The orthogonal coordinate system and the spherical image surface used in the DECARTO model for the QRS presentation and the DECARTO-like graphical presentation of the ST segment are convenient for further data processing and sufficiently flexible in adapting the decartograms to suitable graphical forms for combination with other imaging modalities. The possibilities for the combination are demonstrated by the example of the DECARTO-like presentation of ST-segment deviations compared with other graphical presentation of ST-segment deviations that can be used for the estimation of the area and extent of myocardium at risk in patients with AMI, and with other non-ECG imaging methods that can serve as reference or complementary methods.
Side‐by-Side Comparison of DECARTO-Like Presentation of ST-Segment Deviations with Other ECG-Based Graphical Methods
Several methods for the graphical presentation of the location and extent of the area of myocardium at risk have been recently reported.8,30-33 The methods use different approaches for the estimation of the location and extent of the area at risk, but because these graphical methods are using basically identical or similar graphical presentation, their results can be easily visually compared side by side.
The method described by Andersen et al,33 the ST Compass, is based on the standard 12-lead configuration of the traditional coordination system. It calculates the planar ST vectors for the frontal and horizontal planes from the ST-segment deviations of the 12-lead ECG, and the resultant planar vectors are drawn on the two perpendicular planes with hexaxial lead coordinates of the standard 12-lead ECG. These planes are displayed as circles with additional spatial and/or anatomic description. This method recommends calculating the spatial vector magnitude as a surrogate for the extent of the myocardium at risk because despite the limitation of this estimation, the ST-vector spatial magnitude corresponds better with the spatial character of the injured myocardium. Thus, this method represents a useful bridging from the evaluation of the individual ST-segment deviations of the 12-lead ECG to the vector analysis and visualization of the spatial ST-segment deviations. However, mentally visualizing the relationship of the planar vectors with the anatomic structures of the heart and the coronary artery distribution still requires a great deal of a three-dimensional mental imaging.
Strauss et al31 developed a graphical method that synthesizes a VCG from the 12-lead ECG. The ST-segment vector direction is projected on two image surfaces of the heart—the Mercator surface with a schematic average coronary artery distribution and the polar plot. This approach provides the location of the center of the area at risk with respect to the coronary artery distribution and/or to the anatomic segments of the left ventricle. This method also gives the value of the spatial ST-vector magnitude as an estimate of the extent of the area at risk; however, similarly to the previous method, the extent is not graphically displayed.
The polar image surface suitable for side-by-side comparison with single photon emission computed tomography (SPECT) images was used also by Galeotti et al,32 and the location and extent of the myocardium at risk are estimated from the similarities of the ST-segment deviation of a real patient's ECG with a database of simulated ECGs with known location and extent of ischemia.
A different approach was used by Ubachs et al.30 They estimated the location and extent of the area at risk using the 12-segment model used previously for ECG infarct quantification (QRS score). For the final graphical presentation, both Mercator and polar presentations were used and thus are comparable with polar SPECT or adapted magnetic resonance imaging (MRI) images. Figure 11–6 shows a visual side-by-side comparison of two graphical methods for displaying the ST-segment deviation—the DECARTO-like method and a method described by Ubachs et al30 showing the areas at risk in the two patients.
The methods of ST-segment visualization described here approach the estimation of the location and extent of the area at risk from different perspectives (Table 11–1). Knowing the principles and studying differences and agreement in the results of these ECG methods can contribute to understanding the roles of individual factors that influence the final ST-segment deviation in AMI. A challenge will be to move from descriptive visual comparison to analytical quantitative evaluation of agreements and disagreements between methods.
Table 11–1. Comparison of Methods Developed for the Graphical Presentation of ST-Segment Deviations ||Download (.pdf)
Table 11–1. Comparison of Methods Developed for the Graphical Presentation of ST-Segment Deviations
|Location Estimation||Extent Estimation||Image Surfacea|
|ST-DECARTO8||Orientation of the spatial ST vector (Frank lead, or Dower34 transformation of 12-lead ECG)||Aldrich score (flexible to apply other methods for the extent estimation)||Basically spherical, analytically defined, transformable into any 2D or 3D surface; possible application of any texture|
|Strauss et al31||Orientation of the spatial ST vector (Dower transformation of 12-lead ECG)||A numerical value of the spatial vector magnitude, not displayed||Mercator, polar projections; texture of coronary artery distribution|
|Andersen et al33||Orientation of two planar vectors (horizontal and frontal planes) (calculated from 12-lead ECG)||A numerical value of the spatial vector magnitude, not displayed||Two perpendicular planes of 12-lead ECG (frontal, horizontal) with schematic delineation of lead axes of the standard 12-lead ECG and basic description of directions/anatomic structure; no texture|
|Galeotti et al32||Model based35||Model based35||Polar projection, with the texture of coronary artery distribution; Mercator projection with the texture of coronary artery distribution|
|Ubachs et al30||Model based36,37||Model based36,37||Polar and Mercator projections; texture of coronary artery distribution in the Mercator projection|
Side‐by-Side Comparison with Non-ECG Methods
The non-ECG imaging methods used in cardiology include a variety of two- and three-dimensional images; however, the side-by-side comparison naturally requires the use of identical/comparable images. As was demonstrated earlier, the methods for graphical presentation of area at risk based on the ST-segment deviations tend to use polar or Mercator projections.
