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Zohya Khalique, Dudley J Pennell, What is CMR doing for patients today?, European Heart Journal, Volume 39, Issue 4, 21 January 2018, Pages 266–270, https://doi.org/10.1093/eurheartj/ehx778
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A concise review for today’s use in different conditions from the Royal Brompton Hospital
Cardiovascular magnetic resonance (CMR) is a valuable imaging modality. High spatial resolution offers gold-standard assessment of structure and function, whilst the unique capability of tissue characterization provides diagnostic and prognostic insight into many complex cardiac conditions. Imaging using gadolinium contrast agent can detect thrombus in the early phase, and late gadolinium enhancement (LGE) identifies focal fibrosis enabling the diagnosis of infarction, infiltrative myocardial diseases, and cardiomyopathies (Figure 1). Parametric mapping, based on the relaxation properties of the excited protons, offers potential to characterize diffuse fibrosis and oedema using the measures T1, T2, and extracellular volume (ECV).
These versatile techniques are possible without ionizing radiation, making CMR a safe modality for serial use. Hence CMR is a powerful tool, increasingly being incorporated into European and American guidelines. This article highlights the benefits that CMR brings to cardiac patients.
Coronary artery disease
Cardiovascular magnetic resonance can fully assess CAD within a single scan guiding therapeutic interventions. Cine imaging assesses regional wall motion and with the addition of stress testing can evaluate inducible ischaemia. Current guidelines support the use of CMR as a first line non-invasive strategy for evaluating ischaemia in patients with suspected or stable CAD, with the benefit of not using ionizing radiation.
Negative stress perfusion, CMR has excellent negative predictive values, with annual event rates for cardiac death and non-fatal myocardial infarcts of <1%. The MR-Inform trial suggests CMR-stress perfusion imaging can guide patient management in a high-risk population with the same effectiveness as invasive angiography with fractional flow reserve.
The CE-MARC2 trial showed investigation by CMR spared unnecessary angiography, with no difference in major adverse cardiac event rates at 12 months post-randomization to either CMR or UK angiography driven national guidelines.
After a perfusion scan, LGE CMR can identify reversible myocardial injury that will benefit from revascularization. In patients with prior silent infarcts, LGE is a strong predictor of cardiac events. It also highlights areas of microvascular obstruction and haemorrhage in acute infarction, which portend adverse left ventricular (LV) remodelling and poor prognosis.
Cardiomyopathy
Cardiovascular magnetic resonance is now essential in the assessment of cardiomyopathy. In hypertrophic cardiomyopathy (HCM), CMR is superior to echocardiography for measurement of wall thickness. Perfusion CMR can reveal microvascular perfusion defects, whilst LGE shows areas of fibrosis. A meta-analysis of LGE in HCM has shown prognostic value of fibrosis in predicting increased rates of sudden cardiac death (SCD), cardiovascular mortality, and all-cause mortality. The extent of LGE is associated with an increased risk of cardiac events, even after adjusting for baseline characteristics such as ejection fraction (EF). Cardiovascular magnetic resonance also has a valuable role in distinguishing HCM from HCM phenocopies. Deranged gadolinium kinetics (black blood pool) and elevated T1 and ECV direct towards a diagnosis of cardiac amyloidosis, whilst markedly decreased T1 is in keeping with Fabry’s disease (Figure 2).
In dilated cardiomyopathy (DCM), CMR provides accurate, reproducible volumetric assessment, which is valuable for early diagnosis and for following response to treatment. Late gadolinium enhancement differentiates ischaemic origin from cardiomyopathy and can suggest possible aetiology, such as myocarditis. Mid-wall LGE in DCM is diagnostic and also has prognostic implications. The presence of LGE is associated with increased likelihood of all-cause mortality as well as SCD and heart failure. In DCM, implantable cardioverter defibrillator (ICD) therapy for risk of SCD has largely been driven by EF, but it is recognized that the majority of cardiac arrests are in patients with an EF >35%. Mid-wall LGE is able to identify patient with EF >40% who have an increased risk of SCD who may preferentially benefit from ICD implantation.
