William B. Handler

Design of field-cycled magnetic resonance systems for small animal imaging

This paper presents a design study for a field-cycled magnetic resonance imaging (MRI) system directed at small animal imaging applications. A field-cycled MRI system is different from a conventional MRI system in that it uses two separate and dynamically controllable magnetic fields. A strong magnetic field is used to polarize the object, and a relatively weak magnetic field is used during signal acquisition. The potential benefits of field-cycled MRI are described. The theoretical dependences of field-cycled MRI performance on system design are introduced and investigated. Electromagnetic, mechanical and thermal performances of the system were considered in this design study. A system design for imaging 10 cm diameter objects is presented as an example, capable of producing high-duty-cycle polarizing magnetic fields of 0.5 T and readout magnetic fields corresponding to a proton Larmor frequency of 5 MHz. The specifications of the final design are presented along with its expected electromagnetic and thermal performance.

Simulation of scattering and attenuation of 511 keV photons in a combined PET/field-cycled MRI system

Mixing the imaging modalities of positron emission tomography (PET) and magnetic resonance imaging (MRI) will offer the best soft tissue contrast (MRI) with information about metabolic function (PET). The high magnetic field environment of an MRI system makes the detection of annihilation photons difficult, as the response of standard photo-multiplier tubes is compromised. An approach using field-cycled MRI is discussed here, as field-cycled MRI makes it possible to have long periods of time available for nuclear imaging when there is no magnetic field present. This work focuses upon the effect of the field-cycled MRI upon the nuclear image due to the added material providing additional attenuation of the PET signal, and additional nuclei for scatter. These effects are studied using a Monte Carlo simulation based upon the GEANT libraries. Attenuation effects are shown to be significant, approximately 6% for the RF shield and coil and approximately 24% for the gradients. No significant effect is seen in image quality due to the scattering of the gammas. With these levels of attenuation it is concluded that open gradient coils and shim coils are required around the imaging volume.

Delta relaxation enhanced MR: improving activation-specificity of molecular probes through R1 dispersion imaging.

MR molecular imaging enables high‐resolution, in vivo study of molecular processes frequently utilizing gadolinium‐based probes that specifically bind to a particular biological molecule or tissue. While some MR probes are inactive when unbound and produce enhancement only after binding, the majority are less specific and cause enhancement in either state. Accumulation processes are then required to increase probe concentration in regions of the target molecule/tissue. Herein, a method is described for creating specificity for traditionally nonspecific probes. This method utilizes MR field‐cycling methods to produce MRI contrast related to the dependence of R1 upon magnetic field. It is shown that the partial derivative of R1 with respect to magnetic field strength, R1′, can be used as an unambiguous measure of probe binding. T1‐weighted images and R1′ images were produced for samples of albumin and buffer both enhanced with the albumin‐binding agent Vasovist. For T1 images, samples with low concentrations of Vasovist in an albumin solution could not be differentiated from samples with higher concentrations of Vasovist in buffer. Conversely, the R1′ images showed high specificity to albumin. Albumin samples with a 10‐μM concentration of Vasovist were enhanced over buffer samples containing up to 16 times more Vasovist. Magn Reson Med, 2009.

Field dependence of T1 for hyperpolarized [1-13C]pyruvate

In vivo metabolism of hyperpolarized pyruvate has been demonstrated to be an important probe of cellular glycolysis in diseases such as cancer. The usefulness of hyperpolarized (13)C imaging is dependent on the relaxation rates of the (13)C-enriched substrates, which in turn depend on chemical conformation and properties of the dissolution media such as buffer composition, solution pH, temperature and magnetic field. We have measured the magnetic field dependence of the spin-lattice relaxation time of hyperpolarized [1-(13)C]pyruvate using field-cycled relaxometry. [1-(13)C]pyruvate was hyperpolarized using dynamic nuclear polarization and then rapidly thawed and dissolved in a buffered solution to a concentration of 80 mmol l(-1) and a pH of ~7.8. The hyperpolarized liquid was transferred within 8 s to a fast field-cycling relaxometer with a probe tuned for detection of (13)C at a field strength of ~0.75 T. The magnetic field of the relaxometer was rapidly varied between relaxation and acquisition fields where the sample magnetization was periodically measured using a small flip angle. Data were recorded for relaxation fields varying between 0.237 mT and 0.705 T to map the T(1) dispersion of the C-1 of pyruvate. Using similar methods, we also determined the relaxivity of the triarylmethyl radical (OX063; used for dynamic nuclear polarization) on the C-1 of pyruvate at field strengths of 0.001, 0.01, 0.1 and 0.5 T using 0.075, 1.0 and 2.0 mmol l(-1) concentrations of OX063 in the hyperpolarized pyruvate solution.

