Ultrasound Imaging (Basics) - PowerPoint Presentation

1. Ultrasound Imaging(Basics)

2. Why Ultrasound?Over half a century old technique! Arguably the most widely used imaging technologies in medicine. Portable, free of radiation risk, and relatively inexpensive compared to MRI, CT and PETTomographic, i.e., offering a “cross-sectional” view of anatomical structures. “Real time,”- providing visual guidance for interventional procedures

3. Do you expect any similarities?

4. Most amazing is that sound can actually help us to see what is hidden, just like the way bats 'see'.Bats always have the night shift. They go hunting for things to eat at night where food isn't well lit. Fortunately, bats are gifted with a system of locating things with sound. First they emit sound.

5. The human ear cannot hear below 20 Hz. Elephants can use infra sound. The human ear cannot hear above 20,000 Hz. Bats use ultrasound to locate food. Dolphins use it to communicate. Ultrasound used in medical imaging operate at frequencies way above human hearing: about 2 million Hz - 20 million Hz (2-20 MHz).

6. Sound travels in waves. Ultrasound physics has to do with the higher frequencies of sound. Human hearing is from about 20 cycles per second or 20HZ (a low hum) to about 20,000 cycles per second or 20KHZ. A grasshopper sends out sound waves at 40KHZ. A dog can hear at about 30KHZ and bats send chirps and listens for the echoes at 100KHZ. 

7. Properties of Sound WavesThe number of cycles occurring in one sec of time (cycles per sec)The high frequency wave sounds higher than the low freq waveHigh Frequency WavePeriodTimePressureLow Frequency WavePeriodTimePressurehttp://www.genesis-ultrasound.com/Ultrasound-physics-2.htmlwavelengthCrestTroughAmplitudeFrequencyVelocityWavelengthAmplitudeUnits to describe frequency:Hertz= 1 cycle in one seckHz= 1000 Hz= 1000 cycles per secMHz= 1000000 HertzUS imaging frequency range: 2-12 MHz

8. wavelengthWavelengthLength of space over which one cycle occurs (distance)wavelengthDistanceDistanceGiven a constant velocity, as frequency increases wavelength decreases (V=  x f)Common US frequencies and wavelengths-2.25MHz = 0.6 microns-5.0 MHz = 0.31 microns-10.0 MHz = 0.15 microns

9. High frequency US waves High axial resolution  More attenuation  Superficial structure Ultrasound Wavelength and Frequency Low frequency US waves Lower resolution  Less degree attenuation  Deeper penetrationHigher frequency waves are more highly attenuated than lower frequency waves at a given distance High frequency transducers (10-15 MHz) to image superficial structures (e.g. stellate ganglion blocks) Low frequency transducers (2-5 MHz) to image the lumbar neuraxial structure

10. Velocity Average speed of US in the human body is 1540 m/sec Directly related to the stiffness of media Inversely related to the density of media Slowest in air/gasses fastest in solidsMedium Velocity (m/sec)--------------------------------------------Air 330Fat 1450Water 1480Soft tissue 1540Blood 1570Muscle 1580Bone 4080c =  × f = c / f

11. Amplitude The strength/intensity of the sound wave at any given point in time Represented by the height of the wave Amplitude/intensity decreases with increasing depth Magnitude of the pressure changes along the sound wave Power: rate at which energy is transferred from a sound beam- proportional to the amplitude squared Intensity (Watts/cm2) is the concentration of energy in a sound beam

12. The ultrasound amplitude decreases in certain media as a function of ultrasound frequency (attenuation coefficient) ScN-Sciatic nerve, PA - Popliteal artery.8 MHz 10MHz 12MHzPractical consequence of attenuation: the penetration decreases as frequency increasesAttenuation Coefficient

13. Ultrasound frequency affects the resolution of the imaged object. Resolution can be improved by increasing frequency and reducing the beam width by focusing.8 MHz 10MHz 12MHzFor a constant acoustic velocity, higher frequency US can detect smaller objects and provide a better resolution image.A 0.5-mm-diameter object

14. Spatial ResolutionAxial and Lateral. Axial resolution is the minimum separation of above-below planes along the beam axis. It is determined by spatial pulse length, which is equal to the product of wavelength and the number of cycles within a pulse. Axial resolution = wavelength (λ) × number of cycle per pulse (n) ÷ 2