Myocarditis is an inflammatory cardiomyopathy that has benefitted from multi-parametric CMR imaging. Endomyocardial biopsy is an invasive procedure, subject to sampling error, and not routine practice. Cardiovascular magnetic resonance has taken a role in the non-invasive diagnostic criteria through use of the ‘Lake Louise Criteria’ of T2 weighted oedema imaging and both early and late gadolinium enhancement. Cardiovascular magnetic resonance has showed that a high proportion of patients with chest pain but normal coronary arteries have myocarditis. Cardiovascular magnetic resonance also forms part of the task force criteria for the diagnosis of arrhythmogenic right ventricular cardiomyopathy, in both quantification of right ventricular (RV) size and dysfunction.
Finally, in cardiac siderosis, the use of T2* CMR has revolutionized outcomes for patients with thalassaemia major and transfusion iron overload. Since 2000 in the UK alone, T2* CMR has driven a 71% reduction in the annualized death-rate from iron overload (Figure 3). Serum ferritin and liver iron are poor correlates of cardiac iron loading. However, T2* CMR not only quantifies myocardial iron loading, but also is the most important predictor of heart failure development. Cardiovascular magnetic resonance plays an ongoing substantial role in the care of patients with cardiac siderosis worldwide.
Congenital heart disease
Cardiovascular magnetic resonance is especially beneficial in the assessment of congenital heart disease, in part because the lack of ionizing radiation makes it ideal for serial scanning over a patient’s lifetime. High spatial resolution, comprehensive anatomical delineation, accurate volumetric, vascular and valve flow measurement, and 3D reconstructions render CMR a holistic imaging modality. In addition to anatomical and functional definition, CMR detected fibrosis is associated with adverse clinical outcomes.
In patients with atrial redirection surgery for transposition of the great arteries (TGA), systemic RV LGE is associated with tachyarrhythmias, decompensated heart failure admission, transplantation and death. In patients with a systemic RV (congenital correction of TGA, post-Mustard, and Senning procedures) focal fibrosis associates with RV dysfunction, arrhythmias, and exercise tolerance. In repaired tetralogy of Fallot, LGE was related to impaired exercise capacity, ventricular dysfunction, and clinical arrhythmia, whilst RV LGE predicted arrhythmias.
Cardiac masses
In patients with cardiac masses, CMR is commonly used for morphological assessment and tumour characterization (Figure 4). Gadolinium imaging, T1 and T2 weighted imaging, as well as parametric mapping provide information on tissue composition, whilst perfusion determines vascularity, which is often a marker of malignancy (benign exceptions include haemangioma and myxoma). Cardiovascular magnetic resonance is therefore helpful in differentiating malignant from benign masses and distinguishing normal anatomical variants from pathology.
Valvular diseases
Cardiovascular magnetic resonance assesses valvular flow abnormalities well and can be particularly helpful in the presence of associated structural abnormalities, for example bicuspid aortic valve with coarctation of the aorta, or pulmonary valve disease with RV assessment. Replacement fibrosis in aortic stenosis identified by LGE has been shown to be predictive of 5-year survival rates, and LGE in conjunction with ECV is able to identify patients with adverse LV remodelling, increased troponin and BNP levels as well as reduction in functional capacity.
From today to tomorrow
Cardiovascular magnetic resonance is a highly versatile imaging technique, and continues to expand its capabilities. Multi-parametric mapping adds a new dimension to the unique ability of tissue characterization, improving diagnosis, but also providing novel prognostic information in hitherto difficult to manage conditions. Very new techniques, such as diffusion tensor CMR, will bring new insight into the myocardial microstructure, which has not been imaged previously invivo (Figure 5). Together these could create a CMR guided virtual myocardial biopsy.
Other new areas include 4D flow, which may offer a fuller multiplanar method of assessing flow. Cardiovascular magnetic resonance is also being combined with other techniques in a hybrid fashion, for example PET-CMR (showing promise in cardiac sarcoidosis) and XMR (cath lab combined with CMR) for novel approaches to interventions guided by MR and guided ablation procedures.
Conflict of interest: none declared.