Diffusion tensor magnetic resonance imaging

The microstructure of the heart has a major impact on its mechanical and electrical properties. Diffusion tensor magnetic resonance imaging (DTI) exploits the anisotropic restriction of water diffusion in the myocardium to resolve its microstructure. Recent advances in the field have included the development of acceleration-compensated diffusion-encoded sequences, the investigation of sheet dynamics, and the development of highly accelerated techniques to enable whole heart coverage. Translational studies have demonstrated the utility of DTI in heart failure and other cardiomyopathies. While DTI of the heart remains investigational, ongoing advances in the field will soon allow the technique to be performed reliably and quickly in appropriate clinical scenarios.

Novel CMR techniques for advanced surgical planning

Medical and surgical care for the patient with congenital heart disease (CHD) has advanced greatly over the past 40 years; along with improved surgical and catheter-based techniques, intensive unit care, and overall medical advances, improved outcomes have accrued across a whole host of cardiac defects. This is owed, in no small part, to advances in imaging and cardiovascular magnetic resonance (CMR) which has played an important and growing role in this evolution. Novel CMR techniques 25 years ago, such as gadolinium-based imaging and two-dimensional velocity mapping, are now commonplace. At the cutting edge of novel CMR techniques, in the current era, are computational fluid dynamic modelling, three-dimensional printing, four-dimensional flow imaging, and X-ray magnetic resonance/interventional CMR, which will be the focus of this chapter. The hope is that one day these techniques will be the commonplace ones, aiding in the care of a broad spectrum of CHD.

Pericardial tumours

Primary malignant tumours of the pericardium include the solitary fibrous tumour and pericardial mesothelioma, which is the most common primary malignant pericardial tumour. Finally, intrapericardial germ cell tumours are neoplasms of germ cell origin that arise within the pericardium and mostly occur in infancy or childhood.

Myocardial iron overload

The heart is the target lethal organ in thalassaemia major. Cardiovascular magnetic resonance (CMR) measures iron using the magnetic relaxation time T2*. This allows comparison with the left ventricular function and conventional iron measurements such as liver iron and serum ferritin. The single breath-hold cardiac-gated CMR acquisition takes only 15 seconds, making it cost-efficient and relevant to developing countries. Myocardial T2* of <20 ms (increased iron) correlates with reduced left ventricular ejection fraction, but poor correlation exists with ferritin and liver iron, indicating poor capability to assess future risk. Myocardial T2* of <10 ms is present in >90% of thalassaemia patients developing heart failure, and approximately 50% of patients with T2* of <6 ms will develop heart failure within 1 year without intensified treatment. The technique is validated and calibrated against human heart iron concentration. The treatment for iron overload is iron chelation, and three major trials have been performed for the heart. The first trial showed deferiprone was superior to deferoxamine in removing cardiac iron. The second trial showed a combination therapy of deferiprone with deferoxamine was more effective than deferoxamine monotherapy. The third trial showed that deferasirox was non-inferior to deferoxamine in removing cardiac iron. Each drug in suitable doses can be used to remove cardiac iron, but their use depends on clinical circumstances. Other combination regimes are also being evaluated. Use of T2*, intensification of chelation treatment, and use of deferiprone are associated with reduced mortality (a reduction in deaths by 71% has been shown in the United Kingdom). The use of T2* and iron chelators in the heart has been summarized in recent American Heart Association guidelines.

Infiltrative cardiomyopathy

Abnormal substances can deposit in the myocardium either in the extracellular space (infiltration) or in cells (storage). Infiltration may be cells (inflammatory, histiocytosis, or tumour) or amyloid fibrils [in ventricular myocardium light chain-related (AL) or transthyretin-related (TTR), wild-type or mutant]. Storage may be glycogen (glycogen storage diseases, Danon), lipid (Fabry, Gaucher), mucopolysaccharidoses, or iron. Iron, malignancy, and inflammation (myocarditis) are covered elsewhere. Amyloid and storage diseases are typically systemic multi-organ disease, with ‘red flag’ clinical features often present. They mainly cause heart muscle disease, with hypertrophy mimicking hypertrophic cardiomyopathy. All are relatively rare and often diagnosed late when therapies are less effective. Imaging structural and functional changes provide pointers to the underlying aetiology and additional features may be present (perfusion defects, valve disease, atrial thickening), but it is in myocardial tissue characterization where CMR adds real value. In amyloid, deposition appears to proceed stepwise, with initial subendocardial, and later transmural, late gadolinium enhancement (LGE). Myocardial nulling may be difficult, requiring the phase-sensitive inversion recovery (PSIR) technique. In Fabry disease, a characteristic initial basal inferolateral LGE pattern occurs, later with extensive LGE, leading to dilatation and impairment. Mapping adds value. In amyloid, both native T1 and the ECV are very high. Both are prognostic and candidates for surrogate endpoints in drug development studies. In Fabry disease, native T1 is low, reflecting lipid storage, and may occur early before hypertrophy. The LGE area usually has T2 elevation correlating with blood troponin, which suggests inflammation as part of disease development.

