Page 20 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis

Chapter 1
Imaging modalities
The various medical in vivo imaging techniques each have their own advantages and disadvantages regarding their use in cellular imaging. Optical imaging techniques have been widely used in pre-clinical studies. The limited tissue penetration capability of light, however, to a large extent limits the use of these techniques to small laboratory animals (67). Studies aimed at clinical translatability, have therefore largely focused on US, MRI, PET or SPECT, where the latter three are not limited by signal penetration depths in tissue (68, 69). An overview of different imaging modalities used in preclinical research was recently given by de Jong et.al. (70), the modalities used in this thesis are explained below.
MRI
Magnetic resonance imaging (MRI) offers several advantages for in vivo cell tracking. Through the use of a strong homogenous magnetic field, gradients and radio waves, an image is generated. MRI has high temporal and spatial resolution, excellent tissue contrast and tissue penetration, does not apply ionizing radiation, is non-invasive for serial studies, and simultaneous acquisition of anatomical structure and physiological function can be obtained (71). In this high magnetic field, hydrogen atoms can be manipulated. Our body contains up to 70% of water, which provides an abundant amount of hydrogen atoms. The spin of hydrogen atoms is affected by the radiofrequency (RF) pulse. When the RF pulse is turned off, the hydrogen atoms can return to the original state. This entire process is known as “relaxation.” After positional measurement of the relaxation by receiver coils, the relaxations can be transformed into a MRI image (72). Contrast agents can enhance the relaxation rates (73). Longitudinal and transverse relaxation time (resp. T1 and T2) is being used to characterize different tissues. T1 is the time constant which determines the rate at which excited hydrogen atoms return to equilibrium. T2 is the time constant which measures the time taken for spinning hydrogen atoms to lose phase coherence among the nuclei spinning perpendicular to the main field.
US
Ultrasound imaging, like MRI and CT, is used for diagnostic imaging, but also as a therapeutic tool. Sound frequencies of 1 MHz and higher are used and are also referred to as ultrasound. The human body consists of different structures,
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