DOPPLER SONOGRAPHY

By Dr/ Dina Metwaly

Principles of Doppler ultrasound

General principles Spectral-specific parameters Color-specific parameters Power Doppler imaging Normal flow in arteries Normal flow in veins

General principles of Doppler ultrasound

The Doppler effect

Proposed by Christian Doppler It defined :sound that reflects off a moving

object undergoes a change in frequency. Commonly heard when a vehicle sounding

a siren approaches, passes, & recedes from an observer

RECEIVED frequency1. Higher during approach2. Identical at instant of passing by3. Lower during recession

•The difference in the

transmitted frequency (Ft) and

the received frequency (Fr) is

referred to as the Doppler

frequency shift.

•The angle between the

direction of sound and the

direction of motion of the target

is referred to as the Doppler

angle.

Doppler effect applied to Echocardiography

Transducer emits ultrasound reflected from RBC.

If RBC (flow of blood) moves toward transducer, frequency of the reflected sound’s increases

If RBC (flow of blood) moves away from the transducer, frequency of the reflected sound’s decreases

Goals of Doppler

Detection flow in a vessel Detection direction of flow Detection type of flow: Arterial or venous

Normal or abnormal

Measurement the velocity of flow

Types of Doppler

Continuous wave Doppler Spectral Doppler (duplex): This

combination of gray-scale sonography with pulsed Doppler sonography

Spectral & color Doppler (triplex) Power Doppler

Pulse wave doppler

Short intermittent busts of ultrasound are transmitted.

By varying the delay time between the transmission and reception of the sound wave, it is possible to determine the location (Le., depth) from which the Doppler signal arises the sample volume or Doppler gate.

The waveform that displays the Doppler information has the Doppler frequency shift on the vertical axis and time on the horizontal axis.

The Doppler shift from objects moving toward the transducer is positive and is typically displayed above the baseline, and the shift from objects moving away from the transducer is negative and typically displayed below the line.

As indicated in the Doppler equation, the frequency shift is proportional to the velocity.

So when analyzing blood flow, the size of the waveform varies with the flow velocity.

The frequency shift is also proportional to the cosӨ. At a Doppler angle of 90 degrees (blood flow

perpendicular to direction of sound), the cos Ө = 0 and no Doppler frequency shift will be detected.

On the other hand, at a Doppler angle of 0 degrees (blood flow and sound direction are parallel), the cos Ө = 1 .

Because this is the maximum possible value for the cos Ө, the Doppler frequency shift is maximum at an angle of 0 degrees. Therefore, orienting the transmitted Doppler pulse with respect to the blood vessel so that the Doppler angle is as close to 0 degrees as possible will obtain the largest Doppler signal

Optimizing Doppler angle :Larger the angle, greater the error

Ideally should be zero Usually not possible Do not use angle > 60 Great error in

velocity Angle 90 Complete loss of flow Transducer position Obtain smaller angle Different US systems May be different

results

Color Doppler It provides a real-time image, displaying tissue

morphology in gray scale and blood flow in color. Color Doppler sonography analyzes the phase

information, frequency, and amplitude of the returning echoes.

Signals from moving red blood cells are assigned a color (red versus blue) based on the direction of the phase shift (i. e . , the direction o f blood flow toward o r away from the transducer).

The color shade for each pixel is based on the mean frequency shift arising from that pixel.

High-frequency shifts are assigned a lighter color, and lower-frequency shifts are assigned a darker color

Stationary objects produce no phase shift and are assigned a gray-scale value, as in conventional gray-scale imaging.

Color Doppler

Displayed as color information- Direction- red vs blue (toward or away from

transducer) Velocity- brightness (bright blue higher

velocity) Variance (turbulence)- coded green to give a

mosiac apperance. Overlays this information on 2D images Time consuming (temporal resolution is

especially poor with a large sector window)

Example of Color Doppler

Power Doppler

Power Doppler imaging estimates the power or strength of the Doppler signal rather than the mean frequency shift.

The Doppler detection sequence used in power Doppler imaging is identical to that employed in frequency-based color Doppler imaging.

However, once the Doppler shift has been detected, the frequency components are ignored in lieu of the total energy of the Doppler signal .

The color relate to the moving blood volume rather than the direction or the velocity of flow

Disadvantages of power Doppler imaging

Do not provide velocity of flow Do not provide direction of flow New machines

provide direction of flow in power mode Very motion sensitive (poor temporal resolution)Less

suitable for rapid scan along vessels

Advantages of Color Doppler and Power Doppler

COLOR DOPPLER POWER DOPPLER

Determines flow direction

Slightly more sensitive to detect low-velocity flow

Determines relative flow Angle independen Useful in imaging tortuous vesselst

Less affected by tissue motion

No aliasing

Less affected by probe motion

Distinguish pre-occlusive from occlusive lesions & Increases accuracy of grading stenosis

DOPPLER OPTIMIZATION1. Transducer Frequency

Because the Doppler frequency shift is proportional

to the transmitted frequency, higher-frequency

transducers cause a higher Doppler frequency shift

that is easier to detect.

Therefore, higher-frequency probes result in a

stronger reflection from red blood cells. These effects

improve the sensitivity of higher-frequency probes.

2. Gain Doppler gain is a receiver end amplification

of the Doppler signal that can be applied to either the Doppler waveform or to the color Doppler image. In most situations, the gain should be increased to a maximum value just before the point where random noise begins to obscure the pulsed Doppler waveform, or with color Doppler to the point where color starts to appear in nonvascular spaces of the color image.

The Doppler gain affects only the Doppler portion of the image and does not affect the gray-scale background

3. Power: Stronger or more powerful sound pulses will produce

stronger reflections that are more easily detected. Power output affects both the gray-scale and Doppler

images. In general, increasing the power output improves

Doppler sensitivity . This can be very important in deep abdominal applications where tissue attenuation significantly weakens the Doppler signal.

However, increasing the power output also lead to a number of artifacts. Therefore, power levels should be kept as low as is reasonably achievable to obtain the desired information.