Traceability and calibration in temperature measurement: a clinical necessity

Patient temperature is a fundamental physiological measurement used primarily for observation and diagnosis, for example during surgery, intensive care, recuperation, or treatment. A variety of thermometers are used clinically and these can be separated into two categories, either contact (oral thermometers, rectal thermometers and temporal strips), or non-contact (ear thermometers, temporal thermometers and thermal imagers). To have the maximum confidence in the clinical performance of the temperature measurement instrument it is strongly desirable that the device be traceably calibrated to the International Temperature Scale of 1990 (ITS-90). Lack of traceable calibrations accredited to ISO17025 can lead to unreliability in temperature measurement and in some cases can have a deleterious effect on patient care. The National Physical Laboratory (NPL) maintains and disseminates the ITS-90 for contact and non-contact thermometry in the UK. The importance of accredited traceable calibrations and an outline of contact and non-contact thermometry standards are given here.     

Comparison of non-contact infrared skin thermometers

Non-contact infra-red skin thermometers (NCITs) are becoming more prevalent for use in medical diagnostics. Not only are they used as an alternative means of estimating core body temperature but also to assess the diabetic foot for signs of inflammation prior to ulceration. Previous investigations have compared the performance of NCITs in a clinical setting against other gold standard methods. However, there have been no previous investigations comparing the performance of NCITs in assessing temperature measurement capability traceable to the International Temperature Scale of 1990 (ITS-90). A metrological assessment of nine common NCITs was carried out over the temperature range of 15–45?°C using the National Physical Laboratory’s blackbody reference sources to identify their accuracy, repeatability, size-of-source and distance effects. The results are concerning in that five of the NCITs fell far outside the accuracy range stated by their manufacturers as well as the medical standard to which the NCITs are supposed to adhere. Furthermore, a 6?°C step change in measurement error over the temperature range of interest for the diabetic foot was found for one NCIT. These results have implications for all clinicians using NCITs for temperature measurement and demonstrate the need for traceable calibration to ITS-90.     

Perturbation of the temperature distribution in microwave irradiated tissue due to the presence of metallic thermometers

To date, satisfactory thermal dosimetry during the clinical application of localized hyperthermia can only be achieved using invasive thermometry. However the presence of commonly used metallic thermometers, such as thermocouples, may lead to the distortion of the temperature field due to self-heating of the probe under microwave irradiation. A computer simulation of the effect of this self-heating on the steady-state temperature distribution in plane-microwave irradiated homogeneous tissue has been undertaken and the significance of the effect for clinical hyperthermia dosimetry is assessed. The results indicate that a distortion of the temperature field in the neighborhood of the thermometer by several degrees can occur under adverse conditions.     

Infrared cameras are potential traceable “fixed points” for future thermometry studies

Abstract

The National physical laboratory (NPL) requires “fixed points” whose temperatures have been established by the International Temperature Scale of 1990 (ITS 90) be used for device calibration. In practice, “near” blackbody radiators together with the standard platinum resistance thermometer is accepted as a standard. The aim of this study was to report the correlation and limits of agreement (LOA) of the thermal infrared camera and non-contact infrared temporal thermometer against each other and the “near” blackbody radiator. Temperature readings from an infrared thermography camera (FLIR T650sc) and a non-contact infrared temporal thermometer (Hubdic FS-700) were compared to a near blackbody (Hyperion R blackbody model 982) at 0.5?°C increments between 20–40?°C. At each increment, blackbody cavity temperature was confirmed with the platinum resistance thermometer. Measurements were taken initially with the thermal infrared camera followed by the infrared thermometer, with each device mounted in turn on a stand at a fixed distance of 20?cm and 5?cm from the blackbody aperture, respectively. The platinum thermometer under-estimated the blackbody temperature by 0.015?°C (95% LOA: ?0.08?°C to 0.05?°C), in contrast to the thermal infrared camera and infrared thermometer which over-estimated the blackbody temperature by 0.16?°C (95% LOA: 0.03?°C to 0.28?°C) and 0.75?°C (95% LOA: ?0.30?°C to 1.79?°C), respectively. Infrared thermometer over-estimates thermal infrared camera measurements by 0.6?°C (95% LOA: ?0.46?°C to 1.65?°C). In conclusion, the thermal infrared camera is a potential temperature reference “fixed point” that could substitute mercury thermometers. However, further repeatability and reproducibility studies will be required with different models of thermal infrared cameras.     

