Physics

Tissue harmonics & Contrast harmmonics

• -They are safe.
• -Metabolically inert (doesn't interact)
• - Long-lasting
• -Strong reflector of ultrasound
• -small enough to pass through capillaries

With interaction of microbubbles, a small amount of energy is converted from fundamental to harmonic frequency. This increases the frequency so you get a better image.

During REFLECTION of sound.

proportional or symmetrical

irregular, disproportional, asymmetrical--behaves unevenly

• -administered by injecting
• -enclosed in a shell
• -have different acoustic fingerprints which result in stronger reflections that light up areas

non-linear

The creation of an image from sound reflections at TWICE the fundamental frequency (or 2nd harmonic frequency) of the transmitted sound.

The sound created by the transducer and transmitted into the body.

• -Reduction of acoustic noise
• -Enhances contrast resolution
• -improves borders
• -demonstrates shadowing & enhancement

• -Lower frequency
• -Stronger sound waves
• -High pressure variation

Estimates the amount of contrast harmonics that will be produced.

Low MI

Higher MI (medium)

Highest MI

The stability and longevity in circulation.

During TRANSMISSION of sound.

-Conversion of miniscule amount of energy from fundamental frequency to harmonic frequency.

-Non-linear behavior of speed caused by sound travelling.

-Harmonic frequency caused by variation in speeds.

faster, slower

In the MAIN beam.

Contrast harmonics

stronger

At superficial depth

Weak beam = no harmonics

Medium beam = tiny amount of harmonics

Strong beam = Significant amount of harmonics

Qualitative (an exact #)

Imaging that presents 2-D, cross-sectional, real-time blood flow or tissue motion information along with 2-D, cross-sectional, real-time gray scale anatomic imaging.

-How long it takes for the sound beam to travel to and from the site of the echo

-How strong the echo is

-If there is a Doppler shift present

-The magnitude of the Doppler frequency shift

-The direction of the Doppler shift

the MEAN velocity from the sample volumes and the angle between the ultrasound beam and the blood flow

Pulsed Doppler

A mathematical technique that detects the Doppler shift

The mean and variance of the Doppler shift info. at each location along the scan line.

Autocorrelation

Compares 2 consecutive pulses returning from a given sample volume. Uses time delay to send out the pulses. The phase shift between 4 or more pulses are used to estimate mean velocity which is used to display the corresponding color.

Red indicates flow towards the transducer. Blue indicates flow away from the transducer. Darker colors indicate slower speeds, whereas lightest/brightest colors indicate faster speeds.

stronger signal (amplitude)

Less saturated = faster velocity

more intense saturation = slower velocity

-Colors are arranged vertically

-Slowest flow towards baseline

-Faster flow at ends of scale

-Additional colors added on side

-Side-to-side changes relate to laminar vs. turbulent flow

-Green or yellow used to tag turbulent flow

The sampling frequency

1/2 PRF

-affects lowest frequency that can be displayed

-Only allows frequencies greater than cutoff frequency to be displayed

-Eliminates low flow during diastole

Higher PRF = Higher filter

# of images produced per second.

slower

Temporal resolution

• -Size of color box
• -Position of color box
• -Line density
• -PRF

Decrease the # of lines, decrease color box width, put the color box at a shallower depth

Average of consecutive images to improve signal to noise, creating a smoother color

-Can visualize vessels too small to be seen in gray scale

-Aids in determining where the peak systolic velocities are in order to place sample volume

-Documentation of pathologies such as occlusions, severe stenosis, and turbulent flow

-Can see flow in the entire vessel lumen

-Qualitative (average, not exact) information

-Lowers PRF and frame rate (degrades images, diminishes spatial resolution, more aliasing, and diminishes temporal resolution)

-Blood flow is angle dependent

-Flow direction is arbitrary

-Color may obscure pathology within a vessel

- Color flash

Power Doppler does not show the direction of blood flow.

The strength (amplitude) of the reflected signal is processed. The brightness is directly related to the number of moving blood cells. Displayed in amplitude.

-Improved sensitivity to flow (or slow velocity)

-Unaffected by Doppler angles

-No aliasing since velocity information is ignored

-VERY slow frame rate (lower temporal resolution)

-Susceptible to motion (flash artifact)

-No flow direction

-No velocity measurement

Occurs when Doppler sampling rate is too low and/or when Doppler Shift frequency exceeds half the PRF (Nyquist limit)

-Adjust the scale to its maximum (increases PRF)

- Select lower frequency transducer

-Reposition transducer so area of interest is shallower (increases PRF)

- Shift the base line

Duplicates structure on the other side of a strong reflector. (common around diaphragm and lungs) The image closest to the transducer is usually real.

Moving usually fixes this artifact

ELECTRONIC duplication of spectral info. Spectral info. appears on both sides of the baseline.

Tun the gain down or increase power

Occurs in Color Doppler. Anything in the field of view that moves is shown in color--this generates blotches of color that can obscure the field of view

-Increase wall filter

-Decrease persistence

-Reduce width of the color box.=

The extension of color beyond the region of flow to the adjacent tissue

Decrease the color gain or transmit power.

Random variations in signal detection causes areas without flow to be embedded with color.

Increase the filter or threshold to eliminate the low level echoes.

Useful flow phenomenon: mixture of color is seen within the soft tissue adjacent to the blood vessel as a result of the vessel wall vibration

They can act as a marker of vascular pathology. (Wall vibration is an indication of a severe flow disturbance.

AV fistulae, arterial stenosis, and psuedoaneurysms

Small, strong signal prominent above and below the baseline. (Occurs in systole)