The polar projection is frequently used as one of the possibilities of displaying results for visual comparison with non-ECG imaging methods.8,30-32 The advantage of using the polar projection is the direct visual comparability among different imaging methods and potential possibility for superimposition of the findings. Thus, the ECG-based images can be compared with, for example, SPECT images of perfusion (Fig. 11–7), wall motion, and wall thickening; MRI wall thickening; and echocardiographic images of dyskinesis. However, this projection suffers from a considerable deformation of the basal and apical regions, which must be taken into account in the interpretation of the results.
From left to right: The side-by-side comparison of ST dipolar electrocardiotopography (DECARTO) using Mercator and polar projections and single photon emission computed tomography (SPECT) image. A. A patient with the left descending artery occlusion. B. A patient with right coronary artery occlusion. Although the Mercator projection relates the area at risk to the coronary artery distribution, the polar projection is visually comparable with the polar SPECT image showing the perfusion defect in the corresponding area.
The Mercator projection with the texture of the coronary artery distribution reduces the deformation of the polar projection and illustrates well the anatomic segments of the heart and corresponding coronary artery distribution. This projection is not generally used for displaying results of the non-ECG imaging techniques, but it can be potentially used as the image surface of choice.
The common advantage of planar projections is the ability to display the whole surface, which balances the main disadvantage, which is the deformation of the shape and size of the imaged structures.
Superimposition of the DECARTO Images onto Three-Dimensional Images
The planar image surfaces are not able to display accurately a three-dimensional structure or to capture the individual anatomy. DECARTO allows projection of the decartograms/activation areas onto the "realistic" individual surfaces rendered from other image methods used in cardiology. Such surfaces may be acquired from a variety of clinically available imaging techniques such as SPECT, echocardiography, and MRI. In relation to ECG, these cardiac imaging techniques are used mainly as reference methods for comparative studies. However, they bear basically different and potentially complementary information, including information on the structural characteristics of the cardiac source, which can but does not necessarily influence its function, or on the functional characteristics other than electrical properties.38
The transformation of ECG data into the decartograms creates a two-dimensional, image-like representation of ECG signal that is more suitable for direct comparison with imaging data. The comparison of decartograms and imaging data is done by an approximate merging of a separate coordinate system of imaging data and image sphere used in the DECARTO model, followed by the projection of decartograms onto functional and structural surfaces that were identified in imaging data. This process is called superimposition and allows effective visualization of information about the structural and functional state of the heart and its electrical characteristics derived from the VCG signal. The identification of the surfaces in imaging data is done using standard segmentation and quantification algorithms that are based on the forming of surface according to specific iso-values in data.39 The identified surfaces do not necessarily need to be attributed only to real structures of the heart (as in MRI or echocardiography); they can also describe a functional status of the heart muscle (eg, SPECT imaging).
The main output of data merging is a projection of the information about the spatial localization of a VCG vector end point defined by DECARTO onto the area of defined localization and size of the myocardium, and this can be used to study the relationship between electrogenesis and the myocardial structures and metabolism. However, due to several simplifications of the DECARTO model (eg, assumption about dipole content of the electrical field), the information presented by DECARTO does not need to correspond exactly to the real sequence of activation front in the myocardium. However, the visualization of ECG signal in the same context as cardiac imaging techniques offers an investigative and instructional modality for comparative studies of functional and structural changes of the heart.
The superposition of imaging data and DECARTO data provides the opportunity to study both the diagnostic and prognostic importance of agreements and disagreements of findings of both methods. In the interpretation of the superposition of DECARTO and imaging data, the same parameters embedded in the provided information inherent to both methods are considered. The first group of parameters, which influences the quality of the fusion process, has to do with the solution of merging spatial coordinates of ECG and the imaging data system. These parameters are related to the proper merging of the orthogonal ECG/VCG coordinate center with the center of the anatomic structure or the center of the geometric surface from the heart imaging data.
The second group of parameters is directly related to the diagnostic evaluation of data obtained from the fusion process. Clinically used diagnostic methods have a set of individual decision-making procedures that can differentiate between normal and pathologic findings. Fusion of two diagnostic methods thus increases the number of possible parameters suitable for diagnostic evaluation. In the case of a dynamic system such as the heart, the presence of time dimension in monitoring may lead to a dramatic increase of accessible data describing the functionality of the system. The heterogeneous parameters inherent to these diverse types of diagnostic techniques can be evaluated using multivariate statistics in order to identify new characteristics that could be useful, especially in treatment of normal and borderline pathologic findings.
