Comparison of self-gated cine MRI retrospective cardiac synchronization algorithms

Comparison of Prospective and Retrospective Gated 4D Flow Cardiac MR Image Acquisitions in the Carotid Bifurcation

PURPOSE

To evaluate the agreement of 4D flow cMRI-derived bulk flow features and fluid (blood) velocities in the carotid bifurcation using prospective and retrospective gating techniques.

METHODS

Prospective and retrospective ECG-gated three-dimensional (3D) cine phase-contrast cardiac MRI with three-direction velocity encoding (i.e., 4D flow cMRI) data were acquired in ten carotid bifurcations from men (n = 3) and women (n = 2) that were cardiovascular disease-free. MRI sequence parameters were held constant across all scans except temporal resolution values differed. Velocity data were extracted from the fluid domain and evaluated across the entire volume or at defined anatomic planes (common, internal, external carotid arteries). Qualitative agreement between gating techniques was performed by visualizing flow streamlines and topographical images, and statistical comparisons between gating techniques were performed across the fluid volume and defined anatomic regions.

RESULTS

Agreement in the kinematic data (e.g., bulk flow features and velocity data) were observed in the prospectively and retrospectively gated acquisitions. Voxel differences in time-averaged, peak systolic, and diastolic-averaged velocity magnitudes between gating techniques across all volunteers were 2.7%, 1.2%, and 6.4%, respectively. No significant differences in velocity magnitudes or components ([Formula: see text], [Formula: see text], [Formula: see text]) were observed. Importantly, retrospective acquisitions captured increased retrograde flow in the internal carotid artery (i.e., carotid sinus) compared to prospective acquisitions (10.4 ± 6.3% vs. 4.6 ± 5.3%; [Formula: see text] < 0.05).

CONCLUSION

Prospective and retrospective ECG-gated 4D flow cMRI acquisitions provide comparable evaluations of fluid velocities, including velocity vector components, in the carotid bifurcation. However, the increased temporal coverage of retrospective acquisitions depicts increased retrograde flow patterns (i.e., disturbed flow) not captured by the prospective gating technique.

Monitoring and Synchronization of Cardiac and Respiratory Traces in Magnetic Resonance Imaging: A Review

Synchronization of human vital signs, namely the cardiac cycle and respiratory excursions, is necessary during magnetic resonance imaging of the cardiovascular system and the abdominal cavity to achieve optimal image quality with minimized artifacts. This review summarizes techniques currently available in clinical practice, as well as methods under development, outlines the benefits and disadvantages of each approach, and offers some unique solutions for consideration.

Fetal whole-heart 4D imaging using motion-corrected multi-planar real-time MRI

Purpose

To develop an MRI acquisition and reconstruction framework for volumetric cine visualization of the fetal heart and great vessels in the presence of maternal and fetal motion.

Methods

Four‐dimensional (4D) depiction was achieved using a highly‐accelerated multi‐planar real‐time balanced steady‐state free precession acquisition combined with retrospective image‐domain techniques for motion correction, cardiac synchronization and outlier rejection. The framework was validated using a numerical phantom and evaluated in a study of 20 mid‐ to late‐gestational age human fetal subjects (23‐33 weeks gestational age). Reconstructed MR data were compared with matched ultrasound. A preliminary assessment of flow‐sensitive reconstruction using the velocity information encoded in the phase of real‐time images is included.

Results

Reconstructed 4D data could be visualized in any two‐dimensional plane without the need for highly specific scan plane prescription prior to acquisition or for maternal breath hold to minimize motion. Reconstruction was fully automated aside from user‐specified masks of the fetal heart and chest. The framework proved robust when applied to fetal data and simulations confirmed that spatial and temporal features could be reliably recovered. Evaluation suggested the reconstructed framework has the potential to be used for comprehensive assessment of the fetal heart, either as an adjunct to ultrasound or in combination with other MRI techniques.

Conclusions

The proposed methods show promise as a framework for motion‐compensated 4D assessment of the fetal heart and great vessels.

Self-gated golden angle spiral cine MRI for coronary endothelial function assessment

PURPOSE

Depressed coronary endothelial function (CEF) is a marker for atherosclerotic disease, an independent predictor of cardiovascular events, and can be quantified non-invasively with ECG-triggered spiral cine MRI combined with isometric handgrip exercise (IHE). However, MRI-CEF measures can be hindered by faulty ECG-triggering, leading to prolonged breath-holds and degraded image quality. Here, a self-gated golden angle spiral method (SG-GA) is proposed to eliminate the need for ECG during cine MRI.

