Abstract
Background
This study was conducted to evaluate the chemical composition of eight types of urinary calculi using spiral computerized tomography (CT) in vivo.
Methods
From October 2011 to February 2013, upper urinary tract calculi were obtained from 122 patients in the department of urinary surgery of the First Affiliated Hospital of Soochow University. All patients were scanned with a 64‐detector row helical CT scanner using 6.50 mm collimation before ureterorenoscopy. Data from the preoperative spiral CT scans and postoperative chemical composition of urinary calculi were collected.
Results
The chemical composition analysis indicates that there were five types of pure calculi and three types of mixed calculi, including 39 calcium oxalate calculi, 12 calcium phosphate calculi, 10 calcium carbonate calculi, 8 magnesium ammonium phosphate calculi, 6 carbonated apatite, 21 uric acid/ammonium urate calculi, 10 uric acid/calcium oxalate calculi, and 16 calcium oxalate/calcium phosphate calculi. There were significant differences in the mean CT values among the five types of pure calculi (P < 0.001). Furthermore, we also observed significant differences in the mean CT values among three types of mixed calculi (P < 0.001). Significant differences in the mean CT values were also found among eight types of urinary calculi (P < 0.001). However, no statistically significant difference was observed between the mean CT values of magnesium ammonium phosphate calculi and uric acid/calcium oxalate calculi (P = 0.262).
Conclusion
Our findings suggest that spiral CT could be a promising tool for determining the chemical composition of upper urinary tract calculi.
Keywords: spiral computerized tomography, upper urinary tract calculi, chemical composition
INTRODUCTION
Upper urinary tract calculi are a global health problem occurring with increasing frequency in populations from almost every ethnicity and geographic location 1. According to previous statistics, the male‐to‐female ratio is 3:1, the peak age at presentation is in the third to fifth decades of life, and the clinical manifestation is typical of renal colic 2. As one of the world's three largest highest incidence districts, incidence of urinary calculi in China is up to 1∼5%, and even up to 5∼10% in South China 3. While regional differences are very obvious in the incidence of urinary calculi, the intrinsic and extrinsic factors involved in the development and progression of urinary calculi always include hyperparathyroidism, bedridden, obstruction, infection, etc. 4, 5. Frequently, the true prevalence of urinary calculi is underestimated because many calculi remain asymptomatic and hence go undiagnosed 6. Generally, the great majority of patients with urinary calculi can be diagnosed, to some extent, through medical history, physical examination, ultrasound, or X‐ray examination 7. However, several factors constraint diagnostic accuracy, including the size, amount and position of the calculi, bowel content of patient, obstruction and infection, as well as calculi composition, etc. 8. Furthermore, conventional imaging methods, such as ultrasonography (US) and plain kidneys–ureters–bladder radiographs (KUB) used for urolithiasis are less reliable due to radiolucency of calculi, increased amount of bowel gas, and smaller stone size 9.
Spiral computerized tomography (CT), a relatively safe, rapid, and accurate diagnostic method, is now becoming a frequently used radiographic examination to establish the diagnosis and severity of upper urinary tract calculi and has been widely adopted to evaluate the calculi in patients with renal colic 10, 11. Consequently, the success of spiral CT in determining calculi in renal colic suggests that spiral CT may potentially be a reliable imaging method to diagnosis upper urinary tract calculi 12. The basic advantages of spiral CT are its high sensitivity and specificity in detecting urinary calculi (including nonopaque calculi), celerity, and safety 2, 13. Clinical evidence has confirmed that spiral CT may significantly contribute to the improvement in identifying the chemical composition of upper urinary tract calculi 14, 15. Spiral CT can be conducted to determine the composition and brittleness of upper urinary tract calculi and thus provides a reference standard for the choice of treatment 16. Notably, determination of the chemical composition of upper urinary tract calculi, which is of critical importance in the selection of optimal management, is positively associated with subsequent treatment outcomes 17, 18. In this study, we conducted an in vivo evaluation of the chemical composition of eight types of urinary calculi using spiral CT in a Chinese population, which may contribute to improving the therapeutic effect, reducing complications, preventing stone recurrence, and minimizing unnecessary trauma.
