Magnetic Resonance Imaging with Cardiac MRI basics Michael C. Hoaglin mike@northwestern.edu
ECE381 Professor Max Epstein Northwestern University
June 2, 2004
June 2, 2004
Mike Hoaglin ECE381 Prof. Max Epstein
Magnetic Resonance Imaging (MRI) Introduction MRI can provide medical information unavailable by any other imaging method. Unlike X-ray and CT which have limitations resolving soft tissue due to the high-energy beams used, MRI can have fine detail with soft tissues, due to the imaging substrate: protons. MR saw its first medical installments in the early 1980s. Nuclear magnetic resonance (NMR) spectroscopy, discovered in 1946 by Stanford & Harvard researchers, provided the technological basis for MRI. MRI Physics MR is based upon the principle of spin flipping upon the application of an external magnetic field. Nuclear spin is one of the intrinsic atomic properties, which every atom but Ar and Ce has. A nucleus must not have an even atomic weight and number in order to have spin. Those isotopes with half-integral spin value must have odd atomic weights. The 1H (Protium) nucleus, consisting of a single proton, is the most common probe for MR due to its natural abundance (99.985%) and spin of ½. The human body of course is composed of tissues containing primarily water and fat, which contain a great deal of hydrogen. This again makes protons an excellent human tissue probe. The charged nucleus’ magnetic moment orientation provides the MR signal basis. Applying an external field to a tissue with randomly arranged proton magnetic moments will cause either parallel or antiparallel magnetic moment alignment. The former is of lower energy state, so there are a few more nuclei oriented in this direction than antiparallel. This difference results in a subtle, but detectable magnetic moment. If the tissue is placed inside a 1.5T magnetic field Bo (z-axis), protons precess about the magnetic field at angular frequency proportional to Bo called the Larmor frequency. The relation is as follows: B 0 , where γ is the gyromagnetic ratio. Of all elements in the human body, H has highest γ, also making it an excellent MR probe. In order to obtain imaging, the protons precessing about the external magnetic field B0 must be perturbed by applying a Larmor radio frequency (RF) pulse perpendicular to B0. In the quantum mechanical consideration, assuming thermal equilibrium, the RF pulse provides enough energy (ΔE) at the Larmor frequency to spin flip some parallel spin protons to antiparallel until more protons spin antiparallel, producing another field B1. The RF pulse produced field B1 is much less than the external B0, however its effect is cumulative, so only a few Gauss can cause realignment of the net magnetic field vector. Generally a 90° magnetic moment realignment is achieved. Upon removal of the RF inducing B1, the release of energy to achieve a lower energy equilibrium aligned with B0 has a characteristic relaxation time. The time it takes for a 63% return to full z-axis magnetization is T1. The time it takes for dispersal of the protons oriented in the x-y plane is represented by T2. T2 is defined as the time where 37% of the magnetization in the x-y plane remains. External RF removal signifies t=0. T1 and T2 are obtained from simple exponential solutions, much like the decay of an RC circuit. Gradient coils in 3 axes give fields askew to B0 to allow a gradient of superimposed fields. If the precession frequency is larger than the static field B0, there will be an additive effect and certain proton positions can be obtained. The changing field given off by the alternating magnetic field relaxing toward z-axis magnetization is picked up by x-y plane coils. This multi-frequency signal can be interpreted by Fourier Transform, and the image and contrast is obtained by comparing T1 and T2 values to known values. In diseased tissue, T1 and T2 are increased, thereby altering tissue contrast in the final image. Varying tissue slices can be taken by varying the applied RF and gradient fields.
June 2, 2004
Mike Hoaglin ECE381 Prof. Max Epstein
Equipment requirements The MR equipment required for imaging is vast and complex and can cost upwards of $2 million. Modern full body MR systems use a superconducting solenoid that can surround the patient comfortably. Niobium-titanium alloys allow electric current flow without resistance when maintained at liquid helium temperatures (4K). Complex refrigeration and condensing equipment are required to maintain the liquid phase and superconductivity. Once current flows in the superconducting coils, it continues to flow without an external power source. The decay rate of the magnetic field is on the order of 0.05 PPM/hour, thus excellent field uniformity, temporal stability and strengths can be achieved. This promotes a high signal to noise ratio and combined with a strong external magnetic field (1.5-5T), B0, good image resolution. MRI provides significant intrinsic contrast, but when little signal difference exists between T1/T2 in normal and pathologic tissue, a contrast agent can increase signal difference. Unlike CT agents, MRI contrast agents are not visualized directly, but rather enhance the signal by increasing relaxation times. T1 relaxation agents usually contain a paramagnetic metal ion bound to a chelate complex such as gadolinium-chelate. Water molecules in the coordination sphere of the complex transfer energy from RF pulse most efficiently, thus the tissue water diffuses into the inner sphere, giving up its energy to the metal ion. This results in a larger net magnetization than water in the neighboring tissue. The half-life of these agents is approximately 90 minutes. Brief survey of Cardiac MRI Cardiac MRI is one of the most challenging MRI modalities due to the non-static nature of the cardiac organ. Breath-hold technique and high speed acquisition are essential to avoid artifacts on the image. Coronary artery imaging is an often sought goal of cardiac imaging. These vessels are embedded in a layer of fat next to a large muscle mass and constantly moving. Motion of the heart is addressed by cardiac gating or triggering. This uses an MR technique called cine, which involves EKG leads on the patient. Each heartbeat triggers quick acquisition of part of an image, while the mid-diastole timeframe is used to gather images of other slices of the heart. Cardiac wall motion is an important indicator in cardiac health. The regional wall motion can be visualized in a tagging/saturation technique where the RF pulse saturates certain bands of tissue to be non-magnetic. This occurs by exciting all spins to antiparallel, and eventually they are stimulated to emit all energy. The motion of saturation bands is used to compute myocardial motion, giving accurate muscle stress evaluation throughout the cardiac cycle. Functional cardiac studies provide data for heart models during contraction. Radial thickening and circumferential shortening can be determined from functional data, giving insight into ventricular function. This can detect thinning of the ventricular wall long before any other imaging technique to intervene before myocardial infarction damage. Cardiac MRI will see continuing refinements and will save lives as studies of ventricular function are improved. Real-time imaging techniques will be improved upon improvement of computer systems’ ability to acquire data faster. Cardiac MRI will soon be the study of choice for evaluations of ventricular function and improvements in coronary angiography will approach the goal of completely noninvasive blood flow evaluations. A new exciting development is using MRI to guide probes during surgical procedures. It is definitely a great time to be involved in MRI radiology research.
June 2, 2004
Mike Hoaglin ECE381 Prof. Max Epstein
Bibliography Beutel, Jacob. Handbook of Medical Imaging. Vol. 1. Bellingham, WA: Spie Press (2000). Brown, Mark. MRI Basic Principles and Applications. Hoboken, NJ: Wiley (2003). Guy, Chris. An Introduction to the Principles of Medical Imaging. London: Imperial College Press (2000). Halliday, Resnick, Walker. Fundamentals of Physics. 6th e. New York: Wiley (2001). Li W, Storey P, Chen Q, Li BS, Prasad PV, Edelman RR. “Dark flow artifacts with steady-state free precession cine MR technique: causes and implications for cardiac MR imaging.” Radiology. 2004 Feb;230(2):569-75. National Research Council Institute of Medicine. Mathematics and Physics of Emerging Biomedical Imaging. Washington: National Academy Press (1996).