Ultrasound Imaging Ultr Michael Dadd 2007

Transcription

1 Ultrasound Imaging

2 Ultrasound Physics & Instrumentation - Recommended Reading 1. Diagnostic Ultrasound: Principles and Instruments (7th Ed) Frederick W Kremkau W B Saunders Company 2. Applied Physics & Technology of Diagnostic UltrasoundRoger Gent Milner Publishing (3 Milner St. Prospect South Australia)

3 Commercial Medical Ultrasound Imagers Come in all shapes and sizes Many general purpose ultrasound imagers used by radiologists Some specialised markets such as cardiology and urology Commercial medical images start at less than $50K and go above $300K

4 This Course Ultrasound Imaging Basics Brief History of the Technology Imaging hardware In the patient Doppler Ultrasound Continuous wave Pulsed Doppler Colour Doppler Safety, Artifacts, Clinical Applications Assignment Exam

5 Clinical Applications Abdomen Adult Paediatric Renal Female Pelvis Obstetrics First Second Third Other Areas Scrotum S t Neonatal abdomen Thyroid gland Transvaginal Breast Special procedures e.g. biopsy/theatre/trauma Musculoskeletal l k l l Doppler studies Paediatrics i Male pelvis Neonatal neuro

6 Pioneers in Imaging (1960 s) de Vlieger - Netherlands Toshio Wagai - Japan Howry & Holmes - USA Kossoff, Robinson & Garrett - Australia Kelly-Fry USA Feigenbaum - USA Joyner & Reid - USA

7 How Ultrasound Imaging g Works - an oversimplified view A transducer transmits a short, narrow, pulse of acoustic energy into the patient. The energy travels in a straight line and reflects echoes back to the transducer. The distance to each reflector is calculated from the time of arrival of each echo. The echoes are displayed as dots in a 2D image.

8 The Ultrasound Image In ultrasound images, echoes are produced whenever there are changes in the mechanical properties of the tissues. The image maps the acoustic impedance changes in the tissue

9 Forming One Line Linear Array The transducer emits a short pulse of ultrasound. Tissue variations cause some of the ultrasound to be reflected back to the transducer. These echoes are processed to produce ONE LINE of the image.

10 Forming an Image The position of the beam is moved and another line of echoes obtained. This is repeated until a complete image has been stored in the scan converter.

11 2D Imaging Ultrasound imagers need two essential pieces of information to locate the positions of echo producing structures. 1. the time taken for the transmitted ultrasound pulse to travel to the reflector and back to the transducer. 2. the direction i of the ultrasound beam

12 To maximise i frame rate: Use the least depth of penetration necessary. Decrease the number of lines in the image (narrow angle). Use the minimum practicable number of transmit focal zones. Turn off the colour Doppler when it is not needed.

13 Time to produce one line of P cm Time = Distance Speed P cm Time for one line = 1540 metres/sec Giving approximately 13xP microseconds

14 Doing the arithmetic gives P x N x F = Where P = penetration in cm N = number of ultrasound lines per frame F = frame rate in frames per second

15 Ultrasound Physics Imaging Controls

16 Overview Transmits pulse of ultrasound along a single line of sight. Line remains stationary while echoes are received (typically (yp y200 microseconds). Beam is moved along. Process is repeated until a full frame is completed.

17 Scan Patterns Linear Array Phased Array Curved Linear Array 'Vector' ' Array

18 Signal Processing Gain Signal TGC Processing Image Storage & Display

19 Main Imaging Controls Power Gain Time Gain Compensation

20 Time Gain Compensation

21 Gain Controls Imaging Processing is non-linear A simple gain change does more than change the brightness

22 Time Gain Compensation Compensates for attenuation in tissue Attenuation in db proportional to depth Gain increases over period of reception Gain resets each transmit pulse Attenuation is typically 60dB - 80dB Attenuation ti increases with frequency Display range only 20dB - 30 db Without TGC there would be little depth of penetration

23 Amplitude Compression After TGC there remains a far bigger range of echo sizes than can be displayed This wide dynamic range needs to be compressed down to suit the display The amplitude compression has a major effect on image appearance Diagnostic quality is easily lost

24 Amplitude Compression 2 All ultrasound scanners perform amplitude compression at all times The Dynamic Range control allows the operator to adjust the compression A high dynamic range makes a wide range of echoes visible ibl The image contrast drops Diagnostic ability can be degraded

25 Echo Amplitude Compression Amplification of small echoes is high while the amplitudes of large echoes are strongly gy compressed.

