diff --git a/nrrd/writer.py b/nrrd/writer.py
index d8c0b1c..0a63f59 100644
--- a/nrrd/writer.py
+++ b/nrrd/writer.py
@@ -93,7 +93,7 @@ def _format_field_value(value, field_type):
         raise NRRDError('Invalid field type given: %s' % field_type)
 
 
-def write(filename, data, header={}, detached_header=False, custom_field_map=None,
+def write(filename, data, header={}, detached_header=False, relative_data_path=True, custom_field_map=None,
                           compression_level = 9):
     """Write :class:`numpy.ndarray` to NRRD file
 
@@ -121,6 +121,9 @@ def write(filename, data, header={}, detached_header=False, custom_field_map=Non
         Data to save to the NRRD file
     detached_header : :obj:`bool`, optional
         Whether the header and data should be saved in separate files. Defaults to :obj:`False`
+    relative_data_path : :class:`bool`
+        Whether the data filename in detached header is saved with a relative path or absolute path.
+        This parameter is ignored if there is no detached header. Defaults to :obj:`True`
     custom_field_map : :class:`dict` (:class:`str`, :class:`str`), optional
         Dictionary used for parsing custom field types where the key is the custom field name and the value is a
         string identifying datatype for the custom field.
@@ -183,12 +186,14 @@ def write(filename, data, header={}, detached_header=False, custom_field_map=Non
             else:
                 raise NRRDError('Invalid encoding specification while writing NRRD file: %s' % header['encoding'])
 
-            header['data file'] = data_filename
+            header['data file'] = os.path.basename(data_filename) \
+                if relative_data_path else os.path.abspath(data_filename)
         else:
             data_filename = header['data file']
     elif filename.endswith('.nrrd') and detached_header:
         data_filename = filename
-        header['data file'] = data_filename
+        header['data file'] = os.path.basename(data_filename) \
+            if relative_data_path else os.path.abspath(data_filename)
         filename = '%s.nhdr' % os.path.splitext(filename)[0]
     else:
         # Write header & data as one file

diff --git a/nrrd/tests/test_writing.py b/nrrd/tests/test_writing.py
index 2b8274e..db1b507 100644
--- a/nrrd/tests/test_writing.py
+++ b/nrrd/tests/test_writing.py
@@ -158,7 +158,8 @@ class TestWritingFunctions(unittest.TestCase):
         output_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.nhdr')
         output_data_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.nrrd')
 
-        nrrd.write(output_data_filename, self.data_input, {u'encoding': 'raw'}, detached_header=True)
+        nrrd.write(output_data_filename, self.data_input, {u'encoding': 'raw'}, detached_header=True,
+                   relative_data_path=False)
 
         # Read back the same file
         data, header = nrrd.read(output_filename)
@@ -198,7 +199,8 @@ class TestWritingFunctions(unittest.TestCase):
         output_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.nhdr')
         output_data_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.raw.gz')
 
-        nrrd.write(output_filename, self.data_input, {u'encoding': 'gz'}, detached_header=False)
+        nrrd.write(output_filename, self.data_input, {u'encoding': 'gz'}, detached_header=False,
+                   relative_data_path=False)
 
         # Read back the same file
         data, header = nrrd.read(output_filename)
@@ -208,7 +210,6 @@ class TestWritingFunctions(unittest.TestCase):
 
     def test_write_detached_bz2(self):
         output_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.nhdr')
-        output_data_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.raw.bz2')
 
         nrrd.write(output_filename, self.data_input, {u'encoding': 'bz2'}, detached_header=False)
 
@@ -216,11 +217,10 @@ class TestWritingFunctions(unittest.TestCase):
         data, header = nrrd.read(output_filename)
         self.assertEqual(self.expected_data, data.tostring(order='F'))
         self.assertEqual(header['encoding'], 'bz2')
-        self.assertEqual(header['data file'], output_data_filename)
+        self.assertEqual(header['data file'], 'testfile_detached_raw.raw.bz2')
 
     def test_write_detached_ascii(self):
         output_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.nhdr')
-        output_data_filename = os.path.join(self.temp_write_dir, 'testfile_detached_raw.txt')
 
         nrrd.write(output_filename, self.data_input, {u'encoding': 'txt'}, detached_header=False)
 
