Magnetic resonance imaging brain sequences in pediatrics

Orbit, inner ear, and pituitary lesions

To assess orbital lesions, including optic nerves and optic pathway and the pituitary, dual-echo STIR and T1W SE fat-saturated (FS) sequence in coronal and axial planes are used to suppress orbital fat with a slice thickness of 3 mm and small matrix size to increase tissue contrast. If contrast is given, then FS T1W axial and coronal is used. For pituitary lesions, sagittal and coronal T1W and coronal T2W with additional T1W coronal and sagittal post-contrast are used. For inner ear structures, including internal auditory canal, heavily T2W sequences such as CISS 3D (SIEMENS), FIESTA in GE, and DRIVE in Philips are used apart from the standard.^[9]

We use thin 3 mm slice thickness, axial and coronal STIR or FS T2W, and sometimes oblique sagittal in plane with optic nerves to assess orbit and optic nerves. If contrast is used, then FS T1W axial and coronal are added. Same sequences can be used to assess paranasal sinuses and the pituitary gland with the addition of FS T1W sagittal. Dynamic contrast study in coronal planes for the pituitary gland can be used if indicated. Post-contrast T1 MP-RAGE has not been found useful to assess contrast enhancement in the pituitary gland or orbit due to air (of paranasal sinuses) tissue interface artefacts.

Brain tumors

For characterization of brain tumors in terms of location, size, multiplicity, spread, and complication like hydrocephalus, standard brain sequences with contrast as described above are used. In addition, all children with brain tumors should have their spine imaged, preferably with contrast T1W FS sequence, to detect distant metastases.^[21] It is wise to use a volumetric 3D sequence such as pre-contrast FLAIR or post-contrast T1W (MP-RAGE) without using ear muffs for neuronavigation guided surgery.

Advanced imaging techniques such as MR contrast enhanced dynamic perfusion (DSC), arterial spin labeling perfusion (ASL), MR spectroscopy, and diffusion tensor imaging are usually used to characterize tumor type but rarely required in children. However, these advanced imaging techniques are useful in post-operative children who have received radiation to differentiate between tumor recurrence and radiation necrosis. In children, ASL perfusion study can be useful which does not require contrast.^[22]

Immediate post-operative MRI scans within 72 h are usually performed with contrast to avoid interference of postoperative changes such as edema and blood products with tumor residue.^[21] Sometimes intra-operative MRI scans are done to assess tumor residue, preferably with contrast. It is important to find pre-operative image characteristics of tumors while doing intra-operative scans to use appropriate sequences which can highlight the residual tumor.

Follow-up MRI brain scans in treatment received brain tumors should include imaging of the spine even if there is no residual tumor since early detection of spinal metastases helps improve overall survival.^[23,24]

We prefer to add post-contrast FLAIR axial for brain and FS T1W sagittal sequence for spinal imaging to detect distant metastases. Post-contrast FLAIR is found to be more useful to detect leptomeningeal metastases.^[25]

Infections

Standard imaging protocols should be used along with contrast to characterize the type of lesions. Usually, in meningitis and meningoencephalitis, addition of post-contrast FLAIR along with T1W MT sequence yields better visualization of meningeal enhancement.^[26] In our practice, both post-contrast FLAIR and T1W with MT are found useful; however, 3D T1 MP RAGE sequence is not found to be adequate to evaluate meningeal enhancement [Figure 1]. Sometimes it is useful to find the source of infection such as mastoiditis or sinusitis and add additional sequences accordingly. Screening of spine with post-contrast T1W sagittal FS sequence will show any changes of spinal meningitis.

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Figure 1:

(a) Non-contrast fluid attenuation inversion recovery (FLAIR), (b) post-contrast FLAIR, (c) post-contrast T1W with magnetization transfer (MT), (d) axial reconstruction of 3D T1 magnetization prepared rapid gradient echo (MP-RAGE). In this case of meningitis, meningeal enhancement is clearly visible on post-contrast FLAIR and T1 with MT and not seen in T1 MP-RAGE.

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In cases of ring-enhancing lesions, the addition of heavily T2W sequence, DWI, SWI, MR spectroscopy, or perfusion will help to accurately arrive at the diagnosis. These advanced imaging sequences will help differentiate between metastatic ring lesion, abscess, tuberculoma, or neurocysticercosis. CISS sequence is useful to detect scolex in cysticercosis, SWI is useful to detect calcification in granulomatous disease and detect hemorrhage in hemorrhagic meningoencephalitis, DWI is useful to detect pyogenic abscess and spectroscopy and perfusion to differentiate neoplastic from non-neoplastic lesions.^[27-30]

Hydrocephalus

Hydrocephalus can be communicating or obstructive. In addition to routine imaging protocol, 3D T2W SPACE/ heavily T2W sequences such as CISS 3D and SWI should be added. 3D T2W SPACE and heavily T2W sequences help identify the type. SWI sequence helps to identify any blood products or previous hemorrhage as a cause of communicating hydrocephalus.^[31] Contrast MRI can be useful to detect the infective cause of communicating hydrocephalus.

