Fig. 4

A A bar plot visualizes the number of prospective (n = 19) and retrospective (n = 9) studies included in the systematic review (n = 28) [2,3,4,5, 8, 14, 16, 20, 23, 25,26,27, 31, 33, 34, 39, 41, 42, 45,46,47, 49, 59,60,61,62,63,64]. B A bar plot displays the number of studies for the following years of publications: 1990 (n = 1), 2002 (n = 1), 2004 (n = 1), 2005 (n = 1), 2006 (n = 1), 2007 (n = 1), 2008 (n = 1), 2011 (n = 3), 2013 (n = 3), 2014 (n = 3), 2016 (n = 2), 2017 (n = 1), 2018 (n = 2), 2019 (n = 2), 2020 (n = 3), 2021 (n = 2). C A bar plot shows the sample size of each included study in the systematic review (n = 28). Studies are named numerically 1–28, each number refers to the cited studies in synchronized order [2]. D A bar plot visualizes the number of included studies (n = 28) [2,3,4,5, 8, 14, 16, 20, 23, 25,26,27, 31, 33, 34, 39, 41, 42, 45,46,47, 49, 59,60,61,62,63,64] that use each of the following imaging modalities: “1.5 T MRI” (n = 8), “1.5 T or 0.5 T MRI” (n = 1), “3 T MRI” (n = 5),”0.5-3 T MRI” (n = 2), “Undefined MRI” (n = 1), “CTC” (n = 2), “CT or MRI” (n = 4), “SPECT” (n = 2), “PEG” (n = 3). MRI, magnetic resonance imaging; CT, computed tomography; CTC, computerized tomographic cisternography; SPECT, single-photon emission computerized tomography; PEG, pneumoencephalogram