGC is set to compensate for the lower attenuation in the adjacent tissues High attenuation Hull and East Yorkshire Hospitals NHS Trust Artefacts Reverberation Artefact • Reverberation artefacts arise due to reflections of pulses and echoes by strongly reflecting interfaces • Occur most commonly where there is a strongly reflecting interface parallel to the transducer face • Involves multiple reflections - Initial echo returns to reflecting interface as if it is a weak transmission pulse and returns a second echo (reverberation) Transducer Interface Reverberation Hull and East Yorkshire Hospitals NHS Trust Artefacts Attenuation Artefacts Hull and East Yorkshire Hospitals NHS Trust Artefacts Attenuation Artefacts Hull and East Yorkshire Hospitals NHS Trust Contrast Agents Hull and East Yorkshire Hospitals NHS Trust Contrast Agents Ultrasound Contrast Agents • Ultrasound contrast agents are gas-filled micro-bubbles which are injected into the blood stream • Micro-bubbles will give increased backscatter signal due to the large acoustic impedance mismatch between the gas-filled bubble and surrounding tissue Hull and East Yorkshire Hospitals NHS Trust Contrast Agents Ultrasound Contrast Agents • Micro-bubble suspension is injected intravenously into the systemic circulation in a small bolus • The micro-bubbles will remain in the systemic circulation for a certain period of time • Ultrasound waves are directed on the area of interest and when the micro-bubbles in the blood flow past the imaging window they give rise to increased signal • Allows detection of blood flow where it would otherwise not be seen Hull and East Yorkshire Hospitals NHS Trust Contrast Agents Ultrasound Contrast Agents Name Capsule Gas Bubble Size LEVOVIST Palmitic acid Air 3-5 m SONOVIST Cyano-acrylate Air 2 m DEFINITY Lipid Perfluoropropane 2 m OPTISON Albumin Octafluoropropane 3.7 m SONOVUE Phospholipids SF6 2-3 m SONAZOID Surfactant Fluorocarbon 3.2 m ALBUNEX Albumin Air 4 m Hull and East Yorkshire Hospitals NHS Trust Contrast Agents Targeted Contrast Agents • Targeted contrast agents are under preclinical development • They retain the same general features as untargeted micro-bubbles, but they are outfitted with ligands that bind to specific receptors expressed by cell types of interest • Micro-bubbles theoretically travel through the circulatory system, eventually finding their respective targets and binding specifically • If a sufficient number of micro-bubbles have bound to the target area, an increased signal will be seen Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound The Doppler Effect • The Doppler effect is observed regularly in our daily lives, e.g. it can be heard as the changing pitch of an ambulance siren as it passes by • The Doppler effect is the change in the observed frequency of the sound wave (fr) compared to the emitted frequency (ft) which occurs due to the relative motion between the observer and the source • Consider three situations - Source and observer stationary - Source moving towards observer - Source moving away from observer Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Source and observer stationary Source ) )))) Observer fr = ft The observed sound has the same frequency as the emitted sound (Note: Frequency is the number of cycles per second) Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Source moving towards observer ) )))) f r > ft Causes the wavefronts travelling towards the observer to be more closely packed, so that the observer witnesses a higher frequency wave than emitted Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Source moving towards observer ) ))) ) f r < ft The wavefronts travelling towards the observer will be more spread out, so that the observer witnesses a lower frequency wave than emitted Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound The Doppler Effect • The resulting change in the observed frequency from that transmitted is known as the Doppler shift • The magnitude of the Doppler shift frequency is proportional to the relative velocity between the source and the observer • It does not matter if it is the source or the observer is moving • The Doppler effect enables Ultrasound to be used to assess blood flow by measuring the change in frequency of the ultrasound scattered from moving blood Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound measurement of blood flow • Transducer is held stationary and the blood moves with respect to the transducer • The ultrasound waves transmitted by the transducer strike the moving blood, so the frequency of the ultrasound experienced by the blood is dependent on whether the blood is stationary, moving towards or away from the transducer • The blood then scatters the ultrasound, some of which travels in the direction of the transducer and is detected • The scattered ultrasound is Doppler frequency shifted again as a result of the motion of the blood, which now acts as a moving source • Therefore, a Doppler shift has occurred twice between the ultrasound being transmitted and received back at the transducer Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound measurement of blood flow 2 ft v cos c Doppler frequency shift, fd = fr – ft = Target direction fr = received frequency ft = transmitted frequency c = speed of sound v = velocity of blood = angle between the path of the ultrasound beam and the direction of the blood flow (angle of insonation) Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound measurement of blood flow • The detected Doppler shift also depends on the cosine of the angle between the path of the ultrasound beam and the direction of blood flow • The operator can alter by adjusting the orientation of the transducer on the skin surface cos 1 0 90 • Desirable to adjust to obtain the highest Doppler frequency shift Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound measurement of blood flow If the angle of insonation of the ultrasound beam is known it is possible to use the Doppler shift frequency to estimate the velocity of the blood using the