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Ultrasound is a powerful and inexpensive tool particularly well suited for the diagnosis and monitoring of critically ill patients. While portable bedside sonography may not be the preferred tool for a detailed examination, the development of versatile portable ultrasound machines significantly improves its utility and clinical accuracy.1

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Easy accessibility for major organs of the urinary system makes ultrasound a commonly performed test in critically ill patients. Sonography of the kidneys and bladder in critical care has multiple applications including evaluation of patients with reduced or absent urinary output, complicated urinary tract infections, and fever of unknown origin, renal trauma, and idiopathic hematuria. It is the most useful initial investigation in the early or late period after kidney transplantation. Sonographic study often provides the clinician with a diagnosis or guidance for rapid decision-making necessary for the treatment of critically ill patients. The most important goal of ultrasound evaluation of the urinary system is to identify or rule out a problem that requires prompt, goal-directed surgical or medical intervention to improve the patient's condition. While not intended as a comprehensive formal examination, ultrasound is a convenient bedside monitoring tool for use in the intensive care unit (ICU).

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In addition, many incidental abnormalities may be found during sonographic evaluation of kidneys and bladder. Whereas they may not have an impact on the immediate treatment decision, physicians should be able to recognize them and provide appropriate care if necessary.

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The normal adult kidney is a bean-shaped structure surrounded by a well-defined, smooth echogenic capsule representing Gerota's fascia and perinephric fat. The kidneys have a convex lateral edge and concave medial edge called the hilum. The lower pole is located more laterally and anteriorly than the upper pole. The sonographically measured normal adult kidney is between 9 and 12 centimeters (cm) in length and about 4–5 cm wide.

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The kidney parenchyma surrounds centrally located hyperechoic fatty renal sinus, which contains renal pelvis, calyces, major branches of renal artery and vein, and lymphatic vessels. Parenchyma corresponds to the area between renal sinus and outer renal surface and has two main components: the more echogenic peripherally located cortex and centrally located hypoechoic medulla, which contains renal pyramids (Figure 24.1). The normal renal parenchyma is 1.0–1.8 cm thick. The visible distinction between the cortex and medulla is a sign of a normal kidney. While easily recognized in children and younger patients, it may not always be detectable in the elderly.

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