Dynamic coil selection for real-time imaging in interventional MRI

Interpretable Dimension Reduction for MRI Channel Suppression

Channel suppression can reduce the redundant information in multiple channel receiver coils and accelerate reconstruction speed to meet real-time imaging requirements. The principal component analysis has been used for channel suppression, but it is difficult to be interpreted because all channels contribute to principal components. Furthermore, the importance of interpretability in machine learning has recently attracted increasing attention in radiology. To improve the interpretability of PCA-based channel suppression, a sparse PCA method is proposed to reduce the most coils' loadings to be zero. Channel suppression is formulated as solving a nonlinear eigenvalue problem using the inverse power method instead of the direct matrix decomposition. Experimental results of in vivo data show that the sparse PCA-based channel suppression not only improves the interpretability with sparse channels, but also improves reconstruction quality compared to the standard PCA-based reconstruction with the similar reconstruction time.

Real-Time Magnetic Resonance Imaging

Real-time magnetic resonance imaging (RT-MRI) allows for imaging dynamic processes as they occur, without relying on any repetition or synchronization. This is made possible by modern MRI technology such as fast-switching gradients and parallel imaging. It is compatible with many (but not all) MRI sequences, including spoiled gradient echo, balanced steady-state free precession, and single-shot rapid acquisition with relaxation enhancement. RT-MRI has earned an important role in both diagnostic imaging and image guidance of invasive procedures. Its unique diagnostic value is prominent in areas of the body that undergo substantial and often irregular motion, such as the heart, gastrointestinal system, upper airway vocal tract, and joints. Its value in interventional procedure guidance is prominent for procedures that require multiple forms of soft-tissue contrast, as well as flow information. In this review, we discuss the history of RT-MRI, fundamental tradeoffs, enabling technology, established applications, and current trends. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

Magnetic Resonance Sequences and Rapid Acquisition for MR-Guided Interventions

Interventional MR uses rapid imaging to guide diagnostic and therapeutic procedures. One of the attractions of MR-guidance is the abundance of inherent contrast mechanisms available. Dynamic procedural guidance with real-time imaging has pushed the limits of MR technology, demanding rapid acquisition and reconstruction paired with interactive control and device visualization. This article reviews the technical aspects of real-time MR sequences that enable MR-guided interventions.

Two eyes see more than one: double echo stereoscopic MRA for rapid 3D visualization of vascular structures

OBJECT

A three-dimensional (3D) visualization of the target region during intravascular interventions in real-time is challenging since the acquisition of a time-consuming 3D dataset is required. In this work, a novel stereoscopic double echo sequence for achieving 3D depth perception by sampling only two oblique projection images is presented.

MATERIALS AND METHODS

A double echo (DE) FLASH pulse sequence was developed to acquire continuously stereoscopic image pairs of the vascular target anatomy. Stereo image data were displayed on a stereoscopic 3D LCD monitor in real time after image reconstruction. Phantom experiments followed by a depth perception test were performed to assess the usability of the stereo image pairs for 3D visualization. In an animal experiment the sequence was tested in vivo and was compared with a slower interleaved (IL) sequence variant.

RESULTS

In the phantom experiments an SNR difference of 6 % between left and right image was found which did not influence the depth perception. The DE acquisition was superior to the IL sequence (SNR(DE) = 10.3, 2.3 images/s over SNR(IL) = 7.1, 1.7 images/s), and during contrast enhancement the abdominal arterial vasculature was clearly perceived as a 3D structure.

CONCLUSION

A novel stereoscopic DE pulse sequence can be utilized for the fast 3D stereoscopic visualization of vascular structures in real-time.

Automatic coil selection for channel reduction in SENSE-based parallel imaging

OBJECTIVE

Coil arrays with large number of receive elements allow improved imaging performance and higher signal-to-noise-ratio. The MR systems supporting these arrays have to handle an increased amount of data and higher reconstruction burden. To overcome these problems, data reduction techniques need to be applied, realized either by linear combination of the original coil data prior to reconstruction or by discarding particular data from unimportant coil elements.

MATERIALS AND METHODS

This work focuses on the latter approach and presents an efficient algorithm for automatic coil selection applicable to SENSE imaging. A singular value decomposition (SVD)-based coil selection is proposed that performs a coil element ranking quantifying the contribution of each coil element to the image reconstruction allowing appropriate coil selection. This approach makes use of the coil sensitivity information and takes reduction factor and phase encoding direction into account.

RESULTS

Simulations, phantom and in vivo experiments were performed to validate the SVD-based coil selection algorithm. The proposed approach proved to be computationally efficient without remarkable image quality degradation.

CONCLUSION

The SVD-based approach offers the opportunity for fast automatic coil selection. This could simplify clinical workflow and may, furthermore, pave the way for various 2D real-time and interventional applications.