Cardiopulmonary transit time: A novel PET imaging biomarker of in vivo physiology for risk stratification of heart transplant recipients

Is cardiopulmonary transit time (CPTT) measured by using dynamic rubidium cardiac PET/CT a predictor for cardiac function?

Cardiopulmonary transit time (CPTT) represents the time needed for the circulation of blood from the right to the left ventricle. This parameter can be measured during dynamic acquisition of rubidium ([82Rb]) cardiac PET/CT. To further characterize this marker, we wanted to assess the association between CPTT and parameters of cardiac function derived from echocardiography. Retrospective single center analysis of patients referred to [82Rb]RbCl-PET/CT with rest/stress protocol on an integrated hybrid PET/CT system (Biograph mCT, Siemens Healthineers, Erlangen, Germany) and echocardiography within 100 days. After intravenous injection of 7.5 MBq/kg [82Rb]RbCl dynamic scans with initially 12 × 10 s frames were started. For data analysis a volume of interest (VOI) was drawn in the left and right ventricle using dedicated software. The difference between the peak time for the two time activity curves (TAC) was extracted as CPTT and normalized for heart rate (NCPTT). Associations between NCPTT and echo parameters such as left ventricular ejection fraction (EFEcho) were analyzed using linear regression models. 44 patients (sex: 28 male, 16 female) were enrolled with a time difference between PET and echocardiography of 19.65 ± 23.3 days. 9 patients had a rest CPTT of 0 s, 32 patients 10 s and 3 patients 20 s. The association between EFEcho and NCPTT revealed a significant negative correlation (beta = -0.77; CI: -1.32, -0.22; p = 0.007). Given this association, univariate predictive models for EFEcho were applied. Root mean square error was 6.83% for the EFPET, and 6.0% for NCPTT, which indicates a slightly higher predictive performance for the NCPTT model with a lower error. Pulmonary transit time can be estimated with [82Rb]RbCl-PET/CT, with a high positive association to rest EFEcho. However, smaller time frames than 10 s are needed, for more accurate estimation of cardiac function.

Quantitative analysis of abdominal aortic blood flow by 99mTc-DTPA renal scintigraphy in patients with heart failure

OBJECTIVE

This study aimed to explore the characteristics of abdominal aortic blood flow in patients with heart failure (HF) using 99mTc-diethylenetriaminepentaacetic acid (DTPA) renal scintigraphy. We investigated the ability of renal scintigraphy to measure the cardiopulmonary transit time and assessed whether the time-to-peak of the abdominal aorta (TTPa) can distinguish between individuals with and without HF.

METHODS

We conducted a retrospective study that included 304 and 37 patients with and without HF (controls), respectively. All participants underwent 99mTc-DTPA renal scintigraphy. The time to peak from the abdominal aorta's first-pass time-activity curve was noted and compared between the groups. The diagnostic significance of TTPa for HF was ascertained through receiver operating characteristic (ROC) analysis and logistic regression. Factors influencing the TTPa were assessed using ordered logistic regression.

RESULTS

The HF group displayed a significantly prolonged TTPa than controls (18.5 [14, 27] s vs. 11 [11, 13] s). Among the HF categories, HF with reduced ejection fraction (HFrEF) exhibited the longest TTPa compared with HF with mildly reduced (HFmrEF) and preserved EF (HFpEF) (25 [17, 36.5] s vs. 17 [15, 23] s vs. 15 [11, 17] s) (P < 0.001). The ROC analysis had an area under the curve of 0.831, which underscored TTPa's independent diagnostic relevance for HF. The diagnostic precision was enhanced as left ventricular ejection fraction (LVEF) declined and HF worsened. Independent factors for TTPa included the left atrium diameter, LVEF, right atrium diameter, velocity of tricuspid regurgitation, and moderate to severe aortic regurgitation.

CONCLUSIONS

Based on 99mTc-DTPA renal scintigraphy, TTPa may be used as a straightforward and non-invasive tool that can effectively distinguish patients with and without HF.

Pulmonary Transit Time Assessment by CEUS in Healthy Rabbits: Feasibility, and the Effects of UCAs Dilution Concentration

To evaluate the inter-observer variability and the intra-observer repeatability of pulmonary transit time (PTT) measurement using contrast-enhanced ultrasound (CEUS) in healthy rabbits, and assess the effects of dilution concentration of ultrasound contrast agents (UCAs) on PTT. Thirteen healthy rabbits were selected, and five concentrations UCAs of 1:200, 1:100, 1:50, 1:10, and 1:1 were injected into the right ear vein. Five digital loops were obtained from the apical 4-chamber view. Four sonographers obtained PTT by plotting the TIC of right atrium (RA) and left atrium (LA) at two time points (T1 and T2). The frame counts of the first appearance of UCAs in RA and LA had excellent inter-observer agreement, with intra-class correlations (ICC) of 0.996, 0.988, respectively. The agreement of PTT among four observers was all good at five different concentrations, with an ICC of 0.758-0.873. The reproducibility of PTT obtained by four observers at T1 and T2 was performed well, with ICC of 0.888-0.961. The median inter-observer variability across 13 rabbits was 6.5% and the median variability within 14 days for 4 observers was 1.9%, 1.7%, 2.2%, 1.9%, respectively; The PTT of 13 healthy rabbits is 1.01 ± 0.18 second. The difference of PTT between five concentrations is statistically significant. The PTT obtained by a concentration of 1:200 and 1:100 were higher than that of 1:1, while there were no significantly differences in PTT of a concentration of 1:1, 1:10, and 1:50. PTT measured by CEUS in rabbits is feasible, with excellent inter-observer and intra-observer reliability and reproducibility, and dilution concentration of UCAs influences PTT results.

Prognostic Significance of Myocardial Blood Flow Quantification for Major Adverse Cardiac Events: A Systematic Review and Meta-analysis

Chronic coronary syndromes involve reduced myocardial blood flow (MBF). MBF is a reliable predictor of outcomes, independent of the presence of significant stenosis. Whether MBF can predict major adverse cardiac events (MACE) during long-term follow-up is unknown. PubMed, Embase, Cochrane, CNKI, and WANFANG were searched for papers published up to January 2021. The exposure was the incremental unit of stress MBF (mL/g/min) or low MBF versus high MBF. The imaging examinations included positron emission tomography/computed tomography and coronary magnetic resonance. The study outcome was the occurrence of MACE during follow-up, summarized as time-to-event hazard ratios (HRs) and 95% confidence intervals (CIs). Six studies (300 MACEs in 2326 patients) were included. Four studies presented stress MBF data by unit increments. The pooled HR showed that an increase in stress MBF by 1 mL/g/min is a protective factor for MACE (HR = 0.32; 95% CI, 0.18-0.57; I2 = 62.9%, Pheterogeneity = 0.044). Two studies reported stress MBF as low/high. The results showed that a high-stress MBF was protective against MACEs (HR = 0.43; 95% CI, 0.24-0.78; I2 = 39.5%, Pheterogeneity = 0.199). Quantification of stress MBF using positron emission tomography/computed tomography and coronary magnetic resonance might have incremental predictive value for future MACEs in a population at intermediate to high cardiovascular risk. The results will require validation in large prospective randomized controlled trials.