Magnetic resonance imaging (MRI) has come a long way and is still moving forward with new developments being announced almost every day. Before, there was a clear line between CT and MRI, but that black and white concept is fading away. With the introduction of powerful modules and software, there might come a time in the future where it won’t matter which diagnostic method is performed, granted of course if there are specific indications or contraindications. With newer generation CT scanners to lower dosage while increasing quality, but MRI has no serious side effects related to increase in field strength. This is why researchers and clinicians are waiting with open arms for 7T MRI and beyond.
Whole body 3T sodium MRI (Photo courtesy of Friedrich Wetterling)
What’s sodium and why study it?
MRI takes advantage of the fact that water is located in all tissues and organs, creating a signal which is then formed into an image. However, many people are unaware that MRI can be used to study other elements, like sodium, phosphorus and others. Sodium is a very important element in the human body. The only isotope that is stable is 23Na, which is the most abundant form. In the human body it regulates blood flow, blood pressure, osmotic pressure and pH. Sodium chloride, more commonly known as table salt, is our main source of sodium. You may have heard or read that salt is bad for your health, but abnormally low levels of sodium can also adversely affect health. The American Heart Association recommends ingesting less than 1,500 mg per day, which is considerably less than their previous number 2,300 mg per day. They explain that because hypertension risk is so high among Americans, 1,500 is the number that exemplifies perfect cardiovascular health. Obviously this number is not for everybody, because competitive athletes, people who work in conditions with elevated temperatures, i.e. those who sweat a lot, will require more sodium intake.
So where does the heart come in in all of this? Sodium levels are elevated in cases of acute and subacute myocardial infarction due to three mechanisms: breakdown of ion homeostasis with accumulation of intracellular sodium, extracellular edema formation and in the process of scar formation, increase of extracellular vs. intracellular space as heart cells are replaced by connective tissue. Through many experiments and studies it has also been found that in cases of hypertension, particularly primary aldosteronism, a hormone imbalance, leads to increased extracellular sodium. Studies like these may show the connection between Na+ accumulation and hypertension.
Currently, technological achievements allow researchers and clinicians to measure only Tissue Sodium Concentration or TSC. 23Na-MRI is semi-feasible at 1.5T as shown in several studies, but the scan time and image quality aren’t exactly up to par with 7T and beyond. If this method gains the ability to measure intra- and extracellular sodium content, the possibilities for gaining knowledge could be endless. There are certain points which require immediate attention. The most important is improving coil design. To cement its place as a clinical tool there will have to be dedicated coils, not schlepping together a Frankenstein-esque coil and expecting good results. Coils on the market, like those from MRI.TOOLS GmbH could use a boost in the number of transmission/receiver channels to increase signal to noise ratio and spatial resolution further.
Is it purely research?
Sodium imaging on the whole is by no means purely research. For example, this image of a 23Na MRI scan of a patient before and after chemotherapy shows how this technology can be used to monitor the effect of treatment. It has also shown amazing results in monitoring tissue transplantation, which we have shown in our article about 7.0T’s big beefy brother 20T. When it comes to the heart, there is definitely clinical value in this method, but the question is whether 23Na-MRI can provide information that other more convenient methods can’t. But from the research perspective, it has a lot of advantages over regular 1H MRI and may give us insights into the cellular metabolism of sodium. As of now, software and electronics aren’t at the level to show the true potential of clinical sodium cardiac MRI, but there is room to grow, and as more ≥7.0T machines become available, more methods and tweaks will pop up.
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