Contributors: Fraser McConnell

 Species: Canine   |   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 evaluation the kidneys.
  • A low KVp and high mAs should be used to maximize contrast of abdominal radiographs.
  • A grid is necessary if the depth of tissue is >10 cm.
  • Care should be taken to include the entire urinary tract for most investigations.

Restraint

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.

Indications

Radiographic anatomy

Kidneys

  • The kidneys and ureters are retroperitoneal.
  • The right kidney lies within the renal fossa of the caudate lobe of the liver. It is relatively constant in its position, the 13th rib usually lies over the renal pelvis of the right kidney. The cranial pole of the right kidney is often not visualized due to effacement with the liver.
  • The left kidney is positioned further caudally than the right and lies at the level of the 2nd/3rd lumbar vertebrae Abdomen: normal 01 radiograph lateral. The left kidney is quite mobile and may be situated in the mid abdomen where it may be mistaken for an abdominal mass (especially Labrador retrievers).
  • The kidneys are normally smooth in outline.
  • Normal size is 2 1/2-3 1/2 times the length of the 2nd lumbar vertebra.

Ureters

  • The ureters are not normally visualized unless excretory 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.

Interpretation

  • The entire urinary tract should be on the radiograph.
  • 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.
  • Air introduced into the colon to create a pneumocolon Radiography: large intestine contrast can help to differentiate true renal mineralization from artefact.
  • 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.

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 Kidney small - radiograph.
  • 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.
  • 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 may be due to neoplasia Kidney mass - radiograph VD Kidney mass - radiograph , amyloidosis, renal cysts, hydronephrosis, abscess, hematoma or 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:
    • 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.
  • 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) Kidney calculus - radiograph VD.
  • 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 Ureter calculi - radiograph.
    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.
    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.

Additional studies

Ultrasonography

  • Method of choice for evaluating the structure of the kidney Ultrasonography: kidney.
  • It is faster and less invasive than contrast radiography and allows guided-biopsy or FNA.
  • Ultrasound may be used to identify hydroureter and ectopic ureters.

Contrast

  • Excretory urography Radiography: intravenous urography is required to fully evaluate the integrity, shape, size, position of the ureters.
  • Method of choice for detection of ruptured ureter and involvement of ureters and kidneys in retroperitoneal disease.
  • Most value when evaluating renal disease if kidneys are normal sized, enlarged or irregular.
    Gives limited information about renal function.
  • Ultrasound-guided percutaneous injection of contrast into the ureter is described and allows higher concentration of contrast within the ureter and minimal risk of adverse effects of contrast agents on renal function.

Nuclear medicine

  • Nuclear medicine is the method of choice for evaluation of renal function Scintigraphy: renal for GFR.
  • It is safe, minimally invasive and allows global and individual GFR to be measured.
    This is of particular value when considering nephrectomy Ureteronephrectomy.

CT

  • Limited advantages over conventional contrast radiography, but may allow clearer visualization of the ureters when evaluating for ectopia.
  • Allows 3-D reconstruction of renal vasculature which is important for renal transplantation.