METHODS

SG-GA was tested against retrospectively ECG-gated golden angle spiral MRI (ECG-GA) and gold-standard ECG-triggered spiral cine MRI (ECG-STD) in 10 healthy volunteers. CEF data were obtained from cross-sectional images of the proximal right and left coronary arteries in a 3T scanner. Self-gating heart rates were compared to those from simultaneous ECG-gating. Coronary vessel sharpness and cross-sectional area (CSA) change with IHE were compared among the 3 methods.

RESULTS

Self-gating precision, accuracy, and correlation-coefficient were 7.7 ± 0.5 ms, 9.1 ± 0.7 ms, and 0.93 ± 0.01, respectively (mean ± standard error). Vessel sharpness by SG-GA was equal or higher than ECG-STD (rest: 63.0 ± 1.7% vs. 61.3 ± 1.3%; exercise: 62.6 ± 1.3% vs. 56.7 ± 1.6%, P < 0.05). CSA changes were in agreement among the 3 methods (ECG-STD = 8.7 ± 4.0%, ECG-GA = 9.6 ± 3.1%, SG-GA = 9.1 ± 3.5%, P = not significant).

CONCLUSION

CEF measures can be obtained with the proposed self-gated high-quality cine MRI method even when ECG is faulty or not available. Magn Reson Med 80:560-570, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Retrospective motion gating in cardiac MRI using a simultaneously acquired navigator

A simultaneous acquisition technique of image and navigator signals (simultaneously acquired navigator, SIMNAV) is proposed for cardiac magnetic resonance imaging (CMRI) in Cartesian coordinates. To simultaneously acquire both image and navigator signals, a conventional balanced steady-state free precession (bSSFP) pulse sequence is modified by adding a radiofrequency (RF) pulse, which excites a supplementary slice for the navigator signal. Alternating phases of the RF pulses make it easy to separate the simultaneously acquired magnetic resonance data into image and navigator signals. The navigator signals of the proposed SIMNAV were compared with those of current gating devices and self-gating techniques for seven healthy subjects. In vivo experiments demonstrated that SIMNAV could provide cardiac cine images with sufficient image quality, similar to those from electrocardiogram (ECG) gating with breath-hold. SIMNAV can be used to acquire a cardiac cine image without requiring an ECG device and breath-hold, whilst maintaining feasible imaging time efficiency.

Improved respiratory self-navigation for 3D radial acquisitions through the use of a pencil-beam 2D-T2 -prep for free-breathing, whole-heart coronary MRA

PURPOSE

In respiratory self-navigation (SN), signal from static structures, such as the chest wall, may complicate motion detection or introduce post-correction artefacts. Suppressing signal from superfluous tissues may therefore improve image quality. We thus test the hypothesis that SN whole-heart coronary magnetic resonance angiography (MRA) will benefit from an outer-volume suppressing 2D-T2 -Prep and present both phantom and in vivo results.

METHODS

A 2D-T2 -Prep and a conventional T2 -Prep were used prior to a free-breathing 3D-radial SN sequence. Both techniques were compared by imaging a home-built moving cardiac phantom and by performing coronary MRA in nine healthy volunteers. Reconstructions were performed using both a reference-based and a reference-independent approach to motion tracking, along with several coil combinations. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were compared, along with vessel sharpness (VS).

RESULTS

In phantoms, using the 2D-T2 -Prep increased SNR by 16% to 53% and mean VS by 8%; improved motion tracking precision was also achieved. In volunteers, SNR increased by an average of 29% to 33% in the blood pool and by 15% to 25% in the myocardium, depending on the choice of reconstruction coils and algorithm, and VS increased by 34%.

CONCLUSION

A 2D-T2 -Prep significantly improves image quality in both phantoms and volunteers when performing SN coronary MRA. Magn Reson Med 79:1293-1303, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Fetal cardiac cine imaging using highly accelerated dynamic MRI with retrospective motion correction and outlier rejection

PURPOSE

Development of a MRI acquisition and reconstruction strategy to depict fetal cardiac anatomy in the presence of maternal and fetal motion.

