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Chapter 13

### Clinical Indications

Renal ultrasonography is a commonly employed imaging technique with many useful applications. These include, but are not limited to, evaluation of hydronephrosis, detection and surveillance of nephrolithiasis, characterization of focal renal lesions, and workup of renal failure and hematuria.1-3 While not without limitations, ultrasound offers many advantages over computed tomography (CT) and magnetic resonance imaging (MRI). Ultrasound does not involve ionizing radiation, which is significantly advantageous in certain populations such as pregnant patients and children. Ultrasound can be used safely in patients with allergies to iodinated contrast agents and in patients with cardiac pacemakers. Ultrasound does not carry a risk of contrast-induced nephropathy or nephrogenic systemic fibrosis, and is not limited by renal function. Imaging guidance by ultrasound is frequently employed in renal biopsies, nephrostomy tube, or ureteral stent placements, and intraoperatively in lesion resection and treatment.

Ultrasound is excellent in detection of hydronephrosis and useful in the workup of hematuria, particularly in low- and medium-risk patients as maintained by the European Society of Urogenital Radiology.4 In detection and surveillance of nephrolithiasis, ultrasound offers an alternative to CT imaging in more radiosensitive patients and those with body morphology conducive in sonographic imaging. Ultrasound provides a high level of accuracy in characterization of focal masses as solid, cystic, or mixed, and can be utilized in surveillance of focal renal lesions for stability in conjunction with CT and MRI. Doppler imaging can be applied in the evaluation of cystic or solid lesions and in other renal vascular applications such as identifying arteriovenous fistulas or malformations, screening for renal artery stenosis, detection of renal vein thrombosis, and calculation of resistive indices in the workup of obstructive and medical renal disease. The color comet tail artifact, or “twinkling” artifact, provides additional sensitivity to ultrasound in detection of calcifications, either related to stone disease or complex cystic lesions.

Despite its useful applications, ultrasound possesses many limitations as a modality in renal evaluation. In the setting of acute trauma, ultrasound may not be able to differentiate acute hemorrhage from urine and does not accurately stage the degree of renal injury.5,6 CT serves as the primary imaging modality of choice following blunt or penetrating trauma, and has the advantage of evaluating adjacent organs and vasculature for concurrent injuries. Ultrasound does not reliably visualize the ureter, thus limiting its evaluation of urolithiasis, an assessment more optimally performed by noncontrast CT imaging.7

### Normal Anatomy

The kidneys lie in the retroperitoneum surrounded by Gerota fascia and perinephric fat. Each kidney generally weighs 150 g in the males and 135 g in the females. A typical kidney measures 10–12 cm longitudinally, 5 to 7 cm transversely, and 3 cm in the anteroposterior dimension. While the position of the kidney within the retroperitoneum varies by side, degree of inspiration, body position, and anatomic anomalies, the right kidney is usually located 1 to ...

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