MRI
What is MRI?
MRI is an imaging technique that takes advantage of the property of certain atomic nuclei to vibrate – or “resonate” – when exposed to bursts of magnetic energy. When the hydrogen nuclei resonate in response to changes in a magnetic field, they emit radiofrequency energy. The MRI machine detects this emitted energy, and converts it to an image.
Hydrogen nuclei are used because hydrogen atoms are present in water molecules (H2O), and therefore are present in every tissue in the body.
The images obtained by MRI scanning are remarkably precise and detailed. With current MRI machines, these images are generated as 3-D projections. And once a 3-D MRI image is obtained it can be “sliced” and examined in detail, and in any plane – almost like being able to do exploratory surgery on a computer screen.
Also, subtle differences in the hydrogen atoms between various parts of a tissue – differences caused, for instance, by differences in blood flow or in the viability of the tissue – emit different amounts of energy. These energy differences show up as different shades of gray on the MRI image. Thus, the MRI offers a potential means of detecting areas of cardiac tissue that have poor blood flow (as in coronary artery disease) or that has been damaged (as in a heart attack).
However, there are many technical problems in imaging moving structures like the heart with MRI. Movement of the heart during scanning significantly distorts the image (just as taking a photo of a moving object causes a blurring of the picture), and when the structures you are trying to see are small the movement problem becomes extremely difficult to overcome. Technology is progressing rapidly, however, and commercial MRI machines that can produce high-quality heart images are already being used in many research institutions.
How is cardiac MRI useful today?
While MRI machines abound in the United States, cardiac MRI, because of its complexity, has largely been limited to university hospitals where there is a strong research interest. Accordingly, much of the work with cardiac MRI has been done in the research setting.
Because of the difficulties in producing detailed MRI heart scans, only a few uses of cardiac MRI have become more-or-less routine. MRI has proven very useful in evaluating patients with aortic dissection prior to surgery. The detailed images offered by MRI tell the surgeon precisely where the “tear” in the wall of the aorta begins, and the full extent of the dissection. MRI can also locate and characterize the rare cardiac tumor. And in children with complex congenital heart disease, MRI can help to identify and “sort out” the various anomalies, and to plan potential surgical approaches to treatment.
While such applications of MRI are very helpful, these clinical situations are relatively rare. So cardiac MRI has yet to become a commonly used tool in clinical medicine.
What are some of the potential uses of cardiac MRI?
Once certain limitations are overcome – and that day seems to be rapidly approaching – the uses of cardiac MRI will explode.
MRI has the potential to diagnose heart attacks in patients presenting with chest pain. Not infrequently, a patient coming to the emergency room with chest pain will not have the typical ECG changes seen with myocardial infarctions, and the doctors end up waiting for an hour or two for the results of cardiac enzyme tests. If a heart attack is actually occurring, critical time is thus lost before therapy can begin. MRI can detect myocardial infarction immediately, and can reduce the time it takes to begin definitive treatment.
Strides are being made toward being able to diagnose coronary artery disease with MRI. A new MRI processing technique called “black-blood” MRI seems to be able to distinguish very nicely between normal and atherosclerotic coronary arteries. While further refinements are necessary, such techniques are bringing us very close to the day in which MRI will be able to replace cardiac catheterization for diagnosing coronary artery disease.
MRI can help distinguish between “stable” atherosclerotic plaques and “vulnerable” plaques. Vulnerable plaques are those that are prone to rupture, thus suddenly occluding a coronary artery and causing a myocardial infarction. If vulnerable plaques can be identified, those particular plaques can be targeted for intervention (angioplasty, stent, or bypass), while leaving the stable plaques alone.
MRI has already proven useful in the research setting for identifying restenosis after angioplasty. MRI might thus prove an accurate, noninvasive means of following patients after angioplasty.
MRI has the potential of detecting changes in the tiny blood vessels of the heart – the microvascular circulation – that are completely missed by cardiac catheterization. Detecting such changes seem to be useful in predicting the outcome of patients after a heart attack, and may prove to be useful in assessing patients with cardiac syndrome X, diabetes, and certain other conditions.
Ultimately, MRI may replace the x-ray tube in both diagnostic and therapeutic situations. Research is already being done in animals using MRI to image the coronary arteries – instead of using fluoroscopy – for angioplasty procedures.
What about this week's report from Harvard on using MRI for diagnosing coronary artery disease?
Report in the New England Journal of Medicine constitutes another step forward, but MRI is still quite a ways from being ready to replace cardiac catheterization for most patients. While an accuracy of 72% is encouraging, it is certainly nowhere near the nearly 100% accuracy achieved with cardiac catheterization and coronary angiography. So, aside from the other disadvantages listed below, today the MRI is not accurate enough to substitute for coronary angiography when you really need to know the status of the coronary arteries. Indeed, while progress is ongoing, the MRI today is scarcely better in overall accuracy than the less inconvenient noninvasive tests that are used every day in cardiology.
