Maximizing resection without inflicting new neurological deficits is especially challenging in lesions located in or near eloquent areas. Accurate localization of sensorimotor, visual and language cortex, as well as their associated white matter tracts, is an essential adjunct to operate those cases successfully.

fMRI and DTI of eloquent cortex and white matter tracts are used for preoperative planning and intraoperative navigation.

Preoperative fMRI is intended to detect neuronal metabolic activity while stimulating the cognitive function to infer a map of cerebral eloquent areas. The reliability of motor, visual and language examination in fMRI depends strongly on the preoperative neurological status and the battery of paradigms used to test the cortical function while performing the functional tests (see Chapters 1 and 2 for awake surgery with intraoperative mapping).

The motor tasks are standardized movements of fingers, toes and tongue. Language tasks consist of a battery of paradigms including word generation, sentence reading and responsive naming. Visual stimulation with static and non-static images detects primary and secondary visual areas. Before performing the fMRI, the patients are trained to perform the tasks correctly and with minimal movement of other body regions.

DTI is an MRI technique based on the concept of anisotropic water diffusion in white matter fibers, which enables 3D reconstruction and visualization of the most relevant white matter tracts. This provides information about the relationship of the functional tracts around the tumor mass to plan the best trajectory to reach the lesion safely.

Preoperative structural CT and MRI scans can be fused with fMRI and/or DTI for intraoperative surgical neuronavigation.

Anatomic or tumoral structures of interest for surgery can be highlighted in different colors as reference.

The available software displays the position (trajectory, tool tip) and other location information of the current microsurgical instrument interactively throughout the surgical procedure.

Fiducial markers are placed over the cranial vault before undergoing the preoperative CT or MRI and used for intraoperative registration with the patient under anesthesia.

Patient image registration is performed by touching the center of the fiducial markers with a non-sterile pointer. A reference marker array mounted on the head clamp allows continuous dynamic patient registration, as well as the registration of additional surgical instruments (tool tips, e.g. biopsy forceps or ultrasonic aspirator).

With neuronavigation based on preoperative imaging, tumor margins and anatomic landmarks can become inaccurate due to brain shift occurring after dural opening. Bony landmarks maintain a higher accuracy in these cases.

To decrease the brain shift inaccuracy in supratentorial intra-axial pathology, preoperative imaging can be fused with updated intraoperative imaging. This real-time image data acquisition can be correlated with data obtained from direct cortical and subcortical electrical stimulation during the procedure to increase the precision and safety resection at the tumor margins.

Brain shift is a continuous dynamic process that evolves differently in distinct brain regions. Therefore, only serial imaging or continuous data acquisition can provide consistently accurate image guidance.

Both iCT and iMRI equipment are installed in an operating room (OR) aligned with the surgical table and the head of the patient. Anesthesia is located in the other side, with MR-compatible ventilation and monitoring equipment. An MR-compatible four-point headholder is also used.

The most valuable utility of iCT and iMRI is to evaluate extent of resection, brain shift and progress of surgery, and to monitor possible surgical complications during the procedure.

iMRI is performed either when the neurosurgeon has the impression that the goal of surgery has been met or when a marked brain shift necessitates updating of the navigation system.

Prior to iMRI, the surgical resection cavity is covered with a piece of compressed sponge with hemostatic properties, the wound is then closed with stitches and sterile draping is placed over the head.

Time acquisition of iMRI strongly depends on the set-up timing and hardware/software available (thickness, number, use of contrast, processing sequences or iDTI).

iMRI provides higher resolution than iCT and appears to be the most important tool to maximize the extent of resection in gliomas (especially in low-grade gliomas [LGG] which are very difficult to distinguish from non-tumoral parenchyma).

The extent of brain shift for white matter tracts in patients undergoing tumor resection is the greatest compared to other anatomic structures and has been shown to vary from an inward shift of 8 mm to an outward shift of 15 mm. Although processing of intraoperative DTI data is slower than standard iMRI, DTI may help avoiding damage to white matter tracts and preventing postoperative neurologic deficits.

Despite that intraoperative imaging has demonstrated benefits, two important drawbacks of these tools are:

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  the relatively long image acquisition time, particularly in iMRI/iDTI and,

  
  
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  the financial resources required to acquire and maintain the intraoperative systems.

iUS is less time-consuming and more cost-effective than the other real-time intraoperative imaging modalities (iCT, iMRI and/or iDTI).

Some US probes can be registered with the neuronavigation system allowing simultaneous localization by navigation. Although iUS does not provide the same amount of anatomic and tumoral topographic details compared to iCT or iMRI, the real-time information can be superimposed in the preoperative image data set (CT/MRI/fMRI/DTI) of the navigation system and displayed together allowing real-time neuronavigation.

iUS provides real-time imaging avoiding inaccuracy due to brain shift.

A US monitor is transported to the operating room before the surgery.

The US probe and connecting cable that will be used to acquire US images are covered with a sterile sheath filled with non-sterile gel. In very superficial lesions, a gelatin pad can be used to increase the distance between the US transducer and the cortex/lesion.

Before the dura mater is pierced, a US multiplanar examination can be performed to identify the localization and echo-texture of tumor, parenchyma and the interface with the surrounding brain. Processing the first US takes around 30 seconds to 2 minutes. The images are displayed immediately on the US monitor. Usually US images are also taken after dural opening. If the lesion is deep, several rounds of iUS can be used to redirect the trajectory of the surgical opening to reach the lesion.

When necessary, the cavity is filled with a slow infusion of sterile physiological saline. The average total time spent on iUS examination is less than 15 minutes. iUS is fast and it can be repeated as needed to re-evaluate the existence, amount and localization of residual tumor. If a cavity has been created to reach the tumor, the cavity needs to be filled with a slow infusion of sterile physiological saline to allow the transmission of US waves.

The sonographic criterion for residual tumor has been established as any echogenic region >5 mm in thickness extending from the surgical cavity into the brain parenchyma with well-discriminated borders. A continuous echogenic rim <5 mm is considered a normal finding.

For both HGG and LGG, the tumor margins are blurred; the brain/tumor interface is not clearly visible everywhere and is indistinguishable from edematous brain parenchyma.

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  GB appears hyperechoic compared to brain parenchyma, with heterogeneous appearance and composed of multiple well-defined nodular areas, with diffuse margins and large cysts related to necrotic areas. After administration of contrast agent, GB shows a rapid enhancement (20–30 seconds after injection).

  
  
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  Anaplastic astrocytoma (AA) appears heterogeneously hyperechoic with a diffuse, dense texture. No cystic/necrotic areas are noted.

  
  
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  LGG appears mildly hyperechoic compared with brain parenchyma, has a homogeneous texture also with blurred margins at the brain/tumor interface and microcysts are uncommon.

Even with the aid of Doppler US, iUS has limitations in visualizing cerebral vessels and the perfusion of the lesion.

Intraoperative contrast-enhanced ultrasound (iCEUS) is a novel and fast intraoperative technique that highlights the tumors with the use of contrast agents during iUS. The US contrast consists of micro-bubbles (air or inert gas encapsulated in a layer of proteins or polymers). Micro-bubbles are typically 5 mm in diameter and can therefore be transported into the smallest capillaries. This agent is injected intravenously by the anesthesiologist as a bolus (2.4 mL [5 mg/mL]), followed by a flush of saline solution (10 mL).

In some tumors with ill-defined borders such as gliomas, iCEUS can be helpful delineating the lesion and its boundaries and differentiating tumoral tissue from edematous brain and/or blood products.