An understanding on the role of Imaging in Deep brain stimulation

Deep brain stimulation (DBS) is a surgery to implant a device that sends electrical signals to brain areas responsible for body movement. Electrodes are placed deep in the brain and are connected to a stimulator device. Similar to a heart pacemaker, a neurostimulator uses electric pulses to regulate brain activity. It can help reduce the symptoms of tremor, slowness, stiffness, and walking problems caused by Parkinson’s disease, dystonia, or essential tremor. Successful DBS allows people to potentially reduce their medications and improve their quality of life.

Deep brain stimulation (DBS) has been used to treat intractable pain for several decades. More recently, the use of this technology has proven to be a safe and effective treatment for essential tremor, as well as tremor and involuntary movements associated with Parkinson’s disease, dystonia, and multiple sclerosis, with more than 35,000 DBS implants worldwide. The applications for DBS therapy are expanding rapidly.

The procedure is comparable to that of a cardiac pacemaker in which the pacemaker helps maintain an appropriate cardiac rhythm. DBS is presumed to help modulate dysfunctional circuits in the brain so that the brain can function more effectively. This is accomplished by sending continuous electrical signals to specific target areas of the brain, which block the impulses that cause neurological dysfunctions. These targets are the ventralis intermediate nucleus of the thalamus (Vim), the globus pallidus pars interna (GPi), and the subthalamic nucleus (STN).

Localization of target is the most important step in DBS. Frame-based and Frameless systems are available for image acquisition and localization. Stereotactic frames are fixed on to the patient’s head or fiducial markers are fixed and then MRI and CT images are obtained.

History of Deep brain stimulation

The roots of modern DBS stretch back to the early experimental endeavors in brain stimulation that took place in the late 19th century. Developments in the stimulation of the cerebral cortex of animals laid the foundations of cortical functional localization as we know it today. The first stereotactic frame was developed in the early 1900s, enabling experimentation in the stimulation of deeper brain regions.

Introduction of X-ray pneumoencephalography in 1947 vastly improved surgeons’ ability to localize targets, particularly with the later development of detailed stereotactic atlases. The 1950s saw the introduction of lesioning as a treatment for tremor – stereotactic techniques were used to ablate the ventrolateral or ventroanterior nuclei of the thalamus, with intraoperative electrical stimulation and recording being employed to localize targets. This work was extended by Albe Fessard et al. (1963), who first reported that high-frequency (~100–200 Hz) electrical stimulation of the ventral intermediate thalamic nucleus could substantially alleviate Parkinsonian tremor (Albe Fessard et al., 1963). Implantable spinal cord stimulators first introduced by Shealy and colleagues in 1967 also moved the field closer to the modern use of chronically implanted DBS systems.

The role of Imaging in Deep Brain Stimulation

In our center, we use a Leksell frame-based system. Patients undergo an MRI scan of the brain with specific sequences and protocols on the day prior to surgery. This is done with the patients on their regular medications so the artifacts due to movements are reduced and clear images are obtained. On the day of surgery, the frame is fixed under local anaesthesia in the radiology suite and a CT scan with the frame is obtained. MRI and CT images are loaded onto the Navigation software, “Frame link”, (Medtronic) and co-registered.

The STN in patients with PD is targeted by a direct method, which is visualizing the STN on the scan picture in the appropriate slice. It is super lateral to the red nucleus. The software also has an inbuilt stereotactic atlas which can be overlaid onto the image and used for target localization.

The CT and MRI images are post-contrast images and detailed planning is done to use a trajectory, which does not go through the ventricles, blood vessels. The trajectory, which goes through the longest axis of the STN, is selected. Once the planning is done, X, Y and Z coordinates are obtained and noted down.

