xyz scan data

This version: 25 September 2003
Robin Cleveland
Boston University
robinc@bu.edu

This describes the format for the data taken in an x-y-z scan at the
Acoustic MedBED facility of CenSSIS located in the Aerospace and 
Mechanical Engineering Department at Boston University.

Measurements are taken by moving an acoustic transducer in a one, two 
or three dimensional raster scan with uniform spatial steps over a target.  
The co-ordinate system assumes that the acoustic axis of the acoustic
beam is along the z-axis.  The x-y plane is perpendicular to the acoustic axis.

For the pulse-echo mode  at each location a pulse is transmitted and 
the echo received and digitised.  The data that in these files is a 
short time window of echo signal.

The variables in the Matlab file are:

description
  A structure with some useful meta data associated with the 
acquisition of the data.  Transducer type, settings of the 
electronics, target condition.  Described below.

xvec
  vector with locations in x axis where measurements were made
  
yvec 
  vector with locations in y axis where measurements were made
  
lenwf
  scalar with number of points in the waveform recorded at each 
location

vtwf(lenwf,xvec,yvec,zvec)
  an array with u pto four dimension with the individual voltage-time 
  waveforms (first dimension), as a function of x (second dimension) 
  and y (third dimension) and z (fourth dimension).

t0
  Absolute time associated with first time sample in the vector vtwxy.  
Zero time is defined as when the transducer is first excited by an 
electrical pulse.   If the time of the first sample was not uniform then 
this will be a vector of dimensions xvec,yvec,zvec.

tsample
  Sampling interval


===========

For example, to plot the waveform record at xvec(1) and yvec(1)  
versus time one could use the following Matlab code:

plot(t0+[0:lenwf-1]*tsample,vtwf(:,1,1,1));

The distance in the propagation axis (z-direction) can be obtained 
by multiplying the time base by the sound speed c0 and then dividing
by two to allow for the fact that time corresponds to a 
round-trip distance - there and back.

zvec=(t0+[0:lenwf-1]*tsample)*c0/2;

===========

Some of the components of the description structure.

description.filename=
  Name of file.
  
description.date=
  Date the data was taken.

description.mode=
  Either pulse-echo, through-transmission or pressure measure
  depending on the experiment that was carried out.

description.target=
  Fluid path and the target.  If the fluid path is not mentioned
  it can be assumed to be water.
  
description.transducer=
  Identifies the transducer model
  There is a structure associated with the trnasudcer that
  describes ancillary information about transducer, e.g.,
  frequency, focal length, radius
  
description.transmit=
  Instrument used to transmit the data
  There is also a structure associated with the transmit 
  instrument that describes the pertinent settings used
  
description.receive=
  Instrument use to receive the data
  There is also a structure associated with the receive 
  instrument that describes the pertinent settings used
  
description.digitise=
  Instrument used to digitise the data
  There is also a structure associated with the digitising 
  instrument that describes the pertinent settings used
  
description.location=
  Either Large Ultrasound Tank - LUT at MedBED at BU
  Or     Scanning Acoustic Microscope - SAM at MedBED at BU
  
description.student=
description.contact=
  People responsible for the data
  
description.source=
  Link to the CenSSIS website