More recently, newer CT scanners and newer scanning protocols have allowed for “low dose” NCCT. There is no strict definition for what comprises a “low dose” CT (LDCT), however in general it is accepted that a CT dose <3 mSv constitutes a “low dose” scan [7]. There have been a number of reports evaluating LDCT [11–14]. In a prospective comparative trial, the sensitivity and specificity for detecting ureteral calculi were compared between a standard NCCT (7.30 mSv in males and 10.00 mSv in females) with a LDCT (1.40 mSv in males and 1.97 in females) [11]. The two studies had comparable sensitivities and specificities, with the exception of stones <2 mm. In these cases, the sensitivity of LDCT decreased to 68–79 % compared to 95 % for standard NCCT. In another prospective trial, LDCT (1.6 mSv in males and 2.1 mSv in females) showed similar sensitivity and specificity for detecting ureteral or renal stones compared to standard NCCT (9.6 mSv in males and 12.6 in females) [12]. The authors did find that the LDCT had decreased sensitivity in patients with a body mass index (BMI) >30 kg/m². In patients with a BMI >30 kg/m², the sensitivity for detecting a ureteral calculus was only 50 % with a specificity of 89 %. A third comparative study evaluating the sensitivity and specificity between standard NCCT and LDCT also found equivalent sensitivities between the two imaging modalities for the detection of ureteral calculi [14]. A recent meta-analysis of studies evaluating LDCT for the detection of urolithiasis found pooled sensitivity of 97 % and a pooled specificity of 95 % [7].

Although LDCT has been shown to be highly effective at diagnosing ureteral calculi, it does have limitations. As mentioned above, the sensitivity of LDCT decreases in obese patients [12]. One way to overcome this decrease in image quality in obese patients is to use automatic tube current modulation (ATM) which adjusts the gantry rotation time to optimize the signal-to-noise ratio. When comparing radiation dose for LDCT using ATM in non-obese and obese patients, obese patients were found to have a threefold increase in radiation dose (3.04 mSv vs 10.22 mSv, p < 0.0001) [15]. Given these limitations, the AUA recommendations for imaging ureteral calculi recommend standard dose NCCT in obese patients with suspected ureteral stones [16]. A recent study also evaluated the ability of LDCT to detect uric acid stones in the ureter [17]. This study performed in human cadavers demonstrated decreased sensitivity of LDCT for the detection of uric acid stones, especially for stones ≤3 mm.

Low dose CT is an improvement in regards to radiation exposure over standard NCCT, however it is still more radiation than a KUB. A recent study evaluated the effectiveness of an ultra-low-dose NCCT for the detection of ureteral calculi [18]. The effective dose for the ultra-low-dose NCCT was 0.91 mSv which compares favorably to what has been reported for KUB (0.63 mSv) [8]. When compared to LDCT, ultra-low-dose NCCT was found to have similar sensitivity for stones ≥4 mm [18]. Even lower CT doses have been evaluated. In a study evaluating NCCT with doses of 0.5 mSv in males and 0.7 mSv in females, the sensitivity and specificity was found to be 97 and 95 %, respectively [19].

Plain abdominal radiographs (KUB) are still used in clinical practice for the evaluation of ureteral stones. For radio-opaques stones, they can be used to follow patients who are on a trial of passage. The dose of a KUB can vary based on the machine and settings used. The reported doses have generally been low, between 0.5 mSv and 0.63 mSv [8, 20]. The use of tomograms increase the amount of radiation patients are exposed to. For a series of KUB and tomograms, including one scout KUB image and three tomographic sweeps, the dose has been reported to be 3.93 mSv, more than a LDCT [8]. Intravenous urogram (IVU) is still used to evaluate for obstruction in the setting of ureteral stone and to delineate collecting system anatomy. The dose of radiation from an IVU is primarily dependent on the number of images taken and the technique. Reported doses for IVU have ranged from 0.7 to 3.93 mSv [8, 21].

Digital tomosynthesis (DT) is a novel technology which recreates a number of coronal images from a single tomographic sweep. The study involves a scout plain KUB and a single sweep of the emitter. Digital software then reconstructs a number of slice images at different depths. The advantage of this technology is on each slice, overlying structures are removed to provide clearer imaging. The radiation dose for DT has been measured in a phantom model to be 0.87 mSv which is slightly more than a KUB but significantly less than traditional KUB with tomograms [20, 22].

Renal scans can be useful to evaluate for obstruction and determine differential function of patients with urolithiasis. The radiation exposure to the patient varies with the isotope used. The exposure from a renal scan with MAG3, DMSA and DTPA is 2.6 mSv, 3.3 mSv and 1.8 mSV, respectively [21].

Renal ultrasound is commonly used to evaluate patients with known or suspected ureteral stones. There is no radiation exposure from a renal ultrasound (US) which makes it the ideal first line study for pediatric and pregnant patients. Magnetic resonance imaging (MRI) is another imaging technique that does not expose patients to radiation.

Patients with ureteral stones are at risk for significant radiation exposure. The key to reducing this exposure is appropriate selection of imaging technique and use of radiation free imaging, such as MRI or US, as often as possible. The use of LDCT in lieu of standard NCCT also reduces the amount of radiation patients with urolithiasis are exposed to. The AUA clinical effectiveness review recommends LDCT as first line imaging for patients with renal colic and suspicion for a renal stone with the exception of obese patients, in whom they recommend standard NCCT [22]. It also recommends a KUB at the time of LDCT and if the stone is radio-opaque, the recommendation is for US/KUB for follow up during observation. In general, the panel recommends US and KUB, depending on whether the stone is radio-opaque, for follow up after surgical treatment with shock wave lithotripsy (SWL) or ureteroscopy (URS). By following patients with US and KUB, the radiation dose is reduced nearly 80 % compared to if the patient had been followed with LDCT (0.6 mSv for US/KUB versus 3 mSV for LDCT).

Many urologic procedures in the operating room utilize fluoroscopy. This includes percutaneous nephrolithotomy (PNL), URS and SWL. The radiation from fluoroscopy contributes to patients overall exposure. It also leads to exposure for the urologist. Awareness of the amount of radiation patients are exposed to in the operating room (OR) and understanding techniques to reduce the amount of fluoroscopy will help minimize the overall exposure to both patients and urologists.

Radiation exposure from fluoroscopy can be reported in a number of different ways. Fluoroscopy time (FT) is the simplest measure of exposure, however it does not always correlate with the actual dose the patient received, particularly in obese patients [23]. Dose area product (DAP) is a value reported by the C-arm that can be converted to effective dose (ED) using conversion factors. Effective dose is a calculated dose that relates the amount of radiation absorbed to the risk of malignancy.