Multimodality Cardiac Imaging for Disease Progression in ATTR-CM

Change in FAPI uptake

F-FAPI-74 PET/CT is performed at baseline and 12-month follow-up. Images are acquired on a PET/CT scanner 60 minutes after intravenous injection of F-FAPI-74. The emission scan is obtained from the apex of the lung to the base of the lung. One bed position is acquired (20 minutes, 3-dimensional mode), and the heart is set at the center of the view. Myocardial F-FAPI-74 uptake is quantified by the mean standardized uptakte value of the LV volume of interest (VOI). The epicardial LV contour is drawn on the CT scan and, to correct for blood pool activity, the endocardial border is defined automatically by thresholding for 2 times the mean SUV of the blood pool.

Change in echocardiographic myocardial stiffness

Cardiac intrinsic elastography from echocardiography is performed at baseline and at 12-month follow-up. Ultrahigh-frame rate tissue Doppler data (\>250 frames/sec) are acquired using clinical scanners from the apical fourchamber view while carefully aligning the septal wall with the Doppler beam. The base-to-apex propagation of the late diastolic myocardial stretch wave can be visualized with ultrafast imaging data. The slope of the isovelocity wave front, called intrinsic velocity propagation (iVP) of myocardial stretch (in meters per second) is measured offline. By averaging values for three cardiac cycles, a global value is generated.

Change in extracellular volume (ECV)

Cardiac MRI is performed at baseline and 12-month follow-up. Myocardial LV native T1 maps are acquired in 3 short-axis slices at the base, mid and apical levels using the modified Look-Locker inversion (MOLLI) recovery technique at end-diastole for 3 cycles (5-3-1 R-R durations with 3-4 R-R rest periods) during a single breath hold. T1 mapping is repeated with the same settings 10 minutes after intravenous administration of 0.1 mmol/kg of gadolinium contrast agent. The ECV fraction is estimated by plotting the reciprocal of each segmental myocardial T1 against the reciprocal of the blood pool T1 for native, 10-minute post-contrast MOLLI acquisitions, then calculating linear regression slopes, corresponding to segmental partition coefficients for gadolinium (λGd). Each λGd was multiplied by (1 - hematocrit) to obtain ECV fractions.

Change in BMP4 expression and serum concentration

Serum samples are analyzed. BMP4 levels in the samples were measured using a human BMP4 SimpleStep ELISA Kit following manufacturer's instructions. Endomyocardial biopsies as part of the clinical diagnostic routine from the subgroup of 10 participants with suspicion of cardiac ATTR amyloidosis are analyzed. Cardiac tissue is fixed in 4% formaladehyde and embedded in paraffin. Histopathological changes are evaluated after H\&E staining and the area of percentage of collagen networks per section is determined. For the assessment of BMP4 mRNA expression in tissue, homogenates are generated, samples are centrifuged and the supernatant is used for ELISA measurements.