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Doppler Ultrasonography And Advancements in USG


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Recent advancements in ultrasound technology and principle behind doppler ultrasound is discussed here.

Published in: Health & Medicine
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Doppler Ultrasonography And Advancements in USG

  2. 2. INTRODUCTION  Doppler effect: Change in the perceived frequency of sound emitted by a moving source.  First described by Christian Doppler in 1843.  Same effect that causes a siren on a fire truck to sound high pitched as the truck approaches the listener (the wavelength is compressed) and a shift to a lower pitch sound as it passes by and continues on (the wavelength is expanded). 2
  3. 3.  In ultrasound, the Doppler effect used to measure blood flow velocity.  Ultrasound reflected from red blood cells will change in frequency according to the blood flow velocity.  When direction of blood flow is towards the transducer, the echoes from blood reflected back to the transducer will have a higher frequency than the one emitted from the transducer.  When the direction is away from the transducer, the echoes will have a lower frequency than those emitted.  The difference in frequency between transmitted and received echoes is called the Doppler frequency shift, and this shift in frequency is proportional to the blood flow velocity.
  4. 4. 4
  5. 5. DOPPLER FREQUENCY SHIFT  The Doppler shift is the difference between the incident frequency and reflected frequency.  When the reflector is moving directly away from or toward the source of sound, the Doppler frequency shift (fd) is calculated as where fI is the frequency of the sound incident on the reflector and fr is the frequency of the reflected sound.  Thus, the Doppler shift is proportional to the velocity of the blood cells. 5
  6. 6.  When the sound waves and blood cells are not moving in parallel directions, the equation must be modified to account for less Doppler shift.  The angle between the direction of blood flow and the direction of the sound is called the Doppler angle.  This reduces the frequency shift in proportion to the cosine of this angle. If this angle is known then the flow velocity can be calculated. 6
  7. 7. FACTORS AFFECTING DOPPLER SHIFT 1. Doppler angle 2. Transducer frequency 3. Scattering of US by blood 4. Blood flow characteristic
  8. 8. 1. DOPPLER ANGLE  Also known as the angle of insonation.  Estimated by a process known as angle correction, which involves aligning an indicator on the duplex image along the longitudinal axis of the vessel.  Few considerations that affect the performance of a Doppler examination:  The cosine of 90° is zero, so if the ultrasound beam is perpendicular to the direction of blood flow, there will be no Doppler shift and it will appear as if there is no flow in the vessel  The angle of insonation should also be less than 60° at all times, since the cosine function has a steeper curve above this angle, and errors in angle correction will be magnified
  9. 9. ANGLE EFFECTS  Maximum Doppler shift at 0 degrees minimum at 90 degrees – proportional to the Cosine of the angle between the beam and direction of travel. Direction of movement
  10. 10. Effect of the Doppler angle in the sonogram. Higher-frequency Doppler signal is obtained if the beam is aligned more to the direction of flow. In the diagram, beam (A) is more aligned than (B) and produces higher-frequency Doppler signals. The beam/flow angle at (C) is almost 90° and there is a very poor Doppler signal. The flow at (D) is away from the beam and there is a negative signal.
  11. 11. 11 If the beam direction is perpendicular to the direction of flow, the Doppler frequency is ZERO!
  12. 12. 2. TRANSDUCER FREQUENCY  Increase in transducer frequency causes increased doppler frequency shift.  At frequencies between 2-10 MHz, doppler shift comparatively small & in audible range (shift from 0- 10 KHz) for most physiological motion (for velocity range from 0-100 cm/s).  Change of frequency → measurable → movement of reflector towards/away from transducer.  Depending on ↑ or ↓ in frequency → direction of movement.
  13. 13. c cosθv2f F o D  13
  14. 14. 3. SCATTERING OF US BY BLOOD  Smooth wall of blood vessel → specular reflection → strong echoes.  Us wavelength > size of RBCs → scattering of wave in all directions (Rayleigh-Tyndall scattering). Size of echo from blood → ↓.  Intensity of scattered us ↑ with 4th power of frequency as images of vessel lumen differ significantly with instruments of different frequency.
  15. 15. 4. CHARACTERISTICS OF BLOOD FLOW  Blood (viscous medium) → wall exerts drag effect on moving blood → slow movement near the wall than center.  Non pulsatile flow with low velocity → parabolic velocity profile → laminar flow.  Fast /accelerated flow → same velocity all over → plug flow.  Major artery → plug flow during systole & laminar flow during diastole. Venous flow → laminar.
