It always is difficult to differentiate stones from periureteric calcifications and phleboliths, especially when these are located in the pelvis in the course of the ureter. Phleboliths are calcified concretions within a vein wall that result from thrombosis. The presence of secondary signs of obstruction such as hydronephrosis, ureteric dilatation, nephromegaly, and periureteric and perinephric stranding suggest calculus disease. The most common certain finding is the rim sign, a rim of soft tissue 1 to 2 mm in thickness around the stone secondary to edema of the ureteric wall. The rim sign is reported to have a sensitivity of 50% to 70% and specificity of 92% to 100%.28,29 The absence of secondary signs should not exclude a diagnosis of stones, however. These signs may be absent in patients who have nonobstructing calculi, or they may be seen after recent passage of a stone. A few other features may help in differentiation; phleboliths, for example, are always smooth and round, are not associated with surrounding soft tissue changes, are static in position, and may have central lucency. In 1999, Boridy and colleagues30 described the tail sign seen in phleboliths. The tail of soft tissue is thought to represent the vein, venule, or venous plexus in which the phlebolith is located.
Radiologists should be mindful of the radiation dose used, especially in young patients. With the increasing use of unenhanced helical CT, many patients undergo multiple repeat scans within a short period of time. Dose-lowering strategies, such as decreased milliamperage or increased pitch, can be implemented.31
MR imaging is a noninvasive modality used to assess parenchymal and vascular renal disease. MR urography (MRU) is a more recent tool for evaluating the urinary tract. A stone is diagnosed when a calcific density is seen in the urinary tract, because MR imaging is not able to detect the calcifications directly. T2-weighted MR images demonstrate the stone as a filling defect in the background of a high signal from a urine-filled collecting system. The filling defect, however, can be caused by blood clot or tumor. A stone can be differentiated by the lack of contrast enhancement and by smooth, well-defined margins. It is important to keep in mind the possibility of surrounding soft tissue edema associated with the recent passage of calculus.32
MRU demonstrates the level of obstruction with an accuracy approaching 100%. When T2-weighted (static) MRU is combined with gadolinium-enhanced 3D gradient echo (excretory MRU), the sensitivity for detecting ureteral stones may exceed 90%.33,34
MRU has a more critical role in pediatric and pregnant patients, where there is a greater need to avoid ionizing radiation. The contraindications of MRU are the same as those for general MR imaging, including uncooperative patients and those who have a pacemaker or metallic prosthesis.
Pregnant women who present with acute flank pain are evaluated for nephrolithiasis. Despite renal tract dilatation, urinary stasis, and a degree of obstruction and hypercalciuria, symptomatic renal stone disease is no more common during pregnancy than in the nongravid state, because inhibitors of stone formation, such as magnesium, citrate, and nephrocalcin (an acidic glycoprotein) are excreted in greater concentrations during pregnancy.35