Juliana Serafim da Silveira

Cardiopulmonary exercise testing in the MRI environment.

Maximal oxygen consumption ([Formula: see text]max) measured by cardiopulmonary exercise testing (CPX) is the gold standard for assessment of cardiorespiratory fitness. Likewise, cardiovascular magnetic resonance (CMR) is the gold standard for quantification of cardiac function. The combination of CPX and CMR may offer unique insights into cardiopulmonary pathophysiology; however, the MRI-compatible equipment needed to combine these tests has not been available to date. We sought to determine whether CPX testing in the MRI environment, using equipment modified for MRI yields results equivalent to those obtained in standard exercise physiology (EP) lab. Ten recreationally trained subjects completed [Formula: see text]max tests in different locations; an EP laboratory and an MRI laboratory, using site specific equipment. CMR cine images of the heart were acquired before and immediately after maximal exercise to measure cardiac function. Subjects in all tests met criteria indicating that peak exercise was achieved. Despite equipment modifications for the MRI environment, [Formula: see text]max was nearly identical between tests run in the different labs (95% lower confidence limit (LCL)  =  0.8182). The mean difference in [Formula: see text]max was less than 3.40 ml (kg/min)(-1), within the variability expected for tests performed on different days, in different locations, using different metabolic carts. MRI performed at rest and following peak exercise stress indicated cardiac output increased from 5.1  ±  1.0 l min(-1) to 16.4  ±  5.6 l min(-1), LVEF increased from 65.2  ±  3.3% to 78.4  ±  4.8%, while RVEF increased from 52.8  ±  5.3% to 63.4  ±  5.3%. Regression analysis revealed a significant positive correlation between [Formula: see text]max and stroke volume (R  =  0.788, P  =  0.006), while the correlation with cardiac output did not reach statistical significance (R  =  0.505, P  =  0.137). [Formula: see text]max CPX testing can be effectively performed in the MRI environment, enabling direct combination of physiological data with advanced post-exercise imaging in the same test session.

Quantification of myocardial stiffness using magnetic resonance elastography in right ventricular hypertrophy: initial feasibility in dogs

INTRODUCTION Myocardial stiffness is an important determinant of cardiac function and is currently invasively and indirectly assessed by catheter angiography. This study aims to demonstrate the feasibility of quantifying right ventricular (RV) stiffness noninvasively using cardiac magnetic resonance elastography (CMRE) in dogs with severe congenital pulmonary valve stenosis (PVS) causing RV hypertrophy, and compare it to remote myocardium in the left ventricle (LV). Additionally, correlations between stiffness and selected pathophysiologic indicators from transthoracic echocardiography (TTE) and cardiac magnetic resonance imaging were explored. METHODS In-vivo CMRE was performed on nine dogs presenting severe congenital PVS using a 1.5T MRI scanner. T1-MOLLI, T2-prepared-bSSFP, gated-cine GRE-MRE and LGE (PSIR) sequences were used to acquire a basal short-axis slice. RV and LV-free-wall (FW) stiffness measurements were compared against each other and also correlated to ventricular mass, RV and LV FW thickness, T1 and T2 relaxation times, and extracellular volume fraction (ECV). Peak transpulmonary pressure gradient and myocardial strain were also acquired on eight dogs by TTE and correlated to RV-FW systolic stiffness. Potential correlations were evaluated by Spearmans rho (rs). RESULTS RV-FW stiffness was found to be significantly higher than the LV-FW stiffness both during end-systole (ES) (p=0.002) and end-diastole (ED) (p=0.029). Significant correlations were observed between RV-FW ES and LV-FW ED stiffness versus ECV (rs=0.75; p-value=0.05). Non-significant moderate correlations were found between LV-FW ES (rs=0.54) and RV-FW ED (rs=0.61) stiffness versus ECV. Furthermore, non-significant correlations were found between RV or LV-FW stiffness and the remaining variables (rs<0.54; p-value>0.05). CONCLUSION This study demonstrates the feasibility of determining RV stiffness. The positive correlations between stiffness and ECV might indicate some interdependence between stiffness and myocardial extracellular matrix alterations. However, further studies are warranted to validate our initial observations.

Fast implementation for compressive recovery of highly accelerated cardiac cine MRI using the balanced sparse model.

