Linear array CMOS detectors for laser doppler blood flow imaging

Himsworth, John M. (2011) Linear array CMOS detectors for laser doppler blood flow imaging. PhD thesis, University of Nottingham.

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Abstract

Laser Doppler blood flow imaging is well established as a tool for clinical research. The technique has considerable potential as an aid to diagnosis and as a treatment aid in a number of situations. However, to make widespread clinical use of a blood flow imager feasible a number of refinements are required to make the device easy to use, accurate and safe.

Existing LDBF systems consist of 2D imaging systems, and single point scanning systems. 2D imaging systems can offer fast image acquisition time, and hence high frame rate. However, these require high laser power to illuminate the entire target area with sufficient power. Single point scanning systems allow lower laser power to be used, but building up an image of flow in skin requires mechanical scanning of the laser, which results in a high image acquisition time, making the system awkward to use.

A new approach developed here involves scanning a line along a target, and imaging the line with a 1D sensor array. This means that only one axis of mechanical scanning is required, reducing the scanning speed, and the laser power is vastly reduced from that required for a 2D system.

This approach lends itself well to the use of integrated CMOS detectors, as the smaller pixel number means that a linear sensor array can be implemented on an IC which has integrated processing while keeping overall IC size, and hence cost, lower than equivalent 2D imaging systems.

A number of front-end and processing circuits are investigated in terms of their suitability for this application. This is done by simulating a range of possible designs, including several logarithmic pixels, active pixel sensors and opamp-based linear front-ends. Where possible previously fabricated ICs using similar sensors were tested in a laser Doppler flowmetry system to verify simulation results.

A first prototype IC (known as BVIPS1) implements a 64x1 array of buffered logarithmic pixels, chosen for their combination of sufficient gain and bandwidth and compact size. The IC makes use of the space available to include two front-end circuits per pixel, allowing other circuits to be prototyped. This allows a linear front-end based on opamps to be tested. It is found that both designs can detect changes in blood flow despite significant discrepancies between simulated and measured IC performance. However, the signal-noise ratio for flux readings is high, and the logarithmic pixel array suffers from high fixed pattern noise, and noise and distortion that makes vein location impossible.

A second prototype IC (BVIPS2) consists of dual 64x1 arrays, and integrated processing. The sensor arrays are a logarithmic array, which addresses the problems of the first IC and uses alternative, individually selectable front-ends for each pixel to reduce fixed-pattern noise, and an array of opamp-based linear detectors. Simulation and initial testing is performed to show that this design operates as intended, and partially overcomes the problems found on the previous IC - the IC shows reduced fixed pattern noise and better spatial detection of blood flow changes, although there is still significant noise.

Item Type:Thesis (PhD)
Supervisors:Hayes-GIll, B.R.
Morgan, S.
Uncontrolled Keywords:biomedical electronics, CMOS image sensors, Doppler measurement, laser applications in medicine, laser Doppler, blood flow, CMOS sensor, CMOS linear array
Faculties/Schools:UK Campuses > Faculty of Engineering > Department of Electrical and Electronic Engineering
ID Code:2392
Deposited By:Dr. John Himsworth
Deposited On:22 Mar 2012 15:39
Last Modified:22 Mar 2012 15:39

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