This article will give you a basic understanding of the physics behind Magnetic Resonance Imaging in relation to cardiac imaging.
Basic Physics:
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To give you a full understanding of the physics of Magnetic Resonance Imaging (MRI) would require another 100 editions of this publication, so here is a simplified outline on what this complex imaging modality is. The proper name is actually Nuclear Magnetic Resonance Imaging (NMRI), however in the current world that we live in, the word “nuclear” creates a sense of fear, therefore just MRI is used for consumer comfort.
When you first look at a traditional MRI most people think it is just an overly large CT scanner, however the two imaging systems couldn’t be further apart in the function of obtaining diagnostic images. One uses x-rays (CT), whilst the other uses the effect of magnetic fields on the body’s molecules (MRI).
Excite and Relax:
We all know that the human body is comprised primarily of water (H20) with hydrogen atoms making up 63%, with all parts of the body containing some of the element. In a normal state outside of a strong magnetic field, the hydrogen protons spin randomly at a constant rate in various directions, however in different tissues they have different concentrations. It’s the differences in how they are bound in each type of tissue and the relative concentrations in each tissue that we are measuring.
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MRI components
MRI components
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MRI Sagittal View
MRI Sagittal View
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MRI Axial View
MRI Axial View
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MRI images of the heart
MRI images of the heart
Components:
Primary Magnet:
Looking at Figure 1 above the primary magnet is the largest part of system. It is comprised of a large electrical wire which is coiled, with an electrical current passing through. This results in a strong magnetic field created in the centre of the coil. Left without any extra intervention the electrical current is subject to resistance, reducing the amount of magnetic field strength. Therefore the coiled wire is surrounded by liquid helium at -269 degrees Celsius, becoming a superconducting electromagnet. The magnetic field produced is then 20,000 times the magnetic field of the earth, developing 1.5 to 3 Tesla, the optimum range for medical imaging.
Gradient Magnets:
Referring again to the image right you can see additional smaller magnets called Gradient magnets located closer in to the centre from the primary magnet. There are usually three of these magnets each with a field strength 1/1000th that of the primary magnet. The purpose of these magnets is to precisely adjust the magnetic field so that specific areas of the body can be imaged.
RF Coil:
When the body is placed inside the MRI the magnetic field forces the hydrogen atoms to spin in one of two directions in-line with the field. When all atoms are aligned a radiofrequency is emitted from the inner coil (RF Coil) which causes the hydrogen atoms to change direction and spin in another direction. This transition causes an energy release as the molecules change from their high-energy state to their low-energy state. The absorption of RF energy by the protons, affecting their energy state (not spin direction) is known as resonance. There are different rates of energy loss for different tissues which are displayed as shades of grey.
Confused yet??
This is obviously a quick overview of the workings, and there is much more to talk about. At university we learnt how to do the full mathematical problem solvings of Fourier Transforms, a 4 –page extremely advanced mathematical algorithm which are used to determine the individual tissue frequencies and amplitudes of the resonance signal. Consider yourself lucky not to go through that pain!!
Dangers:
The biggest danger with MRI’s is the presence of metals either inside the patient as part of a medical procedure or brought in accidently. I heard a story once of a doctor walking in with a pen in their pocket. When they were near the patient the magnetic field pulled at very high speed the pen out of the pocket narrowly missing the head of the patient. There was also a case reported in Australia in 2000 whereby a patient with a newly inserted pacemaker was given an MRI. Although the staff had twice asked if they had a pacemaker the patient each time stated “No”. The reason the patient said “No”, was most likely caused by confusion and stress. The patient died as a result of their pacemaker malfunctioning. Systems must be in place to avoid situations like this occurring.
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MRI images of the heart
MRI images of the heart
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MRI images of the heart
MRI images of the heart
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MRI images of the heart
MRI images of the heart
Contrast Enhancements:
Different tissue densities and their proton relaxation times produce the image contrast differences of an MRI image. However sometimes it is difficult to accurately visualise the differentiation such as in the case of demonstrating an infarction. Gadolinium - DTPA is a paramagnetic compound (attracted to a magnetic field) and is commonly used in cardiac MRI as a contrast agent where it appears very bright in certain tissues.
Advantages:
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Visualise structure and function of heart.
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Detect and evaluate coronary heart disease
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Determine extent of tissue damage and viability post MI / chronic heart disease.
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Can create moving images of the pumping function of the heart demonstrating contraction problems and blood flow abnormalities.
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Images can be obtained at any angle allowing physicians to easily obtain images of complex anatomy.
Uses:
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ischemic heart disease,
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myocardial disease,
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right ventricular abnormalities,
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pericardial disease,
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cardiac tumors,
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valvular disease,
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thoracic aortic disease,
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pulmonary artery disease,
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congenital heart disease before and after surgical repair.
Disadvantages:
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MR images are obtained through a process called “gating”, whereby the an ECG is used to acquire images at each stage of the cardiac cycle over several heart beats. If a patient has an irregular rhythm this can reduce the image quality.
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Patients with metallic objects such as pacemakers are unable to be scanned.
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Unlike ultrasound, an MRI is not portable to take to the patient’s bedside.
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Gadolinium can be toxic to patients with impaired kidney function, with haemodialysis recommended in some cases.
Future:
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As with all technology, faster computer processors are allowing for real-time MR images. Several new scanners have this capability however the image quality is at present reduced compared with gating.
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Some medical device companies are developing MRI safe pacemakers. Medtronic is currently trialing the EnRhythm® MRI SureScan™ pacing system, for this purpose.
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Improvements in the development of MRI contrast media agents are occurring all the time, such as what has occurred with cardiac angiography.
Bibliography
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Cluett, J. M.D. 2006, About.com, MRI – What is MRI? Retrieved 1st August 2008, http://orthopedics.about.com/cs/sportsmedicine/a/mri.htm
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http://en.wikipedia.org/wiki/Cardiovascular_magnetic_resonance
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http://www.radiologyinfo.org/en/info.cfm?pg=cardiacmr&bhcp=1
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Calvert, J. & Taylor, T., Death Link to Hospital Scan, Retrieved 1st August 2008. http://www.users.on.net/~vision/misc/pacemaker-death.html
The video displayed at the top was obtained from Wikipedia, author Jccmoon. For image license visit this link.
