Optical Imaging Methods

All diffuse optical measurements were made with the diffuse optical spectroscopic imaging system developed and built at the University of California, Irvine [30; 166]. This is a hybrid DOS instrument that combines multi-spectral frequency-domain and broadband imaging to measure absolute µa,

µ0_s, and the tissue concentrations of oxygenated hemoglobin (HbO2), deoxygenated hemoglobin

(HHb), water, and lipid. The combination of these chromophores permits calculation of tissue total hemoglobin concentration (HbT) and tissue oxygen saturation (StO2). Another optical parameter,

known as the tissue optical index (T OI), defined asT OI= HHb_Lipid·H2O, is also calculated. T OI was derived empirically to maximize contrast between tumor and normal tissue [60].

For data presented herein, DOSI utilizes a single source-detector separation: 2.2cmfor phantom measurements and 2.8cm for tissue measurements. At this single separation, frequency domain measurements are made using laser diodes at six wavelengths (660, 680, 785, 810, 830, and 850nm) that are fiber coupled to an imaging hand-piece that, in turn, is put into contact with the tissue (see Figure 3.3). This hand-piece contains an avalanche photodiode (APD) for detection of the frequency domain signals. To compensate for the lack of multiple source-detector positions, each laser diode is

modulated at 251 to 601 distinct frequencies ranging from 50M Hzto 600M Hz. Three frequency sweeps are performed and averaged for each wavelength, and the absorption coefficientµa and µ0s

are fit using a Levenberg-Marquadt algorithm for the amplitude and phase data at each frequency (see Section 2.6.6). A Mie-scattering framework is assumed for the scatterers, and thus a scattering amplitudeA and scattering powerb are fit using all 6 wavelengths (see Section 2.8.2).

Figure 3.3: DOSI Instrumentation and Hand-piece. This figure comes from [166]. A) DOSI Opto- Electronics Rack. This instrument cart contains the white-light source, laser diodes, spectrometer, frequency analyzer, and instrument control computer for the DOSI system. Several systems similar to this collected all of the DOSI data in Chapters 3, 4, and 5. Note that the instrument is portable and can be wheeled to the patient’s bedside. B) DOSI Hand-piece and Calibration Phantom. The DOSI hand-piece contains an APD for frequency-domain detection and optical fibers coupled to the optical components on the instrument rack. A calibration phantom, placed inside a case for easy alignment with the hand-piece, is also shown. C) DOSI Hand-piece and Reflectance Standard. The DOSI hand-piece is shown in contact with the broadband reflectance standard. Note that the fiber- coupling apparatus can easily be moved to three different positions, using the silver spring-loaded hand screws seen here, to provide multiple source-detector separations.

Broadband spectroscopy is performed using a continuous-wave white-light source and diffraction- grating spectrometer with a range of 650 to 1000nm, both of which are also fiber-coupled to the

tissue via the hand-piece. The best-fitAandbparameters from the frequency-domain scattering fit are then used to correct the CW reflectance spectroscopy for the tissue scattering. Additionally, the absolute absorption coefficients at the 6 frequency-domain wavelengths are used to quantitatively scale the broadband CW absorption spectrum. Finally, the DOSI instrument is calibrated using two standardized phantoms, for the frequency-domain system, and a reflectance standard, for the broadband system, before and after every measurement (see Figure 3.3). Thus, DOSI provides absoluteµa andµ0svalues across the entire 650 to 1000nmrange.

This hybrid modality provides improved quantification of chromophore concentration relative to DOS techniques that use several individual wavelengths because of the increased spectroscopic information across the full biological window. This scheme is particularly beneficial for calculating the lipid and water concentrations because light with wavelength greater than 900nm is more sensitive to these chromophores, and the spectrometer can easily measure the absorption spectrum at these longer wavelengths. Frequency-domain-only instruments often struggle to measure at wavelengths greater than 900nmbecause the required detectors, either PMTs or APDs, typically have very poor sensitivity over this range [247]. Full descriptions of this DOSI instrumentation and analysis technique have been published [30; 166].

During each subject’s Baseline measurement, a grid of∼50−240 points that encompassed both the palpated tumor region and the surrounding normal tissue was measured on the lesion-bearing breast using the DOSI handpiece. A mirrored grid of points was measured on the contralateral breast. These measurement grids were recorded using a hand-marked transparency film that was produced for each subject in order to guide DOSI handpiece placement to the same grid points during the three subsequent measurement time-points (see Figure 3.2). This practice ensured that each measurement point was probing the same tissue region during each longitudinal measurement. Each point measurement lasted approximately 2 to 5 seconds. Since the data analysis is performed independently at each point using a semi-infinite homogeneous model (see Section 2.5.4), this grid will produce separateµa andµ0svalues for each wavelength and each grid point. These independent

measurements can then be smoothed into a two-dimensional topographic image. Figure 3.4 contains a schematic of the DOSI measurement across these spatial grids and a sample image.