Table 2 Factors that modulate guidewire heating during CMR

 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.

 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 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.