1.

Introduction

Recently, there has been a growing interest in investigating both the in vitro and in vivo detection of optical photons from a plethora of radioisotopes using CCD detectors. In earlier works, particular attention has been focused on the detection of Cerenkov photons from beta plus emitters;^{1, 2, 3, 4, 5} these studies have shown that the spectrum of the emitted optical photons follows the predicted 1/λ² shape, typical of the Cerenkov radiation (CR). It is well-known that the emission of CR takes place only when charged particles travel in the medium with a velocity greater than the speed of light in the medium. The energy threshold for CR emission is thus dependent on the medium and, for example, the threshold for a beta particle traveling in water is 263 keV. In the last year, the investigation of weak optical emission coming from a pure gamma emitter such as Tc-99 m was also reported.⁶

One recent study has shown the detection of an intense light emission when using an alpha emitter such as ²²⁵Ac.⁵ In this case given the mass of the alpha particles, the generation of optical photons cannot be explained in terms of Cerenkov effect and, thus, alternative explanations are needed. For example, in Ref. 5 it has been suggested that even if the origin of optical emission from ²²⁵Ac is uncertain it can be linked to CR emitted from the beta minus short-lived daughter nuclides such as ²¹³Bi. This appears to be a plausible and interesting hypothesis; however, a more general explanation of the intense light emission of ²²⁵Ac can be the combination of both Cerenkov photons generated by the beta particles emitted by daughter nuclides and fluorescence induced by alpha particles. The main goal of this proof of principle work was to investigate the latter effect in different experimental conditions.

The detection of ultraviolet fluorescence light emission from nitrogen due to interactions with alpha particles in the atmosphere was investigated, for example, in Ref. 7 and applied to nuclear safeguard issues.

In this paper we have investigated alpha particle induced fluorescence signal by using GEANT4 Monte Carlo (MC) simulations and a commercial CCD-based small animal optical imaging system. More precisely, we measured the light emission induced by a ²⁴¹Am source in different experimental conditions including the imaging of in vivo tissue. This isotope is interesting since the principal decay modes of ²⁴¹Am are mainly alpha emission and low energy gamma radiations. The decay product of ²⁴¹Am is ²³⁷Np which has a long half-life equal to 2.1 × 10⁶ years. These physical properties of ²⁴¹Am are quite useful since they allow us to exclude any possible contribution from Cerenkov light photons generated by high energy electrons and, thus, gave us the possibility to estimate the contribution of alpha particles induced fluorescence only. We believe that the results presented in this work can be useful to describe a possible alternative mechanism for the in vivo detection of alpha emitters used for therapeutic purposes.

For the sake of clarity, in order to distinguish between Cerenkov luminescence imaging and luminescence induced by non-Cerenkov mechanisms we use here the more general term of radioluminescence imaging (RLI).

2.

Materials and Methods

3.

Results

3.1.

RLI Simulation and Acquisition in Air and Under a Plexiglas Slab

Figure 1a shows a frontal RLI of the ²⁴¹Am source placed in air. Figure 1b shows the radiance profiles in the horizontal (x) and vertical (y) directions at the center of the source.

Fig. 1

(a) RLI image of the ²⁴¹Am source in air. (b) The corresponding radiance (p/s/cm²/sr) profiles in the x-y directions at the center of the source.

By looking at the profiles in Fig. 1b one can deduce that the total luminescence signal is a sum of localized intense emission in the active part of the source and a much broader signal. The broader component of the luminescence signal is about 4-cm long, matching quite well the ²⁴¹Am alpha particles range in air.

A lateral view of the air fluorescence induced by the ²⁴¹Am source is presented in Fig. 2. Figure 2a shows the light signal generated by alpha particles. The corresponding Monte Carlo simulations shown in Fig. 2b fits qualitatively well with the experimental findings. Figure 2c shows the fluorescence image obtained by stopping a fraction of alpha particles with a piece of carton as described in Sec. 2. As one can notice by comparing Fig. 2a with Fig. 2c, there is a clear reduction of the fluorescence light signal above the region covered by the carton. This is a rather simple but useful way to show that the causes of the air fluorescence were mainly alpha particles emitted by the ²⁴¹Am source.

Fig. 2

Lateral view (obtained using a mirror) of the ²⁴¹Am source in air. (a) and (b) show, respectively, experimental and MC simulated data. (c) The resulting fluoresce image obtained by using a 1 mm sheet of carton positioned 1 cm above the source. The arrows point at the area above the sheet of carton where there is a clear loss of light signal.

Figure 3a shows the luminescence after covering the alpha source with a Plexiglas slab. In this case the spread of the alpha particles emitted by the ²⁴¹Am source is negligible and the light is localized only above the active part of the source.

