Week 1: MRI physics Flashcards
MRI summary
MRI is based on the magnetization properties of atomic nuclei. A powerful, uniform, external magnetic field is employed to align the protons that are normally randomly oriented within the water nuclei of the tissue being examined. This alignment (or magnetization) is next perturbed or disrupted by introduction of an external Radio Frequency (RF) energy. The nuclei return to their resting alignment through various relaxation processes and in so doing emit RF energy. After a certain period (TE) following the initial RF, the emitted signals are measured. Fourier transformation is used to convert the frequency information contained in the signal from each location in the imaged plane to corresponding intensity levels, which are then displayed as shades of gray in a matrix arrangement of pixels. By varying the sequence of RF pulses applied & collected, different types of images are created. Repetition Time (TR) is the amount of time between successive pulse sequences applied to the same slice. Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal.
Tissue can be characterized by two different relaxation times – T1 and T2. T1 (longitudinal relaxation time) is the time constant which determines the rate at which excited protons return to equilibrium. It is a measure of the time taken for spinning protons to realign with the external magnetic field. T2 (transverse relaxation time) is the time constant which determines the rate at which excited protons go out of phase with each other. It is a measure of the time taken for spinning protons to lose phase coherence among the nuclei spinning perpendicular to the main field.
Longitudinal Relaxation (T1)
This process describes how quickly the magnetic moments of protons return to their equilibrium alignment with the main static magnetic field (B0) after being perturbed by a RF pulse. This process is characterized by an exponential growth (expressed by a constant T1) which differs for different tissues. Each tissue has a different T1 value, which represents the time (in ms) it takes for that tissue to reach 63% of the original magnetization.
Transverse Relaxation (T2)
Characterizes how quickly the magnetic moments of protons lose coherence and dephase in the transverse (xy) plane. This process is characterized by an exponential decay (expressed by a constant T2) which differs for different tissues. Each tissue has a different T2 value, which represents the time (in ms) it takes for that tissue to reach 37% of the original magnetization (and dephasing?).
Different tissues have different…
…T1 and T2 relaxation times, which is what makes them appear differently in MRI images.
Repetition time (TR)
TR is the time between successive RF pulses. It influences T1 relaxation. Longer TR allows for more complete recovery of longitudinal magnetization. This is like giving tissues more time to relax and return to their equilibrium state. Therefore, a short TR will highlight T1 tissue differences.
TE (Echo Time)
TE is the time at which the MRI machine measures the MR signal in the transverse plane after the RF pulse. It affects T2 relaxation. A shorter TE captures signals when the transverse magnetization is still coherent; hence, a longer TE will emphasize T2 effects, because we need to wait some time for the differences in dephasing in different tissues to show.
T/F: TR and TE are adjustable parameters in MRI sequences.
True: By choosing appropriate TR and TE values, we can control the contrast in MRI images.
Short TR (and short TE) emphasizes…
…T1 contrast, making differences in T1 relaxation times more prominent, resulting in T1-weighted images.
T1-weighted MRI enhances the signal of the fatty tissue
Long TE (and long TR) emphasizes…
…T2 contrast, making differences in T2 relaxation times more pronounced, leading to T2-weighted images.
T2-weighted MRI enhances the signal of the water.
Field of view (FOV)
It is a crucial parameter in MRI that defines the size of the anatomical region that is imaged or the area covered by the MRI scan. FOV is typically measured in units of length, such as millimeters or centimeters, and it determines the spatial extent of the MRI image.
Pixel
the smallest discrete unit or picture element in a digital image, representing a single point of color or brightness
Voxel
In MRI, a voxel, short for “volume element,” represents a three-dimensional pixel, serving as the smallest unit in a 3D image. It encompasses a tiny volume within the body, with specific spatial dimensions, and contains information about the tissue properties within that volume, contributing to the construction of detailed 3D MRI images.
Larmor frequency
the specific resonant frequency at which the nuclei of protons in the body precess when exposed to a strong magnetic field in MRI. It is directly proportional to the strength of the magnetic field and is fundamental for the precise manipulation and detection of MRI signals, allowing for the creation of detailed images.
Precession
refers to the circular or spiraling motion of the magnetic moments of atomic nuclei (e.g., protons) when subjected to a strong static magnetic field. This motion occurs at the Larmor frequency and is essential for the generation of MRI signals. It forms the basis for encoding spatial information and creating detailed images of the body’s internal structures in MRI.
Gradient fields
additional magnetic fields that are superimposed on the main static magnetic field (B0) in MRI. These gradient fields are applied along three orthogonal axes: X, Y, and Z. Gradient fields are used to spatially encode the MRI signal by creating variations in the magnetic field strength along these axes.
