Intravascular thermography: Immediate functional and morphological vascular findings

Combined frequency domain photoacoustic and ultrasound imaging for intravascular applications

Intravascular photoacoustic (IVPA) imaging has the potential to characterize lipid-rich structures based on the optical absorption contrast of tissues. In this study, we explore frequency domain photoacoustics (FDPA) for intravascular applications. The system employed an intensity-modulated continuous wave (CW) laser diode, delivering 1W over an intensity modulated chirp frequency of 4-12MHz. We demonstrated the feasibility of this approach on an agar vessel phantom with graphite and lipid targets, imaged using a planar acoustic transducer co-aligned with an optical fibre, allowing for the co-registration of IVUS and FDPA images. A frequency domain correlation method was used for signal processing and image reconstruction. The graphite and lipid targets show an increase in FDPA signal as compared to the background of 21dB and 16dB, respectively. Use of compact CW laser diodes may provide a valuable alternative for the development of photoacoustic intravascular devices instead of pulsed laser systems.

Medical Infrared Imaging of Normal and Dysplastic Elbows in Dogs

OBJECTIVE

To investigate the ability of medical infrared imaging to differentiate between normal canine elbows and those with abnormal elbows (elbow dysplasia).

STUDY DESIGN

Prospective cohort study.

ANIMALS

Dogs with normal (n = 15) and abnormal (n = 14) elbows.

METHODS

Infrared imaging was performed on all dogs and data analyzed via descriptive statistics and image pattern analysis software. Animals with elbow dysplasia had arthroscopic procedures to confirm the presence of elbow disease.

RESULTS

Computer recognition pattern analysis was up to 100% correct in identifying abnormal elbows and normal elbows, with the medial images most consistent. The caudal, lateral, and cranial images correctly identified 83-100% abnormal elbows. The caudal and lateral images correctly identified 83% normal elbows. A significant difference in temperature was found between normal and abnormal elbows for the cranial full region of interest, lateral images, and each quadrant.

CONCLUSION

Medical infrared imaging was able to correctly identify known abnormal and known normal elbows in dogs.

Medical infrared thermal imaging of cats with hyperthyroidism

OBJECTIVE

To determine the usefulness of medical infrared thermal imaging (MITI) as a screening tool for hyperthyroidism in cats, evaluate the need for hair clipping over the ventral aspect of the neck to achieve optimal images, and determine whether there is a change in thermal patterns at 1 and 3 months after radioactive sodium iodide I 131 treatment.

ANIMALS

17 cats with and 12 control cats without hyperthyroidism.

PROCEDURES

All cats underwent MITI first with the hair present and then after the hair was clipped. Each cat with hyperthyroidism was subsequently appropriately treated SC with radioiodide; reevaluations, including MITI before and after hair clipping and measurement of serum thyroxine concentration, were performed 1 and 3 months after treatment.

RESULTS

The MITI had 80.5% and 87.5% accuracy in differentiating hyperthyroid cats from clinically normal cats before and after the hair over the ventral aspect of the neck was clipped. Among cats with an initial serum thyroxine concentration > 4.0 μg/dL, the success rate for MITI-detected response to radioiodide treatment at the 1-month reevaluation was 92.86% in unshaved cats and 85.71% in shaved cats. The success rate for MITI-detected response to radioiodide treatment at the 3-month reevaluation was 100% in unshaved and shaved cats.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that MITI was successful in differentiating between hyperthyroid cats and clinically normal cats and identifying patients with thyroxine concentration within reference interval after radioactive sodium iodide I 131 treatment.

Microwave radiometry: a new non-invasive method for the detection of vulnerable plaque

Atherosclerosis and its consequences are the most rapidly growing vascular pathology, with myocardial infarction and ischemic cerebrovascular accident to remain a major cause of premature morbidity and death. In order to detect the morphological and functional characteristics of the vulnerable plaques, new imaging modalities have been developed. Intravascular thermography (IVT) is an invasive method, which provides information on the identification of the high-risk atheromatic plaques in coronary arteries. However, the invasive character of IVT excludes the method from primary prevention. Microwave radiometry (MR) is a new non-invasive method, which detects with high accuracy relative changes of temperature in human tissues whereas this thermal heterogeneity is indicative of inflammatory atherosclerotic plaque. Both experimental and clinical studies have proved the effectiveness of MR in detecting vulnerable plaque whereas recent studies have also revealed its association with plaque neoangiogenesis as assessed by contrast enhanced carotid ultrasound (CEUS).

