the renal pelvis In the inferior central kidney, the central hilum houses the entry site of the ureter, renal vein, and renal artery (Fig 131.17 ) FIGURE 131.17 Normal kidney Note that on the right side, the liver may be used as an acoustic window Technique The same approach used to perform the FAST should be used to perform a focused image of the kidney A low-frequency abdominal probe will allow for best imaging and, in pediatric patients, a probe with a small footprint is often preferable given the position of the kidneys within the thoracic ribcage The patient can be in the supine or decubitus position The probe marker should be positioned toward the head for the long-axis image and then rotated 90 degrees to perform the short-axis image Obstructive ureteronephrolithiasis may initially cause dilation of the ureter, which can be noted at the level of the central hilum With worsening obstruction and progression to hydronephrosis, the calyces and renal pelvis will dilate and the central region of the kidney will become anechoic ( Video 131.17 ) Identification of an obstructive stone is often difficult but, when noted, may appear hyperechoic and cause shadowing (Fig 131.18 )