Apart from technical characteristics, the usefulness of these functional techniques in the context of neurosurgery is heavily dependent on other parameters such as mobility, ease of use and compatibility with surgical workflow. Several of the functional techniques discussed here will already have a history of clinical use, while others are only used in a pre-clinical stage, but hold promise of applicability in an intra-operative setting. Each of these different axes will be discussed below.
Whether a technique is hand-held or not, can be moved from one operating room to another, or requires its own dedicated space (as is the case of intra-operative MRIs for example (32)), has a huge impact on the technique’s applicability in a clinical and especially intra-operative context. Interruption of the surgical workflow, for example to prepare and move a patient to a scanner, is to be avoided as much as possible. What is more, techniques which allow a patient to wear or even implant the device, move the frontiers of a functional technique beyond that of the operating room alone, opening up possibilities of pre- and post-operative planning or monitoring.
Written from the intra-operative perspective, the ability for a technique to penetrate through the skull (transcranial) might in first instance sound of secondary importance. In the OR-setting, we assume that the tissue of interest (be it the brain or the spinal cord) is already exposed and accessible. Often, intra-operative techniques require even more, in terms of for example an awake patient so that measurements are not disrupted by the use of anesthetics. However, a technique with a transcranial ability has the potential for pre-operative surgical planning, as well as post-operative monitoring, which might add to the intra-operative goals.
Although neurosurgery is per definition invasive in and of itself, a technique’s additional invasiveness or burden to safety can nevertheless be an important consideration to determine its applicability in an intra-operative setting. Some techniques such as PET-scanning require the administration of radioactive tracers for imaging purposes (30). Techniques using Deep Brain Electrodes (DBE) require a higher level of invasiveness due to intra-cerebral electrode-implantations as compared to conventional craniotomy-procedures. Other techniques could be applied transcranially as well, indicating their inherent non-invasive nature.
In the intra-operative or clinical setting in general, techniques are often used in concomitance. Being able to confirm tumor-location with ultrasound in addition to the intra-operatively available MRI, for example, can serve as a confirmation of tumor borders (33). However, some techniques are inherently more difficult to combine with others, due to for example electrical or susceptibility artefacts.
In the intra-operative setting especially, surgeons need to be able to act quickly and with relative certainty based on the feedback they receive from the functional technique. How this information or brain functionality is presented, can greatly influence its effectiveness in an intra-operative setting. One can imagine how volumetric, or 3D-information representing a surgeon’s field of view, might be more intuitive than presenting a single 2D-slice or signal trace representing functionality in part of the regions of interest. However, some techniques do not readily allow for 3D or even 4D, or require significantly longer acquisition times to facilitate this multidimensionality, which is also potentially problematic in an intra-operative setting. On the other hand, the cognitive load of interpreting the results of a technique cannot be too high in an intra-operative setting, which is a challenge for multidimensional techniques.
The level of skill or training a technique requires from a surgical team can influence its intra-operative potential or at least form a significant bottleneck for application. Some techniques, such as continuous neuro-monitoring, require the full-time presence of a trained technician to support the surgical workflow (34), while other techniques are intuitive and less labor-intensive, being close to immediately applicable.
Many of the technical parameters described above, together influence how long a single acquisition and a full acquisition session might take for a technique. In case of fMRI, for example, whole brain imaging can be achieved. However, 30-minute functional acquisition sessions are conventional in the clinical setting in-house (Erasmus MC, Rotterdam, The Netherlands). In the intra-operative setting, interruption of surgical workflow and as a consequence, prolonging the surgical duration, can have severe consequences, including an increased risk of surgical site infection (35).
Patient moet wakker zijn – niet lang vol te houden – iMRI voor tumor kan, maar iFMRI duurt te lang
The costs of acquiring and maintaining the materials and skills for a certain intra-operative technique is a very relevant consideration in view of healthcare economics, and the responsibility to guarantee access to all in need for medical care.
 Continuously updated 3D-data, allowing for e.g. detection of movement in 3D-images