The angular domain imaging system presented in this dissertation represents a
significant advance over previous ADI systems. However, further advancements can be made to improve the system and to optimize the data acquisition process for use as a clinical or commercially available tissue sample imager. This section discusses potential areas of improvement for the ADI system.
4.3.1
Illumination source
The illumination source used in Chapters 2 and 3 consisted of a high powered pulsed diode laser. The pulsed laser source was not optimal for ADI as a pulsed laser source is not required for continuous wave ADI. A comparable CW source such as a fiber coupled diode laser can be obtained at a significantly lower cost, and with improved beam quality and efficiency.
The illumination source was limited to a single wavelength. By imaging a sample with multiple wavelengths, the system will be more sensitive to various endogenous and exogenous sources of contrast. Also, multispectral measurements can be utilized to quantify the concentrations of various local absorbers.
A simple inexpensive multispectral light source can be conceivably constructed by combining several fiber coupled diode lasers. The various lasers can be modulated in intensity to image at different wavelengths, or the image acquisition can be multiplexed to collect multiple images simultaneously. A second approach for multispectral imaging can be performed with the use of a hyperspectral imaging source. A lamp or an LED array can be used as an illumination source. Wavelength discrimination can be performed with various interference filters, or an imaging spectrometer.
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The acquisition time of an ADI imaging system is directly related to the power of the illumination source. Acquisition times can be significantly reduced by using a more powerful laser. The highest admissible power for an ADI system is limited to ANSI limits (e.g. skin exposure >0.2W/cm2 for 400-1400 nm with an exposure time of 10-30000 s) for in vivo applications, or limited by tissue damage thresholds for a tissue sample.
4.3.2
Structured light
In Chapter 2, the contrast enhancement obtained by using structured light was first described. The method involved utilizing a matching angular filter positioned in front of the sample to mask out scattered light. In Chapter 3, the method was improved to use a digital light processor to create highly customizable illumination patterns. While the DLP
illumination method significantly improved the flexibility of the illumination patterns, it was limited by its resolution. The pixel pitch of the DLP was approximately 10 µm wide. The pattern emitted from the DLP was expanded with a 4x beam expander to increase the imaging area. This resulted in an effective pixel pitch of 40 µm. The AFA was translated in 125 µm increments. As a result, the images acquired suffered from various artifacts due to the mismatch between the DLP illumination patterns and the AFA shifting. This can be improved by utilizing a high resolution DLP chip to improve the effective resolution of the system without sacrificing the total illumination area.
A DLP light source represents a single example of a structured light source. Conceivably various other spatial light modulators can be utilized to improve the imaging capabilities of an ADI system. For example, a liquid crystal spatial light modulator can be used to modify both the phase and the amplitude of a beam of light. A phase modulator can be used to improve the collimation of the light source through wavefront correction, while a liquid crystal amplitude modulator can be used in place of a DLP. Each light structuring method provides its own advantages and disadvantages. With respect to ADI, five
characteristics of a spatial light modulator are of interest. These include the spectral response, efficiency, refresh rate, dynamic range and resolution.
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A structured light source can also be replaced with a scanning mirror. A galvanometer scanner or a MEMS scanning micromirror can be used to quickly redirect a laser beam. By scanning a collimated light source quickly and modulating the intensity of the light source, a two-dimensional light source can be constructed. A scanning laser can potentially be more efficient compared with a spatial light modulator.
4.3.3
Detection
In Chapters 2 and 3, an electron multiplying charged coupled device (EMCCD) was used to acquire images. The EMCCD allowed for the collection of images in low light situations. The electron multiplying technology allowed for a reliable gain mechanism to create a camera with high sensitivity and low noise. While a high powered laser was utilized as a light source, the AFA significantly reduced the amount of light reaching the detector by rejecting the majority of the scattered photons.
The studies outlined in Chapters 2 and 3 utilized a telecentric lens to image the exit face of the angular filter array. Conceivably, a camera can be bonded to or positioned against the exit face of the AFA and acquire comparable images. In this imaging arrangement, the size of the detection assembly is significantly reduced, allowing for a more compact system. In addition, the reduction of optical components reduces the cost and improves efficiency. Finally, the proposed system will be less sensitive to alignment errors.