Lighting the Path
Tracing the tracts with Diffusion Tensor Imaging
Imaging has grown much beyond its morphologic aspect to providing
functional and physiologic details. With current non-invasive MRI
techniques, one may assess the Tumour grade, suggest the best possible
surgical approach, prognosticate as well as perform post treatment
follow-ups to see for treatment response, residue and possible recurrence.
Where on one hand non-contrast sequences like Arterial Spin Labelling
used to assess tumour blood flow have the advantage of being non-invasive,
sensitive, cost effective and repeatable, sequences like Diffusion
Tensor Imaging provide further insight to Tumour characteristic and
help in pre-operative planning.
Diffusion Tensor Imaging (DTI) provides superior visualisation and
quantification of Tumour involvement in relation to various white
matter tracts. It can delineate tract invasion and displacement. These
not only helps to discriminate between diffuse and focal brain stem
Tumours, grade these masses but also to guide the best path to be
followed during surgical biopsies and excisions. It has been quite well
established in supratentorial masses but with the present state-of-the-art
imaging techniques, we can achieve considerable level of sensitivity
for brain stem lesions as well.
This was well-projected with a recently performed MRI on a two
year old girl who was experiencing imbalance while walking, making
her prone to frequent falls. A 3.9 x 3.7 x 3.1 cm sized, heterogenous
mixed signal intensity, predominantly T2 hyper-intense, well-defined
lesion was detected with its epicentre in her right half of pons and an
exophytic component extending into the right cerebella-pontine angle
cistern. The lesion was hypo-refused on Arterial Spin Labelling as well
as DSC perfusion.
Diffusion Tensor Imaging clearly depicted the relation of this pontine
mass lesion with the adjoining white matter tracts. The mass was
seen displacing these tracts changing their colour hues rather than
destroying them. The corticospinal tracts, was seen displaced along
its right lateral margin, whereas the right middle cerebellar peduncle
and central tegmental tract were seen effaced along its right lateral
margin. No major white matter tract was identified along the posterior
aspect of the lesion, thereby making this approach suitable for surgery.
On histopathology this pontine mass turned out to be WHO Grade I,
localised pontine pilocytic astrocytoma.
Given the present advanced imaging techniques, Tumour imaging
protocols must include advanced sequences like Arterial Spin Labelling,
DSC perfusion, 3D BRAVO, Neuronavigation, MR spectros-copy as
well as Diffusion Tensor Imaging to provide all possible morphologic as
well as functional details, besides routine morphologic sequences like
T1WI, T2WI, DWI, SWAN and FLAIR.