enh: summarize scenarios first following @sappelhoff's suggestion

src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md

@@ -568,9 +568,15 @@ JSON example:
 ## Fieldmap data
 
 Data acquired to correct for B0 inhomogeneities can come in different forms.
-The current version of this standard considers four different scenarios.
+The current version of this standard considers four different scenarios:
+
+  1. [Phase-difference map](#case-1-phase-difference-map-and-at-least-one-magnitude-image)
+  1. [Two phase maps](#case-2-two-phase-maps-and-two-magnitude-images)
+  1. [Direct *field mapping*](#case-3-direct-field-mapping)
+  1. ["*PEpolar*" fieldmaps](#case-4-multiple-phase-encoded-directions-pepolar)
+
 These four different types of field mapping strategies can be encoded
-using the following image types:
+as described [below](#types-of-fieldmaps), using the following image types:
 
 | **Name**         | `suffix`         | **Description**                                                                                                                                                                                                      |
 | ---------------- | ---------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |