Relative Energy Index to Detect Malignant Microemboli
To the Editor:
Choi et al1 propose a new method for predicting “malignant emboli” based on the product of maximum intensity (measured in decibels) and maximum duration (measured in seconds).1 Unfortunately, this index is based on a misunderstanding of physical principles.
First, Einstein’s equation, E=mc2, describes the equivalence of the rest mass of an object to a certain amount of energy, but that energy is released only in a process wherein the mass is “destroyed,” such as nuclear fusion, nuclear fission, or electron-positron annihilation. Thus, Einstein’s equation cannot be used to characterize the “energy” of an embolus traveling through the cerebral circulation.
Second, Choi et al1 appear to have calculated their relative energy index by using intensity expressed in decibels, which is proportional to the logarithm of intensity rather than intensity itself, and therefore, the index cannot have the units of energy that the authors state.
The determination of embolus size from Doppler methods is a large challenge because the measurements of backscattered power from emboli are significantly affected by Doppler sample-volume shape, the geometric relationship between the sample volume and the vessel, and the embolus trajectory through the blood vessel,2 and as Choi et al1 correctly point out, even under ideal measurement conditions, the backscattered power from emboli does not increase strictly monotonically with size.3 Fortunately, despite these difficulties, in general, larger emboli give rise to larger backscattered signals, and therefore, it is not surprising that there was a “… tendency toward higher risk of recurrent stroke/TIA in the ‘malignant’ microembolic signal (MES) group… ,” as this group also had many more MESs with a very much higher maximum intensity.
The results of the study by Choi et al1 are interesting from an empirical point of view, but the theory on which their equations are based is flawed. It remains to be seen whether their new index can provide any better indication of prognosis than simpler quantities, such as the number and intensity of MES signals.
Choi Y, Saqqur M, Stewart E, Stephenson C, Roy J, Boulanger J-M, Coutts S, Demchuk AM. Relative energy index of microembolic signal can predict malignant microemboli. Stroke. 2010; 41: 700–706.
Martin MJ, Chung EML, Ramnarine KV, Goodall AH, Naylor AR, Evans DH. Thrombus size and Doppler embolic signal intensity. Cerebrovasc Dis. 2009; 28: 398–405.