Magnetic resonance imaging apparatus

Commercially available software for cardiovascular image analysis often has limited functionality and frequently lacks the careful validation that is required for clinical studies. We have already implemented a cardiovascular image analysis software package and released it as freeware for the research community. However, it was distributed as a stand-alone application and other researchers could not extend it by writing their own custom image analysis algorithms. We believe that the work required to make a clinically applicable prototype can be reduced by making the software extensible, so that researchers can develop their own modules or improvements. Such an initiative might then serve as a bridge between image analysis research and cardiovascular research. The aim of this article is therefore to present the design and validation of a cardiovascular image analysis software package (Segment) and to announce its release in a source code format. Segment can be used for image analysis in magnetic resonance imaging (MRI), computed tomography (CT), single photon emission computed tomography (SPECT) and positron emission tomography (PET). Some of its main features include loading of DICOM images from all major scanner vendors, simultaneous display of multiple image stacks and plane intersections, automated segmentation of the left ventricle, quantification of MRI flow, tools for manual and general object segmentation, quantitative regional wall motion analysis, myocardial viability analysis and image fusion tools. Here we present an overview of the validation results and validation procedures for the functionality of the software. We describe a technique to ensure continued accuracy and validity of the software by implementing and using a test script that tests the functionality of the software and validates the output. The software has been made freely available for research purposes in a source code format on the project home page http://segment.heiberg.se
 . Segment is a well-validated comprehensive software package for cardiovascular image analysis. It is freely available for research purposes provided that relevant original research publications related to the software are cited.

/pdf/design-and-validation-of-segment-freely-available-software-40iwqqksv9.pdf

Design and validation of Segment--freely available software for cardiovascular image analysis.

Pulsatile blood flow through the cavities of the heart and great vessels is time-varying and multidirectional. Access to all regions, phases and directions of cardiovascular flows has formerly been limited. Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) has enabled more comprehensive access to such flows, with typical spatial resolution of 1.5×1.5×1.5 – 3×3×3 mm3, typical temporal resolution of 30–40 ms, and acquisition times in the order of 5 to 25 min. This consensus paper is the work of physicists, physicians and biomedical engineers, active in the development and implementation of 4D Flow CMR, who have repeatedly met to share experience and ideas. The paper aims to assist understanding of acquisition and analysis methods, and their potential clinical applications with a focus on the heart and greater vessels. We describe that 4D Flow CMR can be clinically advantageous because placement of a single acquisition volume is straightforward and enables flow through any plane across it to be calculated retrospectively and with good accuracy. We also specify research and development goals that have yet to be satisfactorily achieved. Derived flow parameters, generally needing further development or validation for clinical use, include measurements of wall shear stress, pressure difference, turbulent kinetic energy, and intracardiac flow components. The dependence of measurement accuracy on acquisition parameters is considered, as are the uses of different visualization strategies for appropriate representation of time-varying multidirectional flow fields. Finally, we offer suggestions for more consistent, user-friendly implementation of 4D Flow CMR acquisition and data handling with a view to multicenter studies and more widespread adoption of the approach in routine clinical investigations.

/pdf/4d-flow-cardiovascular-magnetic-resonance-consensus-1gb2j0a91a.pdf

4D flow cardiovascular magnetic resonance consensus statement

BOLD fMRI is hampered by dropout of signal in the orbitofrontal and parietal brain regions due to magnetic field gradients near air-tissue interfaces. This work reports the use of spiral-in trajectories that begin at the edge of k-space and end at the origin, and spiral in/out trajectories in which a spiral-in readout is followed by a conventional spiral-out trajectory. The spiral-in trajectory reduces the dropout and increases the BOLD contrast. The spiral-in and spiral-out images can be combined in several ways to simultaneously achieve increased signal-to-noise ratio (SNR) and reduced dropout artifacts. Activation experiments employing an olfaction task demonstrate significantly increased activation volumes due to reduced dropout, and overall increased SNR in all regions.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.1222

Spiral-in/out BOLD fMRI for increased SNR and reduced susceptibility artifacts.