26 Scan Converter Operation Converts echo sizes to digital representation Calculates position of echo in image Stores echo size in memory location representing the echo position Processes (warps) the image sizes Reads out digital image in TV format

27 Scan Converter Operation Convert Store Convert Analogue Signal to Digital A/D Digital Image in Computer Memory Digital Image to TV Image D/A

28 Pre-Processing Warp GreyScale Store Image in Memory Read Out Image

29 Post Processing Store Image in Memory Warp GreyScale Read Out Image

30 Ultrasound Physics Tissue Harmonic Imaging

31 Harmonic Generation Occurs naturally as sound propagates through tissue In typical ultrasonic scanning conditions, the strength of the second harmonic is, at best, about 20 db below the fundamental Note: 20 db equals 10 times

32 Second Harmonic Amplitude Proportional to the acoustic pressure squared Proportional to the distance along the beam axis Proportional to the ultrasound frequency Depends on the non-linearity parameter of the tissue Inversely proportional to the tissue density and velocity

33 Tissue Harmonic Imaging Transmits at one frequency, say 2 MHz Displays only the harmonics, say 4 MHz While it is possible to display other harmonics, the second harmonic is by far the greatest

34 Tissue Harmonic Imaging

35 Attenuation ti of Harmonics Harmonics are continuously produced The higher frequencies attenuate faster Overall the rate of reduction is similar

36 Artifact Rejection Multiple l reflection artifacts arise in the superficial i layers where there is weak harmonic content. The artifacts at the fundamental frequency are filtered out before display.

37 Optimizing The Pulse frequency overlap

38 Pulses For Tissue Harmonic Imaging The transmitted pulse spectrum is optimized for harmonic imaging The spectrum must be narrower than for normal imagingi The narrower spectrum, lengthens the pulse and reduces axial resolution

39 Pulse Length vs Frequency Spectrum A long pulse implies a narrow frequency spectrum. A narrow spectrum leads to poor axial resolution.

40 Spatial Resolution With THI Often the longer pulse reduces basic axial resolution Reducing noise and clutter in the image effectively improves image resolution As a rule, decrease in axial resolution is compensated for by the improved contrast resolution

41 Pulse Inversion Technique Pulse Inversion adds together two line of echoes from the same line but with one transmit pulse inverted. Much of the fundamental echoes cancel out.

42 Ultrasound Physics Single Element Transducers

43 Ultrasonic Transducers Transducers are the bridge between the electronic world of the imager and the mechanical environment of the patient's body. They convert an electrical pulse from the imager to a mechanical pulse of ultrasound in the patient. Ultrasonic echoes returning from the patient are converted back into electrical signals when they strike the transducer.

44 Beamwidth Definitions

45 A large transducer can focus to a narrow beam over a short distance while a smaller one will have a wider beam over a greater depth

46 Important! In the focal region the beamwidth is diffraction limited. Away from the focal region the beamwidth is controlled by the size of the transducer and the distance to focus.

47 The Diffraction Limit is defined by a cone with an angle of θ in the apex where: sin θ/2 = K λ / D For first off-axis minima, of K is = 1.22.

48 Learn to estimate beamwidth! 50mm 110mm mm 100mm What is the beamwidth at point 1? What is the beamwidth at point 2?

49 Ultrasound Physics Multi-Element Arrays

50 Transducers come in a wide variety of shapes and sizes, each designed for a particular range of clinical applications

51 Transducer Selection The transducer "footprint" t" often determines whether a transducer can be useful in a particular clinical i l situation. ti

52 Transducer arrays can focus and steer the beam. Individual elements are fired at different time delays to control the angle and focus of the transmitted beam On reception the returning signals at individual elements are delayed before combining to form the echo train The focus (and direction) of the received beam can be adjusted continuously.

53 Steering an Array Changing the time delays between pulses changes the angle of the transmitted wave front The order of pulsing controls the direction

54 Transducer Array Focusing Reception dynamic focusing is always active. Transmission focus is over a limited range. Best focus requires multiple focal zones. Each focal zone needs one transmit pulse.

