Magnetic Resonance Imaging (MRI) Fundamentals and Clinical Applications

Comprehensive vocabulary flashcards covering the core principles, physical hardware, pulse sequences, imaging parameters, safety considerations, contrast agents, and diagnostic artifacts in Magnetic Resonance Imaging (MRI).

Magnetic Resonance Imaging (MRI)

An imaging modality that produces cross-sectional views of the body using computer technology by applying a magnetic field and radio waves to generate images reconstructed through mathematical processing without using ionizing radiation; originally known as Nuclear Magnetic Resonance.

Antenna

A device designed for transmitting or receiving radio waves.

Artifact

A spurious finding in or distortion of an image not normally present in the subject, caused by a limitation or malfunction in the MRI hardware or software.

Attenuation

The reduction in energy or amount of a beam of radiation when it passes through tissue or other substances.

Coil

Single or multiple loops of wire designed to produce a magnetic field from electric current flowing through the wire.

Claustrophobia

The psychological fear of having no escape and being confined in closed or small spaces or rooms, recognized as an MRI contraindication.

Cryogenic

Relating to or characterized by extremely low temperatures.

Diffusion

The spontaneous random motion of molecules in a medium.

Fat Suppressed Images

MR images created using specialized pulse sequences such that fat tissue is displayed with a lower, darker signal intensity.

Free Induction Decay (FID)

The faint transient radiofrequency signal emitted by tissues after RF excitation as the net magnetization returns to equilibrium.

Gating

A technique used to organize signal acquisition and reduce motion artifacts from cyclical movement (such as a beating heart) by synchronizing data collection to 'freeze' motion at a specific phase.

Gradient Echo

A fast pulse sequence employing flip angles of less than 90^\\tau often used in 3D imaging to generate T2-weighted images.

Gradient Magnetic Field

A controlled spatial variation or change in the intensity of a magnetic field across distance, measured in units of $\text{mT/cm}$.

Gyromagnetic Ratio

A constant, specific ratio of magnetic moment to angular momentum for each nucleus, expressed in $\text{MHz/T}$; for hydrogen, it is equal to $43\,\text{MHz/T}$.

Inversion Recovery (IR)

A standard MRI pulse sequence available on most imagers ($180^\circ\text{--}90^\circ\text{--}180^\circ\text{--}90^\circ\text{--}180^\circ$) that provides superior anatomical detail and is commonly used for T1-weighted images.

Larmor Frequency

The specific frequency at which a atomic nucleus precesses in a magnetic field, measured in $\text{MHz}$.

Magnetic Moment

A force created when a magnetic dipole is placed in a magnetic field; represents inherent magnetism, measured in Tesla ($\text{T}$).

Magnetization

The large-scale macroscopic magnetic moment resulting from the vector sum of many individual nuclear magnetic moments.

Net Magnetization

The macroscopic vector sum produced by individual hydrogen nuclei aligning with the external magnetic field.

Longitudinal Magnetization ($M_z$)

The component of the net magnetization vector oriented parallel to the main external magnetic field along the z-axis.

Transverse Magnetization ($M_{xy}$)

The net magnetization vector oriented within the transverse plane (XY plane) perpendicular to the main magnetic field.

Perfusion

The microscopic flow of blood through the capillary vascular bed of an organ or anatomical structure.

Precession

The wobbling motion of the rotational axis of a spinning body about a stationary axis, describing a cone in space.

Radio Frequency (RF)

Electromagnetic radiation with frequencies ranging from $0.3\,\text{kHz}$ to $300\,\text{GHz}$, operating between $1\,\text{MHz}$ and $100\,\text{MHz}$ in MRI systems.

Raw Data

The unprocessed spatial frequency information obtained via radio reception of the MRI signal as stored directly by a computer.

Relaxation

The physical process by which excited nuclear spins return from a non-equilibrium state back to thermal equilibrium.

Resonance

The transfer of vibrating energy from one system to another caused by a relatively small periodic stimulus oscillating at or near a natural frequency of the receiving system.

Specific Absorption Rate (SAR)

The measure of RF energy power absorbed per unit mass of biological tissue during irradiation, expressed in Watts per kilogram ($\text{W/kg}$).

Spin Density (SD)

The concentration of hydrogen nuclei within tissue precessing at the Larmor frequency; serves as a principal determinant of the MRI signal.

T1 Relaxation Time

The spin-lattice or longitudinal relaxation time required for longitudinal magnetization to recover to $63\%$ of its maximum value following an RF excitation pulse.

T2 Relaxation Time

The spin-spin or transverse relaxation time required for transverse magnetization to decay to $37\%$ of its initial maximum value due to spin dephasing in the X-Y plane.

Tesla (T)

The SI unit of magnetic field strength, where $1\,\text{T} = 10{,}000\,\text{Gauss}$ ($\text{G}$).

Indomitable

The original name given to the first full-body human MRI scanner, developed and constructed by Dr. Raymond Damadian.

Fourier Transformation (FT)

A mathematical operation that converts a time-domain Free Induction Decay (FID) signal into a frequency-domain NMR spectrum, relating spatial features to distances and occurrence frequencies.

Permanent Magnet

An MRI magnet constructed from bricklike ceramic materials that is simple in design, has low capital and operating costs, and produces negligible fringe fields, but is extremely heavy and limited to low, fixed magnetic field strengths ($\le 0.3\,\text{T}$).

Resistive Magnet

An MRI magnet composed of four large conductive coils with finite resistance; it can be turned off but requires continuous high electric power and water cooling, producing field strengths up to $0.15\,\text{T}$.