15. Common Frequencies for Clinical USDystrophic calcification of the choroidsPortal Vein UltrasoundColor Doppler imaging shows a thrombus in upper PV moderately dilated (14.5 mm) with splenomegaly: Cirrhosis with PV thrombosis.MRI of a large tumor in the left kidney (L) and 12 days following HIFU treatment (R). Ablative therapy 

16. T1: ultrasonic generator, Q1: transmitter, Q2: receiver, T2: converter amplifier, W: water bath, L: light, P: photographic/ heat-sensitive paperUltrasound in Med. & Biol., Vol. 30, No. 12, pp. 1565 - 1644, 2004Dr. Karl Theo Dussik, an Austrian neurologist, was the first to apply US to image the brain.

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18. Interaction Between Ultrasound and Tissue Attenuation Reflection Refraction ScatteringTissue absorbs the ultrasound energy, making the waves disappear. These waves don't return to the probe and are therefore "wasted". The more the body tissues that the ultrasound waves have to cross, the more attenuation the waves suffer. That is one reason why it is more difficult to image deeper structures.True reflectionr=i

19. ReflectionReflection occurs at the boundary/interface between two adjacent tissuesThe difference in acoustic impedance (z) between two tissues causes reflection of the sound wavez= density x velocityReflection from a smooth tissue interface (specular) causes the soundwave to return to the scan headUS image is formed from the reflected echoes

20. ScatteringRedirection of the sound wave in several directionsCaused by interaction with a very small reflector or a very rough interfaceOnly a portion of the sound wave returns to the scan head

21. TransmissionNot all of the sound wave is reflected, therefore some of the wave continues deeper into the bodyThese waves will reflect from deeper tissue structuresTrue reflectionr=i

22. Transducer BasicsGELPropylene glycol (propane-1,2-diol) conductive medium

23. A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated)A Piezoelectric MaterialTetragonal unit cell of lead titanateTransducer (AKA: probe)Piezoelectric crystalEmit sound after electric charge appliedSound reflected from patientReturning echo is converted to electric signal  grayscale image on monitorEcho may be reflected, transmitted or refractedTransmit 1% and receive 99% of the time

24. When a voltage is applied to an piezo electric crystal (shown in red below), it expands. When the voltage is removed, it contracts back into its original thickness.If the voltage is rapidly applied and removed repeatedly, the piezo electric crystal rapidly expands and relaxes, creating ultrasound waves.

25. ListenStrikingPiezoelectric crystal is compressed to generate a voltage

26. AttenuationAbsorption = energy is captured by the tissue then converted to heatReflection = occurs at interfaces between tissues of different acoustic propertiesScattering = beam hits irregular interface – beam gets scattered

27. Acoustic ImpedanceThe product of the tissue’s density and the sound velocity within the tissueAmplitude of returning echo is proportional to the difference in acoustic impedance between the two tissuesVelocities:Soft tissues = 1400-1600m/secBone = 4080Air = 330Thus, when an ultrasound beam encounters two regions of very different acoustic impedances, the beam is reflected or absorbedCannot penetrateExample: soft tissue – bone interface

28. Frequency and ResolutionAs frequency increases, resolution improvesAs frequency increases, depth of penetration decreasesUse higher frequency transducers to image more superficial structuresEx: Equine TendonsPenetrationFrequency

29. Modes of DisplayA modeSpikes – where precise length and depth measurements are needed – ophthoB mode (brightness) – used most often2 D reconstruction of the image sliceM mode – motion modeMoving 1D image – cardiac mainly

30. Ultrasound TerminologyNever use dense, opaque, lucentAnechoicNo returning echoes= black (acellular fluid)EchogenicRegarding fluid--some shade of grey d/t returning echoesRelative termsComparison to normal echogenicity of the same organ or other structureHypoechoic, isoechoic, hyperechoicSpleen should be hyperechoic to liverLiver is hyperechoic to kidneys

31. Diagram illustrating development stage of microbubbles, nanobubbles, and nanodroplets for diagnostic and therapeutic purposes. HIFU = high-intensity focused ultrasound; KDR = kinase domain receptor.Applications of US in Biomedicine

32. High echogenicity Low attenuation Low blood solubility Low diffusivity Ability to traverse pulmonary system Lack of biological effects in repeat exposuresIdeal Characteristics of an Ultrasound Probe