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Performance Validation of a Modified Magnetic Resonance Imaging–Compatible Temperature Probe in Children

Nasr, Viviane G., MD; Schumann, Roman, MD; Bonney, Iwona, PhD; Diaz, Lina, MD; Ahmed, Iqbal, MD, FRCA

doi: 10.1213/ANE.0b013e31824b003e
Technology, Computing, and Simulation: Research Reports
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INTRODUCTION: During magnetic resonance imaging (MRI), children are at risk for body temperature variations. The cold MRI environment that preserves the MRI magnet can cause serious hypothermia. On the other hand, hyperthermia may also develop because of radiofrequency-induced heating of the tissues, particularly in prolonged examinations. Because of a lack of MRI-compatible core temperature probes, temperature assessment is unreliable, and specific absorption rate–related patient heat gain must be calculated to determine the allowable scan duration. We compared an MRI-compatible temperature probe and a modification thereof to a standard esophageal core body temperature probe in children.

METHODS: Children undergoing general anesthesia were recruited, each patient serving as his/her own control. Core body temperature was measured using 3 different devices: (1) a fiberoptic MRI-compatible skin surface temperature probe (MRI-skin) located on the child's skin surface; (2) a fiberoptic MRI-compatible temperature probe modified with a single-use sleeve at the tip (MRI-core), located in the nasopharynx; and (3) a standard temperature monitor (STRD) located in the esophagus or nasopharynx. The Bland–Altman method was used for statistical analysis.

RESULTS: We enrolled 60 children aged 7.8 ± 6 years (mean ± SD) weighing 32.4 (±26.4) kg. The estimated difference between the STRD and MRI-core measurements of core temperature was 0.06°C (confidence interval [CI]: −0.02, 0.15), and between the STRD and the MRI-skin 1.19°C (CI: 0.97, 1.41). According to the Bland–Altman analysis, the 95% limits of agreement ranged from −0.9 to 3.4 and from −1.3 to 1.2 between the STRD and the MRI-skin probe and the MRI-core probe, respectively.

DISCUSSION: Our results show good agreement between standard esophageal measurements of core temperature and core temperature measured using a modified MRI-core probe during general anesthesia in a general surgical pediatric population. The ability to accurately assess core temperature in the MRI suite may safely allow longer scan times and therefore reduce repeat anesthetic exposure, improve patient safety, and enhance the quality of care in children.

Published ahead of print February 24, 2012

From the Department of Anesthesiology, Tufts Medical Center, Boston, Massachusetts.

Lina Diaz is currently affiliated with the Massachusetts Eye and Ear Center, Boston, Massachusetts.

Funding: Department of Anesthesiology. The manuscript was supported by grant no. UL1 RR025752 from the National Center for Research Resources (NCRR). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NCRR.

The authors declare no conflict of interest.

Reprints will not be available from the authors.

This report was previously presented, in part, at the SPA 2010.

Address correspondence to Viviane G. Nasr, MD, Department of Anesthesiology, Tufts Medical Center, 800 Washington Street, Box #298, Boston, MA 02111. Address e-mail to vnasr@tuftsmedicalcenter.org.

Accepted January 4, 2012

Published ahead of print February 24, 2012

The number of children undergoing magnetic resonance imaging (MRI) is increasing in recent years, especially with the additional relevance of cardiac MRI for congenital heart diseases.1 During MRI, patients are at risk of hypo- as well as hyperthermia, but core body temperature monitoring has not been possible for lack of user-friendly, reliable MRI-compatible temperature probes.26 Children seldom tolerate the cold, noisy MRI environment combined with the requirement for immobility for prolonged periods of time and breath holding. Often deep sedation or general anesthesia is needed. Sedation techniques, including general anesthesia, impair thermoregulation, leaving patients at risk for hypothermia.7 In addition, active warming devices are not yet MRI compatible.

