fMRI Imaging and Analysis Concepts

The phenomenon where neighboring slices are excited before nuclei return to equilibrium, causing signal interference.

A type of magnetic field gradient applied briefly during MRI data collection to encode spatial information along the y-axis (usually perpendicular to the image's length).

A type of magnetic field gradient applied continuously during MRI data collection to encode spatial information along the x-axis (usually parallel to the image's length).

A complex representation of MRI signal data, organized by its frequency and phase components. The inverse Fourier transform of k-space produces the final image.

The process of combining spatial encoding from all three axes (slice, phase, and frequency) to collect MRI data and reconstruct an image.

A uniform magnetic field is critical for producing high-quality MRI images. Shimming involves adjusting currents in shim coils to achieve this uniformity.

The process by which a superconducting magnet loses its magnetic field due to a lack of liquid helium or nitrogen. This can cause significant damage to the magnet's structure.

The strength of the magnetic field per unit length. It is measured in T/m or mT/m.

The direction in which the magnetic field strength varies. In MRI, gradients are usually applied in the x, y, or z direction.

The direction of the main magnetic field (usually horizontal in clinical MRI).

The vertical (upward) direction.

NMR signal frequency is directly proportional to the magnetic field strength. This means that a stronger magnetic field will produce a higher frequency signal.

The process of adding a gradient to the magnetic field, causing its strength to vary along a specific direction. This is crucial for localizing NMR signals in MRI.

A change in the signal due to the varying oxygen level in the blood in response to changes in brain activity.

A technique that uses strong magnetic fields and radio waves to create detailed images of the brain's activity.

A software used to analyze fMRI data to identify brain activity during specific tasks.

A slice is a 2D plane that intersects the volume being imaged. In MRI, a slice is usually made in the z-direction, but it can be made in any arbitrary direction.

A gradient pulse applied in the z-direction before the RF pulse, causing resonant frequencies of nuclei to vary along the z-axis. This allows for the selection of a specific slice thickness.

The process of encoding spatial information in MRI, using three orthogonal gradients: slice encoding, frequency encoding, and phase encoding. These gradients are used to create a 2D image.

The ability to select a specific slice thickness using the slice encoding gradient. However, the profile isn't perfect, causing nuclei outside the slice to be excited, leading to blurring.

When imaging multiple slices in a pulse sequence, one slice is excited at a time, while data for the previous slice is collected. This can cause cross-talk between slices.

In MRI, the signal intensity arises from the number of protons within a specific tissue volume. This determines the brightness of the image.

T1 and T2 relaxation times are processes that influence the MRI signal strength. T1 measures the time it takes for protons to align with the magnetic field after a radiofrequency pulse. T2 measures the time they take to lose coherence.

MRI images can be weighted to emphasize different tissue properties. T1-weighted images highlight fat and tissues with short T1 relaxation times; T2-weighted images show water and tissues with long T2 relaxation times.

Contrast agents are substances that alter T1 or T2 relaxation times. This makes certain tissues brighter, improving image clarity.

Specific Absorption Rate (SAR) is the amount of RF energy absorbed by a tissue mass. High SAR increases with strong magnetic fields and big flip angles.

fMRI investigates brain activity during tasks by monitoring changes in blood oxygenation. It exploits the magnetic properties of oxyhemoglobin and deoxyhemoglobin.

Oxyhemoglobin is diamagnetic, repelled by magnetic fields, while deoxyhemoglobin is paramagnetic and attracted to magnetic fields. This difference forms the basis of fMRI signal.

fMRI is used to study brain networks and their connections. Researchers are exploring links between brain functionality and neurodegenerative diseases like dementia or multiple sclerosis.

The change in magnetic field strength per unit distance. It is used to encode spatial information in MRI.

The Larmor frequency (ω) of a nucleus is directly proportional to the magnetic field strength (B) it experiences. This is the foundation of MRI.

A gradient applied in the z-direction (perpendicular to the imaging plane) affects the Larmor frequency of nuclei based on their position along the z-axis. This is used to select and image a specific slice of tissue.

Gradients can be applied in any direction (x, y, or z), or a combination of them, to encode spatial information and enable 3D imaging.

The loud noise during MRI is caused by rapid switching of gradient coils, which generate mechanical vibrations.

The RF pulse excites a specific slice of tissue by rotating the magnetization of the nuclei in that slice.

A sinc pulse is a special type of RF pulse that helps excite a sharp, rectangular slice of tissue for better image resolution.

By adding a third gradient in the slice direction (z), we can encode phase information along the z-axis, enabling 3D imaging.

The ability of MRI to differentiate tissues based on their chemical composition, often visualized as variations in gray-scale intensities. This allows for identifying areas with abnormal tissue structure, like tumors.

MRI technology allows for imaging of blood flowing through vessels, capturing high-intensity signals in thin slices or 3D representations. This helps visualize blood flow patterns and detect abnormalities.

MRI provides flexibility in acquiring images from various perspectives, including axial (horizontal), coronal (frontal), sagittal (vertical), and oblique angles. This allows for a comprehensive examination of the anatomy from multiple viewpoints.

MRI can capture images of long anatomical structures, like the spine, along its length. This is particularly useful for sagittal views, providing a comprehensive view of the entire structure.

The process of nuclear magnetic resonance (NMR) is the foundation of MRI. It involves manipulating the alignment of atomic nuclei within a strong magnetic field, generating signals used for image reconstruction.

MRI utilizes a range of magnetic fields to achieve its imaging goals, including a static main magnetic field (B0) and gradient fields that vary over time.

The strength of the MRI magnet is directly related to the size of the object being scanned, with smaller objects requiring stronger magnets. Field strengths in clinical MRI range from 1.5T to 3T.

Superconductors, materials that allow for near-zero electrical resistance, are crucial for generating strong magnetic fields in clinical MRI systems. They are maintained at extremely cold temperatures through cryogenics.