**3. Anesthesia considerations**

The main challenge during ioMRI anesthesia is to have uninterrupted access to the patient while allowing high-quality real-time MRI images with minimal electrical noise interference. All standard monitors, equipment and anesthesia workstation should have minimal electrical noise interference and MRI compatible. Monitors can be classified as MRI unsafe (prohibited inside suite), MRI conditional (permitted not beyond 5 Gauss line inside the MRI suite), MRI safe (can be allowed freely inside the suite) [9].

Anesthetist should be aware of ECG changes like ST segment changes, P wave abnormalities, AF, Ventricular fibrillation can happen due to static magnetic field, pulse gradient and high frequency field. If these changes occur along with hemodynamic instability then it should be managed accordingly.

All patients should be screened for any metallic objects, piercings, tattoos (lead), metallic implants, pacemakers, deep brain stimulators, implantable defibrillator, vagal stimulator, aneurysmal clips during pre-operative assessment to know the MRI compatibility.

During ioMRI patient access is limited and so all IV lines and tubings should be long, tightly fixed and secured. Proper checklist has to be followed to avoid mishaps and accidents. Anesthesia depth has to be maintained throughout the

procedure either with intravenous or inhalational anesthesia. Nitin et al. [10] reported thermal injuries due to radio frequency energy. It can be avoided with skin to skin packing, avoiding looping of wires, lines and tubing. Noncompatible equipment (Temperature probe and depth monitors and flexometallic tubes) are removed. Nitin Manohar et al. [11] also reported that IONM electrodes can interfere with the signals and produce artifacts affecting the image quality. Patient positioning for ioMRI requires additional attention. Position should be tailored so that head fixed to the head clamp moves freely within the ioMRI bore. ioMRI compatible Mayfield clamps are used for positioning and final position should be confirmed with bore gauge that is provided along with ioMRI compatible operative table, to avoid possible collision with the gantry. During awake procedure or surgery, preoperative counseling regarding the MRI sound, use of ear plugs and sedation should be done.

Emergency drugs should be ready in case of contrast induced reactions or hemodynamic instability during the scan. Mock drills to cope up with emergency situations for all OT and MR staff should be done with everyone knowing all steps and well versed in their roles with even quenching of MRIif needed (**Figures 2** and **3**) (Box 1).

**CHECKLIST FOR INTRA-OP MRI:**

**(To be Completed mandatorily prior to Wheeling-1a; Please Tick on the boxes on checklist** 

• MRI Compatible ECG electrodes: □ • No metallic ornaments: □ • No metallic Implants, Heart valves, Pacemakers in patient: □ • Arms insulated from body: □ • Padding from skin to skin: □ • No staplers, sand bags, metal objects in sheets, sisters check done: □ • OT table sheets tucked under to help sliding: □ • Temp probe removed: □ • Bag free: □ • Warmer disconnected: □ • Antibiotic repeated: □ • Inform MRI Technician: □ • Cautery pad removed: □ • Emergency drugs box sent in: □ • OT personnel moving into the MRI Suite screened for metallic objects: □ • MRI Compatible Monitor Ready: □ • MRI Anaesthesia Machine SWITCHED "ON" CHECKED: □ • OT Table & MRI Table ALIGNED & LEVELLED □ • Infusion pump casing: □ • Long breathing circuit (Double): □ • Long IV line extension (200 ml): □ • Long infusion extension (200 ml): □ • AMBU, Bains circuit ready □ • Compatible Laryngoscope Ready: □ • Relaxant bolus □

**completion)**

**Box 1.** *ioMRI checklist.*

**Figure 2.**

*MRI compatible instruments A. Laryngoscopes, B. Monitor, C. Ventilator, D. Infusion pump and E. ECG electrodes.*

#### **Figure 3.**

procedure either with intravenous or inhalational anesthesia. Nitin et al. [10] reported thermal injuries due to radio frequency energy. It can be avoided with skin to skin packing, avoiding looping of wires, lines and tubing. Noncompatible equipment (Temperature probe and depth monitors and flexometallic tubes) are removed. Nitin Manohar et al. [11] also reported that IONM electrodes can interfere with the signals and produce artifacts affecting the image quality. Patient positioning for ioMRI requires additional attention. Position should be tailored so that head fixed to the head clamp moves freely within the ioMRI bore. ioMRI compatible Mayfield clamps are used for positioning and final position should be confirmed with bore gauge that is provided along with ioMRI compatible operative table, to avoid possible collision with the gantry. During awake procedure or surgery, preoperative counseling regarding the MRI sound, use of ear plugs and

Emergency drugs should be ready in case of contrast induced reactions or hemodynamic instability during the scan. Mock drills to cope up with emergency situations for all OT and MR staff should be done with everyone knowing all steps and well versed in their roles with even quenching of MRIif needed

sedation should be done.

(**Figures 2** and **3**) (Box 1).

