“9.61x6.69” b2815 Tissue Engineering and Nanotheranostics
Magnetic Nanohybrids for Magnetic Resonance Imaging 129
abnormalities of the brain and spinal cord, heart problems,
fibroids, endometriosis, uterine abnormalities and cancerous
tumors 97–100
In 1946, the magnetic resonance (MR) phenomenon was firstly
discovered by Purcell and Bloch, but the first image was produced
in 1973.^99 Meanwhile, Damadian confirmed in 1971 that certain
pathogenic and natural tissues had different proton relaxation
times. Finally, in 1977, Damadian, Minkoff and Goldsmith pro-
duced the first human scan using this technology. Initially, the scan
took 5 h to complete, but the imaging time has been reduced in the
portable MRI.
MR is a basic phenomena in MR imaging, in which nuclei of the
atom like^1 H,^13 C and^31 P are emitting or absorbing some particular
amount of energy from external magnetic fields. Then, magnetic
moment will be produced by these specific nuclei which makes them
MR active. Hydrogen nuclei^1 H is the most frequently used nuclei
for imaging in biological atmospheres because human body has
water and fat containing hydrogen atoms.
Initially, MRI was developed to present molecular imaging non-
invasively. The major problem of MRI in definitive diagnoses is its
lower sensitivity compared with nuclear imaging. However, it was
observed that the development of contrast agents has facilitated MRI
to become the most sensitive imaging technique not only in clinical
diagnosis but also a useful instrument for biomedical research.101,102
Contrast agents enhance sensitivity and/or specificity due to the
shortened relaxation time of protons. Therefore, MR signal intensity
is based on the local value of transverse (1/T 2 ) or longitudinal (1/T 1 )
relaxation rate of water protons. The relaxivities of r1 and r2, which
are generally measured in mM–1s–1, demonstrate the increase in 1/T 1
and 1/T 2 per concentration (M) of contrast agent, respectively.
Therefore, researchers have been getting attracted to this area and
trying to develop promising clinically applicable contrast agents. This
part will focus on some recent important examples of magnetic nano-
hybrids for multifunctionality such as dual MR-imaging, MRI with
photodynamic therapy and MRI contract agents coupled with photo-
thermal therapy.^103
