Microwave Imaging Detects ‘Hot’ Plaque at Potential Risk of Rupture
A noninvasive method of measuring temperature differences in carotid atherosclerotic plaque may be able to identify those lesions at highest risk of rupture, according to findings published in the May 1, 2012, issue of the Journal of the American College of Cardiology.
Researchers led by Konstantinos Toutouzas, MD, of Athens Medical School (Athens, Greece), tested microwave radiometry for the first time in vivo, assessing 34 patients with severe carotid disease scheduled for endarterectomy and 15 healthy controls. Readings were made to compare the temperature difference for each carotid segment (maximal temperature minus minimal temperature) between carotid patients and controls, as correlated with ultrasound and histological analysis.
The imaging system (RTM 01 RES, Bolton, United Kingdom) can detect natural electromagnetic radiation from internal tissues at microwave frequencies, providing temperature measurements at a depth of 1 cm to 7 cm below the skin with an accuracy of ± 0.2º C.
For temperature measurements in each segment by microwave radiometry, intraobserver (0.06º C ± 0.08º C) and interobserver (0.08º C ± 0.11º C) differences were within the accuracy boundaries for the microwave sensor (r = 0.94 for intraobserver, r = 0.89 for interobserver measurements).
Plaque Features Show Temperature Differences
The difference in temperature assessed by radiometry was greater in atherosclerotic carotid arteries identified on ultrasound compared with the arteries of controls (1.39º C ± 0.49º C vs. 0.23º C ± 0.01º C; P < 0.01). Fatty plaques had greater variations in temperature vs. mixed and calcified plaques (P < 0.01). Heterogeneous plaques with ulcerated surface had greater differences in temperature vs. those with homogenous and irregular and regular surfaces (P < 0.01).
On histological analysis, specimen characteristics associated with larger temperature differences via microwave radiometry included:
- Low vs. high extension of calcification (P < 0.01)
- Thin vs. thick fibrous caps (P < 0.01)
- Maximal vs. no or circumscribed expression of inflammatory cells (P < 0.01)
On immunohistochemical analysis, specimen characteristics associated with larger temperature variations via microwave radiometry included intense vs. lower expression of CD3, CD68, and VEGF reactivity (P < 0.01 for each comparison).
The authors conclude that “[temperature changes] of carotid atheromatic plaques can be measured in vivo noninvasively by [microwave radiometry],” and “the in vivo temperature measurements by [microwave radiometry] correlate well with the ultrasound characteristics and histological and immunohistochemical findings.”
In particular, they note, morphological characteristics associated with vulnerable plaque on ultrasound, histology, and immunohistochemical analysis showed greater temperature differences. The new imaging method also showed high temperatures in accordance with histology in 21% of plaques that lacked ultrasound evidence of rupture vulnerability, demonstrating the potential value of combined imaging methods, they add.
Validity of Technology Questioned
However, in a telephone interview with TCTMD, Morteza Naghavi, MD, of Fairway Medical Technologies (Houston, TX), expressed skepticism over the method’s validity. “Microwave radiometry, to my knowledge, does not have thermal resolution within 0.1º C, which you would require to see heterogeneity in plaque temperature,” he said. “Unless there’s been a significant evolution in the field, the whole technology of microwave radiation-based measurement is questionable.”
Dr. Naghavi, who has previously participated in the development of heat-sensing catheters for plaque characterization, also questioned the size of the temperature differences found in the study, on the order of 1º C to 2º C. “For that, you need to have a huge amount of temperature variation in the tissue, which is totally washed by blood with every heartbeat,” he said. “It’s very hard to imagine you can maintain 2 or even 1 degree of temperature variation unless there’s just a huge amount of heat, like an oven.”
His theory is that the researchers are not actually measuring heat but some other factor. “It could be that microwave radiometry is affected by tissue composition and they’re measuring some indirect effect, and temperature is in their imagination,” Dr. Naghavi said, adding that this may also be valuable. “At the end of the day, who cares if they’re measuring temperature or not if you have a device that can detect differences?”
But, he cautioned, the technology still has a long way to go. “The most convenient thing for measuring plaque is ultrasound,” Dr. Naghavi said. “Microwave radiometry at best is going to provide a measure of activity of plaque, but you have to prove what you mean by activity, because heat is not detectable at this level in my view. So the added value over ultrasound is under huge question.”
Radiometry Reliable, Still Developing
In an e-mail communication with TCTMD, Dr. Toutouzas replied to Dr. Naghavi’s critiques, agreeing that the resolution of microwave radiometry is only within 0.2º C, which was the range used previously by thermographic catheters for plaque temperature measurement. “However, taking into consideration that carotid plaque temperature differences in previous ex vivo and in vivo studies were in the range of 1-2º C, microwave radiometry can provide such information reliably with high reproducibility,” Dr. Toutouzas said. “The temperature measurements obtained with microwave radiometry correlated very well with those measured by thermistor-based thermography catheters.”
He acknowledged that other factors, such as neovascularization, may impact the measurements made by the technology. “Thus, microwave radiometry seems to be an indirect indicator of plaque inflammatory cell content,” Dr. Toutouzas said. However, he cited unpublished data in which patients with significant, unstable lesions showed larger variation between contralateral carotid arteries than did healthy subjects. “These findings are suggestive that systemic factors and carotid artery anatomical features have limited impact on microwave radiometry measurements,” Dr. Toutouzas said.
He added that microwave radiometry should be considered a supplementary tool to ultrasound for clinical practice, and that “these preliminary results justify the performance of additional studies for the investigation of the prognostic value of temperature differences in carotid artery disease.”
Toutouzas K, Grassos C, Drakopoulou M, et al. First in vivo application of microwave radiometry in human carotids: A new noninvasive method for detection of local inflammatory activation. J Am Coll Cardiol. 2012;59:1645-1653.
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- Dr. Toutouzas reports no relevant conflicts of interest.
- Dr. Naghavi reports serving as president and CEO of Fairway Medical Technologies and being involved with multiple medical technology companies.