Ultrasound Physics & Doppler

Transcription

1 Ultrasound Physics & Doppler Endocrine University 2018 Mark Lupo, MD, FACE, ECNU

2 Objectives Review the essential components of ultrasound physics in neck sonography Demonstrate the importance of ultrasound artifacts in clinical imaging interpretation Highlight the utility of Doppler flow as a complement to B-mode ultrasound

3 Natural Ability to see with sound

4 Imaging with Sound Create a visual image based on sound waves instead of light Mechanical versus Electromagnetic Energy Passive Sonar - Just listening Active Sonar - Acoustic Echo: Ping a target Reflection of sound waves Creation of a 2-D or 3-D image from reflected sound waves Requires technology and understanding of physics

5 Sound Waves Physics of sound Role of the propagating medium Acoustic Impedance Density, Stiffness, and Speed of Sound Reflection Occurs at Boundaries of Impedance Attenuation

6 Principles of Sound Waves Sound is the Propagation of mechanical energy through matter. No sound in a vacuum Qualities of the transmitting medium influence sound transmission Density Stiffness Acoustic Impedance Speed of Sound (SOS)

7 Sound Waves Sound waves propagate by compression and rarefaction of molecules in space. Molecules of the medium vibrate around their resting position and transfer their energy to neighboring molecules. Waves carry energy, not matter.

8 Characteristics of waves Frequency: # cycles of compression & rarefaction in a sound wave per second (Hz) Wavelength: Distance travelled in one second Amplitude, power, intensity db

9 Pulse of Sound Waves Spatial Pulse Length: Cycle length * # cycles per pulse time spent emitting echoes Determinant of axial resolution (1/2 SPL) Higher Frequency: shorter SPL Better Resolution Lower Frequency: Better Depth Penetration, Lower Resolution Duty Factor (%): Time spent generating pulse (usually 1% - while 99% of the time is spent listening for signal return)

10 Properties of Transmitting Material Speed of Sound = Propagation Speed Constant for a given tissue Not affected by frequency or wavelength Increases with stiffness Decreases with density

11 Propagation Velocity of Common Tissues

12 Properties of Transmitting Material Acoustic Impedance (AI) Reflection (echo) of sound occurs at interfaces of Acoustic Impedance (density * propagation speed) Most tissues have heterogeneous impedance If two tissues have same AI no echo If similar weak echo If very different AIs strong echo Homogeneous medium does not reflect sound Cyst (Anechoic NO echo)

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14 Reflection Reflection is the redirection of a portion of a sound beam back at the interface of tissues of unequal acoustic impedance. The greater the difference in impedance, the greater the reflection. Categories of Reflection Specular Mirror like (e.g., diaphragm) Reflection dependent on angle of incidence Diffuse (Soft Tissue e.g., neck ultrasound) Adjacent structures smaller than wavelength

15 1960s - A-mode of Thyroid A = Amplitude (strength of echo) Solid Cystic Complex Blum, 1971

16 From One Dimensional A-Mode to Two Dimensional B-Mode

17 Mid 1960s - B Mode

18 B Mode Scanner Fujimoto, 1967

19 B-Mode - Image Formation Meritt, 1998

20 B-Mode Thijs, 1971

21 Compound Arm B Mode Scanner

22 Gray Scale High Resolution 1979 and 2010 st e s h sh s t sh A e SM st V A e Scheible 1979 Legend: st = sternothyroid muscle, sh = sternohyoid muscle, arrows = posterior border of thyroid, A = carotid artery, V = jugular vein, sm = sternocleidomastoid muscle, e = esophagus

23 Key elements of an Ultrasound Scanner Sound emitter-sends sound toward the object of interest Sound detector-receives the returning reflected echoes Computer - analyze the detected signal Display shows an image of the object

24 Transducer Transducers both emit sound and receive the reflected echoes necessary for imaging 1% time emitting sound, 99% listening Linear transducer Curvilinear transducer

25 Piezoelectric Crystal Transducers have a naturally occurring or manmade crystal which can convert electrical energy to sound energy and sound energy to electric energy crystal Alternating electric current Sound wave

26 US Makes Assumptions Echo originates w/in main US beam Echo returns after a single reflection Depth defined by round trip time of echo SOS constant in human tissue (1540m/s) Sound beam and echo travel in straight path Acoustic energy is uniformly attenuated

27 Illustrating Physics by Understanding Artifacts Speed of Sound Propagation Artifacts Reflection Artifacts Shadowing and Enhancement Edge Artifact Reverberation Artifact Comet Tail Artifact

28 Speed Displacement (Propagation) Artifact 2009 by Radiological Society of North America Feldman M K et al. Radiographics 2009;29:

29 Speed of Sound - Propagation Artifact Meritt, 1998

30 Attenuation Attenuation of acoustic energy results from Reflection Scatter Absorption Attenuation is frequency dependent Higher frequencies have greater attenuation Less depth of imaging with higher frequencies Artifacts due to attenuation Shadowing Enhancement Time Gain Compensation (TGC) Adjusts for loss of amplitude from echoes returning from the far-field (i.e., compensates for attenuation)

31 Attenuation of sound correction Time Gain Compensation T C G Returning sound

32 Normal Settings

33 Far-Field Not Seen Due to Lack of TGC for Attenuation

34 Increased through transmission Feldman M K et al. Radiographics 2009;29: BEAM ATTENUATED LESS THAN EXPECTED leads to POSTERIOR ENHANCEMENT 2009 by Radiological Society of North America

35 Posterior acoustic enhancement Occurs when the sound beam is attenuated less than expected; i.e. if it travels through fluid (or other medium) rather than tissue The TGC amplifies the returning echo using time (depth) as the only parameter, assuming homogeneous soft tissue in the path

