Ultrasound

By : Saja Abdo

Objectives:

• History• Physics of ultrasound 1. basic principles of sound 2. principles of ultrasound 3. how ultrasound wave impact by tissue• Uses of ultrasound

History

• 1 Jan 1915 Hydrophones covert acoustic energy into electrical energy, and is used for underwater navigation for submarines and iceburgs.

• Use of Ultrasound in Therapy 1 Jan 1920Used as a physical therapy treatment on a soccer team in Europe.

• 1 Jan 1942Neurologist Karl Dussik was the first to use ultrasound for medical diagnosis.

• 1 Jan 1953, First echocardiogram completed through an echo test conducted from a Siemens shipyard.

• 1 Jan 1970, Imaging blood flow through the chambers and different depths of the heart.

• 1 Apr 1986, 3-D ultrasound was developed and 3-D images were captured of a fetus. A 3D ultrasound is acquired by emitting high-frequency sound waves.

•

2 D 3D

• 1 Jan 2000, 4-D ultrasound (real time) was developed . • 2D / 3D / 4D Ultrasound also – Video in 9 Apr 2013.

Physics of ultrasound Basic principles of sound:

• Sound is a mechanical, longitudinal wave that travels in a straight line

• Sound requires a medium through which to travel• The wave travels by compressing and rarefacting matter.• rarefaction can by easily observed by compressing a spring

and releasing it. • Depending on the matter encountered, the wave will travel at

different velocities.

Cycle:• 1 Cycle = 1 repetitive periodic oscillation

Frequency

• Number of cycles per second• Measured in Hertz (Hz)

-Human Hearing 20 - 20,000 Hz-Ultrasound > 20,000 Hz-Diagnostic Ultrasound 2.5 to 10 MHz

Principles of ultrasound

• Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz.

• Diagnostic Ultrasound 2.5 to 10 MHz.Produced by passing an • Produced by passing an electrical current through a piezoelectric

crystal.

MHz to 16MHz

Piezoelectric material

• AC applied to a piezoelectric crystal causes it to expand and contract – generating ultrasound, and vice versa.

• Naturally occurring - quartz.

• Synthetic - Lead zirconate titanate (PZT).

Ultrasound Production

• Transducer contains piezoelectric(elements/ crystals)which produce the ultrasound pulses (transmit 1% of the time).

• These elements convert electrical energy into a mechanical ultrasound wave

The Returning Echo

• Reflected echoes return to the scan head where the piezoelectric elements convert the ultrasound wave back into an electrical signal.

• The electrical signal is then processed by the ultrasound system

Piezoelectric Crystals

• The thickness of the crystal determines the frequency of the scan head

Low Frequency3 MHz

High Frequency10 MHz

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.

• A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body .

• A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz.

Image Formation

• Probe emits a sound wave pulse• measures the time from emission to return of the echo .• Wave travels by displacing matter, expanding and

compressing adjacent tissues.• It generates an ultrasonic wave that is propagated, impeded,

reflected, refracted, or attenuated by the tissues it encounters .

• Electrical signal produces ‘dots’ on the screen Brightness of the dots is proportional to the strength of the

returning echoes.

• Location of the dots is determined by travel time. The velocity in tissue is assumed constant at 1540m/sec

Distance= Velocity Time

Producing an image

• Important concepts in production of an U/S image:

• Propagation velocity• Acoustic impedance• Reflection• Refraction• Attenuation

Propagation Velocity

• Sound is energy transmitted through a medium.

• Each medium has a constant velocity of sound (c).

• Tissue’s resistance to compression density or stiffness.

• Product of frequency (f) and wavelength (λ)c = f λ

• Frequency and Wavelength therefore are directly proportional if the frequency increases the wavelength must decrease.

• Propagation velocity Increased by increasing stiffness

Reduced by increasing density

For example• Bone: 4,080 m/sec• Air: 330 m/sec• Soft Tissue Average: 1,540 m/sec

Acoustic Impedance• Acoustic impedance (z) of a material is the product of its

density and propagation velocity. Z = pc

• Homogeneous mediums reflect no sound.• acoustic interfaces create visual boundaries between

different tissues. • Bone/tissue or air/tissue interfaces with large Δz values

reflect almost all the sound.• Muscle/fat interfaces with smaller Δz values reflect only part

of the energy.

