At some point, high-frame-rate commercial CMOS or CCD cameras
will surely obtain the desirable performance for laser Doppler blood
flowmetry at a reasonable cost.
multiplexers. A digital processing unit implements an eight-tap IIR filter to obtain the flow and, in this case, squaring and
averaging. The total area of the sensor is 5. 9 3 5. 9 mm and the
fill factor of a pixel is 20 percent. The device will provide flow
images at roughly 1 frame/s.
Future challenges
We will combine the 64 3 64 pixel array with wide-field illumination and appropriate imaging optics. We are testing it in a
clinic at the School of Medicine at the University of Southampton in the United Kingdom in late 2009 to study inflammatory
responses. Provided we achieve acceptable performance, we will
construct higher-resolution arrays based on this design. Fixed
pattern noise across the sensor array is a problem when a logarithmic current-to-voltage converter is used. Accurate calibration for fixed pattern noise is therefore required.
At some point, high-frame-rate commercial CMOS or
CCD cameras will surely obtain the desirable performance
for laser Doppler blood flowmetry at a reasonable cost. A
commercial CMOS camera coupled to a high-performance
FPGA may become the instrument of choice for clinicians.
However, there will still be applications in which the small size
of an application-specific integrated circuit will be beneficial—
for example, for pocket-sized Doppler imagers or in capsule
endoscopy. There is also potential in other biomedical applications in which higher frequency shifts are used, such as in
ultrasound modulated optical tomography (which operates at
tens of MHz).
We are also exploring other interesting non-medical applications. For example, Doppler imaging can be used to study
fluid and air flows. It can also be applied in vibrometry, where,
in many cases, the Doppler frequencies are far higher than can
be measured with high-frame-rate cameras. Moreover, it can
image the Doppler shifts obtained from a spinning diffuser.
Different radial positions have varying angular velocities and
produce different Doppler shifts that can be imaged—in this
case by the 32 3 32 pixel sensor.
Conclusion
Smart optical sensors can extract parameters of interest from
incoming signals at the sensor and therefore overcome the
data bottleneck between sensor and processor. This enables
full-field laser Doppler blood flow imaging to be obtained
at a frame rate in which effects due to motion artifacts are
significantly reduced and will allow clinical applications
that involve changes in blood flow to be imaged. Since conventional signal processing cannot be applied due to space
[ Imaging Doppler shifts ]
3 105
5
6. 5
10
6.0
15
5. 5
20
5.0
25
4. 5
30
4.0
The sensor has been used to image the Doppler shifts
obtained from a spinning diffuser.
5 10 15 20 25 30
constraints, the key challenge is to implement the processing
electronics in a small silicon area. t
This research has been funded by the U.K. Department of Health
(NEAT programme) and the U.K. Technology Strategy Board. We are
grateful to our collaborators at Moor Instruments, the Universities of
Exeter and Southampton and the Nottingham University Hospitals
Trust. We thank colleagues at the University of Nottingham, including
D. He, J. Himsworth, N. Hoang, X. Xu and Y Zhu.
Steve Morgan ( steve.morgan@nottingham.ac.uk), Barrie Hayes-Gill and
John Crowe are with the electrical systems and optics research division
at the University of Nottingham in Nottingham, United Kingdom.
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>> A. Serov and T. Lasser. “High-speed laser Doppler perfusion imaging using an integrating CMOS image sensor,” Opt. Express 13( 17),
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