'Fusion' Imaging for DBS Lead Location Safe, Feasible

Pauline Anderson

April 01, 2016

A new fusion imaging technology that combines preacquired magnetic resonance with live ultrasonography imaging to check the lead location in patients undergoing deep-brain stimulation (DBS) is feasible and safe, a new study shows.

The findings suggest the new technology may reduce the need for repeat computed tomography (CT) and MRI in patients having DBS, such as those with Parkinson's disease (PD).

The technology has many advantages over CT and MRI, says lead study author Uwe Walter, MD, professor and deputy chair, Department of Neurology, University of Rostock, Germany.

"It's non-invasive, relatively inexpensive, is easy to perform and is patient friendly," he told Medscape Medical News.

It also doesn't require sedation, said Dr Walter. "Patients with a movement disorder are often moving, and this makes it difficult to do MRI and CT scans because it creates artefacts, so you often need to sedate patients undergoing MRI or CT."

The research was supported by the University of Rostock and published in the March issue of Movement Disorders.

Lead Displacement

For some time now, DBS has been used in patients with movement disorders. Target electrode points differ depending on the type of disorder; for example, for dystonia, the target might be the globus pallidus interna (GPi), but with tremor, it may be the ventro-intermediate thalamic nucleus, and for PD, it's usually the subthalamic nucleus (STN).

There can sometimes be discrepancies between the initial selected target and the final DBS lead location. In some cases, the DBS lead may get displaced after surgery. "Even with extensive planning preoperatively, you don't always have a very precise placement of electrodes," said Dr Walter.

As well, postoperative displacement of an electrode can occur if a patient falls or is in an accident, he said. "So there are situations where you really need postoperative imaging to look at where the tip of the electrode is located."

In these situations, brain imaging to check the lead location is typically performed with CT and MRI.

The new fusion imaging technology allows clear visualization of the DBS lead location by superimposing postoperative ultrasonography and preoperative MRI images.

"You can upload the preoperative MRI or any MRI or CT data set of the patient you have into this ultrasound system, which is designed for fusion imaging," said Dr. Walter.

"You can fuse both images very exactly, and once those images are precisely overlaid, you can freely navigate in 3D [three dimensions] through the ultrasound image and simultaneously through the other methods, for example, from the MRI data set."

According to Dr Walter, the technology combines the advantages of both imaging modalities.

While transcranial ultrasound allows for immediate high-resolution imaging of DBS lead, MRI visualizes the brain with high tissue contrast and is not corrupted by bone structures. As a result, the electrode position in deep brain structures is displayed with higher spatial and temporal image resolution compared with stand-alone MRI."

He added that this could mean less corruption caused by patients' movements, which is important when imaging patients with movement disorders.

Researchers conducted 18 fusion imaging sessions in 15 patients with PD, dystonia, Tourette's syndrome, essential tremor, or epilepsy (mean age, 52.4 years). These patients had undergone DBS of STN, GPi, or the ventro-intermediate or anterior thalamic nucleus, but experts suspected suboptimal lead location or lead displacement.

In all patients, MRI was performed before and 1 to 3 days after the DBS lead was implanted, and before the pulse generator was implanted in the left chest region. These scans were taken to help plan the intervention and document postoperative lead location.

The fusion imaging system comprises a position tracking unit mounted on a high-end ultrasound system, a magnetic field transmitter, and two position sensors, one connected to a transducer bracket and the other fixed at the patient's forehead.

To minimize the possibility of electromagnetic effect on the DBS pulse generator, researchers kept a minimum distance of 65 cm (26 inches) between the magnetic field transmitter and the DBS generator.

They loaded preimplantation MRI data into the ultrasound system for the fusion imaging examination .During each imaging session, investigators monitored the neurologic state of the patient.

The study showed that data registration and matching were feasible in all patients within 10 minutes.

No Neurologic Changes

None of the patients reported discomfort or change of their neurologic symptoms. There was no change in neurologic state or score on motor disability scales.

Of 35 assessed electrode locations, 30 were rated optimal, 3 suboptimal, and 2 displaced. Surgeons reimplanted electrodes after confirming their inappropriate location on CT.

Ultrasonographic fusion imaging isn't necessary after all DBS procedures. For example, ordinary ultrasonography can be used to locate the DBS lead in the STN "very nicely," said Dr Walter.

"You see clearly with ultrasound where the electrode tip is located and whether the tip is the right position."

But to discriminate targets in the GPi or basal ganglia in patients with dystonia or essential tremor, the enhanced fusion imaging appears to be superior, he said.

In addition to better detecting a dislocated electrode, the technology will eventually be used intraoperatively, Dr Walter predicted. "We will be able to upload the MRI data into the ultrasound system and implant the electrodes during the DBS operation using live images."

Dr Walter stressed that with use of this new technology, it's important to ensure that the ultrasonography and MRI images are exactly matched. "This is the main challenge for this fusion imaging method," he said.

But while image coordination is now done with the naked eye, new techniques are being developed to automate this matching, and so should be even more precise, said Dr Walter.

Precise Location

An accompanying editorial stressed that it is "crucial" to have technology to accurately determine electrode location in DBS.

The "really crucial question" that needs to be addressed is how precisely electrode position can be determined using this new technology, Zvi Israel, MBBS, Department of Neurosurgery, Hadassah University Hospital, and Hagai Bergman, MD, PhD, Department of Neurobiology, Hebrew University of Jerusalem, Israel, write.

It's "quite amazing" that after more than 20 years of experience with DBS, the optimal spot for stimulation has not yet been entirely resolved, they point out.

Being able to accurately determine electrode location, both anatomically and physiologically, will play an important role as researchers grapple with this issue, and the new technology might be "a step forward in this direction," they conclude.

Commenting on the new technology for Medscape Medical News, Michael S. Okun, MD, professor and chairman of neurology, University of Florida Health Center for Movement Disorders and Neurorestoration, Gainesville, and national medical director, National Parkinson Foundation, agreed it could reduce the safety implications related to multiple CT scans in some patients.

However, the application of this technique may not always be practical or necessary, and in some cases the imaging could be "duplicative," especially if the patient requires reoperation, said Dr Okun.  

"The most important issue to consider when a clinician suspects a suboptimally placed DBS lead is to check the programming and the clinical response before proceeding to any imaging-based confirmation."  

Imaging is a "critical element "of the care decision for a subset of DBS patients, said Dr Okun. "There may be a limited, but useful, role for this new technique."

The study was supported by the University of Rostock, Germany. 

http://www.medscape.com/viewarticle/861318?src=wnl_edit_tpal