Traditionally, magnetic resonance imaging (MRI) of flow using phase contrast (PC) methods is accomplished using methods that resolve single-directional flow in two spatial dimensions (2D) of an individual slice. More recently, three-dimensional (3D) spatial encoding combined with three-directional velocity-encoded phase contrast MRI (here termed 4D flow MRI) has drawn increased attention. 4D flow MRI offers the ability to measure and to visualize the temporal evolution of complex blood flow patterns within an acquired 3D volume. Various methodological improvements permit the acquisition of 4D flow MRI data encompassing individual vascular structures and entire vascular territories such as the heart, the adjacent aorta, the carotid arteries, abdominal, or peripheral vessels within reasonable scan times. To subsequently analyze the flow data by quantitative means and visualization of complex, three-directional blood flow patterns, various tools have been proposed. This review intends to introduce currently used 4D flow MRI methods, including Cartesian and radial data acquisition, approaches for accelerated data acquisition, cardiac gating, and respiration control. Based on these developments, an overview is provided over the potential this new imaging technique has in different parts of the body from the head to the peripheral arteries.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jmri.23632

4D flow MRI

Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result This is a consequence of Maxwell's equations, ie, within the imaging volume the magnetic field must have zero divergence, and has negligible curl The concomitant, or Maxwell field has been described in the MRI literature for over 10 years In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy) The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields

Concomitant gradient terms in phase contrast MR: Analysis and correction

Maxwell's equations imply that imaging gradients are accompanied by higher order spatially varying fields (concomitant fields) that can cause artifacts in MR imaging. The lowest order concomitant fields depend quadratically on the imaging gradient amplitude and inversely on the static field strength. Time-varying concomitant fields that accompany the readout gradients of spiral scans cause unwanted phase accumulation during the readout, resulting in spatially dependent blurring. Concomitant field phase errors are independent of echo time and, therefore, cannot be detected using Dixon-type field map measurements that are normally used to deblur spiral scan images. Data acquisition methods that reduce concomitant field blurring increase off-resonant spin blurring, and vice versa. Blurring caused by concomitant fields can be removed by variations of image reconstruction methods developed to correct for spatially dependent resonance offsets with nonrectangular k-space trajectories.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291522-2594%28199901%2941%3A1%3C103%3A%3AAID-MRM15%3E3.0.CO%3B2-M

Concomitant gradient field effects in spiral scans.

A method for imaging cancer using a combined PET-MRI system takes advantage of the performance characteristics for both PET and MRI in the context of cancer imaging. MRI is used to assess a large area of the body with high sensitivity for cancer and PET is then used in localized areas of concern to provide physiological information. Optionally, MRI may also then be used to re-scan the localized areas of concern with high spatial resolution and additional tissue contrasts to provide anatomical information and soft tissue contrast to supplement the PET information. The use of a combined PET-MRI system ensures that the imaging data from both modalities is accurately referenced to the same locations in the body.

System, method and apparatus for cancer imaging

An MRI system performs a calibration procedure to calculate optimal compensation parameter values for a pre-emphasis filter that alters the shape of gradient waveforms. The gradient field errors are measured using a series of pulse sequences in which different compensation parameter values are sampled. The measured errors are used to calculate a gradient error function and the optimal compensation parameter values are calculated by finding the minimum in the gradient error function.

Method for compensating an MRI system for time dependent gradient field distortion

A system and method for simultaneously acquiring PET and MR data from a subject of interest with a hybrid PET-MR scanner includes monitoring transmission times of RF and gradient coils of the MR equipment and blanking segments of the PET data stream accordingly. By excluding PET data acquired during active MR transmissions, the remaining PET data used for image reconstruction will provide improved PET image quality.

Alexander Ganin

Papers

Concomitant gradient terms in phase contrast MR: Analysis and correction

Concomitant gradient field effects in spiral scans.

System, method and apparatus for cancer imaging

Method for compensating an MRI system for time dependent gradient field distortion

Exclusion of compromised pet data during simultaneous pet-mr acquisition