Key Points

• 
  

  Cardiac Magnetic Resonance (CMR) provides high spatial and temporal resolution imaging of the beating heart and is the reference standard technique for volumetric analysis of the left ventricle (LV).

  
  
• 
  

  CMR provides accurate morphologic and functional assessment of the right ventricle (RV), which may be difficult to always accurately assess using echocardiography.

  
  
• 
  

  CMR can detect myocardial inflammation and fibrosis to assist with diagnosis and prognostication in heart failure and cardiomyopathies.

  
  
• 
  

  Perfusion imaging can non-invasively evaluate for the presence of coronary artery disease and identify patients at risk of perioperative cardiac complications.

Introduction

Cardiac magnetic resonance (CMR) is a non-invasive imaging modality that allows three-dimensional volumetric analysis and measurement of the left ventriclular (LV) and the right ventricular (RV) function and assessment of abnormalities in wall motion and flow patterns. It also allows characterisation of the myocardium by administering a gadolinium-based, extracellular contrast agent to assess for the pattern and distribution of myocardial inflammation, fibrosis and infarction. Gadolinium is normally restricted to the extracellular and interstitial myocardial spaces, but following changes to the interstitium, such as in myocardial fibrosis or infarction, an increase in the volume of distribution allows more gadolinium to penetrate the myocardium. Although transthoracic echocardiography is the first-line imaging modality of choice in the preoperative diagnosis of most cardiac disorders involving abnormalities of structure and function, patients with poor echocardiographic windows in whom a full evaluation may be suboptimal may benefit from an alternative modality such as CMR. Compared to other imaging modalities such as cardiac Computed Tomography (CT) or myocardial perfusion imaging (MPI), CMR has a higher spatial and temporal resolution, that is, the ability to discriminate between two points in space and time respectively, without exposure to ionising radiation. As such, the utility of CMR in clinical practice has significantly increased in the preoperative assessment of LV function, in myocardial viability and perfusion, the investigation of cardiomyopathies, valvular heart disease and congenital heart disease.

Evaluation of Ventricular Function and Ischaemic Heart Disease

Cine imaging produces moving images of the heart and surrounding structures and allows assessment of ventricular wall thickness, regional wall motion and calculation of LV and RV chamber volumes, mass and function. The flexibility of viewing the heart in any plane is a significant advantage over echocardiography. These measurements are highly reproducible with minimal inter- and intra-observer variability such that CMR is generally regarded as the gold standard method for the quantification of left and right ventricular volumes and systolic function.

Tissue characterisation with late gadolinium enhancement (LGE) can demonstrate a scar pattern in the territory of one or more coronary arteries. Scar characteristically extends from the subendocardium to the epicardium corresponding to the perfusion territory of the coronary arteries. Changes following myocardial infarction can be visualised with excellent spatial resolution, which allows discrimination between subendocardial and transmural infarction, the extent of which correlates with myocardial viability and the success of functional recovery following revascularisation (Figure 5.C.1).

Figure 5.C.1 Sixty-five-year-old lady who presented late with an anterior ST elevation myocardial infarction. Coronary angiography demonstrated occluded mid-left anterior descending artery. CMR LGE imaging confirms almost full thickness scar in the mid to apical inferoseptum and subendocardial scarring in the mid to apical lateral wall (arrows). For details please refer to section Evaluation of Ventricular Function and Ischaemic Heart Disease on page 52.

Myocardial perfusion and ischaemia imaging can be performed as an additional part of the CMR study to assess for coronary artery disease. Under hyperaemic conditions induced by a pharmacologic vasodilator (for example, adenosine or dipyridamole) and injection of a gadolinium-based contrast agent, areas with impaired perfusion appear darker compared to normal myocardium. CMR perfusion imaging has an excellent specificity for the detection of myocardial ischaemia and a negative stress perfusion result is associated with an excellent prognosis in patients with an intermediate risk for cardiovascular events. A large meta-analysis demonstrated that a negative stress study was associated with a very low risk of myocardial infarction or cardiovascular death (0.8 per cent combined annualised event rate) while a positive stress CMR was associated with a 4.9 per cent combined annualised event rate (Lipinski, 2013). Although sequences have been developed to directly visualise coronary arteries, assessment of stenosis severity remains a research application.