心脏磁共振成像

Cardiac MRI offers advantages over alternative modalities. High-frequency ultrasound provides excellent spatial and temporal resolution but suffers from imaging artifacts due to limited penetration in dense tissue. Micro-CT avoids these artifacts but has lower temporal resolution, limited soft tissue contrast, and requires contrast agents that cause radiation damage and renal failure at high doses. Cardiac MRI eliminates ionizing radiation and enables imaging without contrast agents, providing a balanced approach.

FLASH (Fast Low Angle SHot) is an MRI sequence using low flip angle radiofrequency excitations rapidly repeated to induce a steady state pattern in proton motion. The repetition time is much shorter than typical proton relaxation time. This technique allows unexcited hydrogen in blood to produce relatively high signal, enabling rapid cardiovascular imaging and stable snapshots within the cardiac cycle when triggered by physiologic signals.

High-field MRI employs 7-Tesla magnetic field strength, approximately 140,000 times Earth's magnetic field and more than double common clinical scanners at 3-Tesla or 1.5-Tesla. This stronger homogeneous magnetic field causes hydrogen protons to align their rotation axes more effectively. The increased alignment enhances signal detection and enables high-quality soft tissue contrast for studying small animal disease models.

Cardiac MRI monitoring includes three key physiological signals. Electrocardiogram leads detect R-peaks in the cardiac cycle for triggering image acquisition. A pressure-sensitive pillow on the abdomen monitors respiration, allowing imaging during stable expiratory phases. A rectal temperature probe tracks body temperature to control heating module output, ensuring proper animal thermoregulation throughout the procedure.

The two-chamber view along the apex-aortic valve axis visualizes the left ventricle and atrium. The four-chamber view shows all cardiac chambers with bright blood inflow through mitral and tricuspid valves and outflow through aortic and pulmonary valves. Short axis slices perpendicular to the apex-aortic valve axis reveal the left ventricle with distinctly visible papillary muscles, enabling assessment of cardiac function.

Signal dropout regions within the left ventricular lumen indicate fast-moving blood that was originally out of plane and not tagged by radiofrequency wave excitation. This phenomenon occurs because unexcited blood entering the imaging frame produces high signal, but blood moving rapidly out of the imaging plane before excitation appears as signal loss, providing functional information about blood flow dynamics.

Cardiac MRI allows researchers to compare kinematics of healthy versus diseased hearts using controlled murine disease models. This capability enables identification of specific factors contributing to heart disease and study of cardiac remodeling after injury. Additionally, vascular-focused applications assess abdominal aortic aneurysm formation by exploiting blood's high signal intensity to measure vessel expansion and biomechanical property changes.