HOLLI W. HAYNIE

Determining the location of a brain tumor is vital for targeted treatment and surgery. Imaging of the delicate pathways in the brain helps radiologists and surgeons map the anatomy prior to intervention. Conventional MRI, however, does have limitations but radiologists have an advanced tool that visualizes brain anatomy with greater characterization, all thanks to plain old H2O: diffusion tensor imaging (DTI). Used in conjunction with MRI, DTI is a special software that calculates the manner in which water molecules diffuse, or spread out, in various tissues.

As explained by St. Jude literature, how active water molecules diffuse depend on the type of tissue in which they are located. DTI interprets that movement and turns the information into incredible images of red, blue and green; each color representing a different direction pattern (left to right, front to back, head to foot).

While DTI is not a new technology, it has only recently been used in a routine clinical environment. At St. Jude Children’s Research Hospital, radiologists use data derived from DTI to help pediatric neurosurgeons plan brain surgery to an unparalleled level of detail. DTI characterizes the difference between a brain tumor and a nerve tract within the white matter of the brain that the tumor is shoving aside. It also indicates precisely where white matter damage has occurred due to previous treatment.

White matter is made up of a complex organization of fibers which communicate information, similar to that of an electrical wire, to the regions of the cortex that control such functions as sight, cognition and movement.

Water acts differently in the brain than anywhere else in the body. It develops a process called anisotropy, meaning water doesn’t diffuse freely everywhere in all directions. Instead, it diffuses preferentially along the fibers of white matter, not across them. When the fibers are damaged due to treatment or surgery, the connections become inefficient and anisotropy is interrupted. With DTI, this anisotropy indicates precisely where essential fibers are located in relation to the tumor, and whether or not a tumor, or surgery on it, would damage crucial pathways. The benefits of avoiding a cut that leaves a child permanently paralyzed or unable to learn are immense.

“If you have a tumor here, it is important for the surgeon to know, one centimeter this way from the tumor, there are very valuable fibers and not to touch them,” said Dr. Zoltán Patay, section chief of neuroradiology in the diagnostic imaging division of the radiological sciences department.

Sometimes damage is unavoidable, Patay explained, but DTI allows clinicians to know this beforehand so all the risks can be discussed with the patient and family.

Currently DTI surgical mapping is utilized for those patients with tumors in the eloquent cortex, or the areas of the brain in which clinicians know what the fibers represent (motor skills, eyesight, language and memory).

“Our primary interest is getting cognitive function in kids who survive cancer, cancer therapy and other catastrophic diseases,” explained Robert Ogg, PhD, associate member in the translational imaging research division of the radiological sciences department. “Kids are surviving at greater and greater rates, for example in brain tumors, often because of very aggressive treatment in the central nervous system, such as radiation, drugs and surgery.”

Surviving the cancer is only one hurdle. These developing children have quality-of-life issues that affect their ability to learn and remember.

“The long term goal is to understand how we’re affecting the brain and then either develop protective strategies,” Ogg added, “or if not that, then targeted remedial strategies that deal with the specific problems on exactly which parts of the brain are damaged.”

A 2001 article from the Journal of Magnetic Resonance Imaging confirms the most advanced DTI application is fiber tracking in the brain, which, in combination with functional MRI, might open a window on the important issue of connectivity.

While much is identified about the brain’s anatomy, there are still many unknowns about the intricate pathway connections. Scientists simply do not know, Ogg explained, exactly how the brain coordinates a complex activity. With DTI, they can identify major white matter pathways and define or map their direction in space with the ultimate goal of identifying their function.

St. Jude is also using DTI to study the brains of children with sickle cell disease, who are 250 times more likely to have strokes than are healthy individuals. This work might one day help St. Jude researchers design earlier, more aggressive treatment for kids with sickle cell disease.




September 2007