FIGURE 116.20 Ureteropelvic junction obstruction A newborn with left flank mass Ultrasonography of the left flank shows dilated pyelocalyceal system The communicating dilated collecting systems are seen in the periphery of the significantly dilated renal pelvis Initial evaluation in the ED may include plain abdominal films and an US Ultrasonography can differentiate a cystic flank mass ( Fig 116.20 ) that could be a hydronephrotic kidney from a solid tumor such as an adrenal neuroblastoma, and thus, facilitate the proper referral of the child to either a urologist or a pediatric surgeon CT and MRI are superior to other modalities for anatomic detail, and provide anatomic and physiologic information about organs and vascular structures Angiography is indicated for an abdominal mass only if a precise knowledge of segmental vascular anatomy is required or if interventional techniques are contemplated Sacrococcygeal Teratoma