Các mode chế độ Siêu Âm (Ultrasound Modes)

Diagnostic Ultrasound Machine  Diagnostic imaging modes » B-mode imaging B-mode » Color flow imaging BC-mode » Spectral Doppler imaging BCD – mode Transmitter Echo signal processin

Ultrasound Modes

Ravi Managuli

Beamformer Instrumentation

Switch separates high

voltage transmitter from

low voltage receiver

Gain/Amplify Compress (dB) :

Log compression

Pulser Amplification

To compensate depth dependant attenuation

Demodulation

Reject

Pre amplification

All signals are amplified uniformly

Reject Called Threshold

Low level signals are removed

SC Storage (Memory)

Reduce the dynamic range: Equalize larger and smaller echoes Pulser Generates electrical signal

Sonographer controllable

vψ-1 vψScan conversion

  Amplifies signal uniformly

  No change in patient exposure

  Does not change SNR

»   Signal and noise are treated equally

Gain Adjustment

TGC

!   Compensation à Time gain or Depth gain

!   Compensate for the depth dependent attenuation

Depth

TGC

Near gain Delay

Slope : Gain compensated

Far gain : Maximally compensation Knee : Maximally compensated

TGC is only for pulse wave:

For CW wave, there is range ambiguity à So TGC cannot be used

TGC Adjustment

!   Log compression

!   Makes it easy to visualize small level echoes returned from tissues

!   Reduces the dynamic range

Scan Converters

!   Memory is used to store the data for scan converter

!   Also scan converter memory

!   Or scan converter itself

!   Scan Converter converts data suitable for display

!   Coverts polar domain to Cartesian display

to fill the gaps

Such interpolation can

be used to improve spatial resolution

Read vs Write Zoom

!   Read magnification

!   On the stored data

!   Write magnification

!   Ultrasound rescans the ROI

!   Better spatial resolution

Dynamic Range : Post Processing

!   # Shades of Gray Between Black and White

!   Expressed in dB

!   High dynamic range has lots of shades of

gray between black and white

!   Low dynamic range has fewer shades of gray

between black and white

Contrast Resolution

!   To display objects which are similar in strength

!   High contrast as well as high dynamic range are

necessary

!   Dynamic range and Contrast are user selectable

!   By using a different Gray maps provided

Diagnostic Ultrasound

Machine

   Diagnostic imaging modes

»   B-mode imaging (B-mode)

»   Color flow imaging (BC-mode)

»   Spectral Doppler imaging (BCD – mode)

Transmitter

Echo signal processing

Color flow signal processing

Scan conversion

Spectral Doppler processing

RF demodulator

Echo signal processing

Color flow signal processing

Scan conversion

Spectral Doppler processing

RF demodulator

B-mode

Color-flow

Spectral Doppler

Ultrasound System - A Mode

Receiver compensationTime gain

Ultrasound System - M Mode

Ultrasound System - B Mode

Ultrasound : Color Doppler Mode

Various Color Images

Color Flow Angiograpm or Color Power Doppler

Color Doppler

Spectral Doppler

Phase Difference First received signal

Second received signal

Static target leaves no phase difference

Returning echo from static target

Moving target leaves phase difference

φ

Δ

p

cT f

c V

!   !  

p

cT f

c V

π

φ

4 Δ

=

Color Flow Doppler

at only one location

!   Sample gate

!   FFT technique

!   All velocities present

It is the only method available to obtain reproducible, noninvasive, quantitative

parameters on blood flow

Peak systolic End diastolic

RI

PI Average velocity

•  Autocorrelation requires a minimum of 6-16 pulses per scan line

Color Doppler Processing

Color Doppler Imaging

!   Uses 200 – 500 sample gates along each scan line

!   Multiple pulses are sent down each scan line

!   To see the displacement

!   The number of pulses that are sent down each scan line is

called the Packet Size or Ensemble Length

Multiple Pulses

Multiple Gates

Pulse Length Difference

Doppler are almost same

8-12 cycles Narrowband signal 2-3 cycles

Wideband signal

Color Doppler Information

Color Doppler

   Displays average velocity

   Color map provides velocity information

   Narrow band signal

   Packet size varies from 8 to 16

Spectral Doppler

   Display frequency spectrum at one range gate

   Frequency provides detailed velocity information

at that location

   Narrow band signal

   Normally 128 to 256 vectors are acquired at one location

Color Doppler Imaging

Velocity Color Map

—   One color use to encode positive flow

—   Another color to encode negative flow

—   As the Doppler shifts increase, they are assigned lighter

shades of these colors

—   Shows changes in mean velocity by changing shades or

different color

Increasing flow velocity away from the transducer! Zero flow!

