Page 251 - Biomedical Engineering and Design Handbook Volume 2, Applications

230 DIAGNOSTIC EQUIPMENT DESIGN

8.2.1 Field Strength and Signal-to-Noise Ratio (SNR)
As discussed above, the fraction of nuclei that are available for MR interactions increases with static
magnetic field strength, B . Noise in MR scans depends on the square root of the product of 4 times
0
the bandwidth, temperature, Boltzmann’s constant, and the resistance of the object to be imaged. Note
that increased bandwidth (leading to a higher noise floor) may be needed as B inhomogeneity
0
becomes worse. As discussed below, B inhomogeneities may also lead to some signal loss.
0
In magnetic resonance imaging raw data are acquired over some period of time. Raw data are
4
then converted into image data through the use of Fourier transform. Any temporal instability in
B will result in “ghosts” typically displaced and propagating from the desired image. Energy
0
(and signal) in the desired image is diminished by the energy used to form the ghosts. So, image
signal is lost and the apparent noise floor increases with B temporal instabilities. B fields of
0 0
resistive magnets change with power line current fluctuations and with temperature. Resistive
magnets are not used for imaging until some warm-up period has passed. Permanent-magnet B will
0
drift with temperature variations. Superconducting magnets have the highest temporal B stability
0
a few hours after ramping to field.
Another source of B instability is the movement of nearby objects with large magnetic moments
0
such as trucks and forklifts. Such objects may vary the static magnet field during imaging, resulting
in image ghost artifacts propagating from the intended image. This effect depends on the size of the
magnetic moment, its orientation, and on its distance from the magnet isocenter. Siting specifications
typically are designed to prevent such problems. Note that a common misperception is that actively
shielded magnets reduce susceptibility to B instabilities from nearby magnetic moments.
0
Unfortunately, this is not the case.
8.2.2 B Homogeneity
0
Inhomogeneous static magnetic fields can result in apparent T values called T , which are shorter
2*
2
than T . Let the inhomogeneity be ΔB , then T may be expressed as 6
2 0 2*
1 1 γΔ B 0
= + (8.5)
T 2* T 2 2
Spin echo pulse sequences use a 90 RF pulse followed after half an echo time (TE) by a 180 RF
pulse. Spin-echo signals s decay as 7
s ∝ e − ( TT ) (8.6)
/
2
E
Shorter T results in less signal. The static field of MR scanners must be very uniform to prevent sig-
2
nal loss and image artifacts. Typically, B inhomogeneity of MR scanners is about 10 parts per mil-
0
8
lion (ppm) over perhaps a 40-cm-diameter spherical volume (dsv) for imaging. In spectroscopy,
measurements of small frequency shifts must be accurately made, and B inhomogeneity is typically
0
limited to perhaps 0.1 ppm over a 10-cm dsv. 8
Clearly, MR magnets demand high homogeneity of the static magnetic field. In solenoidal
magnets, geometry and relative coil currents determine homogeneity. A perfectly uniform current
density flowing orthogonal to the desired B vector on a spherical surface will produce a perfectly
0
8
uniform B field in the sphere. Patient access needs render such a design impractical. A Helmholtz
0
pair (two coils of the same radius spaced half a radius apart with the same current flowing in the
same direction) is a first approximation to the uniform spherical current density. Typically four or six
primary (not counting active shield coils) coils are used. Higher degrees of homogeneity are possible
as more coils are used.
No matter how clever the magnet design, the local environment may perturb the desired static mag-
netic field. Field “shims,” either in the form of coils (which may be resistive or superconducting) and/or
well-placed bits of ferromagnetic material, are used to achieve the desired magnet homogeneity.