mbll.py

# Authors: Robert Luke <mail@robertluke.net>
#          Eric Larson <larson.eric.d@gmail.com>
#          Alexandre Gramfort <alexandre.gramfort@inria.fr>
#
# License: BSD-3-Clause

import os.path as op

import numpy as np

from mne import __file__ as __FILE__
from mne.io import BaseRaw
from mne.io.constants import FIFF
from mne.utils import _validate_type
from mne.preprocessing.nirs import source_detector_distances, _validate_nirs_info

import constants

def modified_beer_lambert_law(raw, ppf=constants.PPF):
    r"""Convert NIRS optical density data to haemoglobin concentration.

    Parameters
    ----------
    raw : instance of Raw
        The optical density data.
    ppf : array-like
        The partial pathlength factors for the picked frequencies.

    Returns
    -------
    raw : instance of Raw
        The modified raw instance.
    """
    from scipy import linalg
    raw = raw.copy().load_data()
    _validate_type(raw, BaseRaw, 'raw')
    # _validate_type(ppf, 'numeric', 'ppf')
    picks = _validate_nirs_info(raw.info, fnirs='od', which='Beer-lambert')
    # This is the one place we *really* need the actual/accurate frequencies
    freqs = np.array([raw.info['chs'][pick]['loc'][9] for pick in picks], float)
    abs_coef = _load_absorption(freqs)
    distances = source_detector_distances(raw.info, picks='all')
    bad = ~np.isfinite(distances[picks])
    bad |= distances[picks] <= 0
    if bad.any():
        raise ValueError('Source-detector distances are zero on NaN, some resulting '
                         'concentrations will be zero. Consider setting a montage '
                         'with raw.set_montage.')
    distances[picks[bad]] = 0.
    if (distances[picks] > 0.1).any():
        raise ValueError('Source-detector distances are greater than 10 cm. '
                         'Large distances will result in invalid data, and are '
                         'likely due to optode locations being stored in a '
                         ' unit other than meters.')
    if isinstance(ppf, (int, float)):
        ppf = {wavelength: float(ppf) for wavelength in freqs}
    rename = dict()
    for ii, jj in zip(picks[::2], picks[1::2]):
        EL = abs_coef * distances[ii] * np.array([ppf[int(raw.info['chs'][ii]['loc'][9])], ppf[int(raw.info['chs'][jj]['loc'][9])]])[:, None]
        iEL = linalg.pinv(EL)

        raw._data[[ii, jj]] = iEL @ raw._data[[ii, jj]] * 1e-3

        # Update channel information
        coil_dict = dict(hbo=FIFF.FIFFV_COIL_FNIRS_HBO, hbr=FIFF.FIFFV_COIL_FNIRS_HBR)
        for ki, kind in zip((ii, jj), ('hbo', 'hbr')):
            ch = raw.info['chs'][ki]
            ch.update(coil_type=coil_dict[kind], unit=FIFF.FIFF_UNIT_MOL)
            new_name = f'{ch["ch_name"].split(" ")[0]} {kind}'
            rename[ch['ch_name']] = new_name
    raw.rename_channels(rename)

    # Validate the format of data after transformation is valid
    _validate_nirs_info(raw.info, fnirs='hb')
    return raw

def _load_absorption(freqs):
    """Load molar extinction coefficients."""
    # Data from https://omlc.org/spectra/hemoglobin/summary.html
    # The text was copied to a text file. The text before and
    # after the table was deleted. The following was run in matlab
    # extinct_coef=importdata('extinction_coef.txt')
    # save('extinction_coef.mat', 'extinct_coef')
    #
    # Returns data as [[HbO2(freq1), Hb(freq1)],
    #                  [HbO2(freq2), Hb(freq2)]]
    from scipy.io import loadmat
    from scipy.interpolate import interp1d

    extinction_fname = op.join(op.dirname(__FILE__), 'data', 'extinction_coef.mat')
    a = loadmat(extinction_fname)['extinct_coef']

    interp_hbo = interp1d(a[:, 0], a[:, 1], kind='linear')
    interp_hb = interp1d(a[:, 0], a[:, 2], kind='linear')

    ext_coef = np.array([[interp_hbo(freqs[0]), interp_hb(freqs[0])],
                         [interp_hbo(freqs[1]), interp_hb(freqs[1])]])
    abs_coef = ext_coef * 0.2303

    return abs_coef