  16. 16. FLOW PATTERNS  Laminar flow  Highest in center  Zero at wall  Turbulent flow  Larger distribution of velocities 16
  17. 17. BASIC EQUIPMENT  Same probe can be used for Doppler Scanning.  High Q transducer material (transducer will produce narrow range of frequency)  Air backing can be used for maximal sound reflection from backing layer.  Lower frequency probe. Usually 3.5MHZ probe. 17
  18. 18. TYPES OF DOPPLER OPERATION 1. Continuous wave doppler 2. Pulsed wave doppler 3. Duplex scanning 4. Color flow doppler imaging 5. Power doppler 18
  19. 19. 1. CONTINUOUS WAVE DOPPLER  Simplest and least expensive device for measuring blood velocity  Two transducers required, one transmitting the incident ultrasound and other detecting the resultant continuous echoes.  Two elements in one transducer at inclination.  High Q transducer with no backing block  Suffers from depth selectivity with accuracy affected by object motion within the beam path.  Multiple overlying vessels will result in superimposition, making it difficult to distinguish a specific Doppler signal. 19
  20. 20. 20
  21. 21. Advantages:  Simple, Good quality  low noise signal,  No aliasing. Disadvantages:  Lacks depth resolution,  Confusing superimposition, Application: Superficial structures,e.g carotid artery, limb arteries. 21
  22. 22. 2. PULSED WAVE DOPPLER  Combines the velocity determination of continuous wave Doppler systems and the range discrimination of pulse- echo imaging.  Transducer is used in a pulse-echo format, similar to imaging.  The SPL is longer (a minimum of 5 cycles per pulse up to 25 cycles per pulse) to provide a higher Q factor and improve the measurement accuracy of the frequency shift (although at the expense of axial resolution).  Depth selection is achieved with an electronic time gate circuit to reject all echo signals except those falling within the gate window, as determined by the operator. 22
  23. 23.  Each Doppler pulse does not contain enough information to completely determine the Doppler shift, but only a sample of the shifted frequencies measured as a phase change.  Stationary objects within the sample volume do not generate a phase change in the returning echo when compared to the oscillator phase, but a moving object does.  Repeated echoes from the active gate are analyzed in the sample/hold circuit, and a Doppler signal is gradually built up. 23
  24. 24. 24
  25. 25. 25
  26. 26.  Depth (range) controlled by time between pulse transmission & gating on. Axial length ( thickness) determined by time the gate is open.  Width of sampling volume (gated Doppler acquisition area) → width of Doppler beam (pear shaped).  7 μs for ultrasound to travel 1 cm (average soft tissue)  PRF determines the maxm. depth for Doppler shift. So ↑PRF → ↓depth.  Doppler shift detection in pulsed Doppler = 1/2 of PRF.  If Doppler shift frequency > detectable frequency in deep vessels→ aliasing (incorrect shift).
  27. 27. 27
  28. 28.  The discrete measurements acquired at the PRF produce the synthesized Doppler signal.  According to Nyquist sampling theory, a signal can be reconstructed unambiguously as long as the true frequency (e.g., the Doppler shift) is less than half the sampling rate.  Thus, the PRF must be at least twice the maximal Doppler frequency shift encountered in the measurement.  For Doppler shift frequencies exceeding one-half the PRF, aliasing will occur, causing a potentially significant error in the velocity estimation of the blood.  Thus, a 1.6-kHz Doppler shift requires a minimum PRF of 2 *1.6 kHz =3.2 kHz.  One cannot simply increase the PRF to arbitrarily high values, because of echo transit time and possible echo ambiguity. 28
  29. 29. Aliasing of the spectral Doppler display is characterized by "wraparound“ of the highest velocities to the opposite direction when the sampling (PRF) is inadequate. Right: Without changing the overall velocity range, the spectral baseline is shifted to incorporate higher forward velocity and less reverse velocity to avoid aliasing. The maximum, average, and minimum spectral Doppler display values allow quantitative determination of clinically relevant information such as pulsatility index and resistive index.
  30. 30. Advantages:  Identification of source.  Small signal crystal transducer (cardiology).  Blood flow signal from ranges gate Disadvantages:  Quality may suffer d/t background noise (switching).  Aliasing.
