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 NU2-2007 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.

Insertable planar gradient coils offer the potential for significant performance increases in magnetic resonance imaging through higher gradient strength and shorter rise times. Using variational methods to minimize inductance, and thereby to optimize switching speeds, we have analyzed and constructed a biplanar y-gradient coil for insertion into a solenoidal magnet system where z is the magnet axis. We have also analyzed biplanar x-gradient and z-gradient coil designs using the same methods. These biplanar coils offer an advantage over a cylindrical coil of comparable diameter in that they achieve high gradient strengths with relatively short rise times while maintaining patient access. Although the requirement that the currents for the x gradient lie in the same plane as for the y and z gradients increases the stored energy by a factor of 3 with respect to the other two gradients, this stored energy is still smaller by a factor of 2 than that of a comparably constrained x-gradient cylindrical coil. The biplanar coil design offers improved linearity over its single planar coil alternative. The particular designs we have investigated are generally limited to small-volume imaging.

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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.

Ultrashort gradient coils for ultrashort cylindrical MRI systems require new design methods. The challenge is to reduce system length while maintaining performance, e.g., to maintain acceptable linearity and uniformity over a large field of view (FoV). Trading MR system performance to achieve short length is in itself not a challenge. As a system is made shorter, the increasing leakage of the magnetic flux outside the gradient coil enlarges the eddy current effect. We present in this article a new approach to design of short cylindrical gradient coils with 3D current geometry. In contrast to past methods, in this approach the lengths of the gradient primary and shield are arbitrary and genuinely finite throughout the design process, and therefore the process does not require truncation of the current density. We show how to incorporate the residual eddy current effect into the design and control it at any reasonably low level. For a transverse gradient coil we show that the stored magnetic energy is significantly reduced by allowing the current to flow off the primary coil surface toward the shield coil surface along a conical surface that connects the two. For an axial gradient coil we show that allowing the current density to be nonzero at the edges of the primary and shield coils reduces the stored energy as well. We explore the strength of this approach in the example of an ultrashort gradient coil design intended for use in an ultrashort yet whole-body capable notional 1.2-m-long main magnet.