Exploring coronary atherosclerosis with intravascular imaging

Coronary angiography has been widely used for five decades to evaluate a range of vascular pathologies and triage patients to therapeutic interventions. The inability to directly visualize the artery wall with conventional angiographic techniques has stimulated development of a number of intravascular imaging modalities. These approaches have the potential to provide a more comprehensive characterization of the burden, composition and functionality of atherosclerotic plaque, neointimal hyperplasia and allograft vasculopathy that develop within coronary arteries. The ability to use these modalities in vivo and in a serial fashion has provided a greater insight into the factors that underlie the disease process and guide therapeutic interventions.

Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.

Evaluation of thermographic imaging of the limbs of healthy dogs

OBJECTIVE

To describe a thermographic imaging protocol, identify normal thermographic patterns (ie, color map reflecting the skin temperature distribution) for various regions of interest (ROIs) of dog limbs, and evaluate effects of clipping the coat on thermographic patterns and limb temperature in healthy dogs.

ANIMALS

10 healthy dogs.

PROCEDURES

Each dog was thermographically evaluated in the same room (ambient temperature, 21 degrees C) via ROIs that included cranial and caudal views of the body, full lateral body views, full views of the limbs, and views of various limb regions. After initial imaging, the coat was clipped on the forelimbs and hind limbs only. Each dog was then evaluated 15 and 60 minutes and 24 hours after clipping by use of the same protocol.

RESULTS

For each ROI within a category (intact coat and each time point after clipping), mean temperatures were similar among the 10 dogs. Pairwise comparisons for 15 and 60 minutes and 24 hours established patterns of temperature stabilization among the 3 time points. Temperatures did not differ significantly between the left and right limbs. There was a mean success rate of 75% for use of image pattern analysis for recognition of similar thermographic patterns in the forelimbs and hind limbs.

CONCLUSIONS AND CLINICAL RELEVANCE

Thermography can be a viable, noninvasive imaging modality that provides consistent images with reproducible thermal patterns in ROIs examined in healthy dogs. Although the coat had a predictable influence to decrease the mean temperature, thermal patterns remained fairly consistent after the coat was clipped.

Diagnosis and treatment of coronary vulnerable plaques

Thin-capped fibroatheroma is the morphology that most resembles plaque rupture. Detection of these vulnerable plaques in vivo is essential to being able to study their natural history and evaluate potential treatment modalities and, therefore, may ultimately have an important impact on the prevention of acute myocardial infarction and death. Currently, conventional grayscale intravascular ultrasound, virtual histology and palpography data are being collected with the same catheter during the same pullback. A combination of this catheter with either thermography capability or additional imaging, such as optical coherence tomography or spectroscopy, would be an exciting development. Intravascular magnetic resonance imaging also holds much promise. To date, none of the techniques described above have been sufficiently validated and, most importantly, their predictive value for adverse cardiac events remains elusive. Very rigorous and well-designed studies are compelling for defining the role of each diagnostic modality. Until we are able to detect in vivo vulnerable plaques accurately, no specific treatment is warranted.

Influence of catheter design on lumen wall temperature distribution in intracoronary thermography

