import os
#os.mkdir('/tmp/X')
fa = open('/tmp/X/a_file','w')
fmr1 = open('/tmp/X/MR0001.dcm','w')
fmr2 = open('/tmp/X/MR0002.dcm','w')
fmr3 = open('/tmp/X/MR0003.dcm','w')
fmr1.write('fmr1\n')
fmr2.write('fmr2\n')
fmr3.write('fmr3\n')
fa.close()
fmr1.close()
fmr2.close()
fmr3.close()
root='/tmp/X'

print (os.listdir('/tmp/X'))
#['a_file', 'MR0003.dcm', 'MR0001.dcm', 'MR0002.dcm']

i=1
for path, subdirs, files in os.walk(root):
    for name in files:
        print (name)
        os.rename(os.path.join(path, name), os.path.join(path,'MR000'+ str(i)+'.dcm'))
        i=i+1

print (os.listdir('/tmp/X'))
#['MR0003.dcm', 'MR0004.dcm']