Ultrasonography of the kidney and the renal vessels Part I: Normal findings, inherited and renoparenchymatous diseases

Ultrasonography of the kidney and the renal vessels
Part I: Normal findings, inherited and renoparenchymatous diseases

Prof. Dr. med. Jörg Radermacher
Dept. of Nephrology
Klinikum Minden
Friedrichstrasse 17
32427 Minden
Tel: 0571/ 801 – 3021
FAX: 0571/ 801 – 3076
E-Mail: Joerg.radermacher@klinikum-minden.de

Abstract
Renal ultrasonography has become the standard imaging modality in the investigation of
kidneys because it offers excellent anatomic detail, requires no special preparation of patients
is readily available and does not expose the patient to radiation or contrast agents.
Ultrasonography is used to determine the site and size of the kidney and to detect focal lesions
like tumors, cysts and renal stones. Furthermore the presence and urodynamic relevance of
hydronephrosis can reliably be found.
The presence of renoparenchymatous disease as such is also discernible to the experienced
investigator, however most glomerular diseases cannot be further sub classified. Exceptions
are primarily renovascular disorders like hypertensive nephrosclerosis, diabetic nephropathy
or renal vasculitis which can be suspected if the intrarenal resistance index value is increased.

Color Doppler sonography in experienced hand allows the reliable detection and
quantification of renal artery stenosis and increased resistance index values may indicate
irreversible disease.
Ultrasonography has also been found of value in the evaluation of renal transplant kidneys.
Especially in the early transplant course potentially fatal but reversible diseases like renal vein
thrombosis or urinomas are detected with high sensitivity. In the long term course an
increased resistance index value may also predict allograft failure.


Introduction
Renal ultrasonography has become the standard imaging modality in the investigation of
kidneys. Renal size and location can be determined. Solid tumors can be detected and can be
distinguished from renal cysts. Ultrasonography can detect nephrolithiasis and
hydronephrosis. Dilated ureters can frequently be followed up to the location of the occluding
concrement. Different renoparenchymatous diseases, especially the glomerulonephritic
diseases cannot be differentiated be ultrasonography. However, the experienced investigator
can almost always distinguish normal from diseased kidneys. Detection of renal arteries is
reliably possible with Color Doppler sonography. A separate discussion of B-mode and Color
Doppler sonography does not always make sense, because frequently only the combined use
of both techniques (urinary tract obstruction, renal tumors, renal cystic disease, diabetic
nephropathy…) enables the investigator to make a diagnosis. The first part of this review
article will focus on normal findings and renoparenchymatous disease. The second part will

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deal with focal findings including renal tumors. For more in depth information the reader is
referred to books and review articles. 1-6

Patient preparation and ultrasonographic normal findings
Preparation of the patient (fasting, carminatives) is rarely necessary. The investigation should
be performed in a warm, dark room with the patient lying on his back. For relaxation of the
abdominal rectus muscle the investigation should be performed with the upper body elevated
by 30° and possible with slightly bend knees (knee roll). This will cause a slightly more sub
costal position of the kidneys which facilitates the investigation. In rare cases placement of
the patient on his left or right side may become necessary to overcome poor visualization due
to interposition of bowl or to have a better view on the origin of the renal arteries. Changing
patient position however, may greatly prolong investigation time, especially in older patients.
A primary approach with the patient in the prone position is not advised – unless one wants to
perform a renal biopsy - because of poorer kidney visualization through the psoas muscle. In
case of renal biopsy a more dorsal position of the kidneys is achieved by placing the patient
on a bed roll. A 2-5 MHz convex array scanner is usually used in adult sonography. Scanners
with higher frequencies have a lesser penetration depth but allow better visualization of
kidney stones. Massive meteorism may be a lesser problem with the use of a sector scanner.
Harmonic imaging modalities, which are available in most modern machines, allow better
visualization of kidneys (see below). If available a Color Doppler machine with at least a pw-
Doppler should be used.



Figure 1: Better visualization of the right kidney with harmonic imaging and B-mode contrast
enhancement (photopic)

Examination procedure: To compare kidney with liver echogenicity a transhepatic depiction
of the right kidney from a ventrolateral approach is advisable. The caudal pole of the kidney
however is frequently not visible due to interposing bowl gas. Depiction of the whole kidney
in longitudinal and transverse sections is usually possible from a dorsolateral sub costal
approach. A valsalva maneuver will frequently be necessary to see the upper pole of the
kidney. Mobility of a normal kidney should be 3-7 cm. Since deep inspiration may lead to
elongation of kidneys with a concomitant reduction in depth and width all dimensions should

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be measured in the same breath hold position. Otherwise false renal volume calculations may
result.

