5 reasons you need a 3 Tesla for cardiac MRI, and 10 reasons you do not
Magnetic resonance imaging (MRI) has become a cornerstone to diagnose cardiac diseases, allowing clinicians to delve into the intricacies of the heart's anatomy and function. When it comes to choosing the right MRI strength for cardiac imaging, the historical debate between 3 Tesla (3T) and 1.5 Tesla (1.5T) systems often surfaces. Each strength has its unique advantages and challenges. Low-field MRI is a third and increasingly popular option. In this blog post, we will explore the benefits and limitations of each category.
Introduction
MRI is a key technology used in hospitals and medical imaging centers around the world to provide precise diagnoses, and in some cases advanced therapies such as image-guided radiotherapy, focused ultrasounds, or even electrophysiology. There is a dichotomy between high-income countries (HICs) where MRI is widespread, available, and usually based on powerful magnets (1.5T or more) and low to middle income countries (LMICs) where MRI is less developed and often uses less powerful magnets¹.
Figure 1: Number of MRI units in selected countries (per million population). The size of the circle represents the population⁶.
MRI units come in various field strengths, from ultra-high field (7T) to ultra-low field (64 mT). Key opinion leaders, be they radiologists, industry experts or scientists, have hailed the benefits of 3 Tesla (3T) for greater accuracy in cardiac MRI (CMR). 3T MRI has been commercially available for nearly two decades, at a significantly higher price tag than its 1.5T cousin, however, 3T is still not considered the field strength of choice for CMR.
3T
When the first 3T MRI machines started to operate, a controversy called the High-Field-Strength Curmudgeon rocked the industry². Basically, 3T MRI, which used to cost twice as much as 1.5T MRI, was disparaged as being half as efficient. In these early years, the beautiful images from vendors’ marketing materials did not translate into clinical benefits. Fast forward 20 years, and 3T is now an accepted medical standard, and its cost is no longer prohibitive. 3T is often used in research settings to explore emerging techniques, advance our understanding of cardiac diseases, or improve the chances for a research project to be published.
Due to its strong magnetic field, 3T MRI has a high signal to noise ratio (SNR) which results in clearer and more detailed images, for instance, detecting subtle cardiac abnormalities. Furthermore, 3T MRI excels in angiography. Multinuclear spectroscopy is also possible, which may allow advanced metabolic imaging and the study of chemical compounds in-vivo, in the heart. The added SNR serves as a “bank” scientists or clinicians can use to create advanced analyses, for instance quantitative imaging.
However, a strong magnetic field makes MRI more susceptible to motion and susceptibility artifacts which can obscure portions of the cardiac anatomy. Artifacts are not limited to imaging and can affect physiological signals. Due to the “magnetohydrodynamic effect” (MHD), measuring ECG during MRI can be a nightmare for healthcare professionals. This phenomenon is caused by interactions between a strong magnetic field and the electrically conductive blood flowing through the heart and large vessels – mainly the aorta. ECG signals can be so distorted as showing T-waves of the same amplitudes as QRS complexes, thus rendering the detection of R-peaks challenging.
Compared to 1.5T, 3T MRI scanners are less common and may not be as readily available in all healthcare facilities.
1.5T
1.5T, the most established field strength for MRI, provides a good trade-off between image quality and cost. A rumor in the medical industry claims that the world uses 1.5T because GE HealthCare engineers failed to meet the 2 Tesla goal they set for themselves, more than four decades ago. Generally cost-effective to purchase and to maintain, but requiring infrastructure and a reliable power source, 1.5T MRI is widely available in HICs. 1.5T is a high magnetic field, but not so high as to make scanning overly sensitive to patient motion and magnetic susceptibility. When performing CMR, technologists need to take patient respiration and heartbeats into account. Less sensitivity to motion is an asset. 1.5T MRI is suitable for most CMR needs, including assessing cardiac function, detecting structural abnormalities, and evaluating blood flow.
Since 1999³, the use of 2D vectorcardiogram electrocardiograms (ECG) has made CMR practicable at 1.5T, since detecting quintessential QRS complexes is vital to synchronize or gate cardiac scanning. R-peaks, a feature marking the contractions of the heart, are essential to trigger the acquisition of CMR data in a synchronized manner. Finally, recent implantable devices are now “MR Conditional” at 1.5T, meaning patients carrying them can be scanned at 1.5T providing other specific conditions are respected.
