Tobias Frauenrath

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.

Design and application of a four-channel transmit/receive surface coil for functional cardiac imaging at 7T.

To design and evaluate a four‐channel cardiac transceiver coil array for functional cardiac imaging at 7T.

Acoustic method for synchronization of Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging (MRI) of moving organs requires synchronization with physiological motion or flow, which dictate the viable window for data acquisition. To meet this challenge, this study proposes an acoustic gating device (ACG) that employs acquisition and processing of acoustic signals for synchronization while providing MRI compatibility, immunity to interferences with electro-magnetic and acoustic fields and suitability for MRI at high magnetic field strengths. The applicability and robustness of the acoustic gating approach is examined in a pilot study, where it substitutes conventional ECG-gating for cardiovascular MR. The merits and limitations of the ACG approach are discussed. Implications for MR imaging in the presence of physiological motion are considered including synchronization with other structure- or motion borne sounds.

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.

Detailing the use of magnetohydrodynamic effects for synchronization of MRI with the cardiac cycle: A feasibility study

To investigate the feasibility of using magnetohydrodynamic (MHD) effects for synchronization of magnetic resonance imaging (MRI) with the cardiac cycle.

Electrocardiogram in an MRI Environment: Clinical Needs, Practical Considerations, Safety Implications, Technical Solutions and Future Directions

Magnetic Resonance Imaging (MRI) has been listed as the single most important medical innovation, on par with CT scanning (Fuchs & Sox 2001). In current clinical MRI ECG is being using for three major purposes. Firstly, heart motion, blood flow and blood pulsation are commonly dealt with using electrocardiogram (ECG) for synchronization of MR data acquisition with the cardiac cycle (Lanzer et.al. 1984) to address or compensate for cardiac activity related motion artifacts which is of paramount importance for an ever growing portfolio of cardiovascular MR (CMR) and neurovascular MR (NVMR) applications (Assomull et.al. 2007, Kelle et.al. 2008, Kramer et.al. 2008, Kwong & Korlakunta 2008, Niendorf et.al. 2006, Niendorf & Sodickson 2008, Niendorf & Sodickson 2006, Niendorf et.al. 2010, Pennell et.al. 2004, Schwitter 2008). Secondly, ECG is widely used to simultaneously register cardiac activity with MRI; for example to eliminate physiological fluctuations from brain activation maps derived from functional MRI studies (Purdon & Weisskoff 1998). Thirdly, there are an increasing number of clinical applications that require ECG monitoring prior to/after the MR examination while the patient is still in the MR environment but outside of the MR scanners bore using ECG devices as a patient emergency indicator. ECG waveform acquisitions, ECG co-registration and ECG monitoring during MRI pose technical challenges and requires safety measures that will not be familiar to users of other conventional ECG technologies. For all those reasons, the basic principles of using ECG in an MRI environment and their implications for clinical MRI and MRI research are provided in this chapter. Key concepts, technical solutions, practical considerations and safety implications for cardiac gated MRI using electrocardiograms are outlined. Unsolved technical problems and unmet clinical needs are also considered carefully, in an attempt to stimulate the community to throw further weight behind the solutions of remaining issues. Driven by the limitations and motivated by the challenges of ECG, the need for novel cardiac gating/triggering technology is discussed. Current trends, such as the trend towards wireless techniques and the move to acoustic cardiac gating techniques, and their

A system for parallel measurement of glottis opening and larynx position

Abstract The simultaneous assessment of glottal dynamics and larynx position can be beneficial for the diagnosis of disordered voice or speech production and swallowing. Up to now, methods either concentrate on assessment of the glottis opening using optical, acoustical or electrical (electroglottography, EGG) methods, or on visualisation of the larynx position using ultrasound, computer tomography or magnetic resonance imaging techniques. The method presented here makes use of a time-multiplex measurement approach of space-resolved transfer impedances through the larynx. The fast sequence of measurements allows a quasi simultaneous assessment of both larynx position and EGG signal using up to 32 transmit–receive signal paths. The system assesses the dynamic opening status of the glottis as well as the vertical and back/forward motion of the larynx. Two electrode-arrays are used for the measurement of the electrical transfer impedance through the neck in different directions. From the acquired data the global and individual conductivity is calculated as well as a 2D point spatial representation of the minimum impedance. The position information is shown together with classical EGG signals allowing a synchronous visual assessment of glottal area and larynx position. A first application to singing voice analysis is presented that indicate a high potential of the method for use as a non-invasive tool in the diagnosis of voice, speech, and swallowing disorders.