Renal relevant radiology: use of ultrasonography in patients with AKI

Abstract

As judged by the American College of Radiology Appropriateness Criteria, renal Doppler ultrasonography is the most appropriate imaging test in the evaluation of AKI and has the highest level of recommendation. Unfortunately, nephrologists are rarely specifically trained in ultrasonography technique and interpretation, and important clinical information obtained from renal ultrasonography may not be appreciated. In this review, the strengths and limitations of grayscale ultrasonography in the evaluation of patients with AKI will be discussed with attention to its use for (1) assessment of intrinsic causes of AKI, (2) distinguishing acute from chronic kidney diseases, and (3) detection of obstruction. The use of Doppler imaging and the resistive index in patients with AKI will be reviewed with attention to its use for (1) predicting the development of AKI, (2) predicting the prognosis of AKI, and (3) distinguishing prerenal azotemia from intrinsic AKI. Finally, pediatric considerations in the use of ultrasonography in AKI will be reviewed.

Figure 1.

Grayscale ultrasonographic image demonstrating normal kidney size and echogenicity. (A) Grayscale longitudinal ultrasonographic image of a normal right kidney. The kidney is 11.2 cm long and surrounded by the bright rim of renal capsule (yellow arrow) around the parenchyma. The normal parenchyma (red line) forms a thick darker rim around the central bright (echogenic) sinus. Normal renal parenchyma is similar (isoechoic) or slightly darker (hypoechoic) compared with liver and includes the cortex and medulla. The thickness of the renal cortex is measured from the outer border of the medullary pyramids (yellow line) or from the arcuate arteries to the renal capsule. The medullary pyramids (white arrow) contain fluid in parallel tubules, which is anechoic (black) and appear as regularly spaced dark pools at the inner margin of the parenchyma (arrow). Because fat is echogenic, the central renal sinus appears bright. (B) Grayscale transverse ultrasonographic image of normal right kidney showing normal width (5.2 cm) and height (3.4 cm, anterior/posterior measurement) and appearance. (C) Grayscale longitudinal ultrasonographic image of normal right kidney next to a hyperechoic-appearing liver that has steatosis (because fat is echogenic, the accumulation of fat increases the liver echogenicity/brightness). LNG, longitudinal view; MID, midline; RK, right kidney; TRV, transverse.

Figure 2.

Hyperechoic (bright) medulla from medullary nephrocalcinosis. Several kidney diseases may be associated with a hyperechoic medulla, including medullary nephrocalcinosis, sickle cell diseases, and gout. The midline longitudinal image of the right kidney (A) and left kidney (B) demonstrate hyperechoic (bright) medulla (yellow arrow) in a 35-year-old woman with medullary calcinosis. (C) Axial noncontrast computed tomographic image of the same patient demonstrates calcification in medullary pyramids of both kidneys (black arrow). LK, left kidney; LO, long axis; MID, midline.

Figure 3.

Grayscale ultrasonographic image and resistive index (RI) in severe ischemic acute tubular necrosis. (A) Grayscale ultrasonographic image demonstrates normal renal length (11.7 cm), mild increased parenchymal echogenicity, and increased parenchymal thickness, and (B) Doppler imaging demonstrates very high RI (0.95) in a 21-year-old woman with severe ischemic acute tubular necrosis who required hemodialysis for 3 weeks. The patient eventually recovered normal kidney function. (Note: RI was determined in the renal artery near the segmental branch.) EVD, end diastolic velocity; L, left; PSV, peak systolic velocity; RenA, renal artery; RenA TS, renal artery time slope (acceleration time).

Figure 4.

AKI in a patient with multiple myeloma. Grayscale longitudinal ultrasonographic image of the right kidney demonstrates abnormal bright renal parenchyma in a 39-year-old woman with multiple myeloma and AKI.

Figure 5.

Grayscale ultrasonographic image and resistive index in glomerular diseases. Patients with glomerular disease are classically described as having increased kidney size, with normal or increased parenchymal echogenicity, and normal resistive index. (A) Grayscale ultrasonographic image of LK demonstrates increased kidney size (13.1 cm) and normal parenchymal echogenicity and (B) Doppler image demonstrates resistive index of 0.51 in a 50-year-old man with biopsy-proven minimal-change disease. (C) Grayscale ultrasonographic image of RK demonstrating increased kidney size (13.6 cm) and normal parenchymal echogenicity, and (D) normal resistive index (0.63) in 57-year-old man with rapidly progressive GN and biopsy-certain necrotizing and crescentic GN from a pauci-immune GN; creatinine was 4.3 mg/dl at time of biopsy.

Figure 6.

