Image inside datagrid column

<WPFToolkit:DataGridTemplateColumn x:Name="SignalColumn" 
                                  Width="Auto" SortMemberPath="SignalValue"   Header="Signal"
                            HeaderStyle="{DynamicResource ColumnHeaderStyle}"><WPFToolkit:DataGridTemplateColumn.CellTemplate><DataTemplate><Grid><Grid.ColumnDefinitions><ColumnDefinition></ColumnDefinition></Grid.ColumnDefinitions><Grid.RowDefinitions><RowDefinition Height="6"/><RowDefinition/></Grid.RowDefinitions><Image Grid.Column="0" Grid.Row="1" Visibility="Visible" ToolTip="{Binding Path= SignalToolTip}" Width="Auto" VerticalAlignment="Top" HorizontalAlignment="Center"
                                        Source="{Binding Path= Signal}" Stretch="None"></Image></Grid></DataTemplate></WPFToolkit:DataGridTemplateColumn.CellTemplate></WPFToolkit:DataGridTemplateColumn>

Hi,

the above code is for a column inside WPFToolKit:DataGrid , where I want to show a image, here am binding the image source with Signal variable in ViewModel. but the image is not getting displayed. what is wrong with above code?