14 Medical Applications of Magnetic Nanoparticles
331
H 0 , to study, thanks to the hyperfine interactions, the spin dynamics experienced by
the nuclei depending on the environment surrounding them.
Although NMR is a low-sensitivity technique, since it is necessary to collect the
signal of a huge number of nuclei (at least 10
–4 to 10
–6 mol) to detect a macroscopic
nuclear signal (in a common field of 1.41 T used in MRI applications only 1 nucleus
over 40,000 is aligned to H 0 and contributes to the generation of the NMR signal),
clinical MRI is generally exploited by collecting the signal of hydrogen (
1 H) nuclei,
which are characterized by a strong gyromagnetic ratio (γ = 42.5756 MHz/T) and
a high concentration water-rich soft tissues. This allows MRI images to have a very
good space resolution, a property that merged with the MRI capability of investigating
internal organs non-invasively, has made this technique one of the most powerful and
intriguing modalities to perform in vivo imaging both in clinics and in the biological
research field [28].
The parameters measured in a NMR experiment, once the nuclei are driven out of
thermal equilibrium by radiofrequency pulses, are the absorption spectrum, obtained
from the Fourier Transform of the NMR signal, and the relaxation times T 1 and T 2
(nuclear spin–lattice and spin–spin relaxation times, respectively). T 1 measures the
interval for the recovery of the thermal equilibrium of the longitudinal component
of the nuclear magnetization, governed by the interactions between the spins and
the lattice (i.e., the surrounding environment). T 2 describes the vanishing of transverse components of the nuclear magnetization, driven by the interactions among the
nuclear spins. However, other factors besides the atomic and molecular mechanisms
here cited, as the inhomogeneities of the field H 0 , also contribute to the decay of the
transverse magnetization in real NMR experiments, leading to the measurement of
an “effective” T 2 * always faster than the “real” T 2 .
The possibility to produce a MR image of the body comes from the peculiar
characteristics of each human tissue. Soft tissues as fat or muscle, hard tissues as
bones, liquid tissues as blood or cerebrospinal fluid, are all characterized by different
amount of
1 H nuclei (in most cases mainly belonging to water) and different T 1 and
T 2 relaxation times, which allow to collect a pixel-by-pixel signal subsequently
associated to a gray scale (i.e. to an image contrast). In particular, the NMR signal
acquired in an MRI experiment can be expressed as:
S(t) ∝ ρ(
1 H) e
−TE/T 2
1 − e
−TR/T 1
e
−bD
(14.1)
where ρ(
1 H) is the proton density within the analyzed object, TE (echo time) and TR
(repetition time) are characteristic parameters of the sequence which can be adjusted
by the operator, and the last term accounts for diffusive phenomena within the object,
with D the diffusion coefficient of the medium. Choosing different values of TE and
TR compared to the ones of the relaxation times, it is possible to obtain MR images
that are proton density-weighted, T 1 -weighted orT 2 -weighted, thus changing the
image contrast among tissues for obtaining different diagnostic information [29].
In a classic NMR experiment, the signal is collected from all the resonant nuclei
of the system in the presence of the static homogeneous magnetic field H 0 , thus
331
H 0 , to study, thanks to the hyperfine interactions, the spin dynamics experienced by
the nuclei depending on the environment surrounding them.
Although NMR is a low-sensitivity technique, since it is necessary to collect the
signal of a huge number of nuclei (at least 10
–4 to 10
–6 mol) to detect a macroscopic
nuclear signal (in a common field of 1.41 T used in MRI applications only 1 nucleus
over 40,000 is aligned to H 0 and contributes to the generation of the NMR signal),
clinical MRI is generally exploited by collecting the signal of hydrogen (
1 H) nuclei,
which are characterized by a strong gyromagnetic ratio (γ = 42.5756 MHz/T) and
a high concentration water-rich soft tissues. This allows MRI images to have a very
good space resolution, a property that merged with the MRI capability of investigating
internal organs non-invasively, has made this technique one of the most powerful and
intriguing modalities to perform in vivo imaging both in clinics and in the biological
research field [28].
The parameters measured in a NMR experiment, once the nuclei are driven out of
thermal equilibrium by radiofrequency pulses, are the absorption spectrum, obtained
from the Fourier Transform of the NMR signal, and the relaxation times T 1 and T 2
(nuclear spin–lattice and spin–spin relaxation times, respectively). T 1 measures the
interval for the recovery of the thermal equilibrium of the longitudinal component
of the nuclear magnetization, governed by the interactions between the spins and
the lattice (i.e., the surrounding environment). T 2 describes the vanishing of transverse components of the nuclear magnetization, driven by the interactions among the
nuclear spins. However, other factors besides the atomic and molecular mechanisms
here cited, as the inhomogeneities of the field H 0 , also contribute to the decay of the
transverse magnetization in real NMR experiments, leading to the measurement of
an “effective” T 2 * always faster than the “real” T 2 .
The possibility to produce a MR image of the body comes from the peculiar
characteristics of each human tissue. Soft tissues as fat or muscle, hard tissues as
bones, liquid tissues as blood or cerebrospinal fluid, are all characterized by different
amount of
1 H nuclei (in most cases mainly belonging to water) and different T 1 and
T 2 relaxation times, which allow to collect a pixel-by-pixel signal subsequently
associated to a gray scale (i.e. to an image contrast). In particular, the NMR signal
acquired in an MRI experiment can be expressed as:
S(t) ∝ ρ(
1 H) e
−TE/T 2
1 − e
−TR/T 1
e
−bD
(14.1)
where ρ(
1 H) is the proton density within the analyzed object, TE (echo time) and TR
(repetition time) are characteristic parameters of the sequence which can be adjusted
by the operator, and the last term accounts for diffusive phenomena within the object,
with D the diffusion coefficient of the medium. Choosing different values of TE and
TR compared to the ones of the relaxation times, it is possible to obtain MR images
that are proton density-weighted, T 1 -weighted orT 2 -weighted, thus changing the
image contrast among tissues for obtaining different diagnostic information [29].
In a classic NMR experiment, the signal is collected from all the resonant nuclei
of the system in the presence of the static homogeneous magnetic field H 0 , thus