Which thermometer? Factors influencing best choice for intermittent clinical temperature assessment

The objectives of this study are (a) to review the current technologies, (b) to examine comparative costing data for six selected representative devices, and (c) to discuss the clinical factors related to selection of devices for intermittent temperature measurement. Financial estimates indicate that mercury-in-glass thermometers are the cheapest devices. Compact electronic and chemical (phase change) thermometers are cheaper alternatives than multi-patient contact thermometers requiring probe covers and infrared sensing models, which are commonly adopted in hospitals and clinical practice. However, time required to obtain readings will influence overall costs. Rigorous independent clinical research studies are now needed to establish which of these alternative technologies are 'fit for purpose'. As a minimum they should offer comparable clinical accuracy and reliability to mercury-in-glass and be suitable for most clinical measurement situations. Furthermore any additional costs should bring demonstrable benefits to the patient, user and healthcare system.     

Infrared Thermometry for Acute Radiation Syndrome in Mice

Bulletin of Experimental Biology and Medicine - Experiment on female ICR CD-1 mice showed that non-contact infrared thermometry can be used for short-term and medium-term prognosis of animal death...     

SAR distribution in a bio-medium in close proximity with dual segment cylindrical dielectric resonator antenna

The objectives of this study are (a) to review the current technologies, (b) to examine comparative costing data for six selected representative devices, and (c) to discuss the clinical factors related to selection of devices for intermittent temperature measurement. Financial estimates indicate that mercury-in-glass thermometers are the cheapest devices. Compact electronic and chemical (phase change) thermometers are cheaper alternatives than multi-patient contact thermometers requiring probe covers and infrared sensing models, which are commonly adopted in hospitals and clinical practice. However, time required to obtain readings will influence overall costs. Rigorous independent clinical research studies are now needed to establish which of these alternative technologies are ‘fit for purpose’. As a minimum they should offer comparable clinical accuracy and reliability to mercury-in-glass and be suitable for most clinical measurement situations. Furthermore any additional costs should bring demonstrable benefits to the patient, user and healthcare system.     

Assessment of an infra-red non-contact sensor for routine skin temperature monitoring: a preliminary study.

The accuracy and reproducibility of a new non-contact sensor for monitoring skin temperature was examined. Thirty measurements taken by the device were compared with those taken by a commonly used thermocouple contact sensor. The result was a very high correlation coefficient (r = 0.9999). This accuracy was achieved with the probe held at an angle of 90 degrees 1 cm from the skin. Changes in angle and distance were found to contribute to measurement error. Little difference was found between 39 pairs of measurements taken of the left and right sides of subjects. However, intra-subject variability was noted with respect to the dermotomal segmental fields. Inter-tester reliability analysis resulted in a correlation of r = 0.937 involving two independent testers and 26 pairs of measurements. These preliminary data will be used for power calculations to study further the device which we found to be simple to operate, portable, and practical for routine clinical use. This sensor may have applications in the diagnosis of nerve and vascular disorders and in prospective monitoring of skin conditions such as bony areas at risk of pressure ulcers.     

基于纵向弛豫时间的低场磁共振无创测温技术的研究

磁共振成像(MRI)的一些成像参数对温度变化敏感,通过测量成像参数平衡磁化强度M.和纵向弛豫时间T1,可以实现无创的温度测量,该方法尤其适合在低场MRI中使用.本文在介绍磁共振无创测温技术基本原理的基础上,着重分析了低场磁共振无创测温技术中T1值的测量,M.与温度的关系及测温时最佳TR值的选取等问题,提出了增加修正系数等解决方案,并对方案进行检验,结合仿真计算,验证了解决方案的可行性.     

Thermometry considerations in localized hyperthermia

The introduction of local hyperthermia as a method of cancer therapy implies the necessity of quantitative measurements of the thermal dose. Our intention is to describe the nature of the problem, both physically and physiologically, with illustrations drawn from thermographic measurements in phantoms and in animals. The characteristics of a thermometry calibration facility are described. Some measurement problems associated with conventional thermometer probes are mentioned and several new thermometers which were developed for use in the electromagnetic fields are reviewed. We present some of the concepts that will guide the development of noninvasive thermometry. Systemic hyperthermia is not considered. We recommend that other reviews specifically directed toward localized hyperthermia be prepared on the methods of heating and on thermal physiological problems.     

Towards optimized MR thermometry of the human heart at 3T

Catheter ablation using radio frequency (RF) has been used increasingly for the treatment of cardiac arrhythmias and may be combined with proton resonance frequency shift (PRFS) ?based MR thermometry to determine the therapy endpoint. We evaluated the suitability of two different MR thermometry sequences (TFE and TFE‐EPI) and three blood suppression techniques. Experiments were performed without heating, using an optimized imaging protocol including navigator respiratory compensation, cardiac triggering, and image processing for the compensation of motion and susceptibility artefacts. Blood suppression performance and its effect on temperature stability were evaluated in the ventricular septum of eight healthy volunteers using multislice double inversion recovery (MDIR), motion sensitized driven equilibrium (MSDE), and inflow saturation by saturation slabs (IS). It was shown that blood suppression during MR thermometry improves the contrast‐to‐noise ratio (CNR), the robustness of the applied motion correction algorithm as well as the temperature stability. A gradient echo sequence accelerated by an EPI readout and parallel imaging (SENSE) and using inflow saturation blood suppression was shown to achieve the best results. Temperature stabilities of 2 °C or better in the ventricular septum with a spatial resolution of 3.5 × 3.5 × 8mm³ and a temporal resolution corresponding to the heart rate of the volunteer, were observed. Our results indicate that blood suppression improves the temperature stability when performing cardiac MR thermometry. The proposed MR thermometry protocol, which optimizes temperature stability in the ventricular septum, represents a step towards PRFS‐based MR thermometry of the heart at 3 T. Copyright © 2011 John Wiley & Sons, Ltd.     