MATERIALS AND METHODS
Ethics Statement
The Ethics Committee of the First Affiliated Hospital of China Medical University approved our study design. All patients had to sign informed consent in written form to undergo renal colic protocol spiral CT at the time of hospitalization.
Subjects
From October 2011 to February 2013, a total of 122 patients (73 males and 49 females) with upper urinary tract calculi admitted to the Department of Urinary Surgery in the First Affiliated Hospital of Soochow University for ureterorenoscopy were included in this study. These 122 patients included 80 patients with renal calculi and 51 patients with ureteral calculi. The mean age of patients was 48.3 years (range, 21∼84 years).
Methods
Before ureterorenoscopy, all patients were scanned with a 64‐detector row helical CT scanner using 6.50 mm collimation at the energy level of 120 kV at 330 mA. All images were acquired by the picture archiving and communication systems (PACS), and stored for subsequent evaluation by two radiologists blinded to the chemical composition of the calculi during a consensus reading session. For each calculus, a region of interest (ROI) overlying the whole calculi on the slice was obtained for each plane for tissue and bone windows at 120 kV. The absolute Hounsfield units (HU) value of each stone is presented for the central ROI of the plane that passed through the maximum transverse diameter. The average, highest, and lowest CT‐numbers of upper urinary calculi were assessed on the basis of differences in densities measured in HU, which were recorded within the ROI using a conventional soft‐tissue window (window width, 250H; level, 45H) at 120 kV. The visual density of stones was studied using a three‐level scale (level 1: low density; level 2: intermediate density; level 3: high density). Qualitative analysis of eight main components (urate, oxalate, phosphate, carbonate, calcium, magnesium, ammonium, and cystine) in upper urinary tract calculi were detected using the standard urinary calculi analysis kit provided by the Institute of Urolithiasis in the First Affiliated Hospital of Guangzhou Medical College.
Statistical Analysis
Data were presented as mean ± standard deviation (mean ± SD) or frequencies. The χ2 test was used to compare frequencies. One‐way analysis of variance (ANOVA) and Student's t‐test were used for normally distributed variables, whereas the Mann–Whitney's U test was used for nonnormally distributed variables. Comparisons between the two groups for nominal variables were made using the least significant difference (LSD) post hoc test. All statistical analyses were performed using the SPSS 18.0 software (SPSS, Inc., Chicago, IL). P < 0.05 was considered statistically significant.
RESULTS
The analysis of chemical composition indicated that there were five types of pure calculi (n = 90) and three types of mixed calculi (n = 37, Fig. 1), including 39 calcium oxalate calculi (29%), 12 calcium phosphate calculi (9%), 10 calcium carbonate calculi (8%), 8 magnesium ammonium phosphate calculi (6%), 6 carbonated apatite (5%), 21 uric acid/ammonium urate calculi (16%), 10 uric acid/calcium oxalate calculi (8%), and 16 calcium oxalate/calcium phosphate calculi (12%).
Figure 1.
Spiral CT scanning of (A) pure calculi and (B) mixed calculi.
The mean CT values of each type of urinary calculi were as follows: calcium oxalate calculi (1058 ± 71 HU); calcium phosphate calculi (1126 ± 67 HU); calcium carbonate calculi (770 ± 87 HU), magnesium ammonium phosphate calculi (631 ± 76 HU); carbonated apatites calculi (948 ± 90 HU), uric acid/ammonium urate calculi (349 ± 78 HU); uric acid/calcium oxalate calculi (670 ± 73 HU), and calcium oxalate/calcium phosphate calculi (1335 ± 51 HU). The CT values of each type of urinary calculi are shown in Table 1.
Table 1.
