MR Imaging Techniques of Pediatric Brain Tumors

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1		Alok Jaju	--	1337	2022-04-22 05:03:33	\|
2	references added.	Beatrix Zheng	Meta information modification	1337	2022-04-22 08:38:22	\| \|
3	typos corrected.	Beatrix Zheng	+ 3 word(s)	1340	2022-04-22 08:39:21	\|

This entry is adapted from the peer-reviewed paper 10.3390/diagnostics12040961

Imaging plays a central role in the diagnosis, characterization, treatment planning, and disease surveillance of intracranial tumors. Magnetic resonance imaging (MRI) is the mainstay of neuroimaging and provides anatomical details, as well as cellular, vascular, and functional information for brain tumors. Imaging features, in combination with location, demographics, and clinical presentation, can help arrive at an accurate diagnosis or a narrow differential diagnosis.

pediatric brain tumors MRI advanced imaging medulloblastoma embryonal tumors pediatric glioma juvenile pilocytic astrocytoma

1. Introduction

Primary brain tumors are the most common solid neoplasms in children and a leading cause of mortality in this population. Brain tumors have a reported incidence of 6.06 per 100,000 children 0–19 years of age in the United States ^[1]^[2].

2. Imaging Techniques

Due to its speed and availability, computed tomography (CT) of the head without contrast may be the initial modality for screening patients with signs and symptoms suggestive of an intracranial space-occupying lesion. CT can provide useful information about the presence of a mass, its location, associated mass effect, and hydrocephalus. CT may identify solid and cystic components, hemorrhage, and calcifications, but Magnetic resonance imaging (MRI) is required for adequate tumor characterization staging. Post-contrast CT is typically of little added benefit, and a lesion initially identified on non-contrast CT should be further evaluated by MRI.

MRI can be performed on a 1.5 or a 3 Tesla scanner; however, a 3 Tesla scanner has the advantages of better signal-to-noise ratio and faster imaging. MRI study times vary but often require 30–60 min. Therefore, sedation or general anesthesia is typically needed in children younger than six years of age, although this can be assessed on a case-by-case basis. Child life specialists can be very helpful in improving patient compliance. The use of gadolinium contrast material is standard for tumor imaging, although there may be some exceptions.

A conventional brain MRI includes pre- and post-contrast T1-weighted, T2-weighted, fluid-attenuated inversion recovery (FLAIR), and diffusion-weighted sequences. The use of high-resolution, thin section, 3-dimensional imaging, which can be reformatted in multiple planes, is now standard for T1-weighted imaging and is gaining wider acceptance for T2 and FLAIR sequences. In addition, technical advances in imaging methodology such as compressed sensing and parallel imaging allow for the acquisition of high-resolution and thinner slices with a reasonable scan time. The Response Assessment in Pediatric Neuro-Oncology (RAPNO) working groups have published guidelines for minimum essential MRI sequences for different types of pediatric brain tumors, and the reader is referred to those detailed recommendations ^[3]^[4]^[5]^[6].

Diffusion-weighted images measure the mobility of water molecules, which in turn depends on the complexity of cytoarchitecture. High-grade tumors, because of their higher cellularity and high nuclear-to-cytoplasmic ratios, demonstrate restricted diffusion, which can be measured quantitatively as decreased apparent diffusion coefficient (ADC) values ^[7]. Diffusion-weighted imaging is very valuable for the differential diagnosis of brain tumors as well as for the detection of leptomeningeal metastasis, particularly for non-enhancing tumors ^[8]. Susceptibility-weighted imaging (SWI), which is highly sensitive for paramagnetic or diamagnetic compounds and can detect areas of hemorrhage or calcifications, is also commonly included in standard brain imaging ^[9].

2.1. Advanced Techniques

Advanced imaging techniques may provide microstructural, hemodynamic, and metabolic information, and can be performed at most centers, although they do require additional imaging time and post-processing capabilities. Diffusion tensor imaging (DTI) assesses the magnitude as well as the direction of water diffusion and can be post processed to generate white matter tracts. DTI and tractography can be overlaid on anatomical images to provide valuable information about the relationship of the tumor to the major white matter tracts and help guide the surgical approach ^[10].

MR perfusion techniques provide an assessment of tumor vascularity and hemodynamics. Perfusion can be performed using contrast-enhanced techniques, such as dynamic susceptibility (DSC), dynamic contrast-enhanced (DCE) perfusion, or non-contrast arterial spin labeling (ASL) technique. Depending on the method use, MR perfusion can generate hemodynamic parameters, such as relative cerebral blood flow (rCBF), relative cerebral blood volume (rCBV), time to peak (TTP), mean transit time (MTT), and vascular permeability or transfer coefficient (K-trans). These can provide valuable insights into tumor grade, treatment response, and help in differentiating tumor from radiation injury ^[10].

