Parameters of the Magnetic Field

As described in Section 2.3.1.1, acquiring signals at defined multiple locations over a thoracic area permits the reconstruction of cardiac MFM. These maps can be characterized by different measures, and some have been shown to be helpful in separating healthy subjects from patients with IHD, not only under stress but also in a resting condition.

MCG reveals independent, complementary information compared to body surface ECG (Lant et al., 1990; Hanninen et al., 2001). The magnetic field has been analyzed in various studies during the course of the cardiac cycle using different parameters for the quantification of MFM characteristics. One of the most common parameters used has been the MFM orientation. The rationale behind this choice is that ischemia-induced changes in the distribution of magnetic field strength over the precordial thorax will lead to a reorientation of the MFM. The concept has led to various approaches in its quantification in which the spatial distribution of the positive and negative field values, the maximum MFM gradient, or the characteristics of a calculated ECD have been used.

In the first approach, orientation is determined on the basis of the relative positions of two distinct points: one representing the positive field components; the other the negative. These points can be established on the basis of the positive and negative centers of gravity which take both the positions and magnetic field strength values at the registration sites into account. This approach has been examined by Hailer, Van Leeuwen and coworkers, who first attempted to discriminate between healthy subjects and patients who had suffered an acute MI. The course of MFM orientation during the QRS complex was very similar in healthy subjects, whereas those of all MI patients deviated from this course, notably at the R peak (Hailer et al., 2000). In a further study which included 42 CAD patients with and without prior MI, as well as 20 healthy subjects, the time course of orientation was examined over the QT interval (Van Leeuwen et al., 1999a). With a specificity of 90% for the control group, MI patients could be identified with a sensitivity of 85%, and CAD patients without prior MI and with normal resting ECG with a sensitivity of 68%. By examining the constituent X- and Y-axis components during the T wave in the same subject groups, no single component was found to be primarily responsible for the discriminatory power of orientation (Cremer et al., 1999). In further studies examining the influence of the prethoracic area of coverage, a higher sensitivity for CAD could be obtained by positioning sensors symmetrically over a precordial region than by simply including more registration sites which lie distally in the caudal and right-lateral directions (Van Leeuwen et al., 2003c). By applying this approach to a group of 40 patients with suspected CAD, it was found that MFM orientation deviated more clearly from control values in those patients in whom CAD was confirmed on the basis of coronary angiography (Van Leeuwen et al., 2004).

Analogous to the determination of MFM orientation on the basis of the positive and negative centers of gravity, the position of maximum and minimum field strength may be used. This approach, used by Park and Jung (2004) and Chen et al. (2004), registers discontinuities in the course of orientation resulting from the presence of relative extrema associated with MFM nondipolarity. When examining 86 patients with suspected CAD under resting conditions, Park and Jung found that the 53 patients with angiographically confirmed CAD and elevated troponin levels demonstrated greater variation of MFM orientation during the ascending part of the T wave. By comparing 11 patients with ischemia to 51 healthy controls, Chen and coworkers showed that, in the interval up to T-wave peak, the post-exercise stability of MFM orientation was greater in the healthy subjects.

Spatial gradients may be measured or calculated at any point of the MFM, and this forms the basis of an alternative definition of field orientation, namely the direction of the maximum spatial gradient of the magnetic field. This approach has been studied by Hanninen, Takala and coworkers, who examined orientation in the ST segment and at T apex. Analogous to the development of the ST segment depression and T-wave inversion in ECG during exercise as a parameter for myocardial ischemia, these authors determined MFM orientation at rest and post exercise in 17 healthy subjects and in 27 patients with single-vessel disease (Hanninen et al., 2000). They were able to show alterations in the orientation of the field gradient both during the ST segment and at T-wave apex after exercise in the CAD group compared to the control group, whereby changes in ST segment orientation were most profound immediately after cessation of stress, but those in the T wave occurred later. In a second study, the same group included 44 CAD patients with and without MI, and compared the results of various MCG-determined ST segment and T-wave parameters (including orientation) to results obtained in 26 healthy controls (Hanninen et al., 2002) (see also Section 2.3.6.1). The magnetic field orientation at ST segment was found to perform equally well as the other ST parameters, and this parameter helped to explain statistically the presence of ischemia. The application of the gradient method was taken a step further by examining the heart rate dependency of orientation. In a study including 24 patients with single-vessel disease and 17 healthy controls, orientation was determined continuously for 10 minutes after cessation of exercise, and its relationship to heart rate investigated (Takala et al., 2002) (Fig. 2.40). On the basis of regression analysis, it was found that heart rate-adjusted MFM rotation over the ST segment and at T-wave apex improved the detection of ischemia caused by >75% stenosis of the coronary vessels.

Using the same approach, Fenici et al. (2002a) compared orientation at rest in 10 healthy subjects and 10 patients with single- or multivessel disease, collecting the data in an unshielded setting. An abnormal orientation was found in the patient group, particularly during the ST segment.

It is also possible to assess the orientation of cardiac electric activity on the basis of estimating the source parameters (this is described in Section 2.2.6.4). Overall, there appears to be a general agreement that ischemia may be documented by MFM orientation, though direct comparison or the pooling of the results of various studies is difficult due to differences in the approaches used.

Other aspects of the cardiac magnetic field investigated in the context of ischemia include the assessment of field strength and nondipolar content. For example, Tsukada et al. (2000) reported a reduction in both de- and repolarization field strength, based on isointegral QRS and ST-T values, in CAD patients with multi-vessel disease with and without MI compared to healthy subjects. Assessment of the reduction in the coherence of MFM structure is also possible on the basis of

Fig. 2.40. Orientation of the T-wave magnetic field map of successive heart beats at rest and during the recovery from exercise plotted against time. (a) Healthy subject; (b) patient with coronary artery disease (CAD). At rest, value. After exercise, the MFM angle is tilted more in the patient than in the control, and in the control the angle returns faster to the baseline value. The step between two isofield lines in the MFMs shown is 2 pT. (Adapted

Fig. 2.40. Orientation of the T-wave magnetic field map of successive heart beats at rest and during the recovery from exercise plotted against time. (a) Healthy subject; (b) patient with coronary artery disease (CAD). At rest, value. After exercise, the MFM angle is tilted more in the patient than in the control, and in the control the angle returns faster to the baseline value. The step between two isofield lines in the MFMs shown is 2 pT. (Adapted the MFM angle has an approximately constant from Takala et al., 2002, with permission).

the examination of the dipolar and nondipolar content of the maps. The latter may be calculated using the Karhunen-Loeve transformation (KLT). In a study of 15 CAD patients with and without relevant stenosis of the coronary arteries, KLT was used to show that, under pharmacologically induced stress, the patient subgroups could be distinguished on the basis on the nondipolar content of their QRST isointegral maps (Van Leeuwen et al., 1999b). Furthermore, using KLT analysis Stroink et al. (1999) showed that the nondipolar content of MFM was significantly higher in a group of 30 post-MI patients with and without VT compared to a cohort of 76 healthy subjects.

Yamada and coworkers analyzed the tangential components of the cardiac magnetic field in 10 patients who had reversible myocardial ischemia and 10 patients with old MI, documented by scintigraphy, as well as in 10 healthy subjects. The integral values of the tangential components during QRS and JT intervals were calculated. In particular, their quotient showed clearly reduced values not only for the old MI patients but also in the patients with reversible ischemia when compared to the values for healthy subjects (Yamada et al., 2001).

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