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.