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.

Renal stone. Spiral (unenhanced) CT scan demonstrating a right renal stone (yellow arrow).

A 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.

Advantages of Spiral CT

1. The rapid speed, accuracy and ability to image the abdomen in a single breathhold.
2. Thin section images can be obtained to identify small stones within the urinary tract.
3. Multiplanar reconstructions and three-dimensional reconstruction of the urinary tract can be obtained.
4. It is more rapidly performed than IVP.
5. The risks of contrast reaction are eliminated by spiral CT.
6. Both radiolucent and radio-opaque stones can be identified on CT.
7. The radiation dose is equivalent or less than IVP.
8. It has the potential to aid in the diagnosis of extraurinary causes of acute flank pain.
9. 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.
10. 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.
11. Allows direct visualization of stones both on axial images and on the scout CT image in a similar fashion to plain radiography.
12. By utilizing computerized mapping techniques, uric acid stones with relatively low attenuation values can be differentiated from struvite and calcium oxalate calculus stones.
13. Can also image adjacent organs.

Disadvantages for CT scans

1. Unable to identify very rare pure matrix stones of mucoprotein and fibrin.
2. Unable to identify Crixivan stones (see section on Crixivan stones).
3. Does not provide direct physiologic information of the degree of obstruction in patients with kidney stone.
4. CT does not visualize the collection system to evaluate hematuria.
5. Spiral CT is time consuming compared to the standard incremental CT in post-processing time required by radiologists or technicians to review images.

Secondary signs
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.

Secondary signs of obstruction include:

Sensitivity 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.

Stranding 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
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.

Unenhanced 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.

Unenhanced CT scan of the pelvis. Calcifications are consistent with ureteral stone (red arrowhead) and phleboliths (yellow arrowheads).

Alternate pathology
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.

Costs 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 administered.

Non-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.

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