diff --git a/ismrmrd/constants.py b/ismrmrd/constants.py
index 451be1d..e22aef5 100644
--- a/ismrmrd/constants.py
+++ b/ismrmrd/constants.py
@@ -68,6 +68,7 @@
 IMTYPE_REAL      = 3
 IMTYPE_IMAG      = 4
 IMTYPE_COMPLEX   = 5
+IMTYPE_RGB       = 6
 
 # Image flags
 IMAGE_IS_NAVIGATION_DATA  =  1
diff --git a/ismrmrd/hdf5.py b/ismrmrd/hdf5.py
index f391128..f0eb5ee 100644
--- a/ismrmrd/hdf5.py
+++ b/ismrmrd/hdf5.py
@@ -144,12 +144,15 @@ def fileinfo(fname):
 
 
 class Dataset(object):
-    def __init__(self, filename, dataset_name="dataset", create_if_needed=True):
+    def __init__(self, filename, dataset_name="dataset", create_if_needed=True, mode=None):
         # Open the file
-        if create_if_needed:
-            self._file = h5py.File(filename, 'a')
-        else:
-            self._file = h5py.File(filename, 'r+')
+        if mode is None:
+            if create_if_needed:
+                mode = 'a'
+            else:
+                mode = 'r+'
+
+        self._file = h5py.File(filename, mode)
 
         self._dataset_name = dataset_name
 
diff --git a/ismrmrd/waveform.py b/ismrmrd/waveform.py
index f128ffa..936f47f 100644
--- a/ismrmrd/waveform.py
+++ b/ismrmrd/waveform.py
@@ -69,6 +69,9 @@ def from_array(data, **kwargs):
 
         channels, nsamples = data.shape
 
+        if nsamples > np.iinfo(np.uint16).max:
+            raise TypeError(f"Array has {nsamples} samples, which is greater than the maximum of {np.iinfo(np.uint16).max}")
+
         array_data = {
             'version': 1,
             'channels': channels,