Chest-MRI under pulsatile flow ventilation: A new promising technique

Objectives

Magnetic resonance imaging (MRI) of the chest has long suffered from its sensitivity to respiratory and cardiac motion with an intrinsically low signal to noise ratio and a limited spatial resolution. The purpose of this study was to perform chest MRI under an adapted non invasive pulsatile flow ventilation system (high frequency percussive ventilation, HFPV^®) allowing breath hold durations 10 to 15 times longer than other existing systems.

Methods

One volunteer and one patient known for a thymic lesion underwent a chest MRI under ventilation percussion technique (VP-MR). Routinely used sequences were performed with and without the device during three sets of apnoea on inspiration.

Results

VP-MR was well tolerated in both cases. The mean duration of the thoracic stabilization was 10.5 min (range 8.5–12) and 5.8 min (range 5–6.2) for Volunteer 1 and Patient 1, respectively. An overall increased image quality was seen under VP-MR with a better delineation of the mediastinal lesion for Patient 1. Nodules discovered in Volunteer 1 were confirmed with low dose CT.

Conclusion

VP-MR was feasible and increased spatial resolution of chest MRI by allowing acquisition at full inspiration during thoracic stabilization approaching prolonged apnoea. This new technique could be of benefit to numerous thoracic disorders.

Image Quality of Cardiac Magnetic Resonance Imaging in Patients With an Implantable Cardioverter Defibrillator System Designed for the Magnetic Resonance Imaging Environment

BACKGROUND

Recently, magnetic resonance (MR)-conditional implantable cardioverter defibrillator (ICD) systems have become available. However, associated cardiac MR image (MRI) quality is unknown. The goal was to evaluate the image quality performance of various cardiac MR sequences in a multicenter trial of patients implanted with an MR-conditional ICD system.

METHODS AND RESULTS

The Evera-MRI trial enrolled 275 patients in 42 centers worldwide. There were 263 patients implanted with an Evera-MRI single- or dual-chamber ICD and randomized to controls (n=88) and MRI (n=175), 156 of whom underwent a protocol-required MRI (9-12 weeks post implant). Steady-state-free-precession (SSFP) and fast-gradient-echo (FGE) sequences were acquired in short-axis and horizontal long-axis orientations. Qualitative and quantitative assessment of image quality was performed by using a 7-point scale (grades 1-3: good quality, grades 6-7: nondiagnostic) and measuring ICD- and lead-related artifact size. Good to moderate image quality (grades 1-5) was obtained in 53% and 74% of SSFP and FGE acquisitions, respectively, covering the left ventricle, and in 69% and 84%, respectively, covering the right ventricle. Odds for better image quality were greater for right ventricle versus left ventricle (odds ratio, 1.8; 95% confidence interval, 1.5-2.2; P<0.0001) and greater for FGE versus SSFP (odds ratio, 3.5; 95% confidence interval, 2.5-4.8; P<0.0001). Compared with SSFP, ICD-related artifacts on FGE were smaller (141±65 versus 75±57 mm, respectively; P<0.0001). Lead artifacts were much smaller than ICD artifacts (P<0.0001).

CONCLUSIONS

FGE yields good to moderate quality in 74% of left ventricle and 84% of right ventricle acquisitions and performs better than SSFP in patients with an MRI-conditional ICD system. In these patients, cardiac MRI can offer diagnostic information in most cases.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT02117414.

Stress Perfusion CMR in Patients With Known and Suspected CAD: Prognostic Value and Optimal Ischemic Threshold for Revascularization

OBJECTIVES

This study sought to determine the ischemia threshold and additional prognostic factors that identify patients for safe deferral from revascularizations in a large cohort of all-comer patients with known or suspected coronary artery disease (CAD).

BACKGROUND

Stress-perfusion cardiac magnetic resonance (CMR) is increasingly used in daily practice for ischemia detection. However, there is insufficient evidence about the ischemia burden that identifies patients who benefit from revascularization versus those with a good prognosis who receive drugs only.

METHODS

All patients with known or suspected CAD referred to stress-perfusion CMR for myocardial ischemia assessment were prospectively enrolled. The CMR examination included standard functional adenosine stress first-pass perfusion (gadobutrol 0.1 mmol/kg Gadovist, Bayer AG, Zurich, Switzerland) and late gadolinium enhancement (LGE) acquisitions. Presence of ischemia and ischemia burden (number of ischemic segments on a 16-segment model), and of scar and scar burden (number and transmurality of scar segments in a 17-segment model) were assessed. The primary endpoint was a composite of cardiac death, nonfatal myocardial infarction (MI), and late coronary revascularization (>90 days post-CMR); the secondary endpoint was a composite of cardiac death and nonfatal MI.