The CT Values of Eight Types of Urinary Calculi
| 95% CI | ||||||||
|---|---|---|---|---|---|---|---|---|
| Urinary calculi | n | Mean | SD | SE | LL | UL | Min | Max |
| Calcium oxalate | 39 | 1057.79 | 71.449 | 11.441 | 1034.63 | 1080.96 | 912 | 1185 |
| Calcium phosphate | 12 | 1125.92 | 67.353 | 19.443 | 1083.12 | 1168.71 | 1046 | 1210 |
| Calcium carbonate | 10 | 770.30 | 87.469 | 27.660 | 707.73 | 832.87 | 610 | 875 |
| Magnesium ammonium phosphate | 8 | 631.38 | 75.676 | 26.755 | 568.11 | 694.64 | 530 | 720 |
| Carbonated apatites | 6 | 948.33 | 90.608 | 36.991 | 853.25 | 1043.42 | 830 | 1080 |
| Uric acid/ammonium urate | 21 | 349.10 | 77.761 | 16.969 | 313.70 | 384.49 | 213 | 460 |
| Uric acid/calcium oxalate | 10 | 670.20 | 72.718 | 22.996 | 618.18 | 722.22 | 530 | 740 |
| Calcium oxalate/calcium phosphate | 16 | 1335.56 | 50.636 | 12.659 | 1308.58 | 1362.54 | 1263 | 1412 |
SD, standard difference; SE, standard error; 95% CI, 95% confidence interval; LL, lower limit; UL, upper limit; Min, minimum; Max, maximum.
There were significant differences in the mean CT values among five types of pure calculi (P < 0.001). Furthermore, we also observed significant differences in the mean CT values among three types of mixed calculi (P < 0.001). Significant differences in the mean CT values were also found among eight types of urinary calculi (P < 0.001). However, no statistically significant difference in the mean CT values was observed between magnesium ammonium phosphate calculi and uric acid/calcium oxalate calculi (P = 0.262, as shown in Table 2).
Table 2.
Differences in the Mean CT Values of Eight Types of Urinary Calculi
| Urinary calculi | ||||||||
|---|---|---|---|---|---|---|---|---|
| P‐value | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 1 | – | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 |
| 2 | 0.000 | – | 0.005 | 0.000 | 0.000 | 0.001 | 0.000 | 0.000 |
| 3 | 0.000 | 0.005 | – | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 |
| 4 | 0.000 | 0.000 | 0.000 | – | 0.000 | 0.000 | 0.003 | 0.000 |
| 5 | 0.000 | 0.000 | 0.000 | 0.000 | – | 0.000 | 0.262 | 0.000 |
| 6 | 0.000 | 0.001 | 0.000 | 0.000 | 0.000 | – | 0.000 | 0.000 |
| 7 | 0.000 | 0.000 | 0.000 | 0.003 | 0.262 | 0.000 | – | 0.000 |
| 8 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | 0.000 | – |
1, uric acid/ammonium urate calculi; 2, calcium oxalate calculi; 3, calcium phosphate calculi; 4, calcium carbonate calculi; 5, magnesium ammonium phosphate calculi; 6, carbonated apatites calculi; 7, uric acid/calcium oxalate calculi; 8, calcium oxalate/calcium phosphate calculi.
DISCUSSION
Spiral CT is a type of three‐dimensional CT that is becoming frequently used for radiographic examination and is a relatively safe, rapid, and accurate diagnostic method for urinary calculi 12, 19. Clinically, spiral CT has been suggested to have a positive effect on density resolution and contrast, and to be unaffected by interference from overlapping bone and intestinal contents, features that are particularly helpful for obtaining the desired measurements of upper urinary tract calculi 13, 20. Particular in cases where patients have nonfunctioning kidney or iodine allergies, the use of spiral CT performs significantly better on excluding upper urinary tract obstruction or urolithiasis, as well as determining the stone size, location, and values 21. Furthermore, in view of the fast and continuous development of medical science and technology, accompanied by increasingly sophisticated two‐dimensional CT images and 3D reconstruction techniques, the use of spiral CT has become central to a new noninvasive examination method for successfully diagnosing urinary system diseases 14, 15. Previous technologies, including US, necessary X‐ray examination, and plain KUB radiographs, have been demonstrated to be limited by several constraining factors in accurately diagnosing upper urinary tract calculi and in determining the size, composition, amount and position of the calculi, bowel content, obstruction, and infection, etc. 7, 8, 9. Theoretically, 90% of stones can be found by X‐ray and KUB, however the actual display rate of both have been shown to be lower than actual rates, thereby seriously hindering the diagnosis of calculi 22. Therefore, there is strong reason to believe that spiral CT is more powerful in determining the clinical treatment of upper urinary tract calculi. 23. More specifically, spiral CT can be conducted to determine the composition of calculi by measuring stone location, size, and average CT value, which provide a reference for analyzing the composition of upper urinary tract calculi since different calculi components may show different CT values 24. Interestingly, a previous experimental study revealed that the analysis outcome of X‐ray spectroscopy for calculi from a performed percutaneous nephrolithotomy was only able evaluate several calculi, including struvite, uric acid, cystine stones, and calcium stones 23. However, with spiral CT, a total of 11 categories of stone composition are able to be obtained through chemical analysis 23. Therefore, spiral CT may be a more feasible method for accurately determining the chemical composition of the upper urinary tract calculi 25. Indeed, determining calculi composition is of great importance in providing information for appropriately treating urinary calculi, reducing complications, preventing recurrent calculi, and decreasing unnecessary trauma 17, 18. Therefore, we conducted a correlation study on the efficacy of spiral CT in identifying the chemical composition of upper urinary tract calculi.
