In recent years, there has been a proliferation of so-called
open magnet imaging devices. These machines have great marketing
appeal, and they are often billed as being “patient friendly” and capable
of accommodating even those who are claustrophobic or morbidly obese.
These claims are not unwarranted. However, it should be recognized
that even recently designed open systems cannot deliver image quality
commensurate with that of high-field systems. This is readily explained
when it is understood that the open system magnet field strengths
range from one fifth to one tenth those of a high-field imager.
The lower field strength means that the resultant images will be
derived from proportionately weaker radio signals coming from the
patient. Although novel pulse sequences such as constructive interference
in the steady state (CISS) can help to alleviate this problem, as
is usually the case in MRI, this comes at a price: reduced T2 sensitivity
(Fig. 8-7). However, low-field scanners can produce excellent cervical
spine scans, as this region of the body is relatively diminutive.
The small diameter of most necks permits the receiving coil to be close
to the region of interest (cervical spine), maximizing the ability
to detect signals arising from the spinal tissues. Additionally,
low-field units are less susceptible to artifacts arising from vascular
pulsation or respiration, both of which can degrade neck images.
Nevertheless, in general, low-field scanners require more time to
scan the patients, again a function of their operating with weaker
magnets. Longer scan times may not be tolerated by a patient as
a result of pain, with resultant artifacts secondary to patient
motion. Low-field imagers really cannot compete with their high-field
brethren in the production of thin section scans, where signals
are at a premium. Therefore, it is my opinion that “all
imagers are not created equal.” Whenever possible, I prefer
to scan patients with high-field machines to guarantee optimal image
quality. Newer high-field units have a so-called short (magnet)
bore, with flared ends of the scanning aperture, soft lighting,
and abundant ventilation. Such amenities significantly diminish
the patient’s perception of confinement. Where necessary,
the nursing staff can provide either oral or intravenous conscious
sedation, which usually makes it possible to scan even the most
anxious individual.