NANOPARTICLE IMAGINGScintigraphic imaging, also known as gamma photon imaging, is a noninvasive imaging technology that involves the detection of gamma photons emitted from radioactive molecules. It is commonly used in clinical patients following administration of a specific radionuclide-labeled molecule or particle for diagnostic purposes. It has a very high sensitivity for detecting radiolabeled molecules in the body that is not possible with standard X-ray, X-ray computed tomography (CT) imaging, or magnetic resonance imaging (MRI). These high sensitivities of detection in the human body are possible because the relative high energies of the gamma photons can penetrate completely through almost any depth of human tissue and exit from the body to be detected by specialized detectors outside the patient. Because such small quantities of radionuclides can be detected (<0.01-1 nanogram of actual matter), scintigraphic imaging is ideally suited for the imaging of physiologic alterations in living animals
of all sizes. Technological advances over the last 30 years have resulted in great improvements in the resolution and sensitivity of cameras used for scintigraphic imaging [94, 136, 143]. Recently, dedicated imaging systems have been developed for small animal imaging. In addition, dual imaging systems that acquire anatomical CT images along with the scintigraphic images have been developed for both clinical and small animal imaging systems [26, 79, 144]. 4.3 TYPES OF SCINTIGRAPHIC IMAGINGThere are two types of scintigraphic imaging, which are based on either single gamma photon detection or dual-annihilation photon detection. 4.3.1 Single-Photon Imaging Single-photon imaging requires lead collimation, which is located in front of a detector to localize the source of the gamma photons. Single-photon imaging can be either a two-dimensional planar projection imaging or a tomographic three-dimensional imaging known as single-photon emission computed tomography (SPECT) imaging. Commonly used single-photon-emitting radionuclides for nanoparticle imaging are technetium-99m (99mTc) with a 6 hour half-life, indium-111 (111In) with a half-life of 67 hours, and gallium-67 (67Ga) with a half-life of 78 hours. These radionuclides are all widely available from local clinical radiopharmacies. 4.3.2 PET Imaging A second type of imaging known as positron emission tomography (PET) imaging is based on the detection of two annihilation photons that are emitted simultaneously at approximately 180° from each other. These annihilation photons are generated following an annihilation reaction between an emitted positron from the positron-emitting radionuclide and a shell electron in the region of the emitted positron. These dual-photon emissions can be localized along a line by the simultaneous detection of the two 180° emitted photons by crystal scintillation detectors that are part of a ring of detectors surrounding the body. Multiple emissions can be iteratively localized in the body and provide a three-dimensional tomographic image of the distribution of the positron radionuclide. Collimators are not required for this type of imaging, although fairly thick crystals are required to detect the high-energy 511 keV photons that are emitted from all positron emission radioisotopes. PET imaging is always acquired for display of tomographic images, and planar projection imaging is not generally feasible. The most commonly used radionuclides for PET imaging have a relatively short half-life, such as oxygen-15 (2 minute half-life), carbon-11 (20 minute half-life), and fluorine-18 (110 minute half-life). Although some work has been done with shorter-
lived PET agents and liposome imaging [6, 137], for most nanoparticle imaging applications these PET isotopes have too short a half-life to track the nanoparticle in vivo. PET radionuclides with longer half-lives, such as copper-64 (64Cu) (12.7 hour half-life) and iodine-124 (124I) (4.2 day half-life), are more promising for tracking the distribution of long circulating nanoparticles such as liposomes. Recently, several methods have been described for imaging liposomes with 64Cu [100, 119]. 4.3.3 Nanoparticle Scintigraphic ImagingFor either PET or single-photon imaging of nanoparticles, the nanoparticle is radiolabeled using specifically developed methodology and the distribution and uptake monitored with detectors. The emitted photon travels through body tissue and exits the body, whereupon it is detected by a crystal inside the gamma camera. Blood clearance profile of radiolabeled nanoparticles can be modified by adjusting particle sizes or attaching certain molecules, such as polyethylene glycol (PEG). Liposomes can also be molecularly targeted to specifically localize in a disease process or organ of interest [118, 134]. 4.4 METHODS OF LABELING NANOLIPOSOMES WITH
RADIONUCLIDES FOR SCINTIGRAPHIC IMAGINGCommonly used radionuclides for the radiolabeling of liposomes for imaging studies with a gamma camera are 99mTc, 111In, and 67Ga. 99mTc has the most ideal properties of the readily available single-photon-emission diagnostic imaging radionuclides due to its optimal imaging characteristics, which include an ideal photon energy of 140 keV and the fact that it is relatively inexpensive because it can be eluted daily from a commercially available molybdenum-99 (99Mo)/99mTc generator. Because 67Ga, 111In, and 123I are cyclotron products, these agents are more expensive and not always available in every nuclear medicine department. Several detailed reviews have been recently written describing the methodology for radiolabeling liposomes [48, 73, 76]. Important factors to consider in regard to liposome labeling methods are ease of preparation and in vitro and in vivo stability. Ideally, previously manufactured liposomes should be labeled just prior to the initiation of experiments. This situation is ideal because the radionuclides used in clinical imaging and research studies have half-lives of 6 hours to 3 days. The labeling of previously manufactured lipsomes is known as ‘‘after loading’’ or ‘‘remote labeling,’’ in which the preformed liposomes are labeled just prior to the start of the experiment. Also after radiolabeling, the radionuclide should stay firmly associated with the liposomes as excessive instability will obviously lead to complications in interpreting biodistribution results. Stability of the radiolabeled liposomes can be assessed in vitro by incubating the radiolabeled liposomes in serum at body temperature (37°C) and assessing stability. Serum incubation generally mirrors in vivo stability.