Research On The Cerenkov Luminescence Tomography Of Small Living Animals

In recent years, Cerenkov luminescence tomography （CLT） widely raised attentionof many scholars because it can reflect the three-dimensional （3D） distribution of targetmolecule in the small living animals, accurately perform the quantification, and havehigher spatial resolution. CLT has many advantages, such as short imaging time, highsensitivity, high throughput, and the multimodality imaging comprised of optical andnuclear imaging technology using a radionuclide tracer, and has a good applicationprospects in biomedical imaging. Cerenkov luminescence tomography can be appliedfor clinical optical imaging by use of the existing radionuclide-labeled probes andovercome the deficiencies of clinical application for the existing optical imagingtechniques because of the toxicity of the optical molecular probes. Therefore，we canassume that a single radionuclide-labeled molecular probe can be used for the patientswith tumor diagnosis, staging and guide treatment, and surgery-oriented resection, themonitoring and evaluation of all aspects of tumor diagnosis and treatment. Thisdissertation is mainly concerned with the reconstruction method in Cerenkovluminescence tomography of small living animals and in vivo biological applications ofthe CLT. The author’s major contributions are outlined as follows:1. The characteristics of Cerenkov luminescence have been investigated. Firstly,through the in vitro Cerenkov luminescence imaging （CLI） experiment of Na¹³¹I withvarious activities, we studied the correlation between the signal intensity of Cerenkovluminescence emitted from Na¹³¹I and the activities. Secondly, we performed the γimaging of Na¹³¹I with various activities and analyzed the relationship between thenuclear signal and the Cerenkov luminescent signal. Additionally, we acquired theCerenkov luminescence emitted from Na¹³¹I using a set of filters and studied thecharacteristics of Cerenkov luminescence spectrum. Lastly, we investigated thepenetration of Cerenkov luminescence using tissue-mimicking phantom and variousporcine tissues and its influencing factors.2.3D CLT using a heterogeneous mouse model with an implanted Na¹³¹Iradioactive source has been performed and a single photon emission computedtomography （SPECT） imaging validation strategy to verify the results of CLT has beenproposed. Firstly，the four planar luminescent images of the mouse with an implantedNa¹³¹I radioactive source were acquired. A heterogeneous mouse model was thenestablished through the segmentation of the mouse organs on the CT images. Using theadaptive hp-finite element method （hp-FEM） based on diffusion equation （DE）, the3D distribution of the embedded Na¹³¹I radioactive source in the mouse was reconstructedand compared with the real source position obtained from the CT images. Experimentalresults demonstrated that the reconstruction results based on the heterogeneous mousemodel were more accurate in localization than the homogeneous one. Furthermore,SPECT was utilized to verify the results of3D CLT of the mice implanted with theNa¹³¹I radioactive source with various activities. The experimental data showed theability of in vivo CLT to recover the radioactive probe distribution in the heterogeneousmouse model and the potential of a SPECT imaging validation strategy to verify theresults of optical molecular tomography.3. Considering the limitation of the surface flux distribution （SFD）-guidedreconstruction method, in which the permissible source region （PSR） was determinedby the surface flux distribution, in this paper, we presented a SPECT-guidedreconstruction method for CLT, in which a priori information of the PSR from SPECTimaging results was incorporated. Firstly, we constructed an implantation mouse modelwith a Na¹³¹I radioactive source and an physiological mouse model received anintravenous tail injection of Na¹³¹I. The performance of the method was then validatedwith the experimental reconstruction of the two mouse models. Next, a tissue-mimicphantom based experiment was then conducted to illustrate the ability of the proposedmethod in resolving double sources. Compared with the traditional PSR strategy inwhich the PSR was determined by the surface flux distribution, the proposed methodobtained much more accurate and encouraging localization and resolution results andignored the heterogeneity of tissues which can avoid the segmentation procedure of theorgans.4. To efficiently reconstruct the distribution of radionuclide tracer in the livinganimals, a semi-quantitative Cerenkov radiation spectral characteristic-based sourcereconstruction method named the hybrid spectral CLT was presented. On the basis ofthe spectral distribution characteristics of Na¹³¹I through the in vitro experiment, thehybrid optical parameters of mouse organs were computed. The hybrid opticalparameters were employed in the system matrix in the linear relationship between theCerenkov luminescent source and the measured Cerenkov signal on the body surface.Using a single luminescent image acquired without using any filters, the distributions ofboth embedded Na¹³¹I radioactive source in the living mouse and Iodine-131（I-131）uptake in the mouse bladder were reconstructed based on the adaptive hp strategy.Furthermore, the activity of reconstructed Na¹³¹I radioactive source and I-131uptake in the bladder was calculated according to the linear relationship between thereconstructed source energy and activity. Additionally, we compared the reconstructionresults, acquisition and image reconstruction time with that of single-spectral andmultispectral CLT. Integrating the reconstruction results and required time, the hybridspectral CLT showed better performance than the two other methods and had morepractical application values.5.3D noninvasive monitoring of I-131uptake in the thyroid and quantified I-131uptake in vivo using hybrid spectral CLT.3D visualization of longitudinal observationssuggested that the reconstructed energy of I-131uptake in the thyroid increased withacquisition time and there was a robust correlation between the reconstructed energyversus the gamma ray counts of I-131. The ex vivo biodistribution experiment furtherconfirmed the I-131uptake in the thyroid for hybrid spectral CLT. Experimental resultsindicated that hybrid spectral CLT could be potentially used for thyroid imaging toevaluate its function and monitor its treatment for thyroid cancer.6. This study tried to show that aminopeptidase N （APN/CD13） expression can beimaged and quantified with novel CLT. Firstly，an APN/CD13-targeted Asn-Gly-Arg（NGR）-peptide was labelled with the radionuclide131I. Next，through the in vitro CLIexperiment of131I-NGR uptake in the cells, the correlation between the Cerenkovoptical signal intensity versus activity and the HT1080cell numbers was investigated.Lastly, nude mice bearing HT-1080tumors were constructed and then imaged afterinjection with131I-NGR using both planar and tomographic CLI methods. The3Dvisualization CLT results clearly showed that131I-NGR uptake in tumor tissuesrepresented a high expression of the APN/CD13receptor. CLT also allowed quantifying131I-NGR uptake in tumor tissues showing an average activity of0.1388±4.6788E-6MBq in tumor tissues. Our study indicated that131I-NGR combined with CLT allowedus to image and quantify tumor-associated APN/CD13expression noninvasively. Thepromising CLT technique could be potentially used for sensitively evaluating tumorangiogenesis in vivo and has the potential of tumor early detection.