Clinical Applications of Functional MRI, An Issue of Neuroimaging Clinics (eBook)

Jay J. Pillai (Autor)

eBook Download: PDF | EPUB

2014 | 1. Auflage
100 Seiten
Elsevier Health Sciences (Verlag)
978-0-323-32384-0 (ISBN)

Infos

Editor Jay J. Pillai and authors review important areas in Clinical Applications of Functional MRI. Articles will include: Blood Oxygen Level Dependent Functional Magnetic Resonance Imaging for Presurgical Planning; Visual Mapping Using Blood Oxygen Level Dependent Functional MRI; Applications of BOLD fMRI and DTI in Epilepsy; Pretherapeutic fMRI in Children; BOLD fMRI for Presurgical Planning; Brain Tensor Imaging for Brain Malformations: Does it Help?; Technical Considerations for fMRI Analysis; Special Considerations/Technical Limitations of BOLD fMRI; The Economics of Functional MRI: Clinical and Research; Memory Assessment in the Clinical Context Using fMRI: A Critical Look at the State of the Field; Resting State BOLD fMRI for Pre-surgical Planning, and more!

Textauszug

Blood Oxygen Level Dependent Functional Magnetic Resonance Imaging for Presurgical Planning

Meredith Gabriel, BA, Nicole P. Brennan, BA, Kyung K. Peck, PhD and Andrei I. Holodny, MD∗ [e-mail],     Functional MRI Laboratory, Department of Radiology, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10065, USA

∗Corresponding author.

Functional magnetic resonance imaging (fMRI) has become a common tool for presurgical sensorimotor mapping, and is a significant preoperative asset for tumors located adjacent to the central sulcus. fMRI has changed surgical options for many patients. This noninvasive tool allows for easy display and integration with other neuroimaging techniques. Although fMRI is a useful preoperative tool, it is not perfect. Tumors that affect the normal vascular coupling of neuronal activity will affect fMRI measurements. This article discusses the usefulness of blood oxygen level dependent (BOLD) fMRI with regard to preoperative motor mapping.

Keywords

fMRI sensorimotor mapping

Intraoperative motor mapping

BOLD fMRI

Key points

• Functional magnetic resonance imaging (fMRI) has become a common tool for presurgical sensorimotor mapping.

• fMRI is a significant preoperative asset for tumors located adjacent to the central sulcus.

• fMRI has changed surgical options for many patients. This noninvasive tool allows for easy display and integration with other neuroimaging techniques.

• Although fMRI is a useful preoperative tool, it is not perfect. Tumors that affect the normal vascular coupling of neuronal activity will affect fMRI measurement.

Introduction

In the early 1990s, functional magnetic resonance imaging (fMRI) entered neuroimaging as a unique resource in the arsenal of preoperative planning tools for patients with brain tumors. fMRI is a technique that takes advantage of the differences in magnetic susceptibility between oxyhemoglobin and deoxyhemoglobin. It is a less invasive neuroimaging method than its positron emission tomography (PET) predecessor, given that the contrast agent is endogenous.1 fMRI is possible because oxyhemoglobin has a magnetic resonance (MR) signal different from that of than deoxyhemoglobin. When a task is performed, oxygenated blood in excess of the amount needed (termed luxury perfusion) is delivered to the active area. The difference in magnetic susceptibility between deoxyhemoglobin concentrations and oxyhemoglobin concentrations creates the signal in functional imaging. This effect is termed the blood oxygen level dependent (BOLD) signal. fMRI provides good spatial localization (as low as 1 mm) and temporal acquisition resolution (as low as 1 second) but is limited by the resolution of the hemodynamic response (8–30 seconds). The superior spatial resolution is particularly advantageous for mapping peritumoral eloquent areas for treatment planning.2

fMRI can effectively map the sensory and motor areas. The motor gyrus is somatotopically organized, with all body parts represented in a way that is preserved across different people. fMRI can provide a multidimensional map in a single mapping session. Such maps of sensorimotor function help the surgeon to assess the risks of surgery and guide intraoperative mapping techniques.2,3 The primary motor and sensory areas in fMRI are of particular interest for surgical planning, because iatrogenic damage to these areas can cause permanent neurologic deficits. As a result the precise localization of various motor and sensory areas is useful, particularly in the setting of a space-occupying lesion.

