Kidneys are paired, bean-shaped, retroperitoneal organs

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  Function

  
  
  
  
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    Removal of excess water, salts, and wastes of protein metabolism from blood

    
    
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    Regulation of water and electrolyte balance

    
    
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    Secretion of hormones that control blood pressure, bone and blood production

  
  

Located in retroperitoneum, within perirenal space, surrounded by renal fascia (of Gerota)

Each adult kidney is ~ 9-14 cm in length, 5 cm in width, 3 cm in thickness

Both kidneys lie on quadratus lumborum muscles, lateral to psoas muscles, between T12-L3

Kidneys are hollow centrally with renal sinus occupied by fat, renal pelvis, calyces, vessels, and nerves

Renal hilum: Concavity where artery enters and vein and ureter leave renal sinus

Renal pelvis: Funnel-shaped expansion of upper end of ureter

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  Receives major calyces (infundibula) (2 or 3), each of which receives minor calyces (2-4)

Renal papilla: Pointed apex of renal pyramid of collecting tubules that excrete urine

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  Each papilla indents a minor calyx

  
  
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  7-10 papilla per kidney

Renal cortex: Outer part, contains renal corpuscles (glomeruli, vessels), proximal portions of collecting tubules and loop of Henle

Renal medulla: Inner part, contains renal pyramids, distal parts of collecting tubules, and loops of Henle

Vessels, nerves, and lymphatics

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  Artery

  
  
  
  
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    Usually 1 for each kidney

    
    
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    Arise from aorta at about L1-L2 vertebral level

  
  
  
  
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  Vein

  
  
  
  
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    Usually 1 for each kidney

    
    
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    Lies in front of renal artery and renal pelvis

  
  
  
  
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  Nerves

  
  
  
  
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    Autonomic from renal and aorticorenal ganglia and plexus

  
  
  
  
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  Lymphatics

  
  
  
  
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    To lumbar (aortic and caval) nodes

  
  

Ultrasound is 1st-line modality for acute or chronic disease, flank pain, and suspected complications of acute pyelonephritis

Renal capsule

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  Normal kidneys are well-defined due to presence of renal capsule and are less reflective than surrounding fat

Renal cortex

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  Renal cortex has reflectivity that is less than adjacent liver or spleen

  
  
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  If renal cortex brighter than normal liver (hyperechoic), high suspicion of renal parenchymal disease

Medullary pyramids

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  Medullary pyramids are less reflective than renal cortex

Corticomedullary differentiation

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  Margin between cortex and pyramids is usually well-defined in normal kidneys

  
  
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  Margin between cortex and pyramids may be lost in presence of generalized parenchymal inflammation or edema

Renal sinus

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  Echogenic due to the fat that surrounds blood vessels and collecting systems

  
  
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  Outline of renal sinus is variable, smooth to irregular

  
  
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  Renal sinus fat may increase in obesity, steroid use, and sinus lipomatosis

  
  
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  Renal sinus fat may decrease in cachectic patients and neonates

  
  
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  If sinus echoes are indistinct in noncachectic patient, tumor infiltration or edema should be considered

Collecting system (renal pelvis and calyces)

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  Not usually visible in dehydrated patient

  
  
  
  
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    AP diameter of renal pelvis in adults should be < 10 mm

  
  
  
  
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  May be seen as physiological “splitting” of renal sinus echoes in patients with a full bladder undergoing diuresis

  
  
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  Possible obstruction can be excluded by performing postmicturition images of collecting system and looking for ureteral jets in the bladder with color Doppler

  
  
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  Physiological “splitting” of renal sinus echoes is common in pregnancy

  
  
  
  
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    Causes of dilatation of pelvicalyceal system include mechanical obstruction by enlarging uterus, hormonal factors, increased blood flow, and parenchymal hypertrophy

    
    
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    May occur as early as 12 weeks into pregnancy

    
    
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    Seen in up to 75% of right kidneys at 20 weeks into pregnancy, less common on left side, thought to be due to cushioning of ureter from gravid uterus by sigmoid colon

    
    
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    Obvious dilatation of pelvicalyceal system can be seen in 2/3 of patients at 36 weeks

    
    
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    Changes usually resolve within 48 hours after delivery

  
  

Renal arteries

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  Normal caliber 5-8 mm

  
  
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  2/3 of kidneys are supplied by single renal artery arising from aorta

  
  
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  1/3 of kidneys are supplied by 2 or more renal arteries arising from aorta

  
  
  
  
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    Main renal artery may be duplicated

    
    
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    Accessory renal arteries may arise from aorta superior or inferior to main renal artery

    
    
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    Accessory renal arteries enter kidney in hilum or at poles

    
    
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    Extrahilar accessory renal arteries may arise from ipsilateral renal artery, ipsilateral iliac artery, aorta, or retroperitoneal arteries

  
  
  
  
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  Spectral Doppler

  
  
  
  
