Page 262 - Biomedical Engineering and Design Handbook Volume 2, Applications

DESIGN OF MAGNETIC RESONANCE SYSTEMS 241

8.5.2 Gating
Often MR scans need to be synchronized with (gated by) a physiological input such as the electro-
cardiogram or peripheral pulse or possibly respiration. Many MR systems provide the ability to
gate the scanner from these waveforms. It is imperative that the gating waveforms be sufficiently
free of artifacts induced by the MR system (gradient or RF interference or B enhanced “T” waves
0
appearing to be “R” waves) to permit accurate gating. It is also imperative that gating hardware
should not increase the chances of local power deposition in patients. Elimination of conductors
(e.g., by using fiber optic devices) in the scanner greatly reduces local power deposition concerns.
Finally, the gating equipment must not introduce signals that interfere with the scanner and show
up as image artifacts.

8.5.3 Other MR Hardware
Other essential MR hardware may include patient tables, patient comfort systems (pads, lights, air-
flow, or headphones), and venting systems for magnets with cryogens. Patient tables may be used to
transport patients, to move the region of interest into the center of the magnet for the examination,
and to permit rapid removal of patients from scanners during emergencies. Pads may add to patient
comfort and may be essential in reducing concerns of localized RF power deposition. Lighting and
airflow may reduce patient anxiety.

8.6 SAFETY STANDARDS

MR is a rapidly evolving technology. It is imperative that MR safety standards protect patient
safety during exams, while not preventing safe development of future diagnostic techniques.
While many safety standards govern various aspects of MR hardware development, two that are
unique to MR are listed in Table 8.1. The Food and Drug Administration (FDA) published “Non-
Significant Risk Criteria” for magnetic resonance devices. 19 These criteria state under what con-
ditions MR patient studies need investigational device exemption (IDE). The International
Electrotechnical Commission (IEC) 20 developed a widely used MR safety standard. The IEC MR
safety standard is three-tiered. The normal operating mode is for routine scanning of patients. The
operator must take a deliberate action (usually an ACCEPT button) to enter the first controlled
operating mode. This mode provides higher scanner performance, but requires more operator
monitoring of the patient. Finally, there is a second controlled operating mode used only for
research purposes under limits controlled by an Investigational Review Board (IRB). In Table 8.1,
values from the new, recently approved, second edition of the IEC MR Safety Standard are pre-
sented along with FDA criteria.
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Another IEC safety standard also establishes safety criteria for electrical safety and limits sur-
face contact temperatures to 41°C. Note that during high SAR scans, skin temperature approaches
37°C (a 4°C margin for temperature rise). During very low SAR scans, skin temperature is typically
33°C (an 8°C margin).

8.7 NEMA MR MEASUREMENT STANDARDS

The National Electrical Manufacturers Association (NEMA) has developed a number of useful
standards for measurement of MR parameters. The current list of NEMA MR standards is given
in Table 8.2. Note that these standards are now freely available. For more information see
http://www.nema.org/.