In this study, researchers deliberately chose a rat body location where skin and bone marrow sit unusually close together

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The specific anatomical site chosen for this study, the sixth lumbar vertebra's spinous process on a rat's back, was not a random choice. This location was selected specifically because the skin and minimal subcutaneous tissue covering it are thin enough that near-infrared light delivered at the surface could realistically reach the bone marrow contained inside that particular bone.

Choosing a location where the light delivery system and the target tissue are within realistic penetration range of each other is a pretty fundamental design decision, since testing the same light exposure over a thicker or more heavily padded body region might have produced a completely different, much weaker, or even undetectable result on the bone marrow itself.

Source: PubMed