The crown is a symbol of victory.  It too is a symbol of Christ.  He is the spiritual ruler for many and this symbol can be found in the Roman catacombs.  Scholars believe this symbol probably signified the victory of martyrs – for when they lost their life, yet remained faithful until their death they received the crown of everlasting life.  Thus, we also see depictions of the martyrs receiving a crown from Christ  from His hands– or offering their crown to Him