In the subsequent sections we will discuss the role of conventional and molecular imaging in the context of DDD. Before digging into imaging applications we will briefly discuss three important modalities, magnetic resonance imaging (MRI), fluorescence imaging, and PET. Figure 7.2 schematically describes the source of signals, the physical principle leading to image generation, the influence of the (biological) environment on the signals detected, the principle of spatial encoding, and the spatial resolution provided for the respective modality.
Source of signal
Nuclei with magnetic moment, i.e., odd number of protons and/or neutrons
Nuclear magnets align along magnetic field B0. Application of radiofrequency (rf) pulse generates transverse magnetization Mxy, the MRI signal
Fluorescent compounds bioluminescent compounds
Metastable positron emitting radionuclide
Absorption of photon generates excited state S1; from which system relaxes to the ground-state S0 by emission of a photon of lower energy (fluorescence)
Positron (e+) emitted by radio nucleid is anihiliated through interaction with electron (e-), generating two y-photons traveling in opposite direction.
Influence of environment on signal
Relaxation times: T-,, T2, T 2, T1p water diffusion: ADC water exchange rates: kex
Fluorescence quantum yield fluorescence lifetime absorption scattering
Frequency encoding of spatial information
Requires solution of inverse problem of electrodynamics: diffuse photon propagation
Electronic collimation/ coincidence detection: anihiliation event has occured on line of response (LoR)
FIGURE 7.2 Features of imaging modalities MRI, fluorescence imaging, and PET. MR images represent the weighted distribution of tissue protons, predominantly those of water and adipose tissue. The signal is weighted by parameters such as the relaxation times, diffusion properties, or proton exchange rate, which depend on the local environment. Spatial encoding is achieved by applying magnetic field gradients: as the resonance frequency depends on the local magnetic field, the spatial information is directly encoded in frequency information. Spatial resolution in animal imaging is of the order of 100 |lm. Fluorescence imaging measures the distribution of fluorescent molecules (dyes, quantum dots, or fluorescent proteins). As light is heavily scattered by tissue, photon propagates as a diffusion wave. In order to determine the localization and intensity of a fluorescent source the inverse problem has to be solved by iterative procedures. Spatial resolution is of the order of 1 mm. PET measures the distribution of radionuclides that decay by emitting a positron. As antimatter particles, positrons are captured by electrons after traveling through tissue for a short distance (positron range), the annihilation process generating two y-photons traveling in opposite direction. The detector system consists of a ring of scintillation crystals. Coincidence detection of the two photons allows determining a line of response (LOR), on which the annihilation process (not the radionuclide decay) has occurred. Measuring a sufficient number of LORs allows reconstructing the PET image.
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