In previous studies, DECT scans of medium and large phantoms showed good results (100% accuracy, 40/40) by measuring the accuracy and sensitivity of urinary stones according to the setting of the collimating beam width, proving that the clinical use of urinary stones using DECT is possible [3]. In particular, small stone syndromes can cause sufficient pain with urinary stones measuring 3 mm (range 1.5–4.0 mm) compared to that generally caused by urinary tract obstruction; hence, it is important to detect small urinary stones [14]. Their detection using DECT can help radiologists or urologists to identify diseases. New CT technologies have enabled increasingly accurate analyses of urinary stones with the application of tin filters combined with high-energy tubes and wider energy ranges [15, 16].
According to the DECT-based diagnosis and analysis of urinary stones, UA stones consist of light chemical elements (H, C, N, and O), whereas non-UA stones consist of heavy chemical elements (P, Ca, and S), resulting in very different X-ray attenuating properties at high and low peak kilovoltage (kVp). UA stones have higher CT numbers at higher kVp values, whereas non-UA stones have higher CT numbers at lower kVp values [6]. Consequently, the difference between CT numbers at high and low kVp values can be used to improve the prediction accuracy of the CT number approach [17–19]. Furthermore, current third-generation dual-source CT devices are equipped with selective annotation filters for high-energy X-ray tubes to absorb low-energy photons [20, 21].
Slice thickness is an important scanning parameter under the same CT energy conditions. A thinner slice provides better spatial resolution. Conversely, the noise in CT images increases with a decrease in the thickness of the slices. However, because the sensitivity of the urinary stones depends on the detail and spatial resolution, reducing the slice thickness is expected to achieve better sensitivity. The performance of the dual-energy technique for extra-large-sized patients is limited by two factors. Firstly, the 80-kVp images become extremely noisy, increasing the error bars of the data points representing the stones on the dual-energy plot. The second factor is beam hardening, which is more evident in case of extra-large patients [6]. Thus, the larger the phantom size, the lower the accuracy.
We used the reconstruction kernel setting (e.g., SAFIRE kernel strength) and automatic exposure control (AEC), which are currently used in abdominal examinations in clinical trials and investigated the changes in CT parameters that can affect urinary stone analysis. Conditions A in Phantom I, conditions I and K in Phantom II, and conditions Q and S in Phantom III showed the highest sensitivity and highest accuracy. The most common settings for conditions A, I, K, Q, and S are small collimation beam widths (2 × 32 × 0.6 mm) and small slice thicknesses/increments (0.5/0.5 mm). Considering the previously mentioned results, the most accurate urea analysis results were achieved at small slice thickness/increment values. Therefore, a combination of DECT parameters with a tube potential of 80/sn140, collection beam width of 2 x 32 x 0.6 mm, and slice thickness/increment of 0.5 / 0.5 mm may be providing the highest sensitivity and the highest accuracy for small urinary stones, regardless of patient size.
In previous studies, DECT scanning of medium and large phantoms showed good results (100% accuracy, 40/40) by measuring the accuracy and sensitivity of urinary stones according to the collimation beam width setting [3]. Similarly, in our study, a collimation beam width setting of 2 × 32 mm × 0.6 mm under the same condition was better in determining both the sensitivity and accuracy of urinary stones than the setting of 2 × 64 mm × 0.6 mm. In addition, previous studies used relatively large stones with a size ranging from 2 to 7 mm; however, this study used a small stone of 1.5 to 4.0 mm, Evaluating the sensitivity and accuracy of small urinary tract stones according to the difference in slice thickness/incremental of the DECT scan parameters may give a different result compared to a similar analysis using large stones.
In general, DECT system vendors recommend a slice thickness between 1 mm and 2 mm, which is one of the constraints for urinary stone detection, and an overlapping ratio of ‒30%. However, according to the findings of our study, the best result can be obtained with a slice thickness/increment setting of 0.5/0.5 mm instead of 1.5/1.0 mm (P < 0.05). In addition, in the Siemens syngo.via application provided by the vendor, the result of the urinary stone parameter diagram analysis can distinguish UA and non-UA by their color code (Fig. 5D). Vendors have also recommended that stones with diameters less than 3.5 mm may be color- coded incorrectly or not detected at all under other non-standard settings. However, in a study conducted with Phantom III assuming obese patients, condition Q (10 with a diameter < 3.5 mm and 5 with a diameter > 3.5 mm) showed a high urological stone sensitivity of about 72% and a high accuracy of 91%. When setting the DECT scan parameters to detect small-sized urinary stones, the slice thickness and increment must be set to a small value to increase the detection sensitivity and accuracy of urinary stones. However, this experiment has the limitation of being an in-vitro study, and in-vivo research on urolithiasis patients is necessary in the future.