Evaluation of a positron emission tomography (PET)-compatible field-cycled MRI (FCMRI) scanner

Field‐cycled MRI (FCMRI) uses two independent, actively controlled resistive magnets to polarize a sample and to provide the magnetic field environment during data acquisition. This separation of tasks allows for novel forms of contrast, reduction of susceptibility artifacts, and a versatility in design that facilitates the integration of a second imaging modality. A 0.3T/4‐MHz FCMRI scanner was constructed with a 9‐cm‐wide opening through the side for the inclusion of a photomultiplier‐tube–based positron emission tomography (PET) system. The performance of the FCMRI scanner was evaluated prior to integrating PET detectors. Quantitative measurements of the systems signal, phase, and temperature were recorded. The polarizing and readout magnets could be operated continuously at 100 A without risk of damage to the system. Transient instabilities in the readout magnet, caused by the pulsing of the polarizing magnet, dissipated in 50 ms; this resulted in a steady‐state homogeneity of 32 Hz over a 7‐cm‐diameter volume. The short‐ and long‐term phase behaviors of the readout field were sufficiently stable to prevent visible readout or phase‐encode artifacts during imaging. Preliminary MR images demonstrated the potential of the FCMRI scanner and the efficacy of integrating a PET system. Magn Reson Med, 2009.

A new approach to shimming: The dynamically controlled adaptive current network

Magnetic field homogeneity is important in all aspects of magnetic resonance imaging. A new approach to increase field homogeneity is presented that allows dynamic and adaptive control over the flow of current over a single surface using a network of actively controlled solid‐state switches.

First image from a combined positron emission tomography and field-cycled MRI system

Combining positron emission tomography and MRI modalities typically requires using either conventional MRI with a MR‐compatible positron emission tomography system or a modified MR system with conventional positron emission tomography. A feature of field‐cycled MRI is that all magnetic fields can be turned off rapidly, enabling the use of conventional positron emission tomography detectors based on photomultiplier tubes. In this demonstration, two photomultiplier tube‐based positron emission tomography detectors were integrated with a field‐cycled MRI system (0.3 T/4 MHz) by placing them into a 9‐cm axial gap. A positron emission tomography‐MRI phantom consisting of a triangular arrangement of positron‐emitting point sources embedded in an onion was imaged in a repeating interleaved sequence of ∼1 sec MRI then 1 sec positron emission tomography. The first multimodality images from the combined positron emission tomography and field‐cycled MRI system show no additional artifacts due to interaction between the systems and demonstrate the potential of this approach to combining positron emission tomography and MRI. Magn Reson Med, 2011.

Prediction of Force and Image Artifacts Under MRI for Metals Used in Medical Devices

Selection of compatible materials for magnetic resonance imaging (MRI) is a challenging task as severe restrictions are imposed on materials used in and around the scanner due to the static and dynamic magnetic fields involved. Much of the data available for MRI-compatible materials are scattered throughout the literature and are often too device specific. This paper focuses on engineering materials with sufficient strength and stiffness, and with low enough susceptibility to be used in this environment. Experimental results of generic test specimens are used to give comparable performance indicators for candidate materials. As expected, the force varies linearly with susceptibility with good correlation with the theoretical predictions except for brass 360. It is believed the susceptibility for brass 360 in the literature was mistakenly recorded, and our results suggest a value of 112 ppm. The image artifacts were compared based on the radius of the affected area in the image. The theory greatly overpredicts the affected area; however, the trends in terms of susceptibility seem fairly accurate. The size of the artifact increases with susceptibility, echo time, and the use of turbo spin echo over gradient echo sequences. However, the experimental data contradicted the theory by showing no appreciable effect due to bandwidth.

Application and experimental validation of an integral method for simulation of gradient-induced eddy currents on conducting surfaces during magnetic resonance imaging

The time-varying magnetic fields created by the gradient coils in magnetic resonance imaging can produce negative effects on image quality and the system itself. Additionally, they can be a limiting factor to the introduction of non-MR devices such as cardiac pacemakers, orthopedic implants, and surgical robotics. The ability to model the induced currents produced by the switching gradient fields is key to developing methods for reducing these unwanted interactions. In this work, a framework for the calculation of induced currents on conducting surface geometries is summarized. This procedure is then compared to two separate experiments: (1) the analysis of the decay of currents induced upon a conducting cylinder by an insert gradient set within a head only 7 T MR scanner; and (2) analysis of the heat deposited into a small conductor by a uniform switching magnetic field at multiple frequencies and two distinct conductor thicknesses. The method was shown to allow the accurate modeling of the induced time-varying field decay in the first case, and was able to provide accurate estimation of the rise in temperature in the second experiment to within 30% when the skin depth was greater than or equal to the thickness of the conductor.

Results for diffusion-weighted imaging with a fourth-channel gradient insert.

Diffusion‐weighted imaging suffers from motion artifacts and relatively low signal quality due to the long echo times required to permit the diffusion encoding. We investigated the inclusion of a noncylindrical fourth gradient coil, dedicated entirely to diffusion encoding, into the imaging system. Standard three‐axis whole body gradients were used during image acquisition, but we designed and constructed an insert coil to perform diffusion encodings. We imaged three phantoms on a 3‐T system with a range of diffusion coefficients. Using the insert gradient, we were able to encode b values of greater than 1300 s/mm2 with an echo time of just 83 ms. Images obtained using the insert gradient had higher signal to noise ratios than those obtained using the whole body gradient: at 500 s/mm2 there was a 18% improvement in signal to noise ratio, at 1000 s/mm2 there was a 39% improvement in signal to noise ratio, and at 1350 s/mm2 there was a 56% improvement in signal to noise ratio. Using the insert gradient, we were capable of doing diffusion encoding at high b values by using relatively short echo times. Magn Reson Med, 2011.