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calc_ramp.py
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Mon, Jun 10, 18:02

calc_ramp.py
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import numpy as np
from pypulseq.opts import Opts
def calc_ramp(k0: np.ndarray, k_end: np.ndarray, system: Opts = Opts(), max_points: int = 500, max_grad=np.zeros(0),
max_slew=np.zeros(0)):
"""
Join the points `k0` and `k_end` in three-dimensional k-space in minimal time, observing the gradient and slew limits
(`max_grad` and `max_slew` respectively), and the gradient strength G0 before `k0[:, 1]` and Gend after
`k_end[:, 1]`. In the context of a fixed gradient dwell time this is a discrete problem with an a priori unknown
number of discretization steps. Therefore this method tries out the optimization with 0 steps, then 1 step, and so
on, until all conditions can be fulfilled, thus yielding a short connection.
Parameters
----------
k0 : numpy.ndarray
Two preceding points in k-space. Shape is `[3, 2]`. From these points, the starting gradient will be calculated.
k_end : numpy.ndarray
Two following points in k-space. Shape is `[3, 2]`. From these points, the target gradient will be calculated.
system : Opts, optional
System limits. Default is a system limits object initialised to default values.
max_points : int, optional
Maximum number of k-space points to be used in connecting `k0` and `k_end`. Default is 500.
max_grad : float/list, optional
Maximum total gradient strength. Either a single value or one value for each coordinate, of shape `[3, 1]`.
Default is 0.
max_slew : float/list, optional
Maximum total slew rate. Either a single value or one value for each coordinate, of shape `[3, 1]`.
Default is 0.
Returns
-------
k_out : numpy.ndarray
Connected k-space trajectory.
success : bool
Boolean flag indicating if `k0` and `k_end` were successfully joined.
"""
def __inside_limits(grad, slew):
if mode == 0:
grad2 = np.sum(np.square(grad), axis=1)
slew2 = np.sum(np.square(slew), axis=1)
ok = np.all(np.max(grad2) <= np.square(max_grad)) and np.all(np.max(slew2) <= np.square(max_slew))
else:
ok = (np.sum(np.max(np.abs(grad), axis=1) <= max_grad) == 3) and (
np.sum(np.max(np.abs(slew), axis=1) <= max_slew) == 3)
return ok
def __joinleft0(k0, k_end, G0, G_end, use_points):
if use_points == 0:
G = np.stack((G0, (k_end - k0) / grad_raster, G_end)).T
S = (G[:, 1:] - G[:, :-1]) / grad_raster
k_out_left = np.zeros((3, 0))
success = __inside_limits(G, S)
return success, k_out_left
dk = (k_end - k0) / (use_points + 1)
kopt = k0 + dk
Gopt = (kopt - k0) / grad_raster
Sopt = (Gopt - G0) / grad_raster
okGopt = np.sum(np.square(Gopt)) <= np.square(max_grad)
okSopt = np.sum(np.square(Sopt)) <= np.square(max_slew)
if okGopt and okSopt:
k_left = kopt
else:
a = np.multiply(max_grad, grad_raster)
b = np.multiply(max_slew, grad_raster ** 2)
dkprol = G0 * grad_raster
dkconn = dk - dkprol
ksl = k0 + dkprol + dkconn / np.linalg.norm(dkconn) * b
Gsl = (ksl - k0) / grad_raster
okGsl = np.sum(np.square(Gsl)) <= np.square(max_grad)
kgl = k0 + np.multiply(dk / np.linalg.norm(dk), a)
Ggl = (kgl - k0) / grad_raster