Magnetic Resonance Spectroscopy (MRS) provides an analysis of the biochemical composition of the tissue and can be useful for the differential diagnosis and grading of tumors. Proton MRS is the most widely used technique and can be performed as single-voxel or chemical shift imaging (multi-voxel). High-grade tumors typically have elevated choline peaks, decreased n-acetyl aspartate (NAA) peak, and variable lipid/lactate peak. However, there are exceptions, and low-grade tumors such as pilocytic astrocytoma can demonstrate a similar spectroscopic appearance. Short TE MRS (35 milliseconds) allows the detection of additional metabolite peaks, such as myo-inositol (mI, a glial cell marker), glycine, glutamine/glutamate (Glx), taurine (Tau), alanine (Ala), and citrate (Cit), which can be further helpful in tumor characterization. MRS can also be helpful in differentiating tumors from post-treatment change ^[11]^[12].

Functional MRI (fMRI) uses regional changes in brain perfusion induced by brain activation to generate an MRI signal. This can non-invasively map various eloquent areas, such as motor, sensory, language, and visual, during the performance of specific tasks. This information can be used by neurosurgeons to plan the approach and extent of tumor resection ^[13].

2.2. Spine Imaging

Based on the suspected tumor histology, imaging of the spine may be added for the evaluation of cerebrospinal fluid (CSF) dissemination. As with intracranial disease, contrast is typically required for spine evaluation, although high-resolution 3D steady-state T2 imaging, such as constructive interference in steady state, Siemens (CISS), or fast imaging employing steady-state acquisition, GE (FIESTA-C), can be particularly helpful in identifying small lesions and non-enhancing disease along the surface of the spinal cord or cauda equina nerve roots ^[14]. Limited spine imaging with post-contrast T1 and 3D steady-state T2 sequences is usually adequate for routine metastatic surveillance, without the need for pre-contrast imaging, thus decreasing the total imaging and, if applicable, anesthesia time in these patients. As with intracranial disease, DWI can be a valuable technique for metastasis detection in the spine, especially for non-enhancing lesions. However, due to the relatively poor resolution of DWI in the spine, it has limited added value for routine surveillance spine MRI and is often best used as a problem-solving tool ^[15]. It is strongly recommended that spine MRI for initial staging in the setting of a newly diagnosed posterior fossa tumor be performed pre-operatively, as blood products and dural reactions can significantly limit interpretation in the immediate post-operative period. If the spine MRI is not performed pre-operatively, waiting for at least 2 weeks after posterior fossa surgery is recommended to avoid false positive results ^[16]^[17].

Recent advances in artificial intelligence and computer vision in medicine have opened a new frontier in neuro-oncology research. Techniques such as radiomics and deep learning have shown great promise in prospective radiological diagnosis, molecular subtype prediction, and outcome predictions for both pediatric and adult brain tumors ^[18]^[19]^[20].

2.3. Post-Operative and Surveillance Imaging

Post-operative imaging is essential to evaluate the extent of surgical resection, to identify any immediate post-operative complications, and as a baseline for assessing the response to radiation and chemotherapy. In specialized centers, intraoperative MRI may also be used to guide surgical management, with recent advances allowing the use of high-field-strength systems, diagnostic-quality images, and the ability to integrate with the surgical navigation systems, making it an important tool for neurosurgeons ^[21]^[22]^[23].

Post-operative MRI should be performed within 72 h of surgery and preferably within the first 24 h, as distinguishing residual tumors from reactive enhancement on delayed imaging can be challenging ^[24]. In the post-operative period, hemorrhage along the resection margins may demonstrate T1 shortening; therefore, a direct comparison of pre- and post-contrast images obtained in the same imaging plane is important to distinguish hemorrhage from true enhancement. DWI is a very helpful technique for both post-operative and long-term surveillance imaging. In the immediate post-operative period, restricted diffusion along the surgical margins may be related to post-surgical change, and a careful correlation of these areas with contrast enhancement on follow-up imaging should be performed to distinguish post-surgical enhancement from tumors. DWI can also be helpful for assessing residual or recurrent disease in high-grade tumors ^[21].

References

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Fahmideh, M.A.; Scheurer, M.E. Pediatric Brain Tumors: Descriptive Epidemiology, Risk Factors, and Future Directions. Cancer Epidemiol. Biomark. Prev. 2021, 30, 813–821.
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Subjects: Radiology, Nuclear Medicine & Medical Imaging

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Alok Jaju

Maura E Ryan

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Update Date: 22 Apr 2022

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