Introduction to the Physics of Ultrasound

Transcription

1 Introduction to the Physics of Ultrasound Dr MS Abdullatif Critical care consultant RCoA Tutor Stepping Hill hospital Exam Board RCoA MBbCH, MSC, FRCA, FICM, BSE-TOE

2 Objectives Explain the Basics of ultrasound and wave creation Describe Factors that affect US image and discuss image optimization techniques (Terminology & knobology) Review applications of specific imaging modes Choose appropriate transducers

3 Basics of ultrasound and wave creation

4 What is Sound? Sound is a mechanical, longitudinal wave that travels in a straight line Sound requires a medium through which to travel Measured in Hertz (Hz) -Human Hearing 20-20,000 Hz -Ultrasound > 20,000 Hz -Diagnostic Ultrasound 2.5 to 10 MHz (this is what we use!)

5 How can we produce US Piezo-electricity

6 Frequency and wavelength 15 MHz 5 MHz O.1 mm 0.31 mm C = f x λ λ = C / f

7 How does an ultrasound machine make an image? Distance = 1/2 X C (1540 m/s) X T Distance = time

8 US Interaction with tissues

9 US Interaction with tissues Reflection Back to Transducer Scatter In Multiple Directions Refraction Redirection or Bending Absorption Converted to Heat

10 Interactions of Ultrasound with Tissue Reflection The ultrasound reflects off tissue and returns to the scanhead- amount of reflection depends on differences in acoustic media The ultrasound image is formed from reflected echoes Scanhead

11 Interactions of Ultrasound with Tissue Transmission Some of the ultrasound waves continue deeper into the body These waves will reflect from deeper tissue structures Scanhead

12 Reflection Vs Scattering Reflection Large and θ near to 90 Smooth Target size > λ Scattering Small Rough surface Target size < λ

13 Scattering LA RA Uneven surface Small target size

14 Interactions of Ultrasound with Tissue Attenuation The deeper the wave travels in the body, the weaker it becomesdue to processes: reflection, absorption, scattering Air (lung)> bone > muscle > soft tissue >blood > water

15 Factors affecting US image and image optimization techniques (Terminology & knobology)

16 Image Quality and Θ Chest wall

17 Media (tissue) Characteristics Density Stiffness Propagation Speeds Impedance = density X propagation speed

18 Image Quality and Acoustic Impedance (Z) Tissue AI (106 Raylas) Air Lung 0.18 Fat 1.34 Liver 1.65 Blood 1.65 Kidney 1.63 Muscle 1.71 Bone 7.8

19 Image Quality and Z 1.Acoustic coupling Air AI Tissue AI 1.34 Air-tissue great AI mismatch Strong reflection and little transmission Gel 02/02/16

20 Image Quality and Z Hypoechoic Less echogenic than surrounding tissue Hyperechoic More echogenic than surrounding tissue Anechoic Absence of Echoes Isoechoic Same echogenicity as surrounding tissue

21 AI and Echogenicity Echogenicity

22 Goal of an Ultrasound System The ultimate goal of any ultrasound system is to make like tissues look alike and unlike tissues look different

23 What determines how far ultrasound waves can travel? The FREQUENCY of the scanhead The HIGHER the frequency, the LESS it can penetrate The LOWER the frequency, the DEEPER it can penetrate Attenuation is directly related to frequency

24 Frequency vs. Resolution The frequency also affects the QUALITY of the ultrasound image The HIGHER the frequency, the BETTER the resolution The LOWER the frequency, the LESS the resolution Frequency choice is a trade off between resolution and penetration

25 Introduction to the Physics of Ultrasound Adjusting the Frequency

26 Image optimization (Depth) Too shallow Too deep Just right Set the depth to the minimum required to see all structures

27 Introduction to the Physics of Ultrasound Depth

28 Image optimization (Gain) Too little Too much Just Right

29 Introduction to the Physics of Ultrasound Ultrasound Gain

30 Image Optimization (Zoom) 02/02/16

31 Caliper AV diameter Bladder Volume 02/02/16

32 Applications of specific imaging modes

33 US Modes

34 Image formation B and 2D Modes The strength or amplitude (brightness) of each reflected wave is represented by a dot The position of the dot represents the depth from which the returning echo was received These dots are combined to form a complete image 2D phased array

35 Sectors

36 MM

37 Color Doppler Pixels assigned color based on mean velocity of the object Displays direction of blood flow Direction of flow

38 Color Doppler Color Doppler is angle dependent. Therefore there is little or no flow at perpendicular angles. Remember BART Blue Away Red Towards when red bar is on top. Towards Transducer NO FLOW Away from Transducer

39 Color Doppler Color Doppler is angle dependent

40 Probes

41 What is a scanhead? Contains piezoelectric elements/crystals which produce the ultrasound pulses This element converts electrical energy into a mechanical ultrasound wave

42 Anatomy of the Scan Head Backing - dampens sound after pulse is generated Covering - protects transducer face Crystals - converts energy transmits / receives sound Matching Layer - assists in sound transmission

43 Human Hair Single Crystal Microscopic view of scanhead

44 Scanhead Crystals The thickness of the crystal determines the frequency of the scanhead Low Frequency 3 MHz High Frequency 10 MHz

45 Frequency vs. Resolution Format Footprint (mm) Frequency (MHz) Linear L Curved Linear C Phased P Frequency choice is a trade off between resolution and penetration

46 Position of Reflected Echoes Display screen divided into a matrix of PIXELS (picture elements)

47 Reflected Echos Strong Reflections = White dots Diaphragm, gallstones, bone Weaker Reflections = Grey dots Most solid organs, thick fluid No Reflections = Black dots Fluid within a cyst, urine, blood

48 Transducer Orientation Markings are located on one side of transducer only and correspond to orientation marker on screen vertical protrusion

49 Introduction Be to the aware Physics of Ultrasound that you are looking at a 1mm slice Think of a credit card coming out of the end!

50 + Introduction to the Physics of Ultrasound Probes Uses: Vascular access Nerve Blocks 6-12 MHz 02/02/16

51 + Introduction to the Physics of Ultrasound Probes Uses: Abdominal Obs and Gynae 2-5 MHz 02/02/16

52 + Introduction to the Physics of Ultrasound Phased Array TTE Smaller footprint 1-5 MHZ 02/02/16

53 Questions