*Positioning of patient in operative table with MRI compatible Mayfield clamps and navigation probe. Final position is confirmed using bore gauge so that patient moves in and out of MRI machine freely.*

#### **CHECKLIST FOR INTRA-OP MRI:**

#### **(To be Completed mandatorily prior to Wheeling-1a; Please Tick on the boxes on checklist completion)**


#### **Box 1.**

*ioMRI checklist.*

## **4. Role of ioMRI in neurosurgery**

It is useful in both intra and extra axial tumors. Depending upon the situation ioMRI can be used as completion study, residue seeking or guidance for further resection. It is also used in epilepsy surgery, deep brain stimulation (DBS), sterotactic biopsies. Apart from extent of resection and location of residue, ioMRI also gives us information like hematoma in and around operative cavity, ischemia (diffusion restriction), hydrocephalus, location of electrode tips in DBS, proximity to neurovascular bundles which guides surgeon for further planning and proceeding in the same sitting of surgery.

#### **4.1 Intraaxial tumors**

Gliomas are the most common primary brain tumors. These are infiltrating tumors along the subcortical white fibers. In low grade gliomas it is difficult to differentiate from adjacent normal tissue. During surgery extended resection can cause neurological deficits or inadequate resection may leave significant residue which can progress, decreasing overall survival of the patient. Preoperatively MRI is usually done in patients with glioma which gives us valuable information about the nature of the lesion. Important sequences being FLAIR, contrast study, perfusion study, DTI, functional MRI (fMRI)and spectroscopy. Information from these sequences are compared with the ioMRI providing the valuable information for improving the safety and efficacy of the resection [12–14].

ioMRI is an ideal tool for the resection of low grade gliomas (LGGs) because of their superior resolution in differentiating tumor from the surrounding brain, it allows accurate localization of residual tumor. It allows near-real-time assessment of extent of resection and also allows correcting for the brain shift, a disadvantage for Neuronavigation, which happens as surgery progresses. In LGGs, ioMRI flair sequence compared with preoperative image shows the residual tumor. Perfusion study will demonstrate the hyperperfusing area around the surgical cavity and thereby increase the extent of resection. In high grade gliomas (HGGs) usually the lesions are contrast enhancing, post-contrast study in ioMRI will show us the extent of resection of contrast enhancing tumor. Perfusion study helps us to identify the hyperperfusing areas in the non-enhancing part of HGGs which can be resected. Resecting hyperperfusing areas in HGGs will definitively increase the extent of resection and thereby increasing progression free survival [12–14] (**Figures 4** and **5**).

#### **Figure 4.**

*A. Preoperative MRI of a patient showing T2 hyperintense LGG involving right medial frontal lobe B. ioMRI showing residue around the surgical cavity which was excised in the same sitting. C. Post-op MRI showing.*

**Figure 5.**

*A. Preoperative MRI of patient with high grade lesion which was contrast enhancing, involving left caudate and periventricular region. B. ioMRI shows no residue of the contrast enhancing lesion.*

ioMRI is a valuable tool in awake craniotomy. ioMRI shows the location of residual tumor and one can perform fMRI to correlate its position with eloquent areas. If the residue is in the region of BOLD signals of fMRI then one can leave behind the residue without causing any permanent neurological deficits. Intraoperative neuromonitoring will guide us the intactness of long fiber tracts. ioMRI in intraaxial tumors involving or in proximity with these tracts will tell us the extent of residue as well as the intactness of tract using DTI imaging. During resection of the intraaxial tumors, sometimes the wall of cavity collapses and blinds a part of tumor. Deep seated tumor or tumorat difficult angles/corners may be left behind. These tumor residues can be located and resected with ioMRI guidance with additional navigation support from the newly acquired images [12–14].

In the prospective studies done by Senft and colleagues [15] and Hatiboglu and colleagues [16] with ioMRI guidance, it was established that the MRI group had a complete resection of their enhancing tumor compared with the control group [4, 16]. Also in nonenhancing tumors Hatiboglu and colleagues showed increased complete resection from 63–80% with the help of ioMRI [16]. Pamir MN et al. studied 56 patients of LGG who underwent resection with ioMRI. They found that the use of ioMRI increased the number gross total resection of from 31 to 41, up by 32.3% [17]. Coburger and colleagues in their multicenter retrospective assessment of LGGs surgery under ioMRI guidance showed that high-field ioMRI was significantly associated with gross total resection (GTR). WithGTR in 85% of cases compared with 57% with a low-field ioMRI [18]. Similarly ioMRI also used in other intraaxialtumorsandintraventicular tumors, which guides the extent of resection and location of residue.

With ioMRI we had achieved significant reduction in residual tumor volume. The mean residual tumor volume improved from 22.5 cm3 to 11.7 cm3 after ioMRI in 29 patients of LGGs. Also the overall extent of resection improved from 72.9% to 88.4% with ioMRI.

