Contributors: Fraser McConnell

 Species: Feline   |   Classification: Miscellaneous

Introduction

Overview
  • Radiography allows evaluation of kidney number, size, shape and position.

Significant renal disease may be present despite radiological appearance of normal kidneys.

  • Contrast radiography is required for full evaluation of the urinary tract.
  • Radiography is of limited value in assessing renal function Renal function assessment.

Radiographic considerations

  • The ventrodorsal projection is the most useful for evaluating the kidneys.
  • A low KVp and high mAs should be used to maximize contrast of abdominal radiographs.
  • Care should be taken to include the entire urinary tract for most investigations.

Restraint
​GA is preferred for contrast studies, eg excretory urography (EU, IVP, IVU) Radiography: intravenous urography although they may be performed under sedation.

Artifacts

Urine samples should be obtained prior to contrast studies as the contrast media will alter the results of urinalysis.
  • Nipples overlying the kidneys or bladder may be mistaken for calculi.

To identify position of nipples paint them with barium or attach a paper clip (if anesthetized).

Indications

  • Hematuria Hematuria.
  • Dysuria.
  • Urinary incontinence Urinary incontinence.
  • Polycythema.
  • Weight loss (suspect PLN).
  • Anuria.
  • Following trauma to investigate integrity of urinary tract.
  • Investigation of abdominal masses Abdominal organomegaly.
  • Investigation of lower urinary tract disease if unable to catheterize the bladder.

Ultrasonography Ultrasonography: bladder and urinary tract often allows a more complete evaluation of the kidney.

Radiographic anatomy

Kidneys

  • The kidneys and ureters are retroperitoneal.
  • The right kidney is lateral to the spine and caudal to the 13th rib on the VD radiograph   Kidney: normal - IVU 01 VD immediate  .
  • Both kidneys are positioned ventral to L1-L4 but the left may be slightly caudal to the right and they are both very mobile   Kidney: normal - IVU 03 lateral 5 min  .
  • Right lateral recumbancy may separate the kidney shadows more than left lateral.
  • The kidneys are normally smooth in outline   Abdomen: normal (fat) - radiograph lateral  .
  • Normal size is 2.4 - 3 times the length of the 2nd lumbar vertebra.

Smaller in neutered cats - many cats have kidney only 2 x L2 but without demonstrable renal disease.

Ureters

  • The ureters are not normally visualized unless excretory urography Radiography: intravenous urography is performed.
  • They are <2 mm in diameter.
  • The ureters run in a fold of peritoneum from the renal pelvices to the trigone of the bladder.
  • The left ureter inserts into the bladder slightly caudal to the insertion of the right ureter.
  • The ureters curve cranially in a hook shape as they insert into the dorsal wall of the bladder.
  • In cats the ureters insert into the bladder relatively more caudally than in dogs.

Interpretation

  • The entire urinary tract should be on the radiograph   Abdomen: normal (fat) - radiograph lateral  .
  • Care should be taken to assess the rest of the abdominal contents carefully for concurrent disease.
  • The lumbar spine, pelvic inlet, retroperitoneal space and sublumbar lymph nodes (which lie ventral to the 6th lumbar vertebra) should be evaluated for signs of metastatic spread of tumors.
  • Metastatic mineralization of the gastric mucosa and abdominal blood vessels may be seen with chronic renal disease.
  • Poor abdominal detail may be due to rupture of the bladder or lack of abdominal fat due to weight loss.
  • Mineralization of the renal parenchyma may be difficult to assess due to overlying fecal material in the colon.
  • Compression radiographs may be useful in displacing bowel loops from overlying kidney and bladder and help localize area of mineralization.

A wooden spoon may be used to isolate a kidney and compress overlying tissues.

  • Air introduced into the colon to create a pneumocolon Radiography: large intestine contrast can help to differentiate true renal mineralization from artifact.
  • The kidneys should be assessed for alterations in number, size, shape, position and opacity.

Number

  • Alterations in number of kidneys are unusual and are often discovered as an incidental finding.
  • Absence of a kidney may congenital or due to nephrectomy.
  • Renal aplasia is of no clinical significance, the single kidney is often larger than normal due to compensatory hypertrophy.
  • The kidney that is present should be normal in shape with a smooth outline. It may be hard to differentiate congenital absence of a kidney from severe hypoplasia.
  • End stage kidneys may be very small and can be difficult to visualize even with contrast radiography due to poor excretion of the contrast agent.
  • Congenital fusion of kidneys is reported and results in a single kidney that is abnormal in shape, usually sigmoid.
  • A third kidney may be present due to renal transplant for kidney disease.

