Paediatric Neuro - Oncology Imaging

Michael Kean
"MRI for Kid’s"
MRI Unit
Royal Children’s Hospital
Melbourne
AUSTRALIA
FAX +61 3 393455286

Approximately 33% of paediatric CNS tumours occur in patients under the age of 5years. Amongst the most common histologic diagnoses in this group are embryonal neoplasms such as medulloblastoma and primitive neuroextodermal tumours (PNET ) , tumours which have a high propensity for wide spread neuroaxis dissemination. They are the second most common paediatric tumour - exceeded by Leukaemia

This presentation will describe the more common paediatric CNS tumours , the imaging strategies for the complete evaluation of disease (local vs disseminated) and importantly patient care issues.

Tumours of the CNS are classified by the WHO ( World Health Organisation) into two categories primary neuroepithelial tumours and tumours of non-neuroepithelial tissues. Neuroepithelial tumours (gliomas) contain cells derived from the embryonic neuroepithelial tube whilst tumours of non-neuroepithelial tissues ( germinomas / craniopharyngiomas ) originate from the peripheral nervous system , neural crest and other cell types.
Cental nervous system tumours are usually classified histiologically by cell type and degree of malignancy - this differentiation is determined by the morphologic and functionality of the cell as compared to a normal adult cell ( astrocyte ) or embryonic cell type ( medulloblast ) . The degree of malignancy (grade of tumour ) correlates with the loss of histological differentiation (anaplasia ) . In reality the neuroanatomic location, dissemination and respectability play a more significant role in patient’s outcome.
In several large series it has been demonstrated that when the total incidence of paediatric tumours are studied infratentorial and supratentorial lesions occur with equal frequency. Supratentorial lesions are however more frequent in the first 2-3 years of life whereas infratentorial lesions predominate from 4-11 years of age. In patients older than 10 they present with equal frequency. In the first year of life the distribution of lesions is different to the general trends with supratentorial lesions (most common suprasellar astrocytoma grade II or III) seem to dominate. In our recent experience it would seem that Glioblastoma multiforme / PNET are the more common amongst this age group. In the posterior fossa medulloblastoma rather than ependymoma is the more common diagnosis

The symptoms at presentation depend upon the patient’s age, location size and compressive nature of the lesion. Infants generally present with vomiting, irritability, lethargy and increasing head circumference. Older children may present with similar symptoms aswell as seizures, headaches, decreased visual acuity and focal neurological signs. These focal neurologic signs may include nerve palsies, ataxia (truncal or limb) and hemiparesis.

On MRI lesion characteristics ( necrosis , haemorrhage, T1 , T2 , calcification , contrast enhancement - pattern vs intensity ) may vary depending upon tumour classification type of presentation and treatment ( resection , chemotherapy and radiation ) .
 

Posterior Fossa Tumours - Infratentorial Cerebellar Astrocytoma Medulloblastoma Ependymoma Ganglioglioma

Brainstem   Glioma Tectal Pontine diffuse Cervico-medullary

Lesion adjacent to the Third Ventricle Glioma Optic Nerve Hypothalamic Germ Cell Germinoma Pineoblastoma Pituitary Adenoma Malformative type lesions Craniopharyngioma Lipoma Dermoid / Epidermoid Hamartoma

Supratentorial Tumours Glial Tumours Astrocytoma Glioblastoma Ependymoma Oligodendroglioma Choroid Plexus - papilloma / carcinoma Neuronal Tumours Ganglioglioma DNET Neurocytoma Embryonal Tumours PNET Menigioma

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Sedation / Anaesthesia

All pts at initial presentation MUST have a total neuroaxis MRI examination ( Brain / Spine ) before any surgery is performed.
These scans can last 1 hour, and in most cases require the changing of coils so sedation might not be the best option.  The type of sedation used will depend upon the resources of the imaging unit and the availability of MR compatible monitoring equipment. Sedation cases require basic levels of physiological monitoring such as pulse oximetry but in cases of critically ill patients or more intensive levels of sedation where the respiratory drive may be compromised more complex monitoring is appropriate (Pulse Oximetry, Capnography, NIBP) . Our current monitoring is provided using MR Equipment Corporation 9500 (distributed in Australia by MR Devices-Sydney.    Click here for local distributors) which has full physiological monitoring and anaesthetic agent monitoring.

All monitoring equipment used within the magnet environment MUST be MR compatible - if there is any question regarding its compatibility it should not be used.

Sedation Regime

Venous Access
All diagnostic and treatment monitoring scans must be performed using IV contrast. Our current dose rate is 0.6ml per kg (max 25ml) of Magnevist or Omniscan. On presentation all potential venous access points are covered with a vasodilating topical anaesthetic cream - some commercial preparations (eg EMLA) vasoconstrict so the cream needs to be removed 10mins before injection. During treatment these patients have a central line inserted ( infusaport or at our institutional Dual lumen Hickman’s ) which can be accessed to inject MR contrast media. Each hospital Oncology Unit has a policy for appropriate accessing and then heparin locking the line after the procedure. This accessing encroaches into your daily schedule but it is often less traumatic for the patient. Our units policy is to ask the child which they prefer and the tailor the study accordingly - its necessary to liase with nursing staff to coordinate access. If the patient has an Infusaport this can be accessed prior to the examination and the needle-taped insitu.

• clearly defined examination protocol
• protocol that is reproducible
• flexible examination guidelines to reflect changes in technology
• flexible examination guidelines to reflect individual patient / tumour category
• timing issues of the scan – pre or post Op
• treatment protocol requirements
• clear definition as to what is a diagnostic scan
• be careful not to get caught in the trap - diagnostic vs aesthetically publishable images
• pt condition - scan time vs examination sequences vs diagnostic / clinical outcome
• comparison data available – previous imaging

Minimum Examination Requirements

Pre Op Brain / Spine 
within max 36hrs post Op (preferably 24hrs)

Post Op

Low Grade Optic Chiasm Glioma 23/12 old Pt Sagittal T1Post Gd	T1 Sagittal Brainstem Glioma	T2 Sagittal FSE-XL Brainstem Glioma

Coronal T1 Post Gd
This sequence is performed using Magnetisation Transfer for increased conspicuity of meningeal disease. The sequence is performed using short Te and wide bandwidth to reduce data sampling times enabling a reduction in posterior fossa pulsatile artefacts. The TR used should not exceed 700msec ( to maintain adequate T1 weighting ) and this requires a 2 acquisition sequence. The slice coverage must include the anterior cranial fossa as this is an area where metastatic disease is often missed on standard transverse imaging.

Susceptibility Sequence
To detect haemorrhage or Calcium (not always used pre operative but essential post Op)

Sagittal T2
This sequence is important in evaluation of pre + Post treatment Brain Stem Gliomas – in many cases it is far more sensitive to treatment response than Pre contrast FLAIR

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Sagittal T1 Post Gd Spine Series

• Define anatomy
• Staging of disease process
• Status of CSF
• Meningeal disease
• Drop Mets
• Bone involvement

Sagittal T1 Post Gd	Sagittal T1 Post Gd	Sagittal T1 Post Gd + Fatsat

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Imaging Sequence Parameters

Optional Sequences
 

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Alternative 3D FSPGR IR Prep

• TR=21 Te 3 IR 350 Flip 25 10.5kHz 1mm 224/512
• Precontrast T1 volume Acq
• Frameless Stereotaxis
• Less sensitivity / specificity to low uptake mets

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Post Contrast T1 Sequence Options

• CSE vs FSE vs FSPGR ( turboflash )
• FSE - XL short esp , less T1 blurring , efficient T1 sequence ( contrast compatible to CSE ) , susceptible to flow
• Flow comp vs no flow comp - increased TE / reduction slices per TR - ? really necessary - reduction in venous flow artefacts but increased conspicuity of cerebral vessels
• Mag Transfer - reduction in slices per TR , increased posterior fossa venous flow artefacts - should be used to more efficiently delineate leptomeningeal mets and we currently use the mag transfer sequence in the coronal plane
• 2D Fast Spoiled GRE ( FSPGR ) in phase - contrast comparable to T1 at TR > 400 , in phase Te ( 4.2ms at 1.5T) reduces pulsatile flow artefacts
• Spine T1 Fatsat - FSE on GE produces more homogeneous fat suppression - key is to use small FOV + grad shim to improve suppression – external factors such as pt body shape , anatomical boundaries , air tissue interfaces and laryngeal mask may produce ineffective fat or water suppression (inflate laryngeal mask with saline) – We perform fatsat sagittal on the thoracic / lumbar region only .

Spine T1 Trans - CSE vs FSE vs FSPGR

• Non standard sequence / parameter / orientation
• Pt movement
• Flow induced artefacts - contrast based
• Periodic / pulsatile artefacts ( flow / cardiac / resp )
• Sedation / Anaesthesia induces physiological changes - eg higher PCO2 results in increased CSF and vascular pulsatility artefacts
• Sequence induced contrast effects - eg Mag transfer / Fatsat / Inflow etc
• Fatsat inhomogeneities
• Vascular clips
• Contrast enhancement differences between 0.1ml - 0.2ml and 0.3ml/kg

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Recommended Reading:

• Paediatric Neuroimaging – A James Barkovitch : Raven Press
• MRI In Paediatric Neuroradiology – s Wolpert / P Barnres : MosbyYear Book
• Cranial MRI and CT - K Rao , R Zimmerman : Mc Graw Hill Pub
• Intracranial Tumour in Children : MR Imaging findings within 24hrs of Craniotomy ; A Oser , C Moran et al Radiology 1997 , 205;807-812
• Handbook of MRI Techniques - Catherine Westbrook : Blackwell Scientific 1999