<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def dicom_to_nifti(dicom_input, output_file=None):
 """ This is the main dicom to nifti conversion fuction for hitachi images. As input hitachi images are required. It will then determine the type of images and do the correct conversion Examples: See unit test :param output_file: file path to the output nifti :param dicom_input: directory with dicom files for 1 scan """


    assert common.is_hitachi(dicom_input)

    # TODO add validations and conversion for DTI and fMRI once testdata is available

    logger.info('Assuming anatomical data')
    return convert_generic.dicom_to_nifti(dicom_input, output_file)