The target audience for this lecture is clinical neuroradiologists who interpret MR scans of pediatric neuro-oncology patients as well as translational researchers who seek to design MR techniques that improve our diagnostic capabilities both during the initial workup and following therapy.

1. Review the conventional MR imaging features of common and important uncommon pediatric brain tumors and tumor-mimics.

2. Examine the roles of advanced/quantitative MR imaging techniques in refining differential diagnostic considerations and response to therapy.

Pediatric brain neoplasms most frequently arise from glial, neuronal (or mixed glioneuronal), embryonal, and germ cell lineages. Tumor mimics include tumefactive demyelinating lesion, cerebral abscess, reversible splenial lesion, parenchymal hematoma, herpes encephalitis, subacute infarct, radiation necrosis, epidermoid cyst, and arachnoid cyst.

Clinical factors that can help differentiate these entities include age, gender, symptomatology, and acuity of presentation. Important conventional imaging features include location (e.g., intra-axial versus extra-axial, supratentorial, infratentorial, sellar/suprasellar), enhancement pattern, cystic change, central necrosis, white matter infiltration, cortical involvement, calcification, and hemorrhage. Advanced/quantitative MR imaging techniques can further facilitate an appropriate order to diagnoses under consideration.

A multidisciplinary team of neurosurgeons, radiation oncologists, neuro-oncologists, and neuroradiologists can then formulate an appropriate clinical plan. Should the lesion in question be followed with serial imaging? Should additional advanced imaging sequences be added to subsequent MRI protocols? Should a lumbar puncture be performed for cytologic analysis of cerebrospinal fluid? If surgical biopsy is indicated, what should be the extent of resection?

Advanced/quantitative MR techniques can also serve as a valuable adjunct when monitoring response to surgery, chemotherapy, and/or radiation therapy. Surveillance MR imaging frequently incorporates advanced/quantitative techniques, which occasionally reveal concerning findings that may be extremely subtle (or even absent) on conventional sequences. More frequently, new or evolving findings on conventional imaging can be interrogated with advanced techniques that may modulate the level of clinical suspicion for disease recurrence/progression.

Diffusion-weighted imaging (DWI) has two important roles in the diagnosis and management of pediatric brain tumors. First, neoplasms composed of tightly packed cells tend to restrict extracellular water mobility and, therefore, have reduced apparent diffusion coefficient (ADC) values. In general, embryonal tumors (e.g., medulloblastoma, pineoblastoma, primitive neuroectodermal tumor, atypical teratoid rhabdoid tumor) are the most cellular group and appear strikingly dark on ADC maps. DWI can also detect subtle recurrences that are nearly inconspicuous on conventional sequences. Second, DWI can identify devitalized brain tissue adjacent to the surgical resection cavity on immediate postoperative scans. Similar to subacute infarcts, these regions can transiently enhance on short-term follow-up examinations, which could be potentially confused for tumor recurrence.

Perfusion MRI metrics function as a biomarker for tumor neovascularity and capillary permeability. Techniques include dynamic susceptibility-weighted contrast-enhanced (DSC), dynamic contrast-enhanced (DCE), and arterial spin labeling (ASL). As higher grade tumors generally exhibit greater degrees of angiogenesis, perfusion MRI can suggest tumor grading. In a heterogeneous appearing (i.e., possibly mixed grade) lesion, a neurosurgeon might target the most hypervascular region as the ultimate treatment would be dictated by the highest grade component. As a potential pitfall, the solid component of several low-grade brain tumors (e.g., pilocytic astrocytoma, hemangioblastoma) demonstrates vascular hyperplasia. Therefore, perfusion MRI should not be interpreted without also considering conventional imaging features. When conventional imaging features are indistinguishable, perfusion MRI can help differentiate radiation necrosis (hypovascular) from a high grade glioma (hypervascular).

Proton magnetic resonance spectroscopy (¹H-MRS) can provide the metabolic and biochemical profile of a pediatric brain lesion and, similar to perfusion MRI, may intimate tumor grading and distinguish radiation necrosis from a high grade neoplasm. MRS can be performed with single-voxel or multi-voxel techniques. Single-voxel MRS has the advantage of shorter acquisition times but lacks the spatial resolution to interrogate the internal heterogeneity exhibited by many tumors. Multi-voxel spectroscopy, on the other hand, can assist a neurosurgeon who wishes to target the most suspicious region.

In conjunction with conventional sequences, advanced/quantitative MR imaging techniques can refine differential diagnostic considerations, suggest tumor grade, propose targets for stereotactic biopsy, and monitor response to therapy for pediatric brain neoplasms.