Size

It is important to appreciate that significant renal disease may be present with normal sized and shaped kidneys.

Small kidneys

  • Reduced size occurs due to hypoplasia, dysplasia or chronic disease (chronic interstitial nephritis Kidney: chronic kidney disease).
  • Hypoplastic kidneys are normal in shape and may be unilateral or bilateral. Small kidneys may be the result of chronic disease such as chronic interstitial nephritis, pyelonephritis, dysplasia, glomerulonephrosis.
  • With end stage disease the margins of the kidney are often irregular and may be unilateral or bilateral depending on the cause   Kidney: chronic failure - radiograph VD  .
  • A normal margin to the kidneys does not rule out end stage disease.
  • With unilateral disease the contralateral kidney may undergo compensatory hypertrophy and appear larger than normal.

Large kidneys

  • Enlargement of the kidneys   Kidney: lymphosarcoma - radiograph lateral  may be due to neoplasia (lymphosarcoma Lymphoma), FIP Feline infectious peritonitis, amyloidosis, renal cysts, hydronephrosis, abscess, perirenal pseudocysts, hematoma or may be idiopathic.

Most large kidneys have abnormal outlines.All these causes may be unilateral or bilateral and ultrasonography or excretory urography is required to differentiate them.

Position

  • The position of the kidneys alters with body position.
  • On a lateral projection the non-dependant kidney may move ventrally.
  • The kidneys may be displaced by enlargement of other structures.
  • Enlargement of the caudate lobe of the liver may displace the right kidney caudally.
  • Adrenal glands have to be markedly enlarged to displace the kidneys   Adrenal: mass - radiograph lateral  (very rare in cats):
    • Enlargement often displaces the cranial pole of the ipsilateral kidney laterally and possibly ventrally.
    • Severe enlargement may result in displacement of the entire kidney caudally.
  • Retroperitoneal masses and swellings (eg abscess, hemorrhage, neoplasia, granuloma, foreign bodies) displace the kidneys ventrally. In addition there will be loss of the visualization of the normal fascial planes.

Ovarian masses if large will drag the caudal pole of the ipsilateral kidney ventrally. Rare in cats as most female cats are neutered. 

  • Traumatic displacement of the kidneys occurs rarely but displacement of the kidneys through a diaphragmatic rupture is reported (where it may mimic a thoracic mass). Congenital ectopia is reported.

Opacity

Kidneys

  • Gas within the kidneys is usually iatrogenic (following pneumocystography and vesicoureteral reflux).
  • Mineralization in the renal pelvis is most commonly due to uroliths (but can be secondary to chronic inflammation).
  • Mineralization of corticomedullary junction (linear band parallel to cortex) - occurs secondary to hypercalcemic nephropathy.

Ultrasonography is more sensitive for detection of this.

  • Focal mineralization of parenchyma may occur with cysts, abscesses, hematomata or neoplasia.

Ureters

  • Gas in the ureters is invariably iatrogenic (following bladder catheterization).
  • Focal mineralization is usually due to uroliths.

Calculi may be small and easily overlooked.

  • Two orthogonal views are required to localize mineralized material which may be fecal material overlying the ureters on one view.

Appearance

Ureters

  • Most ureteric diseases require contrast studies Radiography: intravenous urography to visualize abnormalities.
  • Megaureter - enlarged ureters may occasionally be visible as linear, homogenous, soft tissue opacities lying between the kidney and bladder dorsal to the colon.

In severe cases the ureters may be massive and lie ventral to the colon and can be mistaken for dilated bowel or uterus.

  • The cause of ureteral dilation, eg retroperitoneal mass, ureteric calculi may be visible on plain radiographs.
  • Ureteral rupture - results in extravasation of urine into the retroperitoneal space   →   swelling of sublumbar tissue with loss of visualization of fascial planes.

EU/IVU is required for definitive diagnosis as sublumbar hematoma or inflammation may give similar appearance.

  • Concurrent injuries may be seen.
  • Focal masses - associated with ureter are rare, eg neoplasia may be seen as soft tissue mass.
  • Differentials include enlarged sublumbar lymph nodes and retroperitoneal masses.