Normal findings


Fig. 2 Normal kidney
Renal shape and -position
The kidneys are located retroperitoneally and slide on the musculus quadratus lumborum and
musculus psoas during in- and expiration. Due to loin lordosis the lower pole of both kidneys
is located more ventrally than the upper pole. The upper pole of the right kidney is transversed
by the 12th and the upper pole of the left kidney by the 11th rib. This means, the right kidney
is usually positioned 1 to 2 cm more caudally than the left kidney. The right kidney surface is
adjacent to the lower liver boarder, so the liver can be used as an ultrasound window for the
right kidney. The kidneys are shaped like beans, convex on the lateral und concave on the
medial sight.
Renal surface and capsule
The kidneys are enclosed by an adipose capsule the thickness of which varies depending on
the general constitution of the patient. This adipose capsule can have high or low echogenicity

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and may be mistaken for the renal parenchyma in patients with shrunken kidneys. However,
the missing mobility during in- and expiration usually allows clear differentiation of
surrounding fatty tissue and renal parenchyma. Kidneys usually have a smooth surface. A
normal variant are renal renculi. These are the remaining signs of fetal lobulation and are
more easily recognizable in the right kidney. Two renculi always surround a medullary
pyramid.

Figure 3. Persisting fetal lobulation of the right kidney (renculation)


Renal parenchyma
An inexperienced investigator should assess the renal parenchyma only in comparison to the
adjacent liver and spleen. The normal renal parenchyma in children age 6 and older and in
adults should be slightly less echogenic than that of liver and spleen. Normal renal
parenchyma from birth until 6 months of age is slightly brighter than that of the liver.
Increased echogenicity compared to liver and spleen in adults is a sensitive but unspecific
sign of renal disease. The normal parenchymal width of 15-25 mm can be measured most
reliably from the basis of a medullary pyramid to the kidneys surface. The renal hilum should
be visible when measuring parenchymal width to avoid underestimation. If such a depiction is
not technically possible a normal parenchymal width can be assumed if the parenchyma-
pelvis relation is normal. The relation of ventral and dorsal parenchymal width to pelvic width
should be 2 : 1. Medullary pyramids are usually less echogenic than the surrounding
parenchyma. Columns of Bertini (columnae renales) are extensions of the renal parenchyma
in the renal pelvis and hypertrophied columns of Bertini should not be confused with pelvic
tumors. The left kidney sometimes has a very wide parenchyma in the medial portion, a so
called splenic notch or dromedary hump. This also is a normal finding and should not be
confused with renal masses. The echogenicity in the area of the hump will be the same as in
the surrounding parenchyma and the medullary pyramids will still be present and the vessel
architecture will be unaffected.

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Abb. 4 Splenic notch (dromedary hump), P denotes medullary pyramids

Renal sinus
The central echogenic part of the kidney (sinus) is composed of the pelvis and the calyces, of
blood and lymphoid vessels and interposed adipose tissue. The normal renal pelvis should not
be fluid filled except in pregnant women. Anechoic areas are frequently due to dilated veins
as can be easily proven by Color Doppler ultrasound. The ureter is located dorsally to the
renal vessels and also should not be visible normally.
Renal vessels
The renal artery usually branches within the renal sinus or extrarenally in 2 to 3 segmental
arteries of first order and these 1st order segmental arteries branch another 2 to 3 times into
segmental arteries of 2nd and 3rd order. When entering the renal parenchyma the vessels are
called interlobar arteries. From these the arcuate arteries branch of in a 90° angle, running
parallel to the kidney surface and the interlobular arteries, which run towards the renal
capsule, originate from the arcuate arteries. The right renal vein is relatively short (4 cm) and
rarely visible from a ventral approach due to overlying bowl gas. The left renal vein runs
between aorta and superior mesenteric artery and can rarely be compressed between these two
vessels (so called nutcracker phenomenon). A retro aortal course of the left renal vein is also
possible.
Estimation of normal renal size
Chronic renal disease frequently leads to renal shrinking. Reliable criteria depicting a
shrunken or enlarged kidney have not been published for adults. A renal length of 9-12 cm is
considered normal, renal length correlating to body length. The correlation is poor however.
Frequently the right kidney is shorter than the left kidney whereas renal function estimated by
szintigraphy and renal volume estimated by CT are equal. A better correlation can be found
between renal volume and body weight or body surface area. Renal volume is frequently
estimated as length x depth x width (cm) / 2. In children with healthy kidneys normal renal
volume (ml) could be estimated as body weight (kg) x 2. The few available data in adult

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populations – which were not evaluated for normal renal function - do not contradict these
findings. A normal renal volume can therefore be defined as body weight (kg) x 2 ± 20%.