Doppler equation v = c fd 2 ft cos In diseased arteries the lumen will narrow and the blood velocity will increase Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound measurement of blood flow Narrowing in artery A1 V1 A2 V2 The flow (Q) remains constant Q = A1 V1 = A2 V2 A = Area V = Velocity Hull and East Yorkshire Hospitals NHS Trust Break Hull and East Yorkshire Hospitals NHS Trust Session 4 Overview Session Aims: • Continuous Wave Doppler US • Pulsed Wave Doppler US • Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Continuous Wave and Pulsed Wave Doppler • Doppler systems can be either continuous wave or pulsed wave • Continuous wave (CW) systems transmit ultrasound continuously • Pulsed wave (PW) systems transmit short pulses of ultrasound • The main advantage of PW Doppler is that Doppler signals can be acquired from a known depth • The main disadvantage of PW Doppler is that there is an upper limit to the Doppler frequency shift which can be detected Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Continuous Wave (CW) Doppler • In a CW Doppler system there must be separate transmission and reception of ultrasound – transducer with two separate elements • The region from which Doppler signals are obtained is determined by the overlap of the transmit and receive ultrasound beams Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Pulsed Wave (PW) Doppler • In a PW Doppler system it is possible to use the same transducer element for both transmit and receive • The region from which Doppler signals are obtained is determined by the depth of the gate and the length of the gate, which can both be controlled by the operator Transducer Gate depth Gate length Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound signal received by transducer The received ultrasound signal consists of the following four types of signal: • echoes from stationary tissue • echoes from moving tissue • echoes from stationary blood • echoes from moving blood The task for the Doppler system is to isolate and display the Doppler signals from blood, and remove those from stationary and moving tissue Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Ultrasound signal received by transducer • Doppler signals from blood tend to be low amplitude (small reflected echo) and high frequency shift (high velocity) • Doppler signals from tissue are high amplitude (large reflected echo) and low frequency shift (low velocity) • These differences provide the means by which signals from true blood flow may be separated from those produced by surrounding tissue Amplitude Tissue Blood Frequency Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Doppler signal processing Transducer Demodulation Separation of the Doppler frequencies from the underlying transmitted signal Demodulator Signal processor High-pass filtering Removal of the tissue signal High-pass filter Frequency estimator Frequency estimation Calculation of Doppler frequency and amplitudes Display Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Demodulation • The Doppler frequencies produced by moving blood are a tiny fraction of the transmitted ultrasound frequency • E.g. If transmitted frequency is 4 MHz, a motion of 1 m/s will produce a Doppler shift of 5.2 kHz, which is less the 0.1% of the transmitted frequency • The extraction of the Doppler frequency information from the ultrasound signal received from tissue and blood is called demodulation • In PW Doppler, need the PRF to be at least twice the maximum Doppler shift frequency in order to avoid ‘aliasing’ (not a problem in CW) • Aliasing is an artefact introduced by under-sampling in which high frequency components take the alias of a low frequency component Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound High-pass Filtering Amplitude Amplitude Tissue High-pass Filtering Blood Blood Frequency Frequency Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Frequency Estimation • A spectrum analyser calculates the amplitude of all the frequencies present within the Doppler signal • In the spectral display the brightness is related to the amplitude of the Doppler signal component at that particular frequency Frequency shift Time Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Aliasing Original signal Aliased signal Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Aliasing Aliased signal Increased PRF Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Duplex Imaging • Duplex imaging combines Doppler information with a real-time B-mode image • This produces a 2D representation of the direction and velocity of the blood flow on a grey-scale image • In a typical display blood flowing towards the transducer is coded as red and blood flow away from the transducer is coded blue Red – towards transducer Blue – away from transducer Green - Variance Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Doppler Displays – Spectral Doppler • Available in CW and PW Doppler • Detailed analysis of distribution of flow • Examine change in flow with time Frequency shift or Velocity Time Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Doppler Displays – Colour Doppler • Available in PW Doppler only • Superimposed Doppler information on underlying B-mode image • Overall view of flow in region • The sign (direction), mean Doppler shift (mean velocity) and variance (turbulence) of Doppler spectrum are usually colour-coded and displayed Red – towards transducer Blue – away from transducer Green - Variance Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Doppler of Common Carotid Artery Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Blockage in Carotid Artery Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Renal Colour Doppler Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Doppler Displays – Power Doppler • In a Colour Doppler image the magnitude of the frequency shift colour encodes the pixel value and assigns a colour depending on blood flow direction • This Doppler signal processing places a restriction on the motion sensitivity since the signals received must be