Discretizing large traceable vessels and using DE-MRI perfusion maps yields numerical temperature contours that match the MR noninvasive measurements.

The success of hyperthermia treatments is dependent on thermal dose distribution. However, the three-dimensional temperature distribution remains largely unknown. Without this knowledge, the relationship between thermal dose and outcome is noisy, and therapy cannot be optimized. Accurate computations of thermal distribution can contribute to an optimized therapy. The hyperthermia modeling group in the Department of Radiotherapy, University Medical Center Utrecht devised a Discrete Vasculature [Kotte et al., Phys. Med. Biol. 41, 865-884 (1996)] model that accounts for the presence of vessel trees in the computational domain. The vessel tree geometry is tracked using magnetic resonance (MR) angiograms to a minimum diameter between 0.6 and 1 mm. However, smaller vessels (0.2-0.6 mm) are known to account for significant heat transfer. The hyperthermia group at Duke University Medical Center has proposed using perfusion maps derived from dynamic-enhanced magnetic resonance imaging to account for the tissue perfusion heterogeneity [Craciunescu et al., Int. J. Hyperthermia 17, 221-239 (2001)]. In addition, techniques for noninvasive temperature measurements have been devised to measure temperatures in vivo [Samulski et al., Int. J. Hypertherminal, 819-829 (1992)]. In this work, a patient with high-grade sarcoma has been retrospectively modeled to determine the temperature distribution achieved during a hyperthermia treatment. Available for this model were MR depicted geometry, angiograms, perfusion maps, as necessary for accurate thermal modeling, as well as MR thermometry data for validation purposes. The vasculature assembly through modifiable potential program [Van Leeuwen et al., IEEE Trans. Biomed. Eng. 45, 596-604 (1998)] was used in order to incorporate the traceable large vessels. Temperature simulations were made using different approaches to describe perfusion. The simulated cases were the bioheat equation with constant perfusion rates per tissue type, perfusion maps alone, tracked vessel tree and perfusion maps, and generated vessel tree. The results were compared with MR thermometry data for a single patient data set, concluding that a combination between large traceable vessels and perfusion map yields the best results for this particular patient. The technique has to be repeated on several patients, first with the same type of malignancy, and after that, on patients having malignancies at other different sites.     

Laboratory and hospital testing of new infrared tympanic thermometers

A patented approach to infrared thermometry based on the use of a standard pyrosensor has resulted in the development of two new infrared tympanic thermometers, one for professional use, the other for home use. Both were tested to evaluate accuracy in the laboratory and to evaluate equivalence to standards, correlation to standards, and precision in human subjects. Accuracy was found to be well within ASTM standards on both models. Mean ear temperatures were 0.2 degrees C below oral and 0.7 degrees C below bladder temperature. Correlations between ear and oral and ear and bladder temperatures were r = .77 to .84. Repeatability in the same ear was very high at r = .95 (left) and .97 (right). Reproducibility between left and right ear ranged from r = .89 to .92.     

A self-reference PRF-shift MR thermometry method utilizing the phase gradient

In magnetic resonance (MR) imaging, the most widely used and accurate method for measuring temperature is based on the shift in proton resonance frequency (PRF). However, inter-scan motion and bulk magnetic field shifts can lead to inaccurate temperature measurements in the PRF-shift MR thermometry method. The self-reference PRF-shift MR thermometry method was introduced to overcome such problems by deriving a reference image from the heated or treated image, and approximates the reference phase map with low-order polynomial functions. In this note, a new approach is presented to calculate the baseline phase map in self-reference PRF-shift MR thermometry. The proposed method utilizes the phase gradient to remove the phase unwrapping step inherent to other self-reference PRF-shift MR thermometry methods. The performance of the proposed method was evaluated using numerical simulations with temperature distributions following a two-dimensional Gaussian function as well as phantom and in vivo experimental data sets. The results from both the numerical simulations and experimental data show that the proposed method is a promising technique for measuring temperature.     

