Ljubomir Manola

Anodal vs cathodal stimulation of motor cortex: A modeling study

OBJECTIVE To explore the effects of electrical stimulation performed by an anode, a cathode or a bipole positioned over the motor cortex for chronic pain management. METHODS A realistic 3D volume conductor model of the human precentral gyrus (motor cortex) was used to calculate the stimulus-induced electrical field. The subsequent response of neural elements in the precentral gyrus and in the anterior wall and lip of the central sulcus was simulated using compartmental neuron models including the axon, soma and dendritic trunk. RESULTS While neural elements perpendicular to the electrode surface are preferentially excited by anodal stimulation, cathodal stimulation excites those with a direction component parallel to its surface. When stimulating bipolarly, the excitation of neural elements parallel to the bipole axis is additionally facilitated. The polarity of the contact over the precentral gyrus determines the predominant response. Inclusion of the soma-dendritic model generally reduces the excitation threshold as compared to simple axon model. CONCLUSIONS Electrode polarity and electrode position over the precentral gyrus and central sulcus have a large and distinct influence on the response of cortical neural elements to stimuli. SIGNIFICANCE Modeling studies like this can help to identify the effects of electrical stimulation on cortical neural tissue, elucidate mechanisms of action and ultimately to optimize the therapy.

ImageGrouper: a group-oriented user interface for content-based image retrieval and digital image arrangement

In content-based image retrieval (CBIR), experimental (trial-and-error) query with relevance feedback is essential for successful retrieval. Unfortunately, the traditional user interfaces are not suitable for trying different combinations of query examples. This is because first, these systems assume query examples are always added incrementally. Second, the query and the result display are done on the same workspace. Once the user removes an image from the query examples, the image may disappear from the user interface. In addition, it is difficult to combine the result of different queries. In this paper, we propose a new interface for Content-based image retrieval named ImageGrouper. ImageGrouper is a Group-Oriented user interface in that all operations are done by creating groups of images. This approach has several advantages. First, the users can interactively compare different combinations of the query examples by dragging and grouping the images on the workspace (Query-by-Group). Because the query results are displayed on another pane, the user can quickly review the results and modify the query. Combining different queries is also easy. Furthermore, the concept of “Image Groups” is also applied for annotating and organizing many images. The Annotation-by-Groups method relieves the user of tedious task of annotating textual information on a large number of images. This method realizes a hierarchical annotation of the images as well as Bulk Annotation. The Organize-by-Group method lets the users manipulate the image groups as “Photo Albums” to organize the images. Finally, the usability of the system is compared with the traditional user interfaces. By incorporating the lessons from the experiments, the usability of ImageGrouper is further improved.

Cathodal, anodal or bifocal stimulation of the motor cortex in the management of chronic pain?

The conditions of motor cortex stimulation (MCS) applied with epidural electrodes, in particular monopolar (cathodal or anodal) and bipolar stimulation, are discussed. The results of theoretical studies, animal experiments and clinical studies lead to similar conclusions. Basically, cortical nerve fibres pointing at the epidural electrode and those normal to this direction are activated by anodal and cathodal stimulation, respectively. Because MCS for the relief of chronic pain is generally applied bipolarly with electrodes at a distance of at least 10 mm, stimulation may actually be bifocal. The polarity and magnitude of a stimulus needed to recruit cortical nerve fibres varies with the calibre and shape of the fibres, their distance from the electrode and their position in the folded cortex (gyri and sulci). A detailed analysis of intra-operative stimulation data suggests that in bipolar MCS the anode of the bipole giving the largest motor response in the pain region is generally the best electrode for pain management as well, when connected as a cathode. These electrode positions are most likely confined to area 4.

Technical Performance of Percutaneous Leads for Spinal Cord Stimulation: A Modeling Study

Objective  To compare the technical performance of different percutaneous lead types for spinal cord stimulation.

