Transitioning from 0.5 to 0.9 mT: Protecting against inadvertent activation of magnet mode in active implants.

The "5 gauss line" is a phrase that is likely to be familiar to everyone working with MRI, but what is its significance, how was it defined, and what changes are currently in progress? This review explores the history of 5 gauss (0.5 mT) as a threshold for protecting against inadvertently putting cardiac pacemakers, implantable cardioverter defibrillators, and other active implantable medical devices into a "magnet mode." Additionally, it describes the background to the recent change of this threshold to 9 gauss (0.

A simple method for estimating the noise level in a signal region of an MR image.

Purpose: A simple extension to the NEMA MS-1 "difference of neighboring pixels" SNR method is presented, which can accurately determine the noise level within a signal region over a wide range of noise levels, image nonuniformities, and artifact levels, as demonstrated by simple simulations and experimental phantom images.

Methods: The new method computes difference of neighboring pixels in the read, phase, and diagonal directions. The variance of these three sets of pixel differences appear to contain the simple sum of the underlying variance of noise and any additional component unique to the read and phase directions, respectively, while the diagonal set of pixel differences contains all three components.

A new single acquisition, two-image difference method for determining MR image SNR.

A new method for computing the signal-to-noise ratio (SNR) of magnetic resonance images is presented. The proposed method is a "difference of images" based technique where two images are produced from one acquisition in which the readout direction field of view (FOV) and matrix size are doubled compared to the phase encode direction. Two "normal" unaliased FOV images are produced by splitting (undersampling) the even versus odd data points in the read direction into two separate raw data sets.

A statistical approach to SENSE regularization with arbitrary k-space trajectories.

SENSE reconstruction suffers from an ill-conditioning problem, which increasingly lowers the signal-to-noise ratio (SNR) as the reduction factor increases. Ill-conditioning also degrades the convergence behavior of iterative conjugate gradient reconstructions for arbitrary trajectories. Regularization techniques are often used to alleviate the ill-conditioning problem.

Measuring magnetic fields generated by DC currents in receive-only coils.

DC decoupling currents applied to receive-only coils during radiofrequency transmission can create stray magnetic fields capable of changing the resonant frequency of nearby nuclei. It is difficult to measure these fields with conventional field-mapping techniques because the fields are not present when the signal is acquired. The stray fields can be measured empirically with cardiac tags.

Open low-field magnetic resonance imaging in radiation therapy treatment planning.

Purpose: To evaluate the possibilities of an open low-field magnetic resonance imaging (MRI) scanner in external beam radiotherapy treatment (RT) planning.

Methods And Materials: A custom-made flat tabletop was constructed for the open MR, which was compatible with standard therapy positioning devices. To assess and correct image distortion in low-field MRI, a custom-made phantom was constructed and a software algorithm was developed.