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Imaging Techniques Used in Pre-surgical and Preprocedural Planning in Neuro-Oncology

There are several imaging techniques with different strengths and weaknesses that can be used for preprocedural planning. These techniques are summarized in Table 49.1 and further described in the subsequent sections.

Table 49.1 Imaging techniques in pre-surgical and preprocedural planning
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Computed Tomography (CT)

CT scanners are widely available and are often used as the initial imaging technique to diagnose and plan neuro-oncological patients’ treatment. CT imaging is a fast technique that provides essential anatomic information about skin and osseous landmarks and is often combined with the intracranial soft tissue information provided by MRI. CT angiography is used to visualize the arterial and/or venous system, providing crucial information for planning purposes. CT images can also be combined with PET for treatment planning purposes, particularly for systemic malignancies. CT myelography has a role in the accurate demarcation of the intradural contents and dural sac for some pathologies (e.g., chordoma) or patients with contraindications for MRI.

Conventional MRI Techniques

Magnetic resonance imaging uses different body tissues’ behavior while exposed to magnetic fields to create images that are particularly advantageous for the visualization of soft tissues and can be combined with the information obtained from other modalities such as CT or PET. Pre-surgical or pre-treatment planning often requires high-resolution MRI sequences that provide detailed anatomic information, clear visualization of the target pathology, and separation from the adjacent normal structures.

Standard MRI protocols for preoperative or pre-therapeutic planning often include the following MRI sequences:

  • Pre-contrast T1-weighted sequences are useful for high-resolution visualization of normal anatomy and baseline pre-contrast assessment of the target pathology.

  • Contrast-enhanced T1-weighted sequences help visualize vascular structures and define enhancing areas (secondary to gadolinium-containing agents’ leakage).

  • T2-weighted sequences provide excellent visualization of water-containing structures such as CSF spaces or cystic components of the target pathology.

  • Other MRI sequences used for pre-surgical or pre-radiation planning include fat-suppressed (T1- or T2-weighted) sequences for the visualization of skull base or orbital pathologies, susceptibility sequences (e.g., susceptibility-weighted imaging, SWI) for the identification of intralesional hemorrhage or mineralization, and advanced MRI sequences as detailed below.

DTI and Tractography

Diffusion tensor imaging is an MR imaging modality that uses the water diffusivity information obtained with particular echo-planar MR sequences for the definition and visualization of white matter tracts. Crucial white matter tracts can be reconstructed noninvasively utilizing this modality, and this information can have a significant impact on pre-surgical or pre-treatment planning [1,2,3,4,5,6,7]. Frequently used white matter structures include the corticospinal tracts, arcuate/superior longitudinal fasciculi, inferior fronto-occipital fasciculi, optic radiations, inferior longitudinal fasciculi, and frontal aslant tracts [2, 8]. Diffusion tensor imaging and other advanced diffusion techniques can also be used to characterize different tumoral components and their differentiation from non-infiltrated brain parenchyma [9].

Functional MRI (fMRI)

Echo-planar sequences can also be used to visualize cortical areas showing increased regional flow due to the blood-oxygen-level-dependent (BOLD) effect during functional tasks (task-based fMRI) [10] or spontaneously during rest (resting-state fMRI or rs-fMRI) [11,12,13,14]. Identifying eloquent areas noninvasively using fMRI allows the surgeon or radiation oncologist to avoid these areas during surgery or radiation planning and assess preprocedural risk before any intervention or treatment [11, 13].

Additional Advanced MRI Sequences

Some specialized MR sequences can provide additional information that improves surgical resection or radiation treatment accuracy [15,16,17,18]. These sequences include the following:

Perfusion Imaging

MR imaging can also be used to assess the volume of saturated protons or intravascular contrast entering a volume of interest over a defined amount of time, providing metrics of tissue perfusion. MR perfusion can be obtained without contrast using specialized subtraction techniques before and after magnetic saturation of intravascular protons. Perfusion techniques using contrast rely on the decrease of signal when using echo-planar images (T2* or DSC perfusion) or increased T1 relaxation produced by the leakage of contrast (T1 or DCE perfusion). Perfusion imaging can result in a more accurate demarcation of tumoral margins in high-grade neoplastic pathologies [19].

MR Spectroscopy (MRS)

MRS uses the properties of different chemical compounds in the MR environment to characterize intracranial pathologies and improve the visualization of subtle areas of pathological involvement that may not be distinctly seen with conventional MR images [20, 21]. MR spectroscopy can also help identify oncometabolites, such as 2-hydroxy-glutarate (2HG), that can be used for pre-surgical mapping or pre-radiation planning [22].

Positron-Emission Tomography (PET)

The high contrast-to-noise ratio produced by some radiopharmaceutical tracers makes the use of PET imaging an attractive technique to improve the accuracy of pre-surgical and pre-radiation planning. Radiotracers such as a fluoro-deoxy-glucose (FDG) will have a high background uptake due to the cerebral cortex’s high metabolic rate and glucose consumption, limiting their value for preprocedural planning for intracranial pathologies. However, PET imaging is beneficial outside of the central nervous system due to the relatively low background uptake in most soft tissues. Other radiotracers have shown their clinical value for the diagnosis and treatment planning of brain tumors, particularly radiolabeled amino acids such as C11-methionine [23], O-(2-[18F]fluoroethyl)-L-tyrosine (FET) [24, 25], and others [26, 27] (Table 49.2).

