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Quantification in CMR: what makes the difference to patient care

Lecture Session III

Quantifying flow and valve regurgitation

Philip Kilner: 2D through plane phase contrast acquisition was reviewed for quantifying aortic valve and pulmonary arterial flow. The importance of choosing the correct valve plane, venc range (25% above expected peak velocity), a large field-of-view to include the chest walls and 10mm slices to improve signal was emphasized. Breath-hold acquisitions are typically used, although non-breath-hold acquisitions are less subject to background offset errors but are longer. As the aortic root moves up in diastole through a fixed plane, this can underestimate regurgitant flow volume. Mitral regurgitation can be quantified indirectly using aortic flow and stroke volume differences, and a cine stack (5mm no gap) perpendicular to the central line of coaptation can assess mitral valve scallops. CMR is particularly useful for pulmonic regurgitation. Static phantoms post scan can correct background offset errors. 4D flow CMR in a single acquisition can provide images for accurate measurements and cuts across desired planes post scan.

Quantifying oedema: T2w, SSFP or T1/T2 mapping?

Erica Dall’Armellina focused on how edema imaging can impact on patient care. Edema imaging can be performed using conventional T2-weighted CMR, whilst mapping techniques can provide direct quantification without reference regions of interest. Edema imaging can be especially useful in patients with chest pain, positive troponins and unobstructed coronaries, such as global myocarditis without LGE. Edema imaging can differentiate acute vs chronic MI to guide intervention. Patients with higher myocardial salvage areas have a better prognosis. T1/T2 mapping are superior to T2W in detecting edema, and useful in acute ischaemic and non-ischaemic cardiac diseases. In acute MI, the severity of injury can be interrogated by the numerical value of T1/T2 relaxation times in the area of injury, infarct and remote myocardium, with higher T1/T2 times generally conferring poorer outcomes .

Quantifying iron load: T2* mapping versus T1 in thalassaemia

Alessia Pepe: T2* CMR has dramatically reduced mortality of patients afflicted with iron overload. T1-mapping is also sensitive to iron (low T1), with good correlation to T2*, and better reproducibility. Whilst T1-mapping can detect iron overload in patients with a “normal T2*>20 ms”, this may be an over-conservative cut-off. Different T1-mapping techniques that have different normal ranges and may be affected by multiple factors, so thresholds may not be directly translatable from one centre to another. It is also challenging to calibrate T1 for iron against biopsy due to the effects of formalin on T1. Confounding factors such as focal and diffuse fibrosis (which raise T1) may impact on diagnostic power. T1-mapping is promising for iron assessment but more work is needed before replacing T2*.

Quantifying ischaemia: can it be done in clinical routine?

Matthias Friedrich: Vasodilatory stress perfusion CMR can be semi-quantified using myocardial perfusion reserve index (MPRI) using the upslope during vasodilation compared to rest. Absolute quantification of perfusion can be performed using several models, with the FERMI deconvolution method being one of the best validated against microspheres, with good reproducibility, although the Fermi model is not corrected for exchange with extracellular space and can be affected by other issues. Perfusion by quantifying flow is only a surrogate marker for ischaemia. BOLD CMR is another method to assess for ischaemia under adenosine stress, and recent animal studies suggest that it may be combine with hyperventilation and then breath-holding  maneuvers in attempt to recruit pulmonary flow reserves in a “Breathing induced myocardial oxygenation reserve protocol”, which is work in progress.

The content of this article reflects the personal opinion of the author/s and is not necessarily the official position of the European Society of Cardiology.