Like the top of your skull, the boney bits on your spine can be less than a mm from the air. Especially if you're a woman or are elderly or are quite sick and r

Like the top of your skull, the boney bits on your spine can be less than a mm from the air. Especially if you're a woman or are elderly or are quite sick and recovering. Many things can affect skin thickness. Also, that depth is probably the FWHM of the decay curve, so it is reaching a bit deeper but at lower amplitudes. Also the scanner has some quirks of its own, so who knows really what the exact specifics of that particular scanner are on that day.