Sparsity‐promoting regularizers can enable stable recovery of highly undersampled magnetic resonance imaging (MRI), promising to improve the clinical utility of challenging applications. However, lengthy computation time limits the clinical use of these methods, especially for dynamic MRI with its large corpus of spatiotemporal data. Here, we present a holistic framework that utilizes the balanced sparse model for compressive sensing and parallel computing to reduce the computation time of cardiac MRI recovery methods.

Aortic Stenosis assessment with a 3-directional phase contrast magnetic resonance technique. Comparison to transthoracic echocardiography

Background Transthoracic Doppler-echocardiography (TTE) is the standard clinical method for diagnosis and staging of aortic stenosis (AS). AS staging is based on measurement of aortic peak velocity, transvalvular gradient, and calculation of aortic valve area. Unidirectional throughplane phase-contrast magnetic resonance imaging (1DPC-MRI) has been widely applied in clinical imaging to quantify aortic peak velocities and flow. Nonetheless, 1DPC-MRI requires accurate positioning of imaging planes perpendicular to flow direction in order to avoid peak velocity underestimation, which can be challenging in patients with multiple or eccentric jets. Therefore PC techniques with multi-directional velocity quantification would likely improve the accuracy of velocity determination, and allow for more accurate grading of AS severity. The aim of this study is to determine whether a rapid technique that is able to capture 3 directions of velocity in a 2D image plane in a single breath-hold (3DPCMRI) provides more accurate estimation of diagnostic parameters compared with the traditional 1DPC-MRI, using TTE as the reference standard.

Quantification of aortic stenosis diagnostic parameters: comparison of fast 3 direction and 1 direction phase contrast CMR and transthoracic echocardiography

BackgroundAortic stenosis (AS) is a common valvular disorder, and disease severity is currently assessed by transthoracic echocardiography (TTE). However, TTE results can be inconsistent in some patients, thus other diagnostic modalities such as cardiovascular magnetic resonance (CMR) are demanded. While traditional unidirectional phase-contrast CMR (1Dir PC-CMR) underestimates velocity if the imaging plane is misaligned to the flow direction, multi-directional acquisitions are expected to improve velocity measurement accuracy. Nonetheless, clinical use of multidirectional techniques has been hindered by long acquisition times. Our goal was to quantify flow parameters in patients using 1Dir PC-CMR and a faster multi-directional technique (3Dir PC-CMR), and compare to TTE.MethodsTwenty-three patients were prospectively assessed with TTE and CMR. Slices above the aortic valve were acquired for both PC-CMR techniques and cine SSFP images were acquired to quantify left ventricular stroke volume. 3Dir PC-CMR implementation included a variable density sampling pattern with acceleration rate of 8 and a reconstruction method called ReVEAL, to significantly accelerate acquisition. 3Dir PC-CMR reconstruction was performed offline and ReVEAL-based image recovery was performed on the three (x, y, z) encoding pairs. 1Dir PC-CMR was acquired with GRAPPA acceleration rate of 2 and reconstructed online. CMR derived flow parameters and aortic valve area estimates were compared to TTE.ResultsReVEAL based 3Dir PC-CMR derived parameters correlated better with TTE than 1Dir PC-CMR. Correlations ranged from 0.61 to 0.81 between TTE and 1Dir PC-CMR and from 0.61 to 0.87 between TTE and 3Dir-PC-CMR. The correlation coefficients between TTE, 1Dir and 3Dir PC-CMR Vpeakwere 0.81 and 0.87, respectively. In comparison to ReVEAL, TTE slightly underestimates peak velocities, which is not surprising as TTE is only sensitive to flow that is parallel to the acoustic beam.ConclusionsBy exploiting structure unique to PC-CMR, ReVEAL enables multi-directional flow imaging in clinically feasible acquisition times. Results support the hypothesis that ReVEAL-based 3Dir PC-CMR provides better estimation of hemodynamic parameters in AS patients in comparison to 1Dir PC-CMR. While TTE can accurately measure velocity parallel to the acoustic beam, it is not sensitive to the other directions of flow. Therefore, multi-directional flow imaging, which encodes all three components of the velocity vector, can potentially outperform TTE in patients with eccentric or multiple jets.

Free-breathing myocardial T2* mapping using GRE-EPI and MOCO for myocardial and hepatic iron overload assessment: a multi-centre study

Background T2* measurement is widely used in the assessment of patients at risk for cardiac and hepatic iron overload (1,2). The conventional breath-hold, ECG-triggered, segmented, multi-echo gradient echo sequence (BH MGRE) used for myocardial T2* quantification (3) is very sensitive to respiratory motion and may not be feasible in patients who are unable to breath-hold. To overcome this limitation, we developed a free-breathing myocardial T2* mapping approach (4). The goal of this study was to investigate the effectiveness of the new technique in patients referred for iron overload assessment at 4 different centers across the world.

