Cancer screening is crucial for timely diagnosis and treatment but poses a slight irony. Computerized tomography and positron emission tomography scans, both valuable imaging techniques that can locate and characterize a tumor, use ionizing radiation which might induce DNA damage and, over repeated scans, could lead to a minor but non-negligible risk of cancer.
A relatively safer alternative is magnetic particle imaging (MPI), which uses non-radioactive & biodegradable iron oxide nanoparticles. A machine then externally applies shifting magnetic fields to which the nanoparticles produce a brightly-lit signal allowing doctors to easily locate the nanoparticle-labelled tumour or nanoparticle-labeled stem cells for diagnosis.
However, existing nanoparticles only react gradually, leading to image blurring in MPI scans and hence hindering its diagnostic value.
To maximize MPI resolution, lead researcher Tay Zhi Wei, a Senior Research Fellow at A*STAR’s Institute of Bioengineering and Bioimaging (IBB) and A*STAR Scholarship recipient, turned to superferromagnetism, which describes changes in the magnetic properties of iron oxide nanoparticles when they form chains.
“We believed that superferromagnetism could produce a more favorable magnetization response due to reinforcing inter-particle magnetic interactions,” said Tay, who closely collaborated with the Conolly Lab from University of California Berkeley and the Rinaldi Lab from University of Florida in the US.
Their hypothesis turned out to be true. The researchers found that when viewed through an MPI scanner, superferromagnetic nanoparticle chains reacted abruptly to an applied field, producing steep signal peaks over narrow windows of magnetic field changes.
Compared with typical MPI nanoparticles, the superferromagnetic chains yielded signal spikes that were 40 times more intense and 10 times narrower—a result that gave rise to better image brightness and spatial resolution.
Traditionally MPI could barely visualize points that were spaced 1.5 mm apart. The use of superferromagnetic particles in MPI, on the other hand, produced sharp images even when the points were 0.6 mm apart. Furthermore, 1-D magnetic measurements indicate that points 0.15mm apart would be the actual limit for resolution improvement.
The far-superior resolution that superferromagnetism achieves could be attributed to the high stability of the nanoparticle chains due to inter-particle interactions. As a result, a stronger magnetic field is needed to initiate a reaction.
“But this is an avalanche effect because each chain member that reverses weakens the initial stability, making it much easier for the next member to reverse,” Tay added.
The overall effect is that a large number of particles react very strongly in a small amount of time, producing high-resolution MPI images with very little background noise. In the future, superferromagnetic nanoparticle chains could facilitate more accurate tracking of radiation-free magnetic labels for applications ranging from cancer to stem cells.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Bioengineering and Bioimaging (IBB).