This last entry of the blog will be focusing on the imaging modalities utilised when assessing the viability of the myocardial tissue before revascularisation. I chose this topic because of my interest on the cardiovascular system and this would help me become familiar with the role of diagnostic imaging in the diagnosis and assessment of cardiovascular diseases. The term myocardial viability will be used to describe the myocardium with reversible contractile dysfunction in patient with coronary artery disease (CAD) (Allman, 2013).
Coronary artery disease is a condition where the arteries supplying blood to the heart muscle have narrowed due to gradual build-up of fatty material on their walls. This is known as atherosclerosis. It can cause a blockage in the coronary arteries which leads to the heart muscle not receiving oxygen-rich blood. If the blood flow is not re-established, it may lead to that part of the myocardium to die and is known as a heart attack (University Hospitals Birmingham NHS Foundation Trust, 2017). Some of the risk factors of CAD are diabetes, hypertension, smoking, lack of exercise, obesity and family history of CAD (NHS Choices, 2017).
The assessment of the viability of myocardium is essential in determining whether a patient is suitable for coronary revascularisation (Briceno et al. 2016; Allman, 2013; Anagnostopoulos et al. 2013). According to the European Society of Cardiology and the European Association for Cardio-Thoracic Surgery (ESC and EACTS, 2014), there are several different methods of myocardial revascularisation. These include coronary artery bypass graft (CABG), balloon angioplasty, and percutaneous coronary intervention with bare-metal stents (BMS) or drug-eluding stents (DES). Several imaging methods, such as single photon emission computed tomography (SPECT), positron emission tomography (PET), cardiac magnetic resonance (CMR) and echocardiography, are utilised in assessment of myocardial viability prior to revascularisation procedures. While these methods focus on determining the functionality of the tissue, it is important to note that there are also structural changes that occur due to prolonged lack of adequeate blood flow to the myocardium (Allman, 2013).
According to Anagnostopoulos et al. (2013) PET myocardial perfusion imaging (MPI) has sensitivity and specificity values around 90 %. It demonstrates metabolic and cellular functions to help differentiate viable myocardium from non-viable (Briceno et al. 2016). The technological advancements over the years have allowed PET to contribute significantly in the diagnosis and management of patients with coronary artery disease. Some of these advantages include determining prognosis for people with suspected or known CAD. The findings are classed in to four categories; 1) normal flow/metabolism (viable), 2) mild matched reduction in flow/metabolism 3) severe matched defect, or 4) mismatch (reduction in resting flow but glucose is still utilised in tissue). All, except number three, have potential for the myocardium to be viable (Allman, 2013).
The limitation of PET is prominent when used with diabetic patients as the tracer (fluorodeoxyglucose) extraction is limited due to the tracers being insulin-sensitive. Additional imaging with PET would be required at a later stage after administrating insulin in order to have diagnostic images, which increases the radiation dose to the patient. This is important information because one of the risk factors for CAD is diabetes (Anagnostopoulos et al. 2013). However, according to Allman (2013) the risk is assessed according to the patient. This is because the benefit outweighs the risk for patients with low survival rate, while it is a significant concern for a relatively healthy patient undergoing viability assessment. Another strength of PET is that it produces images with high resolution allowing the differences of uptake between regions to be assessed. According to ESC and EACTS (2014), nuclear imaging methods, which includes PET, have been found to have high sensitivity values but have a lower sensitivity when used to assess contractile reserve of the tissue.
CMR works by assessing the concentration of gadolinium in the tissue. The contrast media gathers into the interstitial space of non-viable tissue and will remain there for a time. The accumulation also can demonstrate the thickness of scar tissue in the muscle. The area that is not enhanced is considered viable. CMR is also used to assess function such as wall motion and end-diastolic wall thickness. The latter is a very important indicator of functional recovery as it diminishes with decreasing wall thickness. The main limitation of CMR is that it is contra-indicated in patients with severe renal impairment and patients with implanted devices causing rhythm disturbance, device motion and heating of the lead (Allman, 2013). However, due to the lack of PET scanners there is an increase in the use of CMR when assessing myocardial viability (Timmer et al. 2017). Perhaps this is a positive aspect and further research into improving the sensitivity and specificity of CMR would be appreciated as it is non-ionising and would lessen the radiation dose patients may accumulate during their treatment. This is because coronary angioplasty is performed under fluoroscopic guidance, which is why non-ionising diagnostic test would be best for the patient. I had the opportunity to observe a coronary angioplasty procedure and it was a quite long and difficult case. Considering the length and the small area being imaged, the risk of radiation burn seems to be quite high.
According to Briceno et al. (2016) there is a lack of randomised control studies to strongly support the use of late contrast-enhanced CMR to predict the degree of functional recovery following revascularisation. However, there is no mention of this fact by Timmer et al. (2017) which reveals a need for studies with larger sample sizes to be conducted in order to fully assess the usefulness of CMR particularly as it is non-ionising.
This final blog is short because it is a very broad subject and I found it hard to understand all the aspects of imaging the myocardium. However with this exercise I have developed skills which have made me enjoy learning. I hope these blogs have been informative and enjoyable. I have found them to be educational and the improvement in my academic writing skills has boosted my confidence significantly.
Allman, K. (2013) Noninvasive assessment myocardial viability: Current status and future directions. Journal of Nuclear Cardiology [online] 20 (4) pp. 618-637 [Accessed 27 November 2017].
Anagnostopoulos, C., Georgakopoulos, A., Pianou, N., Nekolla, S.G. (2013) Assessment of myocardial perfusion and viability by Positron Emission Tomography. International Journal of Cardiology [online] 167 (5) pp. 1737-1749 [Accessed 27 November 2017].
Briceno, N., Schuster, A., Lumley, M., Perera, D. (2016) Ischaemic cardiomyopathy: pathophysiology, assessment and the role of revascularisation. Heart (British Cardiac Society [online] 102 (5) pp. 397-406 [Accessed 27 November 2017].
NHS Choices (2017) Coronary heart disease: causes. Available at: https://www.nhs.uk/conditions/coronary-heart-disease/causes/ [Accessed 01 December 2017].
The European Society of Cardiology, the European Association for Cardio-Thoracic Surgery (2014) 2014 ESC/EACTS Guidelines on myocardial revascularization. European Heart Journal [online] 35 pp. 2541-2619 [Accessed 30 November 2017].
Timmer, S., Teunissen, P., Danad, I., Robbers, L., Raijmakers, P., Nijveldt, R., van Rossum, A., Lammertsma, A., van Royen, N., Knaapen, P. (2017) In vivo assessment of myocardial viability after acute myocardial infarction: A head-to-head comparison of the perfusable tissue index by PET and delayed contrast-enhanced CMR. Journal of Nuclear Cardiology [online] 24 (2) pp. 657-667 [Accessed 27 November 2017].
University Hospitals Birmingham NHS Foundation Trust (2017) Coronary artery disease. Available at: https://www.uhb.nhs.uk/coronary-artery-disease.htm [Accessed 30 November 2017].