## 4 Results
### Categorization of the Results
#### 4.3.2 Type of Research  
Table 1 presents a breakdown of publications according to the type of research and organ investigated. Most publications (45) focused on diagnostic methods targeting specific diseases or conditions, neurological disorders such as Parkinson's and cardiovascular conditions such as coronary artery disease (CAD). General imaging methods (13) were developed and/or tested with images of a specific organ. However, those methods were developed to enhance image manipulation processes, and, in general, are not exclusively used in some specific diagnostic processes. Therefore, they should be considered in method applications for other organs and diseases. One method, addressed in Table 1 as \"N/A\", uses a Monte Carlo simulated dataset for the generation of random, not organ-specific, SPECT projections [77]. Across the 58 publications, both open and in-house datasets were utilized. The number of datasets, categorized by country, is presented in Table 2. The most frequently used open dataset is the PPMI dataset, a well-recognized, multi-center resource for [PERSON]'s disease research [78]. Additionally, the widely known UCI SPECT Heart dataset appears in two publications [79], while two more datasets which were created in-house during research, later were made publicly available as open data [80; 81]. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Method** & **Organ** & **N** & **Publications** \\ \hline \multirow{6}{*}{Diagnostic methods (N = 45)} & Heart & 17 & [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36] \\  & Brain & 15 & [37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51] \\ \cline{1-1}  & Bones & 7 & [52; 53; 54; 55; 56; 57; 58] \\ \cline{1-1}  & Kidney & 3 & [59; 50; 51; 52; 53; 54; 56; 57; 58] \\ \cline{1-1}  & Thyroid glands & 2 & [62; 63] \\ \cline{1-1}  & Lymph nodes & 1 & [64] \\ \hline \multirow{6}{*}{General imaging methods (N = 13)} & Heart & 6 & [65; 66; 67; 68; 69; 70] \\ \cline{1-1}  & Brain & 3 & [71; 72; 73] \\ \cline{1-1}  & Kidney & 3 & [74; 75; 76] \\ \cline{1-1}  & N/A * & 1 & [77] \\ \hline \hline \end{tabular}  
* The authors used Monte Carlo simulated dataset for generation of not organ-specific, SPECT projections [77].  
\end{table} 
Table 1: Type of research categorized by method and organ. Figure 4: Publishing years for the reviewed studies. A total of 38 datasets were produced and used in the studies covered in this review, with China being the most common source, followed by Japan and Taiwan. Four publications do not explicitly state the medical centers where data were collected, though the authors' affiliations may provide some indication. Notably, publication [35] presents a large-scale international study involving multiple clinical centers across Austria, Italy, the UK, and China at different research phases, making it unique in this review. Additionally, two studies did not use patient data. [PERSON] et al. employed software-generated phantom data [73], while [PERSON] et al. generated synthetic SPECT data [77]. Considering the image modality, planar scintigraphy images are used only in six studies [35; 58; 59; 60; 64; 74], while the rest 52 studies used SPECT images. Moreover, only seven studies include additional data on top of the imaging data, such as biomarker data or/and clinical and demographic data [29; 33; 42; 43; 51; 82; 83]. Finally, it is worth noting that datasets differ in the number of patients from whom data are collected. In-house datasets typically consist of several hundred patients, though some studies explore data from fewer than 100 individuals. For the PPMI dataset, the number of included patients depends on the specific research criteria defined by the authors. The datasets used in these studies vary in scale, structure, and preprocessing approaches, which are relevant factors to be considered, as dataset selection directly impacts model choice. Nuclear imaging data are not always collected through identical processes, nor are the final datasets uniform.