Emerging optical imaging techniques such as hyperspectral imaging (HSI) provide a promising non-invasive solution for intraoperative tissue characterisation with the potential to provide rich tissue-differentiation information over the entire surgical field. Neuro-oncology surgery would especially benefit from detailed real-time in vivo tissue characterisation, improving the accuracy with which boundaries of safe surgical resection are delineated and thereby improving patient outcomes. Current systems are limited by challenges with processing the HSI data because of incomplete characterisation of the optical properties of tissue across the complete visible and near-infrared wavelength spectrum. In this study, we characterised the optical properties of various freshly-excised brain tumours and normal cadaveric human brain tissue using a dual-beam integrating sphere spectrophotometer and the inverse adding-doubling technique. We adapted an integrating sphere to analyse 2 mm-thick tissue samples measuring 4 – 7 mm in diameter and validated the experimental setup with a tissue-mimicking optical phantom. We investigated the different spectral signatures of freshly-excised tumour tissues including pituitary adenoma, meningioma and vestibular schwannoma and compared these to normal grey and white matter, pons, pituitary, dura and cranial nerve tissues across the wavelength range of 400 – 1800 nm. It was found that brain and tumour tissues could be differentiated by their optical properties but the freezing process did alter the tissues’ relative absorption and reduced scattering coefficients. In this work, we have demonstrated a method to characterise the optical properties of small human brain and tumour specimens that may be used as a reference dataset for developing optical imaging techniques.
Elastography measures tissue strain, which can be interpreted under certain simplifying assumptions to be representative of the underlying stiffness distribution. This is useful in cancer diagnosis where tumors tend to have a different stiffness to healthy tissue and has also shown potential to provide indication of the degree of bonding at tumor–tissue boundaries, which is clinically useful because of its dependence on tumor pathology. We consider the changes in axial strain for the case of a symmetrical model undergoing uniaxial compression, studied by characterizing changes in tumor contrast transfer efficiency (CTE), inclusion to background strain contrast and strain contrast generated by slip motion, as a function of Young’s modulus contrast and applied strain. We present results from a finite element simulation and an evaluation of these results using tissue-mimicking phantoms. The simulation results show that a discontinuity in displacement data at the tumor boundary, caused by the surrounding tissue slipping past the tumor, creates a halo of “pseudostrain” across the tumor boundary. Mobile tumors also appear stiffer on elastograms than adhered tumors, to the extent that tumors that have the same Young’s modulus as the background may in fact be visible as low-strain regions, or those that are softer than the background may appear to be stiffer than the background. Tumor mobility also causes characteristic strain heterogeneity within the tumor, which exhibits low strain close to the slippery boundary and increasing strain toward the center of the tumor. These results were reproduced in phantom experiments. In addition, phantom experiments demonstrated that when fluid lubrication is present at the boundary, these effects become applied strain-dependent as well as modulus-dependent, in a systematic and characteristic manner. The knowledge generated by this study is expected to aid interpretation of clinical strain elastograms by helping to avoid misinterpretation as well as provide additional diagnostic criteria stated in the paper and stimulate further research into the application of elastography to tumor mobility assessment.
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