二通り提示します。
imds = imageDatastore(fullfile(currentdirectory,categories),'IncludeSubfolders',true,'FileExtensions','.dcm','LabelSource', 'foldernames');
img = dicomread(imds.Files{1});
size(img)
imds = imageDatastore(fullfile(currentdirectory, categories),'IncludeSubfolders',true,'FileExtensions','.dcm','LabelSource', 'foldernames','ReadFcn',@dicomread);
img=readimage(imds,1);
size(img)