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. Magnetic resonance imaging (MRI) conventionally relies entirely on magnetic field gradients to encode spatial information (1,2). Parallel MRI, as exemplified by the simultaneous acquisition of spatial harmonics (SMASH) (3) and sensitivity encoding (SENSE) (4) techniques, facilitates imaging by partially shifting the burden of spatial encoding from magnetic field gradients to parallel receive coils. One advantage of parallel MRI is that it can improve data acquisition speeds beyond what can be achieved with conventional nonparallel approaches, without imposing additional stress on the gradients. The practical implementations of parallel imaging have typically enabled as much as fourfold accelerations for a wide range of clinical applications.It is well understood that the upper limit of the attainable acceleration factor is equal to the number of parallel receive channels/coils on a scanner system. In practice, however, acceleration is additionally limited by the signalto-noise ratio (SNR) of the system. While the maximum number of parallel receive channels on general-purpose clinical scanners is currently in the vicinity of eight, and can be expected to increase in the near future, the system SNR represents a fundamental challenge and is increasingly limiting as acceleration advances.The SNR limitation on the practice of parallel imaging manifests itself in the well-known scaling by the square root of total acquisition time (5), as well as in a spatially varying noise amplification pattern (3,4) that intrinsically depends on both the k-space sampling pattern and the coil array geometry. As the acceleration factor increases, the noise amplification tends to be aggravated, which, combined with the decrease of noise averaging resulting from the reduction in total acquisition time, leads to an increasingly fast deterioration of image SNR. This property of parallel imaging SNR tends to project a somewhat pessimistic picture for the clinical use of high acceleration factors that exceed the current norm of 2-4. Nevertheless, in the push for higher accelerations, there are various strategies to which one may resort to mitigate SNR deterioration and manage image quality. One such strategy is to raise the baseline SNR by employing, e.g., high-fieldstrength systems, contrast-enhanced (CE) acquisitions, or high-SNR pulse sequences. Anothe...
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.
Purpose:To investigate the accuracy of low signal-to-noise ratio (SNR) T 2 and T* 2 measurements using array coils and optimal B 1 image reconstruction (OBR) compared to the standard root sum of squares (RSS) reconstruction. Materials and Methods:Calibrated gels were used for the in vitro study of T 2 . T 2 and T* 2 measurements were obtained from a volunteer's knee and liver, respectively. T 2 and T* 2 measurements were performed using multiecho spin echo and multiecho gradient echo sequences, respectively. SNR was deliberately kept low. The same raw data were used for both reconstructions. For the in vivo studies the effect of signal averaging was also investigated. Results:The optimal reconstructions demonstrated a lower mean background noise level than RSS. In vitro, the T 2 measurements made with OBR images agreed better with a reference high SNR measurement than measurements made from RSS images; the RSS image results overestimated the T 2. In vivo, increasing the signal averages decreased the difference between the measurements obtained using the OBR and RSS methods, with RSS resulting in longer relaxation times. Conclusion:This work demonstrates improvements to the accuracy of T 2 and T* 2 measurements obtained when OBR is used compared to RSS, particularly in the case of low SNR.
Magnetic resonance analysis of CSF flow can show craniospinal dissociation and limitation of CSF outflow from the ventricles in both obstructive and communicating hydrocephalus; it should help determine the response to shunting in communicating hydrocephalus.
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