dir = NotebookDirectory[] <> "ADNI";
fileNameList = FileNames["*.nii", dir, Infinity];

Partition[Boole[FileExistsQ /@ fileNameList], 29] // ArrayPlot

{"F:\Research\Machine_Learning\Alzheimer_Disease\Dataset\
 ADNI1Screening_1.5T\ADNI\133_S_0912\MPR__GradWarp__B1_Correction__
 N3__Scaled_2\2006-10-09_13_10_30.0\S19884\ADNI_133_S_0912_MR_MPR__
 GradWarp__B1_Correction__N3__Scaled_2_Br_20081008094613870_S19884_
 I119626.nii",
 "F:\Research\Machine_Learning\Alzheimer_Disease\Dataset\
 ADNI1Screening_1.5T\ADNI\036_S_0759\MPR__GradWarp__B1_Correction__
 N3__Scaled\2006-08-22_11_08_33.0\S18094\ADNI_036_S_0759_MR_MPR__
 GradWarp__B1_Correction__N3__Scaled_Br_20070120004026768_S18094_
 I36969.nii",
 "F:\Research\Machine_Learning\Alzheimer_Disease\Dataset\
 ADNI1Screening_1.5T\ADNI\010_S_0422\MPR____N3__Scaled\2006-11-30_
 14_02_07.0\S23188\ADNI_010_S_0422_MR_MPR____N3__Scaled_Br_
 20070731162014912_S23188_I63332.nii",
 "F:\Research\Machine_Learning
 \Alzheimer_Disease\Dataset\ADNI1Screening_1.5T\ADNI\012_S_1133\
 MPR____N3__Scaled\2007-01-12_11_21_49.0\S25016\ADNI_012_S_1133_MR_
 MPR____N3__Scaled_Br_20070711170922052_S25016_I59224.nii"}

{"F:\Research\Machine_Learning\Alzheimer_Disease\Dataset\
 ADNI1Screening_1.5T\ADNI\002_S_0295\MPR__GradWarp__B1_Correction__
 N3__Scaled\2006-04-18_08_20_30.0\S13408\ADNI_002_S_0295_MR_MPR__
 GradWarp__B1_Correction__N3__Scaled_Br_20070319113623975_S13408_
 I45108.nii",
 "F:\Research\Machine_Learning\Alzheimer_Disease\Dataset\
 ADNI1Screening_1.5T\ADNI\002_S_0295\MPR__GradWarp__B1_Correction__
 N3__Scaled_2\2006-04-18_08_20_30.0\S13408\ADNI_002_S_0295_MR_MPR__
 GradWarp__B1_Correction__N3__Scaled_2_Br_20081001114556321_S13408_
 I118671.nii"}