The pooled sensitivity and specificity reflect that LDCT could be a useful test for diagnosing urolithiasis, with no major difference in the reference standard. What is more, ULDCT also has an excellent clinical use according to the subgroup analysis. However, we found that the specificity appears to be low in line with the average effective dose. In our study, ULDCT was defined as effective dose < 1.0 mSv. Similar to ours, a systematic review defining ULDCT as effective dose < 1.9 mSv reported that LDCT and ULDCT have high diagnostic accuracy, sensitivity and specificity despite significant radiation dose reduction in comparison to SDCT[31], though they may not be as effective in detecting stones < 3 mm in size or in patients with a BMI of > 30 kg/m2[32]. Both SDCT and LDCT have high diagnostic accuracy of ureteral uric acid stones, while the detection of uric acid stones is reduced when LDCT is at ≤ 15mAs[33]. Nevertheless, how low can the effective dose actually be still remains unknown.
In addition, the subgroup analysis of patient-based and urolithiasis-based studies shows that the pooled sensitivity of urolithiasis-based studies is much lower than that of the patient-based studies, indicating that the LDCT diagnoses urolithiasis accurately. However, LDCT may be problematic with small stones and evidence of distal ureteral obstruction[34]. The size and location of the stones may cause false-positive and false-negative findings, affecting the sensitivity and specificity. In the studies reporting the size of stone, the sensitivity declines when the size of stone declines[14, 17, 19]. Nevertheless, the thresholds of the stone were 5mm[14], 3mm[19] and 2mm[17] respectively. The specific value of the stone threshold remains uncertian. Phlebolith may cause the false-positive and false-negative findings, especially in distal ureter and VUJ[20, 28]. What is more, heterogeneity of renal stroma may also make it difficult to differentiate between hyperdense pyramids and small calculus, thus causing false-positive findings[19]. In additionm, passed off calculus should be taken into consideration as no calculus was detected on CT with haematuria and positive urine analysis[28].
The quality of the CT images may significantly degrade due to the lower radiation dose, therefore reconstruction techniques were implied in CT images. LDCT of urolithiasis can be feasible in overweight patients with a BMI between 25 and 35 kg/m2 with iterative image reconstruction algorithms[35]. Time has witnessed the development of the reconstruction algorithms during these years. Conventional filtered back projection (FBP) reconstruction technique has been well received due to its quick imaging technology for decades[36]. Iterative reconstruction (IR) algorithms have been introduced to improve image quality with less image noise[37–38]. Advanced modeled IR (ADMIRE), adaptive statistical IR (ASIR), iterative model reconstruction (IMR), sinogram-affirmed iterative reconstruction (SAFIRE), are widely used in current clinical field[39]. Additionally, artificial intelligence (AI), which uses various data mining algorithms such as machine learning, deep learning, and cascading convolutional neural network model, has also been widely applied as feasible and highly-accurate ways in the diagnosis of urolithiasis[40–41].
The first step of managing urolithiasis is accurate diagnosis, and after it is to figure out the components of the stones[42]. The most common urinary stones, calcium oxalate stones, accounts for 75–80%, and then it is uric acid stone and cystine stone taking up for 7–10% and 1%, respectively[43]. However, in a recent meta-analysis including 21 studies found that dual-energy CT (DECT), with pooled sensitivity of 0.88 and specificity of 0.98, is an accurate test for diagnosis of uric acid stones[44]. In general, LDCT is highly accurate for diagnosis of urolithiasis, whereas DECT can accurately differentiate uric acid from non-uric acid stones.