Comparison of cardiac volumetry using real-time MRI during free-breathing with standard cine MRI during breath-hold in children

Virtual MOLLI Target: Generative Adversarial Networks Toward Improved Motion Correction in MRI Myocardial T1 Mapping

BACKGROUND

The modified Look-Locker inversion recovery (MOLLI) sequence is commonly used for myocardial T1 mapping. However, it acquires images with different inversion times, which causes difficulty in motion correction for respiratory-induced misregistration to a given target image.

HYPOTHESIS

Using a generative adversarial network (GAN) to produce virtual MOLLI images with consistent heart positions can reduce respiratory-induced misregistration of MOLLI datasets.

STUDY TYPE

Retrospective.

POPULATION

1071 MOLLI datasets from 392 human participants.

FIELD STRENGTH/SEQUENCE

Modified Look-Locker inversion recovery sequence at 3 T.

ASSESSMENT

A GAN model with a single inversion time image as input was trained to generate virtual MOLLI target (VMT) images at different inversion times which were subsequently used in an image registration algorithm. Four VMT models were investigated and the best performing model compared with the standard vendor-provided motion correction (MOCO) technique.

STATISTICAL TESTS

The effectiveness of the motion correction technique was assessed using the fitting quality index (FQI), mutual information (MI), and Dice coefficients of motion-corrected images, plus subjective quality evaluation of T1 maps by three independent readers using Likert score. Wilcoxon signed-rank test with Bonferroni correction for multiple comparison. Significance levels were defined as P < 0.01 for highly significant differences and P < 0.05 for significant differences.

RESULTS

The best performing VMT model with iterative registration demonstrated significantly better performance (FQI 0.88 ± 0.03, MI 1.78 ± 0.20, Dice 0.84 ± 0.23, quality score 2.26 ± 0.95) compared to other approaches, including the vendor-provided MOCO method (FQI 0.86 ± 0.04, MI 1.69 ± 0.25, Dice 0.80 ± 0.27, quality score 2.16 ± 1.01).

DATA CONCLUSION

Our GAN model generating VMT images improved motion correction, which may assist reliable T1 mapping in the presence of respiratory motion. Its robust performance, even with considerable respiratory-induced heart displacements, may be beneficial for patients with difficulties in breath-holding.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY: Stage 1.

Assessment of the cardiac output at rest and during exercise stress using real-time cardiovascular magnetic resonance imaging in HFpEF-patients

This methodological study aimed to validate the cardiac output (CO) measured by exercise-stress real-time phase-contrast cardiovascular magnetic resonance imaging (CMR) in patients with heart failure and preserved ejection fraction (HFpEF). 68 patients with dyspnea on exertion (NYHA ≥ II) and echocardiographic signs of diastolic dysfunction underwent rest and exercise stress right heart catheterization (RHC) and CMR within 24 h. Patients were diagnosed as overt HFpEF (pulmonary capillary wedge pressure (PCWP) ≥ 15mmHg at rest), masked HFpEF (PCWP ≥ 25mmHg during exercise stress but < 15mmHg at rest) and non-cardiac dyspnea. CO was calculated using RHC as the reference standard, and in CMR by the volumetric stroke volume, conventional phase-contrast and rest and stress real-time phase-contrast imaging. At rest, the CMR based CO showed good agreement with RHC with an ICC of 0.772 for conventional phase-contrast, and 0.872 for real-time phase-contrast measurements. During exercise stress, the agreement of real-time CMR and RHC was good with an ICC of 0.805. Real-time measurements underestimated the CO at rest (Bias:0.71 L/min) and during exercise stress (Bias:1.4 L/min). Patients with overt HFpEF had a significantly lower cardiac index compared to patients with masked HFpEF and with non-cardiac dyspnea during exercise stress, but not at rest. Real-time phase-contrast CO can be assessed with good agreement with the invasive reference standard at rest and during exercise stress. While moderate underestimation of the CO needs to be considered with non-invasive testing, the CO using real-time CMR provides useful clinical information and could help to avoid unnecessary invasive procedures in HFpEF patients.