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    Open systolic window, rapid systolic upstroke occasionally followed by secondary slower rise to peak systole with subsequent diastolic delay but persistent forward flow in diastole

    
    
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    Continuous diastolic flow is present due to low resistance in renal vascular bed

    
    
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    Low-resistance flow pattern is also present in intrarenal branches

    
    
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    Normal peak systolic velocity (PSV) 75-125 cm/s, not more than 180 cm/s

    
    
    
    
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      > 200 cm/s is abnormal

    
    
    
    
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    Resistive index (RI) is (peak systolic velocity – end diastolic velocity)/peak systolic velocity; normal < 0.7

    
    
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    Pulsatility index (PI) is (peak systolic velocity – end diastole velocity)/mean velocity, normal < 1.8

  
  

Renal veins

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  Normal caliber 4-9 mm

  
  
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  Formed from tributaries that coalesce at renal hilum

  
  
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  Right renal vein is relatively short and drains directly into IVC

  
  
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  Left renal vein receives left adrenal vein from above and left gonadal vein from below

  
  
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  Left renal vein crosses midline between aorta and superior mesenteric artery

  
  
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  Spectral Doppler

  
  
  
  
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    Normal PSV 18-33 cm/s

    
    
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    Spectral Doppler in right renal vein mirrors pulsatility in IVC

    
    
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    Spectral Doppler in left renal vein may show only slight variability of velocities consequent upon cardiac and respiratory activity

  
  

Bipolar length is found by rotating transducer around its vertical axis such that the longest craniocaudal length can be identified

Normal size between 10-15 cm

Volume measurements

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  May be more accurate but is time consuming

  
  
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  3D ellipsoidal formula can be used for volume estimation

  
  
  
  
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    Length x AP diameter x transverse diameter x 0.5

  
  
  
  
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  Consistency and changes in volume over time more important

Right kidney

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  Liver used as acoustic window

  
  
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  Transducer placed in subcostal or intercostal position

  
  
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  Varying degree of respiration is useful

  
  
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  Raising patient’s right side and scanning laterally/posterolaterally may be useful

Left kidney

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  More difficult to visualize due to bowel gas from small bowel and splenic flexure

  
  
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  Usually easier to search for left kidney using posterolateral approach with left side raised

  
  
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  Full right lateral decubitus with pillow under right flank and left arm extended above head may be useful in difficult cases

  
  
  
  
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    Spleen can be used as acoustic window for imaging upper pole of left kidney

  
  

Posterior approach for both kidneys

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  Useful for interventional procedures such as renal biopsy, nephrostomy

  
  
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  Use bolster or pillow under the patient’s abdomen to decrease lordosis

  
  
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  Image quality may be impaired by thick paraspinal muscles and ribs shadowing

Renal arteries

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  Origins best seen from midline anterior approach

  
  
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  Right renal artery can usually be followed from origin to kidney

  
  
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  Left renal artery often requires posterolateral coronal transducer scanning position for visualization

Renal veins

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  Best seen on transverse scan from anterior approach

  
  
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  May also be seen on coronal scan from posterolateral coronal

Use highest frequency transducer appropriate for patient body habitus: 2-9 MHz curvilinear or 8-12 MHz linear

Compound and harmonic techniques to decrease artifacts

Color Doppler for global renal perfusion, presence of flow in lesions, segmental hypoperfusion in acute pyelonephritis/infarcts and bladder jets

Spectral Doppler: Renal artery stenosis, arteriovenous fistula

Accessory renal vessels

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  Accurate diagnosis necessary when planning surgery (e.g., resection, transplantation)

  
  
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  Due to limitations of ultrasound, CT arteriography, magnetic resonance angiography, or digital subtraction angiography are more sensitive and accurate

Normal variants may mimic disease

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  Dromedary hump and hypertrophied column of Bertin may be mistaken for renal tumors

Congenital anomalies very common

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  Leading cause of renal failure in children

  
  
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  Early diagnosis important

Congenital structural anomalies include abnormal renal number, position, structure, and vessels

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  Abnormal number: Absence of 1 or both kidneys; supernumerary kidney

  
  
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  Abnormal position: Pelvic kidney, crossed-fused renal ectopia, malrotation, ptosis

  
  
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  Abnormal structure

  
  
  
  
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    Duplication: Results from lack of fusion and commonly produces an enlarged kidney with 2 separate hila and pelvicalyceal systems, these may join or continue as 2 ureters

    
    
    
    
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      Ureters may be completely separate until they join the bladder or join proximal to the bladder

      
      
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      “Duplex kidney”: Bifid renal pelvis with single ureter

    
    
    
    
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    Hypertrophied column of Bertin (lobar dysmorphism; fetal lobulation; hilar lip)

    
    
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    Pelviureteric junction obstruction

  
  
  
  
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  Often accompanied by anomalies of other systems

  
  
  
  
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    VATER acronym: Vertebral, anorectal, tracheoesophageal, radial ray, renal anomalies