ARRT MRI key terms

The rate at which nuclear spins precess in a magnetic field, found by multiplying the gyromagnetic ratio by the field strength. It sets the RF frequency the scanner transmits and receives.

The recovery of longitudinal magnetization toward equilibrium after an RF pulse, driven by energy exchange with surrounding molecules. Tissues with short T1 recover quickly and look bright on T1-weighted images.

The loss of transverse magnetization caused by spins drifting out of phase through local interactions with one another. Tissues with long T2 keep signal longer and appear bright on T2-weighted images.

A faster loss of transverse signal than pure T2 decay, caused by fixed field imperfections and susceptibility differences as well as spin interactions. Gradient echo images are especially sensitive to it.

The interval between successive RF excitation pulses applied to the same slice. A long TR allows more longitudinal recovery and less T1 weighting, and it limits how many slices a sequence can collect.

The interval from the middle of the excitation pulse to the middle of the collected echo. Lengthening TE adds T2 weighting and lowers signal as more dephasing occurs.

The angle by which an RF pulse tips net magnetization away from the main field. Small angles leave more longitudinal magnetization for fast gradient echo sequences but yield less signal per pulse.

A gradient switched on during readout so that position along one axis becomes a difference in precession frequency. The receiver separates the returning signal by frequency to build the readout direction.

A short gradient that makes spins at different positions across the body accumulate different phase angles. Repeating it with varied strength fills the phase direction of k-space line by line.

The raw data matrix that holds the MR signal in frequency and phase form before reconstruction. Lines near the center carry contrast, while outer lines carry fine detail and edges.

A computation that converts the frequency and phase data stored in k-space into a viewable image. The scanner's reconstruction computer performs it automatically after acquisition.

A sequence that applies a 180-degree refocusing pulse after excitation to recover signal lost to spin dephasing. It gives strong T2 contrast and is fairly tolerant of field imperfections.

A sequence that creates an echo by reversing the readout gradient rather than using a refocusing pulse. Short TR and TE plus small flip angles make fast or three-dimensional imaging possible.

A fast readout that fills all of k-space after a single excitation by flipping the readout gradient back and forth. It enables diffusion and functional imaging but can show distortion and loud noise.

The strength of the wanted signal compared with random background noise. It improves with larger voxels, more averages, stronger fields, and better coils, and it trades against resolution and speed.

How strongly a material becomes magnetized in a field, which slightly distorts the surrounding field. It causes signal dropouts and shape distortion near metal, air, and bone.

The small resonance frequency difference between fat and water protons because of their different molecular surroundings. At higher field strengths it produces bright and dark bands at fat-water borders.

A quick, low-resolution scan in two or three planes used to plan where the diagnostic sequences will be placed. The technologist angles and positions slices from these images.

The width and height of the area being imaged, chosen by the operator. A small field of view raises resolution but can cause wraparound if anatomy extends past its edges.

The depth of tissue excited during a scan, controlled by gradient strength and RF pulse bandwidth. Thin slices reduce partial volume averaging but collect less signal.

Often called averages or NSA, this is how many times each k-space line is measured and summed. More excitations raise signal-to-noise ratio but lengthen scan time in direct proportion.

A method that removes fat signal by exciting fat protons with a frequency-selective pulse and spoiling their signal. It helps edema, enhancement, and lesions stand out near fatty tissue.

A technique whose signal depends on how freely water molecules move within tissue. Restricted movement, as in acute stroke or dense tumor, stays bright on the trace image.

A calculated map showing how much water diffusion occurred in each voxel. It removes T2 shine-through from diffusion images, and low values mean restricted diffusion.

Methods for imaging blood vessels, either by making flowing blood bright without contrast or by following a gadolinium bolus through the arteries and veins.

Timing image acquisition to the patient's ECG or pulse so data are gathered at the same point in each heartbeat. It reduces blurring from cardiac motion in chest and spine exams.

The force that pulls ferromagnetic objects toward the magnet, which can injure people or damage equipment. Strict screening and room control are needed because the field is always on.

The amount of RF power absorbed per kilogram of tissue, measured in watts per kilogram. Scanners limit it to avoid heating, and it grows with field strength and fast, high-flip-angle sequences.

Tingling, twitching, or uncomfortable muscle contractions caused when fast-switching gradients induce currents in nerves. It limits gradient strength and speed in very fast sequences.

Loud knocking and banging made when gradient coils vibrate within the magnetic field. It increases with gradient strength and speed, so hearing protection is required in the scan room.

A sudden loss of superconductivity in the magnet coils that releases stored energy and boils off cryogen gas. The room can fill with vapor and the magnetic field collapses rapidly.

A four-level scheme that classifies areas around an MRI suite by magnetic field risk. Zone I is freely accessible and Zone IV is the magnet room itself.

A device label meaning the item may be used in the MRI environment only when specified conditions, such as field strength and scan type, are met. Staff must confirm those conditions before scanning.

A rare, serious disease that thickens skin and internal organs and has been linked to gadolinium contrast in people with severe kidney impairment. Renal function screening guides contrast decisions.

Explaining a procedure's purpose, risks, benefits, and alternatives so the patient can freely agree. In MRI it is typically needed for contrast injection and for research or invasive procedures.

Ways to help anxious patients finish an MRI, such as thorough explanation, feet-first or prone positioning, mirrors, a companion in the room, sedation, and shorter sequences.

Earplugs, headphones, or both worn by patients and staff in the scan room to reduce exposure to loud gradient noise. Headphones can also deliver music and instructions.

An estimate of how well the kidneys filter blood, usually calculated from a creatinine blood test. Low values warn that gadolinium contrast may be unsafe for that patient.

Leakage of intravenously injected contrast into the tissue surrounding the vein. It causes pain, swelling, and sometimes skin injury, and must be reported and monitored.

Placing the patient comfortably and securely with the coil close to the anatomy and the area of interest near the magnet center. Good positioning improves signal and reduces motion.