Discuss how cardiac MRI has revolutionised the diagnosis of human heart failure.

Magnetic resonance imaging (MRI) techniques endure as an evolving clinical tool, providing intricate and comprehensive assessments of anatomical and functional variations in the diagnosis of various injuries and ailments. Particularly, cardiac MRIs have evolved as a revolutionary imaging modality in providing non-invasive, high quality, cross sectional images that enable for precise anatomical delineation in the diagnosis and monitoring of human heart failure (Peterzan et al., 2016; Karamitsos et al., 2009). In considering family history and the onset of any exercise induced symptoms mimicking heart failure, cardiac MRIs are performed as a frontline gold standard method to collate numerical measurements relating to ejection fraction, heart muscle thickness and overall heart size (Kanagala et al., 2018; Stokes et al., 2016). These can in turn identify coronary heart disease, heart attacks, cardiomyopathy, congenital heart disease, diabetes, hypertension, arrythmia, renal disease and obesity as implicated in heart disease. In doing so, cardiac MRIs assess the central changes evidenced in patients with heart failure and provide a revolutionary diagnostic ability. In providing a non-invasive internal view of precise anatomical and functional delineations, cardiac MRIs allow for a revolutionary characterisation and diagnosis of heart failure that other imaging modalities lack.

The strengths and limitations of cardiac MRIs will initially be discussed in underpinning the revolutionary role of various cardiac MRI techniques in the diagnosis of human heart failure. Subsequently, case studies will be utilised to explore the additive nature of cardiac MRIs in clinical settings to imaging modalities such as cardiac CT scans, echocardiography and X-rays. The sole use of cardiac MRIs will also be analysed in the diagnosis of heart failure. Finally, the move from obtaining qualitative diagnostic information towards more refined quantitative assessment methods of imaging will be explored in further emphasising the central role of cardiac MRIs in the diagnosis of heart failure.

The revolutionary role of cardiac MRIs is most significantly attributed to the precise delineation of anatomical structures which accordingly allows for various qualitative and quantitative assessments to be performed. With precise delineation of anatomical structures, it is possible to render myocardial tissue characterisation and reproduce measurements of blood flow and volume (Peterzan et al., 2016). In doing so, regions of abnormal changes are highlighted and become easily distinguishable as symptomatic of heart failure (Fig 1). This is pivotal in underpinning characteristic tissue changes that are recognised with the onset of heart failure without pursuing invasive investigation. Additionally, the contrast mediums used in cardiac MRI exams, primarily gadolinium, are less likely to be contraindicated in allergic reactions than iodine-based radioactive contrast materials utilised in conventional X-ray and cardiac CT scans (Kanagala et al., 2018). While disadvantages persist in determining the suitability of cardiac MRIs for claustrophobic patients and for patients with metallic implants (neurostimulators and pacemakers), the advantages by far outweigh the results that are attained in proceeding with cardiac MRI scans. Perhaps more significantly and on a more global scale, accessibility to MRI scanners resides as an issue for individuals from remote areas and lower GDI nations (Peterzan et al., 2016). In such cases, resorting to less costly imaging modalities such as cardiac CT scans or echocardiography is apprehensible. However, while these factors do prevail, they do not withdraw from the revolutionary role of cardiac MRIs in heart failure diagnosis. These limitations only ground areas of development in forming universally accessible MRI type imaging techniques.

Figure 1. (A): patchy mid-wall late gadolinium enhancement. Arrows elucidate septal mid-wall and inferolateral wall fibrosis (B): autopsy sample from patient depicting the same pattern of fibrosis. This is indicative of the crucial role played by cardiac MRIs in capturing signs of early heart failure progression without the need for invasive investigation. (Taken from Assomull, 2007)

As an additive imaging modality, MRIs are able to be used in conjunction with x-rays, cardiac CT scans and echocardiographs to develop a holistic understanding of heart failure classification and strategies for therapy. Generally, chest x-rays are used to analyse the presence of an enlarged cardiac silhouette, pleural effusion and Kerley B lines to inspect heart failure likeliness (Peterzan et al., 2016; Stokes et al., 2016). In particular, pulmonary oedema is attributed to congestive heart failure. Following pressure increases in blood vessels, fluid is pushed against the alveoli in the lungs. These reflect chronic elevation of left atrial pressure and the thickening of the intralobular septa as a consequence of oedema (Fig 2) (Chalian et al., 2016). Therefore, x-rays allow for initial examination and prognosis of referred patients. However, it is the role of MRIs to correlate these issues recognised in X-rays to functional deficits of the heart in providing a diagnosis (Kanagala et al., 2018). Cardiac MRIs will provide accurate images of cardiac structures in evaluating myocardial disease. Most significantly, cardiac MRIs will detect focal and diffuse myocardial fibrosis and will evidence signs of cardiac amyloidosis, sarcoidosis, hemochromatosis and myocarditis in patients to diagnose heart failure (Karamitsos et al., 2009) (Fig 2). Hence, cardiac MRIs underpin the definitive diagnosis of patients and work superiorly to X-rays in detecting and determining the functional causes of initially abnormal X-ray observations.

