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Encyclopedia of Physical Science and Technology EN008M-395 June 29, 2001 15:52

974 Magnetic Resonance in Medicine

regulations. During the 1980s several manufacturers suc- useofstaticfieldsupto4.0Tand,underappropriatelycon-
cessfully sought FDA approval for their scanners. trolled conditions, an average heat input to the patient’s
As additional experience was gained during the 1990s, body (specific absorption rate or SAR) of 4.0 W/kg. The rf
FDA approval was granted for a variety of scanner en- heating that occurs under standard imaging conditions is
hancements, such as the use of higher field strengths and comparable to that resulting from normal metabolic activ-
a wider variety of RF and gradient coils. ity and is unnoticeable to the patient. Likewise, the possi-
NMR scanners place the patient in an environment that ble effects of dB/dt on nerve excitation and the magneto-
is quite unlike that of any other medical instrument. Ini- hydrodynamic interaction between flowing blood and the
tially, there were several areas of concern that, with expe- static magnetic field appear to require conditions far be-
rience, have become better understood and appear not to yond those used in modern scanners. At the present time
represent a danger to patients. The area of greatest contin- these concerns represent hypothetical, rather than actual,
uing concern is the interaction between the strong, static, hazards of the scanning process.
magnetic field and ferromagnetic substances inadvertently
brought into the region of the scanner. One version of
this problem arises from the fringing field surrounding III. IMAGING TECHNIQUES
the magnet, which can be treacherously strong. Many ob-
jects common in hospitals (e.g., oxygen bottles, mops,
A. Selective Excitation
fans, and hairpins) contain enough magnetic material that
they can be drawn into the magnet with great force and One of the key capabilities of MRI scanners is the ability
rapidity. Such flying objects are extremely dangerous to to excite a single, thin slice of spins within the patient. This
anyone in their path. For this reason, most manufacturers permits the construction of images that have the character
and users of the scanners go to great lengths to limit access of two-dimensional cross-sectional cuts through the pa-
to the vicinity of the scanner. Permanent magnet systems, tient’s anatomy. NMR imaging resembles CT scanning in
and some superconducting systems that have magnetic producing this type of anatomical image. However, MRI
shielding around them, have smaller fringing fields and has a substantial advantage over CT in that it permits the
are less susceptible to this effect. A second version of this location of the slice to be chosen electronically by the op-
problem comes about because some patients have ferro- erator without moving the patient or any components of
magnetic substances implanted within their bodies. This the scanner. In MRI, imaging planes of any orientation
is usually the result of a prior surgical procedure, such as may be chosen, and these planes may also be moved elec-
the clipping of a diseased blood vessel, but in some cases tronically from side to side, top to bottom, or front to back
iron fragments (e.g., shrapnel) have become embedded through the patient’s anatomy.
in a patient’s tissues during some sort of accident or ex- The key to selective excitation is carrying out the rf exci-
plosion. Patients are not always aware of the presence of tation in the presence of a gradient field. Suppose the static
these objects. Therefore care has to be taken before scan- field is uniform across the patient. If a rf pulse is applied at
ning to exclude those patients with possibly dangerous fer- the Larmor frequency, ω 0 = γB 0 , for a time long enough
romagnetic implants. Implanted cardiac pacemakers can to create a 90 pulse, this will excite spins over a large
◦
malfunction, or conceivably, be permanently damaged be- volume of the patient. If, however, the z-gradient coil is
cause of exposure to strong magnetic fields. Therefore used to apply an additional static field, B z = G z z, at the
patients with these devices in place are not normally can- time of the rf pulse, the resonance condition will be met
didates for NMR scanning. strictly only in the plane z = 0. Spins far from this plane
Other areas of initial concern were the effects of the are well off resonance and essentially will be unaffected
static field on normal tissue function; the possibility that by the rf pulse. Spins at z = 0 will be rotated by 90 just
◦
electric fields generated by the rapidly changing gradient as if no gradient field were present. Spins close to, but not
fields (i.e., the “dB/dt” effect) could cause nerve stimula- at, the plane z = 0 will be partially excited. To predict the
tion or irregularities in the cardiac rhythm; tissue heating exact behavior of the excitation, as a function of z, near the
associated with the rf excitation field; and possible effects origin it is necessary to carry out a solution to the Bloch
on blood pressure resulting from forces of interaction be- equations. It turns out that spins in the selected slice, but
tween blood and the static magnetic field. In the scanners slightly off the center plane, will also be flipped through
presently used, all of these effects appear to be readily tol- approximately 90 but will have phase differences with
◦
erable, and in most cases, negligible. The FDA continues those spins at z = 0. This will reduce somewhat the signal
to receive and evaluate designs for more advanced scan- generated by the slice. It can be shown that a more perfect
ners, and the regulatory aspects of scanner safety continue slice profile will result if, instead of using a rf pulse of
to evolve. Of particular note are guidelines approving the constant amplitude, the pulse amplitude is modulated by