Cardiac Magnetic Resonance Imaging (CMR) is the gold standard non-invasive tool for left and right ventricular function, vascular flow, and chamber volume quantification.
Pulmonary & Systemic Flow
Pulmonary flow (Qp) and Systemic flow (Qs) may be calculated non-invasively with cardiovascular magnetic resonance using simple phase-contrast techniques. Velocity encoded phase contrast techniques can quantify the severity of intra-cardiac shunts. Measuring pulmonary blood flow (Qp) in the pulmonary artery and systemic blood flow (Qs) in the aorta provides a noninvasive estimate of Qp/Qs and, thus, quantifies the degree of intra-cardiac shunting. CMR can quantify the amount of valvular regurgitation (e.g., in patients with Valve Disease or Tetralogy of Fallot).
CMR provides non-invasive clear anatomical valvular information that can impact the clinical management of a patient. It is possible to differentiate a bicuspid from a tricuspid aortic valve. CMR reproducibly characterizes aortic valve anatomy and the determined aortic valve area correlates well with cardiac catheterization.
Blood Flow & Velocity
Phase contrast techniques can reliably measure peak velocity and thus peak gradient in aortic stenosis. Valvular information in combination with accurate left ventricular volumes and assessment of thoracic aortic dilatation can assist in planning valvular replacement and, importantly, determine whether the aorta needs intervention as well.
Myocardial Perfusion Imaging
CMR first-pass perfusion has evolved as the non-invasive diagnostic imaging modality of choice for myocardial perfusion imaging standards compared to alternate imaging tools.
Left Ventricular Mass (LVM)
LVM evaluation by CMR permits a 3D high-resolution modeling of the LV free of cardiac geometric assumptions, contrast infusion, acoustic window dependency, or ionizing radiation. Both short-axis & long-axis techniques are highly accurate for quantification of LVM. We use a set of contiguous short-axis slices covering the entire LV from the atrioventricular ring down to the apex, acquired from a cine sequence. Myocardial volume is the area occupied between the endocardial and epicardial border multiplied by the interslice distance. By convention, LVM is measured at end-diastole. Similar to echocardiography, LVM is the product of this volume & the density of the myocardium.
Myocardial Delayed Enhancement
CMR has the ability to assess for myocardial infarction using gadolinium delayed enhancement techniques. The operator can select an inversion time that will “null” normal myocardium resulting in images where viable myocardium appears uniformly dark while a region of myocardial infarction or fibrotic scar appears bright. Dysfunctional but viable myocardium is expected to have functional recovery if revascularised (in the case of hibernating myocardium), with time (in the case of stunned myocardium), or with resynchronization (in the case of dyssynchronous myocardium).
CMR T1 Mapping: State-of-the-art Technology for Myocardial Tissue Characterization:
Mapping of myocardial T1 & T2 relaxation by CMR is a novel concept of quantifiable myocardial tissue characterization and has emerging data show important clinical utility over and beyond what LGE alone can provide. Parametric T1 maps provide a pixel-wise reflection of T1 values, and even without the application of contrast agents, it is possible to detect pathologically important findings that otherwise can be difficult to assess clinically. Clinical application of T1& T2 Mapping is in evaluation of Cardiomyopathies & ischemic Heart Disease. Extracellular volume fraction (ECV) has been shown to quantify the full extent of myocardial fibrosis in non-infarcted myocardium. ECV may predict outcomes at least as effectively as left ventricular ejection fraction.
CMR can accurately quantify iron levels in the heart and liver, particularly as a result of chronic blood transfusions in patients with Thalassemia and Sickle Cell Anemia. Iron overload shortens T2* relaxation properties of the myocardium & liver. Intriguingly, some patients with thalassemia have iron overload in the heart but not in the liver & vice versa. Thus, CMR determinations of iron overload may be better at assessing patient risk than relying on liver biopsy alone and may be used to follow therapy success.
T2 Star Sample Report
LV & RV myocardial strain analysis allows quantification of regional myocardial contractility that is of paramount importance for the diagnosis & management of the patient with cardiac disease. Ejection Fraction is a global assessment of left ventricular performance & does not take into consideration regional contractile dysfunction, which is commonly seen in patients with coronary artery disease and primary myocardial disease.