Magnetic Resonance Imaging: How Does an MRI Work?

Flashcards about MRI, covering the basic principles of how MRI works, including nuclear magnetic resonance (NMR), the role of magnetic fields, radiofrequency pulses, and relaxation times (T1 and T2).

What is an MRI scanner?

A giant magnet that can produce pictures of the inside of the body by using nuclear magnetic resonance to absorb and re-emit radiofrequency energy when placed in a magnetic field.

What is Nuclear Magnetic Resonance (NMR)?

The ability of certain atomic nuclei to absorb and re-emit radiofrequency energy when placed in a magnetic field. This phenomenon is the basis of MRI.

What happens to atomic nuclei in a magnetic field?

Atomic nuclei behave like tiny bar magnets and align with the field.

How does an MRI detect signals from the body?

The MRI sends a radiofrequency pulse to knock the nuclei out of alignment, causing them to precess and emit their own radiofrequency signal, which the scanner detects.

What is T2 relaxation?

The time it takes for the RF signal from the nuclei to die away, which varies among different tissues.

What is T1 relaxation?

The time it takes for the nuclei to realign with the magnetic field, which also varies among different tissues.

What is the Larmour equation?

Omega (precession frequency) = gamma (gyromagnetic ratio) * B (magnetic field strength).

What is the main magnet in an MRI scanner?

A giant, often superconducting magnet that produces a strong, homogeneous magnetic field (B0 field).

What is the role of gradient coils in an MRI scanner?

Alter the B field in three dimensions to localize the RF signal in space, allowing for 3D images of tissue.

What do RF coils do in an MRI scanner?

Send the RF tipping pulse into the tissue and detect the signal from that tissue.

Does MRI use ionizing radiation?

MRI is completely radiation-free.

Which atomic element is primarily used in clinical MRI?

The hydrogen nucleus (a single proton).

What is 'spin' in the context of MRI?

Intrinsic angular momentum that makes a proton behave as if it's spinning.

What is the intrinsic magnetic moment of a proton?

Allows a proton to behave like a tiny bar magnet.

What is the 'flip angle' of a spin?

The angle between the spin and the magnetic field; a larger angle results in a higher signal. The more perpendicular to the field, the higher the signal.

What causes signal decay in MRI?

The NMR signal is lost as spins become dephased over time due to variations in the magnetic fields experienced by each spin.

What does T2 express, and how does it relate to tissue contrast in MRI?

Describes how quickly the MRI signal decays. Tissues with short T2, such as bone and lungs, lose coherence quickly, while tissues with long T2, such as water and blood, retain coherence longer.

What is echo time (TE)?

The time the scanner waits following spin excitation to detect the signal. It's chosen to maximize contrast between tissues.

In a T2-weighted image, what information does each voxel provide?

Each voxel indicates how quickly the processing spins got out of phase with each other.

What can cause the amplitudes for our red and green tissues here to be different?

The differing numbers of protons (again, hydrogen atoms).

TR or time of repetition

The time the scanner waits in between excitations.

What are the stages required to conduct NMR experiments?

We knock spins over, detect their signal as they dephase, wait for them to realign with the field, and then knock them all over again.

M0

Boltzmann magnetization

Define hyperpolarization

Techniques to increase polarization far beyond Boltzmann equilibrium.

What will be covered in the following lectures?

Spin echoes, gradient echoes, and Fourier theory in k-space.

What are spin echoes?

A technique to refocus spins that have dephased due to magnetic field inhomogeneities, creating a signal echo.

What are gradient echoes?

An echo created by using gradient pulses to rephase spins after they have dephased.

What is k-space?

A mathematical space where MRI data is stored, representing the spatial frequencies of the image.

What is Fourier transform?

A mathematical technique to decompose an image into its spatial frequency components, allowing for image reconstruction in MRI.

Define precession frequency

The speed at which the nuclei precess around the external magnetic field.

Define gyromagnetic ratio

A measure of how strongly a nucleus interacts with a magnetic field.

Define nuclear alignment

The alignment of atomic nuclei with an external magnetic field.

Define RF excitation

The process of using radiofrequency pulses to disturb the alignment of atomic nuclei.

What is spin-lattice relaxation (T1)?

