A photoacoustic tomography (hereafter may be abbreviated to PAT) apparatus has been known as one of apparatuses to visualize the information of the inside of a living body. In a measurement by using the PAT apparatus, an arithmetic image of the matter distribution in the inside of a subject to be measured can be obtained by measuring the intensity and the production time of photoacoustic signals produced by a substance (light absorber), which has absorbed light, in the inside of the subject to be measured when the subject to be measured is irradiated with the light.
Here, any substance can be used as the light absorber insofar as the substance produces an acoustic wave because of absorption of the light in a living body. For example, it is possible that a blood vessel, a malignant tumor, or the like in a human body is employed as a light absorber. In addition, it is also possible that molecules of Indocyanine Green (hereafter may be abbreviated to ICG) and the like are administered to a body and are utilized as contrast media. ICG can be favorably used as a contrast medium in the PAT apparatus because of a small irradiation influence on a human body and high absorption of light, which is in the near-infrared wavelength region and which has a high permeability into a living body. Meanwhile, in the present specification, ICG refers to a compound represented by the following structure.

In this regard, the counter ion is not necessarily Na+, but any counter ion, e.g., H+ or K+, may be employed.
However, it has been known that the half-life of ICG in blood is about several minutes and, therefore, is very short.
NPL 1 reports an example in which photoacoustic imaging of cerebral blood vessel of a rat has been performed by using ICG alone. According to this report, the photoacoustic signal is reduced to the same level as that of the blood several tens of minutes after ICG have been administered to the blood alone and, therefore, it is indicated that the administered substance disappears from the blood promptly after administration.
As described above, ICG disappears from the blood several tens of minutes after being administered to the blood alone and it is considered that the degree of accumulation into a tumor is low when some time has passed after administration.