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Tissue Doppler Imaging Echocardiography – Technological advancement or retrograde development?
- George Thomas (16 April 2008)
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George Thomas, Senior Consultant Cardiologist Indira Gandhi Co-operative Hospital, Kochi, India
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gthomas{at}doctor.com George Thomas
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Dear Editor, I read with great interest the article presenting tissue Doppler techniques as technological advances.[1] The authors conclude with the “limitations” of tissue Doppler stating “There is still a significant variability in TDI (tissue Doppler imaging) derived parameters because of differences in machine characteristics and lack of guidelines for standardization of sample positions and machine settings. Therefore it is still difficult to provide firm recommendations for clinical practice. More clinical trials are needed for establishing sensitive and effective parameters for diagnosing specific conditions or detecting abnormalities”. In which case is it technological advances? There are philosophical, methodical and application flaws in tissue Doppler.[2] The basic principles of measurement and Doppler are compromised in this technique. After you make a measurement as per established methods you could have “limitations”. But if you make a measurement paying scant regard to established norms it’s a “blunder”. So you cannot blame “differences in machine characteristics” nor “lack of guidelines for standardization of sample positions and machine settings”. It is like measuring the height of a patient without knowing the required alignment, making random measurements and calling the erroneous measurements as “limitations” of the scale (see figure)! This would be a case of right tool, wrongly used. You cannot sweep the basic flaws under the carpet by labeling them as “limitations”. The authors state “In practice, from the apical window, only longitudinal shortening can be obtained from every segment of the left and right ventricle. These longitudinal velocities reflect both active and passive myocardial motion, which forms the major limitation of the technique”. The problem here is much more than “active and passive myocardial motion”. Doppler will give consistent results only if there is a unique velocity or if the other conflicting velocity vectors are constant. In tissue motion there are several velocity vectors affecting the resultant longitudinal vector. Since we do not know the influence of these conflicting vectors we will consistently get variable results. Compare flow motion and tissue motion to understand this better. The authors also state that “The sample volume should be positioned in the centre of the region of interest. It is recommended to check the sample volume position before acquiring the still frames since the motion of the atrioventricular ring may differ around the basal myocardial segment”. Pray, where are the regions of interest and the points of interrogation? In case of flow Doppler these are unique points - the normal and abnormal orifices. Can we ever define a proper point of interrogation on tissue Doppler? That is also why the authors correctly state “normal values for longitudinal velocities depend on the position of the sample area within the wall”. This is also why, as the authors mention, there are “lack of guidelines for standardization of sample positions”. Doppler strain and E/E’ are incorrect because the foundation is wrong. Besides in the formula for strain calculation, the modulus of a vector quantity (velocity) is incorrectly substituted for a scalar quantity (length) in the original equation. E/E’ also goes against the scientific “principle of parsimony”.[3] When cardiac uses of Doppler were first studied, Doppler recordings were thought to come from the heart tissues and no signals were attributed to blood flow. Consequently the Doppler technique did not interest the cardiologists then! When considering developments in echocardiography, the developments should incrementally add to our knowledge and refine the data collection. In tissue Doppler it is a retrograde development. We use Doppler to study flow because we cannot image flow. When imaging flow becomes a reality, Doppler would be relegated to the background. When we can “see” the tissue there is no need for an indirect Doppler methodology. In this context the authors mention about 2D speckle tracking. They state “Local 2-dimensional tissue velocity vectors are derived from the spatial and temporal data of each speckle” This would be advancement in the right direction. Another problem is the high sensitivity of tissue Doppler compromising on the specificity. An akinetic segment correctly displayed on M-mode is never displayed as 0 velocity on tissue Doppler. This will also affect the temporal data. The authors correctly mention this albeit with a wrong explanation. “However, in patients with ischemic cardiomyopathy, TDI and realtime three-dimensional echocardiography show poor agreement for evaluating the magnitude of intraventricular dyssynchrony and the site of maximal mechanical delay. This may be explained by their respective assessment of longitudinal versus radial timing”. Tissue Doppler can never be a technological advancement. All advancements are built on strong foundations in basic sciences. Here the basic principles of measurement and Doppler are compromised. Making clinical judgments based on this faulty modality would be unacceptable to the scientific cardiologist. Figure 1 References 1. Van de Veire N, De Sutter J, Bax JJ, and Roelandt JR. 2. Thomas G. 3. Thomas G. |
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