Due to increasing availability of pharmaceuticals from many sources, a need is growing to quickly and efficiently
analyze substances in terms of the consistency and accuracy of their chemical composition. Differences in chemical
composition occur at very low concentrations, so that highly sensitive analytical methods become crucial. Recent
progress in dispersive spectroscopy with the use of 2-dimensional detector arrays, permits for signal integration along a
long (up to 12 mm long) entrance slit of a spectrometer, thereby increasing signal to noise ratio and improving the ability
to detect small concentration changes. This is achieved with a non-scanning, non-destructive system. Two different
methods using P&P Optica high performance spectrometers were used. High performance optical dispersion Raman and
high performance optical absorption spectroscopy were employed to differentiate various acetaminophen-containing
drugs, such as Tylenol and other generic brands, which differ in their ingredients. A 785 nm excitation wavelength
was used in Raman measurements and strong Raman signals were observed in the spectral range 300-1800 cm-1.
Measurements with the absorption spectrometer were performed in the wavelength range 620-1020 nm. Both Raman and
absorption techniques used transmission light spectrometers with volume phase holographic gratings and provided
sufficient spectral differences, often structural, allowing for drug differentiation.
The need for parallel spectral analysis of small details in microscopic samples is well recognized in many research
fields. Many instruments were proposed for this purpose, some of them using direct projection of an image produced by
a standard microscope onto entrance slit of a spectrometer. Typical scanning wavelength spectrometers using focusing
reflective gratings have limited imaging performance. These spectrometers also suffer from low light coupling
efficiency, poor spatial and spectral resolution, high acquisition times and low image quality. These significant
concerns are now addressed by a coupling of a high performance imaging spectrometer to one of the readout ports of a
microscope. This spectrometer uses refractive optics, transmission based volume phase holographic (VPH) diffraction
gratings and is equipped with two-dimensional array of photodetectors. Such a system provides a significant advantage
over most currently used microscope coupled spectrometers, resulting in a larger volume of extracted information,
better spectral and spatial resolution, higher SNR and generally better image quality. This is illustrated with examples
of spectral images of various biological samples.
Medical diagnostics and screening are becoming increasingly demanding applications for spectroscopy. Although for many years the demand was satisfied with traditional spectrometers, analysis of complex biological samples has created a need for instruments capable of detecting small differences between samples. One such application is the measurement of absorbance of broad spectrum illumination by breast tissue, in order to quantify the breast tissue density. Studies have shown that breast cancer risk is closely associated with the measurement of radiographic breast density measurement. Using signal attenuation in transillumination spectroscopy in the 550-1100nm spectral range to measure breast density, has the potential to reduce the frequency of ionizing radiation, or making the test accessible to younger women; lower the cost and make the procedure more comfortable for the patient. In order to determine breast density, small spectral variances over a total attenuation of up to 8 OD have to be detected with the spectrophotometer. For this, a high performance system has been developed. The system uses Volume Phase Holographic (VPH) transmission grating, a 2D detector array for simultaneous registration of the whole spectrum with high signal to noise ratio, dedicated optical system specifically optimized for spectroscopic applications and many other improvements. The signal to noise ratio exceeding 50,000 for a single data acquisition eliminates the need for nitrogen cooled detectors and provides sufficient information to predict breast tissue density. Current studies employing transillumination breast spectroscopy (TIBS) relating to breast cancer risk assessment and monitoring are described.
Based on the unique properties of the Volume Phase Holographic (VPH) Grating, P&P Optica has developed a high-performance imaging spectrometer system that has many uses in biophotonic and other research.
These fields include, but are not limited to, optical transillumination spectroscopy for breast cancer detection, photodynamic therapy and other medical optical imaging.
Since the spectrometer is able to image multiple channels simultaneously, there are a multitude of non-biophotonic applications as well, including process control and particulate detection for environmental research.
This paper will outline the unique properties of the spectrometer and why it is valuable for the above applications. The discussion will include information on what Volume Holographic Gratings are and how they allow for the separation of the input and imaging parts of a spectrometer system. The result is a fiber optic spectrometer with superior SNR and efficiency compared to most currently available instruments.
The paper will also highlight research results that have obtained using the high-performance, multi-channel imaging spectrometer.
Mammographic density (MD) has been shown to be a strong risk predictor for breast cancer. Compared to subjective assessment by a radiologist, computer-aided analysis of digitized mammograms provides a quantitative and more reproducible method for assessing breast density. However, the current methods of estimating breast density based on the area of bright signal in a mammogram do not reflect the true, volumetric quantity of dense tissue in the breast. A computerized method to estimate the amount of radiographically dense tissue in the overall volume of the breast has been developed to provide an automatic, user-independent tool for breast cancer risk assessment. The procedure for volumetric density estimation consists of first correcting the image for inhomogeneity, then performing a volume density calculation. First, optical sensitometry is used to convert all images to the logarithm of relative exposure (LRE), in order to simplify the image correction operations. The field non-uniformity correction, which takes into account heel effect, inverse square law, path obliquity and intrinsic field and grid non- uniformity is obtained by imaging a spherical section PMMA phantom. The processed LRE image of the phantom is then used as a correction offset for actual mammograms. From information about the thickness and placement of the breast, as well as the parameters of a breast-like calibration step wedge placed in the mammogram, MD of the breast is calculated. Post processing and a simple calibration phantom enable user- independent, reliable and repeatable volumetric estimation of density in breast-equivalent phantoms. Initial results obtained on known density phantoms show the estimation to vary less than 5% in MD from the actual value. This can be compared to estimated mammographic density differences of 30% between the true and non-corrected values. Since a more simplistic breast density measurement based on the projected area has been shown to be a strong indicator of breast cancer risk (RR equals 4), it is believed that the current volumetric technique will provide an even better indicator. Such an indicator can be used in determination of the method and frequency of breast cancer screening, and might prove useful in measuring the effect of intervention measures such as drug therapy or dietary change on breast cancer risk.
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