Fig. 3

(a) An RLI image of the ²⁴¹Am source under 2.5 mm of Plexiglas. (b) The source was covered with a 1-mm thick piece of carton.

The images obtained by covering the alpha source with a piece of carton in Fig. 3b show a negligible signal in the Plexiglas; this is clear evidence that the main cause of the light was the fluorescence of the source.

The spectra of the light obtained when placing the source in air and under a 2.5 mm Plexiglas slab are shown in Fig. 4.

Fig. 4

RLI spectrum of the ²⁴¹Am source in air (circles) and Plexiglas (square). The spectra were acquired as described in Sec. 2.1.

The spectra in Fig. 4 present different shapes and intensities depending on the materials. It is important to underline here that because the spectra contains only 6 points this cannot be considered to be a detailed precise spectral measurements, and, thus, Fig. 4 provides a rough estimate of the emission spectra of both air and Plexiglas.

3.2.

In Vivo RLI

An in vivo image of luminescence induced by an alpha particle source in a mouse is shown in Fig. 5a. It is interesting to notice that the fluorescence signal can be clearly seen inside the red ROI even if the source activity is quite modest.

Fig. 5

(a) RLI image of the ²⁴¹Am source (white arrow) placed under a control nude mouse (25 g). The spectrum of the emitted light by the animal at the center of the red ROI can be found in (b).

Another interesting aspect is that the source signal is well localized under the mouse shoulder, suggesting a reasonably precise spatial delineation of the luminescence emission region.

4.

Discussions and Conclusions

The results presented in Sec. 3 showed that the light emission of an alpha particles source can be detected using a commercial small animal optical imaging system. Such systems are becoming widely available in biology and preclinical imaging laboratories and, thus, we believe that the results presented in this work will help to expand their range of applications.

The broad component of the radiance profiles shown in Fig. 1b clearly underlines a direct link between the known alpha particles range in air and the extension of the region where the fluoresce light is emitted. The experiment performed by stopping a fraction of the alpha particles (see Fig. 2) confirms as well that the origin of the fluorescence signal are the alpha particles emitted by the ²⁴¹Am source.

The output of Monte Carlo simulations qualitatively agreed with the experimental data obtained by imaging the alpha source in the air. This is quite an interesting aspect of the work that needs to be further developed to simulate, for example, more complex conditions. In this paper the main goal of the MC simulations was to further investigate the cause of the detected fluorescence signal in air. A quantitative comparison between the measured and simulated data was not our priority and was not possible since we did not have a detailed scheme of the imaging apparatus to be implemented in the MC simulations.

Plexiglas and in vivo tissue showed a different light output response both in terms of magnitude and spectral output. As already mentioned in Sec. 3 the measured spectra using the IVIS 200 can be considered as an evaluation of the true emission spectra. A detailed spectral analysis requires the use of a dedicated spectrometer and is well beyond the scope of this paper since our main goal here was to investigate the use of a CCD-based commercial small animal optical imaging system.

In vivo investigation of alpha particles tissue interactions is shown in Sec. 3.2, in particular Fig. 5 provides a clear evidence that alpha source induces a well localized luminescence signal in a mouse tissues. Obviously the generated light photons will suffer from scattering analogously to conventional luminescence imaging, and, thus, the in vivo spatial resolution will ultimately be dependent on this effect.

In our opinion the most interesting aspect of the work presented here is the proof of concept that alpha emitters with no beta emission can be imaged in vivo using standard optical imaging methods commonly used for preclinical experiments.

Most of the alpha emitters used for therapeutic applications also emit gamma rays and, thus, they can be imaged using SPECT; however the percentage of gamma emission is rather low, typically being a few percent of the total decays. This might increase significantly the acquisition time and requires expensive dedicated hardware with respect to optical imaging.

As shown by Miederer ¹⁰ the preclinical study of biodistribution of 225Ac-DOTATOC for neuroendocrine tumor model was performed by scarifying the animals and then counting the organs with a gamma counter. Our optical imaging method could be used to image at different time points the same animal, and, thus without scarifying them. This will allow the reduction of the number of animals used for preclinical protocols and more importantly to provide a better understanding, in a truly molecular imaging sense, of the in vivo alpha emitters biodistri-bution.

This is quite an important and useful aspect because by using the RLI approach we hypothesize it will be possible to perform cost effective preclinical studies of new radiopharmaceuticals, and to infer more precisely the therapeutic outcomes by imaging directly the biodistribution of molecules labeled with alpha emitters.

Future work will be focused on this direction more precisely to develop xenograft mouse tumor models and to use the RLI approach to image the tumor progression.