METHODS

The proposed strategy involves i) acquisition and reconstruction of highly accelerated dynamic MRI, followed by image-based ii) cardiac synchronization, iii) motion correction, iv) outlier rejection, and finally v) cardiac cine reconstruction. Postprocessing entirely was automated, aside from a user-defined region of interest delineating the fetal heart. The method was evaluated in 30 mid- to late gestational age singleton pregnancies scanned without maternal breath-hold.

RESULTS

The combination of complementary acquisition/reconstruction and correction/rejection steps in the pipeline served to improve the quality of the reconstructed 2D cine images, resulting in increased visibility of small, dynamic anatomical features. Artifact-free cine images successfully were produced in 36 of 39 acquired data sets; prolonged general fetal movements precluded processing of the remaining three data sets.

CONCLUSIONS

The proposed method shows promise as a motion-tolerant framework to enable further detail in MRI studies of the fetal heart and great vessels. Processing data in image-space allowed for spatial and temporal operations to be applied to the fetal heart in isolation, separate from extraneous changes elsewhere in the field of view. Magn Reson Med 79:327-338, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Cardiac functional assessment without electrocardiogram using physiological self-navigation

PURPOSE

Electrocardiogram (ECG)-gated cine MRI provides highly accurate functional assessment of the heart. Nevertheless, reliable ECG signals are not always available due to patient's electrophysiology or due to high MR field strengths. Here, a novel framework for cardiac functional assessment using physiological information is presented, which is obtained from MR image data.

METHODS

Multiple long-axis slices rotated around the center axis of the left ventricle are acquired using a 2D Golden Radial acquisition scheme. This sampling approach allows for both real-time data and retrospectively reordered cine images with different temporal resolutions. Functional information from the left ventricle is used for retrospective reordering of the data to reconstruct cine images without an external ECG signal. Afterward, individual 2D cine slices are synchronized using physiological information on the mitral valve closure. The proposed approach was assessed in 15 volunteers and applied in four patients for feasibility.

RESULTS

Physiological gating signals obtained with our approach show great correlation with an ECG reference signal. Functional parameters determined with the presented method show a relative difference of less than 1.3% when compared with an ECG-gated approach.

CONCLUSION

It is successfully demonstrated that functional assessment of the heart is possible using physiological information obtained directly from MR image data.

Robust self-navigated body MRI using dense coil arrays

PURPOSE

To develop a robust motion estimation method for free-breathing body MRI using dense coil arrays.

METHODS

Self-navigating pulse sequences can measure subject motion without using external motion monitoring devices. With dense coil arrays, individual coil elements can provide localized motion estimates. An averaged motion estimate over all coils is often used for motion compensation. However, this motion estimate may not accurately represent the dominant motion within the imaging volume. In this work, a coil clustering method is proposed to automatically determine the dominant motion for dense coil arrays. The feasibility of the proposed method is investigated in free-breathing abdominal MRI and cardiac MRI, and compared with manual motion estimate selection for respiratory motion estimation and electrocardiography for cardiac motion estimation.

RESULTS

Automated motion estimation achieved similar respiratory motion estimation compared to manual selection (averaged correlation coefficient 0.989 and 0.988 for abdominal MRI and cardiac MRI, respectively), and accurate cardiac triggering compared to electrocardiography (averaged temporal variability 17.5 ms).

CONCLUSION

The proposed method can provide accurate automated motion estimation for body MRI using dense coil arrays. It can enable self-navigated free-breathing abdominal and cardiac MRI without the need for external motion monitoring devices. Magn Reson Med 76:197-205, 2016. © 2015 Wiley Periodicals, Inc.

Self-gated fat-suppressed cardiac cine MRI

PURPOSE

To develop a self-gated alternating repetition time balanced steady-state free precession (ATR-SSFP) pulse sequence for fat-suppressed cardiac cine imaging.

METHODS

Cardiac gating is computed retrospectively using acquired magnetic resonance self-gating data, enabling cine imaging without the need for electrocardiogram (ECG) gating. Modification of the slice-select rephasing gradients of an ATR-SSFP sequence enables the acquisition of a one-dimensional self-gating readout during the unused short repetition time (TR). Self-gating readouts are acquired during every TR of segmented, breath-held cardiac scans. A template-matching algorithm is designed to compute cardiac trigger points from the self-gating signals, and these trigger points are used for retrospective cine reconstruction. The proposed approach is compared with ECG-gated ATR-SSFP and balanced steady-state free precession in 10 volunteers and five patients.

