Exporting Simulation Results for Specific Tissues

I am trying to export my simulation results (B1 and SAR) to mat files. In order to save disk space, I want to discard some materials (Background, Coil, Shield etc). I am using the following python script to implement field_mask_filter. When I set ent1='Heart_lumen', I can extract the field results for Heart Lumen only. But, I have 357 entities :). I just want to discard PEC and Background regions, but keep all the other materials in my exports. What is the correct way of coding this?
More specifically, I need learn how to use SetMaterial function for this filter:
field_masking_filter_B1.SetMaterial(['Background'], False)
I tried plenty of combinations for the input arguments but not able to figured it out.
Thanks for your help in advance.

ents = model.AllEntities()
ent1 = 'Heart_muscle'
ent2 = 'Heart_lumen'
ent3 = 'Coil'
ent4 = 'RF Shield'
ent5 = 'Background'

# Adding a new FieldMaskingFilter for B1
inputs = [em_multi_port_sensor_combiner.Outputs["B1(x,y,z,f0)"]]
field_masking_filter_B1 = analysis.core.FieldMaskingFilter(inputs=inputs)
field_masking_filter_B1.Name = "B1-Mask"
field_masking_filter_B1.SetAllMaterials(False)
# field_masking_filter_B1.SetMaterial(ents['Heart_lumen'], False)
field_masking_filter_B1.SetEntities([ents[ent1]])
field_masking_filter_B1.SetEntities([ents[ent2]])
field_masking_filter_B1.UpdateAttributes()
field_masking_filter_B1.Update()
document.AllAlgorithms.Add(field_masking_filter_B1)
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