Simultaneous VO2 and cardiac output measurement to estimate oxygen extraction (a-v)O2

Background Chronic heart failure (CHF) is the leading hospital discharge diagnosis in patients over age 65 [1]. Emerging techniques in cardiovascular magnetic resonance (CMR) have resulted in unique opportunity for improvement of non-invasive assessment of the physiologic and anatomic effects of CHF. We have previously demonstrated the accuracy and feasibility of V̇O2 max measurements in the MRI environment using a modified metabolic cart [2]. Additionally, V̇O2 measures acquired within the MRI have been reported [3]. Existing methods describe non-invasive MRI measurement of whole body oxygen consumption via T2 imaging [4]. The Fick principle states V̇O2 = CO × (A-V)O2; where V̇O2 is oxygen consumption, CO is cardiac output, and (A-V)O2 is arteriovenous oxygen difference. Current modifications required for metabolic cart measures of oxygen consumption in the MRI environment present clinical challenges in widespread application. Using the Fick principle we sought to quantify and compare non-invasive MRI derived V̇O2 (MRI-V̇O2) quantification with metabolic cart measures of V̇O2 .

Peak velocity estimation in aortic stenosis patients using a fast three-directional two-dimensional phase contrast technique in a single breath-hold: comparison to unidirectional phase contrast MRI and transthoracic echocardiography

Background Assessment of aortic valve stenosis (AVS) severity is crucial for valve replacement indication and is typically performed by transthoracic Doppler-echocardiography (TTE). However, TTE may be suboptimal in up to 30% of patients. Unidirectional through-plane phase-contrast magnetic resonance imaging (1Dir PC-MRI) is the most common MRI technique used to quantify peak velocities (Vpeak) and flow (Figure 1A). Nonetheless, 1Dir PCMRI has been shown to underestimate aortic velocities if imaging planes are not prescribed exactly perpendicular to flow direction. Thus, multi-directional velocity quantification would likely improve the accuracy of peak velocity measurements, and allow for more accurate grading of AVS severity. We sought to determine whether a PC technique capable of measuring 3 directions of velocity in a 2D image plane in a single breathhold (3Dir PC-MRI) (Figure 1B) provides more accurate estimation of Vpeak compared to the traditional 1Dir PC-MRI, using TTE as the reference standard.

Highly accelerated phase-contrast MRI-based multi-directional flow imaging for peak velocity estimation in aortic stenosis patients.

Background Aortic stenosis (AS) is the most common valvular disease, and its prevalence is on the rise. Transthoracic echocardiography (TTE) is the current gold standard for diagnosis and grading of AS. However, TTE suffers from inadequate acoustic windows, and misalignment errors. While CMR has emerged as a robust tool for numerous applications, flow analysis by unidirectional phase-contrast MRI (PC-MRI) is known to underestimate velocity if the imaging plane is not set perpendicular to flow direction. Selecting the proper orientation can be challenging as the jet direction may vary with respect to the valve orifice. Thus, multi-directional flow imaging is likely to improve the accuracy of peak velocity (Vpeak) measurements. However, multi-directional acquisition can be prohibitively long, limiting its clinical utility. The purpose of this study is to apply a recently proposed data processing method called ReVEAL [1] to significantly accelerate multi-directional PC-MRI. ReVEAL exploits spatiotemporal sparsity and leverages the relationship between encoded and compensated images to enable highly accelerated PC-MRI.

The feasibility of combining low-level exercise with vasodilator stress in patients referred for stress perfusion cardiac MRI

Background Adenosine, an agonist of the A2a receptor, is widely used for stress CMR. However, target receptors for adenosine are heterogeneous in their location and facilitated physiologic effects. A2b and A3 receptors are responsible for bronchospasm and peripheral arteriolar vasodilation; A1 receptors are responsible for AV block. The rate of adverse reactions with adenosine approaches 80%, consisting of dyspnea, headache, flushing, chest/ abdominal discomfort, angina, ST depression, dizziness, nausea, and dysgeusia. In SPECT protocols, use of vasodilators has been previously established as safe during low level exercise, and results in fewer adverse reactions. Our hypothesis is that adenosine administered during low level exercise treadmill stress cardiac MRI is safe, feasible, and results in diagnostic quality imaging.