RF-Interference with Other Devices:

ECG within MR Environment ++ Thoralf Niendorf1,2

1) Berlin Ultrahigh Field Facility (B.U.F.F.), Max Delbrueck Center for Molecular Medicine, Berlin, Germany

2) Experimental and Clinical Research Center (ECRC), Charité Campus Buch, Humboldt-University, Berlin, Germany

Magnetic Resonance Imaging (MRI) of moving organs requires speed and efficiency

due to physiological motion and flow constraints, which dictate the viable window for

data acquisition. To meet this challenge, various motion compensation techniques

have been proposed to increase immunity to respiratory motion. Blood flow, blood

pulsation, and cardiac motion have been addressed by synchronization strategies

exploiting (i) finger plethysmography [1], (ii) cardiac activity related esophageal wall

motion[2], (iii) invasive left ventricular blood pressure gating[3], (iv) Doppler

ultrasound[4], (v) motion induced changes in the impedance match of RF-coils[5],

(vi) self gating techniques [6-10] (vii) optic acoustic methods [11] and (viii) pulse

oximetry [12].

In current clinical MR practice, cardiac motion is commonly dealt with using

electrocardiographic (ECG) gating techniques [13-15] to synchronize data

acquisition with the cardiac cycle. For this purpose scanners are equipped with extra

hardware for ECG signal detection and processing including: (i) ECG electrodes, (ii)

leads or wireless connections used for ECG signal transfer, (iii) a physiological

monitoring unit for signal processing and TTL trigger generation and (iv) a coupler into

the scanners internal circuitry as illustrated in Figure 1.

ECG, being an inherently electrical measurement with electrically active

components [13], does carry a risk of surface heating of patients’ skin and even of

skin burns resulting from induction of high voltages in ECG electrodes or ECG cables

due to interactions with RF signals used in MRI [16-20]. Various technologies have

been implemented on clinical scanners to safeguard patients with the ultimate goal

of avoiding disasters and injuries due to ECG hardware. These measures involve the

use of (i) ECG electrodes being classified as MR-safe, (ii) ECG leads shorter than the

RF wave length and (iii) high impedance leads, fiber optic leads or wireless

connections for signal transfer. Consequently, user manuals of clinical scanners

outline explicitly that MR-safe electrodes which are made available through the MR

vendor's accessories catalogue must be used. Also, the manufacturer's user manuals

Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)

for RF coils advice to use extra padding for keeping RF coils in a safe distance from

the chest. This measure has been implemented to avoid ECG electrodes being

positioned in areas close to local signal absorption rate (SAR) hot spots caused by the

RF coil's EM fields.

Although ECG is known to be non-diagnostic within the bore of any clinical MR

system due to magneto-hydrodynamic (MHD) effects, there are an increasing

number of indications that require ECG monitoring prior/after to the MR scan or in the

scanner room using conventional 12 leads ECG devices as a patient emergency

indicator. For example, in addition to continuous monitoring during stress testing, ECG

monitoring should resume as quickly as possible after poststress imaging, ideally while

the patient is still on the MRI table [21]. Unfortunately, these clinical needs bear the risk

of leading to RF induced skin burns since patients might undergo an MRI scan without

removing conventional ECG electrodes commonly used in the telemetry unit.

Conventional ECG electrodes are not classified as being MR-safe due to the use of

conductive gels or ferromagnetic components. The FDA's MAUDE data base reports

several skin burns in the last 5 years due to induction of high voltages in ECG

hardware due to interaction with RF signals [20]. The use of non MR safe ECG

hardware has even caused an incident in the MR bore, where high-voltage induction

in ECG wiring caused a fire[16].

