Impact of compressed sensing (CS) acceleration of two-dimensional (2D) flow sequences in clinical paediatric cardiac magnetic resonance (CMR)

Background

Two-dimensional (2D) through-plane phase-contrast (PC) cine flow imaging assesses shunts and valve regurgitations in paediatric CMR, and is considered the reference standard for Clinical quantification Of blood Flow (COF). However, the longer breath-hold (BH) for a flow cine can reduce patient compliance with possibly large respiratory maneuvers altering flow. Compressed sensing (CS) flow has not been widely evaluated in pediatric clinical CMR, although CS flow reduces scan time with persistent accuracy [1–3]. Kocaoglu et al applied CS to ascending and descending aorta and SVC with good results. We used main pulmonary artery (MPA) and sinotubular junction (STJ) planes as usual for clinical CMR.

Purpose

We hypothesise that reduced BH time by modest application of CS to 2D cine through-plane flows (Short BH quantification of Flow) (SBOF) retains accuracy while enabling faster and potentially more reliable paediatric flows. We therefore investigate the variance between conventional COF and new SBOF cine flows in paediatric CMR.

Methods

Paediatric patients were enrolled. Aortic (AO) and MPA COFs were planned from cines of the left and right ventricular outflow tracts. For AO flow the plane was placed at the STJ, and MPA flow was acquired above the pulmonary valve. The same planes were used for COF and SBOF flows at nominally similar parameters except the moderate application of CS exploiting redundancy across the cine frames, SBOF being segmented CS cines not real-time. CVI42 (5.10; Circle CVI) was used for flow analysis (single observer, 3 years' experience). Paired t-tests found the overall differences, and variability was defined at ±2SD for STJSV, MPASV, STJCO, MPACO, and Qp/Qs.

Results and discussion

20 patients (mean age 13.7, range 10–17y) were enrolled (12 CHDs, 7 cardiomyopathies or other diseases). The BH times were COF mean 11.7s (range 8.4–20.9s) vs SBOF mean 6.5s (min 3.6–9.1s). For STJ flows the differences and variabilities between the COF and SBOF flows were SV 6.95±13.6 (ml/beat), CO 0.16±1.35 (l/min) and for MPA flows SV 2.95±12.3 (ml/beat), CO 0.27±0.96 (l/min) and for Qp/Qs were 0.04±0.19 by ml/beat and 0.02±0.23 by l/min. The mean differences were non significant, and variability between SBOF and COF was similar to intrasession repeatability of COF in a separate paediatric population at our centre (unpublished), that might arise from physiological flow changes, possibly in terms of pre and post load and heart rate. With shorter BHs also assisting patient compliance of the SBOF, physiological flow effects might be reduced, although given the variability in COV this was unconfirmed.

Conclusion

Moderate CS applied to clinical segmented-cine paediatric phase-contrast flows in the STJ and MPA planes did not degrade flow repeatability or cause bias. SBOF assists clinical flows by shorter BHs and also may aid compliance and reduce physiological variations.

Funding Acknowledgement

Type of funding sources: None.

Diffusion tensor cardiovascular magnetic resonance predicts adverse remodelling after myocardial infarction

Background

Changes in myocardial microstructure that underlie post-myocardial infarction (MI) left ventricular (LV) remodelling may contribute to progressive deterioration in cardiac function and increased risk of adverse clinical events. Diffusion Tensor Cardiovascular Magnetic Resonance (DT-CMR) derived parameters provide in vivo measures of helix angle (HA) and sheetlet angle (SA), which allow non-invasive characterization of microstructural dynamics associated with cardiac contraction.

Purpose

To evaluate the relationship between DT-CMR metrics of myocardial microstructure with the development of adverse LV remodelling and to determine the relationship between DT-CMR and strain post- MI.

Methods

We performed a longitudinal pre-clinical CMR study whereby DT-CMR, cine imaging and strain were acquired pre-MI and 3 days and 16 weeks post- MI in a pig model. HA (E1A), SA (E2A), and sheetlet angle mobility (ΔE2A = E2Asystole– E2Adiastole), as well as circumferential (Ecc) and radial (Err) strain were calculated at each timepoint and related to change in left ventricular end-diastolic volume (ΔLVEDV) and change in left ventricular end-systolic volume (ΔLVESV) between 3 days post-MI and 16 weeks post-MI. The translational value of this preclinical study was further assessed in six patients with chronic MI.