36 Superficial cyst

37 Posterior Acoustic Enhancement Seen behind cysts, blood vessels and often lymph nodes Can occur with solid nodules, particularly if hypervascular Turn on Doppler to confirm a hypoechoic structure w/ posterior enhancement is a cyst or solid nodule

38 Enhancement is seen behind cysts - But can see it behind homogeneous solid objects as well Benign hyperechoic nodule seen in Hashimoto s Thyroiditis

39 Shadowing LARGE AI MISMATCH 2009 by Radiological Society of North America Feldman M K et al. Radiographics 2009;29:

40 Reflection Artifacts Eggshell Calcification

41 Shadowing and Enhancement

42 Reflection Artifacts Edge Artifact

43 Reverberation Occurs when sound bounces back and forth between two highly reflective surfaces multiple times before returning to the transducer The effect is to produce a step ladder of echoes in longitudinal orientation, each a little smaller Image

44 Reverberation Artifact Meritt, 1998

45 Reverberation Artifact

46 Reverb artifact Adding harmonics

47 Tunneling R IJ Catheter Reverberation Artifact

48 Comet Tail (Ring Down) Artifact

49 Microcalcification vs Comet Tail

50 Focus and Resolution Focused beam width determines Lateral and Azimuthal Resolution Near field (Fresnel Zone) Large variations of intensity Focal Zone - Area of maximal narrowing (Adjustable) Far field (Fraunhofer Zone) Intensity more uniform Pulse duration determines Axial Resolution (axial resolution = 1/2 SPL) Practical Considerations As frequency increases, axial resolution improves, but depth of imaging decreases. The depth of the focal zone is adjustable and indicated on the display

51 Frequency and Resolution With higher frequency the resolution of the near field is improved, but the deep structures are better visualized with the lower frequency.

52 Lateral resolution Ability to discriminate two objects perpendicular to the sound beam The narrower the beam, the better the lateral resolution. Higher frequencies have narrower beams Image 3 MHz 10 MHz

53 Focusing the Beam: Lateral resolution All beams have a focal zone, the narrowest part of the beam which has the best quality sound Near field Far field Focused Focal zone

54 Focal zone

55 Image Optimization Create the sharpest image to allow tissue discrimination. Equipment factors: Quality of Transducer Quality of Electronics Image Enhancement and Compound Imaging User Adjustments: Depth, Gain, Frequency Focal zones Number and Location Compound Imaging Tissue Harmonic Imaging Dynamic range Time Gain Compensation

56 Optimal Gain

57 Image Optimization Dynamic range May increase conspicuity of subtle lesions

58 Adjustment of number and position of focal zones

59 Doppler Physics When an interface is moving with respect to the reflected sound wave, the frequency of the reflected sound is altered. Frequency shifted up by approaching target Frequency shifted down by receding target Degree of frequency shift proportional to velocity Doppler Signal Processing and Display Doppler Frequency Spectrum Color Flow Power Mode

60 Illustration of Doppler Shift

61 Uses of Doppler Vascular Imaging and flow analysis of blood vessels Vascularity of tissue Probability of Malignancy Graves v. Thyroiditis Amiodarone Thyrotoxicosis Image Clarification

62 Color and Power Doppler Meritt, 1998

63 Color and Power Doppler Meritt, 1998

64 Color and Power Doppler Color Doppler Provides information regarding direction and velocity. More useful in vascular studies Power Doppler No information regarding velocity Less angle dependence Less noise Increased sensitivity for detection of flow

65 Doppler Issues High Sensitivity vs. Conventional Power User Variability Grading Scales (for thyroid nodules) 1 (absent vascularity) 2 (mostly peripheral vascularity) 3 (peripheral greater than intranodular) 4 (predominately intranodular)

66 Grade 1 No flow to nodule

67 Grade 2 Peripheral Flow

68 Grade 3 Moderate Central Flow

69 Grade 3 - Moderate Central Flow Suspicious for cystic papillary carcinoma

70 Grade 4 High Velocity Penetrating Flow

71 Increased Intranodular Flow Predictive of Malignancy? B-Mode Power Doppler NO LONGER CONSIDERED AN INDEPENDENT RISK

72 Vascularity - What to do? Consider whether features are suspicious for papillary or follicular cancers. Hypoechoic or features of papillary: Vascularity may be less important Iso or hyperechoic with variable thickness halo: Consider intranodular vascularity as possible risk. AACE & 2009 ATA guidelines do consider vascularity. Korean guidelines do not consider vascularity ATA 2015 guidelines do not consider vascularity **Don t be reassured by absence of vascularity.**

73 Doppler in the diagnosis of Thyrotoxicosis Graves Thyroid inferno Peak systolic velocity (PSV) 8-20 cm/sec Thyroiditis Various vascular patterns absent to hypervascular Thyrotoxicosis Factitia Minimal intrathyroidal vascular flow Peak systolic velocity (PSV) 3-5 cm/sec Amiodarone Induced Type 1 (Grave s like) increased or normal flow Type 2 (Destructive) decreased flow

74 Graves Disease Thyroid Inferno

75 Doppler of Lymph Nodes Suspicious Peripheral Normal Central/Hilar Kim et al, JUM 2013

76 Doppler for the Clarification of interpretation Central Compartment Lymph Nodes? Blood Vessels, Not Nodes

77 Clarification of Interpretation - Cyst?

78 Doppler Before Biopsy

79 Doppler Parathyroid Glands

80 Summary Sound transmission is dependent on the conducting medium. Sound is reflected at interfaces of acoustic impedance mismatch. Resolution is dependent on frequency and beam focal width. Artifacts such as shadowing and enhancement provide useful clinical imaging information. Doppler complements B-mode sonography in the evaluation of nodules, parathyroid, lymph nodes and thyrotoxicosis etiology