Imaging Physics Laboratory

        Limitations in the visual assessment of intermediate severity stenoses by coronary angiography are known to suffer from intra- and inter-observer variability as well as discordance with their true physiologic importance [1-6].  Previous studies have performed functional analyses of stenoses using acquired images to predict pressure gradients [7-9], to estimate coronary flow reserve [10-12], to assess coronary flow through Thrombolysis in Myocardial Infarction (TIMI) frame count [13-15], and to assess functional improvement after coronary intervention [16, 17].  An important index not already estimated from coronary angiography is fractional flow reserve (FFR).  Pressure-based fractional flow reserve (FFR) has proven to aid the cardiologist in evaluating the flow-limiting potential of stenoses as well as the therapeutic gain of angioplasties [18, 19].  

        FFR quantifies the reduction in maximum coronary blood flow from a theoretical maximum normal flow in the presence of a stenosis.  How can FFR be determined if the maximum normal flow is unknown?  The pressure-based approach has elegantly circumvented the need to know the theoretical maximum normal flow by approximately FFR as a ratio of diseased perfusion pressure over the maximum inflow pressure from the aorta.  However, a limitation to the current pressure-based FFR method is the need to insert a pressure wire (0.014”) into distal parts of coronary arteries.

In x-ray imaging, pixel values represent effective attenuation measurements of an object in the beam and can be expressed by the following equation: 

N = N₀e^-ut

Where N is the number of detected photons, N0 is the number of photons incident on the object, u is the attenuation coefficient and is material specific and a function of energy,  tis the object thickness. When imaging two distinct materials, the equation becomes: 

N = N₀e^-(u_j^t_j^+u_a^t_a⁾

Only in limited situations with specific a priori knowledge can information about the thickness of both materials be discerned from a single measurement. 

        Occlusion or partial occlusion of coronary arteries frequently occurs in man while imparting high morbidity and mortality.  Previous studies have shown that the most important determinant of infarct size for a given coronary artery occlusion is the size of myocardium that the artery perfuses [1-5].  Therefore, information about the perfusion territory, or myocardial mass at risk, can be used to determine the potential infarct severity of an occluded artery.  Current clinical applications of radionuclide perfusion imaging and contrast echocardiography utilize this concept by identifying inadequately perfused regions with a radiopharmaceutical tracer or contrast material, respectively, to estimate the potential infarct size.  Alternatively, previous studies have indicated that morphological surrogates for myocardial mass measurements may exist [6-9].  For example, the sum of arterial branch lengths distal to the point of occlusion has been proposed for estimating the corresponding regional myocardial mass at risk [6].