Low-field
While 1.5T and 3T MRI scanners are prevalent in HICs, the healthcare landscape is quite different in LMICs, where the availability of high-field MRI scanners is limited. This is due to cost constraints and infrastructure challenges. As a result, low-field MRI has emerged as a viable and pragmatic option in resource-constrained settings.
Many LMICs struggle with maintaining and servicing high-end medical equipment. Low-field MRI systems are generally more affordable and require less maintenance, making them a sustainable choice for resource-constrained settings. The typical cost of a low-field MRI is around two hundred K$, while the costs of high field MRI range from several hundred K$ for 1.5T to more than one M$ for 3T. Infrastructure wise, low-field MRI systems may not demand the same level of infrastructure and power supply as their high-field counterparts. Installation is a significant cost for high field MRI. Some new MRI designs do not need a dedicated shielded or reinforced room (e.g. Hyperfine), or are even mobile. Mobile MRI units cannot, however, do cardiac imaging.
By introducing low-field MRI scanners in LMICs, healthcare providers can expand access to CMR services and perform basic cardiac assessments that can aid in diagnosing common cardiac conditions such as heart valve diseases, cardiomyopathies, and congenital heart defects. This is crucial for early diagnosis and management of cardiac diseases, which are a significant healthcare burden in many LMICs.
Low-field MRI can also serve as a valuable platform for research and medical training in LMICs. It will allow healthcare professionals to gain experience in cardiac imaging and contribute to local research efforts. However, and especially pertaining to CMR, there are challenges that limit the potential of low-field MRI:
Lower SNR, resulting in less detailed images and impacting the ability to detect subtle cardiac abnormalities.
Lack of specific CMR pulse sequences, in particular perfusion or late gadolinium enhancement.
Lack of contrast for angiography.
Advanced techniques such as quantitative imaging or multi-nuclear spectroscopy.
Lack of cardiac gating/triggering.
However, low-field MRI is inherently less sensitive to patient motion or blood flow induced MHD effects. This is an advantage, especially in stress cardiac applications where it would be desirable to collect diagnostic-quality 12-lead ECG. In the last ISMRM congress, several authors published exciting results using the recent Free.Max 0.55T MRI. Moreover, recent advances have demonstrated that the loss of SNR could be counterbalanced by utilizing dense coil arrays⁴ and artificial intelligence image reconstruction techniques using deep learning⁵.
Conclusion
In the world of CMR, 3T is not a necessity for routine clinical imaging. 1.5T, or even low-field magnets could possibly meet all clinical imaging needs. However, 3T shines in research. Its advanced capabilities, including higher SNR, high angiographic contrast, advanced pulse sequences and even multinuclear spectroscopy, make it the perfect instrument to refine our understanding of the health and diseases of the heart. So, while not essential for everyday practice, 3T MRI is a vital tool in pushing the boundaries of cardiac research. Epsidy is ready to provide valuable solutions to help push these boundaries even further and empower clinical science.
References
C. Qin et al., “Sustainable low-field cardiovascular magnetic resonance in changing healthcare systems,” Eur. Heart J. - Cardiovasc. Imaging, vol. 23, no. 6, pp. e246–e260, Jun. 2022, doi: 10.1093/ehjci/jeab286.
Ross, Jeffrey S., “The High-Field-Strength Curmudgeon.” American Journal of Neuroradiology, Feb. 2004.
S. E. Fischer, S. A. Wickline, and C. H. Lorenz, “Novel real-time R-wave detection algorithm based on the vectorcardiogram for accurate gated magnetic resonance acquisitions,” Magn. Reson. Med., vol. 42, no. 2, pp. 361–370, Aug. 1999, doi: 10.1002/(SICI)1522-2594(199908)42:2<361::AID-MRM18>3.0.CO;2-9.
L. Ying and Z.-P. Liang, “Parallel MRI Using Phased Array Coils,” IEEE Signal Process. Mag., vol. 27, no. 4, pp. 90–98, Jul. 2010, doi: 10.1109/MSP.2010.936731.
G. Zeng et al., “A review on deep learning MRI reconstruction without fully sampled k-space,” BMC Med. Imaging, vol. 21, no. 1, p. 195, Dec. 2021, doi: 10.1186/s12880-021-00727-9.
Health equipment - Magnetic resonance imaging (MRI) units - OECD Data, last access Sep 12, 2023