Chagas’ cardiomyopathy

Chagas’ cardiomyopathy is a major complication emerging from Trypanosoma cruzi infection, appearing in up to 30% of individuals with positive serology and being the principal cause of death from heart failure in some areas of South America. The natural history of Chagas’ cardiomyopathy classically includes two phases: an acute phase, typically with absence of symptoms to mild, non-specific symptoms, and a chronic phase which comprises two forms of disease—an indeterminate (latent, pre-clinical) form and a determinate or clinical form. Patients with the indeterminate form may be asymptomatic for decades, until unidentified triggers initiate disease progression to chronic chagasic cardiomyopathy, manifesting in a broad range of clinical presentations, including cardiac arrhythmias, thromboembolism, heart failure, and sudden death. Non-invasive imaging modalities capable of characterizing not only cardiac morphology and function, but also myocardial tissue remodelling and disease progression, may play an important role in Chagas’ disease. Myocardial late gadolinium enhancement by cardiovascular magnetic resonance (CMR) has been considered the most accurate method to detect myocardial fibrosis in ischaemic and non-ischaemic cardiomyopathy, including Chagas’ cardiomyopathy. Myocardial tissue characterization, uniquely provided by CMR, holds enormous potential within a complex and not completely understood cardiomyopathy, with poor prognosis and limited therapeutic options. This chapter aims to discuss several relevant aspects of Chagas’ cardiomyopathy, focusing on the usefulness of CMR in the diagnosis and risk stratification of affected patients.

Myocarditis

Myocarditis has a high prevalence, especially in young and middle-aged patients. It is the most important differential diagnosis in patients with acute cardiac disease and evidence for cellular injury (positive troponin). In clinical decision-making, it is important to rule in or rule out myocardial inflammation. While endomyocardial biopsy, which remains the gold standard to achieve an aetiopathogenetic diagnosis, can be helpful in patients with heart failure, it is less used in the majority of cases. Cardiovascular magnetic resonance (CMR) imaging has become the most efficient non-invasive diagnostic tool for patients with suspected myocarditis. Its unique value is based on the ability to identify inflammation and myocardial injury, in combination with an accurate assessment of ventricular volumes, as well as regional and global function. In many centres, myocarditis is the most frequent indication for CMR. The diagnostic criteria include markers for myocardial oedema, hyperaemia, and necrosis, while regional or global dysfunction and pericardial effusion serve as supportive criteria. Novel markers, such as quantitative mapping techniques, may allow for even better identification and classification of myocarditis.

Chronic ischaemic heart disease

Chronic ischaemic heart disease (IHD) is one of the most common cardiac conditions worldwide and is generally caused by the consequences of coronary atherosclerosis, including myocardial infarction. Clinical challenges in chronic IHD include detection of myocardial ischaemia in symptomatic patients with suspected coronary artery disease (CAD), evaluation of myocardial viability in patients with established IHD and poor left ventricular ejection fraction (LVEF) when revascularization is considered, as well as risk stratification and identification of patients with chronic IHD at high risk of complications. Cardiovascular magnetic resonance (CMR) can provide vital answers to all three of these challenges. Stress CMR is now increasingly used to detect ischaemia by means of vasodilator stress perfusion or dobutamine stress contractile reserve stress imaging. For viability assessment, late gadolinium enhancement is currently the method of choice to detect myocardial infarction, and low-dose dobutamine stress magnetic resonance can provide additional information to determine viability and guide therapy. Cardiovascular risk in patients with chronic IHD is mainly determined by left ventricular function, most commonly utilizing LVEF, as well as infarct size, infarct characteristics, and ischaemic burden, which can all be measured reliably with CMR. This chapter will review the role of CMR for the detection of myocardial ischaemia, viability, and risk.

Basic MR physics

In magnetic resonance, the properties of protons in tissue giving rise to so-called magnetic moments are exploited. The sum of many magnetic moments yields what is referred to as net magnetization, which can be seen as similar to the magnetization a bar magnet produces. The relation and interaction between magnetic moments, net magnetization, the static magnetic field, and radiofrequency fields are discussed. It is shown that radiofrequency excitation can be used to manipulate the net magnetization, such that it can be detected using radiofrequency antennae or coils. Upon excitation, the net magnetization will recover back to its equilibrium orientation with tissue-specific time constants for the transverse and longitudinal components, which, in turn, are important sources of image contrast in cardiac imaging. The discussion concludes with a foray into susceptibility and chemical shift effects resulting from different molecular environments in which protons can reside and which provide additional image contrast mechanisms.

Interventional CMR (MRI catheterization)

Real-time magnetic resonance imaging (MRI) is currently suitable to guide diagnostic MRI cardiovascular catheterization in patients. This approach is attractive to characterize haemodynamics and function concurrently, especially in the evaluation of cardiomyopathy and pulmonary artery hypertension. Safe clinical guidewires are entering commercial distribution and, combined with passive catheters, allow successful MRI catheterization in adults and children. This chapter briefly describes suitable instrumentation and approaches.