Age determination of the cardiac thrombus: Role of cardiac magnetic resonance imaging

Introduction

Cardiac thrombi, especially LV thrombi, are not infrequent in ischemic heart diseases after myocardial infarction. The medical management of a cardiac thrombus relies on accurate diagnosis and age determination, as acute cardiac thrombus often requires anticoagulant therapy. ¹

Early detection is typically done by echocardiography. However, cardiac magnetic resonance (CMR) has emerged as the preferred imaging modality for cardiac thrombus detection, allowing arbitrary imaging planes and a large field of view. Currently, cine and early gadolinium enhancement (EGE) with long T1-contrast images are used in clinical practice to visualize the cardiac thrombus. However, the age determination of the thrombus still needs to be improved.

The state of the oxygen within the hemoglobin (Hb) of red blood cells trapped in the clot and its water content determine the thrombus’s age. It has long been known that clotting blood passes through several well-defined stages reflecting the state of Hb-oxygenation, which produces alterations in its CMR contrast characteristics.²

Four different hemoglobin species are commonly recognized: oxyhemoglobin (oxy-Hb), deoxyhemoglobin (deoxy-Hb), methemoglobin (met-Hb), and hemichromes, whose structures appear on the right.

These hemoglobin species typically evolve in the order shown over the first 1-2 weeks after initial thrombus formation. Ultimately, the hemoglobin molecule is enzymatically cleaved into multiple small fragments taken by macrophages (hemosiderin).

Oxy-Hb and hemichromes have no unpaired electrons and are weakly diamagnetic (do not affect T1 or T2 relaxation). On the other hand, deoxy-Hb and met-Hb have, respectively, 4 and 5 unpaired electrons per iron atom and are paramagnetic (shorten T1 and T2 relaxation). Therefore, an adequate amount of met-Hb must be generated to produce enough signal; met-Hb must be formed reliably within a thrombus (red thrombus).

The earliest reliable timing of formation is within 8 hours. Follow-up of patients with proven cardiac thrombus has shown that maximum signal is achieved at approximately three weeks, after which the signal intensity plateaus. In the lead-up, the signal development within the thrombus indicates its maturity.

After three weeks, the signal will persist up to, but not beyond, six months. During this phase, the appearance of the thrombus is also characteristic. As the thrombus becomes organized, met-Hb will be transformed into hemosiderin, and the signal is lost. It does not tend to be a uniform process throughout the thrombus length. Instead, it occurs at intervals, with the resultant appearance of islands of high signal, which disappear over the ensuing weeks.³

I. SSFP Cine images: SSFP Cine images: Along the long axis and short axis covering both ventricles for detecting any intercavitary hypodense mass. Acquired using a retrospective electrocardiogram-gated, steady-state free precession sequence during breath-holding at end- expiration. Scan parameters: repetition time = 3.2-3.8 ms; echo time = 1.6-1.9 ms; flip angle = 50-70°; slice thickness = 8 mm without slice gap; matrix = 160 x 256; field of view = 350-400 mm. Temporal resolution was approximately 25 ms.

a. Planned on the cine images that showed the cardiac thrombus

II. T1 weighted images: Acquired using a prospective electrocardiogram-gated, during end-diastole, with breath holding at end-expiration. Scan parameters: repetition time = 960 ms, echo time =15 ms, , flip angle= 180o, slice thickness 8 mm , matrix = 280 x 80 , filed of view 350-400 mm.

III. T2 weighted images: Acquired using a prospective electrocardiogram-gated, during end-diastole, with breath holding at end-expiration. Scan parameters: repetition time = 1,463 ms, echo time = 80 ms, flip angle = 180o, slice thickness= 8mm, matrix = 265 x 80 , filed of view= 350-400 mm

IV. EGE with Long TI:

a. Immediate after injection of a gadolinium-based contrast agent (Magnevist, Schering AG, Berlin, Germany; 0.3 mmol/kg), EGE images were acquired in the same orientation as the cine images using a segmented inversion-recovery gradient echo pulse sequence: repetition time/echo time = 4.01/1.25 ms, flip angle = 15°, matrix = 208 x 256 and a typical voxel size of 1.6x1.3x5.0 mm, inversion time (T1) = 500ms.