In this study, we assessed whether spiral CT with advanced postimage processing can accurately differentiate the chemical composition of upper urinary tract calculi. Nonenhanced spiral CT scans were performed on a total of 131 preoperative patients with upper urinary tract calculi. Our study results showed that we could differentiate the chemical composition of all pure calculi and mixed calculi with absolute HU values at 120 kV, suggesting that the measurement of absolute HU values and HU density via spiral CT scanning with a small collimation size can reveal significant differences among all pure urinary calculi and most of the mixed calculi. A study conducted by Mitcheson et al. and Newhouse et al. confirmed that the chemical composition of all pure calculi could be differentiated by spiral CT scan and found a similar association for mixed calculi 26, 27. Although we still do not fully understand the precise reasons why a spiral CT scan can differentiate the chemical composition of all pure calculi and mixed calculi, one possible explanation is that there were no significant differences between calculi volume and absolute HU values for most calculi in our study. Sheir et al. also found no obvious association between calculi volume and absolute HU values in their study 26. Another explanation may be that the spiral CT examinations used in our study were performed at 120 kV, which almost all previous studies found most useful. Mostafavi et al. also reported in their study that the best single CT parameter for assessing the chemical composition of calculi was the absolute CT‐attenuation value at 120 kV 28. The results of our study also revealed that a spiral CT could differentiate the chemical composition of all calculi besides magnesium ammonium phosphate calculi and uric acid/calcium oxalate calculi. Our findings agreed with Motley et al. and Nakada et al., who found statistically significant differences between pure uric acid calculi and calcium oxalate calculi, but not between uric acid and calcium oxalate calculi, and magnesium ammonium phosphate calculi 29, 30. Although the precise reason why uric acid/calcium oxalate calculi and magnesium ammonium phosphate calculi could not be differentiated from all pure calculi and the mixed urinary calculi is still poorly understood, it may be caused to some extent by calculi composition and size. As Saw et al. concluded, calculi composition and size may influence CT attenuation independently 31. Our results demonstrated that absolute HU values and HU density derived from spiral CT scanning using a small collimation size could uncover statistically significant differences among all pure and most mixed urinary calculi, permitting greater accuracy in the prediction of calculi composition. Furthermore, this technique permits diagnostic conclusions on the basis of a single CT evaluation. In order to improve the accuracy of spiral CT to analyze the chemical compositions of upper urinary tract calculi and to reduce the selection of partial volume effect and interested regions, it is crucial to establish optimal scan parameters and reduce radiation doses to patients. More large‐sample and multicenter research data are needed to support our findings due to the current lack of large‐sample studies and standard examination methods.
In conclusion, our findings provide empirical evidence that spiral CT could be a promising tool for determining the chemical composition of upper urinary tract calculi. Thus, spiral CT may be recommended before selecting a method of treatment for urinary calculi.
ACKNOWLEDGMENTS
We thank the reviewers for their helpful comments on this article.
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