3 Introduction to Brain Imaging
59
reactivity. Furthermore, maps of cerebrovascular reactivity can potentially provide
useful information regarding the nature of the tumour and its boundaries [99].
Cerebrovascular reactivity can be studied with BOLD fMRI, by examining
changes induced by a hypercapnia challenge such as Co2 inhalation or breath-hold
[51, 101]. Hypercapnia induces vasodilation and causes an increase in cerebral
blood flow and oxygen concentration in normal blood vessels, resulting in an
increase in the BOLD signal [65]. In tumours, increased cellularity or necrosis and
low vascular density may cause reduced perfusion and hypoxia [100]. In tumours
with neovasculature, in which there is abnormal microanatomy and cerebrovascular
regulation, blood from a region of the tumour in which the vessels do not dilate
can be redistributed to a responsive tumour region or to surrounding normal tissue,
causing a focal worsening of tumour perfusion. Thus, in tumours with abnormal
cerebrovascular microstructure or abnormal cerebrovascular regulation, low BOLD
reactivity during hypercapnia is expected.
3.2.3 Presurgical DTI Mapping
DTI is a unique method for describing noninvasively the spatial relationship
between brain tumours or lesions and white matter tracts. Combined fMRI and DTI
mapping can reveal both functional regions and their white matter connectivity and
thus provide a more accurate estimate of the proximity of tumour borders to vital
brain networks [42, 92, 108, 125].
DTI can be useful in distinguishing between different effects of space-occupying
lesions such as displacement, disruption, or infiltration of the white matter tracts
by the tumour [23, 49, 108, 134, 135]. White matter fibres may remain intact but
be displaced by a tumour, in which case their anisotropy would be maintained
and their new location and orientation would be detectable on directional DTI
maps. This pattern could confirm the existence of a peritumoural tract that could
potentially be preserved during resection. Alternatively, oedema or infiltration
by the tumour may reduce the anisotropy of tracts, either still leaving enough
directional organisation to remain detectable on DTI maps or completely disrupting
the directional organisation.
DTI-based fibre tractography can be more challenging in cases of brain pathologies with space-occupying lesions, because displaced fibres may not readily be
traced with common methods for seed selection based on anatomical landmarks.
In addition, tumour infiltration and oedema may cause tissue deformation, resulting
in reduced reliability of fibre tracking [38]. Fibre tracking in regions with oedema
can potentially be improved by using sophisticated post-processing algorithms for
separating diffusion properties of neural tissue from surrounding free water [94].
59
reactivity. Furthermore, maps of cerebrovascular reactivity can potentially provide
useful information regarding the nature of the tumour and its boundaries [99].
Cerebrovascular reactivity can be studied with BOLD fMRI, by examining
changes induced by a hypercapnia challenge such as Co2 inhalation or breath-hold
[51, 101]. Hypercapnia induces vasodilation and causes an increase in cerebral
blood flow and oxygen concentration in normal blood vessels, resulting in an
increase in the BOLD signal [65]. In tumours, increased cellularity or necrosis and
low vascular density may cause reduced perfusion and hypoxia [100]. In tumours
with neovasculature, in which there is abnormal microanatomy and cerebrovascular
regulation, blood from a region of the tumour in which the vessels do not dilate
can be redistributed to a responsive tumour region or to surrounding normal tissue,
causing a focal worsening of tumour perfusion. Thus, in tumours with abnormal
cerebrovascular microstructure or abnormal cerebrovascular regulation, low BOLD
reactivity during hypercapnia is expected.
3.2.3 Presurgical DTI Mapping
DTI is a unique method for describing noninvasively the spatial relationship
between brain tumours or lesions and white matter tracts. Combined fMRI and DTI
mapping can reveal both functional regions and their white matter connectivity and
thus provide a more accurate estimate of the proximity of tumour borders to vital
brain networks [42, 92, 108, 125].
DTI can be useful in distinguishing between different effects of space-occupying
lesions such as displacement, disruption, or infiltration of the white matter tracts
by the tumour [23, 49, 108, 134, 135]. White matter fibres may remain intact but
be displaced by a tumour, in which case their anisotropy would be maintained
and their new location and orientation would be detectable on directional DTI
maps. This pattern could confirm the existence of a peritumoural tract that could
potentially be preserved during resection. Alternatively, oedema or infiltration
by the tumour may reduce the anisotropy of tracts, either still leaving enough
directional organisation to remain detectable on DTI maps or completely disrupting
the directional organisation.
DTI-based fibre tractography can be more challenging in cases of brain pathologies with space-occupying lesions, because displaced fibres may not readily be
traced with common methods for seed selection based on anatomical landmarks.
In addition, tumour infiltration and oedema may cause tissue deformation, resulting
in reduced reliability of fibre tracking [38]. Fibre tracking in regions with oedema
can potentially be improved by using sophisticated post-processing algorithms for
separating diffusion properties of neural tissue from surrounding free water [94].