Advanced MR Techniques in Brain Tumor Imaging

Sasan Karimi, MD; Nicole M. Petrovich, BA; Kyung K. Peck, PhD; Bob L. Hou, PhD; Andrei I. Holodny, MD

Disclosures

Appl Radiol. 2006;35(5):9-18. 

In This Article

Functional MRI

Functional MRI is used for the purpose of neurosurgical planning and neurologic risk assessment in the treatment of brain tumors.[20,21] It localizes the eloquent cortices controlling language, motor, and memory functions. The results of an fMRI study can alter a neurosurgical approach to a tumor, suggest that surgery is a safe option in cases in which it might not otherwise have been offered, or steer a clinician away from neurosurgery and toward other treatment options when the risk of damage to the eloquent cortex is high.

Functional MRI is commonly used to map language function.[23,24,25] It is used to localize the areas of the cortex working to support speech and to determine hemispheric dominance for language (Figure 8). Because fMRI is based on a noninvasive, endogenous signal, it has the potential to replace the invasive Wada test (also known as the intracarotid amytal test), which is the current gold standard in language and memory lateralization.[26,27]

Figure 8.

A preoperative blood-oxygen-level--dependent functional MRI performed with language task shows Broca's (green arrow) and Wernecke's (yellow arrow) areas.

Functional MRI has revealed unexpected hemispheric language dominance in both adults and children.[28,29,30] In a case described by our group,[28] a 62-year-old right-handed man with a mostly nonenhancing left temporoparietal glioma in the expected anatomic location of Wenicke's area presented without language impairment. The fMRI revealed split hemispheric dominance for language, with Broca's area localizing to the left hemisphere and Wernicke's to the right hemisphere. Intraoperative electrocortical stimulation confirmed the fMRI results, and the patient underwent gross-total resection. This patient might not otherwise have been offered an operation had the fMRI not suggested atypical language localization.

Language paradigms vary with the location of the tumor. Typically, for frontal lesions, the patient will perform tasks such as generating words to a presented letter or verbs to visually presented nouns, punctuated by periods of rest. For posterior language localization, patients can be asked to name pictures or fill in the appropriate missing word in a sentence. While targeted testing is preferable, often both Broca's and Wernicke's areas are activated during both productive and receptive language tasks, making it not essential to tailor the language task to the lesion location. However, it should be noted that posterior language function (Wernicke's area), like many cognitive tasks, can be difficult to measure in patients.[31] A targeted approach may help lessen the variability in capturing this sometimes elusive language area.

The Wada test produces information about both language and memory function. As a result, the ability to use fMRI to noninvasively measure both language and memory function may further displace the Wada test. There is as yet no commonly accepted set of memory tasks for the purpose of neurosurgical planning. Golby et al[32] use a "novel versus repeated" protocol through which verbal memory and spatial memory can be measured. In this protocol, the patient is presented with novel and repeated images of faces, patterns, scenes, and word pairs (in separate trials). The analysis yields those areas that are active during the novel trials in which the patient encodes the new stimuli. As noninvasive, repeatable techniques, such as fMRI, improve in the measurement of hemispheric dominance for memory and language, invasive tests, with their associated morbidity (such as the Wada test), will be replaced.

Motor mapping, by contrast, is relatively easy for cognitively impaired patients to perform and produces consistent and reliable fMRI maps.[33] Often, patients perform cued movements of the fingers, feet, and/or tongue, depending on the location of the lesion (Figure 9).

Figure 9.

A preoperative blood-oxygen-level--dependent functional MRI with a bilateral finger-tapping paradigm shows the spatial relationship between the tumor and motor activation (green arrow), contralateral motor activation (green arrow), and supplemental motor activation (yellow arrow). Note the relative decrease in activation on the side of the tumor.

Localizing the motor strip and coregistering the results to a surgical scan prior to a neurosurgical intervention can help guide the direct cortical stimulation during an awake craniotomy and possibly shorten operation time. In some cases, using fMRI to confirm the expected location of the motor strip may avoid awake neurosurgery altogether.

Comments

3090D553-9492-4563-8681-AD288FA52ACE

processing....