Ultrasound and Doppler Flashcards Preview

Physics SUM Second Semester > Ultrasound and Doppler > Flashcards

Flashcards in Ultrasound and Doppler Deck (48):
1

Acoustic Impedance (Parameters of Sound)

  • the impedance of a material is the product of its density and the velocity of sound in the material:
  • Z=qv
    • Z-acoustic impedance: Rayls
    • q-density of medium: kg/m3
    • v-velocity of sound in medium: m/s

2

Interactions of ultrasound with matter

  • reflection
  • refraction
  • attenuation

3

Reflection

  • a return of the incident ultrasound energy, as an echo directly back to the transducer when interacting at a boundary with normal incidence
  • reflection of ultrasound arises as a result of differences in acoustic impedence 
  • Various tissue interfaces and reflection coeffecient values:
    • Fat-muscle (0.011)
    • muscle-air (0.999)
    • skull-brain (0.410)

4

The Intensity Reflection Coeffecient (Reflection contd)

  • Ri for perpendicular impedance to a boundary is given by:
    • R= (Z- Z2/ Z+ Z2)^2

5

Refraction

  • a change in direction of the transmitted ultrasound energy that occurs at a boundary interface 

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6

Attenuation

  • a loss of intensity of ultrasound beam resulting from absorption and scattering events
  • is characterized by the attenuation coeffecient 

7

Absorption

  • The absorption of ultrasound waves refers to the conversion of ultrasound energy to thermal energy 

8

Scattering 

  • occurs when the beam encounters an interface that is irregular and smaller than the ultrasound beam
  • occurs when diagnostic ultrasound encouters:
    • a highly irregular or rough surface 
    • heterogeneous tissue as kidney and liver 

9

Ultrasound imaging system is composed of 

  • pulse generator
  • transducer
  • amplifier
  • scan converter
  • image memory
  • display system
  • recording system
  • control panel

10

Ultrasound: How does it work?

  • use very high frequencies-several megahertz
  • makes use of the fact that sound can be reflected
  • a thin layer of gel is placed between the probe and the skin to make sure all the sound enters the body
  • the probe contains a transmitter and a receiver 

11

Ultrasound: How is the image created?

  • millions of sound waves are transmitted every second
  • as the waves reflected at different times, the computer in the ultrasound machine calculates how far the wave travelled before being reflected 

12

Production of Ultrasound 

  • the transducer is a device that can convert one form of energy into another 
    • it is both a transmitter and receiver of the ultrasound signal and it serves a dual role in pulse echo imaging 
  • most important component is a thin approx. 0.5mm  piezoelectric crystal located near the face of the transducer 
  • the transducer uses a piezoelectric crystal, which converts electrical stimuli to sound energy and vice versa 
  • Piezoelectric compounds (means electricity from pressure) are a somewhat compressible material and have a molecular structure composed of a well-defined arrangement of electrical dipoles.
  • when mechanically deformed by an applied pressure, the alignment of the dipoles is disturbed from the equilibrium position and this causes an imbalance of the charge distribution, resulting in an electrical potential to be formed across the crystal surfaces 
  • application of a voltage applied externally through the electrodes induces the mechanical expansion and contraction of the crystal 
  • when a high DC voltage is applied to the crystal surface, it is produced ultrasound beam

13

Mechanical Wave: What are they?

  • Mechanical waves are a local oscillation of material
  • only the energy propogates; the oscillating material does not move far from its initial equilibrium position. 
  • Therefore, mechanical waves transport energy and not material

14

Imaging by Ultrasound

  • The transducer pulses of ultrasounda re transmitted into the body by placing the vibrating the crystal in close contact with the skin, using a specil gel past of acoustic impedance equal to that of the skin to eliminate the air
  • The basis for use of ultrasound in medicine is the partial reflection of the beam by the objects relative to its higher acoustic impedance.

15

Sound: What is it?

  • it is a mechanical energy that propogates through a continuous, elastic medium by the compression and rarefaction of "particles" that comprise it.

16

A mode ultrasounography

Mode

  • A mode is a recording of amplitude of returning echoes versus time

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17

Sound: Compression

  • a mechanical deformation induced by an external energy force with a resultant increase in pressure on the medium

18

Sound: Rarefaction

  • a compression of particle transfer, which adjacents particles with the subsequent reduction in pressure

19

M mode

  • a mode in motion, which converts the variations in signal amplitude of the A mode line into a series of dots along a display oscilloscop. It is useful in heart beating imaging.

20

Ultrasound: Definition

  • it is simply sound that has a very high frequency
  • humans are unable to hear ultrasound, but some animals can hear them
  • Ultrasound over 20,000 Hz, or 20kHz; used in medicine in the technique of ultrasonography
  • Ultrasound is the term given to inaudible, high frequency sound waves, and is also the generic name given to the imaging modality that uses ultrasonic energy

21

Ultrasound is a useful way of examining many of the body's internal organs including:

  • heart and blood vessels, including the abdominal aorta and its major branches
  • liver
  • gallbladder
  • spleen
  • pancreas
  • kidneys
  • bladder
  • uterus, ovaries and unborn child in pregnant patients
  • eyes
  • thyroid and parathyroid glands
  • scrotum(testicles)
  • brain in infants
  • hips in infants

22

Ultrasonography: Definition

  • a radiologic technique in which deep structures of the body are visualized by recording the reflections, (echoes), of ultrasonic waves directed into the tissues

23

Doppler effect

  • The doppler effect is a change in the frequency of a wave, resulting from motion of the wave source or receiver or in the case of a reflected wave, motion of the reflector

24

Doppler ultrasound

  • an unique technique because has the potential to offer information related to function of an organ through blood flow studies and not just morphology
  • Doppler ultrasound is based on the chift of frequency in the carrier ultrasound wave caused by a moving reflector
  • by comparing the incident ultrasound frequency with reflected ultrasound frequency from the blood cells, it is possible to discern the velocity and the direction of the blood.
  • Not only can blood velocity(and indirectly blood flow) be measured, but the information provided by Doppler techniques can be used to create color blood flow maps of vasculature

25

Medical uses of ultrasound requires frequencies in the range of:

  • 2MHz, or 2,000,000 Hz

to

  • 10MHz, or 10,000,000 Hz

26

Uses of Ultrasound in Medicine:

  • Ultrasound is used for examining soft tissue inside the body
  • Parts of the body that may be examined include muscles and unborn babies
  • Blood flow can also be monitored using ultrasound

27

Why use ultrasound?

