MRI. Magnetic Resonance Imaging

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1 MRI Magnetic Resonance Imaging Key Points MRI: The magnet is always on Know the essential components of an MRI system Know general idea of how MRI captures an image; three steps: Magnetic Alignment, Proton Excitation, Signal detection Why do we need liquid helium in an MRI unit? Two degrees K! enables superconduction Know the difference between ramping up, ramping down, and quenching Know what ferromagnetism is and why it matters: image artifacts AND object dislocation within the body AND projectile danger Know what material is being measured & imaged in fmri Know the advantages of MRI compared to CT: no ionizing radiation, and MRI images soft tissue remember: The magnet is always on! not a key point: Fourier analysis explained : D. J. McMahon rev cewood Tesla: A unit of magnetic field intensity necessary to produce a force of one newton, on a charge of one coulomb, moving perpendicular to the direction of the magnetic field at one meter per second. (Earth s magnetic field strength = T) Hydrogen atoms will align to a magnetic field. When they do, the center axis of the atom will have a wobble that is referred to as precession. Precession (that s 5 Gauss, in pre-si units)

2 An MRI study is a three-step process: 1: Magnetic Alignment The patient lies within a large, powerful magnet, where the magnetic field aligns the orientation of the hydrogen nuclei in the body. MRI machines make use of the fact that the human body is 75% water, and rich in protons (H nuclei), which are aligned when in a large magnetic field. When a patient is inside the powerful magnetic field of the scanner, the magnetic spin of the protons becomes aligned with the direction of the magnetic field. Typical MRI units have magnets of 0.8T to 1.5T 3T is being used. 2: Proton Excitation Radio frequency pulses are applied by gradient coils to alter the alignment of the nuclei. This causes the nuclei change their spin, making them emit a signal which is detected by the scanner. The applied pulses have just the right frequency, known as the resonance frequency, to be absorbed and change the spin of the protons in the magnetic field. (Hydrogen atoms in a magnetic field of 1.0 Tesla will absorb RF energy only at their precessional frequency, which is MHz.) 3: Signal detection This relaxation causes a radio frequency signal to be emitted, which is measured with receiver coils (RF coils). Nuclei at different locations precess at different speeds, which allows spatial information to be derived using Fourier analysis of the measured signal. By using gradients in different directions, 2D images or 3D volumes can be obtained in any orientation. After the electromagnetic pulse is turned off, the nuclei relax : the spins return to their previous state and re-align with the static magnetic field.

3 Typical MRI Pulse Sequence: TE = 5 to 100 ms TR = 100 to 2000 ms RF Transmit = RF pulses G X = magnetic gradient, left to right G Y = magnetic gradient, front to back G z = magnetic gradient, head to toe - gradients are 1 to 100 mt/meter SS = slice selection PE = pulse encoding FE = frequency encoding SS = PE = FE = slice selection pulse encoding frequency encoding - this period is sampled between 128 and 512 times, and these samples are plotted on a 2-dimensional grid, producing a readable image.

4 The magnet is superconductive by being surrounded with liquid helium at 2 Kelvin. The magnet assembly is sealed into a stainless steel vessel. This liquid helium vessel is in a stainless steel vacuum chamber (like a thermos bottle.) The magnetic field is ALWAYS ON. Helium Supply shortages have tripled the price. Cost of an H cylinder was $60, now at >$180. Current cost of the helium in a typical MRI unit may be ~$100,000. Liquid Helium, filling MRI Gaseous Helium Gaseous Helium (delivered compressed, in cylinders or trucks) Helium-free MRI? Magnesium diboride (MgB 2 ) superconducts at only 39 K So far, a 0.5 T MRI has been constructed. has promise for use in superconducting low to medium field magnets

5 MRI: basic - patient oriented ~1:00 thru ~2:30 Studies for the head, or a section of limb, use an anatomy-specific RF coil --

