Keywords
intraoperative magnetic resonance imaging - neurosurgery - thermal burns
Introduction
Thermal injuries in magnetic resonance imaging (MRI) scanners are known to be rare
complications. Here we present four cases in which we encountered thermal injuries
during intraoperative 3 Tesla MRI under general anesthesia for neurosurgeries. We
also discuss possible mechanisms causing thermal burn injuries in intraoperative MRI
(IOMRI) and suggest ways to reduce them.
The etiology of MRI-induced thermal injury has not been completely explained and may
be multifactorial. MRI systems require the use of radiofrequency (RF) pulses to create
the magnetic resonance signal. This RF energy is transmitted through free space from
the RF coil to the patient. When conducting materials are placed within this RF field,
a concentration of electrical currents sufficient to cause excessive heating can be
produced and tissue damage may occur.[1] Most burn injuries reported till now occurred where The mechanism, which has been
considered responsible for heating of the coils eventually leading to burns, is the
direct electromagnetic induction. Other mechanisms producing heat in the MRI environment
are heating in a resonant circuit and heating due to the antenna effect.[2]
[3]
[4]
Materials and Methods
We collected a record of all cases that had burns while using a 3-Tesla MRI suite-operating
theater over a period of 1 year, with 150 cases studied from October 2017 to October
2018. There were four such cases noted. Two cases were operated upon in the lateral
position and one each in prone and supine. The patient was moved in between surgery
with sterile drapes into the adjacent MRI suite for scan. IOMRI for neurosurgeries
can pose a higher risk of thermal injuries as the surgeries are long in duration during
which the patient is covered under drapes, so there may be collection of sweat or
moisture in areas where coil touches skin in the body. This can induce thermal injuries
due to excess heat production during an IOMRI scan. Unfortunately, these can be detected
only after the end of surgery.
Results
All cases were neurosurgical cases performed under general anesthesia, two cerebellopontine
angle lesions, one parieto-occipital glioma, and one pituitary adenoma. All patients
were monitored using the MRI-compatible multi-parameter monitor (Tesla, Germany) during
the IOMRI scan. A complete institutional checklist was performed to ensure patient,
personnel, and equipment safety before moving in.
Case 1
Right parieto-occipital glioma operated in the prone position postoperatively had
a superficial burn on the nose ([Fig. 1A]). We hypothesized that this was due to direct contact of nose with the lower MRI
coil or moisture collected between the lower coil and nose in prone position. Therefore,
after this case, we made sure that the lower coil is placed after surgery just before
moving in for the IOMRI scan so that moisture is not collected at the dependent sites.
Fig. 1 (A) Superficial burn on the nose in prone position. (B) Thermal injury between skin folds in the flexed area of neck. (C) Superficial burn under the navigation tracker. (D) Thermal injury under area of BIS sensor on the forehead. BIS, bispectral index.
Case 2
Cerebellopontine angle tumor operated in left lateral position with neck rotation
and lateral flexion had superficial thermal injury in flexed neck region postoperatively
([Fig. 1B]). We postulated that this could be due to collection of moisture or sweat in flexed
neck area leading to heat generation causing burn inside the magnetic field.
Case 3
Pituitary macroadenoma for transnasal transsphenoidal resection with the use of neuronavigation
had a superficial burn postoperatively under the MRI-compatible navigation tracker
site or reference coil (StealthStation EM noninvasive patient tracker; Medtronic-Sofamor
Danek, United States) ([Fig. 1C]). We hypothesized that this could be due to improper contact between the navigation
tracker and patient's skin, allowing moisture to be collected. Because the navigation
tracker ideally needs to be discarded once the glue attaching it to the skin wears
off, we have stopped using the same tracker for multiple cases and discard the same
sooner. This prevents such burn incidents and ensures proper navigation accuracy.
