Gradient coils - Questions and Answers in MRI
Three sets of gradient coils are used in nearly all MR systems: the x-, y-, and z-gradients. Each coil set is driven by an independent power amplifier and creates a gradient field whose z -component varies linearly along the x -, y -, and z -directions, respectively.
In modern MRI, gradient coils able to generate high gradient strengths and slew rates are required to produce high imaging speeds and improved image quality. MRI also requires the use of gradient coils that generate magnetic fields, which vary …
Gradient linearity - Questions and Answers in MRI
This method mirrors the definition of magnetic field homogeneity as % field change over a defined spherical volume (DSV). Gradient linearity issues are often subtle and create minor spatial distortion artifacts and blurring at the extreme margins of MR images.
X- and Y-gradients - Questions and Answers in MRI
To produce an x-gradient the coils are rotated by 90º along the surface of the cylinder. Vintage (ca.1986) MR scanner showing gradients (y-gradient Golay coil marked by red arrow, z-gradient Maxwell coil marked by yellow arrow).
Gradient specifications - Questions and Answers in MRI
Key gradient specifications: Slew rate is defined as Peak gradient strength ÷ Rise time. Both spatial resolution and imaging speed depend upon good gradient performance, and scanners do differ in their gradient specifications.
Open scanner gradients - Questions and Answers in MRI
Gradients for open scanners. All the diagrams I have seen for gradients deal with cylindrical scanners. How are the gradients designed for open-bore scanners? Representative cross-section of an open-bore magnet showing location of planar gradient coils …
Radiofrequency (RF) coils are an essential MRI hardware component. They directly impact the spatial and temporal reso-lution, sensitivity, and uniformity in MRI. Advances in RF hardware have resulted in a variety of designs optimized for specific clinical applications.
Gradient heating - Questions and Answers in MRI
This heating is primarily caused by eddy currents and resistive heating as current is passed through the imaging gradients. Gradient coils are driven by powerful pulse-width modulated (PWM) amplifiers located in an equipment room that is usually directly adjacent to the magnet (scanning) room.
Present gradient technology allows the application of gradient pulses up to 50 mT/m (for whole-body applications) with rise times down to 100 μs. Using dedicated gradient coils, such as head-insert gradient coils, even higher amplitudes and shorter rise times are feasible.
Peripheral Nerve Stimuliation - Questions and Answers in MRI
Simulated distribution of x-direction magnetic gradient (B) and and gradient switching-induced electric (E) fields in a human subject. Note the gradient (B) fields are strongest near the walls of the scanner (immediately adjacent to the coils).