RESULTS

During a follow-up of 2.5 ± 1.0 years, 86 and 32 of 1,024 patients (1,103 screened patients) experienced the primary and secondary endpoints, respectively. On Kaplan-Meier curves for the primary and secondary endpoints, patients without ischemia had excellent outcomes that did not differ from patients with <1.5 ischemic segments. In multivariate Cox regression analyses of the entire population and of the subgroups, ischemia burden (threshold: ≥1.5 ischemic segments) was consistently the strongest predictor of the primary and secondary endpoints with hazard ratios (HRs) of 7.42 to 8.72 (p < 0.001), whereas age (≥67 years), left ventricular ejection fraction (≤40%), and scar burden (LGE score ≥0.03) contributed significantly, but to a lesser extent, in all models with HRs of 2.01 to 3.48, 1.75 to 1.96, and 1.66 to 1.76, respectively.

CONCLUSIONS

In a large all-comer patient cohort with known and suspected CAD, an ischemia burden of ≥1.5 ischemic segments on stress-perfusion CMR was the strongest predictor of the primary and secondary endpoints. Patients with zero or 1 ischemic segment can be safely deferred from revascularizations.

Cardiovascular magnetic resonance in systemic sclerosis: "Pearls and pitfalls"

Systemic sclerosis (SSc) is an autoimmune disease characterized by vascular dysfunction and excessive fibrosis, involving internal organs including the heart. The estimated prevalence of cardiac involvement in SSc is high and remains subclinical until the late stages. It is either primary, related to myocardial inflammation and fibrosis, or secondary, due to pulmonary arterial hypertension (SSc-PAH) or systemic hypertension, in those patients with renal involvement. Cardiovascular magnetic resonance (CMR) is a useful tool for the early assessment of cardiac involvement in SSc. It is the gold standard technique to assess ventricular volumes,ejection fraction, and in particular is very useful to reliably and non-invasively detect myocardial inflammation, early perfusion defects, and myocardial fibrosis. However, the CMR evaluation in SSc may be problematic, because of cardiac and respiratory artefacts, commonly found in these patients. Therefore, a high level of expertise is necessary for both acquisition and interpretation of CMR images in SSc.

In-Vivo Detection and Tracking of T Cells in Various Organs in a Melanoma Tumor Model by 19F-Fluorine MRS/MRI

Background

¹⁹F-MRI and ¹⁹F-MRS can identify specific cell types after in-vitro or in-vivo ¹⁹F-labeling. Knowledge on the potential to track in-vitro ¹⁹F-labeled immune cells in tumor models by ¹⁹F-MRI/MRS is scarce.

Aim

To study ¹⁹F-based MR techniques for in-vivo tracking of adoptively transferred immune cells after in-vitro ¹⁹F-labeling, i.e. to detect and monitor their migration non-invasively in melanoma-bearing mice.

Methods

Splenocytes (SP) were labeled in-vitro with a perfluorocarbon (PFC) and IV-injected into non-tumor bearing mice. In-vitro PFC-labeled ovalbumin (OVA)-specific T cells from the T cell receptor-transgenic line OT-1, activated with anti-CD3 and anti-CD28 antibodies (T_act) or OVA-peptide pulsed antigen presenting cells (T_OVA-act), were injected into B16 OVA melanoma-bearing mice. The distribution of the ¹⁹F-labelled donor cells was determined in-vivo by ¹⁹F-MRI/MRS. In-vivo ¹⁹F-MRI/MRS results were confirmed by ex-vivo ¹⁹F-NMR and flow cytometry.

Results

SP, T_act, and T_OVA-act were successfully PFC-labeled in-vitro yielding 3x10¹¹-1.4x10^{12 19}F-atoms/cell in the 3 groups. Adoptively transferred ¹⁹F-labeled SP, T_OVA-act, and T_act were detected by coil-localized ¹⁹F-MRS in the chest, abdomen, and left flank in most animals (corresponding to lungs, livers, and spleens, respectively, with highest signal-to-noise for SP vs T_OVA-act and T_act, p<0.009 for both). SP and T_act were successfully imaged by ¹⁹F-MRI (n = 3; liver). These in-vivo data were confirmed by ex-vivo high-resolution ¹⁹F-NMR-spectroscopy. By flow cytometric analysis, however, T_OVA-act tended to be more abundant versus SP and T_act (liver: p = 0.1313; lungs: p = 0.1073; spleen: p = 0.109). Unlike ¹⁹F-MRI/MRS, flow cytometry also identified transferred immune cells (SP, T_act, and T_OVA-act) in the tumors.