At other times the MRI's radiofrequency energy can warm body tissue, causing hyperthermia.8,9 A safety standard with respect to the duration of MRI allowed has been added recently by the International Electrotechnical Commission as long-duration patient imaging sequences have become more common.a,10 The maximum allowed specific absorbed energy is 14.4 kJ/kg (240 W/min/kg) per examination. As a safety feature, MR scanners will cease scanner operation if the predicted absorbed energy exceeds the acceptable limit for the patient being scanned. As a consequence, some patients may need additional MRI examinations under anesthesia to complete the needed workup. Accurate core body temperature assessment during MRI could eliminate repeat examinations, reduce anesthetic exposure, and greatly improve a child's safety by detecting either hypothermia or hyperthermia.

We conducted this study to evaluate a current MRI-compatible skin surface temperature probe and a modification of this probe that allows nasopharyngeal or esophageal placement. We validated both devices against a standard esophageal core temperature probe in a pediatric general surgical population.

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METHODS

After receiving IRB approval and written parental/ guardian consent, as well as children assent as indicated, we conducted a prospective study in ASA I or II children younger than 18 years presenting for surgery under general anesthesia in the operating room. Patients with oral or esophageal abnormalities, or those presenting for upper or lower airway surgery or esophageal surgery preventing the insertion of the temperature probes were excluded. All patients received general anesthesia with standard ASA intraoperative monitoring.b During the procedure, each patient had his/her temperature measured with 3 different techniques, serving as their own controls:

  1. An MRI-compatible, fiberoptic skin surface temperature probe (MRI-skin) (Medrad-Veris, Medrad Inc., Warrendale, PA) located on the child's skin surface and affixed using a Tegaderm 3M medical adhesive (3M, St. Paul, MN) as described by the manufacturer (Fig. 1).
  2. The same fiberoptic temperature probe modified with a single-use sleeve at the tip (MRI-core, Fig. 2), located in the nasopharynx.
  3. A standard temperature monitor (STRD) (Vital-Temp, Vital Signs, Inc., Englewood, CO) located in the esophagus or nasopharynx.
Figure 1

Figure 1

Figure 2

Figure 2

Temperature values by all 3 probes were collected simultaneously immediately after insertion and then every 5 minutes thereafter for a total of 30 minutes. The MRI-compatible temperature probes were not used for clinical decision making during each case. Data collected included the age, weight, gender, diagnoses, ambient operating room temperature, anesthetic technique (general, or combined general/regional technique), presence or absence of a warming blanket, type of surgical procedure, and its duration.

The relationship between each method (MRI-skin, MRI-core, and STRD) was determined by Spearman correlation analysis at each time point. The agreement between each method was then visually assessed by Bland–Altman plot at each time point. By averaging the temperature measurements at 7 points in time, the overall limits of agreements (1.96 times the SD of the mean difference) were calculated and compared. To consider the correlation between the repeated measures within each subject, repeated measures of analysis of variance (ANOVA) were performed. For this analysis, we considered the temperature difference between each of the alternative methods and the standard method as an outcome and tested effects of warming and time as covariates. All analyses were conducted by SAS 9.2 (SAS Institute, Inc., Cary, NC). The data are reported as estimated means with 95% confidence intervals (CI) with significance at P < 0.05. According to the Bland–Altman methods,11 60 subjects would allow a CI as narrow as 0.45° SD around the limits of agreement (where SD is the SD of the differences).

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RESULTS

We enrolled 60 children with a mean age of 7.8 ± 6 years (mean ± SD) weighing of 32.4 (± 26.4) kg. Descriptive statistics are presented in Table 1. The estimated temperatures with 95% CI for all patients over time were 36.30°C (36.1, 36.4) for the STRD, 35.1°C (34.8, 35.3) for the unmodified (MRI-skin), and 36.3°C (36.1, 36.4) for the modified MRI (MRI-core) probe.

Table 1

Table 1

The estimated temperature difference between the STRD monitor and the MRI-core probe was 0.06°C with CI (−0.02, 0.15) and between the STRD and the MRI-skin was 1.19°C (0.97, 1.41), which is significant. The difference between the MRI-skin and MRI-core probes was 1.21 (0.94, 1.47), which is also significant.

The MRI-core probe measurements correlate with those of the STRD probe at all times with a correlation coefficient >0.7 (P < 0.05). The MRI-skin probe correlates significantly with the STRD probe starting at the 10-minute time point. However the correlation coefficient remains <0.5 at all time points (maximum 0.44) (Fig. 3).

