Given its lack of ionizing radiation, ultrasound (US) is often the initial modality of choice in evaluating a pediatric patient with suspected urolithiasis and nonemergent abdominal or flank pain [20, 21]. It is also lower in cost than CT, is noninvasive, does not require sedation, and can be performed portably, allowing bedside evaluation of sick patients.

The greatest strength of US in the setting of stone disease is in evaluating for obstruction, or hydronephrosis, easily depicted as fluid distension of the renal pelvis and calices [15] (Fig. 13.4). It can also provide extensive anatomical detail in patients with congenital duplications and other anomalies contributing to stone formation. US can also directly demonstrate stones. Typically, a renal stone appears as a very bright, or echogenic, rounded structure. Due to its high density, the stone will block penetration of the US beam, resulting in a black shadow behind it. This is referred to as posterior acoustic shadowing [2, 16] (Figs. 13.5 and 13.6). The exceptions are drug stones or matrix/protein stones which are of variable density and may not be distinguishable from surrounding soft tissue on ultrasound [16].

Stones are most easily identified in the kidney and in the distended urinary bladder (Fig. 13.7). In general, US is poor at localizing ureteral stones, especially in the absence of ureteral dilatation. Palmer et al. found that the detection rate for ureteral calculi was only 38 % as compared to 90 % for renal stones [22]. Hydronephrosis may suggest a ureteral stone, but even in the presence of hydroureter, it is often difficult to see the ureteral stone if there is overlying bowel gas or surrounding soft tissue which obscures the ureter [2–15] (Figs. 13.8 and 13.9). If the dilated ureter at the level of obstruction can be seen, then the stone may be visible (Fig. 13.10).

A special feature of US is the artifact created by color Doppler imaging of a rough reflective surface, known as “twinkling.” Twinkling refers to the multicolor signal behind the stone simulating turbulence (Fig. 13.11). This can be used as supportive evidence when diagnosing stones by US. One study found that twinkling artifact had a 78 % positive predictive value for nephrolithiasis at subsequent CT. The true-positive rate of twinkling artifact for confirmed calculi at CT, however, was only 49 % with a 51 % false-positive rate [23]. This study used 5 mm CT sections to correlate with twinkle artifact, however, which may have in fact missed smaller stones that were accurately identified by twinkle artifact.

Many studies have found US to lack the sensitivity and accuracy of CT in both identifying and characterizing stones. One study by Passerotti et al. found that US had an overall sensitivity of 76 % and a specificity of 100 % in comparison to CT for urolithiasis in the kidney, ureter, or bladder [24]. Ray et al. performed a literature review and only found a 45 % pooled sensitivity for US [15]. US has been found to both miss stones averaging 2.3 mm [24] and to overestimate stones <5 mm in size by 2 mm [15]. Palmer et al. compared US and CT in symptomatic patients and found US nondiagnostic in 41 % of cases versus 5 % of cases with CT [22]. As a result, a negative US in a symptomatic patient may still require further work-up with CT imaging. Confirmatory CT is also sometimes advocated after an equivocal US and even after a positive US to evaluate stone location and burden for appropriate planning of an intervention [22]. Given the likelihood of requiring subsequent CT, some would advocate beginning with a low-dose CT to decrease the overall number of imaging exams and the burden of cost. For example, Palmer et al. advocated beginning with a CT when evaluating children >11 years old with a positive family history of stone disease [22]. This issue remains controversial, however, and the aforementioned study by Passerotti concluded that although CT is more sensitive than US for the detection of renal stones, the difference between these exams may not be clinically significant; these authors therefore recommend US as the initial imaging exam for children with suspected stones due to the potential adverse effects of ionizing radiation from CT.

As previously mentioned, the sensitivity of US can be increased if used in conjunction with abdominal radiography. Mitterberger et al. found that the combined use of radiography and native tissue harmonic imaging ultrasonography had a sensitivity and accuracy for detecting ureteric calculi of 96 % and 95 %, respectively [25]. Both modalities may be effective in following a patient with known stone disease.

While readily accessible, US is operator dependent and potentially limited by a patient’s body habitus. Skeletal abnormalities such as extreme kyphosis/scoliosis may be prohibitive in visualizing the retroperitoneum with US. These factors must also be considered when choosing between US and CT.

