mri_field_map_reg_scale causing bright pixels outside head

[JeffFessler]

MH reports that putting images from J-SENSE recon code output
https://sigpy.readthedocs.io/en/latest/generated/sigpy.mri.app.JsenseRecon.html#sigpy.mri.app.JsenseRecon
into mri_field_map_reg.m causes amplification of pixels outside the head in this line:

mirt/mri/fieldmap/mri_field_map_reg.m

Line 96 in 43b20b5

[yik yi2] = mri_field_map_reg_scale(yik, etime, arg.fmax);

mirt/mri/fieldmap/mri_field_map_reg.m

Line 148 in 43b20b5

function [yik yi2] = mri_field_map_reg_scale(yik, etime, fmax)

Masking the fieldmaps (using ESPIRT masks) addressed the issue.
Fixing this is a low priority because I recommend using this code instead anyway:
https://github.com/ClaireYLin/regularized-field-map-estimation