Ultrasound. Principles of Medical Imaging. Contents. Prof. Dr. Philippe Cattin. MIAC, University of Basel. Oct 17th, 2016

Transcription

1 Ultrasound Principles of Medical Imaging Prof. Dr. Philippe Cattin MIAC, University of Basel Contents Abstract 1 Image Generation Echography A-Mode B-Mode M-Mode 2.5D Ultrasound 3D Ultrasound 4D Ultrasound 2 Ultrasound Transducers Ultrasound Transducers 2.1 Single Element Transducer Transducer Design Piezoeletric Materials Impedance Matching Impedance Matching (2) Impedance Matching (3) Impedance Matching (4) Impedance Matching (5) Pulse Geometry Pulse Repetition Frequency Contents of :35 2 of :35

2 Images of Transducers Beam Artefacts 2.2 Linear Sequenced Transducer Beam Artefacts 49 Linear Sequenced Transducer 25 Beam Artefacts (2) 50 Linear Sequenced Transducer (2) Linear Sequenced Transducer (3) Linear Sequenced Transducer (4) Linear Sequenced Transducer: Examples 2.3 Phased Array Transducer Phased Array Principle Phased Array Example: Mitral Valve Phased Array Example: IVUS 2.4 Annular Array Transducer 3 The Ultrasonic Field Annular Array Transducer The Ultrasonic Field The Ultrasonic Field (2) Axial Resolution Resolution Multiple Echo Artefacts Multiple Echo Artefacts Multiple Echo Artefacts (2) Multiple Echo Artefacts (3) 5.3 Velocity Artefacts Velocity Artefacts Velocity Artefacts (2) 5.4 Attenuation Artefact Attenuation Artefacts Attenuation Artefacts (2) 5.5 Speckle Speckle Properties of Speckle Speckle Tracking Speckle Tracking (2) Doppler Imaging Doppler Imaging 42 Basics of Doppler Shift 43 Continuous vs Pulsed Doppler 44 Continuous vs Pulsed Doppler (2) 45 Duplex and Colour Velocity Imaging 46 Principles 5 Imaging of Medical Artefacts Imaging 3 of :35 4 of :35

3 Abstract (2) Image Generation Echography Imaging Principle Emission of Ultrasound waves Reflection on tissue boundaries Imaging Frequency (4) depending on application for Intra-Vascular US (IVUS) Fig. 5.1: Principle of Ultrasound imaging 5 of :35 6 of :35

4 Image Generation Image Generation A-Mode (5) B-Mode (6) Fig. 5.2: A-mode Ultrasound The B-Mode or Brightness-Mode encodes the reflected echo strength as grey-values (corrected for the image depth). Interfaces between different tissues are seen as bright regions The B-Mode picture shows a section (slice) through the body who's image depth depends on transducer parameters (frequency, focusing,...) The image display is constructed from scan lines (depends on the transducer design) Fig. 5.3: B-mode Ultrasound showing the four chambers of the human heart 7 of :35 8 of :35

5 Image Generation M-Mode (7) If the frame rate is high enough M-Mode movies can be produced. Fig. 5.4: M-Mode 4-chamber view of the heart 9 of :35 10 of :35

6 Image Generation 2.5D Ultrasound (8) A recent development in Image-Guided Therapy (IGT) is the 2.5D Ultrasound it requires a tracked 2D Ultrasound probe, that is manually pivoted or translated over the patient. The captured 2D slices are then assembled into a sparse 3D data set thus 2.5D. Fig. 5.5: Principle of 2.5D Ultrasound acquisitions 11 of :35 12 of :35

7 Image Generation Image Generation 3D Ultrasound (9) 4D Ultrasound (10) Recent developments in computation equipment allows the visualisation of 3D image sequences in real-time. Recent developments in computation equipment even allow to visualise 4D movie sequences. Fig. 5.6: Surface renderings of 3D Ultrasound data sets Fig. 5.7: 4-dimensional movie of a fetus (week 31) 13 of :35 14 of :35