Primary motor and sensory cortices are distinct in the functions they subserve. However, as a result of significant neuronal reciprocity in the region, injury to either can result in a mixed motor/sensory deficit. For example, injury to the primary motor gyrus usually leads to a permanent, largely irreversible paresis.4 Injury to the sensory cortex, while producing the expected sensory perceptual deficits, can also lead to a similar type of paresis seen with injury to the motor strip as a result of the lack of proprioceptive information. A variety of other deficits is seen with injury to the postcentral gyrus, depending on whether the left or right hemisphere is damaged. Some of these include 2-point discrimination, astereognosis (inability to discern objects by feeling them), and agraphism (inability to write). This article discusses the usefulness of BOLD fMRI with regard to preoperative motor mapping.

Anatomic organization of the sensorimotor system

The 4 main regions that subserve motor control that are of interest to neurosurgeons are the primary motor cortex, the primary sensory cortex, the premotor cortex, and the supplementary motor area (SMA). The motor and sensory gyri taken together are often referred to as one larger area termed the primary sensorimotor cortex.5

The Primary Sensorimotor Cortex

The primary motor cortex, located in the precentral gyrus, is responsible for executing movement (Fig. 1). Its position delineates the frontal from the parietal lobes. The motor gyrus marks the posterior limit of the frontal lobe and the sensory gyrus marks the start of the parietal lobe. The motor gyrus is somatotopically mapped; different body regions are distinctly represented in cortical space in a common (but not steadfast) pattern medially to laterally. Historically, the motor gyrus has been localized using anatomic markers. The most salient anatomic marker of the motor gyrus is the reverse omega portion of the central sulcus (see Fig. 1). This reverse omega typically demarcates the location of the hand motor region of the motor homunculus.6 However, the presence of this marker is occasionally unreliable. Fig. 2 shows a case whereby a reverse omega sign would have incorrectly indicated the position of the motor gyrus. Although cases like this are rare, they do occur. Furthermore, lesions can obscure traditional anatomic markers, making difficult their identification based on visual inspection of MR images alone. Fig. 3 shows a case in which anatomic markers have been obscured by tumor, making anatomic prediction of the location of the motor gyrus impossible without a technique such as fMRI.

Fig. 1 The primary sensory/motor gyrus: The yellow arrows indicate the position of the reverse omega portion of the primary motor gyrus in the posterior frontal lobe. The red arrows show the position of the sensory gyrus.

Fig. 2 Ambiguous anatomy. In rare instances the reverse omega (yellow arrow) does not indicate the position of the central sulcus. Without fMRI (A), this lesion would have been assumed to be in the motor gyrus (B).

Fig. 3 Where is the motor gyrus? fMRI is particularly useful in cases where tumor has obscured normal anatomy. In this case multiple extra-axial lesions make the motor gyrus localization impossible without a technique such as fMRI.

The face/tongue region of the primary motor gyrus is located on the lateral/inferior aspect of the motor gyrus. This region is anatomically just posterior to the area of Broca in the inferior frontal gyrus. Fig. 4 shows an fMRI map of both hand and tongue motor movements acquired simultaneously from an intact patient. Of note, finding the tongue motor region by “pulling down” the sulcus, whereby one first locates the more cephalad component of the central sulcus/reverse omega and follows the sulcus inferiorly, can be misleading and inaccurate. The inferior aspect of the central sulcus moves anteriorly as it is traced inferiorly and shortens, making precise localization of the inferior aspect of the motor gyrus particularly difficult to be discerned anatomically alone. For this reason, fMRI is particularly useful for localizing the face/lips/tongue portion of the motor gyrus at its inferior aspect.

Fig. 4 fMRI showing positions in the primary motor gyrus of the hand (blue arrows) and tongue (yellow arrows) signals.

Another way in which fMRI contributes significantly to motor gyrus localization is in the foot motor region. The foot motor region is located most medially just over the interhemispheric fissure. This region is often localized medial and slightly posterior to the hand motor region in the axial plane (Fig. 5). Direct cortical stimulation (the surgeon’s intraoperative gold standard for functional mapping) of this region is difficult because the sagittal sinus makes the cortex difficult to access. Therefore, fMRI localization of the foot motor region is valuable for presurgical planning.

Fig. 5 Foot motor and supplementary motor (SMA) regions of the primary motor gyrus.

fMRI typically maps these 3 main motor areas (foot, hand, and face/tongue) for neurosurgical planning, partly because these 3 areas span...

Produkt-Details

• Erscheint lt. Verlag: 13.11.2014
• Sprache: englisch
• Themenwelt: Medizin / Pharmazie ► Gesundheitsfachberufe
• Themenwelt: Medizinische Fachgebiete ► Radiologie / Bildgebende Verfahren ► Kernspintomographie (MRT)
• Themenwelt: Medizinische Fachgebiete ► Radiologie / Bildgebende Verfahren ► Radiologie
• ISBN-10: 0-323-32384-7 / 0323323847
• ISBN-13: 978-0-323-32384-0 / 9780323323840