ECG is corrupted by interference with electromagnetic fields and by magneto-

hydrodynamic effects as illustrated in Figure 2. Consequently, artifacts in the ECG

trace and T-wave elevation might be mis-interpreted as R-waves, resulting in

erroneous triggering together with motion corrupted image quality. As (ultra)high field

MR becomes more widespread, the propensity of ECG recordings to interference

from electromagnetic fields and to magneto-hydrodynamic effects is further

pronounced [22-25] as shown in Figure 2. Commonly, signal filtering is applied to

improve the immunity to distortions in the ECG trace. This approach can be

supplemented by advanced vector cardiogram techniques which are designed to

enhance R-wave detection and to alleviate mis-synchronization[14-15]. Despite these

improvements, ECG-gating still needs extra setup time for placing ECG electrodes

and for managing connections, cabling and other auxiliary hardware.

Realizing the constraints of conventional ECG, a MR-stethoscope which uses the

phonocardiogram has been proposed for the pursuit of robust and safe clinical

cardiac gated/triggered MR [25-26]. As shown by the block diagram in Figure 1, the

acoustic gating device comprises three main components: (i) an acoustic sensor

which can be a pressure transducer or an optical microphone for example, (ii) an

acoustic wave guide or a fiber optic lead for signal transfer (iii) a signal processing

unit and (iv) a coupler unit to the MRI system [26]. The acoustic triggering (ACT)

Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)

approach offers suitability for all magnetic field strengths as demonstrated in Figure 2

[22, 27]. ACT presents immunity to electromagnetic interference and magneto-

hydrodynamic effects [22, 27] as illustrated in Figure 3. ACT provides ease of clinical

use, a trigger reliability superior to that of ECG [22, 27-28] as demonstrated in Figure 3

and no risk of high voltage induction and patient burns, which all have practical,

patient comfort and safety implications. Also, acoustical MR synchronization

substantially reduces the complexity of patient preparation by obviating the need to

set up ECG-electrodes and position ECG-leads, and hence may serve to streamline

cardiac gated clinical MR. Acoustic cardiac triggering is galvanically decoupled

from the patient due to the use of an acoustic sensor and an acoustic wave guide,

both being non-electronic components. Acoustic cardiac gating/triggering was

found to meet the demands of several MRI-applications, including breath-hold and

free-breathing acquisition strategies together with prospective and retrospective

triggering regimes [22, 27-28]. The efficacy and reliability of acoustic cardiac gating

has been demonstrated by eliminating the frequently-encountered difficulty of mis-

triggering due to ECG-waveform distortions [22, 27-28] as demonstrated in Figure 4,

which are pronounced at high- and ultra-high magnetic field strengths.

Contrary to the common notion that considers magneto-hydrodynamic effects to be

adverse concomitants of traditional ECG acquired in a magnetic field environment

[21, 29] MHD artifacts can be turned into merits by using MHD effects for

synchronization of MR acquisitions with the cardiac cycle as illustrated in Figure 5. The

magneto-hydrodynamic effect being inherently sensitive to blood flow and blood

velocity provides an alternative approach for cardiac gating, even in peripheral

target areas far away from the commonly used upper torso positions of ECG

electrodes. This feature would be very beneficial to address traveling time induced

motion artifacts and trigger latency related issues raised by ECG-gated peripheral MR

angiography. The applicability of MHD triggering is demonstrated in Figure 6 using

cardiac gated, non-contrast MR angiography (MRA) of the common carotids. The

proposed MHD gating/triggering approach does not require any changes to the MR

system's hardware or software since it connects the trigger signal to the MR-scanner's

standard ECG input.

Another emerging technology which holds the potential to substitute traditional ECG

gating/triggering is ultra wideband (UWB) RF radar which allows for non-contact

detection of myocardium's mechanical activity [30-31]. Since no ionizing radiation is

used, and due to the ultralow specific absorption rate applied, UWB techniques

permit noninvasive sensing of cardiac activity with no potential risks. A demonstrator

was established to prove the feasibility of the simultaneous acquisition of

physiological events by MRI and UWB radar. First in vivo experiments showed

Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)

correlations between the reconstructed UWB signals with physiological signatures

acquired by simultaneous MR measurements, representing respiratory and

myocardial displacements and hence provided encouraging results [30-31].

