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

BACKGROUND: Aortic 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. METHODS: Twenty-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. RESULTS: ReVEAL 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. CONCLUSIONS: By 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 automatic non-rigid motion correction.

BACKGROUND: Measurement of myocardial T2* is becoming widely used in the assessment of patients at risk for cardiac iron overload. The conventional breath-hold, ECG-triggered, segmented, multi-echo gradient echo (MGRE) sequence used for myocardial T2* quantification is very sensitive to respiratory motion and may not be feasible in patients who are unable to breath-hold. We propose a free-breathing myocardial T2* mapping approach that combines a single-shot gradient-echo echo-planar imaging (GRE-EPI) sequence for T2*-weighted image acquisition with automatic non-rigid motion correction (MOCO) of respiratory motion between single-shot images. METHODS: ECG-triggered T2*-weighted images at different echo times were acquired by a black-blood, single-shot GRE-EPI sequence during free-breathing. A single image at a single TE is acquired in each heartbeat. Automatic non-rigid MOCO was applied to correct for in-plane respiratory motion before pixel-wise T2* mapping. In a total of 117 patients referred for clinical cardiac magnetic resonance exams, the free-breathing MOCO GRE-EPI sequence was compared to the breath-hold segmented MGRE approach. Image quality was scored independently by 2 experienced observers blinded to the particular image acquisition strategy. T2* measurements in the interventricular septum and in the liver were compared for the two methods in all cases with adequate image quality. RESULTS: T2* maps were acquired in all 117 patients using the breath-hold MGRE and the free-breathing MOCO GRE-EPI approaches, including 8 patients with myocardial iron overload and 25 patients with hepatic iron overload. The mean image quality of the free-breathing MOCO GRE-EPI images was scored significantly higher than that of the breath-hold MGRE images by both reviewers. Out of the 117 studies, 21 breath-hold MGRE studies (17.9% of all the patients) were scored to be less than adequate or very poor by both reviewers, while only 2 free-breathing MOCO GRE-EPI studies were scored to be less than adequate image quality. In a comparative evaluation of the images with at least adequate quality, the intra-class correlation coefficients for myocardial and liver T2* were 0.868 and 0.986 respectively (p < 0.001), indicating that the T2* measured by breath-hold MGRE and free-breathing MOCO GRE-EPI were in close agreement. The coefficient of variation between the breath-hold and free-breathing approaches for myocardial and liver T2* were 9.88% and 9.38% respectively. Bland-Altman plots demonstrated good absolute agreement of T2* in the interventricular septum and the liver from the free-breathing and breath-hold approaches (mean differences -0.03 and 0.16 ms, respectively). CONCLUSION: The free-breathing approach described for T2* mapping using MOCO GRE-EPI enables accurate myocardial and liver T2* measurements and is insensitive to respiratory motion.

Mitral annular velocity measurement with cardiac magnetic resonance imaging using a novel annular tracking algorithm: Validation against echocardiography.

BACKGROUND: Doppler based mitral annular velocities are an integral part of echocardiographic left ventricular diastolic function assessment. Although these measurements can be obtained by phase contrast cardiac magnetic resonance imaging (PC-CMR), this approach has limitations. The aims of this study were to assess the accuracy and reproducibility of a high temporal resolution steady-state free precession (SSFP) cine acquisition coupled with semi-automated mitral annular tracking to measure tissue velocity, and compare to echocardiography as the reference method. METHODS: High temporal resolution (17 ms) 4-chamber cines were acquired in 25 volunteers using retrospective and prospective gating on a 3.0 T magnet. Mitral annular early (e') and late (a') tissue velocities were derived using a novel algorithm to semi-automatically detect the mitral valve insertion points and track its motion. Additionally, PC-CMR was used to measure mitral inflow early diastolic (E) velocity. Those measurements were also obtained using echocardiography based pulsed and tissue Doppler techniques, on the same day. RESULTS: Subjects were on average 34 ±â€¯14 years-old (48% male). The lateral annulus e' measurements had the best agreement with echocardiography with a concordance correlation coefficient (CCC) of 0.76 and 0.75 for prospectively and retrospectively gated cine CMR respectively. There was no significant difference in the lateral annular tissue velocities between echocardiography (13.8 ±â€¯3.7 cm/s) and prospective (13.4 ±â€¯3.7 cm/s) or retrospective (14.0 ±â€¯3.7) acquisitions. Similarly, CMR measurement of E/e' (a surrogate marker for LV filling pressures) using the lateral e' velocity showed moderate agreement with echocardiography (CCC of 0.56 and 0.51 for prospective and retrospective acquisitions respectively) without a significant difference in ratios (5.3 ±â€¯1.6 and 5.0 ±â€¯1.3) compared to echocardiography (5.2 ±â€¯1.4). Intra- and inter-observer reproducibility of the CMR-based annular velocity measurements was good. CONCLUSION: Measurements of mitral annular tissue velocities can be obtained from SSFP 4-chamber cine images using a semi-automated annular tracking algorithm, and demonstrates moderate agreement with echocardiography. The semi-automated method can provide quantitative mitral annular velocity measurements directly from conventional cine images, thereby providing additional clinically relevant information. The accuracy of this method in patients with diastolic dysfunction remains to be determined.

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.