Intracoronary thermography is a currently used vulnerable plaque detection method. We studied how catheter design and catheter location influence the temperature readings, and thus its capacity to detect vulnerable plaques. Finite element calculations were performed on geometries representing the coronary artery, the vulnerable plaque and the catheter. Catheter material, diameter and location with respect to the plaque were varied. Both flow and no-flow situations were studied. Maximal lumen wall temperature difference without a catheter (DeltaT=0.12 degrees C, flow=75 cm(3) min(-1)) was considered the reference. Presence of a 1.0mm nitinol catheter right under the plaque increased DeltaT to 0.14 degrees C, whereas a 1.0 mm polyurethane catheter increased DeltaT to 0.51 degrees C. The location at which a thermosensitive element should be placed for most optimal temperature readings during a pullback was shown to lie at the catheter edge for the nitinol catheter and at 1.1mm from the catheter edge for the polyurethane catheter. Temperature readings decreased to background temperature when the catheter was in close proximity but not overlapping the plaque. DeltaT decreased approximately by 70% when a gap of 0.2 mm existed between the catheter and the lumen wall. Occlusion of blood flow increased DeltaT values in all cases, but most pronounced for nitinol catheters. A polyurethane catheter increased the temperature readings, since its heat conductivity is lower than that of blood, which makes it a very good choice for heat source detection. Catheter design can contribute to enhanced temperature readings and thus can enable more optimal vulnerable plaque detection.

A numerical study on the influence of vulnerable plaque composition on intravascular thermography measurements

Intracoronary thermography is a technique that measures lumen wall temperatures for vulnerable plaque detection. In this paper the influence of vulnerable plaque composition on lumen wall temperatures was studied numerically. Concerning the vulnerable plaque heat generation, the location of the heat source and its heat production were varied. Concerning the heat transfer, the thermal properties of the lipid core and the location of the vasa vasorum were studied. The heat source location was the main determinant of the lumen wall temperature distribution. The strongest effect was noted when the heat producing macrophages were located in the shoulder region leading to focal spots of higher temperature. The maximal lumen wall temperature was mainly determined by the heat production of the macrophages and the cooling effect of blood. The insulating properties of the lipid core increased lumen wall temperatures when the heat source was located in the cap and the presence of vasa vasorum lowered the temperatures. These results show that the lumen wall temperature distribution is influenced by vulnerable plaque composition and that intracoronary thermography techniques require a high spatial resolution. To be able to couple temperature measurements to plaque vulnerability, intracoronary thermography needs to be combined with an imaging modality.

Assessment of Culprit Plaque Temperature by Intracoronary Thermography Appears Inconclusive in Patients With Acute Coronary Syndromes

OBJECTIVE

Safety and feasibility evaluation of intracoronary temperature measurements in patients with acute coronary syndromes (ACS) using a catheter based thermography system.

METHODS AND RESULTS

Thermography was performed in 40 patients with ACS. A 3.5-F thermography catheter containing 5 thermocouples measuring vessel wall temperature, and 1 thermocouple measuring blood temperature (accuracy 0.05 degrees C) was used. Gradient (deltaTmax) between blood temperature (T(bl)) and the maximum wall temperature during pullback was measured. The device showed satisfactory safety in ACS. Only in 16 patients (40%) deltaTmax was > or = 0.1 degrees C. In 23 patients (57.5%) the highest deltaTmax was found in the culprit segment. DeltaTmax between culprit and adjacent non-culprit segments was observed in patients with transient blood flow interruption during thermography (0.11+/-0.03 versus 0.08+/-0.01; P=0.04), in contrast to patients with preserved flow (0.07+/-0.03 versus 0.06+/-0.02; P=0.058).

CONCLUSIONS

The novel, technically sophisticated intracoronary thermography proved its safety and feasibility. However, we were not able to convincingly and consistently differentiate between different lesions at risk, despite a selection of lesions that should appear most distinct to differentiate. A systematic interruption of flow may be necessary to achieve diagnostic results consistently, although such requirement may unfavorably change the risk-to-benefit ratio of this developing technology.

Intracoronary Thermography for Detection of High-Risk Vulnerable Plaques

Up to two-thirds of acute myocardial infarctions develop at sites of culprit lesions without a significant stenosis. New imaging techniques are needed to identify those lesions with an increased risk of developing an acute complication in the near future. Inflammation is a hallmark feature of these vulnerable/high-risk plaques. We have shown that inflamed atherosclerotic plaques are hot and their surface temperature correlates with an increased number of macrophages and decreased fibrous-cap thickness. Multiple animal and human experiments have shown that temperature heterogeneity correlates with arterial inflammation in vivo. Several coronary temperature mapping catheters are currently being developed and studied. These thermography methods can be used in the future to detect vulnerable plaques, potentially to determine patients' prognosis, and to study the plaque-stabilizing effects of different medications.

