Table 2.

Factors that modulate guidewire heating during CMR

Factor	Heating Impact	How to reduce heating
CMR excitation energy
Radiofrequency excitation, controlled via flip angle α	Heating increases with square of flip angle, α [14]	Reduce α during CMR fluoroscopy
Radiofrequency pulse width duration	Heating decreases linearly with RF pulse width duration	Increase radio frequenchy pulse width duration
Radiofrequency duty cycle	Heating decreases linearly with excitation repetition time (TR)	Prolong TR
Radiofrequency pre-pulses used for CMR magnetization preparation	Heating increases with pre-pulse flip angle and number of RF pulses in preparation, and decreases with RF pulse width duration	Reduce pre-pulses and their SAR characteristics
CMR scanner duty cycle	Additional heat is generated as long as CMR scanning continues	Limit duration of continuous CMR fluoroscopy with a guidewire in place.
Conductive guidewire physical properties
Guidewire insulation	Insulation gaps, such as at the tip, concentrate current density and increase focal heating [8]	Use guidewires that are fully insulated without gaps
Length of conductive materials	Guidewire length > ¼ wavelength λ of the Larmor frequency in vivo (~ 10 cm at 1.5 T) promotes standing waves and therefore heating [13]	Use guidewires having metallic components shorter than ¼λ (not available commercially)
Guidewire configuration
Guidewire position with regard to center of CMR bore	Electrical field minimal at the center of scanner bore (in x & y), greatest closer to wall of scanner bore [21]	Keep guidewire close to scanner centerline and away from walls of scanner.
Guidewire position with regard to patient body	Electrical field is greatest at outer (skin) surface of body. Electrical modeling suggests electrical field is greatest at groin and shoulder during CMR	Use in central blood vessels
Guidewire insertion length with regard to vascular access site	Electrical field is highest at outer edges of scanner bore entrance. Guidewire outside of the body is more likely to couple electrically and heat. In other words, minimal guidewire vascular insertion length is associated with maximal heating	Reduce input energy during scanning. Minimize time with guidewire at minimum insertion length.
Guidewire insulation by catheter	The patient is less exposed to guidewire heating when it is covered by insulating catheter	If guidewire is not in active use, retain its position inside catheter or remove from body during CMR.
Guidewire protrusion length from catheter	A change in insulation with minimum guidewire protrusion outside insulating catheters causes concentration of current density and therefore heating	Reduce input energy during scanning.
Guidewire length	Different guidewire lengths are associated with different degrees of heating, in a non-linear fashion, relating to coupling with scanner electrical field [21]	Select guidewire lengths empirically associated with less heating.
Guidewire diameter	Guidewires with smaller diameter generate more heating [8]	Select larger diameter guidewires as appropriate
Guidewire loops overlapping	Guidewire looping can create a second point of heating at wire contact points, which remains less than or equal to guidewire tip heating	Reduce input energy during scanning.
Guidewire heat is dissipated by conduction and convection into surrounding medium	Blood flow cools heated guidewire dramatically. Testing under static conditions, such as ASTM 2182 phantom, maximizes detected heating	Static phantom testing exaggerates heating to provide a margin of safety predicted in vivo, probably by 10-fold.