The Effect of Pulse Width and Contact Configuration on Paresthesia Coverage in Spinal Cord Stimulation

BACKGROUND:In spinal cord stimulation for the management of chronic, intractable pain, a satisfactory analgesic effect can be obtained only when the stimulation-induced paresthesias cover all painful body areas completely or partially. OBJECTIVE:To investigate the effect of stimulus pulse width (PW) and contact configuration (CC) on the area of paresthesia (PA), perception threshold (VPT), discomfort threshold (VDT), and usage range (UR) in spinal cord stimulation. METHODS:Chronic pain patients were tested during a follow-up visit. They were stimulated monopolarly and with the CC giving each patient the best analgesia. VPT, VDT, and UR were determined for PWs of 90, 210, and 450 microseconds. The paresthesia contours at VDT were drawn on a body map and digitized; PA was calculated; and its anatomic composition was described. The effects of PW and CC on PA, VPT, VDT, and UR were tested statistically. RESULTS:Twenty-four of 31 tests with low thoracic stimulation and 8 of 9 tests with cervical stimulation gave a significant extension of PA at increasing PW. In 14 of 18 tests (low thoracic), a caudal extension was obtained (primarily in L5-S2). In cervical stimulation the extension was predominantly caudal as well. In contrast to VPT and VDT, UR is not significantly different when stimulating with any CC. CONCLUSION:PA extends caudally with increasing PW. The mechanism includes that the larger and smaller dorsal column fibers have a different mediolateral distribution and that smaller dorsal column fibers have a smaller UR and can be activated only when PW is sufficiently large. A similar effect of CC on PA is unlikely as long as electrodes with a large intercontact distance are applied.

Technical Performance of Percutaneous and Laminectomy Leads Analyzed by Modeling

The objective of this study was to compare the technical performance of laminectomy and percutaneous spinal cord stimulation leads with similar contact spacing by computer modeling. Monopolar and tripolar (guarded cathode) stimulation with both lead types in a low‐thoracic spine model was simulated using UT‐SCS software. Dorsal column and dorsal root fiber thresholds were calculated as well as the area of recruited fibers in the dorsal columns, the rostrocaudal span of recruited dorsal root fibers and the energy consumption at discomfort threshold. Tripolar stimulation is superior to monopolar stimulation in the recruitment of the dorsal columns, a percutaneous lead recruits a ∼12% larger dorsal column area than a laminectomy lead does. This difference is reduced when the contact spacing of the lead models is the same. A percutaneous lead with significant wire impedance (140 Ohms) consumes ∼115–240% more energy, whereas the same lead with negligible wire impedance consumes ∼40–85% more energy. A deterioration of all performance parameters is predicted when a percutaneous lead is placed more dorsally in the epidural tissue. When positioned next to the dura mater, a percutaneous lead has a similar performance (fiber recruitment in the dorsal columns and the dorsal roots) as a laminectomy lead with similar contact spacing, but substantially higher energy consumption. The superior clinical performance of the laminectomy lead is most probably due to the difference in volume and insertion technique of the two lead types.

Factors Affecting Impedance of Percutaneous Leads in Spinal Cord Stimulation

Objectives.  Although the load impedance of a pulse generator has a significant effect on battery life, the electrical impedance of contact arrays in spinal cord stimulation (SCS) has not been extensively studied. We sought to characterize the typical impedance values measured from common quadripolar percutaneous SCS contact arrays.

Motor cortex stimulation: role of computer modeling

Motor cortex stimulation (MCS) is a promising clinical technique used to treat chronic, otherwise intractable pain. However, the mechanisms by which the neural elements that are stimulated during MCS induce pain relief are not understood. Neither is it known which of the main neural elements, i.e. cell bodies, dendrites or fibers are immediately excited by the electrical pulses in MCS. Moreover, it is not known what are the effects of MCS on fibers which are parallel or perpendicular to the cortical layers, below or away from the electrode. The therapy and its efficacy are less likely to be improved until it is better understood how it may work. In this chapter, we present our efforts to resolve this issue. Our computer model of MCS is introduced and some of its predictions are discussed. In particular, the influence of stimulus polarity and electrode position on the electrical field and excitation thresholds of different neural elements is addressed. Such predictions, supported with clinical evidence, should help to elucidate the immediate effects of an electrical stimulus applied over the motor cortex and may ultimately lead to optimizations of the therapy.

