Prevalensi Variasi Bentuk Penampang Melintang Sepertiga Apikal Saluran Akar Gigi Premolar Dua dan Molar Satu Rahang Atas serta Molar Satu Rahang Bawah (Analisis dengan Micro-CT) = Prevalence Variation of Apical One-Third Root Canal Cross Section Shape in Maxillary Second Premolars and First Molars and Mandibular First Molars (Analyze with Micro-CT)