  31. 31. 3. DUPLEX SCANNING  Combination of 2D B-mode imaging and pulsed Doppler data acquisition.  Operates in the imaging mode and creates a real-time image.  The Doppler gate is positioned over the vessel of interest with size (length and width) appropriate for evaluation of blood velocity, and at an orientation (angle with respect to the interrogating US beam) that represents the Doppler angle.  When switched to Doppler mode, the scanner electronics determines the proper timing to extract data only from within the user-defined gate. 31
  32. 32.  Most often, electronic array transducers switch between a group of transducers used to create a B- mode image and one or more transducers used for the Doppler information.  The duplex system allows estimation of the blood velocity directly from the Doppler shift frequency, since the velocity of sound and the transducer frequency are known, while the Doppler angle can be estimated from the B-mode image and input into the scanner computer for calculation.  Once the velocity is known, flow (in units of cm3/s) is estimated as the product of the vessel’s cross- sectional area (cm2) times the velocity (cm/s). 32
  33. 33.  Errors in the flow volume may occur. The vessel axis might not lie totally within the scanned plane, the vessel might be curved, or flow might be altered from the perceived direction.  The beam-vessel angle (Doppler angle) could be in error, which is much more problematic for very large angles, particularly those greater than 60 degrees  The Doppler gate (sample area) could be mispositioned or of inappropriate size, such that the velocities are an overestimate (gate area too small) or underestimate (gate area too large) of the average velocity.  Noncircular cross sections will cause errors in the area estimate, and therefore errors in the flow volume. 33
  34. 34.  Advantages:  Selection of site & sample volume for Doppler.  No overlap of information from other vessels/structures.  Measurement of vessel angle in 2-3 sec.  Disadvantages:  Angle restriction if same transducer for imaging & Doppler.  Flow information from one site in image
  35. 35. 4. COLOUR DOPPLER FLOW IMAGING.  Provides a 2D visual display of moving blood in the vasculature which is superimposed on the conventional gray scale image.  Imaging of flow through entire real-time image.  Allowing visualization of blood vessels and their flow characteristics + image of tissue surrounding.  Multiple gate in receiving transducer circuit each gate is controlled by time delay circuit.  These gate arrange close to each other so they can simultaneously sample Doppler shift information across entire lumen of large vessels.  Multiple gate+ real time 35
  36. 36.  Velocities and directions are determined for multiple positions within a subarea of the image, and then color encoded (e.g., shades of red for blood moving toward the transducer, and shades of blue for blood moving away from the transducer). 36
  37. 37. Advantages:  Vessels small on gray scale image can be seen (presence of flow detected with relative ease)  Differentiation between and other structure simplified.  Improved definition of lumen.  Improved measurement of stenosis. Limitations:  Compute mean velocity and image not corrected for doppler angle so it is qualitative .  PRF limited  Detailed study of vessels not possible.  May obscure vascular pathology due improper adjustment.  Other motion like peristalsis, cardiac motion may represented as color which may obscure signal.  High cost.  spatial resolution poorer than gray scale image, noise of slowly moving structure can affect the small echoes returning from the moving blood cell. 37
  38. 38. 5. POWER DOPPLER  A signal processing method that relies on the total strength of the Doppler signal (amplitude) and ignores directional (phase) information.  The power (also known as energy) mode of signal acquisition is dependent on the amplitude of all Doppler signals, regardless of the frequency shift.  This dramatically improves the sensitivity to motion (e.g., slow blood flow) at the expense of directional and quantitative flow information.  Does not give the flow direction or the velocity, but is more sensitive to detecting small amounts of flow, as noise in the image is reduced.  Aliasing artifacts do not occur in Power Doppler  3-dimensional Power Doppler is also possible 38
  39. 39.  Power Doppler produces images that have more sensitivity to motion and are not affected by the Doppler angle (largely nondirectional),  Aliasing is not a problem as only the strength of the frequency shifted signals are analyzed (and not the phase).  Greater sensitivity allows detection and interpretation of very subtle and slow blood flow.  On the other hand, frame rates tend to be slower for the power Doppler imaging mode, and a significant amount of "flash artifacts" occur, which are related to color signals arising from moving tissues, patient motion, or transducer motion. 39
  40. 40. Color flow (top) and power Doppler (bottom) images of the same phantom under the same conditions. The directions of flow toward and away from the transducer are seen in the color flow image (top). The power Doppler image (bottom) displays only the intensity of the Doppler shift
  41. 41. Advantages:  No aliasing  Angle independent  Increased sensitivity to detect low- velocity flow  Useful in imaging tortuous vessels  Increases accuracy of grading stenosis Disadvantages:  Do not provide velocity of flow  Donot provide direction of flow  Very motion sensitive( poor temporal resoultion) 41
  42. 42. SPECTRAL ANALYSIS  The Doppler signal is typically represented by a spectrum of frequencies resulting from a range of velocities contained within the sampling gate at a specific point in time.  With the pulsatile nature of blood flow, the spectral characteristics vary with time.  Interpretation of the frequency shifts and direction of blood flow is accomplished with the fast Fourier transform, which mathematically analyzes the detected signals and generates amplitude versus frequency distribution profile known as the Doppler spectrum
  43. 43.  The intensity of the Doppler signal at a particular frequency and moment in time is displayed as the brightness at that point on the display.  Velocities in one direction are displayed as positive values along the vertical axis, and velocities in the other direction are displayed as negative values. As new data arrive, the information is updated and scrolled from left to right.
