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: // in microseconds pushDuration = 200; pushLine = 32;

// activate long transmits tx.trex = true;

// transmit aperture tx.aperture = 64; // transmit center element tx.centerElement = (pushLine * 10) + 5; // push duration rx.customLineDuration = pushDuration * 1000; // no acquisition for ARFI pulse rx.acquisitionDepth = 0; // add ARFI pulse to the sequence texo->addLine(tx, rx);

// deactivate long transmits tx.trex = false;

// program tracking scanlines ...

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



Acoustic Measurements
Courtesy of Matthew Urban, Mayo Clinic, 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.