8 Ultrasound Transducers Single Element Transducer Ultrasound Transducers (12) Transducer Design (14) The mechanically scanned Ultrasound probes have almost entirely been replaced by electronically scanned multielement array transducers. There exist two basic types of electronically scanned transducers: Sequenced (switched) transducer arrays linear or curvilinear Phased transducer arrays linear annular The device that converts the electrical energy into sound waves is called Transducer. Today's transducers use piezoelectric crystals such as ceramic lead zirconate titanate ( ) to convert the electric into mechanical energy. Fig. 5.9: Basic design of a single transducer Ultrasound head Fig. 5.8: Example of an US transducer 15 of :35 16 of :35

9 Single Element Transducer Single Element Transducer Piezoeletric Materials (15) Impedance Matching (16) Piezoelectric materials have two nice properties: 1. Piezoelectric materials change their shape upon the application of an electric field as the orientation of the dipoles changes. 2. Conversely, if a mechanical forces is applied to the cristal a the electric field is changed producing a small voltage signal. The piezoelectric crystals thus function as the transmitter as well as the receiver! Fig. 5.10: Basic design of a single transducer Ultrasound head There is a large impedance difference ( ) between the piezoelectric cristal and the skin of the patient only a minor part of the energy penetrates the patient's skin. Example: Fig. 5.11: Large impedance difference between the transducer cristal and the patients skin (gel) For a transducer impedance of and a tissue acoustic impedance of the amount of reflected sound energy is given by Eq 3.23 [FundamentalsOfUltrasound.html#(41)] and yields a reflection ratio of, thus roughly of the acoustic energy is reflected. 17 of :35 18 of :35

10 Single Element Transducer Single Element Transducer Impedance Matching (2)(17) Impedance Matching (3)(18) The impedance adaption is solved by attaching a transmission layer or matching layer to the piezoelectric crystal face quarter-wave matching What is the optimal impedance and layer thickness to get the maximum energy into a patients body? Fig. 5.12: US head with a quarter wavelength matching layer for impedance adaption Principle of the Quarter Wavelength Layer The matching layer is optimal when (5.1) From energy preservation it directly follows (5.2) Fig. 5.13: Reflectance model of the quarter wavelength layer The gel coupling medium between the skin and matching layer avoids further signal loss by removing air bubbles. We know that both and are non-zero. Eq 5.1 can thus only be satisfied if 1. they have a phase-shift of and 2. both amplitudes are equal. they then cancel out each other thanks to destructive interference. 19 of :35 20 of :35

11 Single Element Transducer Single Element Transducer Impedance Matching (4)(19) Requirement (1) is straight forward and valid as long as the layer thickness satisfies Impedance Matching (5)(20) Requirement (2) states and yields with Eq 3.23 [FundamentalsOfUltrasound.html#(41)] (5.3) Due to a range of frequencies in the ultrasound pulse the matching layer can never be exactly for all wavelengths less than efficiency. Multiple matching layers are sometimes used to further improve efficiency. this can be simplified to (5.5) (5.4) For our practical example (see this [@]) with and a tissue acoustic impedance of Eq 5.5 yields (5.6) as the optimal impedance for the matching layer. 21 of :35 22 of :35

12 Single Element Transducer Pulse Geometry (21) Ideally, the pulse wave would raise and fall very sharply and contain only one wavelength, but a pulse usually contains several oscillations, see Fig The pulse packet can be characterised by Fig. 5.14: A typical pulse shape The pulse wavelength Its amplitude The Spatial pulse length The Pulse duration The Pulse repetition period and the Pulse repetition frequency Fig. 5.15: Pulses at two different frequencies Example: A and a leaves a period of between pulses. The transducer is thus of the time in receive mode. 23 of :35 24 of :35

13 Single Element Transducer Single Element Transducer Pulse Repetition Frequency (22) Images of Transducers (23) The time between pulses ( must be higher than the return trip which is equivalent to twice the image depth. The maximum is thus defined by (5.7) for an image depth of and the maximum is thus. Fig. 5.16: Maximum pulse repetition frequency A typical value for in practice is. 25 of :35 26 of :35