  44. 44. Pulsed-wave US spectrum displays the maximum, minimum, and average calculated blood flow velocities. The pulsatility index, resistivity index, and systolic-to-diastolic ratio are calculated from these values
  45. 45. The PI will increase as flow is impeded by a stenosis. Care must be taken when the PI is used. Proximal and distal stenoses as well as natural flow resistance from the vascular bed may affect PI measurement. The RI is easier to calculate and is used to evaluate a number of physiologic conditions. Both of these indexes are used to assess the resistance to flow in the vascular system
  47. 47. HARMONIC IMAGING  Harmonic frequencies are integral multiples of the frequencies contained in an ultrasound pulse (a f MHz, upon interaction with a medium, will contain high-frequency harmonics of 2f,3f, 4f and so on, etc.)  These higher frequencies arise through the vibration of encapsulated gas bubbles used as ultrasound contrast agents or with the nonlinear propagation of the ultrasound as it travels through tissues.  Harmonic imaging enhances contrast agent imaging by using a low-frequency incident pulse and tuning the receiver (using a multi frequency transducer) to higher-frequency harmonics.  For harmonic imaging, longer spatial pulse lengths are often used to achieve a higher transducer Q factor.  Improve the sensitivity to the ultrasound contrast agent. 47
  48. 48.  Body wall region close to transducer →less harmonics.  At depth → less harmonics (increasing attenuation of higher order harmonics). Strongest harmonics at greatest pressure amplitudes (near pulse center).  Resultant effect → at near surface good rejection of artifacts & clutter from multiple pulse reflections.  Most useful in technically difficult patients i. e. thick & complicated body wall structures.  Conventional imaging → hazy image (distortion of the transmitted beam by shallow surface layers or to reverberations between the skin and ribs). The distortions and reverberations consist entirely of us energy at the fundamental frequency.
  49. 49.  Peculiarities of harmonic image:  Increase signal-to-noise ratio with better interpretation of image.  Superior lateral & elevational resolution as compared to the fundamental images.  Removal of clutter and haze with more clear and defined image due to side-lobe level reduction in the second harmonic beam. Harmonic beams are narrower with lower side-lobe levels than fundamental ones.
  50. 50. 51
  51. 51. EXTENDED FOV IMAGING  The transducer is slowly translated laterally across the large anatomic region of interest.  During this motion, multiple images are acquired from many transducer positions.  The proper relative positions of the multiple images are determined in the regions of overlap between successive images.  The registered image data are accumulated in a large image buffer and then combined to form the complete large FOV image  Can be used to view whole course of vessels. 52
  52. 52. 53
  53. 53. CODED PULSE EXCITATION  Short, highly focused ultrasound pulses that optimize spatial resolution are generally obtained at higher frequencies.  Higher frequency-more attenuation.  Coded ultrasound pulses are a potential means of overcoming this limitation, providing good penetration at the higher frequencies necessary for high spatial resolution.  The coded pulses are produced with a very specific, characteristic shape, and the resulting echoes will have a similar shape.  These pulse shape locations may be determined with a tight spatial tolerance and are assumed to correspond to the locations of reflective structures in the body.  The end result is an image with good echo signal and good spatial resolution at large depths. 54
  54. 54. Conventional B-mode and coded pulse US images of the liver. The spatial resolution of the coded pulse image is very comparable with that of the 13-MHz conventional image . However, the useful imaging depth is about 7.5 cm for the coded pulse image compared with only about 2.8 cm for the conventional image.
