diff --git a/brainprep/utils/__init__.py b/brainprep/utils/__init__.py
index 069feff0..afb64aed 100644
--- a/brainprep/utils/__init__.py
+++ b/brainprep/utils/__init__.py
@@ -24,6 +24,7 @@
     print_warn,
 )
 from .utils import (
+    bids_basename,
     bvecbval_from_file,
     coerce_to_list,
     coerce_to_path,
@@ -32,6 +33,7 @@
     make_run_id,
     parse_bids_keys,
     sbref_from_file,
+    select_defaced_t1w,
     sidecar_from_file,
 )
 
diff --git a/brainprep/utils/utils.py b/brainprep/utils/utils.py
index f8aea780..f4f7f4b8 100644
--- a/brainprep/utils/utils.py
+++ b/brainprep/utils/utils.py
@@ -33,6 +33,38 @@
     print_warn,
 )
 
+def bids_basename(
+    entities: dict[str],
+) -> str:
+    """
+    Build a BIDS-like file basename from parsed BIDS entities.
+
+    The recognized entities are kept in canonical BIDS order, so that
+    input files differing only by entities such as ``acq`` or ``rec``
+    yield distinct derivative names. The ``mod`` and ``modality`` keys
+    are ignored: the modality is handled by the callers through the
+    file suffix.
+
+    Parameters
+    ----------
+    entities : dict[str]
+        A dictionary of parsed BIDS entities, as returned by
+        :func:`parse_bids_keys`.
+
+    Returns
+    -------
+    basename : str
+        The BIDS-like basename, e.g. ``sub-01_ses-M00_acq-3DT1_run-01``.
+    """
+    order = (
+        "sub", "ses", "task", "acq", "ce", "rec", "dir",
+        "echo", "part", "run", "space", "res", "recording",
+    )
+    return "_".join(
+        f"{key}-{entities[key]}"
+        for key in order
+        if key in entities and entities[key] != ""
+    )
 
 def coerce_to_list(
         value: Any,
diff --git a/brainprep/workflow/defacing.py b/brainprep/workflow/defacing.py
index 254e6abb..57165505 100644
--- a/brainprep/workflow/defacing.py
+++ b/brainprep/workflow/defacing.py
@@ -54,6 +54,7 @@ def brainprep_defacing(
         anatomical_file: File,
         output_dir: Directory,
         keep_intermediate: bool = False,
+        t1w_run_id: str | None = None,
         **kwargs: dict,
     ) -> Bunch:
     """
@@ -82,6 +83,10 @@ def brainprep_defacing(
         If True, retains intermediate results (e.g., reoriented image); useful
         for debugging.
         Default False.
+    t1w_run_id : str | None
+        Optional run identifier to select a specific defaced T1w image when
+        defacing T2w or FLAIR images. Required when multiple T1w images exist
+        in the same session. Default None.
     **kwargs : dict
         entities: dict
             Dictionary of parsed BIDS entities.
@@ -197,10 +202,19 @@ def brainprep_defacing(
                 "defacemask.nii.gz"
             )
         )
-        if len(t1_file) != 1 or len(mask_t1_file) != 1:
+        if (len(t1_file) > 1 or len(mask_t1_file) > 1):
+            if t1w_run_id is None:
+                raise ValueError(
+                    f"Multiple T1w defaced images found: {t1_file}, {mask_t1_file}. "
+                    "Please specify a run identifier run-<t1w_run_id> using the 't1w_run_id' parameter."
+                )
+            t1_file = [f for f in t1_file if f"run-{t1w_run_id}_" in str(f)]
+            mask_t1_file = [f for f in mask_t1_file if f"run-{t1w_run_id}_" in str(f)]
+        if len(t1_file) == 0 or len(mask_t1_file) == 0:
             raise ValueError(
                 f"No T1w defaced image found: {t1_file}, {mask_t1_file}"
             )
+
         t1_file, mask_t1_file = t1_file[0], mask_t1_file[0]
         print_info(f"using T1w: {t1_file}")
         print_info(f"using defacing mask: {mask_t1_file}")