Image Acquisitionand Proces

Image Analysis 191
The Resting Preview VI provides image and transducer verification, and
allows focus and finger placement adjustments to be made while familiarizing the
subject with the procedure. The Resting Data VI collects 30 seconds of analog
input data, providing baseline measurements of the pulse volume and resting finger
force. From these measurements, the Resting Mean global variable is populated,
and target force values for subsequent use during the nail press are computed from
this value. As images are not acquired here, data is gathered at a high collection rate
(1024 Hz) and provides an additional resource of data for any subsequently desired
frequency analysis measurements requiring higher resolution.
The Nail Press VI utilizes its front panel interface to instruct and prompt the
subject accordingly. The test consists of two sequences but starts only after the
occurrence of a hardware trigger. This trigger starts a counter on the DAQ card,
which generates a square pulse wave that serves to initiate each buffer of the image
acquisition. During the initial sequence (the first 10 seconds), the interface provides
an audible prompt and visual instructions to “Rest and Relax” while resting images
and analog input data are gathered at 128 Hz. The subject is then instructed to “press
and hold” in order to reach a desired target force, calculated by adding a load cell
calibrated force to the RESTING MEAN global value. Once the target is exceeded,
a countdown of 10 seconds begins, during which the subject maintains the target
force level. The subject is then prompted to “rest and relax” for 20 additional seconds
of data collection. A browser (Figure 7.37) is created which displays the images
acquired during the press for verification of analysis results, and all images and raw
data are written to file for postprocessing and analysis. The generation of time
stamped data for timing verification purposes was essential, as this VI was expanded
and elaborated upon for functionality. The use of software synchronization was not
problematic at the low image acquisition rate (1 Hz); however, the use of hardware
synchronization, which provides resolution in the order of nanoseconds, will be used
when higher collection rates are desired.
7.5.2.2 Postprocessing and Analysis
Characteristic force and blood volume responses during the nail press are shown in
Figure 7.38. The nail press sequence may be reviewed within the Image Analysis
VI, which cycles through the acquired images at a user-defined speed (Figure 7.39).
As each image is displayed, a histogram is generated and data from this report is
unbundled and saved for further analysis. Although acquired as RGB images, the
histograms are generated from their 8-bit representations. From the histogram data,
selections are made for those classes demonstrating the most significant change
during the press, and an evaluation of the pixel numbers within these bins over time
generates a curve which, not surprisingly, resembles the curves of the force and
blood volume results.
Critical to the automation of the nail press was the development of an algorithm
that could be applied in the analysis of all of the data. Such an algorithm was
developed and incorporated into both the Image Analysis and Data Analysis
VIs. The typical image, force and blood volume data all revealed steep slopes during
the nail press; therefore, the data were analyzed by taking step derivatives along the