CT functional imaging using intracoronary gradient analysis: an indispensable boost for CT coronary angiography.

The saga of CT coronary angiography continues and new exciting developments are on the horizon. We have witnessed the astounding rapid developments in CT technology from 4-, 16-, 64-, 128-, 256-, to 320-slice acquisition, with tube rotational speed increasing from 250 to 87 ms and with ever decreasing radiation exposure from 20 to ,1 mSv with the preservation of image quality. One would think that further developments would not be possible but, lo and behold, at present we see the development of CT coronary angiography combined with CT functional imaging of coronary obstructions. One of the major limitations of CT coronary angiography is the fact that CTCA cannot accurately predict the functional severity of a coronary obstruction and the detection of a coronary diameter stenosis .50% often requires further non-invasive or invasive functional testing. This is not unexpected because the diameter either derived from CT coronary angiography, or also from invasive coronary angiography (with a better temporal and spatial resolution), irrespective of visual or quantitative assessment, is a poor predictor of the functional severity of a coronary obstruction. This issue is further complicated by the tendency to overestimate the stenosis severity by CTCA or by the sometimes occurring inability to grade the stenosis severity due to severe calcification, the presence of a coronary stent or motion artefact. In addition anatomy cannot account for endothelial dysfunction, coronary microvascular disease, or the presence of collateral circulation that affects the functional severity of a coronary obstruction. The severity of obstructive CAD is defined both by vessel morphology and coronary blood flow. The combined information of the coronary anatomic severity and functional severity of an obstruction is now considered crucial for optimal decision-making in stable patients with suspected CAD. The diagnostic performance of non-invasive testing in patients with suspected CAD was dramatically increased using the combination of anatomical and functional imaging by hybrid PET/CT. The addition of CT functional stenosis imaging to CT coronary angiography would blow new life in CT coronary angiography. Recently studies demonstrated the feasibility and reliability of adenosine stress-induced CT myocardial perfusion to analyse the effect of a coronary stenosis on the myocardial blood flow. The method is in its early stage and many obstacles need to be resolved, among others the most pressing being the (unacceptable) increased radiation dose and collection of sufficient data points to determine a precise time attenuation curve before its use in clinical practice. An exciting novel method, CT-FFR, has recently been introduced into clinical practice. This method allows the extraction of ‘stress induced’ quantitative functional information from rest anatomic CT coronary data sets. The method uses computational fluid dynamics with simulated hyperaemia to calculate the fractional flow reserve of a coronary stenosis. CT-FFR correlates well with invasive-derived FFR measurements in patients with suspected CAD. One of the drawbacks at this point of time is that CT-FFR needs extreme computational ability and analysis time which hampers widespread dissemination. Another new CT approach is the measurement of the density values of the passage of contrast bolus through the coronary arteries. This allows the assessment of the gradient of contrast attenuation along the course of the coronary artery. The coronary contrast density depends on the intracoronary blood flow and from the attenuation values one can derive information about the ‘coronary blood flow’ and thus about the functional severity of a coronary stenosis. –8 The data can be extracted from a simple CTCA data set. Earlier reports have shown that intracoronary attenuation gradient analysis correlated with quantitative CT coronary angiography and invasive coronary angiography obtained TIMI flow measurements. –8 However, a study comparing the functional significance of intracoronary gradient analysis with the functional standard of reference intracoronary FFR was lacking. In this journal, Choi et al. present a validation study comparing intracoronary FFR (FFR ,0.80) with two intracoronary gradient analysis methods: the transluminal attenuation gradient (TAG)