  55. 55. 3D US  3D ultrasound imaging acquires 2D tomographic image data in a series of individual B-scans of a volume of tissue.  Forming the 3D dataset requires location of each individual 2D image using known acquisition geometries.  3D ultrasound works by taking thousand of slices in a series (called volume of echoes) the volume are then digitally stored and shaded to produce 3D images that look more life like. 56
  56. 56. 57 Volume sampling can be achieved in several ways with a transducer array: (1) linear translation, (2) freeform motion with external localizers to a reference position, (3) rocking motion, and (4) rotation of the scan
  57. 57. 4D ULTRASOUND  4D Ultrasound represents the difference between video and a still photograph.  Through this technology, three-dimensional image is continuously updated, providing a "live action" view.  The entirely digital platform and very fast processors cope with the large volume of data required to reconstruct 3D images again and again, giving the impression of a moving image.  4D Ultrasound takes multiple 2-dimensional ultrasound images, creates a 3-dimensional image and adds the element of time to the process. The result: live action images of any internal anatomy. 58
  58. 58. Chewing Sleepy Firstwhinge Smiling 59
  59. 59. ELASTOGRAPHY  Stiffness and compliance of different tissue can be assessed .  A set of digital RF echo data from ROI is obtained and the tissue is then compressed usually by transducer and further set of RF echo data obtained.  This is then cross correlated with initial data and any changes in arrival time of pulses from equivalent volume of tissue can be obtained.  An estimate of local tissue strain can be asssessed and from this the degree of tissue stiffness can be assessed.  Tissue stiffness is described by the Young modulus expressed in kilopascals (E = 3γC2).  The elastography methods are based on a common approach: measurement of deformation induced in a tissue by a force. 60
  60. 60.  Elastography is therefore an application, which produces the force coupled with a measurement system for the deformities caused by the force.  There are several types of forces or applications:  static compression induced externally by manual compression or internally by organ motion (heart, vessel, breathing);  dynamic compression induced with a continuous vibration at a given frequency;  impulse compression (transient vibration): induced externally by a transient mechanical impulse (FibroScan®) or internally by an ultrasound impulse (ARFI, SWE), both compression types producing shear waves. 61
  61. 61. ELASTOGRAPHY TECHNIQUES  Impulse elastography:  Uses an external mechanical device (FibroScan) or an internal acoustic radiation force (ARFI and SWE) to induce shear waves in the tissue to be explored.  Shear wave propagation velocity (Vs) is then measured in m/s using ultrasound imaging in the tissue being studied in order to assess its stiffness.  Uni-dimensional transient elastography: FibroScan  Developed around 10 years ago  Based on shear wave, which is generated by an external mechanical impulse and whose speed is measured by an ultrasound one-dimensional probe.  The one-dimensional probe (3.5 MHz) is mounted along the axis of an electro-dynamic transducer (vibrator). 62
  62. 62. 63 • The FibroScan® estimates liver stiffness by measuring the velocity of elastic shear waves in the liver parenchyma generated by the mechanical push. • The propagation velocity is directly related to the stiffness of the medium, defined by the Young modulus. • Stiff tissues exhibit higher shear wave velocities than soft tissues.