Conclusion

SP, T_act, and T_OVA-act were successfully PFC-labeled in-vitro and detected in-vivo by non-invasive ¹⁹F-MRS/MRI in liver, lung, and spleen. The portion of ¹⁹F-labeled T cells in the adoptively transferred cell populations was insufficient for ¹⁹F-MRS/MRI detection in the tumor. While OVA-peptide-activated T cells (T_OVA-act) showed highest infiltration into all organs, SP were detected more reliably by ¹⁹F-MRS/MRI, most likely explained by cell division of T_OVA-act after injection, which dilutes the ¹⁹F content in the T cell-infiltrated organs. Non-dividing ¹⁹F-labeled cell species appear most promising to be tracked by ¹⁹F-MRS/MRI.

Characterization of perfluorocarbon relaxation times and their influence on the optimization of fluorine-19 MRI at 3 tesla

PURPOSE

To characterize and optimize ¹⁹ F MRI for different perfluorocarbons (PFCs) at 3T and quantify the loss of acquisition efficiency as a function of different temperature and cellular conditions.

METHODS

The T₁ and T₂ relaxation times of the commonly used PFCs perfluoropolyether (PFPE), perfluoro-15-crown-5-ether (PFCE), and perfluorooctyl bromide (PFOB) were measured in phantoms and in several different conditions (cell types, presence of fixation agent, and temperatures). These relaxation times were used to optimize pulse sequences through numerical simulations. The acquisition efficiency in each cellular condition was then determined as the ratio of the signal after optimization with the reference relaxation times and after optimization with its proper relaxation times. Finally, PFC detection limits were determined.

RESULTS

The loss of acquisition efficiency due to parameter settings optimized for the wrong temperature and cellular condition was limited to 13%. The detection limits of all PFCs were lower at 24 °C than at 37 °C and varied from 11.8 ± 3.0 mM for PFCE at 24 °C to 379.9 ± 51.8 mM for PFOB at 37 °C.

CONCLUSION

Optimizing ¹⁹ F pulse sequences with a known phantom only leads to moderate loss in acquisition efficiency in cellular conditions that might be encountered in in vivo and in vitro experiments. Magn Reson Med 77:2263-2271, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Four-dimensional respiratory motion-resolved whole heart coronary MR angiography

PURPOSE

Free-breathing whole-heart coronary MR angiography (MRA) commonly uses navigators to gate respiratory motion, resulting in lengthy and unpredictable acquisition times. Conversely, self-navigation has 100% scan efficiency, but requires motion correction over a broad range of respiratory displacements, which may introduce image artifacts. We propose replacing navigators and self-navigation with a respiratory motion-resolved reconstruction approach.

METHODS

Using a respiratory signal extracted directly from the imaging data, individual signal-readouts are binned according to their respiratory states. The resultant series of undersampled images are reconstructed using an extradimensional golden-angle radial sparse parallel imaging (XD-GRASP) algorithm, which exploits sparsity along the respiratory dimension. Whole-heart coronary MRA was performed in 11 volunteers and four patients with the proposed methodology. Image quality was compared with that obtained with one-dimensional respiratory self-navigation.

RESULTS

Respiratory-resolved reconstruction effectively suppressed respiratory motion artifacts. The quality score for XD-GRASP reconstructions was greater than or equal to self-navigation in 80/88 coronary segments, reaching diagnostic quality in 61/88 segments versus 41/88. Coronary sharpness and length were always superior for the respiratory-resolved datasets, reaching statistical significance (P < 0.05) in most cases.

