A 35-year-old nulliparous woman with a history of severe endometriosis and chronic pelvic pain undergoes transvaginal ultrasound, which reveals a 4 cm well-defined mass in the endometrium with heterogeneous echotexture. Histology from endometrial biopsy shows benign smooth muscle proliferation with disorganized arrangement. Which investigation would be most appropriate to confirm the diagnosis and assess the extent of myometrial involvement?