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The Physics of Diagnostic Ultrasound FRCR Physics Lectures Mark Wilson Clinical Scientist (Radiotherapy) Hull and East Yorkshire Hospitals NHS Trust

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Presentation on theme: "The Physics of Diagnostic Ultrasound FRCR Physics Lectures Mark Wilson Clinical Scientist (Radiotherapy) Hull and East Yorkshire Hospitals NHS Trust"— Presentation transcript:

1 The Physics of Diagnostic Ultrasound FRCR Physics Lectures Mark Wilson Clinical Scientist (Radiotherapy) Hull and East Yorkshire Hospitals NHS Trust mark.wilson@hey.nhs.uk Session 3 & 4

2 Session 3 Overview Hull and East Yorkshire Hospitals NHS Trust Session Aims: Recap Image Artefacts Contrast Agents Introduction to Doppler US

3 Recap Hull and East Yorkshire Hospitals NHS Trust

4 Recap Hull and East Yorkshire Hospitals NHS Trust 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 / 

5 Recap Hull and East Yorkshire Hospitals NHS Trust Diagnostic ultrasound utilises the pulse-echo principle Each pulse-echo sequence produces one line of the image Several pulse-echo sequences are needed to compose a full image frame. ) ) ) ) ) D Source of sound Distance = Speed x Time 2D = c x t Sound reflected at boundary Pulse Echo

6 Recap Ultrasound waves undergo the following interactions: Reflection Scatter Refraction Attenuation and Absorption Diffraction Hull and East Yorkshire Hospitals NHS Trust

7 Recap Hull and East Yorkshire Hospitals NHS Trust z1z1 z2z2 p i, I i p t, I t p r, I r Reflection R = Z 2 – Z 1 Z 1 + Z 2 IiIi IrIr = ( ) 2 Intensity Reflection Coefficient (R) Acoustic Impedance z =   k Acoustic Impedance z =  c

8 Recap Hull and East Yorkshire Hospitals NHS Trust 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

9 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

10 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 Hull and East Yorkshire Hospitals NHS Trust c1c1 c 2 (>c 1 ) i t Snell’s Law sin (i) sin (t) c1c1 c2c2 =

11 Recap Attenuation As an ultrasound wave propagates through a medium, the intensity reduces with distance travelled Attenuation describes the reduction in intensity with distance and includes scattering, diffraction, and absorption Attenuation increases linearly with frequency Limits frequency used – trade off between penetration depth and resolution Hull and East Yorkshire Hospitals NHS Trust Distance, d Intensity, I Low freq. High freq. I = I o e -  d Where  is the attenuation coefficient

12 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 Hull and East Yorkshire Hospitals NHS Trust Decibel, dB = 10 log 10 (I 2 / I 1 ) Attenuation and absorption is often expressed in terms of decibels

13 Image Artefacts Hull and East Yorkshire Hospitals NHS Trust

14 Artefacts Hull and East Yorkshire Hospitals NHS Trust 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

15 Artefacts Hull and East Yorkshire Hospitals NHS Trust 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

16 Artefacts Hull and East Yorkshire Hospitals NHS Trust 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 Medium 1 c 1 Medium 2 c 2 > c 1 Target Displayed at

17 Artefacts Hull and East Yorkshire Hospitals NHS Trust 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

18 Artefacts Hull and East Yorkshire Hospitals NHS Trust Acoustic Enhancement Occurs when ultrasound passes through a tissue with low attenuation 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 Image of renal cyst Low attenuation

19 Artefacts Hull and East Yorkshire Hospitals NHS Trust Acoustic Shadowing Occurs when ultrasound wave encounter a very echo dense (highly attenuating) structure 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 Image of Gallstone High attenuation

20 Artefacts Hull and East Yorkshire Hospitals NHS Trust 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 Reverberation Interface

21 Artefacts Hull and East Yorkshire Hospitals NHS Trust Attenuation Artefacts

22 Artefacts Hull and East Yorkshire Hospitals NHS Trust Attenuation Artefacts

23 Contrast Agents Hull and East Yorkshire Hospitals NHS Trust

24 Contrast Agents Hull and East Yorkshire Hospitals NHS Trust 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

25 Contrast Agents Hull and East Yorkshire Hospitals NHS Trust 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

26 Contrast Agents Hull and East Yorkshire Hospitals NHS Trust Ultrasound Contrast Agents NameCapsuleGasBubble Size LEVOVIST Palmitic acidAir 3-5  m SONOVISTCyano-acrylateAir 2  m DEFINITYLipidPerfluoropropane 2  m OPTISONAlbuminOctafluoropropane 3.7  m SONOVUEPhospholipidsSF 6 2-3  m SONAZOIDSurfactantFluorocarbon 3.2  m ALBUNEXAlbuminAir 4  m