The superimposition of the decartogram on the rendered three-dimensional surface positions the activated area on the anatomic structure given by the used imaging method (eg, MRI, SPECT, or coronarography). The superimposition of the QRS decartograms or ST-segment deviation decartograms can visualize, for example, the relationship between the flow in particular coronary arteries and estimated location of MI or area at risk, by superimposing the activated areas/decartograms on a three-dimensional image of the coronary artery distribution.
In the interpretation of the final superimposed images, the following key principles must be considered:
- The principles of the imaging methods (ie, what information is provided by the particular method)
- How to treat the three-dimensional images that are rendered in the case of a fuzzy border (eg, in the case of low signal as demonstrated in a case of a perfusion defect in SPECT; Fig. 11–8)
- What anatomic structure is visualized (eg, the left ventricle or the whole heart)
- How to visualize a structure located intramurally with respect to the surface (eg, a case of a scar in an MRI image that is located intramurally while the DECARTO image is projected on the surface of the heart)
A. The surface representation of the heart rendered from the 2D SPECT image cross-sections. B. The spherical image surface with a decartogram is projected on the rendered surface. In this way, the decartogram is superimposed on the rendered myocardial surface displaying the relation between two functional characteristics of the heart—electrical (DECARTO) and perfusion (SPECT).
The side-by-side comparison of different imaging methods and the superimposition of methods providing different primary information on the structure and function of the heart can thus complement the view of the pathologic process/disease. Imaging ECG signals will contribute to the process of moving from using the ECG as a surrogate to understand better the processes behind particular ECG changes/patterns and their clinical or prognostic relevance.
The superimposition of the decartogram on the SPECT three-dimensional image is an example of a graphical presentation of two methods that provide two complementary sets of information on the function of the heart. The decartogram visualizes depolarization as a sequence of activated points on the image surface, showing the dipolar content of the cardiac electric field. SPECT is a routinely used clinical imaging method that provides information on the function and structure of myocardium based on the retention of radioactive substances depending on cellular viability and integrity of cell membranes. By superimposing the decartogram onto the graphical imaging of viability, the relationships between the viability and electrical activity can be analyzed.
In the interpretation of the superimposition of DECARTO and SPECT, the following factors and limitations of both methods need to be considered:
- Proper definition and position of the coordinate system center of the orthogonal ECG that is used for construction of decartograms need to be ensured, with the center of the real heart surface constructed from SPECT. The image surface constructed from SPECT data can differ from the real surface, especially in cases with severe pathology.
- Differences exist between the anatomic axis/description of the heart and the electrical coordinate system. This requires approximating anatomic structures of the heart (eg, apex, interventricular septum) based on conventionally used views/terms (eg, anterior, posterior, lateral).
- In the clinical evaluation of both SPECT and DECARTO, it is assumed that they provide information mainly about the left ventricle, because it dominates in the image. In reality, DECARTO contains an undefined proportion of information from the right ventricle.
- If the image surface is constructed based on perfusion SPECT data, the surface represents "healthy" myocardium in terms of perfusion. In cases of pronounced pathology (eg, in cases of considerable perfusion defects), the constructed surface can differ considerably from the real anatomic left ventricular surface. Moreover, the shape of the surface depends on the threshold that was used for rendering.
- The information presented by decartogram is based on the representation of the cardiac electrical field as a resultant single dipole and its projection on the selected image surface. This projection, however, does not necessarily correspond to the real sequence of depolarization. For example, the initial part of the depolarization can be projected onto the posterior wall, or the activation areas are projected during depolarization repetitively on the same place.
- DECARTO is constructed primarily from one heartbeat (ie, it visualizes the sequence of activated areas during one depolarization). In contrast, SPECT presents average information from several heartbeats.
These factors/limitations define a framework for the potential use of the superimposition of decartograms and SPECT images. Within this framework, the superimposition of these two methods creates a strong potential for their utilization as illustrative and innovative tools for comparative studies focused on the relationship between functional and structural/anatomic characteristics of the heart. This method of superimposition, including awareness of its limitations, is applicable to other methods as well, such as CT, MRI, and PET, that provide information on the source of the cardiac field from additional aspects.
Limitations of the DECARTO Method
The DECARTO method involves several simplifications related to the basic principles of the method. First, the cardiac electrical field is represented by a single dipole, and DECARTO and the decartograms are primarily defined as circles on the spherical image surface. Second, if the spatial vector is approximated from the standard 12-lead ECG, the result depends on the used formula. Third, the primary image surface is spherical; therefore, deformations due to the projection of this three-dimensional spherical surface onto two-dimensional surfaces (eg, Mercator, polar) can be considerable and will require further optimization of the method. Finally, the relations of the VCG orthogonal coordinate system, the 12-lead hexaxial coordinate system, and the anatomic axis of the heart are considered to be constant and do not consider interindividual variability of patients.