基于空间频域成像的烧伤程度无创定量评估 下载: 779次
The increasing burn mortality rate places an urgent need for accurate diagnosis and treatment of burns. Currently, the third-degree quartile is internationally used to classify the degree of burns based on burn depth, and clinical treatment methods for different degrees of burns are significantly dissimilar. Burn surgeons overestimating the severity of burns can lead to unnecessary surgery, whereas underestimating them leads to treatment delays and worsening of the burn conditions. In addition, studies have shown that burn severity changes dynamically over time, with superficial Ⅱ burn worsening to deep Ⅱ or Ⅲ burns within 48 h of burn occurrence. Therefore, overcoming the defects of subjective judgment using the naked eye and quantitatively monitoring the dynamic changes in the burn degree in real time has become a challenge in the early diagnosis of burns. Burn diagnosis methods based on photonics, such as near-infrared spectroscopy, reflective confocal microscopy, and laser Doppler flowmetry, are developing rapidly. However, their clinical application is limited owing to low accuracy, invasiveness, high detection environment requirements, and high costs. In this study, a noninvasive quantitative method for assessing the burn degree was developed based on spatial frequency-domain imaging (SFDI). Combined with the systematic clustering method and multiparameter dimensionality reduction analysis, the proposed method results in improved classification accuracy of different burn degrees and shortened classification time, thus indicating the potential for early diagnosis of clinical burns.
In this study, the SFDI technique was applied to a rat burn model. First, the backs of Sprague-Dawley (SD) rats were depilated, and a thermostatic iron heated to 100 ℃ was used on the backs of the anesthetized SD rats for 4, 12, and 24 s, respectively, to establish a rat burn model with different burn degrees. Next, the sinusoidally modulated structural patterns were projected onto the surface of each burned area, and the backscattered structural patterns from the tissues were captured using a charge-coupled device (CCD) camera. Subsequently, we used single-snapshot multifrequency demodulation (SSMD) to extract the modulation transfer function (MTF) of light from the burned tissues. Compared with the traditional three-phase shift demodulation method, SSMD only requires a single snapshot to achieve parameter extraction, which significantly suppresses the problem of motion artifacts and improves the signal-to-noise ratio of imaging using filtering technology. Based on the photon diffusion transmission theory, the optical parameters (μa and μ′s) were then recovered using the look-up table method at the 5th, 10th, 30th, 60th, 90th, and 120th minutes after burn. Finally, systematic clustering and multiparameter dimensionality reduction analysis were performed on the optical parameters to quantify and classify different burn degrees.
Different degrees of burns can be effectively distinguished by the relative changes in the two optical parameters at the three wavelengths. The results show that the magnitude of the absorption coefficient positively correlates with the degree of burn. In contrast, the magnitude of the reduced scattering coefficient negatively correlates with the degree of burn. Although the distinction between optical parameters is not significant at the beginning of burns, the optical parameters of the 4 s burn group gradually decrease or gradually recover to the unburned state with observation time. In contrast, the optical parameters of the 12 s and 24 s groups gradually deviate from the normal state (Fig. 6). The burn results are divided into two categories through optimal analysis of systematic clustering. The 4 s group is classified as mild burns, whereas the 12 s and 24 s groups are classified as severe burns. Although the classification accuracy is less than 85% in the first 10 min after burn, it is 100% in the later stages (Table 1). Two new factors (the absorption factor FAC1 and the reduced scattering factor FAC2) reflecting approximately 93% of the original variable information can be generated using the principal component analysis to reduce the dimensionality of the six optical parameters. The results show that the absorption factor, FAC1, distinguishes the degree of burns in a large category (mild burns in the 4 s group and severe burns in the other two groups) and increases the difference between deep Ⅱ degree burn in the 12 s group and Ⅲ burn in the 24 s group. In addition, the assessment of burn severity using principal constituent factors can reduce interference and improve classification accuracy in the early stage after burn (Fig. 9).
The quantitative burn imaging device based on real-time spatial frequency-domain imaging technology has remarkable advantages over existing diagnostic techniques, for example, ease of handling, compact structure, and high precision. Through dynamic monitoring of changes in optical parameters combined with cluster analysis and parameter dimensionality reduction, the degree of burns can be determined through noninvasive assessment, providing a reliable guarantee for the precise treatment of burns. In future studies, we will supplement the pathological verification, characterize additional physiological parameters (such as hemoglobin content, blood oxygen saturation, and melanin concentration) from the optical parameters, and extend this technology to clinical applications so as to significantly reduce the treatment cycle and cost to patients.
