Kidney normal

Coronal scan plane right kidney.

This can be obtained supine or decubitus.

Note the oblique plane of the probe corresponding to the lie of the kidney on the psoas muscle.

Longitudinal ultrasound of a normal kidney.

Transverse scan plane of the kidney.

Normal kidney.

Transverse plane.

Anatomical Variants

In the 1st trimester, the developing kidneys ascend in the foetal abdomen. If the progress is hampered, this can result in:

  • An ectopic kidney if it fails to reach the normal position.
  • Crossed fused ectopia (both on one side)
  • Or a horseshoe kidney if the lower poles fuse.
  • An interruption to the vascular supply to the developing kidney will result in an atrophic, poorly differentiated kidney.

Common anatomical variants

  • Atrophic small kidney
  • Horseshoe kidney
  • Ectopic kidney
  • Duplex kidney
  • Cross fused ectopia
  • Unilateral renal agenesis

Unilateral renal agenesis

Ultrasound image – An absent left kidney indicating agenesis. A case of OHVIRA syndrome in a young teenage female. 


Ultrasound image – The right kidney is present and enlarged due to compensatory hypertrophy.

Courtesy of Callum Linehan.

Extrarenal Pelvis

The renal pelvis can project out of the hilum of the kidney without any obstruction or abnormality.

It will usually be isolated without any calyceal dilatation.

It can be:

  • Congenital
  • Past history of obstruction

An ultrasound of a “baggy” extra-renal pelvis

Ectopic Kidney

Also the result of abnormal or interrupted ascent during embryology.

  • The most common ectopic site is in the pelvis. The kidney will lie obliquely in the ipsilateral iliac fossa.
  • Less commonly, a kidney may ascend to the other side with 2 kidneys on one side of the abdomen. This is called crossed-ectopia. This may result in a single large fused kidney as shown below (crossed-fused-ectopia)

Ultrasound of a cross fused ectopic kidney. The left kidney is fused to the lower pole of the right kidney.

Right renal ectopia with malrotation. 
B-mode and colour Doppler image – Longitudinal to the aorta.

An ectopic right kidney is positioned within the lower abdomen and rotated 90 degrees. 

Note the right renal artery branches anteriorly and courses inferiorly.

Right renal ectopia with malrotation. 

Ultrasound image – Transverse abdomen demonstrating the right kidney in the longitudinal plane. 

Note the aorta relative to the kidney. 

Courtesy of Callum Linehan.

Non-fused cross ectopia

This patient came with right sided pain with the question of possible calculi. It was discovered that there was no kidney in the right renal fossa! As typically discovered incidentally.

Ectopic kidneys may have urological complications, such as, urinary infections, renal calculi,  ureteropelvic junction obstruction, due to their frequent abnormal shape, malrotation, and aberrant vasculature.

Ultrasound discovered the right kidney was sitting below the left kidney. It was rotated and the distal ureteric jet was on the right side.

Colour Doppler Ultrasound showing there are 2 ureteric jets.

B mode ultrasound image demonstrating there is no fusion of the kidneys which is the rarest type.

Ultrasound of a crossed ectopic right kidney not fused to the left kidney.The right ectopic kidney was located in the left hemiabdomen with the hilum anteriorly faced and the vessels coursing unusually across the midline.

Horseshoe Kidney

  • Occurs when there is fusion of the metanephros as they are pushed together during their ascent from the sacral region.
  • Almost always involves fusion of the lower poles.
  • There is an increase incidence of infection, calculi and tumors in horseshoe kidneys.

Horseshoe kidney: A transverse ultrasound view across the midline showing the isthmus across the aorta.

A sagittal ultrasound view of the isthmus of a horseshoe kidney.

Horseshoe kidney: Longitudinal ultrasound view of the horseshoe isthmus

Fused pelvic kidney / ''cake kidney''

A rare fusion anomaly that results in the complete fusion of both right and left kidneys during their ascent from the sacral region. 

This patient presented with reduced eGFR and HTN. Their first presentation for abdomen imaging at 59 years old. 

  • These are almost always located in the pelvis. 
  • No isthmus is present as fusion occurs at the pelvis and polar regions, giving the impression of overlapping kidneys. 
  • Rarely has one ureter.
  • Like the horseshoe variant, it is predisposed to an increased incidence of stones, malignancies and infections. 

Ultrasound image – Transverse.  
A midline pelvic mass is seen anterior to the aortic bifurcation. 
At the superior pole, cortical tissue connects both right and left kidneys. 

Ultrasound image – Transverse.
Surveying through, coritical tissue extends to the inferior pole. 

Ultrasound image – Longitudinal cake kidney (right). 
The cortex and sinus overlap between the renal poles. 

Images courtesy of Callum Linehan.

Ultrasound image – Longitudinal cake kidney (left).
When assessing with colour Doppler, a single vein drained the kidney to the IVC, and multiple renal arteries were originating from the aorta and common iliac arteries.  

Images courtesy of Callum Linehan.

Colour Doppler image – Bladder trigone. 
Two ureteric jets are seen confirming two ureters. 


Role of Ultrasound


To identify the cause of:

  • Flank pain
  • Haematuria (frank or microscopic)
  • Follow-up of previously identified pathology
  • Classification of a mass (Solid V’s cystic)
  • Post surgical complications
  • Guidance of aspiration, biopsy or intervention
  • Post injury


  • The mid to distal ureter is generally obscured by bowel gas.
  • Small lesions at the upper pole of the kidney may be difficult to see due to refractive edge shadowing. This can be overcome with thorough scanning technique.

Patient Preparation

Begin with the patient supine. Each kidney may also need to be examined in the decubitus position. Raise the ipsilateral arm above the patient’s head. 

Equipment setup

Highest frequency curved linear array probe possible. Start with 7MHz and work down to 2 or 3 for larger patients. Assess the depth of penetration required and adapt. Paediatric and thin patients should be scanned with a 7MHz. Good colour / power / Doppler capabilities when assessing vessels or vascularity of a structure. 

What to Check


  • Kidney size (should not be >1cm difference between sides)
  • Cortical thickness(not <10mm)
  • Cortico-medullary differentiation
  • Cortex at least as hypoechoic as the liver
  • Pyramids slightly hypoechoic relative to the cortex
  • No hydronephrosis
  • Renal scarring(beware mistaking prominant lobulations as scars)




A comprehensive examination of the renal tracts should always include assessment of the urinary bladder and, in males,the prostate

Scan longitudinally right subcostally. Visualise the kidney inferior to the right lobe of the liver (RT), or spleen (LT). Place the probe between iliac crest and the lower costal margin to examine in the coronal plane. Ensure the kidney is thoroughly examined from edge to edge. Rotate into transverse. Scan from beyond the superior margin to inferior. Document the normal anatomy and any pathology found, including measurements and vascularity if indicated. 

Basic Hardcopy Imaging

A renal series should include the following minimum images;

  • Both kidneys with length measurements
  • Right kidney long with liver for comparison
  • Both kidneys longitudinal medial and lateral
  • Both kidneys transverse
    • sup
    • mid
    • inf
  • Left kidney long with spleen for comparison
  • Document the normal anatomy. Any pathology found in 2 planes, including measurements and any vascularity.