Chemistry Reference
In-Depth Information
SPECT is similar to PET in terms of using radiotracer materials.
However, SPECT is based on the detection of γ-emitting isotopes
directly, such as
I, whereas PET tracer
emits positrons [30]. SPECT costs less than PET scan due to longer
half-lives and more easily obtained radiotracers. However, SPECT
generates images of lower resolution [24, 30, 31].
99m
Tc,
111
In,
123
I,
125
I, and
131
5.1.2
Magnetic Resonance Imaging
MRI is popular for diagnostic and drug development due to its greater
soft tissue contrast, higher spatial resolution (as good as tens of
micrometers for preclinical studies and ~1 mm for clinical studies)
and good penetration
depth [22, 32]. It does not involve the use of
ionizing radiation, which is used in X-ray and CT scan. However, low
sensitivity for the detection of targeted agents hampers its usage as
compared to PET and SPECT. Owing to the recent development of
powerful, innovative MRI contrast agents, it is now more popular as
a noninvasive imaging modality.
MRI uses a powerful magnetic field to align the nuclear
magnetization of hydrogen atoms in water protons. When the external
magnetic field is applied to these protons, which are oriented in
random directions, they become aligned with the external magnetic
field. They can either align along or opposite to the magnetic field.
The oriented spins remain constant and form net magnetization.
The magnetization can be disturbed from its equilibrium state using
a radio frequency pulse. When the pulse is removed from the system,
the protons realign with the equilibrium states, and MRI machines
measure these relaxation time [33]. There are two types of relaxation
time depending on different pulses used, namely,
(longitudinal
relaxation, the time taken for the protons to realign with the external
magnetic field) and
T
1
(transverse relaxation, the time taken for the
protons to exchange energy with other nuclei).
Based on the relaxation times, contrast agents can be categorized
into
T
2
contrast agents usually include
gadolinium-based chelates or nanoparticles [34] and manganese-
based chelates or nanoparticles [35]. These contrast agents have
unpaired electrons, which give them paramagnetic properties.
T
and
T
contrast agents.
T
1
2
1
T
2
contrast agents include superparamagnetic iron oxide nanoparticles
and rare earth ions such as Dy(III) [21, 36-39].
Several factors affect the quality of MRI contrast agents: water
exchange rate (also called inner-sphere effects), rotational correlation
