Imaging in Stone Diagnosis and Surgical Planning

Emily C. Serrell; Sara L. Best


Curr Opin Urol. 2022;32(4):397-404. 

In This Article

Reducing Radiation Exposure: 'As low as Reasonably Achievable'

Historically a major disadvantage of CT is ionizing radiation exposure, although modern iterative reconstruction algorithms have allowed reduced radiation doses.[10] Stone patients are exposed to radiation in both the workup and treatment. Patients with recurrent urolithiasis may receive more than ten times as much diagnostic imaging as patients without stones.[11] There is increasing recognition of the potential for harm, particularly the increased risk of secondary malignancies. A 2019 modeling study reported radiation exposure per nephrolithiasis episode of 37.3 mSv in 2 years period, with an associated lifetime estimated incidence of secondary malignancy 862.7 cases/year and associated mortality of 545.3 cases/year.[12]

The concept of 'As Low as Reasonably Achievable' (ALARA) has been increasingly emphasized to reduce exposure to both patients and practitioners. The AUA encourages imaging stewardship and specifically endorses the American College of Radiology (ACR) Appropriateness Criteria for Acute Onset Flank pain. This recommends the use of reduced-dose CT, which is defined by a dose length product of 200mGy with an effective dose of 3mSv.[13] Even more recently there has been interest in ultra-low-dose CT with an effective dose of < 2 mSv. A meta-analysis of 12 studies with a pooled 1529 patients compared standard dose CT as a reference and reported the following: low-dose CT (<3.5 mSV) sensitivity 90- 98% and specificity 88–100%; ultra-low-dose CT (<1.9 mSV) sensitivity 72–99% and specificity of 86–100%; similar diagnostic accuracy between low dose (94.3%) and ultra-low-dose (95.5%) CT.[14] Compared to standard CT, low dose and ultra-low-dose CT are associated with a reduction in the dose of radiation up to 78–89%.[15–17]

Despite proven accuracy, adoption of this technology has been slow,[18–20] with an increase from 2% in 2011 to only 7.6% in 2015.[21] Unnecessary radiation exposure is a result of incorrectly protocolled imaging with multiple phases or over-scanning above or below the area of interest.[18,22] Further, there is wide variation both within and between hospitals.[20] In a recent study of patients undergoing a renal colic workup in 51 hospitals from 20 countries in Europe, 52% underwent a 3–5 phase CT whereas only 37% had a single phase scan.[18]

The use of standardized protocols and interventions has been associated with an improved utilization of low-dose CT.[19,23] The 'Dose Optimization for Stone Evaluation' intervention included customized CME modules, personalized consultation, and protocol recommendations for reduced dose CT. It was associated with a significant increase in the number of reduced dose CT's obtained and reduction of 110 mGy/cm annually.[23]

Pregnant women and pediatric patients represent a population for whom ALARA is particularly important. To limit radiation exposure, ultrasound (US) is the initial imaging modality of choice.[24] In children lifetime radiation exposure has long-term implications for increased malignancy risk.[25] The European Association of Urology guidelines encourage clinicians to start with US and consider kidney-ureter-bladder radiography or low-dose CT if US does not provide sufficient information.[26,27] Grivas et al. published an excellent 2020 systematic review regarding imaging for pediatric urolithiasis.[26]

Likewise in pregnant women, fetal radiation exposure is associated with teratogenic and carcinogenic effects. Radiation dose >1Gy is lethal to a fetus, but this is well above modern doses administered for diagnostic imaging. A dose of < 50 mGy is considered a maximum threshold, as doses lower than this have not been associated with teratogenic effects. Notwithstanding, the increased lifetime risk of malignancy is less well understood.[28,29] The 2017 consensus statement from the American College of Obstetrics and Gynecology recommends initial evaluation with nonionizing imaging via US or magnetic resonance imaging (MRI), which may visualize stones as a filling defect.[29,30] If ionizing imaging techniques are necessary, 'they should not be withheld from a pregnant woman.'[29] A 2013 study by White et al. reported lower rate of negative ureteroscopy for patients who underwent low-dose CT with US (4.2%) as compared to women who underwent US alone (23%) or renal US and MRU (20%).[31] Urologists should engage in shared decision-making with pregnant patients in the workup of urolithiasis and work closely with obstetricians and radiologists.[32]