Congenital renal diseases

Renal agenesis or hypoplasia: Usually a chance finding. Unilateral small or missing kidney
with normal architecture and echogenicity. Color Doppler and Doppler ultrasonography also
show normal findings. The contralateral kidney shows compensatory hypertrophy. Caveat:
Unilateral small kidneys due to renal artery stenosis will also results in – age dependant –
hypertrophy of the contralateral kidney. Here altered Doppler signals in the small kidney will
lead to the correct diagnosis. Renal agenesis cannot be proven by ultrasonography (see
ectopy).

Ectopic kidneys: Usually small and malrotated. Located between the urinary bladder and the
normal position. In crossed ectopy both kidneys are located on the same side and are
frequently fused and therefore enlarged. Frequently two or more renal pelvices are present.
Sometimes the kidneys can only be found with CT or MRT.

Horseshoe kidney: Fusion of both lower kidney poles with a parenchymal or connective
tissue bridge (isthmus) ventral to the aorta. Here the fused part may be mistaken for enlarged
lymph nodes. The lower poles of the kidneys are misplaced medially. This malformation is
frequently accompanied by vesicoureteral reflux, stone formation and urinary tract
obstruction.

Double kidney: Most frequent malformation (incidence 0,5-10%). B-mode ultrasonography
shows a parenchymal bridge which completely separates the cranial from the caudal part of
the kidney. Frequently these kidneys are enlarged compared to the contralateral kidney. The
parenchymal bridge can be confused with hypertrophied columns of Bertini. Ultrasonography
cannot prove the presence of double kidneys; this can only be done by an iv-urogramm
showing two renal pelvices and two ureters. Reflux or renal pelvis obstruction occurs more
frequently in double kidneys. Urinary tract obstruction may lead to dilatation of the renal
pelvices in which case a double kidney can be proven by ultrasonography alone.

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Figure 5: Enlarged left kidney with a parenchymal bridge which completely traverses the
renal sinus. Due to crossed ectopy a further small renal parenchymal area and sinus can be
seen in the lower third. This is the hypoplastic fused part of the right kidney.

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Renoparenchymatous diseases

Renoparenchymatous diseases can be classified as uni- or bilateral diseases with small or
large kidneys (see table 1). The method to evaluate kidney size as normal or not has been
shown above. A general rule is that small kidneys depict chronic renal disease and enlarged or
at least normal sized kidneys depict acute and therefore potentially reversible disease. A
further distinguishing feature is parenchymal echogenicity. Diseased kidneys in general show
increased echogenicity, however in the early stages echogenicity may still be normal.
Increasing echogenicity is directly correlated to histopathological findings like global
sclerosis, tubular atrophy, leukocyte infiltration and the number of hyaline casts per
glomerulum. Decreased echogenicity is correlated to the magnitude of interstitial edema 7, 8.
The normal renal parenchyma should be slightly less echogenic than that of the liver. This is
problematic insofar, that it assumes liver parenchyma to be of normal echogenicity. Many
patients, however, have fatty liver disease. Objective and easily applicable methods for
quantification of renal parenchymal echogenicity are missing. Therefore the classification of
renal echogenicity depends to a large amount on investigator experience. A further criterion
separating normal from abnormal kidneys is renal perfusion. The renal resistive index (RI =
decrease of minimal diastolic in relation to maximal systolic Doppler flow velocity, e.g. 0.6 =
60% decrease) has been best evaluated so far. The RI is increased in hypertensive
nephrosclerosis 4 and correlates with the histological severity of glomerulosclerosis and
arterio- and arteriolosclerosis. Table 1 summarizes the major diseases associated with small or
enlarged kidneys.