The future of CMR: All-in-one vs. real-time CMR (Part 2)

Cardiac Magnetic Resonance (CMR) protocols can be lengthy and complex, which has driven the research community to develop new technologies to make these protocols more efficient and patient-friendly. Two different approaches to improving CMR have been proposed, specifically "all-in-one" CMR, where several contrasts and/or motion states are acquired simultaneously, and "real-time" CMR, in which the examination is accelerated to avoid the need for breathholding and/or cardiac gating. The goal of this two-part manuscript is to describe these two different types of emerging rapid CMR protocols. To this end, the vision of all-in-one and real-time imaging are described, along with techniques which have been devised and tested along the pathway of clinical implementation. The pros and cons of the different methods are presented, and the remaining open needs of each are detailed. Part 1 tackles the "All-in-One" approaches, and Part 2 focuses on the "Real-Time" approaches along with an overall summary of these emerging methods.

Major pneumothorax during pediatric cardiac MRI procedure under general anesthesia: step-by-step analysis and importance of a well-known environment and material

BACKGROUND

To perform step-by-step analysis of the different factors (material, anesthesia technique, human, and location) that led to major pneumothorax during an infrequent pediatric cardiac MRI and to prevent its occurrence in the future. Anesthesia equipment used in a remote location is often different than those in operating rooms. For magnetic resonance imaging (MRI), ventilation devices and monitors must be compatible with the magnetic fields. During cardiac MRI numerous apneas are required and, visual contact with the patient is limited for clinical evaluation. Anesthesia-related barotrauma and pneumothorax are rare in children and the first symptoms can be masked.

CASE PRESENTATION

A 3-year-old boy with atrial septal defect (ASD) and suspicious partial anomalous pulmonary venous return was anesthetized and intubated to perform a follow up with MRI. Sevoflurane maintenance and ventilation were performed using a circular CO2 absorber device, co-axial circuit, and 500 mL pediatric silicone balloon. Apneas were facilitated by Alfentanyl boluses and hyperventilation. A few moderated desaturations occurred during the imaging sequences without hemodynamic changes. At the end of the MRI, facial subcutaneous emphysema was observed by swollen eyelids and crackling snow neck palpation. A complete left pneumothorax was diagnosed by auscultation, sonography examination, and chest radiograph. Pneumo-mediastinum, -pericardium and -peritoneum were present. A chest drain was placed, and the child was extubated and transferred to the pediatric intensive care unit (PICU). Despite the anesthesiologist's belief that PEEP was minimal, critical analysis revealed that PEEP was maintained at a high level throughout anesthesia. After the initial barotrauma, repeated exposure to high pressure led to the diffusion of air from the pleura to subcutaneous tissues and mediastinal and peritoneal cavities. Equipment check revealed a functional circular circuit; however, the plastic adjustable pressure-limiting valve (APL) closed within the last 30° rotation. The balloon was found to be more rigid and demonstrated significantly reduced compliance.

CONCLUSIONS

Anesthetists require proficiency is using equipment in non-OR locations and this equipment must be properly maintained and checked for malfunctions. Controlling the human factor risks by implementing checklists, formations, and alarms allows us to reduce errors. The number of pediatric anesthesia performed routinely appeared to be essential for limiting risks and reporting our mistakes will be a benefit for all who care about patients.

First experience with real-time magnetic resonance imaging-based investigation of respiratory influence on cardiac function in pediatric congenital heart disease patients with chronic right ventricular volume overload

BACKGROUND

Congenital heart disease (CHD) is often associated with chronic right ventricular (RV) volume overload. Real-time magnetic resonance imaging (MRI) enables the analysis of cardiac function during free breathing.

OBJECTIVE

To evaluate the influence of respiration in pediatric patients with CHD and chronic RV volume overload.

METHODS AND MATERIALS

RV volume overload patients (n=6) and controls (n=6) were recruited for cardiac real-time MRI at 1.5 tesla during free breathing. Breathing curves from regions of interest reflecting the position of the diaphragm served for binning images in four different tidal volume classes, each in inspiration and expiration. Tidal volumes were estimated from these curves by data previously obtained by magnetic resonance-compatible spirometry. Ventricular volumes indexed to body surface area and Frank-Starling relationships referenced to the typical tidal volume indexed to body height (TTVi) were compared.

