Detection and Characterization of Thrombosis in Humans Using Fibrin-Targeted Positron Emission Tomography and Magnetic Resonance

Abstract

Objectives: The authors present a novel technique to detect and characterize LAA thrombus in humans using combined positron emission tomography (PET)/cardiac magnetic resonance (CMR) of a fibrin-binding radiotracer, [⁶⁴Cu]FBP8.

Background: The detection of thrombus in the left atrial appendage (LAA) is vital in the prevention of stroke and is currently performed using transesophageal echocardiography (TEE).

Methods: The metabolism and pharmacokinetics of [⁶⁴Cu]FBP8 were studied in 8 healthy volunteers. Patients with atrial fibrillation and recent TEEs of the LAA (positive n = 12, negative n = 12) were injected with [⁶⁴Cu]FBP8 and imaged with PET/CMR, including mapping the longitudinal magnetic relaxation time (T₁) in the LAA.

Results: [⁶⁴Cu]FBP8 was stable to metabolism and was rapidly eliminated. The maximum standardized uptake value (SUV_Max) in the LAA was significantly higher in the TEE-positive than TEE-negative subjects (median of 4.0 [interquartile range (IQR): 3.0-6.0] vs 2.3 [IQR: 2.1-2.5]; P < 0.001), with an area under the receiver-operating characteristic curve of 0.97. An SUV_Max threshold of 2.6 provided a sensitivity of 100% and specificity of 84%. The minimum T₁ (T_1Min) in the LAA was 970 ms (IQR: 780-1,080 ms) vs 1,380 ms (IQR: 1,120-1,620 ms) (TEE positive vs TEE negative; P < 0.05), with some overlap between the groups. Logistic regression using SUV_Max and T_1Min allowed all TEE-positive and TEE-negative subjects to be classified with 100% accuracy.

Conclusions: PET/CMR of [⁶⁴Cu]FBP8 is able to detect acute as well as older platelet-poor thrombi with excellent accuracy. Furthermore, the integrated PET/CMR approach provides useful information on the biological properties of thrombus such as fibrin and methemoglobin content. (Imaging of LAA Thrombosis; NCT03830320).

Keywords: MRI; PET; atrial fibrillation; fibrin; left atrial appendage; thrombus.

Figure 1.. Structure and properties of ⁶⁴Cu-FBP8.

(A) The probe consists of a 6 amino-acid cyclic peptide with a high affinity for fibrin, conjugated to a ⁶⁴Cu-containing chelator (NODAGA). (B) Average time activity curves (TACs) from 5 healthy subjects, expressed as SUV (g/ml), show a relatively rapid reduction of the PET signal in the blood and lung, accompanied by renal and hepatic elimination. Image analysis of the blood pool showed that ⁶⁴Cu-FBP8 was eliminated from the blood in a bi-exponential manner typical of radiotracers with extracellular distribution and renal clearance, with an initial distribution phase (alpha t_1/2 = 6.9 ± 3.0 min) followed by an elimination phase (beta t_1/2 = 68 ± 2 min). (C-F) Chemical analysis of blood samples taken from healthy volunteers injected with [⁶⁴Cu]FBP8. (C) High Performance Liquid Chromatography (HPLC) of ⁶⁴Cu-containing moieties in the blood at various times postinjection shows that the majority of ⁶⁴Cu activity is intact [⁶⁴Cu]FBP8 which has a retention time of 12 minutes. (D) [⁶⁴Cu]FBP8 is stable in blood, with >60% of the probe still intact by HPLC one hour after injection. (E) Over 50% of the probe remains functional and able to bind fibrin 1 hour after injection. (F) The clearance of ⁶⁴Cu activity from the blood, measured by venous blood sampling and expressed as %ID/g, indicates a one-hour blood elimination half-life (beta t_1/2 = 57 ± 7 min). (G-H) Biodistribution of [⁶⁴Cu]FBP8 in a healthy control subject shows a substantial reduction in the background thoracic signal within 1 hour of injection, and a near-complete loss of background by 4 hours. Elimination of the probe in the bladder and gallbladder is clearly seen. (H) Magnified view of the thorax at 1 hour showing no evidence of focal [⁶⁴Cu]FBP8 uptake in either the heart or lungs.

Figure 2.. Detection of spontaneous thrombi in the LAA.