CONCLUSION

XD-GRASP represents an attractive alternative for handling respiratory motion in free-breathing whole heart MRI and provides an effective alternative to self-navigation. Magn Reson Med 77:1473-1484, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Current artefacts in cardiac and chest magnetic resonance imaging: tips and tricks

Currently MRI is extensively used for the evaluation of cardiovascular and thoracic disorders because of the well-established advantages that include use of non-ionizing radiation, good contrast and high spatial resolution. Despite the advantages of this technique, numerous categories of artefacts are frequently encountered. They may be related to the scanner hardware or software functionalities, environmental factors or the human body itself. In particular, some artefacts may be exacerbated with high-field-strength MR machines (e.g. 3 T). Cardiac imaging poses specific challenges with respect to breath-holding and cardiac motion. In addition, new cardiac MR-conditional devices may also be responsible for peculiar artefacts. The image quality may thus be impaired and give rise to a misdiagnosis. Knowledge of acquisition and reconstruction techniques is required to understand and recognize the nature of these artefacts. This article will focus on the origin and appearance of the most common artefacts encountered in cardiac and chest MRI along with possible correcting methods to avoid or reduce them.

Cost-effectiveness of functional cardiac imaging in the diagnostic work-up of coronary heart disease

Aims

The aim of this study was to assess the cost-effectiveness of eight common diagnostic work-up strategies for coronary heart disease (CHD) in patients with stable angina symptoms in Switzerland.

Methods and results

A decision analytical model was used to perform a cost-effectiveness comparison of eight common multitest strategies to diagnose CHD using combinations of four diagnostic techniques: exercise treadmill test (ETT), single-photon emission computed tomography (SPECT), cardiac magnetic resonance imaging (CMR), and coronary angiography (CA). We used a Markov state transition model to extrapolate the results over a life-time horizon, from a third-party payer perspective. We used a CHD prevalence rate of 39% in patients and a base-case scenario with 60-year-old male patients with intermediate symptom severity Canadian Cardiovascular Society grading of angina pectoris 2 and at least one cardiovascular (CV) risk factor but without a history of myocardial infarction and without need for revascularization. Among the eight work-up strategies, one strategy was dominant, i.e. least costly and most effective: ETT followed by CMR if the ETT result was inconclusive and then CA if the CMR result was positive or inconclusive. The CMR features a favourable balance between false-negative diagnoses, associated with an elevated risk of CV events, and false-positive diagnoses, leading to unnecessary CA and related mortality. Key parameters guiding the diagnostic strategy are the prevalence of CHD in patients with angina symptoms and the diagnostic costs of CA and CMR.

Conclusion

Cardiac magnetic resonance imaging appears to be a cost-effective work-up strategy compared with other regimens using SPECT or direct CA. Cardiac magnetic resonance imaging should be more widely recommended as a diagnostic procedure for patients with suspected angina symptoms.

Cost-minimization analysis of three decision strategies for cardiac revascularization: results of the "suspected CAD" cohort of the european cardiovascular magnetic resonance registry

BACKGROUND

Coronary artery disease (CAD) continues to be one of the top public health burden. Perfusion cardiovascular magnetic resonance (CMR) is generally accepted to detect CAD, while data on its cost effectiveness are scarce. Therefore, the goal of the study was to compare the costs of a CMR-guided strategy vs two invasive strategies in a large CMR registry.

METHODS

In 3'647 patients with suspected CAD of the EuroCMR-registry (59 centers/18 countries) costs were calculated for diagnostic examinations (CMR, X-ray coronary angiography (CXA) with/without FFR), revascularizations, and complications during a 1-year follow-up. Patients with ischemia-positive CMR underwent an invasive CXA and revascularization at the discretion of the treating physician (=CMR + CXA-strategy). In the hypothetical invasive arm, costs were calculated for an initial CXA and a FFR in vessels with ≥50% stenoses (=CXA + FFR-strategy) and the same proportion of revascularizations and complications were applied as in the CMR + CXA-strategy. In the CXA-only strategy, costs included those for CXA and for revascularizations of all ≥50% stenoses. To calculate the proportion of patients with ≥50% stenoses, the stenosis-FFR relationship from the literature was used. Costs of the three strategies were determined based on a third payer perspective in 4 healthcare systems.

RESULTS

Revascularizations were performed in 6.2%, 4.5%, and 12.9% of all patients, patients with atypical chest pain (n = 1'786), and typical angina (n = 582), respectively; whereas complications (=all-cause death and non-fatal infarction) occurred in 1.3%, 1.1%, and 1.5%, respectively. The CMR + CXA-strategy reduced costs by 14%, 34%, 27%, and 24% in the German, UK, Swiss, and US context, respectively, when compared to the CXA + FFR-strategy; and by 59%, 52%, 61% and 71%, respectively, versus the CXA-only strategy. In patients with typical angina, cost savings by CMR + CXA vs CXA + FFR were minimal in the German (2.3%), intermediate in the US and Swiss (11.6% and 12.8%, respectively), and remained substantial in the UK (18.9%) systems. Sensitivity analyses proved the robustness of results.

