This category includes those parameters inherent to the imaging technique and its underlying technical characteristics. Possible parameters of importance here include spatial and temporal resolution, imaging depth and field of view, each of which will be discussed below within the context of intra-operative use by a neurosurgeon.
The spatial resolution of a technique concerns the physical dimensions that the technique’s smallest unit of measure represents. In case of images, this would concern the physical dimensions represented by a single pixel within the image. The spatial resolution of a functional technique in an intra-operative setting dictates the precision with which (the boundaries of) functional structures can be identified and distinguished, and as a consequence, the precision with which decisions can be made based on these techniques. Ideally, spatial resolution is small enough to readily and safely distinguish what is for example tumor and what is eloquent brain tissue. However, there is also a limit to how useful an increase in spatial resolution can be. With very high spatial resolution, but no ability to act with the same resolution (e.g. no ability to cut or resect on a micrometer-scale), a super high-resolution technique might effectively not lead to an improvement of safety and surgical success in an intra-operative setting.
Not only the spatial resolution but also the depth of the brain which can be covered with that particular resolution, is an important factor dictating a technique’s intra-operative potential.
Depth penetration is often a characteristic inherent to the technique’s contrast mechanism: optical techniques generally present with low penetration due to high scattering by the tissue, whereas ultrasonic scattering is orders of magnitudes weaker, allowing for deeper penetration (25). Often there is also a trade-off between a technique’s spatial resolution and its penetration-depth. Wanting to look at deeper brain regions, such as nuclei or functional tracts, might come at the cost of seeing these structures with less resolution. On the other hand, having access to superficial cortical regions only can expose relevant functional regions, but might not draw the full picture needed in an intra-operative setting. In a neuro-oncological procedure, for example, awake mapping of the eloquent cortical areas can be a powerful tool to ensure safe tumor removal. However, without the ability to identify the involvement of deeper structures such as the pyramidal tract or the arcuate fasciculus, post-operative neurological deficits might nevertheless occur.
The field of view is a third aspect which together with the spatial resolution and depth of penetration dictates how much of the brain can be seen at once or after one acquisition session. In this paper we discuss the field of view on a scale ranging from neuronal level to the whole brain.
The temporal resolution tells us how fast a technique is able to sample the biological substrate which is underlying a technique. Whether a technique’s temporal resolution is ‘sufficient’ to reconstruct the time course of the dynamic process depends on the actual temporal resolution of the underlying biological substrate, as will be discussed in the next chapter.
Whether a technique allows for real-time, intra-operative detection, measurement, imaging or mapping of brain tissue functionality can be of great consequence. Techniques such as functional Magnetic Resonance Imaging (fMRI) allow for pre-operative planning of the surgical procedure based on identification of functional regions, for example in close proximity to the tumor’s borders (26). However, intra-operatively, these images need to be merged with the in-vivo brain anatomy in sight of the surgeon. Due to the inevitable brain shift after cranio- and durotomy, pre-operative images only provide a rough estimation of the 3D-locus of the tumor during surgery (27). In fact, in most clinical settings, these fMRIs are not available intra-operatively due to exceptionally high costs. Techniques which allow for intra-operative acquisition, have the potential to be more in line with the in-vivo anatomy. However, for intra-operative acquisition, new challenges arise such as acquisition time and ease of use (see ‘Intra-Operative Applicability’).