TEE, PET and fused PET/MRI images of a subject with multiple LAA thrombi are shown in two orthogonal planes, (A-C) and (D-F). The thrombi produce echogenic/bright foci (yellow arrows) on the TEE images (A, D). The PET (B, E) and PET/MRI (C, F) images reveal multiple foci of [⁶⁴Cu]FBP8 uptake in the LAA (white arrows) in both planes. In addition, thrombus is also identified in the RAA (green arrow). (G-I) [⁶⁴Cu]FBP8 does not accumulate in the LAA in a subject with spontaneous echo contrast (SEC) but no thrombus. (G) Echogenic signal is seen within the LAA (yellow arrow), but color Doppler (inset) reveals extensive flow consistent with SEC. No evidence of LAA thrombus is seen on the [⁶⁴Cu]FBP8 PET images in the axial (H) or coronal (I) views. (J-L) The maximum SUV value in the LAA (SUV_MAX) accurately distinguishes TEE positive and negative subjects. (J) SUV_MAX in the TEE positive subjects was significantly higher than the TEE negative subjects. (J-K) A SUV_MAX threshold of 2.6 correctly classified all (12/12) TEE positive subjects and 10/12 TEE negative subjects. The area in the rectangle in panel J is magnified in panel K and confirms that a SUV_MAX threshold of 2.6 produces no false negative and only 2 false positive cases. (L) Receiver operating characteristic (ROC) curve demonstrates the robustness of [⁶⁴Cu]FBP8 SUV_MAX in distinguishing TEE positive and negative subjects, with an area under the curve (AUC) of 0.97.

Figure 3.. Integrated analysis of magnetic relaxation (T1) and SUV_MAX in the LAA.

(A-D) Subject with a LAA closure device. (A) T1-weighted image in the coronal plane showing a hyperintense region in the LAA (white arrow). (B) T1 map in the same view demonstrating low values (green-purple; black arrow) in the LAA, similar to the T1 of myocardium (purple) but far lower than the blood pool (orange). (C) Axial T1 map confirms the presence of short-T1 (paramagnetic) species in the LAA, consistent with methemoglobin (met-heme) accumulation in thrombus. (D) Magnified view of the axial T1map fused with the [⁶⁴Cu]FBP8 PET image demonstrates a high degree of the overlap between low T1 and high [⁶⁴Cu]FBP8 uptake. (E-H) Spontaneous LAA thrombus. (E) Axial T1W image showing a hypertintense focus (arrow) in the LAA. (F) Axial T1 map shows a focus of reduced T1 (arrow) in the LAA. (G) Magnified view of T1 map and short T1 focus (arrow). (H) Magnified view of R1 map (units s⁻¹). The thrombus in the LAA (arrow) has a high R1 value. (I) The minimum T1 value (T1_MIN) in the LAA is significantly shorter in the TEE positive than negative group. (J) Probability of having a LAA thrombus based on logistic regression of the z-scores for both SUV_MAX and T1_MIN. WA = Watchman, SP = spontaneous. (K) Biological properties of the LAA (fibrin and met-heme content) in those with precisely aged recent thrombi (WA group) and no thrombi (Neg group). A strong correlation is seen between fibrin content (SUV_MAX) and met-heme content (R1_MAX = 1/T1_MIN). (L) Within the TEE negative group an association may be present between higher SUV and R1 values and thrombotic risk. Factor VL = Factor V Leiden.

Figure 4.. [⁶⁴Cu]FBP8 facilitates whole-body thrombus detection.

(A-C) Right pulmonary artery embolism in a subject with a spontaneous LAA thrombus. (A) Coronal PET image with a focus of high [⁶⁴Cu]FBP8 activity (arrow). (B) Fusion of the PET and Dixon water images reveals that the thrombus is located within a branch of the right pulmonary artery. (C) Volume rendered image showing the pulmonary embolism (arrow,) in the right lung. (D-F) TEE positive subject with a history of a large intracranial bleed. (D) Sagittal PET image, (E) fused PET and T1-weigted image and, (F) fused PET and susceptibility weighted image (SWI). The PET image shows that a low level of [⁶⁴Cu]FBP8 uptake persists at the site of the bleed (arrow).

Central Illustration.. Thrombus in the LAA induced by recent placement of a Watchman LAA closure device.

A large thrombus is seen in the LAA behind the device on (A, B) the coronal PET and PET/MRI images as well as (C, D) the axial PET and PET/MRI images. (E, F) 2D bSSFP cines stacks in the coronal and axial planes have been combined into a single 3D dataset and fused with the 3D PET data. In these multi-modal images the PET and MR data are both displayed in 3D, creating a volumetric depiction of the heart and the thrombus/[⁶⁴Cu]FBP8 containing LAA. Volume rendered images in the oblique coronal and axial planes confirm the presence of thrombus in the LAA. (A-F) The thrombus produced by the closure device is contained within the LAA, and no evidence of thrombus is seen elsewhere in the heart or thorax.