Figure 2. Top Left: Bilateral pleural effusion indicated by arrows. Top Right: horizontal lines extending to periphery denote thickened oedematous interlobular septa following heart failure. Bottom left: short axis cardiac MRI in patient shows mid-myocardial enhancement in mid lateral and inferior segments indicated by arrows. Bottom Right: Fused PET/MR image after glucose diet elucidates patchy areas of intense uptake in lateral and inferior wall. Images highlight sequential method used in localising heart failure diagnosis and classification in patients (Taken from Fishman, 2017; Chalian, 2016).

Similarly, cardiac CT (FDG-PET/CT) scans are used in localising the presence of abnormal activity in the heart. FDG-PET/CT scans evidence abnormalities through high uptake and unusual FDG activity. When using PET with low dose CT scans, it is generally too difficult to localise lesions or to quantify thickened muscular walls in the heart. Therefore, MRIs are used to provide additional information in localising and characterising masses and abnormalities in patient scans (Adigopula & Grapsa, 2018). This is attributed to MRIs providing a 3-5-fold higher difference in image intensity between viable and infarcted myocardium.  For example, if a muscular mass is shown in an FDG-PET/CT scan, MRIs will be used as a complementary imaging modality for precise anatomical localisation. The cardiac MRI component also allows for functional information to be gauged on valve and papillary muscle function which can further be analysed by accurate quantification of ventricular and valvular function and activity (Chalian et al., 2016). Therefore, it is the pivotal role of cardiac MRI images in providing valuable anatomical and functional information that solidifies the ability for definitive diagnosis.

Figure 3. Top Left (a): Axial FDG-PET/CT image evidencing high uptake adjacent to SVC. Bottom Left (b): Four-chamber MR image show lipomatous hypertrophy of interatrial septum. Top Right: Myocardial infarction depicted by short-axis delayed enhancement MRI showing large transmural scar of anterior wall (yellow arrow) and partial thickness scarring of anterior wall (red arrow). Viability of this segment is borderline. Bottom Right: Fused PET/MR image shows no anterior or inferolateral uptake and elucidates viability. Therefore MR images are used collaboratively to determine functional changes that are attributed to CT scans depicting increased uptake. (Taken from Chalian, 2016)

There currently also resides an emerging potential where hybrid PET/MRI scans are employed for cardiovascular disease investigations. Fused PET/MR images can establish further precision by investigating viability assessments of abnormal segments via dye uptake. Practical applications use the high spatial resolution of cardiac MRIs to localise uptake of PET isotopes that are directed towards specific pathophysiological abnormalities (Chalian et al., 2016). Therefore, in combining the anatomical detail of MRIs with the sensitive metabolic information of PET, imaging is able to capture defects of sympathetic innervation in heart failure. This further solidifies a revolutionary role played by MRIs in heart failure diagnosis.

Echocardiography and cardiac MRIs also work collaboratively to elucidate the onset of functional and anatomical delineations in heart failure. Particularly, comparisons are investigated between the two imaging modalities when presenting left ventricular hypertrophy (LVH). Compared to echocardiography, cardiac MRIs offer a substantially improved endocardial visualisation and potential for assessing and evaluating scar tissue (Stokes et al., 2017; Coldea & Lupu, 2012; Valente et al., 2014). Moreover, cardiac MRIs are recognised to elicit high quality assessments of cardiac chamber measurements as a result of increased spatial and contrast resolutions. These further allow for delineation of endocardial and epicardial borders. While, echocardiographs are more cost effective and may be utilised as an initial imaging technique, cardiac MRIs are the gold standard method to determine both quantitative and qualitative evaluation of LVH (Valente et al., 2014). Echocardiographs encounter issues with tracing endocardial contours as a result of limiting contrast resolution. This grounds the revolutionary role of cardiac MRIs in proving high resolution images which allow for distinct tracing by depicting the presence of muscular borders from endocardial features such as trabeculations or papillary muscles. These traces are also now automated with cardiac MRI software.