MRI is a valuable and often most appropriate imaging modality for numerous conditions based on professional society and consensus guidelines Neuroimaging studies are conducted to evaluate the structure and function of the brain in patients with neurodegenerative diseases, including Parkinson’s disease (PD), essential tremor (ET), and dystonia. Functional neuroimaging with MRI holds the potential for a better understanding of neurodegenerative disorders. Modalities like diffusion tensor imaging and functional MRI are becoming increasingly available, but, in the case of functional MRI, are not considered routine clinical practice at this time. Nevertheless, the ACR Appropriateness Criteria for Movement Disorders rate MRI as usually appropriate for movement disorders, including PD, Parkinsonian syndrome, as well as Alzheimer’s disease and dementia.

The most common clinical indication for DBS worldwide is the treatment of advanced PD. Preoperative brain imaging (usually MR imaging) is used principally for the selection of those patients with PD who are candidates for DBS intervention (bilateral GPi or STN DBS). In most cases, the presence of abnormalities on MR imaging such as severe atrophy, leukoencephalopathy, or multiple lacunae contraindicates DBS surgery.

The use of diagnostic imaging has substantially increased over the past few decades, with the largest growth in emerging markets. It is important that patients receive the most appropriate imaging test for their condition. The decision on when MRI is most appropriate can be best understood when considering the following situations: (1) MRI has no equivalent or the alternative would negatively impact patient care and/or outcomes with lower yield, sensitivity, or specificity; (2) There is an alternative to MRI, but involves a procedure or operation and great costs and/or risks, or (3) computed tomography (CT) is an alternative but has a higher radiation dose and may involve less safe contrast injection compared to MRI, as the rate of reactions to CT contrast medium is much higher than to MRI. That being said, there are situations where CT, ultrasound, or nuclear medicine is better than MRI. Additionally, when patients cannot undergo MRI, alternative modalities are employed, such as when they are allergic to intravenous contrast medium. Recently, some devices have received FDA approval for full-body MRI under specific conditions of use. Practitioners should work with manufacturers to understand available options.

CT imaging is suitable for patients who cannot undergo MRI scans due to safety concerns and also there is no risk of tissue damage due to heating, as for example MRI. However, CT scans entail ionizing radiation and provides less tissue contrast, and usually it is required to be co-registered with the pre-operative MRI because the MRI allows a more detailed visualization of the target structures.

Although it has been reported that post-operative acquisition of MRI images on patients with implanted DBS devices may result in severe potential hazards, recent studies have demonstrated that this procedure can be performed safely without causing any adverse effects, especially for ≤ 1.5 T field strength however, it is important to keep in mind that not all currently available DBS stimulating systems on the market are validated for post-operative MRI. MRI should not be applied with multiple implanted electrodes and stimulators, or only with reduced SAR values.

There are several factors that need to be taken into account when choosing the scanning parameters for post-operative MRI acquisition, foremost are: the imaging time, patient safety and image quality. As more MRI scanners with high field strengths (≥ 3 T) are available at clinical centers around the world, it is also necessary to consider the benefits and risks of employing scanners with high field strengths (≥ 3 T) for post-operative imaging and to determine the optimal field strength for this type of application. More powerful imaging with detailed anatomic delineation, such as high-resolution three-dimensional (3D) T2*-weighted images, 3.0 T T2*-FLASH2D and 7.0 Tesla are currently being discussed; however, to the best of our knowledge, not a single study has been published that demonstrates that patients implanted with DBS devices can be safely scanned by 3 T MRI.

Images of different modalities provide complementary information, thus the fusion of inter-modality images may enable a more detailed illustration of the target region and allow a more straightforward comparison of the pre- and post-operative conditions. On MRI, the electrode induces a massive non-linear distortion artifact that makes the determination of the precise electrode position not possible.

Conclusion

DBS is a neurosurgical method, but the role of neuroimaging in successful DBS intervention is critical. Neuroimaging is used for the preoperative selection of patients who will have DBS and to localize the intended target nuclei. In the postoperative period, imaging detects complications that uncommonly accompany the procedure, confirms the position of electrode contacts, and helps explain intended or unexpected effects. The involvement of neuro-radiologists in DBS is mandatory to achieve excellent clinical results consistently.