• Reflection• Refraction• Transmission• Attenuation

Interactions of Ultrasound with Tissue

Refraction

• A change in direction of the sound wave as it passes from one tissue to a tissue of higher or lower sound velocity.

• U/S scanners assume that an echo returns along a straight path.

• Distorts depth reading by the probe.

• Minimize refraction by scanning perpendicular to the interface that is causing the refraction.

Reflection

• The production of echoes at reflecting interfaces between tissues of differing physical properties.

There are 2 types:• Specular - large smooth surfaces • Diffuse - small interfaces or nooks and crannies

Specular Reflection

• Large smooth interfaces (e.g. diaphragm, bladder wall) reflect sound like a mirror.

• Only the echoes returning to the machine are displayed.

• Specular reflectors will return echoes to the machine only if the sound beam is perpendicular to the interface.

Diffuse Reflector• Most echoes that are imaged arise from small interfaces

within solid organs.

• These interfaces may be smaller than the wavelength of the sound.

• The echoes produced scatter in all directions.

• These echoes form the characteristic pattern of solid organs and other tissues.

Transmission

• Some of the ultrasound waves continue deeper into the body.

• These waves will reflect from deeper tissue structures.

Attenuation

• The intensity of sound waves diminish as they travel through a medium

• In ideal systems sound pressure (amplitude) is only reduced by the spreading of waves

• In real systems some waves are scattered and others are absorbed, or reflected

• This decrease in intensity (loss of amplitude) is called attenuation.

• the deeper the wave travels in the body, the weaker it become-3 processes: reflection, absorption, refraction• Air (lung)> bone > muscle > soft tissue >blood > water

Attenuation & Gain

• Sound is attenuated by tissue• More tissue to penetrate = more attenuation of signal• Compensate by adjusting gain based on depth near field / far field• Increase gain = brighter• Decrease gain = darker• Gain settings are important to obtaining adequate images.

bad near fieldbad near field

balancedbalanced

bad far fieldbad far field

Goal of an Ultrasound System

• The ultimate goal of any ultrasound system is to make like tissues look the same and unlike tissues look different

Uses of ultrasound An ultrasound scan can be used in several different ways,

such as monitoring an unborn baby, diagnosing a condition or guiding a surgeon during certain procedures.

• Pregnancy Ultrasound scans are routine procedure for pregnant

women. They produce images of the unborn baby inside the womb, and display them on a monitor.

Most women are offered at least two ultrasound scans during pregnancy:

the first scan (at around eight to 14 weeks) can help to determine when the baby is due.

• the second scan (usually between 18 and 20 weeks) checks for structural abnormalities, particularly in the baby's head or spine.

• Diagnosing conditionsUltrasound scans can help diagnose problems in many parts of your

body, including your:1. liver (cirrhosis) 2. gallbladder (gallstones)3. lymph nodes 4. ovaries5. Testes 6. breasts7. blood vessels (aneurysm) 8. joints, ligaments and tendons9. Skin 10. eyes

• Echocardiogram (ECG) An ultrasound scan can be used to examine the size, shape

and movement of your heart. For example, it can check that the structures of your heart, such as the valves and heart chambers, are working properly and your blood is flowing normally. This type of ultrasound scan is called an echocardiogram (ECG).

• Biopsy Ultrasound can be used to guide doctors during certain

procedures, such as a biopsy (where a tissue sample is taken for analysis). This is to make sure the surgeon is working in the right area and is often used when diagnosing breast cancer.

References• Novelline, Robert (1997). Squire's Fundamentals of

Radiology (5th ed.). Harvard University Press. pp. 34–35.• Bruno Pollet Power Ultrasound in Electrochemistry: From

Versatile Laboratory Tool to Engineering Solution John Wiley & Sons,(2012)

• Corso, J. F. ,"Bone-conduction thresholds for sonic and ultrasonic frequencies". Journal of the Acoustical Society of America 35 (11): 1738–1743. (1963).

• Takeda, S.; Morioka, I.; Miyashita, K.; Okumura, A.; Yoshida, Y.; Matsumoto, K. "Age variation in the upper limit of hearing“, European Journal of Applied, (1992).

• Physiology 65 (5): 403–408. Retrieved 2008• Hearing by Bats (Springer Handbook of Auditory Research, 5.

Art Popper and Richard R. Fay (Editors). Springer, (1995).