In summary, the basic principles of RF interaction with ECG hardware and their

implications for MR research and clinical MRI are provided in this presentation. Key

concepts, technical solutions, practical considerations and safety implications for

cardiac gated MRI using conventional ECG are outlined. Unsolved problems and

unmet 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 conventional 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 implications for daily routine MR applications are surveyed. Furthermore,

demonstrable progress in gating/triggering technology and methodology is shown to

provide further encouragement for the imaging community to tackle solutions of the

outstanding issues. A concluding section of the presentation explores future directions

fueled by a set of alternative gating/triggering techniques.

Fig. 1: Block diagram of left) conventional ECG gating and right) acoustic cardiac triggering (ACT). The signal flow is from the left (signal collection using ECG-electrodes or an acoustic sensor), through the middle (signal processing), to the right (input to MR-systems internal ECG circuitry). For the acoustic approach galvanic decoupling between the patient and the signal conditioning/conversion electronic is accomplished using an acoustic wave guide. For cardiac triggering, the ECG-leads or ACT system are connected to the MR scanner’s standard ECG signal input.

Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)

Fig. 2: Electrocardiogram raw data (ECG), vector cardiogram data (VCG), raw data of the phonocardiogram and the output of the acoustic gating device (ACG) generated from the phonocardiogram obtained at 1.5 T (top), 3.0 T (middle) and 7.0 T (bottom). ECG and VCG waveforms were susceptible to T-wave elevation and other waveform distortions (marked in gray) due to magneto-hydrodynamic effects which were pronounced at 3.0 T and 7.0 T. In comparison, the MR-stethoscope provided raw data of the phonocardiogram and ACG-traces at 1.5 T, 3.0 T and 7.0 T free of interference from electromagnetic fields or magneto-hydrodynamic effects. Warning: Neither, the ECG nor the acoustic waveform are patient emergency indicator.

Fig. 3: Normalized signal traces of the cardiac activity obtained for electrocardiographic (left) and for acoustic (right) measurements at 7.0 T. Note the severe magneto-hydrodynamic effects in the ST segment of the electrocardiograms obtained at 7.0 T. In this example, trigger detection was found to be scattered across several cardiac phases including early systole and diastole as demonstrated by the tick marks. ACT is free of interferences with electromagnetic fields and magneto-hydrodynamic effects. ACT triggered 2D CINE FLASH imaging provided faultless trigger detection , accurate to the peak induced by the 1st heart tone.

Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)

Fig. 4: Four chamber long axis views of the heart obtained at 7.0 T using 2D CINE acquisitions in conjunction with retrospective left) electrographic (ECG) and right) acoustic (ACT) cardiac triggering. In this example, ECG-gated 2D CINE FLASH imaging was prone to cardiac motion artifacts caused by R-wave mis-registration due to T-wave elevation. ECG mis-triggering induced reduction in myocardium/blood contrast and degradation of image sharpness. ACG triggered 2D CINE FLASH imaging provided faultless trigger recognition and hence was immune to the effects of cardiac motion.

Fig. 5: left) Basic scheme of positioning of electrodes used for monitoring magneto-hydrodynamic (MHD) effects due to blood flow in the left carotid artery. right) MHD trace (blue) and trigger detection moments (red) derived from skin surface

Proc. Intl. Soc. Mag. Reson. Med. 20 (2012)

electrodes positioned close to the common carotid artery at 7.0 T using the 3 leads approach.

Fig. 6: Sagittal views of arterial MR angiographies derived from ECG (left) and MHD- (right) triggered acquisitions at 3.0 T using a non-contrast, SSFP based imaging technique. Overall mage quality and richness of anatomic detail of the direct volume renderings obtained from MHD triggered acquisitions compares very well to those derived from ECG triggered acquisitions. References: 1. Langkowski JH, Heller M, Maas R, et al. (1987) Pulse triggering in magnetic

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