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Advanced PTEeXAM > Physics > Flashcards

Physics Flashcards

1
Q

What is rarefaction?

A

areas where molecules are dispersed

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2
Q

Term for the level of sound energy in an area of tissue?

A

Intensity

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3
Q

What is the intensity of sound proportional to?

A

Square of the amplitude

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4
Q

What is maximum intensity output of cardiac ultrasound systems?

A

720 W/cm2

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5
Q

what is the velocity of sound in soft tissue?

A

1540 m/s

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6
Q

What is equation for wavelength in soft tissue?

A

wavelength = 1540 / frequency

1540 m/s is approximate speed of sound in soft tissue

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7
Q

What are the four major events that alters sound energy as it passes through various tissues?

A

Reflection - creates strong echoes directed back toward transducer

Refraction - bends ultrasound beam down a new path

Attenuation - Sound energy converted to heat and beam is dispersed

Scattering - reflections from small objects like RBCs disperse sound energy in all direction

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8
Q

What is the meaning of acoustic impedence?

A

It is a physical property that describes how much resistance an ultrasound beam encounters as it passes through tissue.

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9
Q

What does acoustic impedence (Z) depend on?

A

The density of tissue and the speed of the sound wave.

Z = d x c

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10
Q

What happens to the amount of reflection of a sound wave when the differences between acoustic impedance of two interfacing media is large?

A

Larger amount of energy is reflected back to transducer.

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11
Q

What is specular reflection?

A

occurs when a sound wave encounters a large object with a smooth surface. The reflection is strongest away from interface at an angle equal and opposite to that at which the beam traveled.

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12
Q

What is scattering reflection?

A

When ultrasound beam encounters small or irregularly shaped surfaces like RBCs. Far less energy is reflected back to the transducer.

This is basis for Doppler analysis of RBC movement.

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13
Q

When is refraction of ultrasound beam most pronounced?

A

When sound velocities in the two tissues is large and angle of incidence is acute.

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14
Q

Does refraction occur when angle of incidence is 90 degrees?

A

No. Wave would continue to travel in same direction.

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15
Q

What are the two reasons for attenuation?

A

Dispersion and Absorption

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16
Q

What is dispersion of an ultrasound beam?

A

occurs when the beam diverges over a greater area in the far field.

Irregular tissue causes scattering further dispersing the ultrasound energy.

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17
Q

What is absorption of ultrasound beam?

A

The frictional forces converting ultrasound energy to heat.

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18
Q

What frequency signals have higher absorption?

A

Higher frequencies as this leads to higher fricional forces.

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19
Q

What two variables does the amount absorption depend on?

A

Frequency of ultrasound beam and distance traveled.

20
Q

What transducer component acts as the ultrasonic vibrator and receiver?

A

Ceramic piezoelectric crystal

21
Q

What conducts electric energy to stimulate the piezoelectric crystal and record the voltage signal from returning echoes?

A

Electrodes

22
Q

What component of the transducer acts to rapidly dampen the vibrations of the crystal?

A

backing.

23
Q

What is inside the matrix of the piezoelectric crystal?

A

Polarized molecules that vibrate when an AC current is placed across them.

When they are vibrated by external factors they generate an AC current.

24
Q

What can be used to better focus the ultrasound beam?

A

Larger transducers with high freqency signals

Creating a concave shape in the piezoelectric crystal

Gluing an acoustic lens to the front of the crystal

Electronically with the use of phased array transducers.

25
Q

How does the phased array probe work to focus on a specific object?

A

There are crystals side by side in a linear array. The crystals at ends of array are activated first before those located at the center producing a concave wave front thereby focusing the beam at a selected distance from the transducer face.

26
Q

What is axial resolution?

A

ability of ultrasound system to identify two separate objects that lie along the path of the ultrasound beam axis.

27
Q

What determines axial resolution in ultrasound probe?

A

Bandwidth - resonant frequencies that are emitted about the center frequency.

28
Q

What is the general rule for axial resolution based on wavelength?

A

Axial resolution is about 1.5 times the wavelength of the system.

29
Q

What is lateral resolution of ultrasound probe? Other name for lateral resolution?

A

Ability of the ultrasound system to distinguish between objects that are horizontally aligned and perpendicular to the path of the ultrasound beam.

Also known as Azimuth Resolution

30
Q

What is elevational resolution of ultrasound?

A

Ability of the ultrasound system to distinguish between objects that are vertically aligned and perpendicular to the emitted ultrasound beam.

31
Q

Is lateral or elevational resolution better in ultrasound?

A

Lateralresolution typically 50% greater.

32
Q

What are side lobes in ultrasound beams?

A

They are additional beams of sound emitted that travel off-axis to the main beam These can incorrectly result in reflections off axis to appearn as reflections of the main beam.

33
Q

What is a grating lobe?

A

The constructive interference of side lobes genearted with multielement array transducers.

34
Q

What are two techniques to minimize side lobe artifacts?

A

Lower the gain.

Image structures from multiple planes/windows.

35
Q

What is time gain compensation?

A

Allows the echocardiographer to selectively amplify signals from structures of varying distances from the transducer.

36
Q

What does the electrical processing of an ultrasound machine do to low amplitude signals?

A

They filter them out as they typically represent background noise or speckle.

37
Q

What is B mode?

A

Brightness-mode. Brightness corrrelates with the strength of the returning signal

38
Q

What is M mode on ultrasound?

A

Adds temporal information to B mode by displaying a series of collected b mode images. One dimentional ice pick view through heart and updates the b mode images at a very high rate.

39
Q

What is A mode on the ultrasound?

A

Amplitude mode in which amplitudes of returning signals are represented as a series of horizontal spikes along the vertical axis.

40
Q

How does 2D echocardiography work?

A

It is a modification of B mode where instead of firing ultrasound pulses in a single direction, the transducer in 2D echocardiography sequentially directs the ultrasound pulses across a sector of cardiac anatomy.

41
Q

Is 2D imaging or M mode more precise for demonstrating dynamic motion or the timing of cardiac events?

A

M mode.

42
Q

What is pulse repetition frequency?

A

rate at which sound pulses are transmitted per second

43
Q

What is frame rate?

A

frequency at which sector is rescanned.

44
Q

What is the scan line density?

A

number of lines per degree of the sector.

45
Q

What is tissue doppler imaging?

A

Using conventional doppler principles to measure the velocity and direction of the contracting myocardium compared to doppler signals from blood cells which are high velocity and low amplitude, tissue doppler signals are low velocity and high amplitude.

46
Q

What is equation for amount of sound reflected between two mediums?

A

R = ((Z2-Z1)/(Z2+Z1))^2

Z2 and Z1 are reflection coefficients.

Advanced PTEeXAM (19 decks)

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