27 Contrast Agents Hull and East Yorkshire Hospitals NHS Trust 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

28 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust

29 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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 (f r ) compared to the emitted frequency (f t ) 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

30 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Source and observer stationary ) ) ) ) ) f r = f t The observed sound has the same frequency as the emitted sound Source Observer (Note: Frequency is the number of cycles per second)

31 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Source moving towards observer ) ) ) ) ) f r > f t Causes the wavefronts travelling towards the observer to be more closely packed, so that the observer witnesses a higher frequency wave than emitted

32 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Source moving towards observer ) ) ) ) ) f r < f t The wavefronts travelling towards the observer will be more spread out, so that the observer witnesses a lower frequency wave than emitted

33 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

34 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

35 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Ultrasound measurement of blood flow Doppler frequency shift, f d = f r – f t = 2 f t v cos  c f r = received frequency f t = 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)  Target direction

36 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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 Desirable to adjust  to obtain the highest Doppler frequency shift cos   0 1 90

37 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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 In diseased arteries the lumen will narrow and the blood velocity will increase 2 f t cos  c f d v =

38 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Ultrasound measurement of blood flow The flow (Q) remains constant Q = A 1 V 1 = A 2 V 2 A = Area V = Velocity A1A1 V1V1 A2A2 V2V2 Narrowing in artery

39 Break Hull and East Yorkshire Hospitals NHS Trust

40 Session 4 Overview Hull and East Yorkshire Hospitals NHS Trust Session Aims: Continuous Wave Doppler US Pulsed Wave Doppler US Harmonic Imaging

41 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

42 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

43 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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 Gate depth Transducer Gate length

44 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

45 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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 Frequency Tissue Blood

46 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Doppler signal processing Transducer Demodulator High-pass filter Frequency estimator Display Signal processor Demodulation Separation of the Doppler frequencies from the underlying transmitted signal High-pass filtering Removal of the tissue signal Frequency estimation Calculation of Doppler frequency and amplitudes

47 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

48 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust High-pass Filtering Amplitude Frequency Tissue Blood Amplitude Frequency Blood High-pass Filtering

49 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

50 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Aliasing Original signal Aliased signal

51 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Aliasing Aliased signal Increased PRF

52 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

53 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

54 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

55 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Doppler of Common Carotid Artery

56 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Blockage in Carotid Artery

57 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Renal Colour Doppler

58 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust 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

59 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Suspicious dark lesion

60 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Power Doppler – Circle of Willis Suspicious dark lesion

61 Doppler Ultrasound Hull and East Yorkshire Hospitals NHS Trust Prostate Cancer Suspicious dark lesion

62 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust

63 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust 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.

64 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust 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) Frequency Ideal Actual

65 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust 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

66 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust Bandwidth 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 = F 0 / (F 2 – F 1 ) An increased bandwidth and a decreased SPL reduces the Q Factor High Q = pure ultrasound pulse Amp Freq F 0 is the centre frequency and the lower and upper frequencies (F 2 and F 1 ) are at half the peak amplitude (reduction of 3 dB)

67 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust 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 The energy transmitted in side lobes can reduce image quality (contrast and resolution) Transducer Main lobe side lobes

68 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust 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

69 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust 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 F 0 known as Harmonics Frequency Fundamental Frequency F 0 Second Harmonic 2F 0 Third Harmonic 3F 0

70 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging In harmonic imaging an US pulse is transmitted with fundamental frequency F 0 but due to non-linear propagation the returning echoes also contain energy at harmonic frequencies 2 F 0 and 3 F 0. The imaging system ignores the fundamental frequency component of the echo and forms an image using only the 2 nd 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.

71 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust F0F0 Transmit Pulse Transducer Bandwidth Frequency F0F0 2F 0 Transmit PulseReceived Echo Filtered Echo

72 Harmonic Imaging Hull and East Yorkshire Hospitals NHS Trust Harmonic Imaging Harmonic imaging can only be performed with a wide bandwidth transducer which can respond to both the fundamental frequency and its 2 nd harmonic. The received echoes are passed through a filter which removes frequencies around F 0 and only allows through those near to 2F 0. As ‘acoustic noise’ echoes are mainly at the fundamental frequency, they are suppressed giving a clearer image. To achieve good separation of the received 2 nd 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.

73 The End Hull and East Yorkshire Hospitals NHS Trust


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