William A. Edelstein

Noise Figure Limits for Circular Loop MR Coils

Circular loops are the most common MR detectors. Loop arrays offer improved signal‐to‐noise ratios (SNRs) and spatial resolution, and enable parallel imaging. As loop size decreases, loop noise increases relative to sample noise, ultimately dominating the SNR. Here, relative noise contributions from the sample and the coil are quantified by a coil noise figure (NF), NFcoil, which adds to the conventional system NF. NFcoil is determined from the ratio of unloaded‐to‐loaded coil quality factors Q. Losses from conductors, capacitors, solder joints, eddy currents in overlapped array coils, and the sample are measured and/or computed from 40 to 400 MHz using analytical and full‐wave numerical electromagnetic analysis. The Qs are measured for round wire and tape loops tuned from 50 to 400 MHz. NFcoil is determined as a function of the radius, frequency, and number of tuning capacitors. The computed and experimental Qs and NFcoils agree within ∼10%. The NFcoil values for 3 cm‐diameter wire coils are 3 dB, 1.9 dB, 0.8 dB, 0.2 dB, and 0.1 dB, at 1T, 1.5T, 3T, 7T, and 9.4T, respectively. Wire and tape perform similarly, but tape coils in arrays have substantial eddy current losses. The ability to characterize and reliably predict component‐ and geometry‐associated coil losses is key to designing SNR‐optimized loop and phased‐array detectors. Magn Reson Med, 2009.

MRI acoustic noise can harm experimental and companion animals

To assess possible damage to the hearing of experimental and companion animal subjects of magnetic resonance imaging (MRI) scans.

MRI dynamic range and its compatibility with signal transmission media

As the number of MRI phased array coil elements grows, interactions among cables connecting them to the system receiver become increasingly problematic. Fiber optic or wireless links would reduce electromagnetic interference, but their dynamic range (DR) is generally less than that of coaxial cables. Raw MRI signals, however, have a large DR because of the high signal amplitude near the center of k-space. Here, we study DR in MRI in order to determine the compatibility of MRI multicoil imaging with non-coaxial cable signal transmission. Since raw signal data are routinely discarded, we have developed an improved method for estimating the DR of MRI signals from conventional magnitude images. Our results indicate that the DR of typical surface coil signals at 3T for human subjects is less than 88 dB, even for three-dimensional acquisition protocols. Cardiac and spine coil arrays had a maximum DR of less than 75 dB and head coil arrays less than 88 dB. The DR derived from magnitude images is in good agreement with that measured from raw data. The results suggest that current analog fiber optic links, with a spurious-free DR of 60-70 dB at 500 kHz bandwidth, are not by themselves adequate for transmitting MRI data from volume or array coils with DR approximately 90 dB. However, combining analog links with signal compression might make non-coaxial cable signal transmission viable.

An RF dosimeter for independent SAR measurement in MRI scanners

PURPOSE The monitoring and management of radio frequency (RF) exposure is critical for ensuring magnetic resonance imaging (MRI) safety. Commercial MRI scanners can overestimate specific absorption rates (SAR) and improperly restrict clinical MRI scans or the application of new MRI sequences, while underestimation of SAR can lead to tissue heating and thermal injury. Accurate scanner-independent RF dosimetry is essential for measuring actual exposure when SAR is critical for ensuring regulatory compliance and MRI safety, for establishing RF exposure while evaluating interventional leads and devices, and for routine MRI quality assessment by medical physicists. However, at present there are no scanner-independent SAR dosimeters. METHODS An SAR dosimeter with an RF transducer comprises two orthogonal, rectangular copper loops and a spherical MRI phantom. The transducer is placed in the magnet bore and calibrated to approximate the resistive loading of the scanners whole-body birdcage RF coil for human subjects in Philips, GE and Siemens 3 tesla (3T) MRI scanners. The transducer loop reactances are adjusted to minimize interference with the transmit RF field (B1) at the MRI frequency. Power from the RF transducer is sampled with a high dynamic range power monitor and recorded on a computer. The deposited power is calibrated and tested on eight different MRI scanners. Whole-body absorbed power vs weight and body mass index (BMI) is measured directly on 26 subjects. RESULTS A single linear calibration curve sufficed for RF dosimetry at 127.8 MHz on three different Philips and three GE 3T MRI scanners. An RF dosimeter operating at 123.2 MHz on two Siemens 3T scanners required a separate transducer and a slightly different calibration curve. Measurement accuracy was ∼3%. With the torso landmarked at the xiphoid, human adult whole-body absorbed power varied approximately linearly with patient weight and BMI. This indicates that whole-body torso SAR is on average independent of the imaging subject, albeit with fluctuations. CONCLUSIONS Our 3T RF dosimeter and transducers accurately measure RF exposure in body-equivalent loads and provide scanner-independent assessments of whole-body RF power deposition for establishing safety compliance useful for MRI sequence and device testing.

High performance SPECT system for simultaneous SPECT-MR imaging of small animals

Our goal is to develop a high performance SPECT system for simultaneous SPECT-MR imaging of small animals (SA). The SPECT system has inner diameter (ID) of 15.4 cm and outer diameter of 19.8 cm. It comprises five seamless cylindrical detectors, each with 19 CZT modules (2.54×2.54 cm2, 16×16 pixels). The SPECT system can be operated either stand-alone or as an insert into an MRI system with a minimum 20 cm bore. Cylindrical multipinhole (MPH) collimator sleeves (CSs), made with tungsten powder and solid tungsten pinhole apertures, were designed to provide maximum geometric efficiency under the systems geometric constraints. Different MPH collimators were designed for mouse or rat imaging, and for static high-resolution or dynamic imaging without CS rotation. Sparse-view image reconstruction methods reduce CS rotation. Monte Carlo simulations confirm the SPECT imaging characteristics of 2 MPH CSs that have 18 and 36 pinholes with 1 mm and 1.5 mm system resolution, respectively. Sparse-view 3D MPH image reconstruction with system response modeling indicates that 36 pinholes are sufficient to provide artifact-free images at 1.5 mm resolution without CS rotation. The SPECT system with the 2 MPH CSs, the RF coil, and all mechanical and electronics components have been constructed. Initial experimental phantom and small animal studies demonstrated the high performance and imaging characteristics of the SPECT system. In conclusion, a high performance small animal (SA) SPECT system has been designed and constructed for simultaneous SA SPECT-MRI. Initial subsystem testing has demonstrated excellent SPECT and MRI imaging performance that matches design predictions.

MRI pyschophysics: An experimental framework relating image quality to diagnostic performance metrics

To determine the minimal image quality needed to preserve diagnostic performance relative to arthroscopy in the knee.