code: def test_inv_nchi():
      # See page 5 of the reference paper for tested values
      # Values taken from hispeed.MedianPIESNO.lambdaPlus
      # and hispeed.MedianPIESNO.lambdaMinus
      N = 8
      K = 20
      alpha = 0.01
  
      lambdaMinus = _inv_nchi_cdf(N, K, alpha / 2)
      lambdaPlus = _inv_nchi_cdf(N, K, 1 - alpha / 2)
  
      assert_almost_equal(lambdaMinus, 6.464855180579397)
      assert_almost_equal(lambdaPlus, 9.722849086419043)
  
file_path: dipy/dipy/denoise/tests/test_noise_estimate.py
imports: import numpy
  from numpy.testing import assert_almost_equal, assert_equal, assert_, assert_array_almost_equal, assert_warns
  from dipy.denoise.noise_estimate import _inv_nchi_cdf, piesno, estimate_sigma
  from dipy.denoise.noise_estimate import _piesno_3D
  from dipy.denoise.pca_noise_estimate import pca_noise_estimate
  import dipy.data
  import dipy.core.gradients
  from dipy.io.image import load_nifti_data
  from dipy.testing.decorators import set_random_number_generator
label_desc: Use image node_id 1 for calling a global function with example usage: test_inv_nchi() without return types