MRI Gradients, Coils, and Quality Assurance – Lecture Vocabulary

Key vocabulary covering MRI gradient physics, RF coil types, shielding, artifacts, and quality-assurance concepts discussed in the lecture.

Gradient (in MRI)

A deliberately created variation in magnetic field strength across space used to spatially encode MR signals.

Gradient Coil

One of three additional coil sets (X, Y, Z) that generate secondary magnetic fields to create gradients.

Z-Gradient

Gradient coil pair aligned head-to-foot (longitudinal) that alters field strength along the Z-axis.

X-Gradient

Gradient coil pair aligned left-to-right that alters field strength along the X-axis.

Y-Gradient

Gradient coil pair aligned anterior-to-posterior that alters field strength along the Y-axis.

Helmholtz Coil

Two parallel circular coils used for the Z-gradient design, producing a uniform central field.

Pair Saddle (Pear-Saddle) Coil

Figure-8-shaped coil design used for X and Y gradient sets.

Secondary Magnetic Field

The temporary field produced by gradient coils that adds to or subtracts from B₀.

Pulse Sequence

Timed series of RF pulses and gradients used to excite tissue and collect MR signals.

Slice-Selection Gradient

Gradient applied during an RF pulse to excite a specific slice of tissue.

Phase-Encoding Gradient

Gradient briefly applied to impart position-dependent phase information to spins.

Frequency-Encoding Gradient

Gradient applied during read-out so signal frequency varies with position.

Gradient Amplitude

Maximum strength of a gradient; measured in mT/m or gauss/cm.

Gradient Rise Time

Time required for a gradient to ramp from 0 to set amplitude; given in ms.

Gradient Slew Rate

Rate of change of gradient amplitude (mT/m per ms); combines strength and speed.

Duty Cycle (Gradient)

Percentage of each TR that a gradient may be switched on without exceeding safety limits.

Balanced Gradient

Second gradient of equal magnitude but opposite polarity applied to rephase spins.

Body Coil

Large built-in transceiver coil lining the bore; commonly used for RF transmit.

Local Coil

Detachable coil placed close to the anatomy, usually used for signal reception.

Transmit-Receive (Transceiver) Coil

RF coil capable of both transmitting RF energy and receiving the resulting MR signal.

Surface Coil

Flat or curved single-loop coil that lies directly over superficial anatomy for high SNR.

Volume Coil

Cylindrical coil (e.g., head or knee) that surrounds anatomy, providing uniform coverage.

Linear Coil

Coil with one RF channel polarized in a single direction.

Quadrature Coil

Coil with two orthogonal channels giving circular polarization, 40 % higher SNR and 50 % lower RF power than linear coils.

Phased-Array Coil

Coil containing multiple independent receiver elements/channels for high SNR and parallel imaging.

Parallel Imaging

Technique that uses multi-channel coils to acquire k-space faster, reducing scan time at the cost of some SNR.

RF Coil Decoupling

Electronic detuning of inactive array elements to prevent mutual induction and image artifacts.

Coil Tuning

Adjustment ensuring a coil resonates at the scanner’s center frequency for the targeted anatomy.

Coil Penetration

Depth within the body from which a coil can reliably detect signal; roughly equal to the coil’s radius.

Specific Absorption Rate (SAR)

Measure of RF energy deposited in tissue (W/kg); excessive SAR causes patient heating.

Faraday Cage

Copper/Aluminum RF shield enclosing the scan room to block external radio waves.

Zipper Artifact

Vertical line artifact in MR images caused by RF leakage into the Faraday cage.

Penetration Panel

RF-filtered panel through which all cables enter the scan room without compromising shielding.

ACR Phantom

Standardized cylindrical phantom used for weekly QA tests required for accreditation.

Geometric Accuracy

QA measurement verifying that known phantom dimensions appear correct (±2 mm) on images.

High Contrast Resolution

Ability to distinguish small, closely spaced structures with large signal differences; assessed with 0.9–1.1 mm hole patterns.

Low Contrast Resolution

Ability to detect subtle signal differences; evaluated by counting low-contrast holes in the phantom.

Quality Assurance (QA)

Daily/weekly tests and checks that ensure consistent MR image quality and equipment function.

Quality Control (QC)

More comprehensive, usually weekly or periodic program of performance measurements, documentation, and corrective actions.

Center Frequency

Resonance frequency of hydrogen at the scanner’s field strength (e.g., 63.8 MHz at 1.5 T).