Daniel J. Schaefer

Review of Patient Safety in Time-Varying Gradient Fields

In magnetic resonance, time‐varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (ie, tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%–100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reillys predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5‐T magnetic fields. J. Magn. Reson. Imaging 2000;12:20–29.

Apparatus and method of acquiring physiological gating signals for magnetic resonance imaging of moving objects

An apparatus and method are disclosed which provide signals corresponding to physiological motion of an imaging slice in an MR system for use in synchronizing acquisition of MR data with movement of the slice. The signals are generated by initiating an incident signal of a frequency ω which interacts with the imaging slice and returns a reflected signal of a frequency ω. By mixing the incident and reflected signal, a baseband signal is generated which is indicative of changes in the phase and magnitude relationships between the signals. Because changes in the phase and magnitude relationships between the signals are related in an approximately linear manner to movement of the imaging slice, the baseband signal provides an indication of movement of the imaging slice of sufficient quality to serve as an accurate triggering signal to synchronize acquistion of MR data with movement of the imaging slice.

Alterations in body and skin temperatures caused by magnetic resonance imaging: is the recommended exposure for radiofrequency radiation too conservative?

Abstract Increases in tissue temperature caused by exposure to radiofrequency (RF) radiation are a primary safety concern of magnetic resonance imaging (MRI). Therefore, body and skin temperatures were measured in six subjects before (20 min), during (30min) and after (20 min) MRI procedures performed at specific absorption rates (SARs) six to 10 times higher than the limit recommended by the UK National Radiological Protection Board. Body temperature was unchanged throughout the experiment. Abdominal skin temperature increased significantly (p<0.05) during MRI, decreased significantly post-MRI, but was still significantly (p<0.05) higher than baseline. The highest abdominal skin temperature recorded was 36°C. Upper arm, forearm and chest skin temperatures increased significantly (p<0.05) during MRI and remained elevated post-MRI. The highest skin temperatures recorded on the upper arm, forearm and chest were 38.1, 36.0 and 34.5°C, respectively. Thigh and calf skin temperatures were not significantly chan...

Peripheral nerve stimulation properties of head and body gradient coils of various sizes

Peripheral nerve stimulation (PNS) caused by time‐varying magnetic fields has been studied both theoretically and experimentally. A human volunteer study performed on three different body‐size gradient coils and one head‐size gradient coil is presented in this work. The experimental results were used to generate average PNS threshold parameters for the tested gradient systems. It was found that the average stimulation threshold increases while gradient‐region‐of‐uniformity size decreases. In addition, linear relationships between PNS parameters and diameter of homogeneous gradient spherical volume (DSV) were discovered: SRmin and ΔGmin both vary inverse linearly with DSV. More importantly, the chronaxie value was found to vary inversely linearly with the DSV. This finding indicates that, contrary to the general understanding, the parameter “chronaxie” in the commonly accepted simple stimulation models cannot be considered to be a single‐value, nerve‐specific constant. A modified linear model for gradient‐induced PNS based on these results was developed, which may permit, for the first time, the general prediction of nerve stimulation properties for gradient coils of arbitrary linear region dimension. Magn Reson Med 50:50–58, 2003.

Comparison of the threshold for peripheral nerve stimulation during gradient switching in whole body MR systems

To compare thresholds for peripheral nerve stimulation from gradient switching in whole body magnetic resonance (MR) equipment of different design.

Dosimetry and Effects of MR Exposure to RF and Switched Magnetic Fields

As magnetic resonance (MR) technology evolves, the need for more detailed knowledge of the safety implications of larger and faster gradients and higher average radio-frequency (RF) power scans grows. It is imperative to protect patient safety during MR scans. It is also imperative to avoid overly restrictive safety standards that might compromise patient health by unnecessarily restricting use of beneficial pulse sequences. The purpose of this report is to explore gradient and radio-frequency dosimetry and effects.

Phantom for performance evaluation of a nuclear magnetic resonance scanner

There is provided a phantom for measuring a performance parameter of a nuclear magnetic resonance scanner capable of acquiring NMR data from a study object. The phantom is made up of a housing composed of a substantially non-NMR-active material, and positionable in use within a radio-frequency coil forming part of an NMR scanner. The phantom further includes a first medium disposed within the housing to load the radio frequency coil in a manner similar to that of a patient. The NMR properties of the medium are further selected such that an NMR signal, if any, produced by the medium occurs at a time other than when a performance parameter of said NMR scanner is measured. In the preferred embodiment, the phantom comprises two concentrically disposed cylinders of different diameters and a pair of end plates fastened to opposite ends of the cylinders. The first medium is disposed within the volume defined between the side walls of the cylinders and the end plates. A second medium is disposed within the volume defined within the innermost cylinder and generates the NMR test signals.

Spiral Scan Peripheral Nerve Stimulation

Time‐varying magnetic fields induce electric fields that can cause physiological stimulation. Stimulation has been empirically characterized as a function of dB/dt and duration based on experiments using trapezoidal and sinusoidal gradient waveforms with constant ramp time, amplitude, and direction. For two‐dimensional (2D) spiral scans, the readout gradient waveforms are frequency‐ and amplitude‐modulated sinusoids on two orthogonal axes in quadrature. The readout gradient waveform therefore rotates with amplitude and angular velocity that are generally not constant. It does not automatically follow that spiral stimulation thresholds can be predicted using available stimulation models. We scanned 18 normal volunteers with a 2D spiral scan and measured global thresholds for axial, sagittal, and coronal planes. We concluded that the stimulation model evaluated accurately predicts slew rate‐limited spiral mean stimulation thresholds, if the effective ramp time is chosen to be the half‐period at the end of the spiral readout. J. Magn. Reson. Imaging 2000;12:164–170.