Ultrasound Imaging (Basics)

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

Do you expect any similarities?

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.

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).

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. 

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= 1000000 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 http://www.genesis-ultrasound.com/Ultrasound-physics-2.html

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

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

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 330 Fat 1450 Water 1480 Soft tissue 1540 Blood 1570 Muscle 1580 Bone 4080 c =  × f  = c / f

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

Attenuation Coefficient 8 MHz 10MHz 12MHz 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

Ultrasound frequency affects the resolution of the imaged object. 8 MHz 10MHz 12MHz 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.

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

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). 

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. 1565 - 1644, 2004

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.

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

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

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

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

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

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

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

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.

Piezoelectric crystal is compressed to generate a voltage Striking Listen

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

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 = 1400-1600m/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

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

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

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

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.

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