@@ -228,7 +228,7 @@ class TestWritingFunctions(unittest.TestCase):
         data, header = nrrd.read(output_filename)
         self.assertEqual(self.expected_data, data.tostring(order='F'))
         self.assertEqual(header['encoding'], 'txt')
-        self.assertEqual(header['data file'], output_data_filename)
+        self.assertEqual(header['data file'], 'testfile_detached_raw.txt')
 
 
 if __name__ == '__main__':

Print relative data file name in the header
In the detached header, the data file name is printed with abspath. That means, if someone moves the data around, than data file location is invalid. Instead, how about we write the data file with basename?

https://github.com/mhe/pynrrd/blob/master/nrrd/writer.py#L186 and https://github.com/mhe/pynrrd/blob/master/nrrd/writer.py#L191

I think the specification also suggests that:

> Breaking the dataset into a header and one or more data files raises a new concerns, namely that the header file can't know if the data file has been erased, renamed, or moved. NRRD provides no means to overcome these problems once they've been created. On the other hand, moving the header and data files together to a new place is a common operation, and is supported by the special semantics associated with the data filename:
    If the filename (given either directly from "filename", or generated from "format", or listed in a "LIST") does not begins with "/", it is taken to be the filename of the data file, relative to the location of the detached header file. The NRRD reader is responsible for constructing the full data filename from the filename of the detached header and "filename". Obviously, if the detached header has been passed to the reader not as a file name, but as a FILE* (such as stdin), this filename construction is impossible.
    Otherwise, if "filename" does start with "/", then filename gives the full path of the data file. It is passed directly to fopen() without any interpretation.
Note: as of NRRD0004, the signifier of a header-relative file changed from the presence (at the beginning of the filename) of "./", to the absence of "/". Since essentially all uses of detached headers assumed header-relative data files, the explicit "./" flag was deemed unnecessary. With this change, it becomes impossible for a header to refer to the data file relative to the current working directory of the reader (which may be different that the directory of the header), but that's probably a good thing. 


Suggestion:
> header['data file']= os.path.basename(data_filename)

Happy to submit a PR.

cc: @ihnorton 

nrrd/tests/test_writing.py::TestWritingFunctions::test_write_detached_ascii nrrd/tests/test_writing.py::TestWritingFunctions::test_write_detached_bz2 nrrd/tests/test_writing.py::TestWritingFunctions::test_write_detached_gz nrrd/tests/test_writing.py::TestWritingFunctions::test_write_detached_raw_as_nrrd
nrrd/tests/test_writing.py::TestWritingFunctions::test_write_ascii_1d nrrd/tests/test_writing.py::TestWritingFunctions::test_write_ascii_2d nrrd/tests/test_writing.py::TestWritingFunctions::test_write_ascii_3d nrrd/tests/test_writing.py::TestWritingFunctions::test_write_bzip2 nrrd/tests/test_writing.py::TestWritingFunctions::test_write_bzip2_level1 nrrd/tests/test_writing.py::TestWritingFunctions::test_write_custom_fields_with_custom_field_map nrrd/tests/test_writing.py::TestWritingFunctions::test_write_custom_fields_without_custom_field_map nrrd/tests/test_writing.py::TestWritingFunctions::test_write_detached_raw_odd_extension nrrd/tests/test_writing.py::TestWritingFunctions::test_write_fake_encoding nrrd/tests/test_writing.py::TestWritingFunctions::test_write_gz nrrd/tests/test_writing.py::TestWritingFunctions::test_write_gz_level1 nrrd/tests/test_writing.py::TestWritingFunctions::test_write_raw