Table 49.2 Properties of PET radiotracers commonly used in neuro-oncology
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Combined PET-MRI

The prospective real-time and accurate co-registration produced by the simultaneous acquisition of MRI and PET images can further improve the planning steps’ accuracy before surgery or radiotherapy [28,29,30].

The Use of Imaging Techniques in Specific Clinical Scenarios

Pre-surgical Planning for Intracranial Tumors

The combination of high-resolution CT (without or with contrast) and high-resolution contrast-enhanced brain MRI is frequently used as the imaging work-frame for pre-surgical planning. These pre-surgical imaging datasets are typically fused using specialized software, and surgical trajectories can be planned before the procedure. Preoperative imaging has a significant positive effect on the surgical procedure’s performance and outcome [48].

Lesions that are near eloquent areas (e.g., Wernicke’s or Broca’s areas, peri-Rolandic regions, primary visual cortex) can be particularly challenging and would benefit from the use of fMRI [49, 50] and DTI [1, 2, 6,7,8, 51] for pre-surgical planning (Fig. 49.1). These techniques will help the surgeon avoid functionally eloquent cortical areas or white matter tracts near the target lesion. A significant number of studies show the crucial clinical value of fMRI and DTI in the pre-surgical planning of patients with brain tumors [17, 49, 50, 52,53,54,55]. Identifying tumoral infiltration beyond the expected margins based on conventional sequences using MR perfusion and MR spectroscopy can also impact pre-surgical planning. Still, these sequences’ use for this purpose is more variable in clinical practice [56]. Despite the additional information provided by these sequences, the surgical debulking is often limited by the potential morbidity produced by more extensive resections.

Fig. 49.1
figure 1

Example of MRI sequences obtained for pre-surgical planning in a young patient with a left temporal IDH1 mutant anaplastic astrocytoma. Axial FLAIR (a), T2 (b), and postcontrast T1-weighted images (c), choline/creatine maps from a multivoxel MRS (d), single-voxel MRS (SVS) using an echo time of 97 ms (e), snapshots of language fMRI activations (f), and left arcuate fasciculus tractography (g). Heterogeneously decreased FLAIR signal within the tumor is compatible with a “T2-FLAIR mismatch” (compared to the tumor’s homogeneous hyperintensity on the T2-weighted image) and suggestive of an IDH mutant tumor. Multivoxel MRS shows a gradient of decreasing choline/creatine ratios from the tumor into the mesial left temporal lobe (d). Single-voxel MRS showed a small 2HG peak at around 3.5 ppm after special post-processing of this spectrum. The SVS MRS also displayed elevated choline/creatine and decreased N-acetyl acetate/creatine ratios consistent with an intermediate grade astrocytoma. The patient showed left hemispheric dominance for language, and the left arcuate fasciculus appeared partially effaced/infiltrated by the mass

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Pre-radiation Planning for Intracranial and Skull Base Tumors

The accurate demarcation of the target pathological volume, exclusion of normal tissues adjacent to the target, and definition of areas for different radiation exposure are the core principles of pre-radiation planning. The following terms are often used in pre-radiation planning and are necessary to understand the role of imaging during pre-radiation planning:

  • Gross tumor volume (GTV) represents the total neoplastic volume shown on conventional MRI, CT, or PET images.

  • Clinical target volume (CTV) includes the GTV and suspected surrounding areas likely containing microscopic tumoral infiltration.

  • Internal target volume (ITV) includes the CTV and a margin of error due to motion. This volume is often dismissed for cranial radiosurgery if motion artifacts are minimal.

  • Planning target volume (PTV) includes the ITV and a potential margin of error due to geometrical errors during the planning process.

  • Treated volume (TV) is the total tissue volume that receives the planned radiation dose and is the same as the prescription isodose volume for practical purposes.

  • Irradiated volume includes all tissues exposed to a significant level of radiation.

A similar combination of high-resolution CT and structural MRI datasets is often used for pre-radiotherapy planning for intracranial malignancies [57, 58]. There is preliminary evidence of a potential beneficial role of MRS [56, 59,60,61,62], MR perfusion [19], and DTI [18, 63, 64] techniques to define areas of microscopic tumoral infiltration outside of the GTV target. Radiation planning can also benefit from structural and functional connectivity information to minimize the collateral damage adjacent to the treated areas [65,66,67]. Recent publications explore the possibility of using deep learning and combined multimodal imaging to improve radiation planning’s accuracy and effectiveness [68,69,70] (Fig. 49.2).

Fig. 49.2
figure 2

Example of pre-radiation planning . Axial FLAIR (a), axial, and coronal T1-weighted images (b, c) and fused radiation maps (d). This patient with a right frontal diffuse astrocytic glioma (IDH-wild-type, EGFR-amplified, molecular features of GBM) received a standard chemoradiation regimen after surgical debulking of his right frontal tumor

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