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Renal Ultrasound: Sonography of the Kidneys

Ultrasonography of the kidneys is frequently indicated in urological patients. Indications include (but are not limited to): determination of size and localization of the kidneys, abdominal pain, flank pain, hematuria, proteinuria, lower urinary tract symptoms, abdominal trauma, (recurrent or febrile) urinary tract infections, increased retention parameters, nephrolithiasis, diagnosis and follow-up of renal tumors and cysts, kidney transplantation, planning and performing surgical procedures such as renal biopsy or percutaneous nephrolithotomy. Review literature: (Singer et al., 2006).

Examination Technique

The transducer used for renal ultrasonography is a sector or curved array probe with 3.5–5 MHz. The examination of the kidneys is done in supine position and with inspiration of the patient. In case of unfavorable conditions, a lateral decubitus position is helpful, whereby the side to be examined is raised by 30 degrees. Each kidney is screened in sagital and transverse plane [fig. normal findings in renal ultrasonography]. The upper pole is further dorsal than the lower pole, the transducer must be tilted accordingly.

Normal findings in renal ultrasonography (right side): sagittal plane (left) and transverse plane (right). Length and width are measured in the sagittal plane and the thickness or parenchymal pyelone index in the transverse plane. Spine (1), vena cava (2), psoas muscle (3), gall bladder (4), liver (5), upper pole (6), lower pole (7). With kind permission, Dr. J. Haak, Königsbrunn.

Normal Findings in Renal Ultrasound

Size if the Kidney

• Length: 100–115 mm
• Width: 50–70 mm
• Thickness: 30–50 mm

The size of the parenchyma is measured from the convex outer edge to the tip of a papilla, the normal value is 13–18 mm. The kidney volume of 110–200 ml and can be calculated by using the following formula:

Renal parenchyma pyelon index:

The relationship between the dorsal parenchyma to pyelon to ventral parenchyma is normally 1:1:1. With increasing age, the kidney atrophy shifts the renal parenchyma pyelon index to 1:2:1.

Renal Ultrasound Anatomy

The kidney is a bean-shaped organ, which has usually a smooth convex lateral organ boundary. The renal parenchyma is homogeneously hypoechoic, sometimes the medullary pyramids are somewhat less echogenic than the cortex. The surrounding fat tissue and the pyelocalyceal system with parapelvic fat is hyperechoic. Medial to the pyelocalyceal system, sections of the great vessels and the psoas muscle are visible.

Contrast enhanced ultrasound (CEUS):

CEUS is indicated for the evaluation of cystic lesions of the kidney and for suspected renal infarction.

Doppler-Sonography of the Renal Vessels

Indications for renal Doppler sonography:

• Suspected renal artery stenosis or renal hypertension
• Suspected renal infarction
• Assessment of renal function after transplantation

Standard flow rates of the renal artery:

• Peak systolic velocity (PSV): 80–150 cm/s
• End-diastolic velocity (EDV): 20–50 cm/s

Resistive index of the kidney

The resistive index (RI) is calculated with the peak systolic velocity (PSV or V_PSV) and the end-diastolic velocity (EDV or V_EDV) using following formula:

RI=(V_PSV-V_EDV)/V_PSV

The location for the assessment of PSV and EDV are the arcuate arteries or interlobar arteries at the border between cortex and medulla. The standard value for the resistive index is 0.5 to 0.7. A RI of >0.7 or a side difference of more than 0.1 are seen in hydronephrosis, renal transplant graft rejection or intrinsic renal disease. An RI of less than 0.5 is a sign for a renal artery stenosis.

References

  Deutsche Version: Sonographie der Nieren