The major advantage of plain radiography of the kidney, ureters and bladder (KUB) is assessing whether the stone is radio-opaque or not. While most calcium containing stones (calcium oxalate monohydrate, calcium oxalate dihydrate, and calcium phosphates) are easily visible, struvite, apatite and cystine stones are semi-opaque and faintly visible on KUB. The radiolucent stones (uric acid, ammonium urate, and xanthine) are not seen on KUB unless they are mixed composition. Therefore, in some clinics KUB is routinely used in the evaluation of patients with renal colic. While it is valuable in the follow-up of patients with opaque residual fragments or in the reassessment of the location of opaque ureteral calculi, it fails to detect non-opaque calculi. Because of its low sensitivity (45–58 %) and specificity (69–77 %) rates, KUB is not accepted as a proper imaging method for the initial diagnosis of urolithiasis [1, 2]. Another study presented that KUB could demonstrate stones in only 59 % of patients who had a diagnosis of urolithiasis with computed tomography (CT) [1]. Based on this proof, Kennish et al. argue that KUB should be omitted as a primary imaging method for ureteric colic if CT is available [2]. They recommend KUB for additional imaging in patients with urolithiasis diagnosed with CT. In another study Lamb et al. demonstrated that treatment was altered significantly in 17 % of patients with KUB in addition to CT due to the detection of stone radiolucency or change in stone size and position [3]. Besides the evaluation of stone radio-opacity, the baseline KUB helps to image the bony structures, phleboliths and calcified masses that can mimic a stone so that it can serve as a baseline image that can be compared with future images.

The CT “scout” film (screening digital radiograph) is obtained during axial CT scan simulating KUB. Studies evaluating both imaging methods show that the stone visibility rate on the CT scout (17–49 %) is significantly lower than the rate of KUB (40–63 %, p < 0.001) [4, 5]. Reduced contrast between stone and soft tissue resulting from lower spatial resolution and the high kilovoltage settings of CT scout leads to a decrease in sensitivity and specificity of CT scout in the detection of ureteral calculi [5]. Although KUB is recommended only if the stone is not visualized on CT scout, Foel and colleagues presented that definitive assessment rates increase with KUB and CT [6, 7]. Therefore, they recommend baseline KUB for patients with ureteral stones diagnosed with CT.

Plain X-ray (KUB) is a proper imaging method for follow-up for opaque ureteral stones having the advantages of being lower cost and less radiation exposure (0.5–1 millisievert (mSv) dose of radiation exposure) when compared to computed tomography [7].

Ultrasonography (US) is a common imaging method used by urologists because it is cost-effective, radiation free, non-invasive and repeatable [7]. The sensitivity of US in detecting urolithiasis varies in the range of 19–96 % [8]. While the sensitivity and specificity are higher for stones located in the renal pelvis, renal calices, the ureteropelvic junction, and the ureterovesical junction, detection of ureteral calculi is more difficult and requires expertise. The deep location of the mid ureter and presence of intestinal gas may hinder the visualization of the ureter and calculi. However, the stone might be detected by re-examination with US due to the migration of the stone to the distal portion of the ureter which is more visible. Therefore, US can be used for follow-up in patients with ureteral calculi.

In a study comparing the diagnostic accuracy of CT, intravenous urography (IVU), and US for ureteral calculi, the sensitivity was found to be 94, 52, and 19 %, respectively [9]. US may also detect secondary signs (hydronephrosis, ureteral dilation, lack of ureteral jet flow) that may be related to obstruction or stricture. To increase the accuracy of US, a combination of KUB and Doppler US is described in the literature. It is shown that KUB or Doppler US, in addition to US, increases sensitivity (79–97 %) and specificity (91–100 %) in the detection of ureteral calculi [10]. For radiopaque stone formers, combining US and KUB is regarded as an optimal imaging modality [9].

The twinkling sign, a sonographic artifact behind the stone detected using Doppler US, increases the detection rate of ureteral calculi [11]. Similarly, native tissue harmonic imaging (NTHI), a new sonographic technique, provides higher image quality and precise measurement of stone size. It gives more details even in obese patients [12]. Mitterberger et al. presented that KUB with NTHI-US has a comparable specificity and sensitivity rate with CT in the detection of urolithiasis [12].