Figure 6: Right kidney with increased echogenicity as a sensitive but unspecific sign of renal
disease. Compare the echogenicity of the kidney with that of the overlying liver


Diabetic nephropathy
Diabetic nephropathy is the most frequent renoparenchymatous disease, accounting for 40%
of cases of incident terminal renal failure. Diabetic nephropathy is almost always associated
with enlarged kidneys prior to terminal renal failure. Echogenicity increases with increasing
stages of renal failure; however in earlier stages of disease echogenicity is frequently normal.
A further diagnostic criterion is an increased RI (see below) however, the increased RI occurs
relatively late in the course of diabetic nephropathy when other signs like microalbuminuria
are also already present. Even when terminal renal failure is present diabetic kidneys remain
relatively enlarged as opposed to kidneys from patients with glomerulonephritic or renal
interstitial disease. Diminished parenchymal width or a kidney with a low volume in a patient
with diabetes can hint at superimposed hypertensive nephrosclerosis. Renal scars, abscesses
and papillary necrosis can also occur in diabetic nephropathy.

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Glomerulonephritis and vasculitis
There is no specific ultrasonographic sign for glomerulonephritic disease. Depending on the
degree of renal functional impairment kidneys frequently lose volume and almost always
show increased echogenicity but no renal scaring. Increased echogenicity is less notable in
IgA nephropathy, minimal change disease and membranous glomerulonephritis and
corticomedullary differentiation is better preserved than in proliferative and interstitial
glomerulonephritis. In acute glomerulonephritis kidneys are enlarged or of normal size, the
parenchyma is frequently of increased depth and echogenicity but normal or even decreased
echogenicity can also occur. A renal segmental artery RI value of 0.80 or greater is a bad
prognostic sign 9. About 3-5 years after initiation of dialysis treatment secondary cyst
formation frequently occurs. These cysts may undergo malign transformation. For this reason
yearly control ultrasonography is indicated


Figure 7: Condensed and small right kidney in preterminal renal failure due to chronic IgA
nephropathy


Hypertensive nephrosclerosis
Hypertensive nephrosclerosis is frequently associated with small and echo dense kidneys and
parenchyma of diminished thickness. The renal resistive index is frequently elevated above
0.80 and this is a bad prognostic sign.

Amyloidosis
Enlarged kidneys with thick parenchyma, greatly increased echogenicity, preserved
corticomedullary differentiation, medullary pyramids with low echogenicity and an
increased renal resistive index (> 0.7-0.8) are the unspecific hallmarks of renal amyloidosis.

Acute renal failure
In acute intrarenal failure kidneys are frequently enlarged, parenchymal echogenicity is
increased, medullary pyramids appear to have low echogenicity and renal RI is severely
increased. In acute prerenal failure renal echogenicity and renal RI is frequently normal.
Patients with severely increased echogenicity appear to have a worse prognosis.

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Hemolytic uremic syndrome
Due to endothelial damage occlusion of small intrarenal vessels is the hallmark of this
disease. This process is accompanied by an increased renal RI value (> 0.80). In addition to
the clinical presentation and laboratory parameters like elevated LDH, thrombocytopenia and
fragmentocytes the increased RI can be used as a diagnostic clue.

Hepatorenal syndrome
According to B-mode ultrasonography the kidneys appear completely normal. However, a
renal RI ≥0.70 in patients with liver cirrhosis and ascites is associated with a 20-30 fold
increased likelihood for the development of hepatorenal syndrome.


Tubulointerstitial diseases

Pyelonephritis
Rarely the renal pelvis is filled with echogenic material. As in all acute diseases the kidneys
are enlarged. Due to affection of pararenal tissues renal motility is frequently impaired.
Chronic pyelonephritis with impaired renal function is frequently associated with reduced
renal volume and renal scars. The scars traverse the parenchyma and pyelectasia is frequently
seen next to a scar. Renal echogenicity is increased in a patchy pattern and the
corticomedullary differentiation is lost. Renal parenchymal width is diminished locally or
generalized. Xantogranulomatous pyelonephritis is a variant, which in 2/3 of cases occurs due
to pelvic obstruction with infected renal stones (struvite stones = magnesium-ammonium-
phosphate). This variant results in a chronic, putrid and fatty inflammation with complete
destruction of the renal architecture. The destructed area may be confused with renal tumors
or cysts. Renal stones or calcifications can frequently be found in affected areas. Definitive
diagnosis relies on a CT-investigation

(Medullary) nephrocalcinosis

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