"""Field map generator module."""
import numpy as np
[docs]
def make_b0map(shape, b0range=(-300, 300), mask=None):
"""
Make radial B0 map.
Parameters
----------
shape : tuple[int]
Matrix size. Only supports isotropic matrices.
b0range : tuple[float], optional
Frequency shift range in [Hz]. The default is (-300, 300).
mask : np.ndarray
Spatial support of the objec. If not provided,
build a radial mask with radius = 0.3 * shape
Returns
-------
np.ndarray
B0 map of shape (*shape) in [Hz],
with values included in (*b0range).
mask : np.ndarray, optional
Spatial support binary mask.
"""
assert np.unique(shape).size, ValueError("Only isotropic matriex are supported.")
ndim = len(shape)
if ndim == 2:
radial_mask, fieldmap = _make_disk(shape)
elif ndim == 3:
radial_mask, fieldmap = _make_sphere(shape)
if mask is None:
mask = radial_mask
# build map
fieldmap *= mask
fieldmap = (b0range[1] - b0range[0]) * fieldmap / fieldmap.max() + b0range[0] # Hz
fieldmap *= mask
# remove nan
fieldmap = np.nan_to_num(fieldmap, neginf=0.0, posinf=0.0)
return fieldmap.astype(np.float32), mask
[docs]
def make_t2smap(shape, t2svalue=15.0, mask=None):
"""
Make homogeneous T2* map.
Parameters
----------
shape : tuple[int]
Matrix size.
t2svalue : float, optional
Object T2* in [ms]. The default is 15.0.
mask : np.ndarray
Spatial support of the objec. If not provided,
build a radial mask with radius = 0.3 * shape
Returns
-------
np.ndarray
T2* map of shape (*shape) in [ms].
mask : np.ndarray, optional
Spatial support binary mask.
"""
assert np.unique(shape).size, ValueError("Only isotropic matriex are supported.")
ndim = len(shape)
if ndim == 2:
radial_mask, fieldmap = _make_disk(shape)
elif ndim == 3:
radial_mask, fieldmap = _make_sphere(shape)
if mask is None:
mask = radial_mask
# build map
fieldmap = t2svalue * mask # ms
# remove nan
fieldmap = np.nan_to_num(fieldmap, neginf=0.0, posinf=0.0)
return fieldmap.astype(np.float32), mask
def _make_disk(shape, frac_radius=0.3):
"""Make circular binary mask."""
ny, nx = shape
yy, xx = np.mgrid[:ny, :nx]
yy, xx = yy - ny // 2, xx - nx // 2
yy, xx = yy / ny, xx / nx
rr = (xx**2 + yy**2) ** 0.5
return rr < frac_radius, rr
def _make_sphere(shape, frac_radius=0.3):
"""Make spherical binary mask."""
nz, ny, nx = shape
zz, yy, xx = np.mgrid[:nz, :ny, :nx]
zz, yy, xx = zz - nz // 2, yy - ny // 2, xx - nx // 2
zz, yy, xx = zz / nz, yy / ny, xx / nx
rr = (xx**2 + yy**2 + zz**2) ** 0.5
return rr < frac_radius, rr
