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 PET
Tomographic, 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 Waves
Frequency
Velocity
Wavelength
Amplitude
Units to describe frequency:
Hertz= 1 cycle in one sec
kHz= 1000 Hz= 1000 cycles per sec
MHz= Hertz
US imaging frequency range: 2-12 MHz
wavelength
Crest
Trough
Amplitude
High Frequency Wave
Period
Time
Pressure
Low Frequency Wave
Period
Time
Pressure
The number of cycles occurring in one sec of time (cycles per sec)
The high frequency wave sounds higher than the low freq wave

8. Wavelength Length of space over which one cycle occurs (distance)
Given 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. Ultrasound Wavelength and Frequency
High frequency US waves High axial resolution  More attenuation  Superficial structure
Low frequency US waves Lower resolution  Less degree attenuation  Deeper penetration
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
Higher frequency waves are more highly attenuated than lower frequency waves at a given distance

10. Medium Velocity (m/sec)
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 solids
Medium Velocity (m/sec)
Air
Fat
Water
Soft tissue 1540
Blood
Muscle
Bone
c =  × 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. Attenuation Coefficient
8 MHz MHz MHz
The ultrasound amplitude decreases in certain media as a function of ultrasound frequency (attenuation coefficient)
ScN-Sciatic nerve, PA - Popliteal artery.
Practical consequence of attenuation: the penetration decreases as frequency increases

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

14. Spatial Resolution
Axial 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 US
Dystrophic calcification of the choroids
Portal Vein Ultrasound
Color Doppler imaging shows a thrombus in upper PV moderately dilated (14.5 mm) with splenomegaly: Cirrhosis with PV thrombosis.
Ablative therapy 
MRI of a large tumor in the left kidney (L) and 12 days following HIFU treatment (R). 

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

18. Imaging: Therapy: B-mode imaging: Improved contrast
Doppler Ultrasound: Improved contrast and signal strength
Perfusion Imaging: Imaging where micro bubbles are deliberately collapsed to measure how rapidly the blood refills an organ or suspected tumor.
Targeted Imaging
Therapy:
Thrombolysis: USCAs are collapsed to clear a blood clot
Angiogenesis: Bubbles in vasculature are popped to break open target blood vessel.
Sonoporation: Opening of cellular membrane by USCA and ultrasound exposure.
High intensity focused ultrasound (HIFU): Already an established practice for burning target tissues; use bubbles to increase heating.

19. Cardiac US imaging frequency range
Wavelength and Frequency
Wavelength and frequency are inversely related
The unit frequency is Hertz (Hz) = 1 cycle in one sec
Cardiac US imaging frequency range
TTE
2-3 MHz
IVUS
10-40 MHz
TEE
3.5-7 MHz

20. Interaction Between Ultrasound and Tissue
Attenuation
Reflection
Refraction
Scattering
Tissue 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

21. Reflection
Reflection occurs at the boundary/interface between two adjacent tissues
The difference in acoustic impedance (z) between two tissues causes reflection of the sound wave
z= density x velocity
Reflection from a smooth tissue interface (specular) causes the soundwave to return to the scan head
US image is formed from the reflected echoes

22. Scattering Redirection of the sound wave in several directions
Caused by interaction with a very small reflector or a very rough interface
Only a portion of the sound wave returns to the scan head

23. Transmission True reflection r=i
Not all of the sound wave is reflected, therefore some of the wave continues deeper into the body
These waves will reflect from deeper tissue structures

24. Transducer Basics G E L
Propylene glycol (propane-1,2-diol) conductive medium

25. A Piezoelectric Material
A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated)
Tetragonal unit cell of lead titanate
Transducer (AKA: probe)
Piezoelectric crystal
Emit sound after electric charge applied
Sound reflected from patient
Returning echo is converted to electric signal  grayscale image on monitor
Echo may be reflected, transmitted or refracted
Transmit 1% and receive 99% of the time

26. 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.

27. Piezoelectric crystal is compressed to generate a voltage
Striking
Listen

28. Attenuation
Absorption = energy is captured by the tissue then converted to heat
Reflection = occurs at interfaces between tissues of different acoustic properties
Scattering = beam hits irregular interface – beam gets scattered

29. Acoustic Impedance
The product of the tissue’s density and the sound velocity within the tissue
Amplitude of returning echo is proportional to the difference in acoustic impedance between the two tissues
Velocities:
Soft tissues = m/sec
Bone = 4080
Air = 330
Thus, when an ultrasound beam encounters two regions of very different acoustic impedances, the beam is reflected or absorbed
Cannot penetrate
Example: soft tissue – bone interface

30. Frequency and Resolution
As frequency increases, resolution improves
As frequency increases, depth of penetration decreases
Use higher frequency transducers to image more superficial structures
Ex: Equine Tendons
Frequency
Penetration

31. Modes of Display A mode B mode (brightness) – used most often
Spikes – where precise length and depth measurements are needed – ophtho
B mode (brightness) – used most often
2 D reconstruction of the image slice
M mode – motion mode
Moving 1D image – cardiac mainly

32. Ultrasound Terminology
Never use dense, opaque, lucent
Anechoic
No returning echoes= black (acellular fluid)
Echogenic
Regarding fluid--some shade of grey d/t returning echoes
Relative terms
Comparison to normal echogenicity of the same organ or other structure
Hypoechoic, isoechoic, hyperechoic
Spleen should be hyperechoic to liver
Liver is hyperechoic to kidneys

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

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