Rinaldo Cubeddu

A solid tissue phantom for photon migration studies

A solid tissue phantom made of agar, Intralipid and black ink is described and characterized. The preparation procedure is fast and easily implemented with standard laboratory equipment. An instrumentation for time-resolved transmittance measurements was used to determine the optical properties of the phantom. The absorption and the reduced scattering coefficients are linear with the ink and Intralipid concentrations, respectively. A systematic decrease of the reduced scattering coefficient dependent on the agar content is observed, but can easily be managed. The phantom is highly homogeneous and shows good repeatability among different preparations. Moreover, agar inclusions can be easily embedded in either solid or liquid matrixes, and no artefacts are caused by the solid-solid or solid-liquid interfaces. This allows one to produce reliable and realistic inhomogeneous phantoms with known optical properties, particularly interesting for studies on optical imaging through turbid media.

Age-related changes in the morphology, absorption and fluorescence of melanosomes and lipofuscin granules of the retinal pigment epithelium

The morphological and spectral characteristics of purified populations of melanosomes and lipofuscin granules from the human retinal pigment epithelium (RPE) were studied with respect to donor age. All melanosome and lipofuscin fractions exhibited the typical ultrastructural appearance associated with these granules. Absorption profiles of both melanin and lipofuscin granules demonstrated an increased optical density of the granules with increasing age. The former was associated with an overall increase of melanin within the granules. Melanosomes were weakly fluorescent; emission in the blue decreased with increasing age while emission in the red increased. The fluorescent intensity of lipofuscin granules increased with age. These results provide support for the concept that melanogenesis is occurring within the human RPE throughout life and that pigment granules within the RPE undergo age-related modifications during life.

Determination of visible near-IR absorption coefficients of mammalian fat using time- and spatially resolved diffuse reflectance and transmission spectroscopy

In-vivo optical spectroscopy and the determination of tissue absorption and scattering properties have a central role in the development of novel optical diagnostic and therapeutic modalities in medicine. A number of techniques are available for the optical characterization of tissue in the visible near-IR region of the spectrum. An important consideration for many of these techniques is the reliability of the absorption spectrum of the various constituents of tissue. The availability of accurate absorption spectra in the range 600 to 1100 nm may allow for the determination of the concentration of key tissue constituents such as oxy- and deoxy-hemoglobin, water, and lipids. The objective of the current study is the determination of a reliable absorption spectrum of lipid(s) that can be used for component analysis of in-vivo spectra. We report the absorption spectrum of a clear purified oil obtained from pig lard. In the liquid phase above 36 degrees C, the oil is transparent and thus suitable for collimated transmission measurements. At room temperature, the oil is a solid grease that is highly scattering. The absorption and scattering properties in this solid phase are measured using time- and spatially resolved diffuse reflectance spectroscopy. Using these three independent measurement techniques, we have determined an accurate estimate for the absorption spectrum of mammalian fat.

In vivo optical characterization of human tissues from 610 to 1010 nm by time-resolved reflectance spectroscopy

A fully automated system for time-resolved reflectance spectroscopy based on tunable mode-locked laser sources and on time-correlated single-photon counting for the detection of time-resolved reflectance data was applied to the evaluation of the optical properties of biological tissues (arm, abdomen and forehead) in vivo from 610 to 1010 nm. The scattering decreases progressively with increasing wavelength, while the absorption line shapes show the typical spectral features of the principal tissue components (haemoglobin, water and lipid), with different weights depending on the tissue type. The best fit of the absorption spectra measured in vivo with the spectra of the pure constituents yielded information on the percentage composition of the different tissues. The interpretation of transport scattering spectra with Mie theory provided information on tissue structure.

Nondestructive quantification of chemical and physical properties of fruits by time-resolved reflectance spectroscopy in the wavelength range 650–1000 nm

Time-resolved reflectance spectroscopy can be used to assess nondestructively the bulk (rather than the superficial) optical properties of highly diffusive media. A fully automated system for time-resolved reflectance spectroscopy was used to evaluate the absorption and the transport scattering spectra of fruits in the red and the near-infrared regions. In particular, data were collected in the range 650-1000 nm from three varieties of apples and from peaches, kiwifruits, and tomatoes. The absorption spectra were usually dominated by the water peak near 970 nm, whereas chlorophyll was detected at 675 nm. For all species the scattering decreased progressively with increasing wavelength. A best fit to water and chlorophyll absorption line shapes and to Mie theory permitted the estimation of water and chlorophyll content and the average size of scattering centers in the bulk of intact fruits.