Superconducting Magnet

An MRI magnet containing coils made of a superconducting metal alloy exhibiting zero electrical resistance at cryogenic temperatures, providing high field strengths (e.g., $2\,\text{T}$) and high field homogeneity without continuous power consumption.

Dewar

A specialized double-walled metal container with an insulating vacuum between the walls, used to house liquid cryogens in a superconducting magnet.

Cryogens

Cooling agents such as Liquid Nitrogen (outer chamber) and Liquid Helium (innermost chamber) used to optimize and sustain superconductivity in an MRI scanner.

Shim Coils

A configuration of individual windings (typically 30) used in the process of 'shimming' to maximize the homogeneity of the main magnetic field and prevent image degradation.

Gradient Coils

Three sets of secondary coils (X, Y, and Z) that generate gradient magnetic fields for spatial localization: the Z gradient performs slice selection (transaxial), the X gradient performs frequency encoding (sagittal), and the Y gradient performs phase encoding (coronal).

RF Coils (RF Probe)

A rigid transceiver assembly consisting of an antenna coil, structural plastics, fiberglass, and insulators positioned inside the gradient coils closest to the patient to transmit RF pulses and receive MR signals.

Surface Coils

The simplest design of RF receive coils, placed directly on or adjacent to specific superficial body parts (such as the spine, shoulder, or TMJ) to provide high local sensitivity.

Endocavitary Coil

A specialized receiver coil inserted into body orifices to image the rectum and cavity walls with high resolution.

Neurovascular Coil

A dedicated multi-channel RF coil engineered specifically for simultaneous high-resolution imaging of the brain and neck vasculature.

Paired Saddle Coil

A coil geometry commonly used for knee joint imaging and employed in the construction of X and Y gradient coils.

Helmholtz Pair Coil

A coil configuration consisting of two circular coils oriented parallel to one another; used for Z gradient coils and as RF coils for pelvis and cervical spine imaging.

Birdcage Coil

An RF coil design that provides the highest RF field homogeneity of all coil types, predominantly utilized as a transceiver coil for head imaging and occasionally for extremities.

Faraday Cage

A continuous conductive wire-mesh or copper enclosure lining the MRI examination room to attenuate extraneous external sources of radiofrequency interference.

Fringe Magnetic Field

The stray static magnetic field that extends outside the physical bore of the MRI scanner into adjacent areas, capable of interfering with electronic devices and causing projectile hazards.

Repetition Time (TR)

The time elapsed from the beginning of one pulse sequence to the initiation of the subsequent sequence, or the time between two consecutive $90^\circ$ excitation pulses.

Echo Time (TE)

The time interval between the center of the initial $90^\circ$ RF excitation pulse and the peak of the recorded spin echo signal.

Spin Echo (SE)

The most widely utilized MRI pulse sequence ($90^\circ\text{--}180^\circ\text{--}90^\circ\text{--}180^\circ$), also designated as the Carr-Purcell Meiboom-Gill (CPMG) sequence.

Gradient Refocused Echo (GRE)

A fast imaging sequence that uses RF flip angles of less than $90^\circ$ and magnetic field gradient reversals to refocus transverse magnetization, serving as the basis for sequences like FLASH, FISP, and GRASS.

Quenching

The rapid and uncontrolled boil-off of liquid cryogens (helium) cooling a superconducting magnet coil, resulting in explosive escape of cold gas, risk of asphyxiation and frostbite, and loss of the magnetic field.

Gadolinium

A rare-earth metallic contrast agent exhibiting paramagnetic properties, low toxicity, and few side effects, used to shorten tissue T1 relaxation times.

Teslascan

A manganese-based paramagnetic contrast agent specifically utilized for the detection, characterization, localization, and evaluation of liver lesions.

Feridex

An iron-oxide mixture and superparamagnetic contrast agent used specifically to detect and diagnose liver lesions.

Magic Angle Artifact

An MRI artifact caused by an orientation of ordered collagen fibers at approximately $55^\circ$ to the main magnetic field, leading to increased T2 relaxation time and false bright signals within tendons.

Partial Volume Artifact

An imaging artifact manifesting as a loss of contrast and blurred margins between adjacent tissues due to insufficient spatial resolution or voxel thickness; resolved by acquiring thinner slices.

Metal Artifact

An artifact presenting as severe signal dropouts, bright spots, and spatial distortions caused by local magnetic field inhomogeneities from metallic or ferromagnetic objects; managed by removing the metal, avoiding GRE, and using short echo times.

Motion Artifact

An artifact presenting as ghosting and blurring along the phase-encoding axis caused by physiological or gross movement of the patient; mitigated by gating, motion compensation, signal averaging, or antispasmodics.

Herringbone Artifact

An image artifact characterized by a widespread crisscross or static pattern across the entire slice, caused by electromagnetic emissions or discrete RF interference spikes.

RF Overflow / Data Clipping Artifact

An artifact characterized by image non-uniformity and washed-out signal contrast, occurring when received signal amplitudes exceed the dynamic range of the analog-to-digital converter; resolved by manually decreasing receiver gain.

Aliasing Artifact (Wrap-Around)

An artifact occurring when anatomical structures extend beyond the prescribed field of view (FOV), causing anatomy outside the FOV to wrap around and overlap onto the opposite side of the image.

Zipper Artifact

An artifact presenting as a straight line or band of alternating dots running through the center of the image, caused by external RF interference penetrating the scan room or internal equipment hardware problems.

Staircase Artifact

An artifact presenting as stepped or staircase-like pixel steps across reconstructed images, caused by acquiring overly thick slices without overlapping.