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
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 (UTIs), 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).
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.
Sonographic Anatomy of Urinary Tract
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 cm in length and about 4–5 cm wide.
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 21-1). 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.
(A) Normal renal anatomy. C = calyx; P = pyramid; RA = main renal artery; RV = main renal vein. (B) Normal kidney. Longitudinal view of the kidney demonstrates peripheral hypoechoic universally thick parenchyma and central hyperechoic renal sinus. Note the echogenic white Gerota’s fascia. Parenchyma is less echogenic then liver. (C) The cortical echogenicity is equal of that of the liver. Several slightly hypoechoic renal pyramids are seen. C = cortical echogenicity; L = liver. (D) Portable ultrasound of the normal right kidney. Note less contrast appearance but renal contour, parenchyma and renal sinus are clearly identified.
Parenchymal homogeneity is determined in comparison with that of adjacent liver and spleen. Normally, the renal cortex is hypoechoic or isoechoic to the liver (right kidney) and hypoechoic to spleen (left kidney). The collecting system of the kidney is not usually visible with ultrasound because calyces and pelvis are collapsed within renal sinus. The normal ureters measure approximately 8 mm wide and are difficult to evaluate sonographically. However, proximal or distal ends of significantly dilated ureter (hydroureter) can be seen.
The shape and appearance of the normal bladder depends on the degree of distention. When empty, the bladder lies behind the symphysis pubis. On longitudinal transabdominal view, the full bladder has a teardrop-shaped anechoic appearance, with distinct wall, while on the transverse view it appears rectangular. The thickness of the bladder wall varies with the degree of bladder filling. When mildly distended or empty, the bladder wall is thick and irregular. With full distension, the normal bladder wall is thin and smooth and does not exceed 4–5 mm in thickness (Figure 21-2).2
The spectrum of urologic ultrasound includes gray-scale and Doppler evaluation of the kidneys and the bladder. In accordance with American Institute of Ultrasound in Medicine practice guideline, the examination of kidneys should include longitudinal and transverse views and assessment of the cortex and renal pelvis. Renal echogenicity may be compared with echogenicity of the adjacent liver or spleen. Kidneys and perirenal regions should be assessed for abnormalities (Table 21-1).3
|Longitudinal diameter||Easy to obtain and reproduce with less intra- and interobserve variation. Average length is between 9 and 12 cm|
|Parenchymal thickness||Measurements between renal surface and hyperechoic sinus, normally >1 cm|
|Kidney margins||Sharp and regular. V-shaped indentation may represent persistent fetal lobulation. The outline depression with rounded angles indicates inflammatory or ischemic scars|
|Parenchymal echogenicity||Cortex hypo/isoechogenicity compared with that of liver or spleen is usually normal, while hyperechogenicity indicates a diffuse parenchymal pathology. Medulla is slightly less echogenic than cortex|
|Collecting system||Visible only when dilated (hydronephrosis), mostly secondary to mechanical obstruction|
|Calcifications||Small hyperechogenic lesions are nonspecific and may represent small stones, vascular or intraparenchymal calcifications. Larger stones are easily diagnosed by the characteristic posterior acoustic shadowing|
|Renal/extrarenal masses||Solid masses are usually neoplastic and require further evaluation with CT or MRI. Simple renal cyst is anechoic thin-walled space occupying lesion with good through-transmission and no internal echoes|
|Resistive index||Color Doppler study of renal perfusion. Normal RI <0.70. High resistance pattern indicates decrease perfusion of various causes|
Sonographic evaluation of a critically ill patient is typically limited by a supine position, lack of patient cooperation, presence of monitoring devices, tissue changes (e.g., bowel gas, edema, ascites), postsurgical incisions, and dressings.
Kidneys demonstrate a significant mobility with respiration (about 2–3 cm), which complicates the evaluation of patients on a ventilator.4
Commonly, a sector or curved-array transducer (3–5 MHz) is used, while higher frequency probes (5–7 MHz) with higher space resolution may be necessary to evaluate children, thin patients, and transplanted kidneys. Imaging of the urinary tract must always include evaluation of both kidneys and the bladder.
