Recently, planar fluorescence imaging has been extensively applied for in vivo
small animal imaging. At the same time, molecular imaging with fluorescence techniques has emerged in personalized medicine applications [32
]. Nevertheless, imaging the distribution of the fluorophores located deep in a highly scattering medium with high quantitative accuracy still requires tomographic imaging techniques rather than projection imaging. Indeed, even though a lesion may be detected in the reconstructed image, the true location, size, and most importantly, the true concentration of the fluorophore cannot be recovered unless proper a priori
information is available [31
]. A system that can reveal quantitatively accurate fluorophore concentration as an image will have a great potential in a number of applications covering a broad range from stem cell to cancer research.
Until now, a number of groups have explored the improvement in the FT reconstruction when the background optical property is obtained from the DOT measurement. Hervé et al.
have shown that the fluorophore concentration and location can be recovered more accurately by correcting the attenuation heterogeneity a priori
to the fluorescence reconstruction [26
]. Furthermore, Tan and Jiang also demonstrated that DOT-guided FT can recover the fluorophore concentration more accurately [25
]. However, as presented in Cases 2 and 3, even though the background heterogeneity is corrected, a small inclusion deeply embedded inside the medium cannot be recovered accurately. On the other hand, a few previous studies have also used the structural a priori
information to improve FT reconstruction using phantoms with homogeneous background [24
]. However, as we presented in Cases 2 and 3, the structural a priori
information alone is not enough to recover quantitatively accurate fluorophore concentration images if the background is heterogeneous. In this study, we confirm that both functional and structural a priori
information are crucial to recover the fluorophore concentration accurately especially for a small inclusion embedded in a heterogeneous background. For example, when the functional a priori
information is applied alone, the reconstructed ICG concentration map is substantially improved particularly for Cases 2 and 3. As illustrated in the , the recovered ICG concentration in the inclusion improved approximately four and ten times for Cases 2 and 3, respectively. Nevertheless, the error in the recovered ICG concentration in the inclusion is still very high, around 80% for both cases. On the other hand, applying the structural a priori
alone makes a substantial impact only for Case 2, in which the error in the recovered ICG concentration is reduced to 28%. Please note that the background for the third case is more complex than the other two cases. Accordingly, these results reiterate that the structural a priori
information alone would not be enough for small inclusions located in a highly heterogeneous background. In any case, only when both functional and structural a priori
information are applied during FT reconstruction can the fluorophore concentration be obtained within a 2% error for both cases. Indeed, our phantom experiment results are highly consistent with the previous simulation results [24
Most previous studies have been targeting mouse imaging, and therefore noncontact FT systems were generally optimized and validated for small-sized objects [10
]. For instance, Hervé et al.
used a 15 mm thick slab phantom [26
]. Likewise, Koenig et al.
utilized an imaging chamber with 15 mm thickness [37
]. In this study, we applied 41 mm diameter cylindrical phantoms, which resemble larger animal models such as rat. We also believe that these experimental studies with larger phantoms will pave the way for translation of the FT technique for clinical applications such as breast cancer imaging or brain imaging.
In conclusion, we have built a noncontact CCD-based multiprojectional FT system that could obtain both FT and DOT measurements congruently in the same settings. We evaluated the importance of both functional and structural a priori
information using multimodality phantoms with different compartments representing heterogeneous optical background. In practice, the functional a priori
information can be obtained by acquiring additional DOT measurements prior to the FT measurement. In our study, homogeneous scattering coefficient distribution was assumed. However, in reality the scattering coefficient can also be very heterogeneous. In order to effectively separate the absorption and scattering coefficients during the reconstruction, time-dependent DOT measurements are required. This can be achieved by using a frequency-domain or time-domain DOT system. The structural a priori
information, on the other hand, can be obtained from a high resolution anatomical imaging modality such as MRI, as demonstrated in this paper. It should be noted that other modalities may also be used to obtain structural a priori
information. A multimodality imaging system that can acquire both structural and functional a priori
information in the same setting would be preferable due to its superior coregistration potential. Several examples of hybrid systems such as DOT MRI and positron-emission tomography MRI have recently emerged as truly integrated multimodality systems [17
]. In essence, our preliminary phantom studies have validated that a system that can acquire DOT, FT, and anatomical images in the same setting would be very promising for quantitative in vivo