72 – Return to Table of Contents Magnetic Resonance Imaging Quality Control Manual
IV. Annual MRI System Performance Evaluation
A. Magnetic Field Homogeneity
Homogeneity refers to the uniformity of the main magnetic eld strength
B
0
over a designated volume. Magnetic eld inhomogeneity is usually
specied in parts per million (ppm) of the magnetic eld strength
over a spherical volume (DSV=diameter of spherical volume). e
actual homogeneity will be inuenced by a variety of factors, including
imperfections in the magnet manufacturing, the degree to which the B
0
magnetic eld is perturbed by external ferromagnetic structures or, in the
case of clinical scans, the presence of the patient within the eld and the
degree to which the above inuences can be compensated using magnetic
elds produced by shim and/or gradient coils. e most common problem
caused by magnet inhomogeneities at high eld strength is diculty in
obtaining uniform fat suppression. Inhomogeneities also can contribute
to geometrical distortion of images (particularly at low eld strengths),
adversely inuence image signal uniformity, increase the severity of wrap
artifacts, and compromise SNR in some fast imaging sequences.
is is sometimes a dicult test to perform independently. If the
magnetic eld homogeneity test cannot be performed, the physicist
should note this in the report, and the facility must arrange for the service
engineer to provide the medical physicist/MRI scientist with a copy of
the most recent eld map, which should be led as an attachment to the
report. Test results should demonstrate that magnetic eld homogeneity
is within manufacturer’s specications and was performed within the
last six months. If the medical physicist/MRI scientist has an alternate
method of accurately assessing magnetic eld uniformity, it is acceptable,
providing the report includes a description of the methodology used.
If a magnet is perfectly homogeneous over the imaging volume, all of
the water protons (also referred to as spins) will precess at the same
frequency, the magnet center frequency, which is directly proportional
to the strength of the magnet. Aer applying an RF excitation pulse, and
in the absence of any imaging gradients, a Fourier transform (FT) of the
resulting signal will exhibit a strong, narrow peak at that center frequency.
If the magnet were perfectly homogeneous, one would expect the FT to
have a peak at only one frequency (i.e., be a delta function). However,
random spin-spin interactions temporarily cause some protons to precess
a little faster than the center frequency, whereas others will temporarily
precess more slowly. is results in spreading of the peak with the
full-width half-maximum (FWHM) of the frequency peak related to the
average T2 time constant. A long T2 will have a narrow peak (little spin-
spin interaction), and a short T2 will have a very broad peak (substantial
spin-spin interaction). Along with these random spin-spin interactions,
anything that causes imperfections in the static magnetic eld will cause
this spectral peak to spread. e greater the imperfections and the more
inhomogeneous the magnetic eld, the wider the peak. Although it is
quick and easy to perform, monitoring the spread of the spectral peak is
a crude and insensitive method of assessing magnet homogeneity. is
measurement contains no information regarding spatial variations of the
magnetic eld.
OBJECTIVE
GENERAL THEORY