ARFI

Introduction
Acoustic Radiation Force Imaging, or ARFI, is a method where high power ultrasonic waves are used to displace tissue for the purpose of tissue characterization and measurements.

The basic process is as follows:
 * Acoustic waves deposits energy into the tissue.
 * This energy generates a force that causes displacement of the tissue.
 * These motions are used to derive information about the tissue.



Pulse Sequencing
The basic sequence for programming ARFI is as follows:
 * Acquire imaging line/lines (used as a reference)
 * Apply the push pulse (a long transmit)
 * Acquire imaging line/lines afterward to use for tracking



System Requirements
To perform the sequence above, an ultrasound system must be composed of:
 * Programmable sequencer
 * Transmit pulses with long durations up to 500 microsec



Texo Implementation (6.1)
The Texo SDK can be used to program the ARFI sequence. Below is some pseudo-code for programming the Texo sequence:

// transmit aperture tx.aperture = 64; // transmit center element tx.centerElement = pushLine + 0.5; // fill out the entire transmit pulse train (48 cycles) tx.pulseShape= "+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-"; // repeat the transmit as many as required (maximum 255) tx.txRepeat = 20; // push duration rx.customLineDuration = pushDuration * 1000; // no acquisition for ARFI pulse rx.acquisitionDepth = 0;

// add a long transmit pulse to the sequence texo->addLine(tx, rx);

// program tracking scanlines ...

Electronic Measurements
The figures below show some measurements from an oscilloscope attached to a Sonix RP system.



Acoustic Measurements
The needle hydrophone results can be shown below.



Displacement Measurements
The figure below shows the resulting displacement from an ARFI sequence.
 * 100 microsec push duration
 * 1 kHz RF sampling
 * Displacement profile at push location shown below



Remaining Issues

 * Hold over effect
 * The switching from short pulses to long pulses introduces artifacts in the echo signal that appear in the displacement estimates. The user need to make sure enough delay exist between the push pulse and the imaging pulse. This can be controlled by the customLineDuration parameter.
 * Voltage Droop
 * Ideally the amplitude should stay steady during the transmit push pulses.