Randy Otto John Giaquinto

Highly parallel volumetric imaging with a 32-element RF coil array.

The improvement of MRI speed with parallel acquisition is ultimately an SNR‐limited process. To offset acquisition‐ and reconstruction‐related SNR losses, practical parallel imaging at high accelerations should include the use of a many‐element array with a high intrinsic signal‐to‐noise ratio (SNR) and spatial‐encoding capability, and an advantageous imaging paradigm. We present a 32‐element receive‐coil array and a volumetric paradigm that address the SNR challenge at high accelerations by maximally exploiting multidimensional acceleration in conjunction with noise averaging. Geometric details beyond an initial design concept for the array were determined with the guidance of simulations. Imaging with the support of 32‐channel data acquisition systems produced in vivo results with up to 16‐fold acceleration, including images from rapid abdominal and MRA studies. Magn Reson Med 52:869–877, 2004.

Coupling and decoupling theory and its application to the MRI phased array

In classical MRI phased‐array design, optimal coil overlapping is used to minimize coupling between nearest‐neighbor coils, and low input impedance preamplifiers are used to isolate the relatively weak coupling between non‐nearest neighbors. However, to make the complex sensitivities of phased‐array coils sufficiently distinct in parallel spatially‐encoded MRI, it is desirable to have no overlapping between coils. Also, if phased arrays are used as transmit coils in MRI, one can no longer rely on the low input impedance of the preamplifiers for decoupling. Here a coupling and decoupling theory is introduced to provide a better understanding of the relations between coupled and uncoupled signals in the MRI phased array, and to offer a new method for decoupling phased‐array coils without overlapping the nearest coil pairs. The new decoupling method is based on the assumption that any n‐element phased array can be decoupled by a 2n‐port interface system between phased array and preamplifiers. The detailed analysis and the experimental results show that a four‐port interface can be used to decouple a two‐element phased array. Furthermore, the four‐port interfaces can serve as building blocks to construct a 2n‐port decoupling interface. This new method allows one to place the coil elements anywhere that could optimize parallel spatial encoding without concern for coupling between the coils. The method can also be used for phased‐array transmit coils. Magn Reson Med 48:203–213, 2002.

Toward single breath-hold whole-heart coverage coronary MRA using highly accelerated parallel imaging with a 32-channel MR system†

Coronary MR angiography (CMRA) is generally confined to the acquisition of multiple targeted slabs with coverage dictated by the competing constraints of signal‐to‐noise ratio (SNR), physiological motion, and scan time. This work addresses these obstacles by demonstrating the technical feasibility of using a 32‐channel coil array and receiver system for highly accelerated volumetric breath‐hold CMRA. The use of the 32‐element array in unaccelerated CMRA studies provided a baseline SNR increase of as much as 40% over conventional cardiac‐optimized phased array coils, which resulted in substantially enhanced image quality and improved delineation of the coronary arteries. Modest accelerations were used to reduce breath‐hold durations for tailored coverage of the coronary arteries using targeted multi‐oblique slabs to as little as 10 s. Finally, high net accelerations were combined with the SNR advantages of a 3D steady‐state free precession (SSFP) technique to achieve previously unattainable comprehensive volumetric coverage of the coronary arteries in a single breath‐hold. The merits and limitations of this simplified volumetric imaging approach are discussed and its implications for coronary MRA are considered. Magn Reson Med, 2006.

Mapping of the prostate in endorectal coil-based MRI/MRSI and CT: a deformable registration and validation study.

The endorectal coil is being increasingly used in magnetic resonance imaging (MRI) and MR spectroscopic imaging (MRSI) to obtain anatomic and metabolic images of the prostate with high signal-to-noise ratio (SNR). In practice, however, the use of endorectal probe inevitably distorts the prostate and other soft tissue organs, making the analysis and the use of the acquired image data in treatment planning difficult. The purpose of this work is to develop a deformable image registration algorithm to map the MRI/MRSI information obtained using an endorectal probe onto CT images and to verify the accuracy of the registration by phantom and patient studies. A mapping procedure involved using a thin plate spline (TPS) transformation was implemented to establish voxel-to-voxel correspondence between a reference image and a floating image with deformation. An elastic phantom with a number of implanted fiducial markers was designed for the validation of the quality of the registration. Radiographic images of the phantom were obtained before and after a series of intentionally introduced distortions. After mapping the distorted phantom to the original one, the displacements of the implanted markers were measured with respect to their ideal positions and the mean error was calculated. In patient studies, CT images of three prostate patients were acquired, followed by 3 Tesla (3 T) MR images with a rigid endorectal coil. Registration quality was estimated by the centroid position displacement and image coincidence index (CI). Phantom and patient studies show that TPS-based registration has achieved significantly higher accuracy than the previously reported method based on a rigid-body transformation and scaling. The technique should be useful to map the MR spectroscopic dataset acquired with ER probe onto the treatment planning CT dataset to guide radiotherapy planning.

128-channel body MRI with a flexible high-density receiver-coil array.

To determine whether the promise of high‐density many‐coil MRI receiver arrays for enabling highly accelerated parallel imaging can be realized in practice.

Large field-of-view real-time MRI with a 32-channel system.