RESULTS

The difference of ECG and self-gating trigger times has a variability of 13 ± 11 ms (mean ± SD). Qualitative reviewer scoring and ranking indicate no statistically significant differences (P > 0.05) between self-gated and ECG-gated ATR-SSFP images. Quantitative blood-myocardial border sharpness is not significantly different among self-gated ATR-SSFP ( 0.61±0.15 mm -1), ECG-gated ATR-SSFP ( 0.61±0.15 mm -1), or conventional ECG-gated balanced steady-state free precession cine MRI ( 0.59±0.15 mm -1).

CONCLUSION

The proposed self-gated ATR-SSFP sequence enables fat-suppressed cardiac cine imaging at 1.5 T without the need for ECG gating and without decreasing the imaging efficiency of ATR-SSFP.

ECG-based gating in ultra high field cardiovascular magnetic resonance using an independent component analysis approach

BACKGROUND

In Cardiovascular Magnetic Resonance (CMR), the synchronization of image acquisition with heart motion is performed in clinical practice by processing the electrocardiogram (ECG). The ECG-based synchronization is well established for MR scanners with magnetic fields up to 3 T. However, this technique is prone to errors in ultra high field environments, e.g. in 7 T MR scanners as used in research applications. The high magnetic fields cause severe magnetohydrodynamic (MHD) effects which disturb the ECG signal. Image synchronization is thus less reliable and yields artefacts in CMR images.

METHODS

A strategy based on Independent Component Analysis (ICA) was pursued in this work to enhance the ECG contribution and attenuate the MHD effect. ICA was applied to 12-lead ECG signals recorded inside a 7 T MR scanner. An automatic source identification procedure was proposed to identify an independent component (IC) dominated by the ECG signal. The identified IC was then used for detecting the R-peaks. The presented ICA-based method was compared to other R-peak detection methods using 1) the raw ECG signal, 2) the raw vectorcardiogram (VCG), 3) the state-of-the-art gating technique based on the VCG, 4) an updated version of the VCG-based approach and 5) the ICA of the VCG.

RESULTS

ECG signals from eight volunteers were recorded inside the MR scanner. Recordings with an overall length of 87 min accounting for 5457 QRS complexes were available for the analysis. The records were divided into a training and a test dataset. In terms of R-peak detection within the test dataset, the proposed ICA-based algorithm achieved a detection performance with an average sensitivity (Se) of 99.2%, a positive predictive value (+P) of 99.1%, with an average trigger delay and jitter of 5.8 ms and 5.0 ms, respectively. Long term stability of the demixing matrix was shown based on two measurements of the same subject, each being separated by one year, whereas an averaged detection performance of Se = 99.4% and +P = 99.7% was achieved.Compared to the state-of-the-art VCG-based gating technique at 7 T, the proposed method increased the sensitivity and positive predictive value within the test dataset by 27.1% and 42.7%, respectively.

CONCLUSIONS

The presented ICA-based method allows the estimation and identification of an IC dominated by the ECG signal. R-peak detection based on this IC outperforms the state-of-the-art VCG-based technique in a 7 T MR scanner environment.

Estimation of the timing of carotid artery flow using peripheral pulse wave-gated MRI

PURPOSE

To investigate the relationship between peripheral pulse wave (PPW)-gating and the carotid systolic pulse wave in a large clinical patient cohort, and to establish a process for correct estimation of delay time from PPW-gating to foot (ie, beginning) or peak times of carotid systolic pulse waves.

MATERIALS AND METHODS

Subjects comprised 209 patients scanned using 3T magnetic resonance imaging (MRI) for PPW-gated phase contrast images at the common carotid artery. Stepwise multiple regression analysis was conducted for the relationship between foot or peak times and the following factors after excluding correlated factors with coefficients ≥0.5: pulse rate (PR); systolic blood pressure; diastolic blood pressure; height; body weight; body mass index; Brinkman index; diabetes mellitus; hypertension; and hyperlipidemia.

RESULTS

PR showed significant correlation with foot (r = -0.86, P < 0.001) and peak (r = -0.87, P < 0.001) times. The following equations were derived: foot time (msec) = -8.55 × PR + 993.1 and peak time (msec) = -9.21 × PR + 1142.3. No other factors showed significant correlations.

CONCLUSION

PR was the only factor showing significant relationships to foot and peak times of carotid artery flow. The derived equations will facilitate various kinds of noncontrast MR acquisition with simple PPW-gating.

Influence of the trigger technique on ventricular function measurements using 3-Tesla magnetic resonance imaging: comparison of ECG versus pulse wave triggering

BACKGROUND

Three Tesla cardiovascular magnetic resonance imaging (3T-CMR) is increasingly used in clinical practice. Despite many advantages one drawback is that ECG signal disturbances and artifacts increase with higher magnetic field strength resulting in trigger problems and false gating. This particularly affects cardiac imaging because most pulse sequences require ECG triggering. Pulse wave (PW) triggering is robust and might have advantages over ECG triggering.

PURPOSE

To evaluate differences in left ventricular (LV) function as an integral part of most CMR studies between ECG- and PW-triggered short-axis imaging using 3T-CMR.

MATERIAL AND METHODS

Forty-three patients underwent multiple short-axis cine imaging for LV-function assessment with ECG and PW triggering using standard multibreath hold steady-state free precession. LV-volumes (EDV, ESV), ejection fraction (EF), and mass were determined by slice summation. LV-wall motion was assessed by using a 4-point scoring scale. Bland Altman statistics for inter-observer variability were performed.

RESULTS

ECG triggering failed in 15 patients (34.8%). Thus, analysis was performed in 28 patients (13 with impaired LV function). Difference in volumes (EDV 0.13 ± 1.8 mL, ESV 0.59 ± 1.1 mL), EF (-0.32 ± 0.6%) and mass (0.01 ± 1.1 g) between ECG and PW triggering were very small and significant only for ESV and EF (p ≤ 0.011). In patients with impaired LV function (n = 19) differences were not significant (p ≥ 0.128). Wall motion scores did not differ between ECG and PW triggering (p ≥ 0.295). Inter-observer variability for function measurements was low.

CONCLUSION

Short-axis cine imaging for LV-function assessment can accurately be performed using PW triggering on 3T magnets, and may be used in clinical practice when ECG triggering is disturbed.

Motion correction using coil arrays (MOCCA) for free-breathing cardiac cine MRI

In this study, we present a motion correction technique using coil arrays (MOCCA) and evaluate its application in free-breathing respiratory self-gated cine MRI. Motion correction technique using coil arrays takes advantages of the fact that motion-induced changes in k-space signal are modulated by individual coil sensitivity profiles. In the proposed implementation of motion correction technique using coil arrays self-gating for free-breathing cine MRI, the k-space center line is acquired at the beginning of each k-space segment for each cardiac cycle with 4 repetitions. For each k-space segment, the k-space center line acquired immediately before was used to select one of the 4 acquired repetitions to be included in the final self-gated cine image by calculating the cross correlation between the k-space center line with a reference line. The proposed method was tested on a cohort of healthy adult subjects for subjective image quality and objective blood-myocardium border sharpness. The method was also tested on a cohort of patients to compare the left and right ventricular volumes and ejection fraction measurements with that of standard breath-hold cine MRI. Our data indicate that the proposed motion correction technique using coil arrays method provides significantly improved image quality and sharpness compared with free-breathing cine without respiratory self-gating and provides similar volume measurements compared with breath-hold cine MRI.

Acoustic cardiac triggering: a practical solution for synchronization and gating of cardiovascular magnetic resonance at 7 Tesla

BACKGROUND

To demonstrate the applicability of acoustic cardiac triggering (ACT) for imaging of the heart at ultrahigh magnetic fields (7.0 T) by comparing phonocardiogram, conventional vector electrocardiogram (ECG) and traditional pulse oximetry (POX) triggered 2D CINE acquisitions together with (i) a qualitative image quality analysis, (ii) an assessment of the left ventricular function parameter and (iii) an examination of trigger reliability and trigger detection variance derived from the signal waveforms.

RESULTS

ECG was susceptible to severe distortions at 7.0 T. POX and ACT provided waveforms free of interferences from electromagnetic fields or from magneto-hydrodynamic effects. Frequent R-wave mis-registration occurred in ECG-triggered acquisitions with a failure rate of up to 30% resulting in cardiac motion induced artifacts. ACT and POX triggering produced images free of cardiac motion artefacts. ECG showed a severe jitter in the R-wave detection. POX also showed a trigger jitter of approximately Δt = 72 ms which is equivalent to two cardiac phases. ACT showed a jitter of approximately Δt = 5 ms only. ECG waveforms revealed a standard deviation for the cardiac trigger offset larger than that observed for ACT or POX waveforms.Image quality assessment showed that ACT substantially improved image quality as compared to ECG (image quality score at end-diastole: ECG = 1.7 ± 0.5, ACT = 2.4 ± 0.5, p = 0.04) while the comparison between ECG vs. POX gated acquisitions showed no significant differences in image quality (image quality score: ECG = 1.7 ± 0.5, POX = 2.0 ± 0.5, p = 0.34).

CONCLUSIONS

The applicability of acoustic triggering for cardiac CINE imaging at 7.0 T was demonstrated. ACT's trigger reliability and fidelity are superior to that of ECG and POX. ACT promises to be beneficial for cardiovascular magnetic resonance at ultra-high field strengths including 7.0 T.

Self-gating MR imaging of the fetal heart: comparison with real cardiac triggering

PURPOSE

To investigate the self-gating technique for MR imaging of the fetal heart in a sheep model.

MATERIAL AND METHODS

MR images of 6 fetal sheep heart were obtained at 1.5 T. For self-gating MRI of the fetal heart a cine SSFP in short axis, two and four chamber view was used. Self-gated images were compared with real cardiac triggered MR images (pulse-wave triggering). MRI of the fetal heart was performed using both techniques simultaneously. Image quality was assessed and the left ventricular volume and function were measured and compared.

RESULTS

Compared with pulse-wave triggering, the self-gating technique produced slightly inferior images with artifacts. Especially the atrial septum could not be so clearly depicted. The contraction of the fetal heart was shown in cine sequences in both techniques. The average blood volumes could be measured with both techniques with no significant difference: at end-systole 3.1 ml (SD±0.2), at end-diastole 4.9 ml (±0.2), with ejection fractions at 38.6%, respectively 39%.

CONCLUSION

Both self-gating and pulse-wave triggered cardiac MRI of the fetal heart allowed the evaluation of anatomical structures and functional information. Images obtained by self-gating technique were slightly inferior than the pulse-wave triggered MRI.

Cardiac flow component analysis

In a chamber of the heart, large-scale vortices are shown to exist as the result of the dynamic blood flow and unique morphological changes of the chamber wall. As the cardiovascular flow varies over a cardiac cycle, there is a need for a robust quantification method to analyze its vorticity and circulation. We attempt to measure vortex characteristics by means of two-dimensional vorticity maps and vortex circulation. First, we develop vortex component analysis by segmenting the vortices using an data clustering algorithm before histograms of their vorticity distribution are generated. The next stage is to generate the statistics of the vorticity maps for each phase of the cardiac cycle to allow analysis of the flow. This is followed by evaluating the circulation of each segmented vortex. The proposed approach is dedicated to examining vortices within the human heart chamber. The vorticity field can indicate the strength and number of large-scale vortices in the chamber. We provide the results of the flow analysis after vorticity map segmentation and the statistical properties that characterize the vorticity components. The success of the cardiac measurement and analysis is illustrated by a case study of the right atrium. Our investigation shows that it is possible to utilize a data clustering algorithm to segment vortices after vorticity mapping, and that the vorticity and circulation analysis of a chamber vorticity can provide new insights into the blood flow within the cardiovascular structure.

Comparison of left ventricular function assessment using phonocardiogram- and electrocardiogram-triggered 2D SSFP CINE MR imaging at 1.5 T and 3.0 T

OBJECTIVE

As high-field cardiac MRI (CMR) becomes more widespread the propensity of ECG to interference from electromagnetic fields (EMF) and to magneto-hydrodynamic (MHD) effects increases and with it the motivation for a CMR triggering alternative. This study explores the suitability of acoustic cardiac triggering (ACT) for left ventricular (LV) function assessment in healthy subjects (n = 14).

METHODS

Quantitative analysis of 2D CINE steady-state free precession (SSFP) images was conducted to compare ACT's performance with vector ECG (VCG). Endocardial border sharpness (EBS) was examined paralleled by quantitative LV function assessment.