55 Focusing an Array Pulsing starts ts at the outer elements followed, after delays, by the inner elements A curved (focused) wave front results

56 Focus and Steering The delays for focusing and steering an array can be combined to produce a focused beam at an angle to the transducer Changing g the delays after each line of ultrasound produces a swept, focused beam

57 Steering on Reception 1 A wave front arriving at the transducer at an angle, reaches each element at different times The signal received at each element must be time aligned to achieve a strong, clean echo Each element is connected to an adjustable delay element

58 Steering on Reception 2 The time delay is electronically adjustable, usually under computer control These delay times determine the angle and focus of the array during reception Continuously adjusting the delays allows focus to be maintained throughout the whole beam

59 Dynamic Focus The signal delay elements used for focus and steering during reception are electronically adjustable. It is therefore feasible to continuously change the delays so that t the transducer is always focussed on the echoes being received at that time. This is called Dynamic Focus

60 Dynamic Focus During reception the effective radius of the transducer is continuously changed to be in focus for the echoes arriving at that time

61 Transducer focusing Reception dynamic focusing is always active. Transmission focus is over a limited range. Best focus requires multiple focal zones. Each focal zone needs one transmit pulse.

62 Multiple Transmit Focus Each transmit beam is optimised for a particular depth in the patient Only echoes from the region of best focus is stored from each individual transmitted pulse

63 Multiple Transmit Focus Each transmit pulse in the group is sent along the same line of sight One full line of ultrasound takes longer to obtain & frame rate drops How much longer?

64 Dynamic Aperture During reception the receive beamwidth, at any instant, is determined d by the wavelength, the receive delay settings and the size of the aperture. This aperture is altered as the echoes are being received to control the beam pattern. In particular, small apertures are used for echoes arising close to the transducer.

65 Sidelobes Sidelobes (and grating lobes) are additional beams of energy transmitted at angles away from the main beam. Sidelobes are low energy levels. Sidelobes introduce artifact, particularly when they intersect strong reflectors. They present a significant problem.

66 Sidelobes beam centre

67 Apodization Apodization is the main means of reducing sidelobes with array transducers. The strength of the signals in the outer elements in the apertures are reduced. This reduction smears the sidelobe production.

68 Apodization Apodization can be applied at both transmission and reception. The result is reduced sidelobes at the expense of a wider main beam.

69 Ultrasound Physics Propagation & Reflection

70 Frequency The number of cycles per second In clinical ultrasound frequencies range between 1 MegaHertz and 20 MegaHertz

71 Wavelength Wavelength is the distance occupied by one cycle of ultrasound in the medium. At one instant as a wave propagates p through a tissue, the wavelength is the distance between the same point on two successive cycles of the wave.

72 Speed of Propagation The speed of propagation of an ultrasound pulse in tissue is the rate at which the ultrasound energy travels through the tissue. The speed in most soft tissue is approximately 1540 m/s. The speed in fat is about 1400 m/s. The speed in bone is very high. The speed in air is very low.

73 Typical Values Blood Fat Muscle metres/sec metres/sec metres/sec Bone metres/sec

74 Velocity = frequency x wavelength

75 Refraction When an ultrasound beam passes from one medium to another, the direction of propagation can change - its path becomes bent. This change in direction is called "refraction".

76 Snell s Law V1 V2 = Sin 1 Sin 2

77 Refraction increases The further the angle of incidence differs from 90 degrees. The greater the ratio of the speeds in the two tissues

78 Refraction Artifacts transducer real artifact The imager always puts the echo on the transducers line of sight Refraction results in errors in position o Calculate the positional errors in real tissue combinations!

79 Attenuation Ultrasound loses energy very quickly (attenuates) as it travels through h biological tissue. It is the major limit to the maximum depth which h can be successfully imaged at any given ultrasound frequency.

80 Absorption The major component of attenuation of ultrasound in soft tissue is absorption where the lost energy is converted into heat in the tissue. This consideration imposes maximum power levels which are considered free of risk in clinical examinations and a general agreement to use the lowest practical power levels at all times.

81 Absorption Absorption in db proportional to depth of penetration. Absorption in db proportional to frequency A 5 MHz ultrasound echo reflected from a perfect reflector 5 cm deep in liver tissue will be reduced to about 1/20 of its original size. If the same reflector is situated 10 cm in the liver (twice the distance) the echo will be about 1/400 of its original size.