Noncontrast computed tomography (CT) has surpassed many other imaging modalities in diagnosing and confirming stone disease; it is overall considered the gold standard imaging technique for urolithiasis. CT provides accurate information regarding the location and size of the stone and relevant surrounding anatomy [15] (Figs. 13.12, 13.13, 13.14, and 13.15). It can also be used to measure stone density to provide preliminary information regarding stone composition which may help direct therapy [18]. CT imaging has a high sensitivity and specificity independent of stone location (96–99 % sensitivity and 96–97 % specificity) [26, 27]. Using thin slice collimation of <3 mm, the sensitivity and specificity are virtually 100 % [15]. CT is typically used in the emergency setting due to its speed and high sensitivity/specificity as well as its ability to rule out other pathology. CT for the evaluation of renal stones does not require IV contrast, and in fact, contrast enhancement of the renal parenchyma and excretion of contrast into the collecting system may obscure small stones. Stones which are radiolucent or too small to be seen on abdominal radiographs can generally be seen on CT.

Secondary signs of ureteral stones can also be seen on CT, such as hydronephrosis, proximal ureteral dilatation, unilateral renal enlargement, decreased renal attenuation due to edema, and perinephric or periureteral edema (Figs. 13.16, 13.17, 13.18, and 13.19). In the event that no stone is seen on the CT scan, these secondary signs can suggest recent passage of a stone. A method to distinguish a ureteral stone from a phlebolith is to look for the “rim sign” – circumferential soft tissue density representing the ureter encircling the stone, which is not found with a phlebolith (Fig. 13.20). If the obstructing stone is associated with infection, pyelonephritis and ureteritis can also be demonstrated using contrast-enhanced CT (Fig. 13.21).

Despite all the advantages of CT imaging, a major concern is its use of ionizing radiation, particularly as it pertains to the pediatric patient. CT utilizes significantly more ionizing radiation than the other imaging modalities: the effective dose of an abdominal/pelvic CT is 4.3–8.5 mSv as compared to an abdominal radiograph which is 0.2–0.7 mSv and MRI or US which is 0 mSv [18]. To put it further in perspective, the effective dose of a single noncontrast CT (stone protocol) for a child is about 1–2 years worth of background radiation (3 mSv) [12]. As a result, it is not necessarily the automatic first choice of imaging in the pediatric population and should be thoughtfully used in these patients who may require multiple imaging procedures in the future.

As a result, there has been keen interest in low-dose CT imaging techniques to try to reduce the amount of radiation to which the patient is exposed without affecting its diagnostic capability. Newer CT protocols have been shown to decrease the radiation-measured entrance dose by 60–90 % [28], with published effective doses for a low-dose CT of 0.98–1.5 mSv as compared to 4.3–8.5 mSv for a standard abdominal/pelvic CT [18]. Techniques such as tube current modulation, low voltage protocols, and other adjustments to alter image acquisition and reconstruction can significantly reduce patient radiation exposure [18, 29]. In a model looking at effective dose and lifetime risk of cancer, low-dose imaging (40 mA, 10 ms, 125 kVp) as compared to standard dose (80 mA, 25 ms, 125 kVp) decreased the lifetime attributable risk of cancer from 23–144 cases per 100,000 to 5–31 cases per 100,000 [30]. Diagnostic accuracy has been shown to be maintained with low-dose protocols. Karmazyn et al. found that reducing the dose of radiation did not significantly reduce the detection of ureteral or kidney stones in children weighing <50 kg [31].

It is important to remember that even with lower-dose imaging techniques, there is no threshold under which radiation is safe. A study from the Netherlands found a linear-no-threshold model for the induction of chromosomal damage induced by ionizing radiation, seeing an induction of micronuclei in human fibroblasts in doses as low as 20 mGy. The damage was increased if cells were exposed during the S-phase of the cell cycle [32]. The only way to most effectively decrease the amount of radiation to which a patient is exposed is to choose nonionizing studies when possible (i.e., US). If CT has to be used, use it sparingly and limit the scan to only the area of interest [13].

Magnetic resonance imaging (MRI) is a very good modality for imaging the anatomy and function of the genitourinary tract using MR urography protocols and has the added benefit of lacking ionizing radiation. Its use in patients with urolithiasis, however, is limited [33]. MRI is unable to directly visualize stones; stones present instead as signal voids. It is also expensive, requires sedation for most young children, and is limited in its availability, especially in the emergent setting and at night [18].