Bulk optical properties and tissue components in the female breast from multiwavelength time-resolved optical mammography

We present the results of a clinical study about optical properties and bulk composition of the female breast. The clinical study involved more than 150 subjects that underwent optical mammography. A multiwavelength time-resolved mammograph designed to collect time-resolved transmittance images of the breast at different wavelengths in the range 637 to 980 nm is used to this purpose. From the absorption spectrum of the breast, the concentrations of the main tissue constituents, i.e., oxygenated and deoxygenated hemoglobin, lipid, and water, are obtained for a subset of 113 breasts. The lipid content of breast is estimated for the first time on such a large number of subjects. The total hemoglobin concentration, blood oxygen saturation, lipid, and water content of breast is correlated to demographic information collected during the trial. As expected, breast optical properties and components undergo huge variations among different subjects. Different constituents, however, show interesting correlation with clinical parameters such as age, breast size, body mass index, and mammographic parenchymal pattern. These results suggest that optical measurements on breasts can be exploited to obtain relevant information on breast tissue composition.

Performance assessment of photon migration instruments: the MEDPHOT protocol

We propose a comprehensive protocol for the performance assessment of photon migration instruments. The protocol has been developed within the European Thematic Network MEDPHOT (optical methods for medical diagnosis and monitoring of diseases) and is based on five criteria: accuracy, linearity, noise, stability, and reproducibility. This protocol was applied to a total of 8 instruments with a set of 32 phantoms, covering a wide range of optical properties.

Clinical trial of time-resolved scanning optical mammography at 4 wavelengths between 683 and 975 nm

The first time-resolved optical mammograph operating beyond 900 nm (683, 785, 913, and 975 nm) is presently being used in a clinical trial to test the diagnostic potential of the technique in detecting and characterizing breast lesions. Between November 2001 and October 2002, 101 patients with malignant and benign lesions were analyzed retrospectively. Scattering plots, as derived from a homogeneous model, and late gated intensity images, to monitor spatial changes in the absorption properties, are routinely used. The intensity images available at four wavelengths provide sensitivity to the main tissue constituents (oxy- and deoxyhemoglobin, water, and lipids), in agreement with expected tissue composition and physiology, while the scattering plots mirror structural changes. Briefly, tumors are usually identified due to the strong blood absorption at short wavelengths, cysts to the low scattering, and fibroadenomas to low absorption at 913 nm and high at 975 nm, even though the optical features of fibroadenomas seem not to be uniquely defined. The effectiveness of the technique in localizing and discriminating different lesion types is analyzed as a function of various parameters (lesion size, compressed breast thickness, and breast parenchymal pattern). .

Time-resolved optical mammography between 637 and 985 nm: clinical study on the detection and identification of breast lesions

The first time-resolved optical mammograph operating beyond 900 nm was tested in a retrospective clinical study involving 194 patients with malignant and benign lesions, to investigate the diagnostic potential for the detection and characterization of breast lesions. For the first part of the study (101 patients with 114 lesions), the system was operated at 683, 785, 913 and 975 nm. Subsequently, to improve the spectral content of optical images, the number of wavelengths was increased (up to 7) and the spectral range was extended (637-985 nm). Late gated intensity and scattering images provide sensitivity to tissue composition (oxy- and deoxyhaemoglobin, water and lipids) and physiology (total haemoglobin content and oxygen saturation), as well as to structural changes. Tumours are typically identified because of the strong blood absorption at short wavelengths (637-685 nm), while cysts are characterized by low scattering, leading to a detection rate of approximately 80% for both lesion types, when detection is required in both cranio-caudal and oblique views. The detection rate for other benign lesions, such as fibroadenomas, is presently much lower (<40%). The effectiveness of the technique in localizing and identifying different lesion types was analysed as a function of various parameters (lesion size, compressed breast thickness, age, body mass index, breast parenchymal pattern). The possibility that physiologic changes due to the development of a malignant lesion could affect the entire breast was investigated. The capacity to assess the density of breast based on the average scattering properties was also tested.

A semiconductor detector for measuring ultraweak fluorescence decays with 70 ps FWHM resolution

The performance of a new single-photon avalanche photodiode particularly suitable for the detection of fast and ultraweak light pulses is described. The advantages of this device over other available detectors are discussed. The electronic circuitry developed allows the measurement of fluorescence decays with a time resolution of 70 ps FWHM (full-width at half-maximum) and a data acquisition rate of up to 50 kHz.