data_dir='#image directory"
img=nib.load(os.path.join(data_dir,'ADNI_941_S_1311_MR_MPR__GradWarp__B1_Correction__N3__Scaled_Br_20080703170241434_S51039_I112538.nii'))                           #loading the image
img_data=img.get_data()                                                     #accessing image array
multi_slice_viewer(img_data)
plt.show()

it will load your nii file using nib.load