Sgl = (Ggl - G0) / grad_raster
okSgl = np.sum(np.square(Sgl)) <= np.square(max_slew)
if okGsl:
k_left = ksl
elif okSgl:
k_left = kgl
else:
c = np.linalg.norm(dkprol)
c1 = np.divide(np.square(a) - np.square(b) + np.square(c), (2 * c))
h = np.sqrt(np.square(a) - np.square(c1))
kglsl = k0 + np.multiply(c1, np.divide(dkprol, np.linalg.norm(dkprol)))
projondkprol = (kgl * dkprol.T) * (dkprol / np.linalg.norm(dkprol))
hdirection = kgl - projondkprol
kglsl = kglsl + h * hdirection / np.linalg.norm(hdirection)
k_left = kglsl
success, k = __joinright0(k_left, k_end, (k_left - k0) / grad_raster, G_end, use_points - 1)
if len(k) != 0:
if len(k.shape) == 1:
k = k.reshape((len(k), 1))
if len(k_left.shape) == 1:
k_left = k_left.reshape((len(k_left), 1))
k_out_left = np.hstack((k_left, k))
else:
k_out_left = k_left
return success, k_out_left
def __joinleft1(k0, k_end, G0, G_end, use_points):
if use_points == 0:
G = np.stack((G0, (k_end - k0) / grad_raster, G_end))
S = (G[:, 1:] - G[:, :-1]) / grad_raster
k_out_left = np.zeros((3, 0))
success = __inside_limits(G, S)
return success, k_out_left
k_left = np.zeros(3)
dk = (k_end - k0) / (use_points + 1)
kopt = k0 + dk
Gopt = (kopt - k0) / grad_raster
Sopt = (Gopt - G0) / grad_raster
okGopt = np.abs(Gopt) <= max_grad
okSopt = np.abs(Sopt) <= max_slew
dkprol = G0 * grad_raster
dkconn = dk - dkprol
ksl = k0 + dkprol + np.multiply(np.sign(dkconn), max_slew) * grad_raster ** 2
Gsl = (ksl - k0) / grad_raster
okGsl = np.abs(Gsl) <= max_grad
kgl = k0 + np.multiply(np.sign(dk), max_grad) * grad_raster ** 2
Ggl = (kgl - k0) / grad_raster
Sgl = (Ggl - G0) / grad_raster
okSgl = np.abs(Sgl) <= max_slew
for ii in range(3):
if okGopt[ii] == 1 and okSopt[ii] == 1:
k_left[ii] = kopt[ii]
elif okGsl[ii] == 1:
k_left[ii] = ksl[ii]
elif okSgl[ii] == 1:
k_left[ii] = kgl[ii]
else:
print('Unknown error')
success, k = __joinright1(k_left, k_end, (k_left - k0) / grad_raster, G_end, use_points - 1)
if len(k) != 0:
if len(k.shape) == 1:
k = k.reshape((len(k), 1))
if len(k_left.shape) == 1:
k_left = k_left.reshape((len(k_left), 1))
k_out_left = np.hstack((k_left, k))
else:
k_out_left = k_left
return success, k_out_left
def __joinright0(k0, k_end, G0, G_end, use_points):
if use_points == 0:
G = np.stack((G0, (k_end - k0) / grad_raster, G_end)).T
S = (G[:, 1:] - G[:, :-1]) / grad_raster
k_out_right = np.zeros((3, 0))
success = __inside_limits(G, S)
return success, k_out_right
dk = (k0 - k_end) / (use_points + 1)
kopt = k_end + dk
Gopt = (k_end - kopt) / grad_raster
Sopt = (G_end - Gopt) / grad_raster
okGopt = np.sum(np.square(Gopt)) <= np.square(max_grad)
okSopt = np.sum(np.square(Sopt)) <= np.square(max_slew)
if okGopt and okSopt:
k_right = kopt
else:
a = np.multiply(max_grad, grad_raster)
b = np.multiply(max_slew, grad_raster ** 2)
dkprol = -G_end * grad_raster
dkconn = dk - dkprol
ksl = k_end + dkprol + dkconn / np.linalg.norm(dkconn) * b
Gsl = (k_end - ksl) / grad_raster
okGsl = np.sum(np.square(Gsl)) <= np.square(max_grad)
kgl = k_end + np.multiply(dk / np.linalg.norm(dk), a)
Ggl = (k_end - kgl) / grad_raster
Sgl = (G_end - Ggl) / grad_raster
okSgl = np.sum(np.square(Sgl)) <= np.square(max_slew)
