/usr/lib/python2.7/dist-packages/nibabel/mriutils.py is in python-nibabel 2.1.0-1.
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 | # emacs: -*- mode: python-mode; py-indent-offset: 4; indent-tabs-mode: nil -*-
# vi: set ft=python sts=4 ts=4 sw=4 et:
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#
# See COPYING file distributed along with the NiBabel package for the
# copyright and license terms.
#
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"""
Utilities for calculations related to MRI
"""
from __future__ import division
__all__ = ['calculate_dwell_time']
GYROMAGNETIC_RATIO = 42.576 # MHz/T for hydrogen nucleus
PROTON_WATER_FAT_SHIFT = 3.4 # ppm
class MRIError(ValueError):
pass
def calculate_dwell_time(water_fat_shift, echo_train_length, field_strength):
"""Calculate the dwell time
Parameters
----------
water_fat_shift : float
The water fat shift of the recording, in pixels.
echo_train_length : int
The echo train length of the imaging sequence.
field_strength : float
Strength of the magnet in Tesla, e.g. 3.0 for a 3T magnet recording.
Returns
-------
dwell_time : float
The dwell time in seconds.
Raises
------
MRIError
if values are out of range
"""
if field_strength < 0:
raise MRIError("Field strength should be positive")
if echo_train_length <= 0:
raise MRIError("Echo train length should be >= 1")
return ((echo_train_length - 1) * water_fat_shift /
(GYROMAGNETIC_RATIO * PROTON_WATER_FAT_SHIFT *
field_strength * (echo_train_length + 1)))
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