Diagnostic ultrasound ( PDFDrive )

CHAPTER 52 The Pediatric Urinary Tract and Adrenal Glands 1801
prevalence of stones occurs in the southeastern United States;
the geographic distribution of stone disease in children has not
been reported. 92 Approximately 70% of children with urolithiasis
have an underlying predisposing condition such as urinary stasis,
hypercalciuria, or chronic infection. Approximately 40% to
60% of stones are predominantly composed of calcium oxalate,
10% to 20% are mainly calcium phosphate, 10% to 25% are
mixed stones containing both calcium oxalate and calcium
phosphate, 17% to 30% are magnesium ammonium phosphate
(struvite or infection related), 6% to 10% are cystine, and 2% to
10% are uric acid. 93 Ultrasound is the initial imaging study of
choice, with noncontrast CT reserved for cases in which ultrasound
is nondiagnostic. 94 Urinary tract stones appear as foci of
increased echogenicity with or without associated acoustic
shadowing. In general, acoustic shadowing is present when the
stone is 5 mm or more in diameter. 95 he color comet tail artifact
“twinkle sign” is a rapidly alternating color Doppler signal that
occurs immediately deep to the object that causes it, and appears
as a linear aliased band of color 96 (see Fig. 52.37C). his artifact
can be useful in the diagnosis of small stones in the absence of
hydronephrosis or hydroureter. Color Doppler evaluation of the
ureteral jet may be helpful in diagnosing obstruction because it
is absent or of diminished velocity in the setting of high-grade
obstruction. 17
RENAL TRAUMA
Ater blunt abdominal trauma, the kidney is the most commonly
injured organ in children. he pediatric kidney is believed to
be at increased risk of injury compared with the adult kidney
because of its larger size relative to the abdomen, and less protection
from unossiied ribs, weaker abdominal muscles, and
minimal perinephric fat. 97 Renal trauma in children is oten
associated with other organ injury, particularly of the liver
and spleen.
Most children with clinically signiicant kidney trauma have
hematuria. Asymptomatic, microscopic hematuria is a low-yield
sign for the presence of renal injury, whereas patients with gross
hematuria have a much higher incidence of renal injury (22%)
than those who do not (8%). 98 Preexisting and oten clinically
silent renal abnormalities such as hydronephrosis or ectopic
kidney may make the kidney more susceptible to injury by minor
trauma.
CT is the primary imaging modality for suspected blunt
abdominal trauma in the pediatric patient. 99,100 Ultrasound is
used primarily in the follow-up of injuries found on CT. 101
Parenchymal contusion is the most common kidney injury. It is
characterized by microscopic hemorrhage and edema and is best
depicted by CT, although ultrasound may show distortion of the
normal renal architecture. Perirenal hematoma may occur as a
complication of renal injury, and is subcapsular or perinephric
in location. Renal hematoma can vary in appearance, but is usually
echogenic at irst and becomes hypoechoic as it liqueies. Injury
to the renal collecting system leads to extravasation of urine. A
urinoma results when the extravasated urine is limited to the
perirenal space, which can be diicult to distinguish from
hypoechoic blood by ultrasound. When the hematoma results
from injury to the vascular pedicle, there will also be absent low
in the renal vessels and/or within the renal parenchyma.
he introduction of ultrasound contrast agents into clinical
practice has permitted their use in the assessment of patients
with mild to moderate blunt abdominal trauma, particularly
when there is clinical concern but conventional ultrasound reveals
no solid organ injury (Fig. 52.41, Video 52.6). Menichini and
colleagues 102 and Valentino and colleagues 103 have shown that
CEUS is nearly as accurate as contrast-enhanced CT in depicting
solid organ injuries in children. One important limitation of the
technique, however, is that the ultrasound contrast agent is not
iltered by the kidney, so a urinoma cannot be readily distinguished
from a hematoma.
RENAL VASCULAR DISEASE
Doppler Sonographic
Examination Technique
Infants and small children are examined without special preparation,
but they may be given clear liquids to drink during the
examination to calm them, to increase hydration, and to provide
an acoustic window through the luid-illed stomach. Sedation
is rarely necessary. he older child is best examined ater a 4- to
6-hour fast (to reduce intestinal gas) only if a detailed examination
of the main renal artery is planned. Color and spectral Doppler
examinations are performed. Power Doppler may also be useful
to evaluate the presence of vascular low. Doppler settings should
be adjusted for maximal detection of slow low using the highest
possible transducer frequency, relatively small color area of
interest, low pulse repetition frequency, and low wall ilter. Pulse
repetition frequency is augmented if aliasing occurs. A small
sample volume and an insonating angle of 60 degrees or less are
needed. 104
he aorta is examined by means of an anterior let paramedian
as well as a let axillary approach, with longitudinal and transverse
views. he main renal arteries are oten best imaged through
the lanks, although an anterior midline approach can also be
used. Color Doppler is used to trace the renal arteries, which
are then examined with serial pulsed Doppler samples, especially
in areas of high-velocity low. Even if the entire renal artery
cannot be outlined because of overlying intestinal gas, the retrocaval
portion of the right renal artery and the hilar arteries
can usually be analyzed. A segmental or interlobar artery in each
third of the kidney (upper, middle, and lower) is then studied
with spectral Doppler, and the RI or pulsatility index is
calculated.
Normal Vascular Anatomy and
Flow Patterns
he intrarenal arteries and veins and their relationship to the
renal cortex, pyramids, and calyces are exceptionally well outlined
with color Doppler ultrasound. 105 he main renal artery or arteries
divide in the renal hilum to form several pairs (anterior and
posterior) of segmental arteries. hese course toward the pyramids
and divide into the interlobar branches, which follow the periphery