code: def __len__(self):
      return len(self.img_list)
  
file_path: MedicalNet/datasets/brains18.py
imports: import math
  import os
  import random
  import numpy
  from torch.utils.data import Dataset
  import nibabel
  from scipy import ndimage
label_desc: Use image node_id 3 for calling the BrainS18Dataset obj's underlying member method code with example usage: obj.__len__() and returns: len