  63. 63.  Acoustic Radiation Force Impulse (ARFI) Imaging mode elastography:  A new method for quantifying mechanical properties of tissue, without manual compression, by measuring the shear wave velocity induced by acoustic radiation and propagating in the tissue.  Developed by Siemens.  Provides a single uni-dimensional measurement of tissue elasticity like the FibroScan, although the measurement area can be positioned on a two-dimensional Bmode image.  The region is a 1 × 0.5 cm rectangular, which can be freely moved in the two-dimensional Bmode image to a maximum depth of 8 cm from the skin plane.  The measurement is expressed in m/s, expressing shear wave speed, travelling perpendicular to the shear wave source.  The technique has been implemented on the ultrasound probe designed for abdominal imaging. 64
  64. 64. 65
  65. 65.  Shear Wave Elastography (SWE):  Was introduced in 2005.  Relies on the measurement of the shear wave propagation speed in soft tissue; like ARFI, does not require an external vibrator to generate the shear wave. Based on the generation of a radiation force in the tissue to create the shear wave.  The ultrasound probe produces a very localized radiation force deep in the tissue of interest that induces a shear wave, which then propagates from this focal point.  Several focal points are then generated almost simultaneously, in a line perpendicular to the surface of the patient's skin.  This creates a conical shear wave front, which sweeps the image plane, on both sides of the focal point. The progression of the shear wave is captured by the very rapid acquisition of ultrasound images (up to 20,000 images per second), called UltraFast Imaging. 66
  66. 66. 67  A high-speed acquisition is necessary to capture the shear wave as it moves at a speed in the order of 1 to 10 m/s.  Comparison of two consecutive ultrasound images allows the measurement of displacements induced by the shear wave and creates a “movie” showing the propagation of the shear wave whose local speed is intrinsically linked to elasticity.  The propagation speed of the shear wave is then estimated from the movie that is created and a two-dimensional color map is displayed, for which each color codes either the shear wave speed in meters per second (m/s), or the elasticity of the medium in kilopascals (kPa).  This color map is accompanied by an anatomic reference gray scale (or B-mode) image. This quantitative imaging technique is a real-time imaging mode.
  67. 67.  Static elastography:  Developed by Hitachi.  The operator manually applies gentle pressure with the ultrasound probe in order to induce in the underlying tissues a deformation.  Only the deformed tissue subject to the manual compression is measured, rather than a direct measurement of elasticity. The deformation is considered to be inversely proportional to elasticity.  A color map of tissue elasticity is obtained: this is a qualitative approach. In more recent systems, the deformation of the liver parenchyma as a result of vascular beating or respiration alone has also been used. 68
  68. 68. TRANSDUCER DEVELOPMENTS 69  Multielement array transducers:  Possible for US beam focus and stear electronically, different group of elements can be used for different tasks e.g pulse doppler.  Beam can be manipulated in two direction thus improving resolution.  Now days 2D array are available possible for volume scan(3D and 4D possible)
  69. 69.  Small sized transducers:  Can be mounted on catheter to pass into coronary arteries.  Specialized transducer like trans rectal and TVS(endo cavitary). 70
  70. 70. ECHO-ENHANCING AGENT  Most agents comprised of encapsulated microbubbles of 3 to 5 micrometer diameter containing air, nitrogen, or insoluble gaseous compounds such as perfluorocarbons.  Encapsulation materials, such as human albumin, provide a container for the gas to maintain stability for a reasonable time in the vasculature after injection.  Response of US to microbubble depends on acoustic power.  At low power (MI -0.1-0.3) they reflect and scatter Us.  At moderate power (MI abt 0.5) they resonate frequency of US.  At high power (MI-0.7-0.8) microbubble oscillate nonlinear fashion and production of harmonic signal which are specific to agent .  More than this power micro bubble disrupted and brust. 71
  71. 71. SPATIAL COMPOUND IMAGING  Aims to improve image quality by averaging several coplanar ultrasound frames into a single image.  Mechanism:  Repeated us beam steering in same tissue by an array transducer using parallel beams oriented in different directions→ echoes from multiple aquisition of same tissue → averaged → single composite image.  Multiple beams for same tissue (vs one beam in conventional B-mode imaging) hence more time for data acquisition & reduced frame rate.
  72. 72. set of us beam lines for conventional imaging Two additional set of us beam lines for spatial compound imaging. Upto nine sets are used.
  73. 73.  Advantages of compound imaging:  greater spatial details of lesions and tissue in the FOV with decreased image graininess (due to speckle). Speckle varies with beam line direction, averaging echoes from different directions average out and smooth the speckle with less grainy image.  Improved contrast & margin definition.  Reduction of enhancement & shadowing artifacts. Conventional image of breast mass Spatial compound imaging of breast mass
  74. 74. ELECTRONIC SECTION FOCUSING  Electronic focusing of US pulse in the lateral direction by transducer arrays produces great benefits compared with mechanical focusing.  Most transducers in current clinical use still employ mechanical focusing (curved piezoelectric elements or acoustic lenses) in section thickness direction → resultant highly variable section thickness → great difficulty in accurately visualizing small structures.  Electronic focusing is employed with more than one-dimensional single row of piezoelectric elements {referred to as 1.25D, 1.5D, 1.75D, or full 2D (two-dimensional) arrays} as per the number of element rows and the level of independence with which the individual rows may be excited.  Full-size, filled 2D array transducers are not yet commercially available, only 1.25D arrays are available.  The greater the number of rows, the greater the ability for full electronic focusing which provide multiple, user-selectable elevational focusing and more uniform section profiles.