RESULTS

Indexed RV end-diastolic volume (RV-EDVi) and indexed RV stroke volume (RV-SVi) increased during inspiration (RV-EDVi/TTVi: RV load: + 16 ± 4%; controls: + 22 ± 13%; RV-SVi/TTVi: RV load: + 21 ± 6%; controls: + 35 ± 17%; non-significant for comparison). The increase in RV ejection fraction during inspiration was significantly lower in RV load patients (RV load: + 1.1 ± 2.2%; controls: + 6.1 ± 1.5%; P=0.01). The Frank-Starling relationship of the RV provided a significantly reduced slope estimate in RV load patients (inspiration: RV load: 0.75 ± 0.11; controls: 0.92 ± 0.02; P=0.02).

CONCLUSION

In pediatric patients with CHD and chronic RV volume overload, cardiac real-time MRI during free breathing in combination with respiratory-based binning indicates an impaired Frank-Starling relationship of the RV.

Real-time MRI: a new tool of radiologic imaging in small children

Real-time MRI (rt-MRI) in children is a new imaging technique that combines the advantages of US - at frame rates of up to 50 images per second - with the quality and features of MRI. Although still subject of research, it has become a standard tool in the diagnostic portfolio of two pediatric radiology departments in Germany. Based on ultrashort acquisition times, any detrimental effects of macroscopic movements of the child and the physiological movements of the organs are negligible. Especially in pediatric brain imaging, rt-MRI has already proven its value. With suitable indications, rt-MRI can reduce anesthesia and sedation examinations in children below 6 years of age by 40% due to its very short examination time and its robustness to motion. There is a high level of acceptance among parents and referrers when diagnostic possibilities and limitations are communicated correctly.

CONCLUSION

Completely new diagnostic possibilities arise in the imaging of the moving lung, the beating heart, joint movements, and speaking and swallowing, as demonstrated in this video-backed review.

WHAT IS KNOWN

• MRI in moving children has been burdened with severe artifacts. • Gross motion usually has to be handled by sedation and periodic motion of the heart and lungs has to be compensated with time-consuming techniques until now.

WHAT IS NEW

• Real-time MRI allows image acquisition with up to 50 frames per second similar to ultrasound frame rate. • Real-time MRI proofs to be very promising for imaging children, reducing examination time and sedation rate drastically.

External Hardware and Sensors, for Improved MRI

Complex engineered systems are often equipped with suites of sensors and ancillary devices that monitor their performance and maintenance needs. MRI scanners are no different in this regard. Some of the ancillary devices available to support MRI equipment, the ones of particular interest here, have the distinction of actually participating in the image acquisition process itself. Most commonly, such devices are used to monitor physiological motion or variations in the scanner's imaging fields, allowing the imaging and/or reconstruction process to adapt as imaging conditions change. "Classic" examples include electrocardiography (ECG) leads and respiratory bellows to monitor cardiac and respiratory motion, which have been standard equipment in scan rooms since the early days of MRI. Since then, many additional sensors and devices have been proposed to support MRI acquisitions. The main physical properties that they measure may be primarily "mechanical" (eg acceleration, speed, and torque), "acoustic" (sound and ultrasound), "optical" (light and infrared), or "electromagnetic" in nature. A review of these ancillary devices, as currently available in clinical and research settings, is presented here. In our opinion, these devices are not in competition with each other: as long as they provide useful and unique information, do not interfere with each other and are not prohibitively cumbersome to use, they might find their proper place in future suites of sensors. In time, MRI acquisitions will likely include a plurality of complementary signals. A little like the microbiome that provides genetic diversity to organisms, these devices can provide signal diversity to MRI acquisitions and enrich measurements. Machine-learning (ML) algorithms are well suited at combining diverse input signals toward coherent outputs, and they could make use of all such information toward improved MRI capabilities. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 1.