3 Introduction to Brain Imaging
55
such as contrast-enhanced MRI, fMRI, and DTI maps can also be registered to the
intrasurgical images, to make this information more readily available during the
surgery [87].
3.2.1 Structural MRI
MRI is used to detect cerebral tumours or lesions that may cause seizures. In brain
tumour patients, resection of primary tumours can improve survival, functional
capability, and the effect of adjuvant therapies, provided that postsurgical neurological deficits can be avoided [4, 52, 128]. In epilepsy patients, resection of an
MR-visible lesion near the site of seizure onset, such as mesial temporal sclerosis
(scarring of the hippocampus) or cortical dysplasia (cellular maturation abnormality), dramatically increases the chances of freedom from seizures [116]. The use
of higher-field MR scanners and thinner image slices and the implementation of
advanced sequences such as fluid-attenuated inversion recovery (FLAIR) improve
the odds of detecting subtle focal cortical dysplasia [27]. However, in approximately
25% of epilepsy surgery candidates, structural MRI produces inconclusive evidence
of brain lesions or evidence that is inconsistent with scalp EEG results. In some
of those cases, positron emission tomography (PET) can be used to characterise
cerebral metabolism and indicate the location of subtle abnormalities that can cause
seizures. This information can further be used to guide the positioning of intracranial
electrodes to confirm the seizure locus [63, 103].
3.2.2 Presurgical fMRI Mapping
The main application of fMRI for neurosurgical planning is for the mapping
of sensorimotor, language, and memory functions, because these functions are
considered most vulnerable to neurosurgical procedures and most important for
postsurgical quality of life (Fig. 3.1). Reports suggest that presurgical planning
using fMRI may reduce surgical time, affect decisions regarding the targeted extent
of resection [95, 127], and improve surgery outcome [124]. Compared to the gold
standard cortical stimulation mapping [26, 39], fMRI is noninvasive and performed
presurgically, without the time constraints and possible side effects on cognitive
function associated with the surgical environment. The fast T2*-weighted imaging
capabilities required for fMRI are a standard feature on MRI clinical systems, and
the implementation of audiovisual stimulation and response equipment needed for
most fMRI activation paradigms is relatively low cost.
Nevertheless, major obstacles to clinical deployment of fMRI presurgical
mapping remain, including the following: (1) a lack of standardised acquisition
paradigms and analysis procedures for sensitive and specific functional mapping
in individual patients [102, 119], (2) spatial imprecision in presurgical maps due
55
such as contrast-enhanced MRI, fMRI, and DTI maps can also be registered to the
intrasurgical images, to make this information more readily available during the
surgery [87].
3.2.1 Structural MRI
MRI is used to detect cerebral tumours or lesions that may cause seizures. In brain
tumour patients, resection of primary tumours can improve survival, functional
capability, and the effect of adjuvant therapies, provided that postsurgical neurological deficits can be avoided [4, 52, 128]. In epilepsy patients, resection of an
MR-visible lesion near the site of seizure onset, such as mesial temporal sclerosis
(scarring of the hippocampus) or cortical dysplasia (cellular maturation abnormality), dramatically increases the chances of freedom from seizures [116]. The use
of higher-field MR scanners and thinner image slices and the implementation of
advanced sequences such as fluid-attenuated inversion recovery (FLAIR) improve
the odds of detecting subtle focal cortical dysplasia [27]. However, in approximately
25% of epilepsy surgery candidates, structural MRI produces inconclusive evidence
of brain lesions or evidence that is inconsistent with scalp EEG results. In some
of those cases, positron emission tomography (PET) can be used to characterise
cerebral metabolism and indicate the location of subtle abnormalities that can cause
seizures. This information can further be used to guide the positioning of intracranial
electrodes to confirm the seizure locus [63, 103].
3.2.2 Presurgical fMRI Mapping
The main application of fMRI for neurosurgical planning is for the mapping
of sensorimotor, language, and memory functions, because these functions are
considered most vulnerable to neurosurgical procedures and most important for
postsurgical quality of life (Fig. 3.1). Reports suggest that presurgical planning
using fMRI may reduce surgical time, affect decisions regarding the targeted extent
of resection [95, 127], and improve surgery outcome [124]. Compared to the gold
standard cortical stimulation mapping [26, 39], fMRI is noninvasive and performed
presurgically, without the time constraints and possible side effects on cognitive
function associated with the surgical environment. The fast T2*-weighted imaging
capabilities required for fMRI are a standard feature on MRI clinical systems, and
the implementation of audiovisual stimulation and response equipment needed for
most fMRI activation paradigms is relatively low cost.
Nevertheless, major obstacles to clinical deployment of fMRI presurgical
mapping remain, including the following: (1) a lack of standardised acquisition
paradigms and analysis procedures for sensitive and specific functional mapping
in individual patients [102, 119], (2) spatial imprecision in presurgical maps due