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Hospital Healthcare Europe

Magnetic field imaging (MFI) and its application in cardiology

Stephen Taylor
1 January, 2008  

Stephen Taylor
Editorial Director
Hospital Healthcare Europe

At the University Clinic of Jena, Germany, a novel diagnostic system has just been installed in the clinical environment. Magnetic field imaging (MFI) addresses two of the main concerns in cardiology: risk stratification before implantation of an implantable cardioverter defibrillator (ICD); and the early detection of stress-induced ischaemia. In both cases, MFI can deliver results earlier, faster and more efficiently than the classic electrophysiological and imaging methods.

In Europe every year more than 300,000 people die from sudden cardiac death (SCD). Many of these deaths could have been prevented if these people had received appropriate treatment before, for example with an ICD. On the other hand, a total of 80–90% of ICDs are implanted into patients that do not benefit from this intervention. Furthermore, each year approximately 750,000 heart attacks occur in Europe and lead to more than 210,000 deaths. Any improvement in diagnosis and early detection is of major interest for cardiologists and patients.

Physical background of magnetic field imaging
MFI is a highly innovative and noninvasive cardiac diagnostic tool that does not involve any other type of energy and does not require any contrast agent. MFI records, displays and analyses the changes in the magnetic field generated by the heart itself during all phases of the heartbeat. From this high-speed, high-resolution recording the electrophysiological functionality of the heartbeat is displayed in different modalities. In this way pathologic changes of the cardiac muscle function in terms of both excitation and activation can be visualised.

The general principle underpinning MFI is based on two laws of nature:

  • Cell activity in the human body is connected to electric activity (Galvani, Italy; 1780).
  • Any electric current is associated with a corresponding magnetic field (Ørsted, Denmark; 1820).

The recording and interpretation of the electric signals is well known from the ECG, but the magnetic signals, acquired, processed and displayed with a modern MFI system, contain very important additional information that cannot be found in a surface ECG. It is only since the 1980s that magnetic signals could be recorded in sufficient quality using superconducting quantum interference devices (SQUIDs), the most sensitive magnetic sensors in the world. As the magnetic signals of the heart are approximately one million times weaker than the earth’s magnetic field, high-end acquisition electronics and noise reduction concepts are necessary to provide reliable and reproducible data.

In comparison with the electrical signals, which are influenced by the differently conductive tissues of the body and varying resistance of the skin, magnetic signals travel through the body almost without disturbance and are recorded contact-free. Furthermore, the magnetic signals of the so-called vortex currents, which occur regularly in every heartbeat, can be measured with an MFI system, but they cannot be recorded electrically on the body surface. These heart internal vortex currents include important information for an advanced and more accurate cardiac diagnosis.

With regard to ischaemia diagnosis, MFI can also detect and visualise the electrophysiologic effects of microcirculation disturbances in the cardiac muscle and possibly lead to a focused, early-stage treatment.

Innovative contact-free data acquisition and highly sophisticated software for safe and fast diagnosis
MFI diagnosis provides many advantages: MFI is fast, contact-free, radiation-free, noninvasive, it does not require any external magnetic field, and no other external energy sources (to be absorbed or reflected by the patient) and no contrast agents are necessary. The data recording of a rest MFI takes only two to three minutes, is purely passive and absolutely harmless for the patient. The special data acquisition and patient handling software makes the procedure very easy for the nurse performing it. The analysis system of the MFI system, which can be used independently from the acquisition system, provides results in less than a minute. Depending on the indication, the data can be displayed and analysed in various modalities – for example, in specific curve functions or as 3D animations. This simplifies the use and the interpretation of the results, saving time and resources.

The system installed in the University Clinic of Jena, Germany, is an Apollo CXS, manufactured by BMDSys GmbH. This system is equipped with a magnetic sensor array consisting of 55 gradiometric SQUID sensors in the measurement plane plus an additional eight sensors in a second and third plane for noise compensation. As already underlined, SQUIDs are the most sensitive magnetic sensors available today. The sensor array of the Apollo CXS has a diameter of 28 cm and covers the whole relevant area of the chest in one pass, acquiring the data with a time resolution of less than a microsecond.

The preconditions for the establishment of MFI in the clinical routine are good. For example, the Apollo CXS has an independent acquisition unit that can be installed as a self-contained unit outside the hospital, if there is not sufficient or appropriate room available inside the hospital. The footprint of the acquisition unit is less than 20 m². This system also is developed with the clinical routine in mind. According to Torsten Kruemmel, CEO of BMDSys GmbH: “Our system can be operated by a nurse while the acquired data can be made available to multiple cardiologists that can work in parallel on the analysis of different patient data. This way an optimised patient flow and a high efficiency of the investment can be achieved.”

From an economic standpoint, MFI diagnosis is very promising, as the duration of the data acquisition is very short (two minutes for a rest MFI), and more than one analysis system can be connected to the data acquisition unit – hence more than 60 patients can undergo an MFI procedure per shift. Besides the optimisation of the diagnostic portfolio of a hospital, MFI can also help to reduce costs by optimising patient management.

Study results confirm the high potential of MFI
In the 1990s, scientists from Berlin, Germany, found out that the magnetically acquired heart signal, especially the QRS complex, when filtered in a special way in patients with documented ventricular tachycardia (VT), shows a typical fragmentation pattern. It was also shown that on the basis of the magnetically acquired QRS fragmentation a stable differentiation of postmyocardial infarction patients with and without elevated risk of VT is possible. These results are part of the scientific basis on which the modules for this risk stratification are built. Looking at the output of the MFI analysis system of a patient with a high risk of VT, the magnetically acquired and accordingly analysed QRS complex shows many extrema (see Figure 1). The QRS-complex is fragmented. On the contrary, the similarly acquired and filtered QRS complex of a post-MI patient at low risk for VT shows only few extrema (see Figure 2).

[[HHE07_fig1_Ca25]]

[[HHE07_fig2_Ca25]]

In Jena the MFI RISTI study (Magnetic Field Imaging for Risk Stratification before Implantation of an ICD) has been started. The goal of this study is to increase the accuracy of the indication for the implantation of ICDs.

In a recent paper presented at the 34th International Congress on Electrocardiology and 48th International Symposium on Vectorelectrocardiology in Instanbul on stress-induced ischaemia, entitled “Stress testing in coronary artery disease by magnetic field imaging”, it was shown using a functional 3D model that there is a strong difference at the maximum of the T-wave in the pseudocurrent distribution comparing rest and stress situation in patient with ischaemia. For validation, the same measurement with the same protocol was repeated on a patient after successful treatment with a stent, showing that there was no difference detectable between rest and stress situations.

Outlook on MFI application in the clinic
MFI is now available for clinical routine use with high-impact applications in cardiology. The main fields of use are the risk stratification of VT and the detection of stress-induced ischaemia. Further possible applications are already under scientific discussion. As MFI is harmless, the procedure can be repeated without having any negative effects on the patient, which gives the cardiologist the opportunity to observe the patient’s changes in condition and make appropriate treatment decisions.

The noninvasiveness of MFI also makes it an ideal tool during pregnancy, as it can detect the cardiac signal of an unborn child (from the 4th month of pregnancy). Even in cases where the vernix caseosa impairs the electric detection of fetal heart activity, MFI is able to provide an overview of the time evolution of the electrical activity. Hence it is possible to monitor the development of the fetal heart throughout pregnancy.

MFI can be carried out with minimal distress to the patient. In preparation for an MFI recording, the patient does not need to undress, only remove any metal objects, such as watches and belts. Modern implants do not normally disturb the recording, as they are generally made of a nonmagnetic material.