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The use of functional MRI to study appetite control in the CNS.

De Silva A, Salem V, Matthews PM, Dhillo WS - Exp Diabetes Res (2012)

Bottom Line: In the present absence of any safe or effective centrally acting appetite suppressants, a better understanding of how appetite is controlled is vital for the development of new antiobesity pharmacotherapies.Early functional imaging techniques revealed an attenuation of brain reward area activity in response to visual food stimuli when humans are fed-in other words, the physiological state of hunger somehow increases the appeal value of food.The hypothalamus acts as a central gateway modulating homeostatic and nonhomeostatic drives to eat.

View Article: PubMed Central - PubMed

Affiliation: Division of Diabetes, Endocrinology and Metabolism, Imperial College London, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK.

ABSTRACT
Functional magnetic resonance imaging (fMRI) has provided the opportunity to safely investigate the workings of the human brain. This paper focuses on its use in the field of human appetitive behaviour and its impact in obesity research. In the present absence of any safe or effective centrally acting appetite suppressants, a better understanding of how appetite is controlled is vital for the development of new antiobesity pharmacotherapies. Early functional imaging techniques revealed an attenuation of brain reward area activity in response to visual food stimuli when humans are fed-in other words, the physiological state of hunger somehow increases the appeal value of food. Later studies have investigated the action of appetite modulating hormones on the fMRI signal, showing how the attenuation of brain reward region activity that follows feeding can be recreated in the fasted state by the administration of anorectic gut hormones. Furthermore, differences in brain activity between obese and lean individuals have provided clues about the possible aetiology of overeating. The hypothalamus acts as a central gateway modulating homeostatic and nonhomeostatic drives to eat. As fMRI techniques constantly improve, functional data regarding the role of this small but hugely important structure in appetite control is emerging.

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Related in: MedlinePlus

Schematic of T1 and T2 relaxation. MRI utilises the behaviour of protons within varying magnetic fields to produce signals which can be converted into images. Each hydrogen nucleus in the brain can be thought of as a vector (in the z and x-y planes) representing the strength and direction of its magnetic field as it spins on its axis (its magnetic dipole moment, MDM). The MDMs of the imaged protons try to align with the main external magnetic field of the scanner (referred to here as B0 and conventionally shown along the z axis in 3D coordinates). A second magnetic field (in the form of a short radiofrequency RF pulse) is applied, which flips all of the MDMs from alignment in the z direction into the x-y plane (a). Before application of the RF pulse the, amplitude in the z-axis is maximal while the amplitude in the x-y plane is zero. Just after application of the RF pulse the, amplitude in the z-axis is zero (a) while the amplitude in the x-y plane is maximal (d). During relaxation, the amplitude in the z-axis will slowly increase ((b) and (d)) while the amplitude in the x-y plane slowly decreases ((e) and (f)).  T1 relaxation is the time taken for the z vector to regain in strength, whereas T2 relaxation is the time taken for the x-y vector to decay. These changing magnetic vectors invoke their own RF signals, which are picked up by the receiver coils and interpreted into information about the proton density of the subject being scanned.
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fig2: Schematic of T1 and T2 relaxation. MRI utilises the behaviour of protons within varying magnetic fields to produce signals which can be converted into images. Each hydrogen nucleus in the brain can be thought of as a vector (in the z and x-y planes) representing the strength and direction of its magnetic field as it spins on its axis (its magnetic dipole moment, MDM). The MDMs of the imaged protons try to align with the main external magnetic field of the scanner (referred to here as B0 and conventionally shown along the z axis in 3D coordinates). A second magnetic field (in the form of a short radiofrequency RF pulse) is applied, which flips all of the MDMs from alignment in the z direction into the x-y plane (a). Before application of the RF pulse the, amplitude in the z-axis is maximal while the amplitude in the x-y plane is zero. Just after application of the RF pulse the, amplitude in the z-axis is zero (a) while the amplitude in the x-y plane is maximal (d). During relaxation, the amplitude in the z-axis will slowly increase ((b) and (d)) while the amplitude in the x-y plane slowly decreases ((e) and (f)).  T1 relaxation is the time taken for the z vector to regain in strength, whereas T2 relaxation is the time taken for the x-y vector to decay. These changing magnetic vectors invoke their own RF signals, which are picked up by the receiver coils and interpreted into information about the proton density of the subject being scanned.