if okGsl:
k_right = ksl
elif okSgl:
k_right = kgl
else:
c = np.linalg.norm(dkprol)
c1 = np.divide(np.square(a) - np.square(b) + np.square(c), (2 * c))
h = np.sqrt(np.square(a) - np.square(c1))
kglsl = k_end + np.multiply(c1, np.divide(dkprol, np.linalg.norm(dkprol)))
projondkprol = (kgl * dkprol.T) * (dkprol / np.linalg.norm(dkprol))
hdirection = kgl - projondkprol
kglsl = kglsl + h * hdirection / np.linalg.norm(hdirection)
k_right = kglsl
success, k = __joinleft0(k0, k_right, G0, (k_end - k_right) / grad_raster, use_points - 1)
if len(k) != 0:
if len(k.shape) == 1:
k = k.reshape((len(k), 1))
if len(k_right.shape) == 1:
k_right = k_right.reshape((len(k_right), 1))
k_out_right = np.hstack((k, k_right))
else:
k_out_right = k_right
return success, k_out_right
def __joinright1(k0, k_end, G0, G_end, use_points):
if use_points == 0:
G = np.stack((G0, (k_end - k0) / grad_raster, G_end))
S = (G[:, 1:] - G[:, :-1]) / grad_raster
k_out_right = np.zeros((3, 0))
success = __inside_limits(G, S)
return success, k_out_right
k_right = np.zeros(3)
dk = (k0 - k_end) / (use_points + 1)
kopt = k_end + dk
Gopt = (k_end - kopt) / grad_raster
Sopt = (G_end - Gopt) / grad_raster
okGopt = np.abs(Gopt) <= max_grad
okSopt = np.abs(Sopt) <= max_slew
dkprol = -G_end * grad_raster
dkconn = dk - dkprol
ksl = k_end + dkprol + np.multiply(np.sign(dkconn), max_slew) * grad_raster ** 2
Gsl = (k_end - ksl) / grad_raster
okGsl = np.abs(Gsl) <= max_grad
kgl = k_end + np.multiply(np.sign(dk), max_grad) * grad_raster
Ggl = (k_end - kgl) / grad_raster
Sgl = (G_end - Ggl) / grad_raster
okSgl = np.abs(Sgl) <= max_slew
for ii in range(3):
if okGopt[ii] == 1 and okSopt[ii] == 1:
k_right[ii] = kopt[ii]
elif okGsl[ii] == 1:
k_right[ii] = ksl[ii]
elif okSgl[ii] == 1:
k_right[ii] = kgl[ii]
else:
print('Unknown error')
success, k = __joinleft1(k0, k_right, G0, (k_end - k_right) / grad_raster, use_points - 1)
if len(k) != 0:
if len(k.shape) == 1:
k = k.reshape((len(k), 1))
if len(k_right.shape) == 1:
k_right = k_right.reshape((len(k_right), 1))
k_out_right = np.hstack((k, k_right))
else:
k_out_right = k_right
return success, k_out_right
# =========
# MAIN FUNCTION
# =========
if np.all(np.where(max_grad <= 0)):
max_grad = [system.max_grad]
if np.all(np.where(max_slew <= 0)):
max_slew = [system.max_slew]
grad_raster = system.grad_raster_time
if len(max_grad) == 1 and len(max_slew) == 1:
mode = 0
elif len(max_grad) == 3 and len(max_slew) == 3:
mode = 1
else:
raise ValueError('Input value max grad or max slew in invalid format.')
G0 = (k0[:, 1] - k0[:, 0]) / grad_raster
G_end = (k_end[:, 1] - k_end[:, 0]) / grad_raster
k0 = k0[:, 1]
k_end = k_end[:, 0]
success = 0
k_out = np.zeros((3, 0))
use_points = 0
while success == 0 and use_points <= max_points:
if mode == 0:
if np.linalg.norm(G0) > max_grad or np.linalg.norm(G_end) > max_grad:
break
success, k_out = __joinleft0(k0, k_end, G0, G_end, use_points)
else:
if np.abs(G0) > np.abs(max_grad) or np.abs(G_end) > np.abs(max_grad):
break
success, k_out = __joinleft1(k0, k_end, G0, G_end, use_points)
use_points += 1
return k_out, success

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