Ultrasound physics chapter 19

Doppler shift is also called

Doppler frequency

Process of extracting the low doppler frequency from the transducers carrier frequency is called

Demodulation

Relative motion between ___ and ___ creates frequency change

Sound source and receiver

This physical principle of frequency change is used to measure ___ of blood in circulation

Velocity

If the reflected sound has a higher frequency than the transmitted sound the doppler shift is?

Positive

When doppler shift is positive blood is moving in which direction

Toward the transducer

The equation for doppler shift is

Reflected frequency - transmitted frequency

What is created when transmitted sound waves strike moving red blood cells?

Doppler shift

When blood cells move away from the transducer the doppler shift is

Negative

Doppler frequencies indicate ___, not speed

Velocity

Speed is purely a

Magnitude

Indicates the distance that a red blood cell moves in 1 second

Speed

Units of speed

Cm/s (distance over time)

Velocity is defined by a ___ and a ___

Magnitude and direction

Doppler shift equation (the longer equation)

2 x velocity of blood × transducer freq x cosine ÷ propagation speed

Doppler shift is ___ related to velocity of blood

Directly

Why is the there a 2 in the doppler equation?

2 doppler shifts

What are the 2 doppler shifts that occur

1. Sound wave strikes RBCS

2. Transducers reception of the sound wave

Modern ultrasound systems measure the ___ ___ between received and transmitted sound waves

Frequency difference

X axis of doppler spectrum represents

Time

Y axis of doppler spectrum represents

Doppler shift or velocity

Relationship between doppler shift and frequency

Direct

Doppler shift has a unit of

Hertz

Measurement of doppler frequency depends on the relationship between

Direction of blood flow and direction in which sound waves propogates

When blood cells are moving ___ to sound beam the entire velocity is measured

Parallel

When an angle exists between the direction of flow and the sound beam the measured velocity is

Less than true velocity

The percentage of true velocity that is measured depends on the ___ of the angle between sound beam and direction of flow

Cosine

What is the relationship between cosine and doppler shift

Direct

When the angle between direction of motion and sound is 0° or 180° flow is

Parallel

Cosine of 0°

1

Cosine 0° indicates what direction of flow

Toward the transducer

Cosine -180° is

-1

Cosine -180° indicates what direction of flow

Away from transducer

When the direction of flow is 90° the measured velocity is

0

The cosine of 60° is

0.5

The cosine of 30°

0.87

The cosine of 120°

-0.5

The cosine of 150°

-0.87

The most common clinic doppler devices are

Bidirectional

Commonly used signal processing technique for bidirectional doppler

Phase quadrature of quadrature detection

Are arterial velocities higher/lower than venous velocities

Higher

Continuous wave doppler requires __ crystals in the transducer

2

Greatest advantage of continuous wave doppler us it's ability to

Accurately measure very high velocities

Primary disadvantage of continuous wave doppler is that

The exact location of the moving blood cells cannot be determined

The limitation of continuous wave doppler is that the exact location of moving blood cells cannot be determined and this is called

Range ambiguity

Another disadvantage of continuous wave doppler is its lack of

TGCs

A dedicated continuous wave transducer is simple because it does not create

Anatomic images

Dedicated continuous wave transducers contain __ elements, each shaped like a __

2, semicircle

Simultaneous imaging and doppler imaging is called

Duplex imaging

Dedicated continuous wave transducers do not use

Backing material

Dedicated continuous wave transducers do not use backing material which results in what

Undampened signal, narrow bandwidth, high q factor and high sensitivity

In pulsed wave doppler how many pzt crystals are necessary?

1

Greatest advantage of pulsed doppler is

Being able to select the exact location where velocities are measured

The advantage of pulsed doppler is called

Range resolution, range specificity, or freedom from range ambiguity artifact

With a pulsed doppler the sonographer positions a small marker called the ___ on a 2d image

Sample volume or gate

During pulsed doppler the transducer emits a sound pulse and then waits the time of flight before listening for a reflection, if the rrasnducer relieves an echo the echo was produced at the

Gate

Primary disadvantage of pulsed doppler is

Inaccurate measurement of high velocity signals

When high velocity flow is incorrectly displayed as traveling in the opposite direction the inaccuracy is called

Aliasing

Pulse doppler transducers are imaging transducers that contain

Backing material

Since pulsed doppler transducers have backing material they have characteristics similar to those of imaging transducers which are -

Low quality factor, Low sensitivity, wide bandwidth

The most common error associated with doppler ultrasound

Aliasing

And alias is a

False identity

Aliasing occurs only with ___ doppler

Pulsed

Occurs when the doppler sampling rate is to low in comparison to the measured blood velocities

Aliasing

Nyquist limit is ___ of the PRF

1/2

Nyquist limit equation

PRF/2

Highest doppler frequency that can be measured without appearance of aliasing

Nyquist limit

2 ways to avoid aliasing are

Raise nyquist limit or reduce doppler shift

The sampling rate of pulsed doppler is the systems

PRF

When sample volume is deep the PRF Is ___ and the nyquist limit is ___

Low, low

Since deeper sample volumes have a lower PRF ___ happens

Aliasing

Higher/lower frequency transducers create more aliasing

Higher

5 techniques used to avoid aliasing artifact

1 adjust scale to max

2 select new ultrasonic view with shallower sample volume

3 select lower frequency transducers

4 use baseline shift

5 use continuous wave doppler

When the sonographer adjusts the scale to its max the ___ is also increased to its max

PRF

The advantage of maximizing PRF is

Raises the nyquist limit and aliasing is less likely to appear

Disadvantage of raising the PRF to max is

Reduces sensitivity to low velocities

What are the disadvantages to selecting a new ultrasonic view with a shallower sample volume

None

Advantage of selecting a lower frequency transducer

Lower shifts occur with lower frequency transducer

Disadvantages of selecting a lower frequency transducer

No significant disadvantages l

Advantage of zero baseline shift

High velocity flows are displayed in proper direction

Advantage of using continuous wave doppler

Aliasing never appears with continuous wave doppler

Eliminating aliasing improves the ability to measure the ___ ___ with doppler

Maximum velocity

Gray shades on a doppler spectrum are related to

Amplitude of reflected signal or number of blood cells creating reflection

Form of 2d doppler where velocity information is coded into colors and superimposed on a 2 dimensional gray scale anatomic image

Color flow doppler

Color doppler provides information on the location of flow and therefore

Is a pulsed ultrasound technique, has range resolution, is subject to aliasing

Color doppler provides information regarding direction of flow and is considered

Semi quantitative

Spectral doppler measures

Peak velocity

Color doppler measures

Mean velocities

Color doppler uses a dictionary or a ___ ___ to concert measured velocities into colors that appear on the image

Lookup table

The two most commonly used dictionaries are called

Velocity mode and variance mode

What do the colors in a velocity mode map mean

Flow direction and velocity

The black region in the middle of the color map indicates

No doppler shift

With color doppler multiple ultrasound pulses are used to accurately determine blood velocities, these multiple pulses are called a

Packet or ensemble

Packets composed of a larger number of Pulses have 2 advantages which are

More accurate velocity measurement and increased sensitivity to low flow

Packets with more pulses have 3 disadvantages which are

More time needed to acquire data, reduced frame rate, decreased temporal resolution

Power mode only identifies the presence of a

Doppler shift

Power mode is ___ color doppler

Non directional

Power doppler is also called

Energy mode or color angio

In power mode the ___ of the reflected signal is processed without regard to the bloods direction or speed

Strength (amplitude)

The amplitude of the reflection is ___ related to the number of moving blood cells

Directly