The emergence of parallel MRI techniques and new applications for real‐time interactive MRI underscores the need to evaluate performance gained by increasing the capability of MRI phased‐array systems beyond the standard four to eight high‐bandwidth channels. Therefore, to explore the advantages of highly parallel MRI a 32‐channel 1.5 T MRI system and 32‐element torso phased arrays were designed and constructed for real‐time interactive MRI. The system was assembled from multiple synchronized scanner‐receiver subsystems. Software was developed to coordinate across subsystems the real‐time acquisition, reconstruction, and display of 32‐channel images. Real‐time, large field‐of‐view (FOV) body‐survey imaging was performed using interleaved echo‐planar and single‐shot fast‐spin‐echo pulse sequences. A new method is demonstrated for augmenting parallel image acquisition by independently offsetting the frequency of different array elements (FASSET) to variably shift their FOV. When combined with conventional parallel imaging techniques, image acceleration factors of up to 4 were investigated. The use of a large number of coils allowed the FOV to be doubled in two dimensions during rapid imaging, with no degradation of imaging time or spatial resolution. The system provides a platform for evaluating the applications of many‐channel real‐time MRI, and for understanding the factors that optimize the choice of array size. Magn Reson Med 52:878–884, 2004.

32-element receiver-coil array for cardiac imaging.

A lightweight 32‐element MRI receiver‐coil array was designed and built for cardiac imaging. It comprises an anterior array of 21 copper rings (75 mm diameter) and a posterior array of 11 rings (107 mm diameter) that are arranged in hexagonal lattices so as to decouple nearest neighbors, and curved around the left side of the torso. Imaging experiments on phantoms and human volunteers show that it yields superior performance relative to an eight‐element cardiac array as well as a 32‐element whole‐torso array for both traditional nonaccelerated cardiac imaging and 3D parallel imaging with acceleration factors as high as 16. Magn Reson Med, 2006.

A broadband phased-array system for direct phosphorus and sodium metabolic MRI on a clinical scanner

Despite their proven gains in signal‐to‐noise ratio and field‐of‐view for routine clinical MRI, phased‐array detection systems are currently unavailable for nuclei other than protons (1H). A broadband phased‐array system was designed and built to convert the 1H transmitter signal to the non‐1H frequency for excitation and to convert non‐1H phased‐array MRI signals to the 1H frequency for presentation to the narrowband 1H receivers of a clinical whole‐body 1.5 T MRI system. With this system, the scanner operates at the 1H frequency, whereas phased‐array MRI occurs at the frequency of the other nucleus. Pulse sequences were developed for direct phased‐array sodium (23Na) and phosphorus (31P) MRI of high‐energy phosphates using chemical selective imaging, thereby avoiding the complex processing and reconstruction required for phased‐array magnetic resonance spectroscopy data. Flexible 4‐channel 31P and 23Na phased‐arrays were built and the entire system tested in phantom and human studies. The array produced a signal‐to‐noise ratio improvement of 20% relative to the best‐positioned single coil, but gains of 300–400% were realized in many voxels located outside the effective field‐of‐view of the single coil. Cardiac phosphorus and sodium MRI were obtained in 6–13 min with 16 and 0.5 mL resolution, respectively. Lower resolution human cardiac 23Na MRI were obtained in as little as 4 sec. The system provides a practical approach to realizing the advantages of phased‐arrays for nuclei other than 1H, and imaging metabolites directly. Magn Reson Med 43:269–277, 2000.

Accelerated spectroscopic imaging of hyperpolarized C-13 pyruvate using SENSE parallel imaging.

The ability to accelerate the spatial encoding process during a chemical shift imaging (CSI) scan of hyperpolarized compounds is demonstrated through parallel imaging. A hardware setup designed to simultaneously acquire 13C data from multiple receivers is presented here. A system consisting of four preamplifiers, four gain stages, a transmit coil, and a four receive channel rat coil was built for single channel excitation and simultaneous multi‐channel detection of 13C signals. The hardware setup was integrated with commercial scanner electronics, allowing the system to function similar to a conventional proton multi‐channel setup, except at a different frequency. The ability to perform parallel imaging is demonstrated in vivo. CSI data from the accelerated scans are reconstructed using a self‐calibrated multi‐spectral parallel imaging algorithm, by using lower resolution coil sensitivity maps obtained from the central region of k‐space. The advantages and disadvantages of parallel imaging in the context of imaging hyperpolarized compounds are discussed. Copyright

MRI in the Neonatal ICU: Initial Experience Using a Small-Footprint 1.5-T System

OBJECTIVE The objective of our study was to develop a small 1.5-T MRI system for neonatal imaging that can be installed in the neonatal ICU (NICU) and to evaluate its performance in 15 neonates. SUBJECTS AND METHODS A 1.5-T MR system designed for orthopedic use was adapted for neonatal imaging. Modifications included raising and leveling the magnet, construction of a patient table, and integration of imaging electronics from a high-performance adult-sized scanner. The system was used to perform MR examinations of the brain, abdomen, and chest in 15 medically stable neonates using standard clinical protocols. The scanning time was limited to 60 minutes. The MR examinations were performed without administering sedation to the patients. ECG, heart rate, oxygen saturation, and temperature were monitored continuously throughout the examination. The images were evaluated by two pediatric radiologists for overall study quality, motion artifact, spatial resolution, signal-to-noise ratio, and contrast. RESULTS All 15 neonates were successfully imaged without sedation. No adverse MRI-related events were noted. In total, 19 brain and seven abdominal examinations were performed. Six chest and two cardiac examinations were also obtained. Gross (versus physiologic) subject motion proved to be the most influential factor in determining overall study and image quality. High-quality diagnostic images were obtained at each anatomic location. CONCLUSION The customized neonatal MRI system provides state-of-the-art MRI capabilities in the NICU.