6 Setting up the magnetic field, Ramping Up and Ramping Down: The current in the magnet coils that makes the magnetic field is produced by a Magnet Power Supply. The current is around 550A and 10v DC. The process of starting this perpetual current is called Ramping Up the magnet. Shutting down in an orderly way (by diverting the current) is Ramping Down. HDCL - current probe MPS - Magnet Power Supply Axial view of the magnetic field around the core: Horizontal view of the magnetic field around the core:

7 Ferromagnetism Ferromagnetic does not mean that a metal contains iron, but rather that the material has magnetic properties like iron. Ferromagnetic metals are iron, cobalt and nickel. Ferromagnetism is a property not just of the chemical make-up of a material, but of its crystalline structure and microstructure. This is why some stainless steels (Austenitic like 304, 18/8, and A2) are non-magnetic, even though they contain Iron and Nickel. Ferromagnetic materials are dangerous hazards in an MRI suite, frequently becoming projectiles that can injure or kill. Ferromagnetic materials, even micro-sized, also create artifacts in the MRI image. Susceptibility (χ) is a measure of the extent a particular substance becomes magnetized when placed in an external magnetic field. χ below zero indicates a diamagnetic material. Susceptibility Artifact Magnetic disasters - Any ferromagnetic item (iron, steel, etc.) can be quickly drawn into the core. Susceptibility artifact due to braces Susceptibility artifact due to iron particles in mascara Susceptibility artifacts from screws in femur Susceptibility is a measure of the extent a substance becomes magnetized when placed in an external magnetic field. Both ferromagnetic and diamagnetic susceptibility creates artifacts in the MRI images. Materials that concentrate the field are called paramagnetic, superparamagnetic, or ferromagnetic, depending on the magnitude of the effect. Materials that disperse the main field are called diamagnetic. The Calcium in Cortical (hard) bone is one of these. Observe all precautions regarding entering the MRI room, and use caution with any tools or clinical equipment taken into the room.

8 Graphics for MRI safety: Safe for MRI area Caution for MRI Prohibited in MRI MRI Safe Equipment Quench buttons vent the helium to atmosphere in an emergency. An MRI machine holds between 1400 and 2000 liters of liquid He (LHe) Current price for LHe is $14-$22 per liter. and Cost for filling includes flushing the system with liquid Nitrogen, then gaseous Helium, then LHe, and watching a lot of it boil off until the job is done.

9 The Quench button vents the helium to atmosphere as a gas. In the superconducting state, the resistance of the magnet coil windings is zero and hence no energy is required to maintain current flow. If the coil temperature rises above the superconductivity threshold (Tc), the windings suddenly develop a finite resistance. The several-dozen amperes of circulating current passing through this elevated coil resistance create heat. This heat causes a sudden, explosive boil-off of liquid helium liters of liquid He (LHe) fills a 2 cubic meter volume. As a gas, this same Helium expands to 1500 cubic meters or 52,000 cubic feet. (this is 5 times the volume of the entire generic MRI suite shown earlier.) Although gaseous helium is lighter than air and will float to the top of the room, large quantities can completely displace oxygen from the entire room and if inhaled may cause loss of consciousness within 10 seconds with the possibility of asphyxia and death. Patients and staff must therefore be evacuated immediately from the scanner room if a quench occurs. fmri: Functional MRI: An MRI procedure that measures brain activity by detecting associated changes in cerebral blood flow. Similar to MRI but uses the change in magnetization between oxygen-rich and oxygen-poor blood as its basic measure. Esp useful for candidates for surgery or radiation therapy. Also used for clinical studies of new drugs, psychology, etc.

10 Comparisons with Computerized Tomography: Open MRI: more patient accessible, but lower field strength > CT uses ionizing radiation > CT is better for detecting bone and calcified components > MRI better for soft tissue, making it better for studies of tumor, aneurysms, etc. > MRI can be tailored to increase image contrast > MRI can produce slices in any plane, where CT scans are limited in the number and orientation of slices The Major Makers and their Models: Suggested web references: GE Siemens Philips Hitachi Toshiba (Signa brand) (Magnetom) (Achieva, Intera, Panorama) (Aaltaire, Airis) (Vantage, Opart, Ultra) The Basics of MRI by J. P. Hornak, Ph.D.: MRI safety: Magnetic disasters: