Fabian Hezel

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

BackgroundTo 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.ResultsECG 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).ConclusionsThe applicability of acoustic triggering for cardiac CINE imaging at 7.0 T was demonstrated. ACTs 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.

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.

Modular 32-channel transceiver coil array for cardiac MRI at 7.0T

To design and evaluate a modular transceiver coil array with 32 independent channels for cardiac MRI at 7.0T.

Two-Dimensional Sixteen Channel Transmit/Receive Coil Array for Cardiac MRI at 7.0 T: Design, Evaluation, and Application

To design, evaluate, and apply a 2D 16‐channel transmit/receive (TX/RX) coil array tailored for cardiac magnetic resonance imaging (MRI) at 7.0 T.

Progress and promises of human cardiac magnetic resonance at ultrahigh fields: A physics perspective

A growing number of reports eloquently speak about explorations into cardiac magnetic resonance (CMR) at ultrahigh magnetic fields (B0≥7.0 T). Realizing the progress, promises and challenges of ultrahigh field (UHF) CMR this perspective outlines current trends in enabling MR technology tailored for cardiac MR in the short wavelength regime. For this purpose many channel radiofrequency (RF) technology concepts are outlined. Basic principles of mapping and shimming of transmission fields including RF power deposition considerations are presented. Explorations motivated by the safe operation of UHF-CMR even in the presence of conductive implants are described together with the physics, numerical simulations and experiments, all of which detailing antenna effects and RF heating induced by intracoronary stents at 7.0 T. Early applications of CMR at 7.0 T and their clinical implications for explorations into cardiovascular diseases are explored including assessment of cardiac function, myocardial tissue characterization, MR angiography of large and small vessels as well as heteronuclear MR of the heart and the skin. A concluding section ventures a glance beyond the horizon and explores future directions. The goal here is not to be comprehensive but to inspire the biomedical and diagnostic imaging communities to throw further weight behind the solution of the many remaining unsolved problems and technical obstacles of UHF-CMR with the goal to transfer MR physics driven methodological advancements into extra clinical value.

High spatial resolution and temporally resolved T2* mapping of normal human myocardium at 7.0 Tesla: an ultrahigh field magnetic resonance feasibility study.

Myocardial tissue characterization using T2 * relaxation mapping techniques is an emerging application of (pre)clinical cardiovascular magnetic resonance imaging. The increase in microscopic susceptibility at higher magnetic field strengths renders myocardial T2 * mapping at ultrahigh magnetic fields conceptually appealing. This work demonstrates the feasibility of myocardial T2 * imaging at 7.0 T and examines the applicability of temporally-resolved and high spatial resolution myocardial T2 * mapping. In phantom experiments single cardiac phase and dynamic (CINE) gradient echo imaging techniques provided similar T2 * maps. In vivo studies showed that the peak-to-peak B0 difference following volume selective shimming was reduced to approximately 80 Hz for the four chamber view and mid-ventricular short axis view of the heart and to 65 Hz for the left ventricle. No severe susceptibility artifacts were detected in the septum and in the lateral wall for T2 * weighting ranging from TE = 2.04 ms to TE = 10.2 ms. For TE >7 ms, a susceptibility weighting induced signal void was observed within the anterior and inferior myocardial segments. The longest T2 * values were found for anterior (T2 * = 14.0 ms), anteroseptal (T2 * = 17.2 ms) and inferoseptal (T2 * = 16.5 ms) myocardial segments. Shorter T2 * values were observed for inferior (T2 * = 10.6 ms) and inferolateral (T2 * = 11.4 ms) segments. A significant difference (p = 0.002) in T2 * values was observed between end-diastole and end-systole with T2 * changes of up to approximately 27% over the cardiac cycle which were pronounced in the septum. To conclude, these results underscore the challenges of myocardial T2 * mapping at 7.0 T but demonstrate that these issues can be offset by using tailored shimming techniques and dedicated acquisition schemes.

16-channel bow tie antenna transceiver array for cardiac MR at 7.0 tesla

To design, evaluate, and apply a bow tie antenna transceiver radiofrequency (RF) coil array tailored for cardiac MRI at 7.0 Tesla (T).

Assessment of the right ventricle with cardiovascular magnetic resonance at 7 Tesla

BackgroundFunctional and morphologic assessment of the right ventricle (RV) is of clinical importance. Cardiovascular magnetic resonance (CMR) at 1.5T has become gold standard for RV chamber quantification and assessment of even small wall motion abnormalities, but tissue analysis is still hampered by limited spatial resolution. CMR at 7T promises increased resolution, but is technically challenging. We examined the feasibility of cine imaging at 7T to assess the RV.MethodsNine healthy volunteers underwent CMR at 7T using a 16-element TX/RX coil and acoustic cardiac gating. 1.5T served as gold standard. At 1.5T, steady-state free-precession (SSFP) cine imaging with voxel size (1.2x1.2x6) mm3 was used; at 7T, fast gradient echo (FGRE) with voxel size (1.2x1.2x6) mm3 and (1.3x1.3x4) mm3 were applied. RV dimensions (RVEDV, RVESV), RV mass (RVM) and RV function (RVEF) were quantified in transverse slices. Overall image quality, image contrast and image homogeneity were assessed in transverse and sagittal views.ResultsAll scans provided diagnostic image quality. Overall image quality and image contrast of transverse RV views were rated equally for SSFP at 1.5T and FGRE at 7T with voxel size (1.3x1.3x4)mm3. FGRE at 7T provided significantly lower image homogeneity compared to SSFP at 1.5T. RVEDV, RVESV, RVEF and RVM did not differ significantly and agreed close between SSFP at 1.5T and FGRE at 7T (p=0.5850; p=0.5462; p=0.2789; p=0.0743). FGRE at 7T with voxel size (1.3x1.3x4) mm3 tended to overestimate RV volumes compared to SSFP at 1.5T (mean difference of RVEDV 8.2±9.3ml) and to FGRE at 7T with voxel size (1.2x1.2x6) mm3 (mean difference of RVEDV 9.3±8.6ml).ConclusionsFGRE cine imaging of the RV at 7T was feasible and provided good image quality. RV dimensions and function were comparable to SSFP at 1.5T as gold standard.

