Biomedical subjects

Christopher J Hardy

Publications and source records attributed to Christopher J Hardy.

9 recordsLinked to original sources

PubMed2007

Aortic pathophysiology by cardiovascular magnetic resonance in patients with clinical suspicion of coronary artery disease.

PURPOSE: To correlate cardiovascular magnetic resonance (CMR)-based measurement of aortic pulse wave velocity (PWV) with serum markers for atherosclerosis and plaque burden in the thoracic aorta. METHOD: Individuals with risk factors for coronary atherosclerosis underwent CMR pulse wave velocity examination of the descending thoracic aorta and computed tomography for coronary calcium scoring. Inversion recovery images allowed quantification of aortic plaque. Serum lipids and c-reactive protein levels were measured. RESULTS: Mean PWV did not correlate with presence of aortic plaque (p = 0.55). Subgroup analysis showed no significant correlation with PWV and total plaque. PWV and pulse pressure correlated (PP) (R2 0.38, p = 0.0003), but PWV and other predictor variables did not. Total plaque area correlated with aortic diameter (p = 0.0066). CONCLUSIONS: In patients with suspected coronary artery disease, aortic pulse wave velocity reflects increased aortic stiffness demonstrated by elevated pulse pressure, but does not directly correlate with aortic plaque or serum markers for arterial disease.

Adult↗

PubMed2006

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.

Algorithms↗

PubMed2006

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.

Coronary Angiography↗

PubMed2005

Rapid volumetric MRI using parallel imaging with order-of-magnitude accelerations and a 32-element RF coil array: feasibility and implications.

RATIONALE AND OBJECTIVES: Many clinical applications of Magnetic Resonance Imaging are constrained by basic limits on imaging speed. Parallel MRI relaxes these limits by using the sensitivity patterns of arrays of radiofrequency receiver coils to encode spatial information in a manner complementary to traditional encoding with magnetic field gradients. Until now, parallel MRI has been used to achieve modest improvements in imaging speed; order-of-magnitude improvements have been elusive given fundamental losses in signal-to-noise ratio. The goal of this work was to demonstrate that, with appropriate hardware and careful SNR management, rapid volumetric imaging at high accelerations is in fact feasible. MATERIALS AND METHODS: Contrast-enhanced MRI with an axial 3D spoiled gradient echo imaging sequence was performed in healthy adult subjects using a 32-element RF coil array and a prototype 32-channel MR imaging system. Large imaging volumes were prescribed, in place of traditional limited slabs targeted only to suspect regions. RESULTS: As much as 16-fold net accelerations of imaging were achieved repeatably using this approach. The use of large 3D volumes allowed comprehensive anatomical coverage at clinically useful spatial and/or temporal resolution. The need for careful, time-consuming, and subject-specific scan prescription was also eliminated. CONCLUSION: The highly parallel imaging approach presented here allows previously inaccessible volumetric coverage for time-sensitive MRI examinations such as contrast-enhanced MRA, and simultaneously provides a substantially simplified imaging paradigm. The resulting capability for rapid volumetric imaging promises to combine the strengths of MRI with some of the advantages of alternative imaging modalities such as multidetector CT.

Adult↗

PubMed2004

Lumped-element planar strip array (LPSA) for parallel MRI.

The recently introduced planar strip array (PSA) can significantly reduce scan times in parallel MRI by enabling the utilization of a large number of RF strip detectors that are inherently decoupled, and are tuned by adjusting the strip length to integer multiples of a quarter-wavelength (lambda/4) in the presence of a ground plane and dielectric substrate. In addition, the more explicit spatial information embedded in the phase of the signals from the strip array is advantageous (compared to loop arrays) for limiting aliasing artifacts in parallel MRI. However, losses in the detector as its natural resonance frequency approaches the Larmor frequency (where the wavelength is long at 1.5 T) may limit the signal-to-noise ratio (SNR) of the PSA. Moreover, the PSA's inherent lambda/4 structure severely limits our ability to adjust detector geometry to optimize the performance for a specific organ system, as is done with loop coils. In this study we replaced the dielectric substrate with discrete capacitors, which resulted in both SNR improvement and a tunable lumped-element PSA (LPSA) whose dimensions can be optimized within broad constraints, for a given region of interest (ROI) and MRI frequency. A detailed theoretical analysis of the LPSA is presented, including its equivalent circuit, electromagnetic fields, SNR, and g-factor maps for parallel MRI. Two different decoupling schemes for the LPSA are described. A four-element LPSA prototype was built to test the theory with quantitative measurements on images obtained with parallel and conventional acquisition schemes.

Equipment Design↗

PubMed2004

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.

Equipment Design↗

PubMed2004

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.

Abdomen↗

PubMed2003

Coronary MR angiography: respiratory motion correction with BACSPIN.

PURPOSE: To improve the signal-to-noise ratio (SNR) of breath-held coronary magnetic resonance angiography (CMRA) without increasing the number or duration of breath holds. MATERIALS AND METHODS: In this BACSPIN (Breathing AutoCorrection with SPiral INterleaves) technique, a single breath-held electrocardiogram (ECG)-gated multi-slice interleaved-spiral data set is acquired, followed by repeated imaging of the same slices during free breathing. Each spiral interleaf from the breath-held data set is used as a standard for comparison with corresponding acquisitions at the same interleaf angle during free breathing. The most closely matched acquisitions are incorporated into a multi-slice, multi-average data set with increasing SNR over time. In-plane translations of the coronary artery can be measured and compensated for each accepted acquisition before combination with the other acquisitions. RESULTS: CMRA was performed on six volunteers, with improved SNR and minimal motional blurring. In some cases, breath holding could be dispensed with completely and the average respiratory position used as a reference. CONCLUSION: BACSPIN provides a promising method for CMRA with improved SNR and limited breath-holding requirements.

Coronary Vessels↗

PubMed2002

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.

Equipment Design↗