Ischemia Imaging With CT-FFR



Ischemia Imaging With CT-FFR



Introduction


Traditionally, clinicians investigating chest pain in the stable patient population had to decide between the mutually exclusive functional or anatomic testing. With the advent of fractional flow reserve computed tomography (FFRCT), a unique opportunity for simultaneous functional and anatomic assessment of coronary artery disease (CAD) is now feasible in the noninvasive setting.


A particular advantage of FFRCT is its ability to adjudicate lesion-specific pressure loss as with the invasive gold standard of fractional flow reserve (FFR). This ability to adjudicate anatomy and hyperemic physiology allows for the determination of lesion-specific treatment planning. Importantly, early clinical experience suggests that this modifies treatment decision making in a large majority of patients, and this has been shown to have favorable implications on cost effectiveness and resource allocation.



Technology: Function from Anatomy


FFRCT prerequisites




FFRCT computation process




The computational fluid dynamics (CFD) Navier Stokes equations are based on the principles of conservation of mass and balance of momentum to derive the coronary flow and pressure. Recent advances in artificial intelligence and machine learning have enabled more rapid, reproducible, and robust coronary segmentation by iterating and improving the segmentation of the coronaries, permitted the application of the finite element method used by FFRCT technology, where velocity and pressure are solved by a parallel supercomputer to create a mesh of complete epicardial coronary tree FFRCT.


Currently, the most commonly used and FDA-approved platform for FFRCT is “Heartflow.” Data is transferred offsite for central advanced post processing.




Onsite vendor-based platforms are not widely available.






Interpretation of FFRCT


FFRCT provides FFR values throughout the coronary tree as well as lowest FFRCT value at distal end of each vessel analyzed. This is distinct from invasive fractional flow reserve (FFRINV) in which pressure measurements are determined based on the operator, typically to assess pressure and flow loss across a focal lesion.





Clinical Evidence


Refinements in FFRCT technology and improved physiological modeling, particularly microcirculatory resistance, have led to an improvement in diagnostic accuracy when referenced with FFRINV. Per vessel diagnostic accuracy was 86% and 87% in the more recent NXT and PACIFIC trails, respectively.


Compared with CTA, DeFACTO and PACIFIC trials showed that FFRCT had marked increased sensitivity without significant increase in specificity. On the contrary, NXT showed significantly higher specificity compared with CTA. These contradictory sensitivity and specificity distributions could be explained by differences in CAD prevalence and inclusion criteria of respective studies. In PROMISE, FFRCT was shown to be more predictive of coronary revascularization and major adverse cardiac events (MACE) than severe stenoses (> 70%) on CTA. Within a subanalysis of the PACIFIC trial, FFRCT was shown to be superior in identifying lesion-specific pressure loss when referenced with FFRINV as compared with Tc-99m tetrofosmin SPECT, [15O]H2O PET, and CTA in provided CTA images where evaluable.


In the PLATFORM trial, FFRCT combined with CTA was compared with the usual care to determine the need for ICA. Combining FFRCT with CTA resulted in 60% of ICA being canceled with a significant reduction in the burden of nonobstructive disease, decrease in the total number of ICA referrals, decrease in the percentage of ICA for stenoses < 50%, and from 73% to 12% with no events in those for whom ICA had been canceled through 1 year.


In clinical practice, FFRCT has been shown as an effective clinical tool in determining whether subjects should undergo ICA or continue with medical therapy alone while enriching the population referred for ICA and achieving higher percutaneous coronary intervention (PCI):ICA ratio. Deferring ICA through a strategy of CTA and selective FFRCT in moderate stenosis (30-70%) in patients with stable chest pain has been shown by Norgaard et al to prevent further testing in 2/3 of patients with favorable outcomes at 3-year follow-up.


In the RIPCORD study performed on 200 patients of the NXT trial, cardiologists were asked to decide on management PCI vs. coronary artery bypass graft (CABG)] based on findings of CTA. Later, FFRCT data was given and a consensus decision was again solicited. With the use of FFRCT data, the management plan was changed in 36% of patients. There was a 30% decrease in PCI and 18% change of vessel with overall change of 44%. Among those patients, 12% were reallocated from optimal medical therapy to PCI.


Finally, ADVANCE, a large prospective multicenter international registry, assessed the real-world impact of FFRCT. Results at 90 days revealed a change in treatment plan in 2/3 of patients as compared with CCTA alone and no MACE reported in FFRCT > 0.80.


With a forward-looking approach, Serruys and colleagues have begun to investigate the potential of CTA/FFRCT to help plan complex coronary revascularization decision making. In SYNTAX II, a noninvasive anatomical syntax was shown to overestimate the invasive gold standard of angiographic Syntax score. However, the noninvasive functional Syntax of combined CTA and FFRCT was similar to the invasive functional gold standard of ICA and FFR. Building on these results, Syntax III REVOLUTION study sought to assess whether noninvasive anatomical and functional data would lead to similar clinical decision making when provided to heart teams as invasive ICA and FFR. The study was positive with a Cohen Kappa of 0.81 well above the prespecified endpoint of a Kappa of 0.60.

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Apr 6, 2020 | Posted by in CARDIOVASCULAR IMAGING | Comments Off on Ischemia Imaging With CT-FFR

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