Mentions: MRI utilises the behaviour of hydrogen nuclei, which consist of single protons that possess angular momentum (spin). As soon as an external magnetic field (B0) is applied, the protons in tissue tend to align with this, causing their spins to precess about a circular path around B0. Here, they are in a low-energy state. Generation of MRI images requires application of a radiofrequency (RF) pulse at 90 degrees to B0. The protons will then “tip” to align with the RF pulse. In doing so, they gain energy. After the RF pulse is switched off, the protons realign with B0. Spins return to the low-energy state by emitting the absorbed energy, also in the form of a radio wave. The emitted energy can be measured by the receiver coil and converted to images. By slightly altering the strength of the magnetic field (and therefore the frequency of the emitted radiation) using gradient coils across the volume to be imaged, spatial information can be inferred. The T1 relaxation time is a time constant referring to the realignment of spins with B0 in the longitudinal plane after the RF pulse is switched off. The T2 relaxation time, on the other hand, is a time constant referring to the dephasing of spins in the transverse plane after the RF pulse is switched off (Figure 2). T1 and T2 vary depending on the tissue being imaged. The strength of magnetic resonance signal obtained for a particular tissue depends primarily on the proton density. However, by altering the time between successive RF pulses and therefore the degree of T1 and T2 relaxation, the image can be weighted towards one or the other of these tissue-specific properties. It should be noted that, in practice, due to localised inhomogeneities in the externally applied magnetic field, T2 is shorter than expected for any particular tissue. This apparent T2 is referred to as T2*.


The use of functional MRI to study appetite control in the CNS.

De Silva A, Salem V, Matthews PM, Dhillo WS - Exp Diabetes Res (2012)

Schematic of T1 and T2 relaxation. MRI utilises the behaviour of protons within varying magnetic fields to produce signals which can be converted into images. Each hydrogen nucleus in the brain can be thought of as a vector (in the z and x-y planes) representing the strength and direction of its magnetic field as it spins on its axis (its magnetic dipole moment, MDM). The MDMs of the imaged protons try to align with the main external magnetic field of the scanner (referred to here as B0 and conventionally shown along the z axis in 3D coordinates). A second magnetic field (in the form of a short radiofrequency RF pulse) is applied, which flips all of the MDMs from alignment in the z direction into the x-y plane (a). Before application of the RF pulse the, amplitude in the z-axis is maximal while the amplitude in the x-y plane is zero. Just after application of the RF pulse the, amplitude in the z-axis is zero (a) while the amplitude in the x-y plane is maximal (d). During relaxation, the amplitude in the z-axis will slowly increase ((b) and (d)) while the amplitude in the x-y plane slowly decreases ((e) and (f)).  T1 relaxation is the time taken for the z vector to regain in strength, whereas T2 relaxation is the time taken for the x-y vector to decay. These changing magnetic vectors invoke their own RF signals, which are picked up by the receiver coils and interpreted into information about the proton density of the subject being scanned.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3376546&req=5

