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

• The number of cycles occurring in one sec of time (cycles per sec)• The high frequency wave sounds higher than the low freq wave

High Frequency Wave

Period

Time

Pres

sure

Low Frequency Wave

Period

Time

Pres

sure

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

wavelength

Crest

Trough

Amplitude • Frequency• Velocity• Wavelength• Amplitude

• Units to describe frequency:Hertz= 1 cycle in one seckHz= 1000 Hz= 1000 cycles per secMHz= 1000000 HertzUS imaging frequency range: 2-12 MHz

wavelength

Wavelength

• Length of space over which one cycle occurs (distance)

wavelength

Distance 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

• 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 penetration

Higher 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

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 solids

Medium Velocity (m/sec)--------------------------------------------Air 330Fat 1450Water 1480Soft tissue 1540Blood 1570Muscle 1580Bone 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

The ultrasound amplitude decreases in certain media as a function of ultrasound frequency (attenuation coefficient) ScN-Sciatic nerve, PA - Popliteal artery.

8 MHz 10MHz 12MHz

Practical consequence of attenuation: the penetration decreases as frequency increases

Attenuation Coefficient

• 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 12MHz

For a constant acoustic velocity, higher frequency US can detect smaller objects and provide a better resolution image.

A 0.5-mm-diameter object

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 USDystrophic 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.

MRI of a large tumor in the left kidney (L) and 12 days following HIFU treatment (R).

Ablative therapy

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

Dr. Karl Theo Dussik, an Austrian neurologist, was the first to apply US to image the brain.

Imaging:•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.

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

TTE2-3 MHz

IVUS10-40 MHz

TEE3.5-7 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 reflectionr=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 directionsCaused by interaction with a very small reflector or a very rough interfaceOnly a portion of the sound wave returns to the scan head

Transmission

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

True reflectionr=i

Transducer Basics

GEL

Propylene glycol (propane-1,2-diol) conductive medium

A piezoelectric disk generates a voltage when deformed (change in shape is greatly

exaggerated)

A Piezoelectric Material

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.

ListenStriking

Piezoelectric crystal is compressed to generate a voltage

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

Penetration

Frequency

Modes of Display

• A mode– 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

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

• High echogenicity• Low attenuation• Low blood solubility• Low diffusivity• Ability to traverse pulmonary system• Lack of biological effects in repeat exposures

Ideal Characteristics of an Ultrasound Probe