14 Linear Sequenced Transducer Linear Sequenced Transducer The Linear sequenced array transducer consists of many (up to 128) individual transducer elements arranged in groups. As the near-field of a very narrow single element beam would be very small, groups of elements are grouped and pulsed simultaneously (usually 8 to 32 elements) wider beam with improved resolution at depth A scanning motion is obtained by shifting an element one at a time As only a small number of transducer elements are active at a time (8 to 32) the electronics is rather simple, compared to phased array designs. (25) Fig. 5.17: Commonly used linear array designs in diagnostic imaging 27 of :35 28 of :35

15 Linear Sequenced Transducer Linear Sequenced Transducer Linear Sequenced Transducer (2) (26) Linear Sequenced Transducer (3) (27) By adapting the delays (shifting the phases) of the individual elements linear steering is possible. Fig. 5.18: Phased linear steering By delaying or phasing the excitation pulses, linear arrays can be focused. It is even possible to switch between multiple focal points. The frame rate is then, however, reduced. Fig. 5.19: Beam is focused by adapting the delays Fig. 5.20: Focal point can be changed 29 of :35 30 of :35

16 Linear Sequenced Transducer Linear Sequenced Transducer Linear Sequenced Transducer (4) (28) Linear Sequenced Transducer: Examples (29) The position of the narrow section of the beam is controlled by the aperture size (number of elements in the group). The number of scan lines can be virtually doubled if two groups having different sizes are used. Fig. 5.21: Different aperture size depending on number of active elements Fig. 5.22: Thoracic diaphragm wall (Provided by GE Healthcare) Fig. 5.23: Liver image 31 of :35 32 of :35

17 Phased Array Transducer Phased Array Principle In Phased array transducers, the transmit pulses are applied to all elements via an element individual delay allowing a swiveled wavefront. A wavefront angle of requires very narrow elements of about dimensions. (31) Fewer number of elements (48 to 128) compared to linear arrays smaller footprint Transmit, receive and delay electronics for each element separately In receive mode individual element delays are introduced that enable the transducer to be direction sensitive Phased arrays allow for miniaturised probe designs tiny catheter sized ultrasound probes for intra-luminal inspection Fig. 5.24: Phased array switching can produce either planar or focused beams 33 of :35 34 of :35

18 Phased Array Transducer Phased Array Transducer Phased Array Example: Mitral Valve (32) Phased Array Example: IVUS (33) Fig. 5.27: Catheter sized Ultrasound device Fig. 5.28: Example image of a coronary artery Fig. 5.29: Fluoroscopic contrast image of the cardio-vascular tree Fig. 5.25: Phased array transducer head Fig. 5.26: Example image captured with a phased array transducer (Mitral valve stenosis) 35 of :35 36 of :35

19 Annular Array Transducer Annular Array Transducer The Annular array transducer consists of concentric transducers operated as a phased array, see Fig (35) Excellent image quality, since lateral resolution at depth can be controlled by signal phasing The overall depth of focus can be controlled by the delay between pulsing signals Can not be steered electronically mechanical wobbling Doppler imaging is not possible due to the mechanical wobbling producing interfering signals Fig. 5.30: (a) Design of an annular array transducer, (b) scan pattern achieved by the mechanical scan head Annular array transducers are used when fine detail is important such as in fetal examinations (obstetrics). 37 of :35 38 of :35

20 The Ultrasonic Field 39 of :35 40 of :35

21 The Ultrasonic Field (37) The Ultrasonic Field The shape of a single flat element transducer is split in two zones: the near-field or Fresnel zone [ /wiki/fresnel_diffraction] and the far-field or Fraunhofer zone [ /wiki/fraunhofer_diffraction], Fig. 5.31: Beam profile of a single transducer The Ultrasonic Field (2) The length of the near-field is governed by (5.8) and the divergence of the far-field by (38) see Fig (5.9) The near-field retains the width of the transducer, the beam then spreads out in the far-field decreasing the lateral resolution. where is the radius of the transducer and the wavelength. Resolution at depth is best with a wide transducer at high frequency Fig. 5.32: Examples of various US fields 41 of :35 42 of :35

