## 3 Methodology
### Search Strategy
#### 3.1.1 Preliminary Screening:Addressing the RQ1  
**Search string.** To identify relevant research, comprehensive search queries were constructed using the following search keywords: SPECT, single-photon emission, scintigraphy, renal, kidney, CKD, GFR, glomerular filtration rate, chronic kidney disease, machine learning, artificial intelligence, and deep learning. The keywords \"GFR\" and \"glomerular filtration rate\" are selected due to their strong connection with CKD. As for the techniques used, we have opted for broad keywords, such as \"machine learning\", to avoid limiting the search to known, popular, or expected ML methods. To enhance the specificity of the search and reduce irrelevant results, the keywords were combined using the boolean operators: AND and OR. This Boolean search strategy, ensured that only articles containingat least one term were retrieved. For each of the selected databases, the search queries were constructed similarly, following the respective syntax rules. For example, the WoS search query used was: \"TS = ((SPECT OR \"single-photon emission\" OR \"scintigraphy\") AND (renal OR kidney OR CKD OR GFR OR \"glomerular filtration rate\" OR \"chronic kidney disease\") AND (\"machine learning\" OR \"artificial intelligence\" OR \"deep learning\"))\".  
**Databases.** The search was performed in three databases, Web of Science (WoS), Scopus, and PubMed. PubMed is a well-known archive of biomedical and life sciences journal literature that covers extensive research for medical and clinical literature. Scopus is a free, large, multidisciplinary database of peer-reviewed literature that provides a comprehensive overview of the world's research output in the fields of science, technology, medicine, social science, and arts and humanities. And finally WoS, is a free platform that accelerates and delivers novel research from premier scholarly and scientific venues. The last search was performed in December 2024.  
**Inclusion/exclusion criteria.** In all three databases, searches included all collections, document types, and publication years, with results limited to English language publications. For the WoS database, the \"exact search\" option was not selected to allow for a broader search. These settings were chosen to maximize the number of relevant studies, given the anticipated limited volume of literature in this domain. All returned papers were screened by title and abstract and filtered by the following exclusion criteria:  
1. Not related to planar scintigraphy or SPECT image modalities;
2. Not related to the CKD prediction;
3. Not related to this scoping review;
4. Not written in English language.  
Exclusion criterion one will help to filter out papers that consider dual modalities, such as SPECT/CT or SPECT combined with ultrasound. Exclusion criterion two filters out papers related to deep learning applications on planar and SPECT images if are not specific to renal imaging, as well as those focused on renal imaging but unrelated to CKD. Those publications will be manually added to the broader screening. Finally, exclusion criterion three will filter out studies unrelated to the research scope, such as those on radiopharmaceutical design or dosage.