Results

ΔE2A over the whole LV (global ΔE2A) at 3 days post-MI correlated significantly with ΔLVEDV (R2=0.89, p=0.0013, Fig.1A;) and ΔLVESV (R2=0.81, p=0.0055, Fig.1B). Global Ecc at 3 days post-MI also correlated with both ΔLVEDV (R2=0.75, p=0.012, Fig.1C) and ΔLVESV (R2=0.71, p=0.018, Fig.1D). Global Err at 3 days post-MI did not show significant correlation with either ΔLVEDV (R2=0.32, p=0.19, Fig.1E) or ΔLVESV (R2=0.35, p=0.17, Fig.1F). Global ΔE2A correlated strongly with global Ecc 3 days post-MI (R2=0.9, p=0.00099, Fig.1G) but less strongly with global Err 3 days post-MI (R2=0.57, p=0.049, Fig.1H). Global ΔE2A at the chronic stage correlated significantly with ejection Fraction (EF), in both clinical (R2=0.87, p=0.007) and preclinical data (R2=0.87, p=0.0024). Global ΔE2A correlated well with LVEDV (clinical: R2=0.72, p=0.033; preclinical: R2=0.8, p=0.0066) and LVESV (clinical: R2=0.78, p=0.020; preclinical: R2=0.89, p=0.0013). In vivo E1A maps at 16 weeks post-MI and ex vivo DT-CMR demonstrated reduced right-handed helix angles in the endocardium of the infarct region.

Conclusion

ΔE2A measured at 3 day post-reperfused MI is strongly correlated with the development of increased end-systolic and end-diastolic volumes, and may therefore serve as a novel CMR early predictor of adverse LV remodelling after reperfused MI. Strong correlations between ΔE2A, LV volumes and EF in a small cohort of stable patients with remodelled hearts after chronic MI confirm the feasibility of performing these measurements in patients and the plausibility of further evaluation of ΔE2A as a predictor of adverse remodelling after reperfused MI.

Sheetlet mobility predicts volume change

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): National Institutes of Health by the Division of Intramural Research (NHLBI, NIH, DHHS); British Heart Foundation

Diffusion tensor cardiovascular magnetic resonance detects altered myocardial microstructure in patients with acute st-elevation myocardial infarction

Background

Diffusion Tensor Cardiovascular Magnetic Resonance (DT-CMR) can quantify metrics of tissue integrity (mean diffusivity [MD] and fractional anisotropy [FA]) and changes in laminar microstructures (sheetlets), which reorientate from more wall-parallel in diastole (DIA) towards wall-perpendicular in systole (SYS) as the myocardium thickens, quantified by E2 angle [E2A]. Microstructural changes after STEMI may provide new insights into adverse LV remodelling and risk stratification.

Methods

In vivo DT-CMR was performed 3–5 days after PPCI for first presentation STEMI (N=19, mean age 57±9, 79% male). DT-CMR was acquired in 2 short-axes (SYS & DIA) using a STEAM-EPI sequence. 12 segment analysis of MD, FA, E2A and E2A mobility (ΔE2A = E2ASYS − E2ADIA) was performed. Infarct (INF) segments were defined as >25% LGE, adjacent (ADJ, located contiguous to INF) and remote (REM, all other segments). Wilcoxon signed rank tests were used with threshold P<0.017 (Bonferroni corrected).

Results

See Table.

MD in both SYS and DIA was significantly higher in INF and ADJ regions compared to REM. FA in both SYS and DIA was lower in the INF and ADJ compared to REM. E2ADIA was higher in INF, indicating a more wall-perpendicular orientation of sheetlets, compared to ADJ and REM zones. E2ASYS in INF was significantly reduced, indicating a more wall-parallel orientation of sheetlets, compared to ADJ and REM regions, resulting in significantly reduced sheetlet mobility (ΔE2A).

Conclusions

Microstructural changes can be detected after acute STEMI by in vivo DT-CMR. Zonal changes in MD and FA may suggest loss of barriers to water diffusion and altered cardiomyocyte organisation, respectively. We provide the first report of reduced sheetlet mobility after acute STEMI in INF. Ongoing work is evaluating the mechanisms and prognostic importance of altered sheetlet mobility after STEMI.

Funding Acknowledgement

Type of funding source: Foundation. Main funding source(s): British Heart Foundation Clinical Research Training Fellowship

Relationship between cardiac diffusion tensor imaging parameters and anthropometrics in healthy volunteers

BACKGROUND

In vivo cardiac diffusion tensor imaging (cDTI) is uniquely capable of interrogating laminar myocardial dynamics non-invasively. A comprehensive dataset of quantative parameters and comparison with subject anthropometrics is required.

METHODS

cDTI was performed at 3T with a diffusion weighted STEAM sequence. Data was acquired from the mid left ventricle in 43 subjects during the systolic and diastolic pauses. Global and regional values were determined for fractional anisotropy (FA), mean diffusivity (MD), helix angle gradient (HAg, degrees/%depth) and the secondary eigenvector angulation (E2A). Regression analysis was performed between global values and subject anthropometrics.

RESULTS

All cDTI parameters displayed regional heterogeneity. The RR interval had a significant, but clinically small effect on systolic values for FA, HAg and E2A. Male sex and increasing left ventricular end diastolic volume were associated with increased systolic HAg. Diastolic HAg and systolic E2A were both directly related to left ventricular mass and body surface area. There was an inverse relationship between E2A mobility and both age and ejection fraction.

CONCLUSIONS

Future interpretations of quantitative cDTI data should take into account anthropometric variations observed with patient age, body surface area and left ventricular measurements. Further work determining the impact of technical factors such as strain and SNR is required.