82 Attenuation Practical Limits The attenuation (loss of strength) of the ultrasound increases rapidly as the frequency increases. This places a practical limit on the maximum depth of tissue that may be imaged at a particular frequency. This depends on many factors including the ultrasound imager control settings. Very approximately, maximum penetrations are: 2.5 MHz: 20 cm 3.5 MHz: 15 cm 5 MHz: 10 cm 10 MHz: 5 cm

83 Attenuation Artifacts The shadow is formed when some lines of ultrasound are attenuated in a localized l area. Everything behind the shadowing structure appears weaker than they would without the shadowing.

84 A calculus reflects a large proportion of the energy, resulting in a bright echo area followed by a well defined, strong acoustic shadow. The shadow follows exactly the direction of the ultrasound beam.

85 Echo Enhancement Artifacts Any localised area of low attenuation ation can result in visible echo enhancement in the image.

86 Ultrasound Physics Image Resolution

87 Image Resolution The quality of a real time image depends on the ultrasound imager's ability to separately distinguish features in the image. Adequate resolution is needed both in time (for moving structures) and in space. In space, three resolutions must be considered. * Axial resolution * Lateral resolution * Slice thickness (array transducers)

88 Axial Resolution Axial Resolution is the ability of an imager to separate the images of two reflectors lying in the direction of the ultrasound beam. reflectors image

89 Lateral Resolution Lateral Resolution defines the ability of the imaging system to resolve two reflectors lying in a direction perpendicular to the beam direction. reflectors direction of scan image

90 Beamwidth Artifacts The images of the reflectors are spread in the direction the beam is scanning. When the reflectors are at equal distances to the transducer the images run into each other.

91 The artifact is at a constant distance from the transducer in the direction of the scan echo free area strong echo structure

92 Slice Thickness > Beamwidth lateral resolution slice thickness

93 Slice Thickness slice thickness lateral resolution Electronic focussing in array transducers works only within the scan plane. Perpendicular to the scan plane only a weak degree of fixed focus is available.

94 Slice Thickness Artifact The artifactual echoes come from outside the plane of scan 0 degrees 90 degrees

95 Ultrasound Ut Physics Doppler

96 Doppler Shift 1 When ultrasound is reflected from a moving red cells, the received frequency differs from the one transmitted. This change in frequency is known as the Doppler shift.

97 Doppler Shift 2 The Doppler shift is proportional to the velocity of the cells in the direction of the beam. Doppler shift is maximum when the beam is aligned with the flow direction. Doppler shift is zero when the beam is at right angle to the flow.

98 Doppler Effect The crucial factor that produces the Doppler effect is that the path travelled by the ultrasound varies during the time the ultrasound is travelling through h the tissue.

99 Doppler Effect The Doppler effect can occur in the classical l situation ti where the receiver is moving relative to the transducer or the usual clinical situation where the ultrasound is reflected from moving particles back to approximately the same point it started from.

100 Doppler Effect In the reflection case both the path length to the reflector and the path length back to the receiver are changing. The size of the Doppler effect is therefore double that obtained in a one-way transmission.

101 Doppler Equation F Doppler = 2 x F 0 x V x cos θ C where F 0 = ultrasound frequency V = reflector velocity C = velocity of ultrasound θ = angle between ultrasound beam and direction of particle motion

102 Continuous Wave Doppler This is the simplest form of Doppler and transmits a continuous sinusoidal ultrasound wave. A second transducer receives the reflected signal and the size of any Doppler shifts measured. As the wave is continuous it is not possible to know where reflectors causing the frequency shift are situated.

103 Continuous Wave Doppler Continuous wave Doppler is commonly used in situations ti where the anatomy is well known and uncomplicated such as in assessment of peripheral circulation. As it has no significant limitation on measuring high velocities it is also used for specific applications such as regurgitant g jets.

104 Pulsed Doppler (also spectral or range gated Doppler) A pulse of ultrasound is transmitted into the patient. Only echoes received over a specific short period (the range gate) are taken for processing and any Doppler frequency shifts calculated apply only to a small volume of tissue known as the sample volume.

105 The Sample Volume The sample volume is the volume of tissue that is contributing to the Doppler frequency shifts. Its length, in the direction of the beam, is defined ed by the range gate while the other two dimensions are defined by the size of the beam. In the case of an array transducer, these dimensions are the beamwidth and the slice thickness at the position of the sample volume.