Grayscale ultrasonographic image in CKD. (A) Grayscale longitudinal ultrasonographic image of a 67-year-old man with stage 4 CKD (creatinine, 4.0 mg/dl) and long-standing hypertension. Kidney size is small (8.78 cm) and renal parenchyma is thin. The finding of small kidneys or a thin parenchyma is diagnostic of CKD. (B) Grayscale longitudinal ultrasonographic image of RK demonstrates kidney size of 11.4 cm, abnormal bright (diffusely increased echogenicity) renal parenchyma, although parenchymal thickness is normal, in a 32-year-old man with CKD (creatinine 3.3 mg/dl) due to severe poorly controlled hypertension (BP, 230/130 mmHg at time of examination).

Figure 7.

Doppler imaging and resistive index. (A) Normal color and spectral Doppler ultrasonogram of the right main renal artery (MRA) demonstrates normal peak systolic velocity (98.5 cm/sec), resistive index (0.67), and acceleration time (32 milliseconds). (B) Normal color and spectral Doppler ultrasonogram of the right superior segmental renal artery (SEG ART SUP) demonstrates normal resistive index (0.68). (C) Spectral Doppler image of left inferior segmental renal artery demonstrates an abnormally high resistive index of 0.76 in a 39-year-old woman with multiple myeloma and AKI. Resistive index is determined by the following formula: (peak systolic velocity − end diastolic velocity)/(peak systolic velocity); in A and B, peak systolic velocity is indicated by the long yellow arrows, and end diastolic velocity is indicated by the short purple arrows. AO, aorta.

Figure 8.

AKI due to hepatorenal syndrome. (A and C) Grayscale longitudinal ultrasonographic images of the right and left kidneys demonstrates normal renal parenchyma in a 51-year-old man with cirrhosis, ascites, end-stage liver disease due to hepatitis C, and AKI due to hepatorenal syndrome (serum creatinine, 2.5 mg/dl; low urinary sodium <10 mEq/L). (B and D) Spectral Doppler ultrasonograms of segmental renal artery in the same patient demonstrates an increased resistive index of 0.74 in the right kidney and 0.77 in the left kidney. SEG ART MID, middle segmental artery.

Figure 9.

Grayscale ultrasonographic images demonstrating varying degrees of hydronephrosis. Grayscale longitudinal ultrasonographic image of the right kidney obtained at different time periods in the same patient demonstrates mild (A), moderate (B), and severe (C) hydronephrosis in a 59-year-old man with metastatic prostate cancer and creatinine level of 15 mg/dl. Grayscale longitudinal ultrasonography of both kidneys demonstrates severe right (D) and severe left (E) hydronephrosis in a 54-year-old man with AKI (creatinine increase of 1.2–1.5 mg/dl), benign prostatic hypertrophy, and acute urinary retention. MED, medial.

Figure 10.

Grayscale ultrasonographic images of hydronephrosis due to obstructing stones. Grayscale longitudinal ultrasonographic image of the right kidney demonstrates hydronephrosis (A) with a dilated ureter (B) (red arrow) due to an 8-mm hyperechoic, obstructing stone (yellow arrow) in the proximal ureter in a 26-year-old woman with right flank pain. (C) Grayscale longitudinal ultrasonographic image of the left kidney demonstrates hydronephrosis containing complex fluid due to a large, echogenic, staghorn calculus (blue arrows) in the renal pelvis causing pyonephrosis in a 60-year-old man who presented with sepsis; (D) color Doppler ultrasonogram does not demonstrate flow, confirming that it is a dilated collecting system containing urine and not blood vessels in the central renal sinus (urine and blood are both anechoic [black]; color Doppler imaging can be used to distinguish urine [no flow] from blood in vessels [flow is present]).

Figure 11.

Hydronephrosis without functional obstruction. Grayscale and color Doppler ultrasonogram of a 59-year-old woman with right flank pain demonstrate mild right hydronephrosis (A) and hydroureter (yellow arrow) (B) ureteric jet (C) (white arrow) in the bladder. No calculus was seen on computed tomography of the kidneys, ureters, and bladder done at the same time. Thus, this likely represents a recently passed calculus or vesico-ureteric reflux and hydronephrosis without functional obstruction is present. (C) The utility of Doppler imaging to assess for functional obstruction by assessing the ureteric jet. When the bolus of urine being transmitted through the ureter reaches the terminal portion, it is ejected forcefully into the bladder through the vesicoureteric junction. This creates a jet of urine that can be seen within the urinary bladder on grayscale and color Doppler ultrasonography (white arrow; jet appears red). LNG, longitudinal; MED, medial.