22 The Ultrasonic Field The Ultrasonic Field Axial Resolution (39) Resolution (40) Axial resolution defines the ability to resolve two closely placed surfaces parallel to the direction of the beam and is determined by the spatial pulse length (SPL): the higher the frequency, the shorter the SPL the better the axial resolution BUT the higher the frequency, the lower the depth. Lateral Resolution: The Lateral Resolution depends on the beam focusing Aperture Resolution Axial Resolution: Frequency Resolution Frequency Attenuation find an optimum between resolution and penetration depth. Fig. 5.33: Lateral resolution depends on size of focal point In general: Ultrasound devices have better axial than lateral resolution! 43 of :35 44 of :35

23 Doppler Imaging Doppler Imaging For simple cases where the transducer is in line with the flowing medium (blood) the observed frequency is given by (5.10) where is the velocity of sound in the medium, the velocity of the blood and the Ultrasound frequency. (42) Fig. 5.34: Doppler effect as we know it from emergency siren Basics of Doppler Shift For a transducer with an incident angle of, the observed frequency is given by (5.11) where is the Doppler shift (frequency change) and the angle between the sound beam and the direction of the blood flow. Note that Doppler shift increases as transducer is aligned with the vessel axis ( gets smaller) Doppler shift can be positive or negative Relative Doppler shift is small for blood flow rates Doppler Imaging (43) Fig. 5.35: Basic Doppler geometry 45 of :35 46 of :35

24 Doppler Imaging Continuous vs Pulsed Doppler (44) Continuous Wave Doppler Simple design The transmitted and received signals are often electronically mixed [ /wiki/electronic_mixer] (additive) and low-pass filtered to form an audible signal Fig. 5.36: Continuous wave Doppler principle Fig. 5.37: The transmitted and received signals are mixed 47 of :35 48 of :35

25 Doppler Imaging Doppler Imaging Continuous vs Pulsed Doppler (2) (45) Duplex and Colour Velocity Imaging (46) Pulsed Wave Doppler Allows to select the tissue depth by limiting the frequency analysis to echo pulses that are received at specific time intervals after pulse generation gated Analysis at multiple depths is possible Fig. 5.38: Pulsed Doppler principle The design on the right allows to combine a pulsed Doppler with a real-time M-Mode Ultrasound. Blood flowing towards the transducer is coded red and blood flowing away is coded in blue. Fig. 5.40: Linear array with Doppler transducer used for combining flow in duplex imaging Fig. 5.39: B-Mode image with Doppler information 49 of :35 50 of :35

26 Imaging Artefacts Beam Artefacts Beam Artefacts Problem: Ultrasound processing assumes that the echos originated from within the main beam. Notes: (49) US beam has a complex 3D shape with low-energy off-axis lobes Strong reflectors outsize the main beam might generate a detectable signal will be displayed as coming from within the main beam! Best recognised in regions expected to be anechoic Fig. 5.42: Multiple copies of the same structure are caused by the side lobes Fig. 5.41: US beam with the side lobes and grating lobes 51 of :35 52 of :35

27 Beam Artefacts (2) Problem: A high echogenic object in the far-field may produce a signal strong enough to be detected. The object then appears as originating from within the main beam. Note 1: Beam width artefacts are best recognised when a structure that should be anechoic - such as the bladder - contains peripheral echos Note 2: Beam Artefacts (50) Multiple Echo Artefacts By adjusting the focal zone to the level of interest improves image quality Fig. 5.43: High echogenic objects in the far-field appear as originating from the main beam, (e) US image of a partially filled bladder that shows echoes (arrow) in the expected anechoic urine, (f) Same anatomical structure after adjusting the focal zone 53 of :35 54 of :35

28 Multiple Echo Artefacts Problem: US assumes that an echo returns to the transducer after a single reflection and that the depth of an objects is related to the time for this round trip. Notes: (52) closely spaced parallel structures they have a triangular shape Two highly reflective parallel surfaces reflect the beam forth and back multiple echoes are recorded and displayed (Reverberation artefact) Only the first reflection is properly positioned Comet tail artefact is a special form of reverberation at Fig. 5.44: Reverberation artefacts 55 of :35 56 of :35

