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Renal (Kidney) Ultrasound: Probe, Settings, Views and Measurements

A kidney scan is one of the most useful things a clinic ultrasound can do: it shows whether the kidneys are blocked, small, scarred or carrying stones and cysts, without radiation or contrast. How the kidney looks and why, the optimal probe and settings, the standard views and measurements, and how hydronephrosis is graded.

Also called Renal ultrasound, Kidney ultrasound, KUB ultrasound

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What a renal scan answers

  • Is there hydronephrosis — dilatation of the collecting system that may mean obstruction?
  • Are both kidneys present, in the usual place and of similar size?
  • Are the kidneys small and bright, a pattern seen in long-standing kidney disease?
  • Are there stones, simple cysts, or a solid mass that needs further imaging?
  • How much urine is in the bladder before and after voiding, and is the bladder wall normal?

How the kidney looks, and why

Each layer of the kidney has a different structure, so it reflects sound differently. Knowing why helps you recognise what is abnormal.

Schematic of a right kidney in long axis seen through the liver, with cortex, darker medullary pyramids and a bright central sinus, callipers from upper to lower pole, and five small kidneys illustrating SFU hydronephrosis grades 0 to 4.
Long axis through the liver window. Measure from pole to pole on the longest image. The SFU grades on the bottom row describe how far the dark, urine-filled collecting system spreads into the calyces and thins the parenchyma.

Cortex

Mid-grey. In adults it is normally no brighter than the neighbouring liver or spleen; a cortex brighter than the liver is a sign of medical renal disease.

Medullary pyramids

Darker triangles inside the cortex, because they contain more fluid and fewer reflecting interfaces. They are especially prominent in infants.

Central sinus

Bright, because fat and vessels around the collecting system create many strong interfaces.

Urine

Black, because fluid has no internal reflectors. A dilated pelvis or calyx shows as black branching spaces in the bright sinus, with brighter tissue behind (posterior enhancement).

Stones

Much stiffer than tissue, so they reflect strongly and leave a dark acoustic shadow. On colour Doppler many stones show a twinkling artefact behind them.

What is optimal: probe and settings

SettingOptimal starting pointWhy
ProbeCurvilinear, about 2–5 MHz for adults; micro-convex or linear for children and thin adultsPenetration to the kidneys, which lie deep in the flank
PresetRenal or abdominalSets dynamic range, persistence and harmonics for solid organs
DepthKidney fills about two-thirds of the screenLarger image, higher frame rate
FocusAt the level of the kidney or the structure of interestNarrowest beam where you are looking
HarmonicsOnCleaner cyst walls and sharper stone shadows
Colour DopplerLow scale for perfusion; colour over stones for twinkle; colour over the bladder base for ureteric jetsEach answers a different question
GainLiver and kidney cortex shown as mid-grey on the same imageLets you compare cortical brightness fairly

Read next Why frequency, depth and focus matter

Positions, windows and standard views

Right kidney

Patient supine or turned onto the left side. Scan through the liver at the right flank or anterior axillary line, with the breath held in deep inspiration to bring the kidney down.

Left kidney

Patient turned onto the right side. Scan further back, in the posterior axillary line, through the spleen; bowel gas often blocks an anterior approach.

Prone

Useful in children and when ribs or gas block the flank views; the kidneys lie just beneath the back muscles.

Views to record

Long axis through the longest length, then transverse at the upper pole, hilum and lower pole for each kidney, plus the bladder in two planes.

Measurements

  • Kidney length: pole to pole on the longest long-axis image. In a study of 665 adult volunteers, median length was 11.2 cm on the left and 10.9 cm on the right; length correlated with height and decreased with age (Emamian et al., 1993).
  • Compare the two sides. A kidney that is clearly smaller than its partner, or smaller than the patient’s earlier scans, needs explaining.
  • Parenchymal thickness at mid-pole helps judge chronic scarring and thinning from long-standing obstruction.
  • Bladder volume and post-void residual: length × width × height × a shape correction factor, calculated by most systems’ bladder package.

Read next Bladder scanners and post-void residual

Hydronephrosis: how it is graded

Grading describes how far the black, urine-filled collecting system spreads. It is a description, not a diagnosis of obstruction: a full bladder, a roomy extrarenal pelvis or pregnancy can all dilate a normal system.

