Learn Everything About MRI with Flashcards

Main magnet (Bo), radio frequency source, magnetic field gradients (to generate TVMFs), computer system, image processor, Magnets ( design, field strength and direction), RF Coils ( types & function), Gradient Coils ( Location and performance rating), Computer, Operating console (location & function), and Equipment room.

The main functions of the Bo is it aligns nuclei into parallel and anti-parallel states. It also affects low-state nuclei generation and the stronger Bo, the more low-state nuclei are created, so we get a stronger signal.

The main function of shim is it maintains the Bo homogeneity which directly affects the image quality

The function of RF is to disturb nuclei and transfer energy to nuclei through resonance.

The function of the gradient coils is to locate signal and rephase nuclei.

• Diamagnetism: Diamagnetic materials have paired electrons, their magnetic moments cancel each other out. In the absence of the magnetic field, there is no net magnetic moments, and substances with diamagnetic susceptibility weakly attract or repel magnetic lines of force in the presence of the main Bo magnetic field.

• Paramagnetism: The ability of substances with positive magnetic exposure to attract and repel magnetic lines of force. Such substances contain 1+ unpaired electron on their orbits. They produce additional magnetic fields to B0 (line up with external magnetic field). They produce T1 & T2 relaxation times of tissue.

• Super-paramagentism: The property of substances with large positive magnetic susceptibility which strongly attract or repel magnetic lines of force. These substances have a single magnetic domain and can be magnetized in a magnetic field. Magnetic susceptibility of these substances are much larger than those of paramagnetics and similar to those of ferromagnetic materials. Super-paramagnetic substances do not exhibit residual magnetism when the external field is removed. They exhibit an intermediate positive magnetic susceptibility compared to the next type of magnetism. Substances are hemosiderin, superparamagnetic iron oxide – SPIO.

• Ferromagnetism: The property of substances with large positive magnetic susceptibility that becomes magnetized within a magnetic field and remain magnetized after being removed from the magnetic field. Substances are iron and cobalt containing material which have have high positive magnetic fields.

They measure the flow and area of a magnetic field. Isocenter is the center of the magnetic field and the fringe field is the part of the magnetic field that extends away from it. The further we get from isocenter, the weaker the magnetic field.

• Tesla vs. Gauss / 1Tesla=10,000 Gauss
• Magnetic strength of the Earth / 0.5G
• 5 Gauss line / Line that demarcates point of safety for unsafe patients and materials
• Homogeneity / Unit of measure – Parts per Million (ppm) • Measures uniformity of the MRI magnetic field

Line that demarcates point of safety for unsafe patients and materials.

Permanent: No electrical power or cryogen needed. Alnico (iron, cobalt, nickel); ceramic bricks. Field direction – vertical. Low fringe field (safe). Can be sited near public areas.

I believe they feel more comfortable since it’s more like a donut hole and doesn’t feel as crammed and tight.

Passive Shimming: Achieved by placing small ferromagnetic plates around the warm bore. Performed by scanning the phantom during the installation.