29 Multiple Echo Artefacts Multiple Echo Artefacts Multiple Echo Artefacts (2) (53) Multiple Echo Artefacts (3) (54) Problem: Problem: Liquids trapped in a tetrahedron of air bubbles create a continuous sound wave that is transmitted back to the transducer Notes: Ring-down artefacts are displayed as a line or series of parallel bands extending posterior to a gas collections Fig. 5.45: Oscillating air bubbles Mirror artefacts are caused by structures indirectly (from the backside) hit by the Ultrasound beam. Notes: The display shows an imaginary object mirrored and equidistant from the highly reflective interface (e.g. diaphragm) The true object is always closer (proximal) to the transducer Fig. 5.46: The black arrows show the beam path producing mirror images. The crosses mark the real structure, whereas the arrow points to the mirrored structure. White marks the diaphragm. 57 of :35 58 of :35

30 Velocity Artefacts Velocity Artefacts Problem: Ultrasound imaging assumes a constant speed of sound in human tissue of. Depending on the type of tissue it can, however, travel faster or slower than this. Notes: As adjacent beams not necessarily travel through the same tissues, speed displacements can occur (56) Fig. 5.47: Speed displacement artefact caused by different tissue speeds Velocity Artefacts (2) Problem: The Ultrasound beam may undergo refraction while traveling through tissues Snell's law. The Ultrasound devices assume that the acoustic waves travel on a straight line. Notes: Structures can appear wider than they actually are Structures can be duplicated Velocity Artefacts (57) Fig. 5.48: Refraction artefact 59 of :35 60 of :35

31 Attenuation Artefact Attenuation Artefacts (2) Attenuation Artefact (60) Attenuation Artefacts Problem: As an Ultrasound beam travels through the body its energy becomes attenuated. Ultrasound equipment compensates this effect during amplification (Time gain compensation). Echoes that take longer to return are more amplified. The image thus appears more uniform. (59) Fig. 5.49: Shadowing artefact caused by a strong attenuator Similar to the strong attenuator artefact, the artefact caused by a weak attenuator brightens the image distally to the transducer. Fig. 5.50: Artefact caused by a weak attenuator 61 of :35 62 of :35

32 Speckle Speckle (62) Most tissues appear in Ultrasound as being filled with tiny scatter like structures Speckle. Speckle is a result of interference between multiple scattered echoes produced within the volume of the incident Ultrasound pulse In fact most of the signal intensity seen in Ultrasound images results from scatter interactions Fig. 5.52: Ultrasound pulse scattered off tiny reflectors Fig. 5.51: Irregular interference pattern caused by multiple scatterers somewhat randomly distributed. The speckle pattern thus appears random too 63 of :35 64 of :35

33 Speckle Speckle Properties of Speckle (63) Speckle Tracking (64) The appearance of speckle is not completely random, but follows physical principles: Speckle patterns are quasi random. Speckle patterns stay reasonably stable even with organ motion. The size of the speckle cells depends on the lateral dimension and axial pulse length see regions "a" and "b" The orientation of the speckle cells reflects the orientation of the acoustic wave, thus the direction of the beam lines see regions "c" and "d" The speckle pattern does not change with time but with varying transducer position/orientation and organ configuration Fig. 5.53: Liver Ultrasound image with varying speckle patterns Fig. 5.54: Speckle pattern comparison in the myocardium Fig. 5.55: M-mode speckle pattern in the septum of the myocardium of Fig 5.54 that nicely follows the septal motion 65 of :35 66 of :35

34 Speckle Speckle Tracking (2) (65) As the speckle patterns stay reasonably stable, simple template matching allows to accurately track regions in the image, e.g. the myocardium. Note: Fig. 5.56: Template tracking As the speckle pattern will not repeat perfectly, the search should be done from frame to frame danger of drift Reverberations will also degrade tracking performance The lower lateral resolution will result in a smeared speckle patterns tracking is less effective. If the frame rate is too low poor tracking because of large changes If the frame rate is too high poor tracking because of the reduced lateral resolution Fig. 5.57: Motion of the myocardium tracked using multiple regions If multiple regions are simultaneously tracked, deformations of organs e.g. the myocardium (heart muscle) can be measured. 67 of :35 68 of :35