Biomedical Engineering Reference
In-Depth Information
strong T
2
-effect. Relaxivity values of 0.311 and 71.3 s
− 1
m
M
− 1
were reported at
60 MHz for
r
1
and
r
2
, respectively. The corresponding
r
2
/
r
1
ratio was 229, far greater
than in the commercially available nanoparticulate agents.
Morales and coworkers [75] presented their results of studies with dextran-
stabilized iron oxide nanoparticle cluster suspensions, where the primary particle
core (produced by the laser-induced pyrolysis of iron pentacarbonyl vapors) was
varied in size. The clusters were typically 50-100 nm in size, but the strongest
determinant of an increased
r
2
/
r
1
ratio was found to be the primary particle size.
It was suggested that this was due to an increased saturation magnetization of the
cores, which manifested as a higher global moment of the cluster, and resulted in
a greater fi eld gradient which the diffusing water molecules could pass through;
in other words, the relaxation was due to outer sphere effects.
4.5
Application of Magnetic Nanomaterials in MRI
Magnetic nanomaterials have numerous current and potential clinical applications
as MRI contrast agents, and some of these will be considered below. In contrast,
nonclinical applications of superparamagnetic nanoparticles are currently very
limited in number, with most involving the biosensing [184, 185] and imaging of
proteins or peptide assemblies [186, 187]. Such applications will not be discussed
in this chapter.
4.5.1
Current Clinical Applications
At present, only limited SPIO- and USPIO-type magnetic nanoparticle-based con-
trast agents have been used in human clinical applications. Selected nanoparticu-
late agents, which have been approved for clinical applications or clinically tested
are listed in Table 4.1.
Clinical applications for MRI of the liver, spleen, lymph nodes, bone marrow,
kidneys and atherosclerotic plaques are based on accumulation of nanoparticles
in the RES cells (e.g., macrophages); that is, an indication of the extent of cell
labeling.
4.5.1.1
Gastrointestinal Tract and Bowel Imaging
MRI with fast-imaging sequences, which provide the ability to acquire motion-free
T
1
- and T
2
-weighted images of static fl uids, have opened up new developments in
imaging and diagnostics of the gastrointestinal tract (GIT) [188]. Overall, MRI can
successfully compete with computed tomography (CT) in both GIT and bowel
imaging [189]. According to earlier reports, oral contrast-enhanced CT has a higher
sensitivity (83%) than MRI, using superparamagnetic iron oxide particles (67%)
in the detection of GIT pathologies. However, the specifi city for CT was only 68%
compared to 89% for MRI. Thus, SPIO MRI is more specifi c than CT [190].