The right kidney is best examined in the supine or left lateral decubitus position through the liver, which serves as an acoustic window. The probe should be placed along the right lateral subcostal margin in the anterior axillary line, scanning through the liver to locate the right kidney. After visualization of the whole kidney, the optimal longitudinal view is obtained by slowly adjusting the probe’s position up and down or side to side. The kidney is traditionally measured in the longest axis (length and width) because the longitudinal diameter has minor inter- and intraobserver variations. If needed, a transverse plane (short-axis view) can be obtained by rotating the probe 90° and evaluating upper, mid, and lower portions of the kidney separately.
The left kidney is typically less visible due to its location in a more superior position, the lack of the sonographic window generated by the liver and the overlying small bowel, and gastric gas. If possible, placing the patient in the right lateral decubitus position with the probe positioned in the posterior axillary line or left costovertebral angle may improve visualization. If bowel gas obscures the kidney (especially left) and reflects the ultrasound waves, the transducer can be positioned in the mid- or posterior axillary line.5
The bladder can be examined only when it is distended. Sonographic evaluation is usually performed from a transabdominal location, with the patient in a supine position. A probe is placed 1 cm above the symphysis and angled laterally, inferiorly, and superiorly. Most commonly, the transverse scan is obtained first. A normal bladder is located in the midline without deviation, and appears symmetric, smooth, and without irregularities of inner surface. On the longitudinal scan, the bladder is oriented toward the umbilicus and tapered anteriorly. Transverse and longitudinal scans provide a fairly accurate calculation of the urine volume within the bladder. If needed, postvoid residual urine volume can be automatically calculated.
Detailed bladder ultrasound may have a limited application in critically ill patients with draining indwelling Foley catheters. However, it can provide immediate bedside diagnosis of urinary retention in patients with decreased or absent urinary output.
Color Doppler Ultrasound
Complex Doppler studies have not been used routinely as a bedside test in critically ill patients. However, the technical improvement of portable ultrasound machines with color and power Doppler equipment enables the ability to combine gray-scale ultrasound with limited Doppler study in the critical care setting. Changes in the perfusion of renal parenchyma are commonly associated with different renal pathology. Color-flow and spectral Doppler studies are able to provide noninvasive, indirect global assessment of renal blood flow and identify the vessels at the level of the renal hilum and in the renal parenchyma. Because the spatial resolution of gray-scale sonography is much lower than frequency resolution, the Doppler study is able to detect the arteries on the basis of the flow and not the anatomic size.2 The Doppler spectral tracing reflects a low vascular resistance and classically has a ski slope appearance. From the many different indices introduced to quantify blood flow, the most commonly used single parameter is the resistive index (RI), a ratio between end-diastolic velocity and peak systolic velocities. Restrictive index is a physiological parameter reflecting the degree of renal vascular resistance. Normal renal blood flow has a low resistance pattern, with a flow maintained throughout diastole. The normal RI values are 0.58 ± 0.10.2 Values >0.70 are considered abnormal and may be due to lower arterial patency, although major clinical significance is observed for values >0.80. Doppler signals are commonly obtained from renal artery or interlobar arcuate arteries at the corticomedullary junction and border of medullary pyramids. However, the identification of these areas requires more training and experience in performing Doppler ultrasound. The test is routinely performed to evaluate the transplanted kidney. The RI has been proposed to assist with the differential diagnosis between obstructive and nonobstructive hydronephrosis, or diagnosis of acute obstruction when dilatation has not yet developed. A minority of patients with obstructive renal failure may not show hydronephrosis due to dehydration or decompression caused by rupture of calyceal fornix. High intrarenal pressure and changing renal hemodynamics due to the release of vasoactive substances and vasoconstriction secondary to obstruction cause an increase in intrarenal arterial resistance measured by a higher RI. While the diagnostic accuracy of RI still remains controversial due to a wide range of results, a normal RI may still be helpful in arguing against the presence of obstruction.6–8 Color-flow Doppler ultrasound is frequently performed for the evaluation of the patency of the ureter. Jet phenomenon should be seen in the bladder when the urine bolus from the ureter is being propelled into the bladder cavity due to periodic peristalsis (1–12 jets per minute). Ureteral jets are usually identified during transverse bladder scanning as a color projecting into the bladder lumen from lateral posterior border and coursing superior and medial (Figure 21-3). While most critically ill patients have indwelling Foley catheters, bedside evaluation of ureteral jets may be limited due to the empty bladder.