Intracoronary thermography: does it help us in clinical decision making?

The concept of the "vulnerable" plaque has recently emerged to explain how quiescent atherosclerotic lesions evolve to cause clinical events. The morphologic and immunologic determinants specific for the vulnerable plaque have been reported: a large lipid core (>or=40% plaque volume) composed of free cholesterol crystals, cholesterol esters, and oxidized lipids impregnated with tissue factor; a thin fibrous cap depleted of smooth muscle cells and collagen; an outward (positive) remodeling; inflammatory cell infiltration of fibrous cap and adventitia (mostly monocyte-macrophages, some activated T cells, and mast cells); and increased neovascularity. Despite the large amount of information regarding the morphological characteristics of remote lesions, we lack studies with functional assessment of non-culprit lesions. Coronary thermography is a technique for functional assessment of coronary atherosclerotic plaques. Several catheter designs have been proposed. There are catheters with thermistor(s) and wires with thermal sensors at the distal tip. All designs have several advantages and disadvantages. Despite the current limitations of coronary thermography, we gained important pathophysiological and clinical information regarding the vulnerability of atheromatic plaques. It has been documented both experimentally and clinically that increased heat generation is associated with increased macrophage concentration within the plaque. The correlation between local inflammatory involvement and local heat generation has also been observed with the peripheral inflammatory markers such as C-reactive protein. Whether systemic treatment, with agents such as statins or interventional techniques, such as drug-eluting stents, will have an impact on stabilizing vulnerable plaques need to be determined in future studies. In conclusion, although there are several techniques for evaluating morphologically atheromatic plaques, thermography is a promising method for the functional assessment of vulnerable plaque and has been introduced into clinical practice, with a good predictive value for clinical events in patients with increased temperature in the atherosclerotic plaque.

Elevated Plaque Temperature in Non-Culprit De Novo Atheromatous Lesions of Patients With Acute Coronary Syndromes

OBJECTIVES

We investigated whether there is increased temperature in non-culprit lesions, and the correlation of clinical syndrome with heat production of non-culprit lesions.

BACKGROUND

There is a controversy regarding whether there is widespread inflammation involving non-culprit lesions, or whether inflammatory involvement is limited to the culprit lesion. Coronary thermography assesses the local inflammatory involvement in atherosclerotic lesions.

METHODS

We included patients suffering from stable angina (SA) or acute coronary syndrome (ACS). All patients had two or more angiographically detectable lesions at different arteries. Culprit lesions should be identified in all patients. Patients with chronic total occlusions and multiple significant lesions at the culprit vessel were excluded. We measured at each non-culprit lesion the temperature difference (DeltaT) between the atherosclerotic plaque and the proximal vessel wall temperature.

RESULTS

The study population included 42 patients: 23 with SA, 19 with ACS. The DeltaT in non-culprit lesions was 0.08 +/- 0.07 degrees C. Patients with ACS had a higher temperature difference in non-culprit lesions compared with patients with SA (ACS 0.11 +/- 0.08 degrees C vs. SA 0.05 +/- 0.06 degrees C; p < 0.01). The mean value of DeltaT in non-culprit lesions was higher in the untreated group compared with the treated group with statins (0.11 +/- 0.10 degrees C vs. 0.06 +/- 0.05 degrees C; p = 0.05).

CONCLUSIONS

The results of this study show that heat is generated in non-culprit lesions. Moreover, in patients with ACS, temperature difference is increased compared with patients with stable angina.

In vivo temperature heterogeneity is associated with plaque regions of increased MMP-9 activity

AIMS

Plaque rupture has been associated with a high matrix metalloproteinase (MMP) activity. Recently, regional temperature variations have been observed in atherosclerotic plaques in vivo and ascribed to the presence of macrophages. As macrophages are a major source of MMPs, we examined whether regional temperature changes are related to local MMP activity and macrophage accumulation.