Theoretical Investigation Into Longitudinal Cathodal Field Steering in Spinal Cord Stimulation

Objective.  When using spinal cord stimulation (SCS) for chronic pain management, precise longitudinal positioning of the cathode is crucial to generate an electrical field capable of targeting the neural elements involved in pain relief. Presently used methods have a poor spatial resolution and lack postoperative flexibility needed for fine tuning and reprogramming the stimulation field after lead displacement or changes in pain pattern. We describe in this article a new method, “electrical field steering,” to control paresthesia in SCS. The method takes advantage of newer stimulator design and a programming technique allowing for “continuous” adjustment of contact combination while controlling stimulation current for each contact separately.

MEPs to identify the optimal cathode position in MCS

Objective. In the subthalamic area, the subthalamic nucleus (STN) is the main target for deep brain stimulation (DBS) in idiopathic Parkinson’s disease (PD). We aimed at analysing relationships between the MRI anatomy and spontaneous neuronal activity in order to confirm the potential of microelectrode recording to assist in the determination of the optimal surgical target. Methods. Thirty trajectories were analysed. Procedures (8 bilateral surgeries) were performed under local anaesthesia after identification on 1.5T MRI (T2 weighted) of anatomical structures, STN, above STN (zona incerta, forel’s fields) and substantia nigra (SN). Spontaneous neuronal activity was recorded (30 seconds, 0.5 mm step), simultaneously along the distal 10 mm on a central (covering optimally STN) and on a 2 mm anterior track. Intra operative X-Ray controls checked that the exploration tracts followed the planed trajectories. Chronic DBS (on central tract 14 times out of 16) dramatically improved PD. We calculated cell numbers and mean firing rates (MFR, average firing on 30 seconds) on 276 isolated neurons (after threshold and principal component analysis) regrouped according to anatomical structures. MFR were compared with a non parametric statistical test. Results. 60% of spontaneously active cells were in the dorsal and lateral STN. MFR increased entering STN and SN (p<0.03): 5.88 +/- 1.06 spike.sec-1 above STN; 9.23 +/- 1.22 spike.sec-1 in STN; 14.46 +/- 2.32 spike.sec-1 in SN. Conclusions. Anatomical boundaries and unitary recordings seem to be linked. We found a maximum number of spontaneously active neurons in the dorsal and lateral STN which is also known as a good surgical target for DBS in severe PD.Objectif : plusieurs auteur ont decrit une somatotopie dans le globus pallidus (GP) de l’homme, a partir des enregistrements electrophysiologiques effectues au cours de neurochirurgies, en considerant des mouvements volontaires ou passifs des patients, ou encore sur les effets cliniques des stimulations cerebrales profondes (SCP) effectuees pour traiter les mouvements anormaux. Le present travail montre une somatotopie des mouvements indesirables (dystonies) observee dans le pallidum d’un patient parkinsonien pendant la stimulation aigue. Ces donnees sont mises en correspondance avec l’anatomie IRM. Methode : un patient parkinsonien souffrant principalement de dyskinesies des membres et du tronc (16 ans d’evolution de la maladie, 68 ans) a beneficie d’une CSP bilaterale du GP interne (GPi) en condition stereotaxique avec une approche en visee directe. Une IRM T2 permet de discerner les differentes structures pallidales : GP externe (GPe) et les subdivisions du GPi (laterale GPil et mediale GPim). Des mouvements dystoniques controlateraux ont ete observes a l’aide de 2 x 3 electrodes paralleles et separees de 2 mm pendant la stimulation effectuee sous anesthesie locale. Les mouvements dystoniques concernant le visage, les membres superieurs et inferieurs ont ete visualises dans les structures anatomiques etudiees. Resultats : chacune des structures GPe, GPil et GPim semble presenter une organisation rostro-caudale avec le visage represente dans la partie superieure, les membres superieurs dans la region intermediaire et les membres inferieurs dans la portion inferieure. Conclusion : l’analyse des effets moteurs indesirables (dystonie) est ici un outil interessant pour dessiner une somatotopie. Cette somatotopie pallidale, observee chez un patient, doit cependant etre validee.