MRI is an imaging technique that takes advantage of the property of certain atomic nuclei to vibrate – or “resonate” – when exposed to bursts of magnetic energy. When the hydrogen nuclei resonate in response to changes in a magnetic field, they emit radiofrequency energy. The MRI machine detects this emitted energy, and converts it to an image.
Hydrogen nuclei are used because hydrogen atoms are present in water molecules (H2O), and therefore are present in every tissue in the body.
The images obtained by MRI scanning are remarkably precise and detailed. With current MRI machines, these images are generated as 3-D projections. And once a 3-D MRI image is obtained it can be “sliced” and examined in detail, and in any plane – almost like being able to do exploratory surgery on a computer screen.
Also, subtle differences in the hydrogen atoms between various parts of a tissue – differences caused, for instance, by differences in blood flow or in the viability of the tissue – emit different amounts of energy. These energy differences show up as different shades of gray on the MRI image. Thus, the MRI offers a potential means of detecting areas of cardiac tissue that have poor blood flow (as in coronary artery disease) or that has been damaged (as in a heart attack).
However, there are many technical problems in imaging moving structures like the heart with MRI. Movement of the heart during scanning significantly distorts the image (just as taking a photo of a moving object causes a blurring of the picture), and when the structures you are trying to see are small the movement problem becomes extremely difficult to overcome. Technology is progressing rapidly, however, and commercial MRI machines that can produce high-quality heart images are already being used in many research institutions.
How is cardiac MRI useful today?
While MRI machines abound in the United States, cardiac MRI, because of its complexity, has largely been limited to university hospitals where there is a strong research interest. Accordingly, much of the work with cardiac MRI has been done in the research setting.
Because of the difficulties in producing detailed MRI heart scans, only a few uses of cardiac MRI have become more-or-less routine. MRI has proven very useful in evaluating patients with aortic dissection prior to surgery. The detailed images offered by MRI tell the surgeon precisely where the “tear” in the wall of the aorta begins, and the full extent of the dissection. MRI can also locate and characterize the rare cardiac tumor. And in children with complex congenital heart disease, MRI can help to identify and “sort out” the various anomalies, and to plan potential surgical approaches to treatment.
While such applications of MRI are very helpful, these clinical situations are relatively rare. So cardiac MRI has yet to become a commonly used tool in clinical medicine.
What are some of the potential uses of cardiac MRI?
Once certain limitations are overcome – and that day seems to be rapidly approaching – the uses of cardiac MRI will explode.
MRI has the potential to diagnose heart attacks in patients presenting with chest pain. Not infrequently, a patient coming to the emergency room with chest pain will not have the typical ECG changes seen with myocardial infarctions, and the doctors end up waiting for an hour or two for the results of cardiac enzyme tests. If a heart attack is actually occurring, critical time is thus lost before therapy can begin. MRI can detect myocardial infarction immediately, and can reduce the time it takes to begin definitive treatment.
Strides are being made toward being able to diagnose coronary artery disease with MRI. A new MRI processing technique called “black-blood” MRI seems to be able to distinguish very nicely between normal and atherosclerotic coronary arteries. While further refinements are necessary, such techniques are bringing us very close to the day in which MRI will be able to replace cardiac catheterization for diagnosing coronary artery disease.
MRI can help distinguish between “stable” atherosclerotic plaques and “vulnerable” plaques. Vulnerable plaques are those that are prone to rupture, thus suddenly occluding a coronary artery and causing a myocardial infarction. If vulnerable plaques can be identified, those particular plaques can be targeted for intervention (angioplasty, stent, or bypass), while leaving the stable plaques alone.
MRI has already proven useful in the research setting for identifying restenosis after angioplasty. MRI might thus prove an accurate, noninvasive means of following patients after angioplasty.
MRI has the potential of detecting changes in the tiny blood vessels of the heart – the microvascular circulation – that are completely missed by cardiac catheterization. Detecting such changes seem to be useful in predicting the outcome of patients after a heart attack, and may prove to be useful in assessing patients with cardiac syndrome X, diabetes, and certain other conditions.
Ultimately, MRI may replace the x-ray tube in both diagnostic and therapeutic situations. Research is already being done in animals using MRI to image the coronary arteries – instead of using fluoroscopy – for angioplasty procedures.
What about this week's report from Harvard on using MRI for diagnosing coronary artery disease?
Report in the New England Journal of Medicine constitutes another step forward, but MRI is still quite a ways from being ready to replace cardiac catheterization for most patients. While an accuracy of 72% is encouraging, it is certainly nowhere near the nearly 100% accuracy achieved with cardiac catheterization and coronary angiography. So, aside from the other disadvantages listed below, today the MRI is not accurate enough to substitute for coronary angiography when you really need to know the status of the coronary arteries. Indeed, while progress is ongoing, the MRI today is scarcely better in overall accuracy than the less inconvenient noninvasive tests that are used every day in cardiology.
posted by Heart Scan @ 6:02 AM
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