106 Doppler Spectral Display frequency shift seconds At any time in the cardiac cycle, a Doppler shift will be displayed as a dot. Strong signals produce brighter dots while a range of frequency shifts results in a vertically thick trace.

107 Spectral Trace In this spectral trace, the broadening of the trace at the top of the image results from an increased range of Doppler shifts being received. In this case the appearance is artifactual but could represent turbulent flow.

108 Angle Correction The Doppler spectrum displays frequency shifts. To convert these frequencies into velocities we need the Doppler angle. The operator feeds this information to the equipment by setting a cursor along the vessel.

109 Angle Correction Angle not set Angle set If angle correction has not been set, machines assume a Doppler angle of zero. Some calibrate the display in frequency, rather than velocity.

110 Doppler Sampling 1 In pulsed Doppler the echoes from the range gate are received for a brief period once for each transmitted pulse. This one sample allows only a very rough estimate of the received frequencies to be made. Each additional sample received from the following gpulses improves the estimate.

111 Doppler Sampling 2 The size of the Doppler shift that can be estimated from these returning samples is directly limited by the rate at which these samples are being received. This is the same rate at which the interrogating pulses are being transmitted (the Pulse Repetition Frequency).

112 Doppler Sampling 3 The maximum Doppler shift that can be unambiguously estimated is PRF/2 This is known as the Nyquist Limit

113 Frequency Aliasing Aliasing occurs at all Doppler shift frequencies above the Nyquist Limit it That is, whenever the maximum Doppler shift exceeds half the P.R.F. (Pulse Repetition Frequency) Aliasing represents ambiguity - there is no way for the equipment to reliably define the frequency shift

114 Doppler Aliases When The Pulse Repetition Frequency Is Too Low frequency aliasing

115 Estimating Doppler Shift Some frequency information is obtained from each ultrasound line. A reasonable estimate requires repeated lines on the same path. In spectral Doppler the line does not move. In colour Doppler, the line moves on after a few repetitions (say 8 times.)

116 IMPORTANT! Doppler Sampling Limits The maximum PRF is set by how long it takes to get echoes back (i.e. depth of penetration) The maximum Doppler shift before aliasing a is set by the maximum PRF Calculate typical examples say an artery 10cm away from the transducer

117 Colour Doppler - Overview Colour Doppler quickly provides approximate estimates of frequency shifts in a multitude of range gates Colour Doppler is ideal for indicating overall flow patterns, presence of flow and as a guide to locating a spectral Doppler range gate for accurate measurement.

118 Superimposing the Colour The basic colour image is of lower resolution than the greyscale image. Sophisticated signal processing is needed to decide where the colour will be displayed.

119 The Colour Bar The colours are defined in the colour bar on the far left of the image. The scale is shown incorrectly in cm/s. There is no way of introducing angle correction.

120 Calculating the Colour Each line of sight in the colour box is divided into multiple range gates. The echoes from each range gate are processed separately in parallel. A typical system might have 128 range gates along each line.

121 Calculating the Colour A series of pulses is sent out along one line of sight and the echoes from each range gate are separately processed. In this example, the echoes from eight consecutive pulses are used to estimate the frequency shift in each range gate.

122 Colour Quality The accuracy of the Doppler shift estimate is improved by using more repeated lines for the calculation at the expense of reduced frame rate. In applications, such as cardiac imaging where high frame rates are often essential, lower colour quality is accepted. Frame rate can also be increased by narrowing the colour box and using less lines of colour.

123 Colour Doppler Aliasing i 1 Colour Doppler is a form of pulsed Doppler and therefore subject to frequency aliasing.here positive flow will display as blue/green. As the flow speed increases, the display will become more green. As the speed increases above the Nyquist limit of 52, the display will switch to the most negative flow and be displayed as orange.

124 Power Doppler 1 Is closely related to Colour Doppler Instead of displaying frequency shift, the power of the Doppler signal is displayed Displays the quantity of moving reflectors, rather than their speed

125 Power Doppler 2 Does not contain flow direction Is more sensitive to low flow Has better spatial resolution Requires averaging over time

126 Ultrasound Physics Imaging Artifacts

127 Artifacts Imaging artifacts result when the conditions encountered in the tissue don't match exactly with the assumptions made when constructing the image. Artifacts are images formed using strictly applied laws of physics. In order to understand the artifactual appearance, it is necessary to identify where these imaging assumptions have broken down.

