336
M. Avolio et al.
(red arrow), which partially exceed the theoretical limit. Indeed, these estimates were
obtained by theoretical modeling and have not been experimentally verified on human
beings yet. In the absence of a thorough clinical study on safety limits and regulations
about human body exposure to AMF, the human tolerance limit routinely accepted
by the scientific and physician communities, concerns a constraint on the maximum
value of the product H 0 ν which is recommended not to exceed 4.84 × 10
8 Am
−1 s
−1
[35] or 5 × 10
9 Am
−1 s
−1 [36] for total or partial body exposure, respectively. To
better understand the need of a reformulation of this issue, we would like to remind
that the first of this threshold values, also known as Brezovich condition, which is the
most popular and cited, dates by 1988, and was empirically established by recording
the subjective discomfort of a few adults directly exposed to an increasing AMF.
14.4 Design of MNPs as Contrast Agents and Heat
Mediators
In common clinical applications of MRI, the static field μ 0 H 0 has a maximum value
of 1.5 T. Lowest values of μ 0 H 0 , around 0.2–0.5 T, are sometimes applied for the
imaging of the joints of the body. Higher values of μ 0 H 0 , and particularly systems
working from 3 to 11 T, would allow to reach outstanding values of contrast, spatial
resolution and signal-to-noise ratio in the final image. However, these systems are
nowadays being tested only for research purposes, since the application of such strong
fields on the human body is susceptible to (i) high values of SAR, (ii) low RF field
penetration through the body at high resonant frequencies, and (iii) other potentially
dangerous side effects, which need to be carefully evaluated before moving to the
clinical practice [19–21]. The amplitude of the magnetic field gradients is instead
much weaker, since for clinical applications it reaches a maximum value of about
45 mT/m.
To increase the contrast of the MR image and, especially, to better define the
borders of a lesion are the main goals of MRI in clinics. For this purpose, magnetic
CAs are administered to the patient and targeted to the organ or tissue of interest.
Here, they generally shorten the characteristic relaxation times of the tissue, allowing
its better detection in the MR image [37]. Molecules that mainly shorten T 1 are called
positive CAs, since according to (14.1) they increase the brightness of the interested
region. On the contrary, negative CAs have the property of shortening mainly T 2 ,
and consequently to darken a portion of the image.
The efficiency of a CA is quantified through the longitudinal (r 1 ) or transversal
(r 2 ) relaxivity, which is defined as
r i =
1
C
1
T i,obs
−
1
T i,diam
, i = 1, 2, . . .
(14.9)
M. Avolio et al.
(red arrow), which partially exceed the theoretical limit. Indeed, these estimates were
obtained by theoretical modeling and have not been experimentally verified on human
beings yet. In the absence of a thorough clinical study on safety limits and regulations
about human body exposure to AMF, the human tolerance limit routinely accepted
by the scientific and physician communities, concerns a constraint on the maximum
value of the product H 0 ν which is recommended not to exceed 4.84 × 10
8 Am
−1 s
−1
[35] or 5 × 10
9 Am
−1 s
−1 [36] for total or partial body exposure, respectively. To
better understand the need of a reformulation of this issue, we would like to remind
that the first of this threshold values, also known as Brezovich condition, which is the
most popular and cited, dates by 1988, and was empirically established by recording
the subjective discomfort of a few adults directly exposed to an increasing AMF.
14.4 Design of MNPs as Contrast Agents and Heat
Mediators
In common clinical applications of MRI, the static field μ 0 H 0 has a maximum value
of 1.5 T. Lowest values of μ 0 H 0 , around 0.2–0.5 T, are sometimes applied for the
imaging of the joints of the body. Higher values of μ 0 H 0 , and particularly systems
working from 3 to 11 T, would allow to reach outstanding values of contrast, spatial
resolution and signal-to-noise ratio in the final image. However, these systems are
nowadays being tested only for research purposes, since the application of such strong
fields on the human body is susceptible to (i) high values of SAR, (ii) low RF field
penetration through the body at high resonant frequencies, and (iii) other potentially
dangerous side effects, which need to be carefully evaluated before moving to the
clinical practice [19–21]. The amplitude of the magnetic field gradients is instead
much weaker, since for clinical applications it reaches a maximum value of about
45 mT/m.
To increase the contrast of the MR image and, especially, to better define the
borders of a lesion are the main goals of MRI in clinics. For this purpose, magnetic
CAs are administered to the patient and targeted to the organ or tissue of interest.
Here, they generally shorten the characteristic relaxation times of the tissue, allowing
its better detection in the MR image [37]. Molecules that mainly shorten T 1 are called
positive CAs, since according to (14.1) they increase the brightness of the interested
region. On the contrary, negative CAs have the property of shortening mainly T 2 ,
and consequently to darken a portion of the image.
The efficiency of a CA is quantified through the longitudinal (r 1 ) or transversal
(r 2 ) relaxivity, which is defined as
r i =
1
C
1
T i,obs
−
1
T i,diam
, i = 1, 2, . . .
(14.9)