## 1 Introduction
### ROI Concept  
The ROI conception was proposed in the literature to prevent losing some information after averaging the analytical signal from an area which resulted in bulk analysis only. The initialism ROI means \"region of interest\" and signifies a carefully selected part of the sample. Such areas can be treated as an integrity and often analyzed as a whole, despite the collection of more detailed data. Distinctions of ROIs are performed either before the analysis by the separation of anatomical or histological parts of the tissue, or after analysis, e.g., if elemental distribution maps indicate particulate areas. The nervous system is the main biological sample which is either divided into ROIs or only selected ROIs are subjected to analysis. [PERSON] et al. [46] distinguished three ROIs in mouse brains (entire hemisphere, hippocampus, and cortex) by manual extraction with the use of a copper distribution map. Maximum copper contents located along the ventricles identified the edges. The mouse brain was also in the field of interest for [PERSON] et al. [47] in the context of neurodegenerative disease that can influence the very local distribution of elements. Based on the previously published \"The Mouse Brain in Stereotaxic Coordinates\" atlas [48] and comparison of elemental distribution maps (Zn, Cu, Fe, Mn, C, and P) with histological image after cresyl violet staining, the researchers [47] covered the whole brain section area with ROIs. If absolute calibration was not possible, signal intensities were normalized to the average of the reference region, which was the corpus callosum for the most of elements. Univariate one-way analysis of variance (ANOVA) was used to compare group means, and some statistically significant differences were found, especially in the periventricular zone and interpenduncular nucleus between the control group and those treated with MTPP (inducing Parkinson's disease) [47]. [PERSON] et al. [49] selected ROIs as coronal sections, as described elsewhere by [PERSON] et al. [50]. [PERSON] et al. [51] analyzed Gd concentrations in brain tissue and distinguished two main ROIs: (1) the dentate nucleus (DN), divided afterwards into four more detailed parts, although still treated as a single ROI; and (2) the surrounding region. Separate Gd contents in each ROI determined by LA-ICP-MS strongly differed from the total content quantified by ICP-MS with solution nebulization, which confirmed use of targeted analyses of a solid sample [51]. The authors stated that the Gd content may vary considerably, even between two adjacent laser spots [51]. The second well-known LA-ICP-MS-supported medical area is metal-containing xenobiotic analysis. Imaging of Pt in thin layers of blood cells taken from patients treated with cisplatin performed with LA-ICP-MS was advantageous in view of short analyses and simple sample preparation. Blood cell thin films were stained in order to find the ROIs containing white blood cells; thus, in this case, ROIs were selected before the laser ablation [52]. In [PERSON] et al. 's study [53], histopathological analysis indicated the ROI, which was separated by drawing a line around the tumor surface. Then, targeted Gd analysis was possible, showing the distribution of Gd-containing contrast agents. [PERSON] et al. [54] proposed the statistical use of ROIs on the example of Gd deposition from GBCA-gadolinium-based contrast agents. The authors identified the area of DCN (deep cerebellar nuclei) by the increased \({}^{57}\)Fe transient signals. The mean Gd concentration in the DCN was calculated by averaging the data obtained from three ROIs of the approximate area 880 \(\upmu\)m \(\times\) 880 \(\upmu\)m and of 500 data points each. Similarly, three regions of interest outside the DCN were selected for averaging the Gd concentration in the surrounding tissue [54]. [PERSON] et al. [55] analyzed the iatrogenic gadolinium as well as lanthanum (from lanthanum carbonate) deposition patterns.