METHODS AND RESULTS

Plaques were experimentally induced in rabbit (n=11) aortas, and at the day of sacrifice, a pull-back was performed with a thermography catheter. Hot (n=10), cold (n=10), and reference (n=11) regions were dissected and analysed for smooth muscle cell (SMC), lipids (L), collagen (COL), and macrophage (MPhi) cell densities (%); a vulnerability index (VI) was calculated as VI=MPhi+L/(SMC+COL). In addition, accumulation and activity of MMP-2 and MMP-9 were determined with zymography. Ten hot regions were identified with an average temperature of 0.40+/-0.03 degrees C (P<0.05 vs. reference) and 10 cold regions with 0.07+/-0.03 degrees C (P<0.05 vs. hot). In the hot regions, a higher macrophage density (173%), less SMC density (77%), and a higher VI (100%) were identified. In addition, MMP-9 (673%) activity was increased. A detailed regression analysis revealed that MMP-9 predicted hot regions better than macrophage accumulation alone.

CONCLUSION

In vivo temperature measurements enable to detect plaques that contain more macrophages, less SMCs, and a higher MMP-9 activity.

Intravascular electric impedance spectroscopy of atherosclerotic lesions using a new impedance catheter system

Newer techniques are required to identify atherosclerotic lesions that are prone to rupture. Electric impedance spectroscopy (EIS) can characterize biological tissues by measuring the electrical impedance over a frequency range. We tested a newly designed intravascular impedance catheter (IC) by measuring the impedance of different stages of atherosclerosis induced in an animal rabbit model. Six female New Zealand White rabbits were fed for 17 weeks with a 5% cholesterol-enriched diet to induce early forms of atherosclerotic plaques. All aortas were prepared from the aortic arch to the renal arteries and segments of 5-10 mm were marked by ink spots. A balloon catheter system with an integrated polyimide-based microelectrode structure was introduced into the aorta and the impedance was measured at each spot by using an impedance analyzer. The impedance was measured at frequencies of 1 kHz and 10 kHz and compared with the corresponding histomorphometric data of each aortic segment.Forty-four aortic segments without plaques and 48 segments with evolving atherosclerotic lesions could be exactly matched by the histomorphometric analysis. In normal aortic segments (P0) the change of the magnitude of impedance at 1 kHz and at 10 kHz (|Z|(1 kHz) - |Z|(10 kHz), = ICF) was 208.5 +/- 357.6 Omega. In the area of aortic segments with a plaque smaller than that of the aortic wall diameter (PI), the ICF was 137.7 +/- 192.8 Omega. (P 0 vs. P I; p = 0.52), whereas in aortic segments with plaque formations larger than the aortic wall (PII) the ICF was significantly lower -22.2 +/- 259.9 Omega. (P0 vs. PII; p = 0.002). Intravascular EIS could be successfully performed by using a newly designed microelectrode integrated onto a conventional coronary balloon catheter. In this experimental animal model atherosclerotic aortic lesions showed significantly higher ICF in comparison to the normal aortic tissue.

Temperature distribution in atherosclerotic coronary arteries: influence of plaque geometry and flow (a numerical study)

Intravascular coronary thermography is a method that may detect vulnerable, atherosclerotic plaques and is currently evaluated in a clinical setting. Active macrophages or enzymatic heat releasing processes in vulnerable plaques may act as heat sources. To better understand the parameters of influence on thermographic measurements, numerical simulations have been performed on a model of a coronary artery segment containing a heat source. Heat source parameters and flow were varied to study their influence on temperatures at the lumen wall. Maximal temperature differences at the lumen wall increased when the source volume increased and they differ with the source geometry. The simulations showed that blood flow acts as a coolant to the lumen wall. Blood flow decreased maximal temperatures depending on the source geometry, source volume and the maximal flow velocity. Influence of flow was highest for circumferentially extended sources, up to a factor 3.7, and lowest for longitudinally extended sources, down to a factor 1.9. When cap thickness increased, maximal temperatures decreased and the influence of flow increased. This study shows that correct interpretation of intravascular thermographic measurements requires data on the flow and on the morphologic characteristics of the atherosclerotic plaque.