Adaptive averaging for improved SNR in real-time coronary artery MRI

Air-coupled ultrasonic diffuse-wave techniques to evaluate distributed cracking damage in concrete

In this study, we investigated the suitability of applying air-coupled ultrasonic diffuse-wave techniques to concrete structures for the evaluation of arbitrarily distributed micro-cracking damage. Air-coupled test results were compared with those obtained using a conventional full-contact measurement system. Three different micro-cracking damage levels were simulated by embedding varying amounts of low-stiffness polypropylene fibers in concrete samples. Two principal diffuse-wave parameters, diffusivity and dissipation, were determined using air-coupled and full-contact test configurations. Wave frequencies of 300-600 kHz were employed, which set up significant ultrasonic scattering owing to the heterogeneous characteristics of concrete components (e.g., aggregates, micro-cracks expressed by fibers, and pores). In addition, the sensitivities of diffusivity and dissipation to the number of measurement locations were examined. The results demonstrated that the air-coupled method can provide an equivalent reliability to the full-contact method, allowing a much faster and flexible data collection. The spatial averaging of 20 arbitrarily selected data (measured at different locations) yielded sufficiently accurate diffuse-wave parameters, showing less than a 5 % difference from the average of 32 spatially different data.

The use of parallel imaging for MRI assessment of knees in children and adolescents

BACKGROUND

Parallel imaging provides faster scanning at the cost of reduced signal-to-noise ratio (SNR) and increased artifacts.

OBJECTIVE

To compare the diagnostic performance of two parallel MRI protocols (PPs) for assessment of pathologic knees using an 8-channel knee coil (reference standard, conventional protocol [CP]) and to characterize the SNR losses associated with parallel imaging.

MATERIALS AND METHODS

Two radiologists blindly interpreted 1.5 Tesla knee MRI images in 21 children (mean 13 years, range 9-18 years) with clinical indications for an MRI scan. Sagittal proton density, T2-W fat-saturated FSE, axial T2-W fat-saturated FSE, and coronal T1-W (NEX of 1,1,1) images were obtained with both CP and PP. Images were read for soft tissue and osteochondral findings.

RESULTS

There was a 75% decrease in acquisition time using PP in comparison to CP. The CP and PP protocols fell within excellent or upper limits of substantial agreement: CP, kappa coefficient, 0.81 (95% CIs, 0.73-0.89); PP, 0.80-0.81 (0.73-0.89). The sensitivity of the two PPs was similar for assessment of soft (0.98-1.00) and osteochondral (0.89-0.94) tissues. Phantom data indicated an SNR of 1.67, 1.6, and 1.51 (axial, sagittal and coronal planes) between CP and PP scans.

CONCLUSION

Parallel MRI provides a reliable assessment for pediatric knees in a significantly reduced scan time without affecting the diagnostic performance of MRI.

Free-breathing cine MRI

Standard MRI cine exams for the study of cardiac function are segmented over several heartbeats and thus require a breath-hold to minimize breathing motion artifacts, which is a current limitation of this approach. The purpose of this study was to develop a method for the measurement and correction of respiratory motion that is compatible with cine imaging. Real-time images were used to measure the respiratory motion of heart, to allow translations, rotations, and shears to be measured and corrected in the k-space domain prior to a final gated-segmented reconstruction, using the same data for both purposes. A method for data rejection to address the effects of through-plane motion and complex deformations is described (respiratory gating). A radial k-space trajectory was used in this study to allow direct reconstruction of undersampled real-time images, although the techniques presented are applicable with Cartesian k-space trajectories. Corrected and uncorrected free-breathing gated-segmented images acquired over 18 sec were compared to the current standard breath-hold Cartesian images using both quantitative sharpness profiles (mm(-1)) and clinical scoring (1 to 5 scale, 3: clinically acceptable). Free-breathing, free-breathing corrected, and breath-hold images had average sharpness values of 0.23 +/- 0.04, 0.38 +/- 0.04, and 0.44 +/- 0.04 mm(-1) measured at the blood-endocardium interface, and clinical scores of 2.2 +/- 0.5, 4.2 +/- 0.4, and 4.7 +/- 0.5, respectively.

Contrast-enhanced 4D radial coronary artery imaging at 3.0 T within a single breath-hold

Coronary magnetic resonance angiography data are usually acquired during mid-diastole of each heartbeat to minimize cardiac motion related artifacts. The proper trigger delay time, which may vary widely among subjects, must be determined individually for each subject before data acquisition to achieve optimal image quality. These complications could be resolved by acquiring contiguous cardiac phase images through the cardiac cycle. In this study, we used a radial sampling technique to acquire 3D cine coronary artery images at 3 T within a single breath-hold. An extravascular, paramagnetic contrast agent was i.v. administered to improve the blood signal intensity. Relatively high temporal resolution and spatial resolution were achieved simultaneously with radial sampling, parallel data acquisition, and interleaved sliding window image reconstruction. Volunteer studies demonstrate the feasibility of this technique in acquiring 4D coronary artery images and the flexibility in postprocessing of 3D image sets.