  • For antenatal and postnatal urinary tract dilatation, many units now use the UTD classification, which combines the anteroposterior pelvic diameter with calyces, parenchyma, ureters and bladder (Nguyen et al., 2014).
  • In adults, reports commonly describe hydronephrosis as mild, moderate or severe, together with the cause if seen.
  • If the bladder is very full, rescan the kidneys after the patient has voided; mild dilatation often resolves.
  • Parapelvic cysts can mimic hydronephrosis; unlike a dilated pelvis, they do not connect with each other.
Society for Fetal Urology (SFU) grades, used mainly in infants and children (Fernbach et al., 1993)
SFU gradeUltrasound appearance
0No dilatation; the central sinus is intact
1Renal pelvis visible, no calyces seen
2Pelvis dilated, a few calyces visible
3Nearly all calyces dilated, parenchyma normal
4As grade 3, with thinning of the parenchyma

Kidney stones and cysts on ultrasound

Stones

Look for a bright focus with a clean shadow, and twinkle on colour Doppler. Small stones and stones in the middle ureter are easily missed; a normal scan does not exclude a small stone.

Simple cysts

Round, black, thin-walled and with posterior enhancement. Cysts with septa, solid parts or internal flow need further imaging.

Ureteric jets

Colour Doppler over the bladder base shows urine squirting in from each ureter. A jet from both sides argues against complete obstruction.

Equipment checklist for renal scanning

  • A curvilinear probe with good penetration on heavy patients, and harmonics on that probe.
  • Colour Doppler sensitive enough for ureteric jets and stone twinkle.
  • Measurement and bladder-volume packages included, not licensed extras you did not expect.
  • DICOM export so that kidney lengths can be compared between visits.
  • MDA registration for the system and every probe (Act 737).

Read next Ultrasound for GP clinics

Related guides

Bladder Scanner: Portable Bladder Volume MeasurementA bladder scanner measures urine volume at the bedside in seconds, without a catheter. How it works, where it helps, where it can mislead and what to check before buying.Abdominal Ultrasound in the Clinic: A Systematic SurveyAbdominal scanning is often the first application a GP clinic adds after early pregnancy. The probe and preset, how to prepare the patient, a survey that covers every organ in the same order each time, the findings clinics see most, and when to refer.How Ultrasound Works: From Piezoelectric Crystal to ImageAn ultrasound scanner is a very fast sonar. The probe sends a short burst of sound, times every echo that comes back, and draws each one at the depth its delay implies. Here is how the probe, the gel, the timing and the choice of frequency fit together, and how to set them for each exam.

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Sources and further reading

  1. Emamian SA et al. Kidney dimensions at sonography in 665 adult volunteers. AJR 1993;160:83–6
  2. Fernbach SK et al. Ultrasound grading of hydronephrosis: the Society for Fetal Urology system. Pediatr Radiol 1993
  3. Nguyen HT et al. Multidisciplinary consensus on the classification of urinary tract dilation (UTD). J Pediatr Urol 2014
  4. AIUM — Practice parameters for ultrasound examinations

Frequently asked questions

What is the normal size of a kidney on ultrasound?
In a study of 665 adult volunteers, median kidney length was 11.2 cm on the left and 10.9 cm on the right. Length varies with height and falls with age, so compare the two sides and earlier scans as well.
Which probe is used for a kidney ultrasound?
A curvilinear probe of about 2–5 MHz for adults. A higher-frequency micro-convex or linear probe gives more detail in children and thin adults.
Do I need a full bladder for a kidney ultrasound?
A comfortably full bladder helps assess the bladder itself. The kidneys can be scanned either way, but a very full bladder can cause mild dilatation, so the kidneys may be rechecked after voiding.
Can ultrasound see kidney stones?
Many stones in the kidney show as bright spots with a shadow, and often twinkle on colour Doppler. Small stones, and stones in the ureter, can be missed, so a normal scan does not rule out a small stone.
What does hydronephrosis look like on ultrasound?
Black, urine-filled spaces spreading through the bright centre of the kidney. Grades describe how far it spreads into the calyces and whether the kidney tissue has thinned.

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