Case 4
Cerebellopontine angle tumor operated with bispectral index (BIS) monitoring had a
superficial burn on the forehead, in the area of BIS sensor, which was removed prior
to the IOMRI scan ([Fig. 1D]). Our hypothesis was that this burn would be due to the moisture collected under
the BIS sensor due to prolonged duration of surgery. After this case, we started cleaning
and drying the site of BIS sensor before going for IOMRI.
All cases had a dermatology consultation, and since all of them were grade I burns,
they healed completely without sequelae or scarring with just bacitracin and silver
sulfadiazine dressings.
Discussion
A review of the areas burned in these cases clearly brings out the causative mechanism
([Table 1]).
Table 1
Review of the burned areas along with mechanisms of prevention and duration of surgery
|
Cases
|
Mechanism
|
Region of burn
|
Prevention
|
Surgery and duration (surgery + MRI)
|
|
Abbreviations: BIS, bispectral index; MRI, magnetic resonance imaging.
|
|
1 and 2
|
Most-dependent portion of the patient in contact with MRI coils
|
Nose (prone)
Neck (lateral)
|
Place coil just before shifting to the MRI suite
|
Case 1: right parieto-occipital craniotomy and excision
Duration: 8 h (7 + 1)
Case 2: retromastoid suboccipital craniotomy and excision of vestibular schwannoma
Duration: 14 h (13 + 1)
|
|
3 and 4
|
Glued electrodes/trackers have moisture
|
Fore head (navigation/BIS electrodes)
|
-
Remove and clean the BIS sensor/tracker
-
Use only till the glue perfectly sticks
|
Case 3: endoscopic endonasal transsphenoidal decompression of pituitary adenoma
Duration: 10 h (9 + 1)
Case 4: retromastoid suboccipital craniotomy and excision of vestibular schwannoma
Duration: 14 h (13 + 1)
|
Because electrocardiographic (ECG) electrodes were removed while shifting the cases
to the MRI room, none of the patients had any burns over the chest region where these
were placed. Again, because new ECG electrodes were used in every single case, there
were no burns, thus proving our hypothesis. Since application of these measures from
October 2018, we have not had any similar burns cases in the further 50 cases conducted
since then.
Based on our experience with intraoperative MRI (> 200 cases), we suggest following
measures to prevent these thermal mishaps during surgeries.[5]
We should only use monitoring devices that are cleared for use with MRI during scanning.
Placement of ECG leads ([Fig. 2A]) should be as close as possible, and wires should be braided and cables should be
of short length ([Fig. 2B]). All areas that are likely to experience thermal burn injuries should be padded
for thermal insulation (e.g., areas of skin-to-skin contact such as fingers, toes,
axilla, and groin or skin-to-machine contact) ([Fig. 2C, D]).
Fig. 2 (A) Closely placed MRI-compatible ECG leads made of carbon. (B) Braided MRI-compatible ECG wires with short connection ends to prevent looping.
(C) Packing in areas of skin-to-skin contact (finger webs and axilla). (D) Patient under the drapes going into intraoperative MRI with upper coil visible.
ECG, electrocardiogram; MRI, magnetic resonance imaging.
There are many factors that may cause thermal injuries in intraoperative MRI-guided
injuries. ECG cables should not form closed loops as they can pick up radiofrequency
signals and heat up.[6] All monitoring cables should be braided, not form loops and covered with padding
to provide insulation. There should be no collection of moisture between skin folds
and minimal soiling with surgical fluids. At the end of the surgery, all patients
should be thoroughly checked for any thermal injury.
There has been only one other series of 57 cases with a total of 1,093 electrodes
placed who underwent surgery in an IOMRI setting. In this prospective trial only 1
case had Grade I burns (1.75%) which co-relates well with our series.[7] With further experience and knowledge about the mechanisms, we should be able to
eliminate any such cases in the future.
Conclusion
Most thermal injuries caused by MRI occur as a result of direct contact with the MRI
coil, equipment, cables or wires, improper patient positioning, or due to collection
of moisture under the drapes. Increased awareness among OT personnel, knowledge of
mechanisms, institute-based checklist, and protocols can minimize these thermal injuries.