  • Ultrasound is very safe, there is no evidence that it does harm to the body, (or baby in pregnancy scans)
  • X-Rays are potentially dangerous, particularly for young children and pregnant women, (damage to unborn baby)

28

Doppler angle

  • the angle of incidence between the ultrasound beam and the estimated flow direction(parallel to the long axis of the vessel)

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29

Th ratio of ankle and arm arterial pressure

  • Physician measures your blood pressure in your ankle and in your arm(physician will use an ordinary blood pressure cuff and an ultrasound device)
  • Physican will compare the 2 numbers to determine your rtio of ankle and arm
  • Normally the blood pressure in your ankle and arm should be about equal
  • If your ankle pressure is half your arm pressure(or lower), your leg arteries are probably narrowed
  • The ratio of ankle and arm helps your physician diagnose arterial disease in the legs

30

Types of Wave: Longitudinal Wave

  • the particles of the elastic medium vibrate in the direction of wave propagation

31

Types of Waves: Transverse Wave

  • the particles of the elastic medium vibrate perpendicular to the direction of wave propogation

32

The Physical Characteristics of Ultrasounds

  • Ultrasonic waves require a flexible medium to spread
  • in water and soft tissues dominate longitudinal waves
  • Bones also propogate transverse waves

33

Doppler shift

  • Doppler shift is dependent on the isonating frequency, the velocity of moving blood and the angle between the sound beam and direction of moving blood, as expressed in the Doppler equation
  • Df = 2fvcosθ/c
  • Df- doppler shift frequency
  • f-transmitted frequency
  • v-blood velocity
  • c-speed of sound
  • θ-angle between the ultrasound beam and the direction of moving blood
  • The equation can be rearranged for blood velocity and this value is calculated by Dopper US machine:
  • v=Dfc/2fcosθ
  • Doppler is used to detect and measure blood flow and the major reflector is the red blood cell

34

Parameters of Sound: Frequency

  • it is a number of times the wave repeats itself, (or a cycle) over one second
    • 1 Hz - 1 oscillation per second
    • 1kHz - 1000 oscillations per second
    • 1MHz - 1,000,000 oscillations per second

 

35

Parameters of Sound: Propogation Speed

  • a distance traveled per unit time of the energy pulse and is equal to the wavelength divided by the period (or therefore multiplied by the frequency) 

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36

Limitations

  • Ultrasound waves are distrupted by air or gas therefore ultrasound is not an ideal imaaging technique for air filled bowel or organs obscured by the bowel
  • Ultrasound has difficulty penetrating bone and therefore can only see the oouter surface of bony structures and not what lies within

37

Parameters of Sound: Relationship with Variables

  • Propogation speed = Wavelength + Frequency
  • The speed of sound in air: 330 m/s
  • The speed of sound in soft tissue: 1540 m/s
  • The speed of sound in bone: 4000 m/s

38

Types of Ultrasonography

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39

Production of Ultrasound

  • When mechanically deformed by an applied pressure, the alignment of th edipoles s dosturbed from equillibrium position and this causes an imbalance of the charge distribution, resulting in an electrical potential to be formed across the crystal surfaces
  • Medical ultrasound systems use very high frequencies-several megahertz
  • Ultraousnd imaging makes use of the fact that sound can be reflected

40

Parameters of Sound: Pressure Difference

  • it is a sound energy that causes particle displacement to occur in the propogation medium
  • sometimes, a pressure difference can be measured as an amplitude, (up and down travel).

41

Scattering

  • occurs when the beam encounters an interface that is irregular and smaller than the ultrasound beam
  • Scattering occurs when diagnostic ultrasound encounters a highly irregular or rough surface(A) or heterogenous tissue such as kidney and liver(B)

A image thumb
42

An ultrasound imaging system is composed of

  • pulse generator
  • transducer
  • amplifier
  • scan converter
  • image memory
  • display system
  • recording system
  • control panel

43

Absorption of ultrasound

  • Absorption of ultrasound waves refer to the conversion of ultrasound energy to thermal energy

44

Interactions

  • reflection
  • refraction
  • attenuation

45

Parameters of sound

The relationship represents

  • Propogation speed=wavelength x frequency
  • The speedof sound in air: 330m/s
  • The speed of sound in soft tissue:1540m/s

The speed of cound in bone: 4000m/s

46

Pressure difference

  • A sound energy causesparticle displacement to occur in the propogation medium
  • Sometimes a pressure difference can be measured as an amplitutde(up and down travel)

47

Acoustic impedance

  • the impedance of a material is the product of its density and the velocity of sound in the material
  • Z=qv
  • Z-acoustic impednce=Rayls
  • q-density of medium kg/m^3
  • V-velocity of sound in medium m/sec

48

B mode

A mode is a brightness mode used for mapping the acoustic impedance variations of the tissues into a 2D, 3D and 4D display