extracted to determine the velocity (magnitude of Doppler shift) and direction (phase shift) • Power Doppler encodes the strength of the Doppler shifts (amplitude, intensity, power) with colours and ignores directional (phase) information • In Power Doppler the magnitude of the Doppler signal is displayed rather than the Doppler frequency shift (e.g. the density of the red blood cells is depicted rather than their velocity) • Power Doppler therefore exhibits increased sensitivity to slow flow rates at the expense of directional and quantitative flow information Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Suspicious dark lesion Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Power Doppler – Circle of Willis Suspicious dark lesion Hull and East Yorkshire Hospitals NHS Trust Doppler Ultrasound Prostate Cancer Suspicious dark lesion Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Introduction • Harmonic imaging of tissue is useful in suppressing weak echoes caused by artefact (acoustic noise) which cloud the image and make it difficult to identify anatomical features. • These echoes (often called clutter) are particularly noticeable in fluid filled areas (e.g. heart or cyst) and are a common problem when imaging large patients. • Harmonic imaging is possible by utilising harmonic frequencies of the ultrasound pulse. Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging The Ultrasound Pulse • Ideally the US pulse would rise and fall very quickly and contain only one frequency (or wavelength) • In reality the US pulse contains a finite range of frequencies (or wavelengths) Actual Ideal Frequency Frequency Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Spatial Pulse Length (SPL) The spatial pulse length (in mm) is defined as n Where is the wavelength and n is the number of cycles A wider range of wavelengths and more cycles produces a longer SPL Use a higher frequency and shorter US pulse to give smaller SPL and improved resolution For Diagnostic US typical SPL values range from 0.3 to 1.0 mm SPL Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Bandwidth Amp • Bandwidth describes the spread of US frequencies the transducer can transmit/receive • The frequency content may be specified in terms of the Q factor: Q factor = F0 / (F2 – F1) • An increased bandwidth and a decreased SPL reduces the Q Factor • High Q = pure ultrasound pulse Freq F0 is the centre frequency and the lower and upper frequencies (F2 and F1) are at half the peak amplitude (reduction of 3 dB) Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Side Lobes • Ideally US pulse energy appears as a single front travelling in forward direction • Some energy travels off in different directions called side lobes side lobes Main lobe • The energy transmitted in side lobes can reduce image quality (contrast and resolution) Transducer Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Non-linear Propagation • In the description of propagation of sound waves given in the first session the wave propagated with a fixed speed determined by the properties of the medium • This is a good approximation to reality when the amplitude of the wave is small, but at higher pressure amplitudes the effects on non-linear propagation become noticeable • The speed at which each part of the wave travels is related to the properties of the medium and to the local particle velocity, which enhances or reduces the speed • In the high pressure (compression) parts of the wave this results in a slight increase in speed • In the low pressure (rarefaction) parts of the wave this results in a slight decrease in speed Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Non-linear Propagation • As the wave propagates into the medium the compression parts catch up with the rarefaction parts • The compression parts become taller and narrower in amplitude, while the rarefaction become lower in amplitude • The rapid changes in pressure in the compression parts of the wave appear in the pulse spectrum as high frequency components, these are multiples of the fundamental frequency F0 known as Harmonics Fundamental Frequency F0 Second Harmonic 2F0 Third Harmonic 3F0 Frequency Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Harmonic Imaging • In harmonic imaging an US pulse is transmitted with fundamental frequency F0 but due to non-linear propagation the returning echoes also contain energy at harmonic frequencies 2 F0 and 3 F0. • The imaging system ignores the fundamental frequency component of the echo and forms an image using only the 2nd harmonic component. • The effective ultrasound beam (the harmonic beam) is narrower than the conventional beam because non-linear propagation occurs most strongly in the highest amplitude parts of the transmitted beam (i.e. near beam axis). • Weaker parts of the beam such as side lobes and edges of the main lobe produce little harmonic energy and are suppressed in relation to the central part of the beam. • Harmonic imaging also reduces acoustic noise from weak echoes and reverberations. Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Transmit Pulse Received Echo Filtered Echo Transducer Bandwidth Transmit Pulse F0 Frequency F0 2F0 2F0 Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Harmonic Imaging • Harmonic imaging can only be performed with a wide bandwidth transducer which can respond to both the fundamental frequency and its 2nd harmonic. • The received echoes are passed through a filter which removes frequencies around F0 and only allows through those near to 2F0. • As ‘acoustic noise’ echoes are mainly at the fundamental frequency, they are suppressed giving a clearer image. • To achieve good separation of the received 2nd harmonic frequencies from the fundamental frequencies, the frequency spectra of the pulse and received echoes must be made narrower than in normal imaging. • Reduction of the frequency range results in an increase in pulse length and a reduction in axial resolution. Hull and East Yorkshire Hospitals NHS Trust The End Hull and East Yorkshire Hospitals NHS Trust