## 3 Results
### Abdomen vs. Subcutaneous Tubes  
The fluorescent tubes were implanted in two locations--subcutaneously and in the abdomen. It is interesting to investigate if and how these locations affected the accuracy of estimation. The tubes implanted subcutaneously were located in a depth of less than 1 mm under skin surface, whereas the tubes implanted in the abdomen were implanted in a depth of 1-5 mm. It is worth mentioning that in some cases where the mice fur was rather thick, the mice were shaved, while in other cases the fur wasn't removed. The shaving was performed in order to reduce the attenuation caused by the thick black fur. The shaving was performed both for the subcutaneous tubes and the abdomen tubes. Since the fluorescence intensity ratio was calculated, and it is assumed that the absorption and emission spectra of the fur is almost flat in the used wavelengths, the shaving should not affect the accuracy of the results. The shaving only assisted in achieving higher fluorescence intensities, reducing the measurement time and perhaps increasing the signal to noise ratio. In addition, although the distance of the subcutaneous tubes from the mouse skin was less than the distance of the abdomen tubes, the thickness of the fur in the subcutaneous tubes was higher than in the abdomen. This is due to the fact that the fur in the leg area, which was chosen for the subcutaneous implantation, is thicker than the fur in the abdomen area. Therefore, although the energy attenuation due to the mouse tissue was rather small in the subcutaneous tubes, there was additional attenuation due to the fur. When comparing the mean error in the subcutaneous tube with the abdomen tubes, as presented in Figure 2a, we notice a slight overestimation in the abdomen tubes (4.95% \(\pm\) 31.22%) compared to a slight under-estimation in subcutaneous tubes (\(-\)3.58% \(\pm\) 22.08%). However, in our opinion this is not due to higher effective attenuation in the subcutaneous tubes, since in most subcutaneous tubes the fluorescence energy measured was higher than the abdomen tubes. Furthermore, it is noticeable in Figure 2a that when taking into account the standard deviation, there is a complete overlap between the subcutaneous and the abdomen errors. In addition, the results are based only on measurements of 4 tubes from each type. It may also be noticed in Figure 1 that both types of tubes showed overestimation in some cases and underestimation in others. Therefore, we assume that the negative mean error in the subcutaneous tubes and the positive mean error in the abdomen tubes do not necessarily imply a consistent underestimation or overestimation, correspondingly. On the other hand, when looking at the mean absolute error, as presented in Figure 2b, we do see a trend of higher absolute errors in the abdomen tubes compared to the subcutaneous tubes. The mean absolute error for the abdomen tubes was 23.13% \(\pm\) 17.14% and for the subcutaneous tubes: 16.59% \(\pm\) 11.73%. This is also noticeable in Figure 1, where the star symbols representing the subcutaneous data are all in the range of about 25% error, while two out of four circle symbols representing the abdomen data have errors of about 35%. The higher absolute errors in the fluorescence measurements taken from the abdomen tubes are not surprising and are actually expected. As mentioned earlier, the tissue overlying the fluorescent tubes is thicker for the tubes implanted in the abdomen. The thick tissue causes more absorption and scattering events thus increasing the effective attenuation. As a result, the intensity of the fluorescence signal decreases and the signal to noise ratio decreases too, resulting in higher errors. In addition, in some