Fig. 1: Dual PET/MRI imaging made possible using magnesium iron oxide magnetic nanoparticles (top right, TEM image) incorporating radioactive iodine. Nanoparticles were injected into rat’s paw (left). In the combined PET/MRI image of the rat (bottom right), sentinel lymph nodes are detected more accurately than ever before.Copyright © 2008 Jinwoo Cheon and Jeongsoo Yoo

Doctors today have a wide variety of imaging technologies to help with the diagnosis of diseases. To improve picture quality, researchers have been trying to use two or more imaging techniques at the same time. The main challenge is to develop tracer particles that can be seen by more than one type of detector.

Now Jinwoo Cheon at Yonsei University in Seoul, Jeongsoo Yoo at Kyungpook National University in Daegu and co-workers1 have developed a hybrid nanoparticle probe that is compatible with two imaging systems—positron emission tomography (PET) and magnetic resonance imaging (MRI). The combination of both techniques could help in assessing the spread of cancer through the body.

“Currently, the most powerful imaging tool for cancer diagnosis is a PET/CT fusion scanner,” says Yoo. “However CT, which is based on X-ray irradiation, does not provide high resolution anatomical information for soft tissues such as the brain or spinal cord, and patients are exposed to high radiation. In contrast, MRI is harmless because it is based on radiofrequency waves, and its spatial resolution for soft tissue is superior to CT.”

The researchers made new probes from magnetic nanoparticles of manganese iron oxide coated in albumin, a common blood protein. The albumin was impregnated with a radioactive isotope of iodine that is detectable by PET.

The hybrid probes were around 32 nanometres in diameter—perfect for moving around within the lymphatic system, through which cancer often spreads. The major goal is identification of the ‘sentinel’ lymph nodes, which act as the first line of defense against cancer.

The probes were tested by injecting them into a rat’s paw. The researchers unambiguously identified two different lymph nodes—but only when PET and MRI images were overlaid. MRI revealed the clear body shape of the rat, while PET highlighted bright spots at the lymph nodes where the probes had accumulated.

“PET/MRI fusion imaging will be very helpful in the early diagnosis of various biological targets, including brain tumors and neurodegenerative diseases,” says Cheon. Other hybrid nanoparticles similar to the ones described here could provide insight into cell migration and drug delivery.