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make_sinc_pulse.py
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Mon, Jun 10, 22:16

make_sinc_pulse.py
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import math
from types import SimpleNamespace
import numpy as np
from pypulseq.make_trap_pulse import make_trapezoid
from pypulseq.opts import Opts
def make_sinc_pulse(flip_angle: float, system: Opts = Opts(), duration: float = 0, freq_offset: float = 0,
phase_offset: float = 0, time_bw_product: float = 4, apodization: float = 0,
center_pos: float = 0.5, max_grad: float = 0, max_slew: float = 0, slice_thickness: float = 0,
delay: float = 0, use: str = None):
"""
Creates a radio-frequency sinc pulse event and optionally accompanying slice select and slice select rephasing
trapezoidal gradient events.
Parameters
----------
flip_angle : float
Flip angle in radians.
system : Opts, optional
System limits. Default is a system limits object initialised to default values.
duration : float, optional
Duration in milliseconds (ms). Default is 0.
freq_offset : float, optional
Frequency offset in Hertz (Hz). Default is 0.
phase_offset : float, optional
Phase offset in Hertz (Hz). Default is 0.
time_bw_product : float, optional
Time-bandwidth product. Default is 4.
apodization : float, optional
Apodization. Default is 0.
center_pos : float, optional
Position of peak. Default is 0.5 (midway).
max_grad : float, optional
Maximum gradient strength of accompanying slice select trapezoidal event. Default is 0.
max_slew : float, optional
Maximum slew rate of accompanying slice select trapezoidal event. Default is 0.
slice_thickness : float, optional
Slice thickness of accompanying slice select trapezoidal event. The slice thickness determines the area of the
slice select event. Default is 0.
delay : float, optional
Delay in milliseconds (ms). Default is 0.
use : str, optional
Use of radio-frequency sinc pulse. Must be one of 'excitation', 'refocusing' or 'inversion'.
Returns
-------
rf : SimpleNamespace
Radio-frequency sinc pulse event.
gz : SimpleNamespace, optional
Accompanying slice select trapezoidal gradient event. Returned only if `slice_thickness` is provided.
gzr : SimpleNamespace, optional
Accompanying slice select rephasing trapezoidal gradient event. Returned only if `slice_thickness` is provided.
"""
valid_use_pulses = ['excitation', 'refocusing', 'inversion']
if use is not None and use not in valid_use_pulses:
raise ValueError(
f'Invalid use parameter. Must be one of excitation, refocusing or inversion. You passed: {use}')
BW = time_bw_product / duration
alpha = apodization
N = int(round(duration / 1e-6))
t = np.arange(1, N + 1) * system.rf_raster_time
tt = t - (duration * center_pos)
window = 1 - alpha + alpha * np.cos(2 * np.pi * tt / duration)
signal = np.multiply(window, np.sinc(BW * tt))
flip = np.sum(signal) * system.rf_raster_time * 2 * np.pi
signal = signal * flip_angle / flip
rf = SimpleNamespace()
rf.type = 'rf'
rf.signal = signal
rf.t = t
rf.freq_offset = freq_offset
rf.phase_offset = phase_offset
rf.dead_time = system.rf_dead_time
rf.ringdown_time = system.rf_ringdown_time
rf.delay = delay
if use is not None:
rf.use = use
if rf.dead_time > rf.delay:
rf.delay = rf.dead_time
try:
if slice_thickness == 0:
raise ValueError('Slice thickness must be provided')
if max_grad > 0:
system.max_grad = max_grad
if max_slew > 0:
system.max_slew = max_slew
amplitude = BW / slice_thickness
area = amplitude * duration
gz = make_trapezoid(channel='z', system=system, flat_time=duration, flat_area=area)
gzr = make_trapezoid(channel='z', system=system, area=-area * (1 - center_pos) - 0.5 * (gz.area - area))
if rf.delay > gz.rise_time:
gz.delay = math.ceil((rf.delay - gz.rise_time) / system.grad_raster_time) * system.grad_raster_time
if rf.delay < (gz.rise_time + gz.delay):
rf.delay = gz.rise_time + gz.delay
except:
gz = None
gzr = None
if rf.ringdown_time > 0:
t_fill = np.arange(1, round(rf.ringdown_time / 1e-6) + 1) * 1e-6
rf.t = np.concatenate((rf.t, rf.t[-1] + t_fill))
rf.signal = np.concatenate((rf.signal, np.zeros(len(t_fill))))
# Following 2 lines of code are workarounds for numpy returning 3.14... for np.angle(-0.00...)
negative_zero_indices = np.where(rf.signal == -0.0)
rf.signal[negative_zero_indices] = 0
return rf, gz, gzr

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