Fringing Fields
A large, powerful magnet produces a strong magnetic field in the region surrounding it as well as in its interior. This fringing field can produce undesirable effects such as erasing…
Magnetic Field Homogeneity
The necessary degree of spatial uniformity of the field can be achieved only by carefully placing the coils at specific spatial locations. It is well known that a single loop…
Superconducting Magnets
Since the early 1980s, the use of cryogenically cooled superconducting magnets has been the most satisfactory solution to the problem of producing the static magnet field for MRI scanners. The…
Resistive Magnets
The first whole-body scanners, manufactured in the late 1970s and early 1980s, used four to six large coils of copper or aluminum wire surrounding the patient. These coils are energized…
Permanent Magnets and Electromagnets
Both these magnet types use magnetized materials to produce the field that is applied to the patient. In a permanent magnet, the patient is placed in the gap between a…
Static Field Magnets
The main field magnet is required to produce an intense and highly uniform, static magnetic field over the entire region to be imaged. To be useful for imaging purposes, this…
Fundamentals of MRI Instrumentation
Three types of magnetic fields — main fields or static fields (B2), gradient fields, an radiofrequency (RF) fields (B1) — are required in MRI scanners. In practice, it is also…
Defining Terms
Gyromagnetic ratio γ : An intrinsic property of a nucleus. It determines the Larmor frequency through the relation ω0 = −γ B0 . k-space: The reciprocal of object space, k-space describes MRI data acquisition…
Contrast Mechanisms
The tremendous clinical utility of MRI is due to the great variety of mechanisms that can be exploited to create image contrast. If magnetic resonance images were restricted to water…
Fast Imaging
The 2D FT scan of k-space has the disadvantage that the scan time is proportional to the number of phase encodes. It is often advantageous to trade off SNR for…
