A 28-year-old woman with irregular menstrual cycles presents for dating of pregnancy. Transabdominal ultrasound at 14 weeks (by last menstrual period) shows a single intrauterine gestational sac with a crown-rump length (CRL) of 68 mm. The biometric parameters suggest an age of 16 weeks. There is also a small hypoechoic lesion in the fetal thorax measuring 8 mm with a thin echogenic rim. What is the most appropriate next investigation to characterize this thoracic lesion?