It could be used in a general sense to describe a state of reduction or lessening in a specific medical context. For example:

Bone Density:

In the field of osteoporosis or bone health, rarefaction may refer to a reduction in bone density. This can be associated with conditions where there is a loss of bone mass, leading to weakened bones.
Radiology:

Rarefaction might be used in radiology or imaging to describe a region of reduced density in a tissue or organ. For instance, rarefaction could be mentioned in the context of an ultrasound where sound waves encounter areas with less density.
Blood Vessels:

Rarefaction could potentially be used in the context of blood vessels to describe a reduction in the density of blood vessels in a certain area, which might be indicative of a vascular condition.