128 Image Artifacts fall into three main areas. Structures in the image which are not real. Real structures which do not appear in the image. Real structures which have the wrong shape, size or intensity. All these appearances can arise from a variety of different sources.

129 Artifact or Real? To be sure that a structure in an image is an artifact, it is important to be able to describe in detail how the artifactual image is being formed. Otherwise,,you run the risk of overlooking important pathology. One difficulty with artifacts is that they vary with the type of transducer and scanning being performed. The appearance is often a combination of a variety of distortions.

130 Attenuation Artifacts The shadow is formed when some lines of ultrasound are attenuated in a localized area. Everything behind the shadowing structure appears weaker than they would without the shadowing.

131 Shadowing A calculus reflects a large proportion p of the energy, resulting in a bright echo area followed by a well defined, strong acoustic shadow. The shadow follows exactly the direction of the ultrasound beam.

132 Echo Enhancement Artifacts Any localised area of low attenuation can result in visible echo enhancement in the image.

133 Slice Thickness Artifact The artifactual echoes come from outside the plane of scan 0 degrees 90 degrees

134 Mirror Artifacts The scanner knows only the direction that the beam left the transducer and the time taken to receive the echo. The scanner displays an artifactual image in the original line of sight of the transducer.

135 Mirror Artifacts - Distortion The size, position and shape of the artifactual image depends on the position and shape of the reflecting mirror and the scan pattern of the transducer.

136 Mirror Artifacts in the Diaphragm

137 Range Ambiguity Artifacts Pulse 1 Pulse 2 TG C

138 Reverberation Artifact Strong reflectors close together th can set up waves continuously bouncing backwards and forwards. Many copies of the original interfaces are repeated deep into the patient. t

139 Clinical Applications Abdomen - Adult 550 Paediatric 50 Renal 300 Female Pelvis 150 Obstetrics - First 150 Second 250 Third 150 Other Areas 400 Scrotum 30 Neonatal abdomen 30 Thyroid gland 30 Transvaginal 30 Breast 30 Special procedures 30 e.g. biopsy/theatre/trauma Musculoskeletal 30 Doppler studies 50 Paediatrics 30 Male pelvis 30 Neonatal neuro 30 Miscellaneous 50

140 Ultrasound Physics - Safety Considerations

141 Thermal Index -Definition Thermal Index is defined d as the ratio of the in situ acoustic power to the acoustic power required to raise tissue temperature by 1 degree Centigrade

142 Thermal Index The thermal index gives a relative guide to how severe any heating effects are likely to be with the equipment settings in use. It accounts for changes such as pulse length but cannot have any knowledge of the tissue being scanned (e.g. a bone-brain b interface)

143 Mechanical Index - Definition Mechanical Index is defined as a ratio related to the predicted cavitation threshold

144 Mechanical Index The mechanical index gives a relative indication of the likelihood of producing mechanical bioeffects. It cannot have any knowledge of the sensitivity to damage of the tissue being scanned.

145 Ultrasound Physics y Contrast Agents

146 Contrast Agents Contrast material consists essentially of gas bubbles bbl The size of the bubbles is well controlled The gas is encased in a skin Gas is highly reflective

147

148 Contrast Agents The bubbles can resonate The frequency at which they resonate depends on their size Bubbles decay Bubbles get trapped The bubbles can burst

149 Contrast Agent Backscatter

150 Contrast Agents With bubbles of contrast material, harmonics can arise from resonance Often a relatively high level of harmonics will be returned Transmit a 1MHz pulse can produces harmonics in return Transmit at 1MHz, receive at 2 MHz This can add to contrast between blood and tissue

151 Contrast Agents Reflect The bubbles reflect ultrasound The frequency at which they reflect best depend on their size Bubbles of one size reflect strongly at one frequency A range of bubble sizes reflect a range of frequencies

152 Safety and Contrast Agents We have deliberately introduced gas bubbles bbl into the system These bubbles burst, sometimes deliberately, at higher powers Watch your MI (Mechanical Index) You have the same responsibilities as with normal imaging i (ALARA)

153