The renal collecting system forms part of the
echogenic central echo complex and is frequently not identifiable
as a separate structure. Slight collecting system dilatation is
a common normal finding during a diuresis or when the bladder
is quite full. In these cases the dilatation resolves when the
bladder is emptied. Hydronephrosis is simply a dilatation of the
renal collecting system. Hydronephrosis is seen as anechoic fluid
in the renal collecting system and pelvis separating the central
sinus echoes. Dilated calyces lose their sharp angular margins
and become blunted. When the hydronephrosis is marked, the entire
collecting system is outlined as a series of connecter fluid-filled
Calculi, or stones, can form anywhere within the urinary tract.
Most stones original within the kidney (nephrolithiasis).
The clinical presentation varies depending on the size or location
of the stone or whether the stone is being passed. Calculi located
in the kidney or proximal portion of the ureter may cause either
no pain or dull flank pain, whereas lower back pain radiating
down the pelvis may be caused by a stone in the distal ureter
or bladder. Severe, sharp pain (renal colic) is usually caused
by the passage of a stone down the urinary tract. Other clinical
symptoms may include nausea, vomiting, fever, chills, and, depending
on the presence of obstruction, oliguria. The laboratory findings
may consist of hematuria, white blood cells, and bacteria.
A staghorn calculus is a stone that fills the renal pelvis and
extends into the infundibulum and calyces, causing dilation of
the calyces. Stones can cause obstruction of the renal collecting
System, or they may pass into the ureter and obstruct it, causing
a hydroureter. The three most common sites of obstruction are
the ureteropelvic junction (UPJ), the point at which the ureter
crosses over the pelvic brim, or the location at which the ureter
enters into the urinary bladder. Stones may also pass into the
urinary bladder or, in rare cases, obstruct the urethra.
The ultrasound diagnosis of urinary calculi is based on the demonstration
of a highly echogenic focus that produces an acoustic shadow.
Relatively small stones can be detected, especially if there is
hydronephrosis. The surrounding fluid media "urine" helps in their
visualization. The presence of posterior acoustic shadowing varies
according to the size and composition of the stone. Very small
stones may not have posterior acoustic shadowing. Patients with
urinary tract calculi may have a color Doppler artifact. The artifact
is called a "twinkle artifact", which
is imaged as a rapidly changing color posterior to the stone with
a cornet tail (scan 7).
A calculus in the kidney may cause obstructive hydronephrosis.
Stones that pass into the ureter may obstruct it. Absence of a
unilateral ureteral jet may occur with complete ureteral obstruction.
A partial ureteral obstruction may cause absence of a ureteral
jet or decreased blood flow, which has a pattern that resembles
a burning candle (candle sign) (scan
8). Sonographic documentation of a ureteral stone is difficult
to obtain because of the small size of the calculi, posterior
location of the ureter, lack of fluid surrounding the stone, and
adjacent bowel gas.
Bladder stones appear as high intensity echoes within the bladder,
have an associated acoustic shadow. The color Doppler examination
is of minor importance in the evaluation of patients with stone
disease. The stone need not be calcified to be detected by ultrasound.
Unless embedded in the bladder wall, the calculi shift to the
dependent portion of the bladder with change in patient position.
The stones can be extremely difficult to identify when there is
no hydronephrosis. There are other normal structures in the renal
pelvis that may cause a focal area of bright echogenicity although
they will not cause distal sonic shadowing. This includes focal
areas of hyperechoic renal hilar fat and blood vessels. Ultrasound
evaluation for renal stones can be extremely helpful in patients
with an unexplained filling defect on an intravenous urogram.
The differential diagnosis of non-calcified stone, tumor, and
blood clot can be difficult radiographically. A non-calcified
stone, such as a uric acid stone, will still cause a bright echo
reflection and distal sonic shadowing. Blood clots and epithelial
tumors, on the other hand, will be less hyperechoic and will not
cause distal sonic shadowing.