fig2: Schematic of T1 and T2 relaxation. MRI utilises the behaviour of protons within varying magnetic fields to produce signals which can be converted into images. Each hydrogen nucleus in the brain can be thought of as a vector (in the z and x-y planes) representing the strength and direction of its magnetic field as it spins on its axis (its magnetic dipole moment, MDM). The MDMs of the imaged protons try to align with the main external magnetic field of the scanner (referred to here as B0 and conventionally shown along the z axis in 3D coordinates). A second magnetic field (in the form of a short radiofrequency RF pulse) is applied, which flips all of the MDMs from alignment in the z direction into the x-y plane (a). Before application of the RF pulse the, amplitude in the z-axis is maximal while the amplitude in the x-y plane is zero. Just after application of the RF pulse the, amplitude in the z-axis is zero (a) while the amplitude in the x-y plane is maximal (d). During relaxation, the amplitude in the z-axis will slowly increase ((b) and (d)) while the amplitude in the x-y plane slowly decreases ((e) and (f)).  T1 relaxation is the time taken for the z vector to regain in strength, whereas T2 relaxation is the time taken for the x-y vector to decay. These changing magnetic vectors invoke their own RF signals, which are picked up by the receiver coils and interpreted into information about the proton density of the subject being scanned.
Mentions: MRI utilises the behaviour of hydrogen nuclei, which consist of single protons that possess angular momentum (spin). As soon as an external magnetic field (B0) is applied, the protons in tissue tend to align with this, causing their spins to precess about a circular path around B0. Here, they are in a low-energy state. Generation of MRI images requires application of a radiofrequency (RF) pulse at 90 degrees to B0. The protons will then “tip” to align with the RF pulse. In doing so, they gain energy. After the RF pulse is switched off, the protons realign with B0. Spins return to the low-energy state by emitting the absorbed energy, also in the form of a radio wave. The emitted energy can be measured by the receiver coil and converted to images. By slightly altering the strength of the magnetic field (and therefore the frequency of the emitted radiation) using gradient coils across the volume to be imaged, spatial information can be inferred. The T1 relaxation time is a time constant referring to the realignment of spins with B0 in the longitudinal plane after the RF pulse is switched off. The T2 relaxation time, on the other hand, is a time constant referring to the dephasing of spins in the transverse plane after the RF pulse is switched off (Figure 2). T1 and T2 vary depending on the tissue being imaged. The strength of magnetic resonance signal obtained for a particular tissue depends primarily on the proton density. However, by altering the time between successive RF pulses and therefore the degree of T1 and T2 relaxation, the image can be weighted towards one or the other of these tissue-specific properties. It should be noted that, in practice, due to localised inhomogeneities in the externally applied magnetic field, T2 is shorter than expected for any particular tissue. This apparent T2 is referred to as T2*.

Bottom Line: In the present absence of any safe or effective centrally acting appetite suppressants, a better understanding of how appetite is controlled is vital for the development of new antiobesity pharmacotherapies.Early functional imaging techniques revealed an attenuation of brain reward area activity in response to visual food stimuli when humans are fed-in other words, the physiological state of hunger somehow increases the appeal value of food.The hypothalamus acts as a central gateway modulating homeostatic and nonhomeostatic drives to eat.

View Article: PubMed Central - PubMed

Affiliation: Division of Diabetes, Endocrinology and Metabolism, Imperial College London, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK.

ABSTRACT
Functional magnetic resonance imaging (fMRI) has provided the opportunity to safely investigate the workings of the human brain. This paper focuses on its use in the field of human appetitive behaviour and its impact in obesity research. In the present absence of any safe or effective centrally acting appetite suppressants, a better understanding of how appetite is controlled is vital for the development of new antiobesity pharmacotherapies. Early functional imaging techniques revealed an attenuation of brain reward area activity in response to visual food stimuli when humans are fed-in other words, the physiological state of hunger somehow increases the appeal value of food. Later studies have investigated the action of appetite modulating hormones on the fMRI signal, showing how the attenuation of brain reward region activity that follows feeding can be recreated in the fasted state by the administration of anorectic gut hormones. Furthermore, differences in brain activity between obese and lean individuals have provided clues about the possible aetiology of overeating. The hypothalamus acts as a central gateway modulating homeostatic and nonhomeostatic drives to eat. As fMRI techniques constantly improve, functional data regarding the role of this small but hugely important structure in appetite control is emerging.

Show MeSH
Related in: MedlinePlus