Figure 3

Figure 3

The Bland–Altman analysis indicated that the 95% limits of agreement ranged from −0.9 to 3.4 between the STRD and the MRI-skin probe and from −1.3 to 1.2 between the STRD and the MRI-core probe. The interval width of agreement for the MRI-core probe is narrower and its estimated temperature is closer to the STRD measurements in comparison with the MRI-skin values, indicating a good agreement between standard esophageal and the MRI-core monitor that increases with time (Fig. 4).

Figure 4

Figure 4

Use of a warming blanket does not affect the difference between the temperatures measured with STRD and MRI-core over time (Fig. 5). However, the skin temperature measurement tended to converge with both core temperature measurements when a warming blanket is used as noted by the decrease of the measurement differences over time between STRD and the MRI-skin probes (P < 0.05) as well as between the MRI-core and the MRI-skin probes (P < 0.01, Fig. 5). After adjusting for room temperature, the results remained unchanged; the room temperature did not affect temperature measurements.

Figure 5

Figure 5

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DISCUSSION

Our study shows a much closer correlation between standard esophageal core temperature measurements and those obtained by the MRI-core in comparison with the MRI-skin probe. The difference between the MRI-skin probe and both core probes decreased over time (Fig. 5). This could have been due to redistribution or warming blanket use that might have caused a decrease in measured temperature differences between the MRI-skin probe and both the standard (P = 0.02) and the MRI-core probe (P = 0.01) over time, mostly by an increase in the MRI-skin temperature reading. It is possible that the approximation between MRI-skin and the core temperatures measured is solely related to the close proximity of the MRI-skin probe to the warming blanket and not an actual increase in skin temperature as a result of body warming. However, it is likely that both phenomena have contributed to this result. In fact, the warming blanket did not affect the measurement differences between the standard probe and the MRI-core probe, suggesting a steady state comparison between the 2. The correlation between both these probes reached a stable maximum within 15 minutes after insertion, with a correlation coefficient >0.8 (Fig. 3). The MRI-skin probe never achieved a correlation coefficient >0.5, irrespective of a time factor. In our study we controlled for the effect of room temperature, which could theoretically have affected our readings.7

A limitation of our study is the non-MRI environment. Although the MRI suite would have been an ideal setting for this performance validation, the MRI-incompatible standard esophageal temperature probe prohibited this location.

This study confirms the possibility of real-time core body temperature assessment during MRI in children under general anesthesia by using a simple modification of a current MRI-compatible skin temperature probe. Benefits of this option include the ease of application, close correlation with the current “gold standard” in core temperature assessment, and subsequently the possibility to dismiss the limitations of calculated allowable scan times in favor of physiologically determined durations. With the current concern for possible neuromodulatory effects from anesthetic exposure in childhood and other risks, ultimately we expect decreased exposure of children to repeat anesthetics for scan completions.12 Procedural safety and quality of care would improve with the widespread application of routine temperature monitoring during MRIs, allowing early intervention for hypo- or hyperthermia.

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DISCLOSURES

Name: Viviane G. Nasr, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Viviane G. Nasr has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Roman Schumann, MD.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Roman Schumann has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Iwona Bonney, PhD.

Contribution: This author helped conduct the study, analyze the data, and write the manuscript.

Attestation: Iwona Bonney has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Lina Diaz, MD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Lina Diaz has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Iqbal Ahmed, MD, FRCA.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Iqbal Ahmed has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

This manuscript was handled by: Dwayne R. Westenskow, PhD.

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ACKNOWLEDGMENTS

The authors wish to thank Yoojin Lee, MS, MPH, Statistical Associate, Biostatistics Research Center, Institute for Clinical Research and Health Policy Studies at Tufts Medical Center, for expert assistance with statistical analysis of the data.

a International Electrotechnical Commission. Medical electrical equipment. Part 2–33. Particular requirements for the safety of magnetic resonance equipment for medical diagnosis. Geneva, Switzerland: International Electrotechnical Commission, 2010.
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b The American Society of Anesthesiologists. Standards for basic anesthetic monitoring. Effective July 2011. Available at: http://www.asahq.org/For-Members/Clinical-Information/Standards-Guidelines-and-Statements.aspx. Accessed June 2011.
Cited Here...

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