The use of infrared thermometry for the detection of fever

The use of tympanic thermometry is attractive in primary care, but a recent systematic review highlighted the paucity of data comparing tympanic thermometry with conventional methods. We report a study of 94 preschool children presenting to primary care in the United Kingdom (UK) with acute cough in whom tympanic infrared and axillary mercury thermometry are compared. Infrared thermometry showed poor agreement, poor sensitivity and high specificity. Infrared thermometry is too insensitive to be used as a screening test for fever, but when fever is already suspected, for example by touch, it may be useful as a 'rule in' test.     

基于超声成像的眼压非接触测量方法

目的 目前市场上主流眼压测量设备以接触式和刺激性激励为主,使用过程中极易给患者带来不适,更有甚者会导致角膜感染。为了解决这一问题,提出一种超声成像眼压非接触式测量方法。方法 对该方法进行有限元分析与实验验证,得到角膜形变仿真计算结果,制作模拟眼并搭建眼压测量实验系统,建立眼压、角膜形变与压力的数学模型,并对该模型进行准确性和重复性验证。结果 在某一眼压下,角膜的压平直径和顶点位移分别与压力存在幂函数与线性关系;在恒定压力下,压平直径和顶点位移均与眼压存在幂函数关系,可作为眼压测量指标。通过该方法测得的眼压平均相对误差在8%以下,相对平均偏差在9%以下,重测信度均大于0.9。结论 该方法测量结果准确,重复性较好。研究结果为眼压的非接触测量提供一种有效途径,具有较好的应用前景。     

Artefacts in intracavitary temperature measurements during regional hyperthermia

For adequate hyperthermia treatments, reliable temperature information during treatment is essential. During regional hyperthermia, temperature information is preferably obtained non-invasively from intracavitary or intraluminal measurements to avoid implant risks for the patient. However, for intracavitary or intraluminal thermometry optimal tissue contact is less natural as for invasive thermometry. In this study, the reliability of intraluminal/intracavitary measurements was examined in phantom experiments and in a numerical model for various extents of thermal contact between thermometry and the surroundings. Both thermocouple probes and fibre optic probes were investigated. Temperature rises after a 30 s power pulse of the 70 MHz AMC-4 hyperthermia system were measured in a tissue-equivalent phantom using a multisensor thermocouple probe placed centrally in a hollow tube. The tube was filled with (1) air, (2) distilled water or (3) saline solution that mimics the properties of tissue, simulating situations with (1) bad thermal contact and no power dissipation in the tube, (2) good thermal contact but no power dissipation or (3) good thermal contact and tissue representative power dissipation. For numerical simulations, a cylindrical symmetric model of a thermocouple probe or a fibre optic probe in a cavity was developed. The cavity was modelled as air, distilled water or saline solution. A generalised E-Field distribution was assumed, resulting in a power deposition. With this power deposition, the temperature rise after a 30 s power pulse was calculated. When thermal contact was bad (1), both phantom measurements and simulations with a thermocouple probe showed very high temperature rises (>0.5 degrees C), which are artefacts due to self-heating of the thermocouple probe, since no power is dissipated in air. Simulations with a fibre optic probe showed almost no temperature rise when the cavity was filled with air. When thermal contact was good, but no power was dissipated in the tube (2), artefacts due to self-heating were not significant and the observed temperature rises were very low ( approximately 0-0.1 degrees C). For the situation, with tissue representative power dissipation (3), a temperature rise of approximately 0.23 degrees C was observed for both measurements and simulations. A clinical example of a regional hyperthermia treatment of a patient with a cervix uteri carcinoma showed that the artefacts observed in the case of bad thermal contact also affect the steady-state temperature measurements. Good tissue contact must be assured for reliable intraluminal or intracavitary measurements.     

电磁场中热电偶测温的热传导误差分析

肿瘤热疗过程中通常采用热电偶进行温度监测。在排除了较大的电磁干扰误差后，热传导误差（ＴｈｅｒｍａｌＣｏｎｄｕｃｔｉｏｎＥｒｒｏｒｓ）是热电偶探头在电磁加热场中测温时的主要误差来源。本文论述产生热传导误差因素，指出使用低热传导性材料、避免焊点过大、避免较大温度梯度等措施可以减小这种误差。     

Dosimetry of Sr-90 ophthalmic applicators

Sr-90 ophthalmic applicators are commonly used for the treatment of superficial eye disorders. Although a variety of dosimetric devices such as film, thermoluminescent dosimeters (TLD's), ion chambers, and radiochromic foils have been used to measure the peak dose at the applicator surface, there is no internationally agreed upon calibration procedure. Recently, large discrepancies among calibrations of the same applicator at three institutions have been reported. Here we describe a technique to obtain the peak dose rate at the applicator surface using LiF TLD's. The technique can be used for the calibration of flat as well as curved surface applicators. Results for two flat and three concave applicators are presented. Our measurement of the surface dose rate for one of the flat applicators is compared with those obtained by four other institutions, each using different dosimetric devices.