Ultrasound Physics: Doppler and Harmonics

cavitation

interaction of US with tiny bubbles in tissue and
liquid

stable cavitation

gas bubbles oscillate (expand and contract)

inertial (transient) cavitation

gas bubbles rapidly oscillate

What is used to monitor interial cavitations

mechanical index (MI)

hydrophone

ultrasound transducer used to measure acoustic pressure fields

beam plots

measures intensities, key word Schlieren

Thermal index

• estimates the potential for tissue heating during ultrasound

• higher TI = greater potential for temperature rise

• preset by machine

Three Thermal Indices

• TIS= soft tissue

• TIB= bone

• TIC= cranial bone

Maximum intensities (SPTA)

• 100mW/cm sq. - unfocused

• 1W/cm sq. - focused

What is hemodynamics?

The study of blood flow or circulation.

How does Doppler ultrasound evaluate blood flow?

It assesses the characteristics of blood flow and differentiates normal from abnormal characteristics.

What is resistance in the context of blood flow?

The force placed upon blood that is determined by the terminal end of the blood stream/ supply

What characterizes high resistive arterial flow?

It has significant resistance that slows blood flow before reaching capillary vessels.

What characterizes low resistive arterial flow?

It has minimal resistance and demands continuous blood flow.

Examples of highly resistive vessels:

-external carotid

-SMA

-Peripheral arteries

Examples of vessels with low resistance:

-internal carotid

-renal artery

-celiac artery

Do low resistive vessels need blood flow all of the time?

yes

How can you differentiate between high and low resistance?

high often has a component that goes below the baseline

Venous side has the _____ pressure but _____ volume of blood.

lower; higher

What facilitates venous flow back to the heart?

Lower right atrial pressure, valves within the veins, calf muscle pump, and respiration.

How does respiration affect venous flow?

It can change diameter vessel facilitates propulsion of flow as well as changes the pressure within the thoracic and abdominal cavities.

What is the formula for calculating Doppler frequency shift?

Doppler frequency shift = received frequency - transmitted frequency.

-difference falls between audible range

What indicates a positive Doppler shift?

When the received frequency is higher than the transmitted frequency, displayed above the baseline.

What indicates a negative Doppler shift?

When the received frequency is lower than the transmitted frequency, displayed below the baseline.

The doppler effect is caused by a change in _____ caused by motion relative to the observer.

wavelength

Flow toward observer, wavelength becomes _____, frequency becomes _____

shorter; higher

Flow away from observer, wavelength becomes _____, frequency becomes _____

longer; lower

What is the significance of the Doppler shift in ultrasound?

It provides directionality of blood flow relative to the transducer.

What is the best angle for accurate velocity calculations in Doppler ultrasound?

Zero or parallel to the flow, where cosine of the angle is 1.0.

What cosine value is used for vascular Doppler measurements?

A cosine value of 0.5 is used, use a 60-degree angle or less.

What happens when the transducer is positioned at 90 degrees to blood flow?

The Doppler shift is zero, as the cosine of the angle is 0.

What is the Doppler equation?

fDop = (2 * f0 * v * cos(θ)) / c, where fDop is the Doppler shifted frequency.

What is the difference between Pulsed Wave (PW) and Continuous Wave (CW) Doppler?

PW has good range resolution but suffers from aliasing, while CW has no range resolution and does not suffer from aliasing.

What does spectral Doppler measure?

It detects various frequencies and velocities displayed in blood vessels, showing that not all red blood cells travel at the same velocity.

What causes spectral broadening in Doppler ultrasound?

Increased variation in the velocities of red blood cells traveling through a vessel.

What is the recommended angle for Doppler measurements relative to the vessel wall?

The angle must be parallel to the vessel wall and less than 60 degrees.

What is the purpose of the baseline adjustment in Doppler ultrasound?

To accommodate high velocities, such as those in the internal carotid artery.

What is the role of Fast Fourier Transform in Doppler instrumentation?

It is a mathematical technique used to process spectral Doppler data.

What is the PRF in ultrasound?

Pulse Repetition Frequency is the frequency at which sampling (pulses are transmitted) occurs.

Is PRF the same as transducer frequency?

No

Transducer frequency- MHZ

PRF- kHz

How does sample gate placement affect PRF?

The deeper the sample gate, the less the PRF will be.

What is aliasing in Doppler ultrasound?

Aliasing is a 'wrapping around effect' that occurs when the velocity exceeds the maximum limit

Maximum limit is called_____

Nyquist limit

What is the Nyquist limit in Doppler ultrasound?

The Nyquist limit= 1/2 PRF

What does the horizontal axis represent in the Doppler spectral format?

The horizontal axis represents time.

What does the vertical axis represent in the Doppler spectral format?

The vertical axis represents frequency shift or velocity.

What is the significance of the color bar in Doppler imaging?

The color bar presents flow and directionality and average flow velocities

What does the black line on the color bar represent?

the baseline; represents doppler shift

What is autocorrelation in ultrasound?

the technique used in equipment to process color Doppler imaging.

What are the key differences between Color Doppler and Power Doppler?

Color Doppler provides directionality, mean velocity mode, is angle-dependent, and displays two colors.

Power Doppler has improved sensitivity, is not angle-dependent, Amplitude mode, displays one color, and does not provide info on directionality.

What adjustments can be made to optimize Doppler imaging?

Adjustments include modifying PRF, wall filter (acts like a reject at the baseline), and gain levels.

What are the characteristics of fast or dynamic flow in Doppler imaging?

Fast flow is associated with lower Doppler frequency, higher PRF, higher wall filter, high frame rates, moderate color box width.

What are the characteristics of slow flow in Doppler imaging?

Slow flow is associated with higher Doppler frequency, lower PRF, lower wall filter settings, medium frame rates, decrease color box width.

What are some uses of Doppler ultrasound in vascular imaging?

Uses include evaluating carotids and arteries for stenosis, assessing veins for deep venous thrombosis, and accessing grafts for dialysis.

How is Doppler ultrasound utilized in obstetrics?

Doppler ultrasound is used for assessing the umbilical cord for fetal distress.

What is the role of harmonics in ultrasound imaging?

Harmonics improve lateral resolution and reduce artifacts like grating lobes and reverberations.

What is the disadvantage of using harmonics in ultrasound?

Harmonics may reduce axial resolution

What is the FDA approval status of contrast agents in sonography?

Contrast agents are only FDA approved for use in echocardiography.

What is the purpose of compound imaging in ultrasound?

Compound imaging generates beams from various angles to enhance resolution and detect pathology.

-iBeam, crossbeam

What are the advantages of compound imaging?

It improves resolution and information within the image and allows echoes to be received posterior to strong attenuators.

What are the concerns associated with compound imaging?

It can reduce enhancement and shadowing artifacts that are useful for diagnostics.

What is a panoramic feature in ultrasound?

A panoramic feature completes the image for you.