Parallel transmit excitation at 1.5 T based on the minimization of a driving function for device heating

PURPOSE

To provide a rapid method to reduce the radiofrequency (RF) E-field coupling and consequent heating in long conductors in an interventional MRI (iMRI) setup.

METHODS

A driving function for device heating (W) was defined as the integration of the E-field along the direction of the wire and calculated through a quasistatic approximation. Based on this function, the phases of four independently controlled transmit channels were dynamically changed in a 1.5 T MRI scanner. During the different excitation configurations, the RF induced heating in a nitinol wire immersed in a saline phantom was measured by fiber-optic temperature sensing. Additionally, a minimization of W as a function of phase and amplitude values of the different channels and constrained by the homogeneity of the RF excitation field (B1) over a region of interest was proposed and its results tested on the benchtop. To analyze the validity of the proposed method, using a model of the array and phantom setup tested in the scanner, RF fields and SAR maps were calculated through finite-difference time-domain (FDTD) simulations. In addition to phantom experiments, RF induced heating of an active guidewire inserted in a swine was also evaluated.

RESULTS

In the phantom experiment, heating at the tip of the device was reduced by 92% when replacing the body coil by an optimized parallel transmit excitation with same nominal flip angle. In the benchtop, up to 90% heating reduction was measured when implementing the constrained minimization algorithm with the additional degree of freedom given by independent amplitude control. The computation of the optimum phase and amplitude values was executed in just 12 s using a standard CPU. The results of the FDTD simulations showed similar trend of the local SAR at the tip of the wire and measured temperature as well as to a quadratic function of W, confirming the validity of the quasistatic approach for the presented problem at 64 MHz. Imaging and heating reduction of the guidewire were successfully performed in vivo with the proposed hardware and phase control.

CONCLUSIONS

Phantom and in vivo data demonstrated that additional degrees of freedom in a parallel transmission system can be used to control RF induced heating in long conductors. A novel constrained optimization approach to reduce device heating was also presented that can be run in just few seconds and therefore could be added to an iMRI protocol to improve RF safety.

Muscle Na+ channelopathies: MRI detects intracellular 23Na accumulation during episodic weakness

BACKGROUND

Muscle channelopathies such as paramyotonia, hyperkalemic periodic paralysis, and potassium-aggravated myotonia are caused by gain-of-function Na+ channel mutations.

METHODS

Methods: Implementation of a three-dimensional radial 23Na magnetic resonance (MR) sequence with ultra-short echo times allowed the authors to quantify changes in the total muscular 23Na signal intensity. By this technique and T2-weighted 1H MRI, the authors studied whether the affected muscles take up Na+ and water during episodes of myotonic stiffness or of cold- or exercise-induced weakness.

RESULTS

A 22% increase in the 23Na signal intensity and edema-like changes on T2-weighted 1H MR images were associated with cold-induced weakness in all 10 paramyotonia patients; signal increase and weakness disappeared within 1 day. A 10% increase in 23Na, but no increase in the T2-weighted 1H signal, occurred during cold- or exercise-induced weakness in seven hyperkalemic periodic paralysis patients, and no MR changes were observed in controls or exercise-induced stiffness in six potassium-aggravated myotonia patients. Measurements on native muscle fibers revealed provocation-induced, intracellular Na+ accumulation and membrane depolarization by -41 mV for paramyotonia, by -30 mV for hyperkalemic periodic paralysis, and by -20 mV for potassium-aggravated myotonia. The combined in vivo and in vitro approach showed a close correlation between the increase in 23Na MR signal intensity and the membrane depolarization (r = 0.92).

CONCLUSIONS

The increase in the total 23Na signal intensity reflects intracellular changes, the cold-induced Na+ shifts are greatest and osmotically relevant in paramyotonia patients, and even osmotically irrelevant Na+ shifts can be detected by the implemented 23Na MR technique.

ECG-gated 23Na-MRI of the human heart using a 3D-radial projection technique with ultra-short echo times

Pathological changes in tissue often manifest themselves in an altered sodium gradient between intra- and extracellular space due to a malfunctioning Na+-K+ pump, resulting in an increase in total sodium concentration in ischaemic regions. Therefore, 23Na-MRI has the potential to non-invasively differentiate viable from non-viable tissue by detecting concentration changes of intra- and extracellular sodium. As the in vivo sodium signal shows a bi-exponential T2 decay, with a short component of less than 1 ms, the accurate quantification of the total sodium content requires imaging techniques with ultra-short echo times (TE) below 0.5 ms. A 3D-radial projection technique has been developed which allows the acquisition of ECG-triggered sodium images of the human heart with a TE of 0.4 ms. With this pulse sequence 23Na-MRI volunteer measurements of the head or the heart were performed in less than 18 min on a 1.5-T clinical scanner with an isotropic resolution of 10 mm3. The signal to noise ratio of the radial projection technique is twofold higher than that of a Cartesian gradient echo pulse sequence (TE = 3.2 ms). Radial 23Na-MRI provides a tool for clinical studies, aiming at the differentiation of viable and non-viable tissue.