Increasing flow velocity toward the transducer!

Variance Color Map

!   Variance color map

!   In addition to velocity and direction variance mode

distinguishes laminar flow from turbulent flow

Turbulent information is also color coded

!   Positive direction

!   Negative direction

Variance Turblance

Turbulence Velocity

Color Frame Rate?

—  B-mode and Color frame rate

—  Inversely proportional to packet size

—  For real-time interactive feedback frame rate should be high

vectors vectors

fps

B C

E

PRF C

Frame Rate

!   In the color-Doppler mode

!   Reducing the ROI reduced the number of color

vectors

!   Thus in Color-mode, frame rate can be increased

by

vectors vectors

fps

B C

E

PRF C

+

×

=

Power Doppler Imaging (PDI)

.

Power Doppler Imaging (PDI)

Power Doppler (Color Angio)

!   Detects presence of flow

!   Non-directional color Doppler

!   Is used whenever extreme sensitivity to low/slow flow is

!

Power Doppler pros and

cons

!   Advantages

!   Disadvantages

!   Sensitive to noise

Adjust Aliasing

!   Increase PRF (also called scale)

!   Increases Doppler shift range

!   Select range gate at shallow depth

Wall Filter

!   Amplitude due to clutter (wall motion)

!   Is typically 40dB or more (100 times)

!   If not removed, display is littered with clutter

!   Difficult to detect Doppler shift due to

blood flow

!   Wall motion frequency (velocity) are much

lower than Doppler frequency

!   Wall filter is used to remove them

!   Only Doppler shift is remained

Amplitude to clutter

Amplitude to flow

After wall filter

Wall Filter

!   Increasing wall filter

!   Decreases low velocity signals

!   Flash artifacts

!   Decreasing wall filter

!   Increases low velocity signals

!   For example to visualize end-diastolic velocity

Wall filter à

No flow seen

Decreased wall filter

Fast Speed

Other Controls for Doppler

!   Steer : To steer the Doppler beam

!   Doppler angle correct

!   To inform the system at what

angle the blood vessel is sampled

Angle Correct

Wrong Angle

Wrong Gain

Wrong Gain

PRF Setting

Wrong PRF

PRF Setting

Wrong PRF

Inside a Commercial Ultrasound

Machine

!   Tissue Doppler Imaging

!   Tissue Elasticity Imaging

!   BW and Power

!   Real-time virual sonography

Tissue Harmonic

!   Fundamental à many artifact arise in first few centimeters

!   Harmonic images are created by deeper structures

!   No artifact due to near regions

!   The relationship between tissue harmonic and signal strength is non-linear

!   Weak signals (side lobe, grating lobe) do not create harmonic

!   Medium signals (elevation beam) small and

!   Strong signal (main beam) create strong harmonics

!   Thus artifacts due to grating lobe, side lobes, elevational beams

!   Are very small

Less noise Good spatial resolution

Clear boundary and

good contrast resolution

Harmonics

Contrast Harmonics : During

reflection

!   Contrast harmonics are created when ultrasound interacts with a microbubble

!   Non linear behavior of microbubbles when sound strikes them

!   Tissue harmonics are created during transmission

!   Determined by the shell and gas structure

!   Determines the interaction of microbubbles with ultrasound

!   The likelihood of bubble rupture increasing with increasing MI à Causing

harmful effect

!   MI increases with frequency and high negative pressure

!   The peak MI in the focal is estimated by the scanner

!   Mechanical index and harmonic creation is non-linear

!   < 0.1 MI à No harmonics

!   0.1 to 1.0 à Moderate

!   > 1.0 à Very strong harmonics à Strong reflection

!   Bubble does not burst

!   Transient cavitation à Inertial or normal cavitation

!   Occurs at higher MI

!   Bubble burst releasing energy

!   Not considered clinically significant

Low Cost US Machine High Performance US Machine

Guided by US machine Ultrasound-Enhanced Drug Delivery System

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