[6] Currently, the most common breast imaging techniques are X-ray mammography, ultrasounds, MRI and PET.

[citation needed] X-ray mammography is widely spread for breast screening, thanks to its high spatial resolution[7] and the short measurement time.

[10] Ultrasounds are non-invasive and they are used especially on young women,[11] who are usually characterized by dense breasts, but the images interpretation depends on the operator's experience.

MRI shows a good correlation with the tumour dimensions and is claimed to be the best method for the identification and characterization of lesions.

[12] Even though there is no verified long-term health risk from the magnetic fields employed during an MRI, it is not used as first investigative tool because of the high costs and the elevated duration of the exam.

[13] Finally, PET allows the early evaluation of the metabolic changes of the tumour,[14] but it is very expensive and requires the administration of a radioactive tracer.

On the contrary, optical mammography is cheap, efficient also on dense breasts, and devoid of any side effect, so that it can be used to track the evolution of the patient's condition on a daily basis.

[15] However, being still under development, there is a lack of standardization in data analysis among the research groups dealing with it, and it suffers from low spatial resolution.

For this reason, a "multimodal approach" is suggested, where optical mammography is complementary to another conventional technique, so that also the diagnostic efficacy is improved.

[10][15] Biological tissues are diffusive media, which means that light attenuation during propagation is due not only to absorption, but also to scattering.

represents the probability per unit length that an absorption event takes place, while the scattering coefficient

[6] Light propagation through highly diffusive media is typically described through the heuristic approach of the radiative transport theory, sided by the so-called “diffusion approximation”: scattering is assumed to be isotropic and strongly dominant over absorption.

This is fairly accurate for example for the breast tissue, in the red and near infrared spectral range (between 600 and 1100 nm), known also as "therapeutic window".

[19] Blood strongly absorbs in the red spectral range, whereas collagen, water and lipids have their absorption peaks at wavelengths longer than 900 nm.

The distinction between oxy and deoxy-haemoglobin is due to the presence of a second large peak in the case of oxy-haemoglobin.

The use of multiple laser sources allows to investigate the breast constituents' concentrations of interest, by selecting some specific wavelengths.

[29] Based on the number and position of sources and detectors, an optical mammograph can produce bidimensional or three-dimensional breast constituents' maps.

[citation needed] In time-domain measurements, short light pulses of the order of hundreds of picoseconds are delivered to the breast and its optical properties are retrieved from the features of the re-emitted pulses, which have undergone delay, broadening and attenuation.

[25][30] Time-correlated single photon counting is fundamental to cope with the low-level output signal.

In general, the CW approach is combined with the frequency domain one, in order to reinforce the strengths of both.

[19] A dense breast is characterized by a meaningful amount of fibrous tissue, relatively to the adipose one.

Studies demonstrate that the variation in concentration with respect to the healthy tissue is statistically more marked in the case of malignant tumours than benign ones.

One of the possible strategies is the administration of neoadjuvant therapy, whose goal is to shrink the tumour size before surgery.

[38] Studies show that if the therapy is efficient, then the water, collagen and hemoglobin contents of the lesion show a decreasing behaviour over time, which suggests that the initially fibrous tissue acquires features similar to the adipose one.

[4][39] Optical measurements in correspondence with therapy sessions could track its evolution, so to assess the patient's response to it.

Moreover, it is believed that therapy effectiveness could be predicted even on the first day of treatment on the base of initial breast constituents' concentrations.