#### **4.2 Extraaxial tumors**

#### *4.2.1 Sella-suprasellar tumors*

ioMRI role is well established in the pituitary tumors. It is important for both functional and nonfunctional pituitary adenomas. With the advent of endoscopic

#### **Figure 6.**

*A. Pre-operative MRI showing pituitary macroadenoma B. ioMRI showing residue (arrow) in left side which was removed in the same sitting after ioMRI.*

pituitary excision the extent of resection has significantly increased compared to that of the microscopic approach, though the complete excision is still around 50–60%. The residual disease is strongly associated with complications like postoperative hemorrhage, need for adjuvant radiotherapy or hormonal therapy, significant higher risk of adenoma regrowth and possibly reduced life expectancy. Thus Gross total resection is recommended for both NFPAs and FPAs. With ioMRI one can locate the residue and chase it. In functional adenomas complete resection is mandatory to achieve cure, with ioMRI it is achievable. With ioMRI normal pituitary can be identified and preserved so that we can avoid post-operative hypopituitarism [19–21].

In various large series involving non-iMRI-guided transsphenoidal endoscopic resection of pituitary tumors, Dehdashti AR et al. [22] reported gross total resection of 79% and Serra C et al. [23] reported between 44–88%, while on analysis of studies that involved iMRI guided eTSS for PAs, average initial gross total resection rates at iMRI was only 51% which was increased to 73% help of ioMRI guided resection [24]. Berkmann et al. [25] observed new onset hypopituitarism of any one of the axes in 29% patients in iMRI guided resection group versus 45% in control group operated without iMRI guidance. They also observed that post-operatively RT was required in 3 patients in group without ioMRI compared to that none of the patients in ioMRI group.

ioMRI also used in craniopharyngiomas, one can assess whether adequate decompression has been achieved like decompression of optic chiasm. It also gives us information about contrast enhancing residue if any that is accessable for resection.

In our centre with ioMRI, we achieved gross total resection rate from 52–80% (p value <0.05) in 57 patients of pituitary macroadenoma (**Figure 6**).

#### *4.2.2 Other extra axial lesions*

Large extra axial lesions in the CP angle and skull base are difficult to excise completely due to its relations with cranial nerves, blood vessels and vital neural structures like brainstem. Due to its complexitysometimes surgeons lose the direction or leave behind large residues. In such cases ioMRI gives valuable information about volume and location of the residual lesion. When gross total resection of skull base lesions is not feasible then ioMRI can be a used for tailored tumor resection.

#### **Figure 7.**

*A&B preoperative CT image showing CP angle meningioma, C. ioMRI showing the residue along the brainstem which was difficult to mobilize hence it was left behind and size was less than 2 cm which was subjected for SRT, D post\*-operative CT showing residue without any operative site hematoma. In this patient ioMRI helped to guide the surgeon to stop further resection and safely subjected for SRT without causing any neurological deficits.*

With ioMRI one can achieve maximum safe resection and decrease the size of residue so that it can become suitable for sterotactic radiosurgery [26] (**Figure 7**).

Mario Giordano et al. [27] recruited 19 patients of para-sellar meningiomas includes clinoidal, tuberculum sellae, and cavernous sinus who underwent surgical resection using intraoperative MRI. In 7(37%) of 19 patients, further tumor resection was performed based on information from the ioMRI. 56% of patients with cavernous sinus meningioma benefited by ioMRI by further safe resection of tumor. Dr. Chakraborty et al. [28] conducted a retrospective review of 70 operations performed on 66 patients with intracranial meningiomas. Among them 30 were skull base meningiomas. 9(12.8%) patients required additional tumor resection based on ioMRI findings, and in 4 patients (6%), ioMRI imaging allowed for the avoidance of further dissection near-critical neurovascular structures (**Figure 8**).

Hussam Metwali et al. [29] performed a retrospective analysis of 15 patients with skull base chordomas with ioMRI. 8 patients had complete resection confirmed by ioMRI. Out of 7 patients 3 hadtumor residual requiring further resection was located in the clivus and in 4 patients in the intradural space. All the intradural residue patients had significant improvement in preoperative deficits which was possible with ioMRI guidance for locating the residue. Joseph C. Dort et al. [30] did a prospective, non-randomized, cohort study on 31 patients with skull base lesions.

#### **Figure 8.**

*A&B ioMRI images of sphenopetroclival meningioma, initially tumor was decompressed by subtemporal approach but due to intra operative bleeding and hard calcified which was unable to mobilize. In this case ioMRI helped surgeon to approach by retromastoid craniotomy and achieve further resection of tumor as safely as possible in the same sitting. C&D. post-operative MRI images showing residual tumor. Volume of the residue is significantly reduced compared to that of in ioMRI.*

All these patients underwent surgery in a 1.5-Tesla ioMRI suite. 11 out of 31 patients surgical course altered with the help of ioMRI and maximum safe resection was achieved. ioMRI is a valuable adjunct to skull base surgery.

In our center we did ioMRI in 30 skull base meningiomas. 16 patients had residual lesion, of which 12 patients had subjected for further resection in same sitting. They had significant reduction in volume of residual lesion. we achieved Simpson grade 2 excision in 6 out of 12 patients. Other 4 patients, residual lesion was not chased due to its proximity to neurovascular structures.