RESULTS

Unlike VCG, ACT provided signal traces free of interference from EMF or MHD effects. In the case of correct R-wave recognition, VCG-triggered 2D CINE SSFP was immune to cardiac motion effects-even at 3.0 T. However, VCG-triggered 2D SSFP CINE imaging was prone to cardiac motion and EBS degradation if R-wave misregistration occurred. ACT-triggered acquisitions yielded LV parameters (end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), ejection fraction (EF) and left ventricular mass (LVM)) comparable with those derived from VCG-triggered acquisitions (1.5 T: ESV(VCG) = (56 +/- 17) ml, EDV(VCG) = (151 +/- 32) ml, LVM(VCG) = (97 +/- 27) g, SV(VCG) = (94 +/- 19) ml, EF(VCG) = (63 +/- 5)% cf. ESV(ACT) = (56 +/- 18) ml, EDV(ACT) = (147 +/- 36) ml, LVM(ACT) = (102 +/- 29) g, SV(ACT) = (91 +/- 22) ml, EF(ACT) = (62 +/- 6)%; 3.0 T: ESV(VCG) = (55 +/- 21) ml, EDV(VCG) = (151 +/- 32) ml, LVM(VCG) = (101 +/- 27) g, SV(VCG) = (96 +/- 15) ml, EF(VCG) = (65 +/- 7)% cf. ESV(ACT) = (54 +/- 20) ml, EDV(ACT) = (146 +/- 35) ml, LVM(ACT) = (101 +/- 30) g, SV(ACT) = (92 +/- 17) ml, EF(ACT) = (64 +/- 6)%).

CONCLUSIONS

ACT's intrinsic insensitivity to interference from electromagnetic fields renders it suitable for clinical CMR.

Feasibility of cardiac gating free of interference with electro-magnetic fields at 1.5 Tesla, 3.0 Tesla and 7.0 Tesla using an MR-stethoscope

OBJECTIVES

To circumvent the challenges of conventional electrocardiographic (ECG)-gating by examining the efficacy of an MR stethoscope, which offers (i) no risk of high voltage induction or patient burns, (ii) immunity to electromagnetic interference, (iii) suitability for all magnetic field strengths, and (iv) patient comfort together with ease of use for the pursuit of reliable and safe (ultra)high field cardiac gated magnetic resonance imaging (MRI).

MATERIALS AND METHODS

The acoustic gating device consists of 3 main components: an acoustic sensor, a signal processing unit, and a coupler unit to the MRI system. Signal conditioning and conversion are conducted outside the 0.5 mT line using dedicated electronic circuits. The final waveform is delivered to the internal physiological signal controller circuitry of a clinical MR scanner. Cardiovascular MRI was performed of normal volunteers (n = 17) on 1.5 T, 3.0 T and 7.0 T whole body MR systems. Black blood imaging, 2D CINE imaging, 3D phase contrast MR angiography, and myocardial T2* mapping were carried out.

RESULTS

The MR-stethoscope provided cardiograms at 1.5 T, 3.0 T and 7.0 T free of interference from electromagnetic fields and magneto-hydrodynamic effects. In comparison, ECG waveforms were susceptible to T-wave elevation and other distortions, which were more pronounced at higher fields. Acoustically gated black blood imaging at 1.5 T and 3.0 T provided image quality comparable with or even superior to that obtained from the ECG-gated approach. In the case of correct R-wave recognition, ECG-gated 2D CINE SSFP imaging was found to be immune to cardiac motion effects -even at 3.0 T. However, ECG-gated 2D SSFP CINE imaging was prone to cardiac motion artifacts if R-wave mis-registration occurred because of T-wave elevation. Acoustically gated 3D PCMRA at 1.5 T, 3.0 T and 7.0 T resulted in images free of blood pulsation artifacts because the acoustic gating approach provided cardiac signal traces free of interference with electromagnetic fields or magneto-hydrodynamic effects even at 7.0 Tesla. Severe ECG-trace distortions and T-wave elevations occurred at 3.0 T and 7.0 T. Acoustically cardiac gated T2* mapping at 3.0 T yielded a T2* value of 22.3 +/- 4.8 ms for the inferoseptal myocardium.

CONCLUSIONS

The proposed MR-stethoscope presents a promising alternative to currently available techniques for cardiac gating of (ultra)high field MRI. Its intrinsic insensitivity to interference from electromagnetic fields renders it suitable for clinical imaging because of its excellent trigger reliability, even at 7.0 Tesla.