  75. 75. US images of a phantom with small anechoic spheres in conventional B mode & electronic focusing in section thickness direction.
  76. 76. MINIATURIZATION  Modern full-size US scanners are relatively portable and inexpensive.  The general trend toward hardware miniaturization and the use of dedicated integrated circuitry is making possible even smaller and less expensive US scanners, with few (if any) compromises in general image quality and available features.  This trend toward lower cost and smaller systems has great potential to radically alter the overall role of US in medicine.
  77. 77. Commercially available small US scanners from two vendors. The left scanners are 13.3 in (33.8 cm) tall and weigh 5.7 lb (2.6 kg), whereas the right scanners weigh about 3 lb (1.4 kg). The dedicated electronics module attached to the transducer cable weighs 10 oz (0.3 kg) and is 7.6 in (19.3 cm) long.
  78. 78. FUSION TECHNOLOGY  An emerging technique in the field of abdominal imaging with translation possibilities to neuroradiology.  Involves co-registered display of live ultrasound with a reference series from another modality, such as CT, MRI or PET.  As the ultrasound exam is performed the fusion system continuously generates reformatted planes from the reference series matching the oblique imaging planes of the ultrasound transducer.  The reformatted planes are displayed either as an overlay or side-by-side with the live ultrasound.  This display enhances interpretation of ultrasound by enabling a direct comparison with the reference images from the same view angle. 79
  79. 79.  Fusion imaging makes use of a tracking system to localize ultrasound transducers and other devices relative to the patient.  Optical and electromagnetic systems are available, the latter being most commonly used.  Various software tools are also used to bring the reference series into alignment with the tracking system for fusion display.  Tracking sensors are also incorporated into some interventional devices such as introducers and ablation needles, enabling the display of needle location as an overlay on live ultrasound image.  For the fusion imaging technique, a variety of tracking methods are available, including optical, image-based, and EM tracking. 80
  80. 80. 81 Fusion imaging and contrast- enhanced ultrasonography (US)- guided radiofrequency ablation (RFA) of a 1-cm hepatocellular carcinoma (HCC) in a patient with a hepatitis B virus carrier
  81. 81. 82
  82. 82. WIRELESS TRANSDUCER  They feature proprietary ultra-wideband (UWB) wireless radio to help ensure images are transferred at high speeds, with full data integrity and free from interfering with other medical equipment—and all without cables.  These breakthrough cable-free innovations help provide uninterrupted, hassle-free, real-time imaging that allows you to scan in the position that is comfortable for you and safe for your patient. 83
  83. 83. SMARTPHONE ULTRASOUND  Inexpensive, which will lead to even wider use, especially in rural and third-world areas  Widespread use in Emergency Rooms, and in the surgical and medical wards, as well as in office practice  A great start for diagnosing all types of medical problems, as a screening device, such as vascular problems, gallstones, kidney stones, abdominal masses, and other problems.  The model shown is a Mobisante which is the world’s first smartphone-based ultrasound imaging system, the MobiUS SP1 ultrasound system. 84
  84. 84. CONCLUSIONS  Modern US equipments are based on many of the same fundamental principles employed in the initial devices used for human imaging over 50 years ago.  USG is relatively inexpensive, portable, safe, and real-time in nature, all of which make it one of the most widely used imaging modalities in medicine.  B-mode US continues to experience very significant evolution in capability with the US science, technology, and applications are expanding at a brisk pace and are far from mature with exciting developments in future.
  85. 85. FIND MORE AT:  The essential physics of medical imaging; Bushberg JT, Seibert JA  AAPM/RSNA Physics Tutorial for Residents: Topics in US  Ultrasound elastography in liver; Frulio N, Trilaud H.  Fusion imaging of real-time ultrasonography with CT or MRI; Lee MW  86
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