Diagnosis of Stones
Unenhanced Computed Tomography [Helical (Spiral) CT Scan]
Computed Tomography (CT) Scan has assumed a greater and increasing
importance with regard to urolithiasis. CT Scan has the ability
to detect radiolucent calculi such as uric acid stones. Unlike ultrasounds,
CT Scans can image the entire ureter and differentiate among the
various causes of ureteral obstruction. Therefore, both radial opaque
and non-opaque stones in the ureter can easily be demonstrated and
to identify the cause for ureteral obstruction by CT Scan. CT Scan
can detect stones as small as 3 mm.
stone. Spiral (unenhanced) CT scan demonstrating a right renal
stone (yellow arrow).
spiral or helical CT has gained widespread acceptance at numerous
institutions as becoming the imaging modality of choice for studying
patients with suspected urinary calculi.
of Spiral CT
The rapid speed, accuracy and ability to image the abdomen in
a single breathhold.
Thin section images can be obtained to identify small stones within
the urinary tract.
Multiplanar reconstructions and three-dimensional reconstruction
of the urinary tract can be obtained.
It is more rapidly performed than IVP.
The risks of contrast reaction are eliminated by spiral CT.
Both radiolucent and radio-opaque stones can be identified on
The radiation dose is equivalent or less than IVP.
It has the potential to aid in the diagnosis of extraurinary causes
of acute flank pain.
The imaging of the entire genitourinary tract system (including
the opposite kidney) can ideally be performed in less than 5 minutes
while an IVP requires an average of 80 minutes.
CT does not necessitate the checking of a separate blood test
for BUN/Creatinine level. This avoids further delays while awaiting
the completion of lab tests, allowing for more rapid triage of
emergency room patients.
Allows direct visualization of stones both on axial images and
on the scout CT image in a similar fashion to plain radiography.
By utilizing computerized mapping techniques, uric acid stones
with relatively low attenuation values can be differentiated from
struvite and calcium oxalate calculus stones.
Can also image adjacent organs.
for CT scans
Unable to identify very rare pure matrix stones of mucoprotein
Unable to identify Crixivan stones (see section on Crixivan stones).
Does not provide direct physiologic information of the degree
of obstruction in patients with kidney stone.
CT does not visualize the collection system to evaluate hematuria.
Spiral CT is time consuming compared to the standard incremental
CT in post-processing time required by radiologists or technicians
to review images.
In some patients, urinary tract stones may not be readily apparent
on CT scan due to volume averaging, small stone size, low stone
attenuation, sparseness of retroperitoneal fat or recent passage
of stone. Secondary signs of obstruction including hydronephrosis,
hydroureter, renal swelling, periureter edema, perinephric edema
and edema at the ureteral vesicle junction especially when seen
in combination are strong evidence that an acute ongoing obstructive
process was present.
signs of obstruction include:
and specificity of each associated secondary sign are described
above. The combination of unilateral ureteral dilatation and unilateral
perinephric streaking have a positive predictive value of 99% for
the presence of a stone and a negative predictive value of 95% for
the exclusion of ureteral stones. The absence of a hydroureter and
hydronephrosis on Spiral CT images should prompt a search for a
diagnosis other than an obstructing ureteral stone.
of perinephric fat is defined as linear areas of soft tissue attenuation
in the perinephric space, which can result from any acute process
or injury to the kidney. When unilateral perinephric stranding is
identified it is most frequently the result of calyceal rupture
or perinephric inflammation. Perinephric stranding is probably the
manifestation of increased pressure in the collecting system in
the early phase of ureteral obstruction. Perinephric stranding is
easily identified on Spiral CT and less commonly seen on IVP.
Phleboliths, defined as focal calcified venous thrombi, are frequently
seen along the normal anatomical course of the lower ureter. They
are usually the result of injury to the vein wall commonly from
venous hypertension and are composed of concentric calcified strata
around a central kernel. Typically, phleboliths are rounded with
a central lucency and are seen in the true pelvis often below the
distal ureter. A limitation of a non-contrast CT is in the evaluation
of stone disease when differentiating a pelvic phlebolith from a
stone within the ureter, especially in patients with a paucity of
retroperitoneal and pelvic fat. Circumferential periureteral edema,
or the soft tissue "rim" sign, described as a rim of soft tissue
attenuation seen around the circumference of an intraureteral calculus
on non-contract CT, can also help differentiate ureteral calculi
from phleboliths. Theoretically, phleboliths will not show a "rim"
sign. Since larger stones result in stretching of the ureteral wall,
the "rim" sign tends to be more commonly associated with the presence
of smaller stones.
helical CT findings that have the most significant predictive value
in diagnosing phleboliths include the presence of a central lucency,
a bifid peak at the profile analysis and a "comet" sign. The "comet"
sign refers to the adjacent eccentric, tapering soft-tissue mass
corresponding to the non-calcified portion of pelvic vein contiguous
to a phlebolith.
CT scan of the pelvis. Calcifications are consistent with ureteral
stone (red arrowhead) and phleboliths (yellow arrowheads).
CT scanning has the distinct ability to accurately detect and diagnose
a variety of disorders other than urinary tract stones that can
present with acute flank pain. Patients presenting with renal colic
might in fact be suffering from numerous alternative GU disorders
including pyelonephritis, renal cell carcinoma, perinephric hematoma,
transitional cell carcinoma, duplicated collecting system, bladder
outlet obstruction, emphysematous cystitis or UPJ obstruction. The
broad spectrum of pathologies outside the GU tract which can mimic
renal colic but are accurately established with non-contrast CT
is board including adnexal masses, appendicitis, diverticulitis,
Crohn's disease, pancreatitis, cholecystitis, lymphoma, ruptured
spleen, renal artery aneurysm, vertebral masses and adrenal masses.
are often stated as a reason to avoid non-contrast CT. However,
some institutions have adopted a billing change where non-contrast
CT is charged as a "limited" CT since there is no oral contrast
CT is a safe and rapidly performed procedure in the evaluation of
patients with suspected renal stones. With the increasing use of
spiral CT in the evaluation of patients with renal calculi non-contract
CT may one day become the procedure of choice in evaluating patients
with suspected renal stones.