Lack of radiation exposure and contrast administration, widespread availability and low cost are the main advantages of US. However, the requirement of expertise and low sensitivity for mid ureter, especially in obese patients, are regarded as limitations. Because of this, US is used as the primary imaging method in children and pregnant patients [7, 8]. Additionally, it is recommended as the first step imaging modality for the follow-up of patients treated with SWL or URS or in symptomatic patients who passed the stone to assess the relief of the obstruction [8].

Regarded as the traditional imaging method used for the diagnosis of acute flank pain, intravenous urography provides information about renal function, the grade of hydro-ureteronephrosis, stone location, anatomical abnormalities of the pelvicaliceal system and ureter, and tumors in the collecting system and ureter. On the other hand, non-visualization of radiolucent stones with X-ray and difficulty differentiating the filling defect sign from other etiologies (e.g. urothelial tumors) are regarded as its major limitations. The requirement of a contrast medium injection, proper bowel preparation and adequate hydration are other concerns about the method. The administration of contrast may lead to contrast nephrotoxicity, a 5–10 % risk of allergic reaction, and the most severe complication of an anaphylactic reaction, with the risk of 1/100,000 [8]. The IVU procedure may be more time consuming due to the requirement of delayed films in patients with delayed urinary extraction related to obstruction. Chen et al. presented that room time for IVU was almost three times higher than the time for CT scan [13].

With the introduction of CT for the assessment of flank pain, the utility of IVU is significantly decreased. The studies prospectively comparing CT and IVU in patients with flank pain presented that the sensitivity and specificity of IVU (51–87 %, 92–100 %, respectively) are significantly lower than those of CT (94–100 %, 92–100 %, respectively) (p = 0.015) [7, 14]. With the widespread availability of CT in emergency rooms, it has replaced IVU for the radiological evaluation of patients with acute flank pain.

Contrast studies can be helpful in the assessment of obstruction and renal function after treatment of a ureteral stone. Although the ureteral stricture rate after URS (1–4 %) and SWL (0–2 %) is reported to be very low, contrast studies (IVU, diuretic renography or contrast CT) can be used in the assessment of patients who have symptoms or persistent hydronephrosis after definitive treatment [8].

The usage of unenhanced helical CT for the diagnosis of acute flank pain was initially reported by Smith et al. in 1995 [15]. In recent years with its widespread usage, computed tomography, is regarded as the gold standard imaging method for the initial diagnosis of acute ureteric colic because it has the highest sensitivity (94–100 %) and specificity (92–100 %) rates among imaging modalities [7, 8]. The ability to diagnose non-urologic conditions mimicking renal colic, such as appendicitis, diverticulitis, pancreatitis, cholelithiasis, and lumbar discitis, its short examination time, and its high sensitivity and specificity rates make it the most useful and common imaging method utilized in emergency rooms.

The determination of precise stone size is essential in decision making for the proper treatment of ureteral calculi. While ureteral stones <5 mm have the spontaneous passage rate of 77 %, only 39 % of the ureteral calculi >7 mm pass spontaneously [16]. While KUB may lead to overestimation of stone size by 20 %, measurement with CT is closer to the actual stone size. However, Kishore et al. could not show significant correlation between actual stone size and CT measurement for distal ureteral stones [17].

Computed tomography reveals other stone related parameters [number of calculi, stone location, stone volume, stone attenuation measured in Hounsfield units (HU), and skin-to-stone distance (SSD)] that affect the selection of the proper treatment regime, in order to achieve the best results. Hounsfield unit attenuation and SSD are important factors affecting the success of SWL. Pareek et al. presented that SSD measured on CT is a powerful predictor of the success of SWL for renal calculi [18]. In this study, SSD of >10 cm was found to be the most sensitive and specific point on the curve, illustrating the relationship of the success of SWL with SSD. Stone volume measured using a three-dimensional (3D) reconstruction of the preoperative CT is also a predictor of success for SWL in the treatment of upper urinary tract calculi.