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



Overview
The Texo SDK can be used to program the ARFI sequence as follows:
 * Access to the API that can build a custom sequence of individually programmable scanlines on the Sonix systems.
 * From version 1.8, the SDK provides extended transmissions pulses that can be used for ARFI
 * On the Sonix RP, ensure to set tx=2 when calling init to enable CW mode, which is used for ARFI. Note that this will limit the maximum transmit aperture to 64 elements, tx=3 allows for 128 maximum aperture, but no CW mode.

Programming
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 (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.
 * 200 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.
 * Focusing time delays
 * Currently only half of the transducer is using focusing time delays and the other half transmits plane waves.
 * Voltage Droop
 * Ideally the amplitude should stay steady during the transmit push pulses.
 * Voltage Amplitude
 * Higher amplitudes will increases the applied force which subsequently will increases the displacements.