CSF flow study can add value in detecting the cause of hydrocephalus. It can be done across the aqueduct or foramen magnum in case of Chiari malformation. It is also useful to detect flow patterns in intracranial arachnoid cysts and in shunt malfunction.^[32]

Metabolic disease and leukodystrophy

In evaluating children with leukodystrophy, majority of the time, standard brain protocol is sufficient. In addition, spine screening with T2W sagittal can be done to assess spinal cord involvement and MR spectroscopy to detect metabolite abnormalities. MR spectroscopy can show pathognomonic findings in a few leukodystrophies like Canavan disease and metabolic diseases like mitochondrial cytopathies.^[33]

Demyelinating diseases

In addition to routine standard imaging protocols, imaging of orbits and spinal screening should be performed, preferably with contrast. Orbital coronal STIR is used to detect optic neuritis and enhancement on coronal FS T1W. Spinal screening can be done with T2W sagittal and post-contrast T1W FS sagittal to detect spinal lesions and their enhancement. Post-contrast FLAIR will be useful especially to detect periventricular lesions and their enhancement. 3D FLAIR or sagittal FLAIR adds value to detect corpus callosal lesions. Similar protocols should be followed in follow-up scans for studying the comparative images.^[34]

Epilepsy

Imaging protocols in children with epilepsy differ from routine imaging protocol. A few centers use axial T2W fast spin-echo, 3D T1W volume acquisition reconstructed in three planes, coronal T2W fast spin-echo, coronal FLAIR (or 3D FLAIR), and hippocampal T2-relaxometry. In children less than 2 years, coronal T2W is replaced by dual-echo STIR sequence. The coronal sequences are used with planes perpendicular to hippocampus.^[9] T2 relaxometry is rarely used today, but it is useful to obtain T2 values in cases of mesial temporal sclerosis, especially if it is bilateral.^[35]

In our institute, we use 3D FLAIR, 3D MP-RAGE T1W, 3D TSE single slab SPACE, DWI axial, FLAIR axial parallel to hippocampus, coronal dual-echo STIR or T1W IR, and T2W coronal perpendicular to hippocampus with 3 mm slice thickness and SWI. With the use of these sequences, most of the migrational anomalies, temporal and extratemporal lesions, and calcifications/hemorrhagic lesions can be detected. We recommend use of 3D T2W SPACE sequence which is a single slab 3D T2W image (PHILIPS) with the ability to reconstruct in all three planes without degrading image quality.

In some children with refractory epilepsy and normal MRI in the first 2 years of life, a second scan after brain maturation (at least 6 months apart) to detect the focus of epilepsy is required.^[36]

The use of 3 tesla MRI is more useful in detecting the focal epileptogenic lesions as compared to 1.5T MRI due to better image uniformity, signal to noise ratio, and spatial resolution.^[37,38]

Functional MRI is specifically useful in the presurgical evaluation of the sensorimotor cortex or language lateralization. Ictal/interictal PET or SPECT are additional modalities to detect seizure focus apart from MR spectroscopy, magnetoencephalography, MT imaging, and diffusion tensor imaging.^[39]

Children presenting with non-traumatic brain hemorrhage

Apart from standard imaging protocol, 3D time of flight (TOF) MR angiography, 2D or 3D phase-contrast MR venogram, and contrast can be used in non-traumatic brain hemorrhage. Sometimes if the child is not stable, CT angiography can be performed. MRI with MR angiogram or venogram should be performed after 72 h of ictus to avoid signal interference from hemorrhage. Contrast sequences are useful to detect any underlying tumor or vascular malformation such as cavernoma or small arteriovenous malformation.^[9,40] We additionally add SWI sequence to detect any previous petechial hemorrhages, which could support the diagnosis of vasculitis or multiple cavernomas.

FAST/ultrafast MRI sequences can be used in unstable patients such as FAST FLAIR, EPI FLAIR, EPI DWI, single-shot FSE T2W/T1W by use of parallel imaging technique or susceptibility-weighted angiography, and HASTE sequence in coronal or axial planes, which reduce the scan time significantly and avoid the need for CT scan.^[41-43]

Children presenting with stroke

The most important causes of stroke in children are cardiac, hematologic, oncologic, infective, vasculopathy/vasculitis, trauma, or drug-induced vasculopathy.^[44] The imaging protocols should be set to detect any of these causes. 3D TOF MR angiography along with DWI and SWI should be done. In 80% of stroke cases, abnormalities are found in the intracerebral arteries, so doing MR angiography is very important.^[45] Extracranial MR angiography with axial dual-echo STIR and FAT SAT T1W sequences of the neck will help detect arterial dissection of neck vessels and also identify vascular wall hemorrhage, especially in case of trauma.^[46] Rapid and ultrafast MRI sequences can be additionally used to fast screen the suspected stroke patients where stroke mimics can be identified by FLAIR sequence.^[47]

Children with non-accidental brain trauma

Although CT scan is the preferred modality in non-accidental brain trauma, MRI is done usually after 3–4 days to detect hypoxic brain injury by DWI (which is better than CT), thin subdural hematoma, or multiple petechial subcortical hemorrhages by adding gradient SWI sequences and also FLAIR. Sagittal T2W imaging of the cervical spine is also recommended to evaluate brainstem or cord injury which could be the result of violent shaking, in turn causing hypoxic injury.^[48-50]

Children presenting with global developmental delay (GDD)/intellectual disability

Pediatric neurologist very often comes across children presenting with GDD. The first investigation offered is MRI. This is because MRI is found abnormal in almost 54.7% of cases with common structural abnormality seen in white matter, corpus callosum, and ventricles. In 39.6% of cases, MRI is helpful in clinching the right diagnosis either directly or indirectly which can be confirmed by other investigations. The protocols used are Axial DWI, TSE T1W, TSE T2W, FLAIR, PD, T2TIRM, coronal T1TIR, and T2 TSE and sagittal T1 TSE.^[51] For initial screening, these protocols can be used but should be modified according to the findings. For example, if there is hydrocephalus or leukodystrophy, then appropriate additional sequences should be added. Thus, it is very important to look at the imaging findings during the scan to modify the protocols to avoid redo or repeat imaging.

Disease-specific recommended MRI protocols are summarized in [Table 1].