Describes the time it takes for excited nuclei to return to their equilibrium state by releasing energy to the surrounding environment.

What is spin-spin relaxation (T2)?

Describes the time it takes for excited nuclei to lose phase coherence due to interactions with neighboring nuclei.

Define repetition time (TR)

The interval between successive RF excitation pulses in an MRI sequence.

Define echo time (TE)

The time between the RF excitation pulse and the peak of the echo signal.

What is T1-weighted imaging?

A weighting in MRI that enhances differences in T1 relaxation times between tissues.

What is T2-weighted imaging?

A weighting in MRI that enhances differences in T2 relaxation times between tissues.

What is bandwidth?

The range of frequencies contained in an RF pulse.

What is slice selection?

The process of using magnetic field gradients to selectively excite spins in a specific location.

What is frequency encoding?

Magnetic field gradients used to spatially encode the signal in the frequency domain.

What is phase encoding?

Magnetic field gradients used to spatially encode the signal in the phase domain.

What are motion artifacts?

Artifacts caused by the movement of the patient during the scan.

What are susceptibility artifacts?

Artifacts caused by metallic objects in or near the imaging field.

What is signal-to-noise ratio (SNR)?

A measure of the signal strength relative to the noise level.

What is spatial resolution?

A measure of the level of detail that can be seen in an image.

What is a multi-echo sequence?

A sequence that uses multiple spin echoes to improve image quality and contrast.

What is echo-planar imaging (EPI)?

A sequence that uses rapid gradient switching to acquire data quickly.

What is gadolinium-based contrast agent?

A contrast agent that contains gadolinium ions, used to enhance the visibility of blood vessels and certain tissues.

What is superparamagnetic iron oxide (SPIO)?

A type of contrast agent that contains iron oxide nanoparticles, used for liver and spleen imaging.

A technique that measures the random movement of water molecules in tissues

Diffusion-weighted imaging (DWI)

What is apparent diffusion coefficient (ADC)?

A measure of the magnitude of diffusion in a specific direction.

What is diffusion tensor imaging (DTI)?

A technique used to visualize the white matter tracts in the brain by measuring the diffusion of water molecules.

What is fractional anisotropy (FA)?

A measure of the directionality of diffusion in a tissue.

A technique that measures the blood flow in tissues

Perfusion-weighted imaging (PWI)

What is cerebral blood volume (CBV)?

A measure of the volume of blood in a given tissue.

What is cerebral blood flow (CBF)?

A measure of the rate of blood flow in a given tissue.

What is fat suppression?

A technique that uses RF pulses to selectively suppress the signal from fat or water.

What is STIR (short tau inversion recovery)?

A technique that uses a short tau inversion recovery pulse to suppress the signal from fat.

What is spectral fat saturation?

A technique that uses a chemical shift selective pulse to suppress the signal from fat.

What is metal artifact reduction sequence (MARS)?

A technique that reduces artifacts from metallic implants by adjusting the bandwidth and other parameters.

What is parallel imaging?

A technique that compensates for motion artifacts by acquiring data at multiple time points.

What is real-time MRI?

A technique used to acquire images in real-time, allowing for dynamic imaging of moving structures.

What is high-field MRI?

A technique that uses a higher magnetic field strength to improve image quality and SNR.

What is low-field MRI?

A technique that uses a lower magnetic field strength to reduce costs and improve accessibility.

What is pulse sequence optimization?

The process of optimizing MRI parameters to achieve the best possible image quality for a given clinical indication.

What is quality assurance in MRI?

The process of ensuring that the MRI scanner is operating within specified safety limits and performance standards.

What is MR safety?

The study of the effects of MRI on the human body.

What is the MRI suite?

A zone immediately surrounding the MRI scanner, where access is restricted to trained personnel.

What are ferromagnetic objects?

Metallic objects that can be attracted to the MRI scanner, posing a safety risk.

What is SAR (specific absorption rate)?

The specific absorption rate, which is a measure of the amount of RF energy absorbed by the body during an MRI scan.

What is functional MRI (fMRI)?

A technique that uses MRI to measure the function of the brain by detecting changes in blood flow.

What is magnetic resonance spectroscopy (MRS)?

A technique that uses MRI to measure the chemical composition of tissues.

A technique that combines MRI with other imaging modalities, such as