1. A method for analyzing blood vessels in the presence of intervening unwanted spectral signal, the method comprising: irradiating blood vessel walls through intervening fluid; collecting spectral responses; determining spectral responses of the blood vessel walls from the collected spectral responses by applying a filter generated using a spectral response of the intervening fluid to remove or deemphasize a contribution of the intervening fluid relative to a signal from the blood vessel walls; and assessing states of the blood vessel walls in response to the determined spectral responses of the blood vessel walls.