MRI Physics: Electromagnetism and Pulse Sequences

Vocabulary flashcards covering key electromagnetism concepts, physics laws, gradient functions, and pulse sequence types from MRI lecture notes.

Faraday's First Law

The principle stating that a change in magnetic field while in the presence of a coil of wire will produce a voltage.

Faraday's Second Law

The principle stating that the amount of electromagnetic induction produced is equal to the rate of changing flux and the number of wire coils.

Electromotive Force (EMF)

A term used to describe induction or voltage produced when conductive materials are exposed to a changing magnetic field, represented mathematically by E = -\frac{d\frac{\text{d}}{\text{dt}}\text{\Phi}_B}{\text{dt}}.

Magnetic Flux

A non-moving direction of force produced in all magnetic fields that travels from the North pole to the South pole of a magnet.

RF Coil

An antenna used in MRI that acts like a coil of wire to detect changing magnetic fields produced by precessing hydrogen nuclei and induce a measurable voltage signal.

Right-Hand Rule

A rule indicating that moving a magnetic field towards a conductive material produces a clockwise current, while moving the magnetic source away produces a counterclockwise current.

Magnetophosphenes

A visual sensation of flashing lights caused by voltages induced in the optic nerve when a patient is moved through magnetic flux.

Peripheral Nerve Stimulation (PNS)

The induction of electrical current in human tissues during MRI caused by changing magnetic fields, which can lead to discomfort and muscle twitching.

Eddy Currents

Small swirls of electrical current produced in a conductive material (such as patient tissue or system components) as it moves through a strong magnetic field or experiences changing gradient fields.

Lenz's Law

The law stating that eddy currents generated in a conductor create an electromagnetic field with opposing magnetic flux at $90^\text{o}$ to the electrical field, opposing the original magnetic motion.

Ohm's Law

The physical law describing the proportional relationship between voltage ($V$), current ($I$), and resistance ($R$), expressed as $V = I \times R$.

Resistance ($R$)

The impedance or total opposition that a circuit or system offers to the flow of an alternating electric current, which generates thermal energy and heat loss.

Ohmic Heating

Thermal energy produced in a conductor or patient tissue as electrical resistance opposes induced current flow, presenting potential risk with long linear conductive implants.

Net Magnetization Vector (NMV)

The cumulative magnetic vector created by the excess number of precessing hydrogen nuclei pointing in alignment with the primary magnetic field.

Pulse Sequence

The set of programmed instructions given to time-varying magnetic fields (RF pulses and magnetic gradients) to perform MRI data acquisition.

Physical Gradients

The three magnetic field gradients (Z, Y, and X) defined relative to the patient's physical body axes to achieve slice localization.

Z-Gradient

The physical gradient activated along the superior-to-inferior axis (head to foot) to select axial imaging planes.

Y-Gradient

The physical gradient activated along the anterior-to-posterior axis of the patient to select coronal imaging planes.

X-Gradient

The physical gradient activated along the right-to-left axis of the patient to select sagittal imaging planes.

Anatomical Planes

Standard spatial orientation planes (Sagittal, Coronal, and Axial/Transverse) defined along the X, Y, and Z axes of a patient during MRI imaging.

Slice Selection Gradient

The magnetic gradient activated simultaneously with an RF pulse to isolate a specific slice thickness and orientation plane of image.

Phase Encoding Gradient

The magnetic gradient activated to spatially localize and determine which specific line of k-space receives collected signal data.

Frequency Encoding Gradient

Also called the readout gradient, this gradient is activated during echo time ($TE$) to alter hydrogen precessional frequencies while sampling signal.

Fourier Transformation

The mathematical algorithm used to convert raw frequency and phase data stored in k-space into a reconstructed visual image.

Spin Echo (SE) Sequence

A pulse sequence consisting of a $90^\text{o}$ RF excitation pulse followed by one or more $180^\text{o}$ refocusing pulses to eliminate field inhomogeneities and acquire true $T2$ weighting.

Gradient Echo (GE) Sequence

A rapid pulse sequence using flip angles of $90^\text{o}$ or less and gradient reversals instead of $180^\text{o}$ RF refocusing pulses, yielding $T2^*$ weighted contrast.

Steady-State Imaging

Pulse sequence techniques (such as SPGR, TRUEFISP, SSFP, FLASH, and FIESTA) that maintain transverse magnetization by utilizing very short repetition times ($TR$) and small flip angles.

Echo Planar Imaging (EPI)

An ultra-fast pulse sequence technique used in DWI, perfusion, and fMRI where phase and frequency encoding gradients alternate rapidly to fill k-space in single or multiple shots.