Design and performance evaluation of a whole-body Ingenuity TF PET-MRI system.

The Ingenuity TF PET-MRI is a newly released whole-body hybrid PET-MR imaging system with a Philips time-of-flight GEMINI TF PET and Achieva 3T X-series MRI system. Compared to PET-CT, modifications to the positron emission tomography (PET) gantry were made to avoid mutual system interference and deliver uncompromising performance which is equivalent to the standalone systems. The PET gantry was redesigned to introduce magnetic shielding for the photomultiplier tubes (PMTs). Stringent electromagnetic noise requirements of the MR system necessitated the removal of PET gantry electronics to be housed in the PET-MR equipment room. We report the standard NEMA measurements for the PET scanner. PET imaging and performance measurements were done at Geneva University Hospital as described in the NEMA Standards NU 2-2007 manual. The scatter fraction (SF) and noise equivalent count rate (NECR) measurements with the NEMA cylinder (20 cm diameter) were repeated for two larger cylinders (27 cm and 35 cm diameter), which better represent average and heavy patients. A NEMA/IEC torso phantom was used for overall assessment of image quality. The transverse and axial resolution near the center was 4.7 mm. Timing and energy resolution of the PET-MR system were measured to be 525 ps and 12%, respectively. The results were comparable to PET-CT systems demonstrating that the effect of design modifications required on the PET system to remove the harmful effect of the magnetic field on the PMTs was negligible. The absolute sensitivity of this scanner was 7.0 cps kBq(-1), whereas SF was 26%. NECR measurements performed with cylinders having three different diameters, and image quality measurements performed with IEC phantom yielded excellent results. The Ingenuity TF PET-MRI represents the first commercial whole-body hybrid PET-MRI system. The performance of the PET subsystem was comparable to the GEMINI TF PET-CT system using phantom and patient studies. It is conceived that advantages of hybrid PET-MRI will become more evident in the near future.

Application of the SUSHI method to the design of gradient coils.

An approach to potential improvements in magnetic field shielding for a gradient coil system with cylindrical geometry is presented, utilizing "supershielding" conditions for the currents on both the primary and the secondary coils. It is demonstrated that the field can be strongly suppressed everywhere outside a cylindrical shield coil radius, even though the finite-length active shield only partially surrounds a primary coil. The supershielding method, which is aimed at controlling eddy currents, still has sufficient freedom to maintain the desired magnetic field behavior inside the imaging volume. The trade-off is an additional primary current oscillation and increased current peaks and field energy. This method has been applied to design short transverse and axial gradient coils, giving substantially improved shielding compared to an apodization method. Magn Reson Med 45:147-155, 2001.

Novel quadrature birdcage coil for a vertical B(0) field open MRI system.

A birdcage coil possesses various intrinsic resonant modes. One of the modes produces sinusoidal current distribution around the rung elements of the birdcage and generates a highly uniform B(1) field in the transverse plane. Another mode is the end-ring resonant mode, in which there are no currents in the rungs of the birdcage, leaving the currents flowing in the top and bottom end-rings. Thus, this mode behaves like a "Helmholtz pair," producing a highly uniform B(1) field along the axis of the birdcage. By combining the sinusoidal resonant mode and the end-ring resonant mode at the same frequency, a quadrature birdcage coil for a vertical B(0) field is made possible. In particular, the coil dimension can be chosen such that the two end-ring loops behave as a Helmholtz pair, assuring the optimized B(1) field uniformity around the coil center region. The coil is well suited for a vertical B(0) MRI system, as both B(1) fields are perpendicular to the vertical B(0) field. To mimic a vertical B(0) field orientation, an existing horizontal bore MRI system was used with the RF coil oriented to simulate a "vertical" B(0) field. This is a report of the successful quadrature implementation of this novel birdcage coil.

Moment method analysis of mutual interaction in MRI phased array coils.

We describe the use of a computational method for determining the overlap distance minimizing the mutual interaction between two adjacent coils that constitute a part of a phased array system used in MRI. The method is based upon the method of moments, and the analysis is carried out at a target imaging frequency to obtain the overlap distance. For a variety of complex RF phased array coils, we can determine, using the proposed approach, the overlap distance between nearest neighbor block element RF coils such that the mutual interaction is nullified or minimized. We give experimental results to validate the proposed approach. When compared with the experimental data, our theoretical prediction is in excellent agreement.

A hybrid inverse approach applied to the design of lumped-element RF coils.

A combination of inverse procedures is employed in the design of radio-frequency (RF) coils with specific examples in, but not restricted to, magnetic resonance imaging. The first inverse procedure is the use of functional methods for the optimization of coil characteristics subject to restrictions on the field behavior. Continuous current distributions are derived from analysis of the fields they are required to produce. To make use of these distributions at a desired frequency, the method of moments is applied as a second inverse procedure to a discretized version of the current distribution. The advantage of this hybrid technique is that it provides a computational algorithm for optimization of feeding, tuning, impedance matching and other aspects of RF coil design. A prototype RF coil has been built using the engineering values predicted by the theory. Experimental results including images acquired from the prototype coil are presented.

Exact temporal eddy current compensation in magnetic resonance imaging systems.

A step-response method has been developed to extract the properties (amplitudes and decay time constants) of intrinsic-eddy-current-sourced magnetic fields generated in whole-body magnetic resonance imaging systems when pulsed field gradients are applied. Exact compensation for the eddy-current effect is achieved through a polynomial rooting procedure and matrix inversion once the 2 N properties of the N-term decay process are known. The output of the inversion procedure yields the required characteristics of the filter for spectrum magnitude and phase equalization. The method is described for the general case along with experimental results for one-, two-, and three-term inversions. The method's usefulness is demonstrated for the usually difficult case of long-term (200-1000-ms) eddy-current compensation. Field-gradient spectral flatness measurements over 30 mHz-100 Hz are given to validate the method.