Myocardial T(2) (*) mapping free of distortion using susceptibility-weighted fast spin-echo imaging: A feasibility study at 1.5 T and 3.0 T

This study demonstrates the feasibility of applying free‐breathing, cardiac‐gated, susceptibility‐weighted fast spin‐echo imaging together with black blood preparation and navigator‐gated respiratory motion compensation for anatomically accurate T  2* mapping of the heart. First, T  2* maps are presented for oil phantoms without and with respiratory motion emulation (T  2* = (22.1 ± 1.7) ms at 1.5 T and T  2* = (22.65 ± 0.89) ms at 3.0 T). T  2* relaxometry of a ferrofluid revealed relaxivities of R  2* = (477.9 ± 17) mM−1s−1 and R  2* = (449.6 ± 13) mM−1s−1 for UFLARE and multiecho gradient‐echo imaging at 1.5 T. For inferoseptal myocardial regions mean T  2* values of 29.9 ± 6.6 ms (1.5 T) and 22.3 ± 4.8 ms (3.0 T) were estimated. For posterior myocardial areas close to the vena cava T  2* ‐values of 24.0 ± 6.4 ms (1.5 T) and 15.4 ± 1.8 ms (3.0 T) were observed. The merits and limitations of the proposed approach are discussed and its implications for cardiac and vascular T  2* ‐mapping are considered. Magn Reson Med, 2009.

Ophthalmic Magnetic Resonance Imaging at 7 T Using a 6-Channel Transceiver Radiofrequency Coil Array in Healthy Subjects and Patients With Intraocular Masses

ObjectivesThis study was designed to examine the feasibility of ophthalmic magnetic resonance imaging (MRI) at 7 T using a local 6-channel transmit/receive radiofrequency (RF) coil array in healthy volunteers and patients with intraocular masses. Materials and MethodsA novel 6-element transceiver RF coil array that makes uses of loop elements and that is customized for eye imaging at 7 T is proposed. Considerations influencing the RF coil design and the characteristics of the proposed RF coil array are presented. Numerical electromagnetic field simulations were conducted to enhance the RF coil characteristics. Specific absorption rate simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Phantom experiments were carried out to validate the electromagnetic field simulations and to assess the real performance of the proposed transceiver array. Certified approval for clinical studies was provided by a local notified body before the in vivo studies. The suitability of the RF coil to image the human eye, optical nerve, and orbit was examined in an in vivo feasibility study including (a) 3-dimensional (3D) gradient echo (GRE) imaging, (b) inversion recovery 3D GRE imaging, and (c) 2D T2-weighted fast spin-echo imaging. For this purpose, healthy adult volunteers (n = 17; mean age, 34 ± 11 years) and patients with intraocular masses (uveal melanoma, n = 5; mean age, 57 ± 6 years) were investigated. ResultsAll subjects tolerated all examinations well with no relevant adverse events. The 6-channel coil array supports high-resolution 3D GRE imaging with a spatial resolution as good as 0.2 × 0.2 × 1.0 mm3, which facilitates the depiction of anatomical details of the eye. Rather, uniform signal intensity across the eye was found. A mean signal-to-noise ratio of approximately 35 was found for the lens, whereas the vitreous humor showed a signal-to-noise ratio of approximately 30. The lens-vitreous humor contrast-to-noise ratio was 8, which allows good differentiation between the lens and the vitreous compartment. Inversion recovery prepared 3D GRE imaging using a spatial resolution of 0.4 × 0.4 × 1.0 mm3 was found to be feasible. T2-weighted 2D fast spin-echo imaging with the proposed RF coil afforded a spatial resolution of 0.25 × 0.25 × 0.7 mm3. ConclusionsThis work provides valuable information on the feasibility of ophthalmic MRI at 7 T using a dedicated 6-channel transceiver coil array that supports the acquisition of high-contrast, high–spatial resolution images in healthy volunteers and patients with intraocular masses. The results underscore the challenges of ocular imaging at 7 T and demonstrate that these issues can be offset by using tailored RF coil hardware. The benefits of such improvements would be in positive alignment with explorations that are designed to examine the potential of MRI for the assessment of spatial arrangements of the eye segments and their masses with the ultimate goal to provide imaging means for guiding treatment decisions in ophthalmological diseases.