CONCLUSIONS

A CMR + CXA-strategy for patients with suspected CAD provides substantial cost reduction compared to a hypothetical CXA + FFR-strategy in patients with low to intermediate disease prevalence. However, in the subgroup of patients with typical angina, cost savings were only minimal to moderate.

High-field dissolution dynamic nuclear polarization of [1-13C]pyruvic acid

Greater than 60% solution-state polarization of [1-¹³C]pyruvic acid polarized at 7 T and 1.0 K can be measured after rapid transfer to an MRI scanner magnet.

An intact small animal model of myocardial ischemia-reperfusion: Characterization of metabolic changes by hyperpolarized 13C MR spectroscopy

Hyperpolarized carbon-13 magnetic resonance spectroscopy ((13)C MRS) enables the sensitive and noninvasive assessment of the metabolic changes occurring during myocardial ischemia-reperfusion. Ischemia-reperfusion models using hyperpolarized (13)C MRS are established in heart preparations ex vivo and in large animals in vivo, but an in vivo model in small animals would be advantageous to allow the study of reperfusion metabolism with neuroendocrine and inflammatory responses intact with the option to perform a greater number of experiments. A novel intact rat model of ischemia-reperfusion is presented that incorporates hyperpolarized (13)C MRS to characterize reperfusion metabolism. Typically, in an in vivo model, a tissue input function (TIF) is required to account for apparent changes in the metabolism of injected hyperpolarized [1-(13)C]pyruvate resulting from changes in perfusion. Whereas the measurement of a TIF by metabolic imaging is particularly challenging in small animals, the ratios of downstream metabolites can be used as an alternative. The ratio of [(13)C]bicarbonate:[1-(13)C]lactate (RatioBic/Lac) measured within 1-2 min after coronary release decreased vs. baseline in ischemic rats (n = 10, 15-min occlusion, controls: n = 10; P = 0.017 for interaction, 2-way ANOVA). The decrease in oxidative pyruvate metabolism [RatioBic/Lac(Ischemia)/RatioBic/Lac(Baseline)] modestly correlated with area at risk (r = 0.66; P = 0.002). Hyperpolarized (13)C MRS was also used to examine alanine production during ischemia, which is observed in ex vivo models, but no significant change was noted; metrics incorporating [1-(13)C]alanine did not substantially improve the discrimination of ischemic-reperfused myocardium from nonischemic myocardium. This intact rat model, which mimics the human situation of reperfused myocardial infarction, could be highly valuable for the testing of new drugs to treat reperfusion injury, thereby facilitating translational research.

Incidental extracardiac findings on cardiac MR: Systematic review and meta-analysis

PURPOSE

To perform a systematic review and meta-analysis to calculate the pooled prevalence of incidental extracardiac findings (IEFs) on cardiac magnetic resonance (MR) and to determine factors influencing reported prevalences.

MATERIALS AND METHODS

We examined studies published in the literature using the MEDLINE database. Studies reporting IEFs on cardiac MR were included. Meta-analysis provided pooled prevalences of total, minor, major IEFs, and major IEFs with patient management changes using a random-effects model. Heterogeneity and inconsistency (I-squared) between studies as well as publication bias were assessed.

RESULTS

Twelve studies including 7062 patients (mean age: 52 years, range: 0.5-93 years, 4476 male/2586 female) and 7122 cardiac MR examinations were considered in the meta-analysis. Overall, the pooled prevalence of total IEFs was 35% (95% confidence interval [CI]: 23-47%). The pooled prevalence of minor and major IEFs were 17% (95% CI: 9-26%) and 12% (95% CI: 7-18%), respectively. Newly diagnosed major IEFs changed patient management in 1% (95% CI: 1-2%) of the study population. A high heterogeneity and inconsistency (I-squared >74%) between studies without publication bias were observed, notably due to IEFs recording method (P < 0.002) and formal training of cardiac MR readers (P < 0.006).

CONCLUSION

Major IEFs may be found in 12% of patients undergoing cardiac MR examination and change the management in 1% of patients. Readers' training for the evaluation of noncardiac structures increases reported prevalence.