Raman chemical imaging, a new tool in kidney stone structure analysis: Case-study and comparison to Fourier Transform Infrared spectroscopy

Background and objectives
The kidney stone’s structure might provide clinical information in addition to the stone composition. The Raman chemical imaging is a technology used for the production of two-dimension maps of the constituents' distribution in samples. We aimed at determining the use of Raman chemical imaging in urinary stone analysis.
Material and methods
Fourteen calculi were analyzed by Raman chemical imaging using a confocal Raman microspectrophotometer. They were selected according to their heterogeneous composition and morphology. Raman chemical imaging was performed on the whole section of stones. Once acquired, the data were baseline corrected and analyzed by MCR-ALS. Results were then compared to the spectra obtained by Fourier Transform Infrared spectroscopy.
Results
Raman chemical imaging succeeded in identifying almost all the chemical components of each sample, including monohydrate and dihydrate calcium oxalate, anhydrous and dihydrate uric acid, apatite, struvite, brushite, and rare chemicals like whitlockite, ammonium urate and drugs. However, proteins couldn't be detected because of the huge autofluorescence background and the small concentration of these poor Raman scatterers. Carbapatite and calcium oxalate were correctly detected even when they represented less than 5 percent of the whole stones. Moreover, Raman chemical imaging provided the distribution of components within the stones: nuclei were accurately identified, as well as thin layers of other components. Conversion of dihydrate to monohydrate calcium oxalate was correctly observed in the centre of one sample. The calcium oxalate monohydrate had different Raman spectra according to its localization.
Conclusion
Raman chemical imaging showed a good accuracy in comparison with infrared spectroscopy in identifying components of kidney stones. This analysis was also useful in determining the organization of components within stones, which help locating constituents in low quantity, such as nuclei. However, this analysis is time-consuming, making it more suitable for research studies rather than routine analysis.

研究背景与目的
除结石成分外，肾结石的结构亦可提供临床相关信息。拉曼化学成像（Raman chemical imaging）是一种可用于生成样本内成分分布二维图谱的技术。本研究旨在探讨拉曼化学成像在尿路结石分析中的应用价值。

材料与方法
本研究采用共聚焦拉曼显微分光光度计（confocal Raman microspectrophotometer）对14例尿路结石样本进行拉曼化学成像分析。所有样本均根据其非均一的成分与形态特征进行筛选。拉曼化学成像操作覆盖结石的全部切片区域。数据采集完成后，先进行基线校正，再通过多元曲线分辨-交替最小二乘法（MCR-ALS）进行分析。最后将分析结果与傅里叶变换红外光谱（Fourier Transform Infrared spectroscopy）获取的光谱进行比对。

研究结果
拉曼化学成像可成功识别绝大多数样本中的化学成分，包括一水草酸钙、二水草酸钙、无水尿酸、二水尿酸、磷灰石、鸟粪石（struvite）、透钙磷石（brushite），以及白磷钙石（whitlockite）、尿酸铵和药物等稀有成分。但由于自发荧光背景极强，且此类弱拉曼散射体的浓度极低，因此无法检测到蛋白质成分。即使碳磷灰石和草酸钙占结石总成分的比例不足5%，仍可被准确识别。此外，拉曼化学成像可清晰呈现结石内部的成分分布：不仅可精准识别结石核心，还可清晰显示其他成分形成的薄层结构。在1例样本的中心区域，本研究成功观测到二水草酸钙向一水草酸钙的转化现象。且根据所处位置的不同，一水草酸钙呈现出不同的拉曼光谱特征。

研究结论
与红外光谱技术相比，拉曼化学成像在肾结石成分识别方面具有较高的准确性。该分析方法还可有效揭示结石内部的成分排布规律，有助于定位含量较低的成分（如结石核心）。但该分析过程耗时较长，因此更适用于科研研究，而非常规临床检测。