Figure 4. Paired echocardiography and cardiac MRI from 4 patients presenting with maximal wall thickness identified precisely by only the cardiac MRI imaging modality. Regions of thickening are indicated by arrows (Taken from Valente et al., 2014)

In light of the additive role of cardiac MRIs, it is also pertinent to discuss the sole role of cardiac MRIs in the diagnosis of heart failure. The revolutionary role of cardiac MRIs can holistically be attributed to the broad-spectrum utility provided by MRIs in imaging various cardiac health problems (Fig 5). This elucidates the vital role of cardiac MRIs in the anatomical and functional characterisation of heart failure without the need to resort to secondary imaging modalities (Peterzan et al., 2016).

Figure 5. The broad-spectrum capabilities of cardiac MRI imaging techniques in distinct clinical settings. These underpin the revolutionary role of cardiac MRIs in heart failure assessment and subsequent tailorable diagnostic ability for HF characterisation (Taken from Chalian, 2016)

Contrast studies with gadolinium in cardiac MRIs prove invaluable when differentiating between ischemic and non-ischemic cardiomyopathy (Thompson & Maredia, 2017). The onset of late gadolinium enhancement (LGE) is fundamental in differentiating between the ischemic nature of cardiomyopathy (Francone, 2014; Boonyasiranant & Flamm, 2010). For example, patients presenting with dilated non-ischemic cardiomyopathy generally evidence a mid-wall distribution of LGE (Fig 6). LGE is a risk for heart failure. As a result, cardiac MRIs are able to elucidate the characterisation of heart failure by utilising non-ionising contrast agents.

Figure 6. Short axis cardiac MRI image in patient with dilated cardiomyopathy presenting mid-wall LGE distribution (Taken from Peterzan et al., 2016).

Moreover, the sole role of cardiac MRIs can also be examined in cases of heart failure with suspected iron overload. This is particularly pronounced in patients with thalassaemia, using T2 cardiac MRI for definitive diagnosis. In patients with heart failure and possible iron overload, cardiac MRI with T2 provides definitive diagnosis (Anand & Janardhanan, 2016; Peterzan et al., 2016). Heart failure will generally develop following prolonged periods of iron overload resulting in stiffened and weakened heart muscles.  Thus, if patients were diagnosed with heart failure, the aetiology of the disease would be attributed to the iron overload cardiomyopathy marking the use of cardiac MRIs as invaluable to heart failure assessment and diagnosis (Fig 7).

Figure 7. T2 cardiac MRIs in two patients presenting with thalassaemia. Left: iron loading of heart, spared liver. Right: iron loading of liver, heart spared (Taken from Peterzan et al., 2016)

Cardiac MRIs can also provide quantitative data during imaging. For example, the most common form of quantification is carried out in utilising highly reproducible measurements of ventricular volume, muscle thickness, myocardial mass and the flow of blood across heart valves. Ejection fraction is also a critical measurement used in analysing a weakened left ventricle (Agha et al., 2018). If a patient presents with a reduced ejection fraction (generally 40% or less), it is classified as a heart failure. Following imaging procedures and computerised quantifications, abnormalities are evidenced and able to be analysed on various planes with mathematical inputs. Measurements can also encompass ventricular size and function to establish heart failure characterisation and possible aetiology (Boonyasiranant & Flamm, 2010). Myocardial viability is therefore underpinned through quantitative cardiac MRI techniques.

T1-mapping techniques extend beyond standardised MRI techniques and allow for understanding of the pathophysiological processes culminating in heart failure development in a wide range of diseases. In determining diffuse myocardial fibrosis (DMF) and extracellular volume calculation, T1 mapping techniques allow for characterisation processes and diagnosis (Adam et al., 2017; Radenkovic et al., 2017). T1 mapping can also be pivotal in monitoring DMF measures in response to therapy. This develops diagnostic confidence in supplement to standard MRIs.

Holistically, these quantifiable measures suggest an inherent revolutionary role embedded in cardiac MRIs during heart failure diagnosis.

Conclusively, cardiac MRIs have profoundly established a revolutionary take on human heart failure assessment and diagnosis. It is the additive, sole and quantifiable capacities of cardiac MRIs that underpin the pivotal role played by cardiac MRIs in heat failure diagnosis, characterisation and aetiology determination. Through discussion of these factors it is unquestionable that MRIs are a superior imaging modality in the qualitative and quantitative assessment of heart failure diagnosis.

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