Page 31 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

28 CHAPTER 2 Diagnostic imaging in cancer

2.2.2 Kinds of magnetic probes to enhance MRI contrast
In general, there are two types of MRI CA, one of the groups is related to the enhanc-
ing T -weighted MRI and the other is related to the enhancing of T - and T *-weighted
2
1
2
MRI [18]. Two main types of MRI CA are based gadolinium and iron oxide for T -
1
weighted imaged as well as T and T *-weighted image, respectively. Both catego-
2
2
ries are well-coated nanoparticles [2]. The application of nanoparticles as a CA has
been increased not only for MRI, for the other imaging systems as well. The main
advantages of using nanoparticles as a contest agent are increasing blood circulation
time, targeting to desired side of action, and increasing the CA concentration in the
targeted tissue. The most used MRI CAs is shown in Table 2.1. Gadolinium-based
CAs mainly are divided into two groups: (1) intravascular, interstitial and extracel-
lular fluid agents applied for examining lymphatic system and vessels and (2) blood
pool MRI CA used for angiography.
Since gadolinium in ionic form has undesirable cell toxicity in in vivo media,
3+
considerable efforts have been done to connect this ion with ligands. Gd chelates
possess high thermodynamic stability and high kinetic, improving the images of
interstitial spaces. Furthermore, Gd chelates cannot pass through blood brain barrier
3+
and mostly are administrated to improve the CA of blood vessels and tumors. Due to
their hydrophilic nature, Gd chelates filter by the kidney and undergo renal clear-
3+
3+
2+
ance. It should be noted that transition elements (such as Mn , Fe , and Cu ) are
2+
used as a CA for MRI due to having a large number of unpaired electrons, suitable
for longitudinal MRI imaging. So far, gadolinium-based CAs are the most used CA
for positive imaging in MRI. Based on the application, there are large variety of CAs
from ionic to nonionic and linear to nonlinear with different ratios of Gd to ligand.
3+
Table 2.1 Types of MRI contrast agent.
MRI contrast enhancer
Gadolinium Iron oxide
Gadopentetate dimeglumine (Magnevist) Ferumoxsil (Lumirem or GastroMark)—silicon
coating
Gadoterate meglumine, (Dotarem) Ferristene (Abdoscan)—sulfonated styrene-
divinylbenzene copolymer citrate coating
Gadoteridol (ProHance) Ferumoxide (Endorem)—dextran coated-
Fe3O4
Gadodiamide (Omniscan) Ferucarbotran (Resovist)—carboxyldextran
coating
Gadobutrol (Gadovist) Ferumoxtran-10 (Sinerem or Combinex)—
dextran coating
Gadobenate dimeglumine (MultiHance) Feruglose (Clariscan)—PEG starch coating
Gadoversetamide (OptiMark)
Gadoxetic acid (Primovist or Eovist)
Gadofosveset (Vasovist)