1 引言
烧伤是指由热力、电流、化学物质、放射线等因素导致的皮肤组织损伤,居全球意外伤害死因的第六位[1-2]。据世界卫生组织(WHO)统计[3],全球每年因烧伤导致的死亡病例超过30万。烧伤不仅导致沉重的经济负担,还给患者带来了严重的生理和心理危害[4-5],社会影响广泛而巨大,已成为全球性公共卫生问题。
烧伤治疗的重点是及时准确地评估出烧伤深度。对于不同深度的烧伤,目前国际上惯用Ⅲ度四分法[6]将烧伤程度划分为Ⅰ度、浅Ⅱ度、深Ⅱ度和Ⅲ度。Ⅰ度烧伤仅伤及表皮,浅Ⅱ度烧伤可伤及真皮层的上层,深Ⅱ度烧伤则伤及真皮层的下层,较长时间接触热源会伤及皮肤全层及其以下的各种组织器官从而导致Ⅲ度烧伤。Jackson[7]将烧伤组织描述为三个带:最接近热源的凝固带(出现焦痂或坏死),该区域组织发生了不可逆转的坏死与蛋白质变性;下方的淤滞带,此区域由于毛细血管渗漏、细胞膜被破坏而出现水肿,而且大分子蛋白中度变性,血流速度缓慢;最下方的充血带,此区域血流速度逐渐加快,尤其是烧伤7 d后特别明显。针对不同程度的烧伤,临床治疗手段存在明显差异:浅表烧伤一般通过给药自动愈合;深度烧伤必须要对创口进行手术切除和移植;介于两者之间的Ⅱ度真皮层烧伤,烧伤外科医生通常难以直接依靠肉眼判断,若仅仅只是损伤到真皮层的上层,则只需敷药依靠基底层细胞或皮肤附属器上皮细胞就可修复创面,但若触及到真皮层的下层,则治疗方案将与深度烧伤相同。此外,相关研究表明烧伤程度会随时间动态变化[8],浅Ⅱ度创伤在烧伤后的48 h内可能会恶化为深Ⅱ度或Ⅲ度烧伤。因此,对烧伤程度进行准确、连续监测尤为重要。高估烧伤的严重程度可能意味着不必要的手术,而低估将导致治疗延误,从而加重病情。因此,克服肉眼主观判断的缺陷并实时定量监测烧伤程度的动态变化,成为烧伤早期诊断的难点。
目前,国内外开展了多种基于生物光子学的烧伤诊断方法,且部分已在临床中试用。其中,近红外光谱法(NIRS)[9-10]可以实现对烧伤组织的非接触近红外光谱成像,通过表征组织内部各类化合物的浓度,就可大致区分出浅部和全层烧伤,但对浅部和深部烧伤的界线划定仍不够准确。反射式共焦显微镜(RMCM)[10-11]法作为一种“光学活检”方法,通过连续调整聚焦深度可以精确判断损伤程度,然而,该方法具有检测区域小且需要与伤口直接接触等不足。现有的唯一被美国食品药品监督管理局(FDA)批准可用于诊断烧伤程度的激光多普勒流量仪(LDI)[10,12],对检测条件的要求较高,需要一定的环境温度和平衡时间,而且设备价格昂贵、体积大,限制了其在临床上的广泛应用。因此,研制出一种简便、快速、非入侵性的烧伤程度诊断方法是十分有必要的。空间频域成像(SFDI)技术[13-21]因具有大视野、非接触、定量检测等优点,已在血液动力学检测、癌症诊断、烧伤评估等临床上获得了广泛应用。如:康旭等[22]采用SFDI技术提取出了皮肤组织的光学参数,并准确测量出了组织的血氧饱和度;曹自立等[23]利用SFDI成像系统检测出了多种病变皮肤(例如带状疱疹、湿疹样皮肤炎等)组织与正常皮肤组织在光学参数、生理参数上的较大差异;Nguyen等[14]将SFDI技术应用于猪烧伤模型成像中,通过反演出的光学参数二维图,直观地展现了烧伤部位组织与周边正常组织的光学参数差异。
本课题组研究了基于SFDI的烧伤程度无创定量评估方法,该方法利用单次快照解调法对组织的后向散射结构图案进行快速解调,即可反演出与组织结构、生理特性紧密相关的光学参数(吸收系数μa与约化散射系数μ′s),从而能有效、快速地评估烧伤程度,实时监测烧伤进程。实验结果表明,烧伤导致皮肤组织的两个光学特性参数发生显著变化,且不同的烧伤深度表征出的光学参数随时间的变化趋势有所不同。本文方法为烧伤程度的定量评估提供了一种极具潜力的评估途径。
2 方法与材料
2.1 基于单次快照多频解调技术的空间频域成像(SSMD-SFDI)原理
根据光与组织的相互作用原理[24],光子在生物组织中传输时将被吸收与散射。组织对光的吸收与散射能力分别由吸收系数(μa)与约化散射系数(μ′s)这两个光学参数来描述。吸收系数取决于组织中各发色团的浓度,而约化散射系数则与组织的结构尺寸紧密相关。因此,当烧伤引起皮肤组织中各发色团的浓度改变以及结构变性后,势必会影响吸收系数与约化散射系数的变化。SFDI技术是一种全新的非接触、无损、大视场定量成像技术,能够快速同时解析出生物组织的吸收系数与约化散射系数,为烧伤的功能化定量成像提供了一种重要途径。
SFDI的核心是将生物组织作为一个低通信号系统,通过投影正弦调制的结构图案到组织表面,利用CCD采集组织后向散射的结构图像,并通过解调技术提取出组织的调制传递函数(MTF),再由查表法或蒙特卡罗模拟反演出光学参数。假设入射到样品表面的结构光图案以及样品后向散射的光强图案分别为
最后,根据光子扩散传输理论[26-27],组织的调制传递函数与吸收系数、约化散射系数满足
图 1. 快速计算光学参数(μa和 )的双频调制传递函数(直流与交流)查表法
Fig. 1. Two-frequency MTF (DC versus AC) lookup table for rapid calculation of optical parameters (μa and )
需要特别说明的是,在SFDI的各种解调方法中,本研究采用了独具优势的SSMD方法。相较于传统的三相移解调法需要采集三幅图像才能解调计算交流分量和直流分量,SSMD仅需单次快照就可以实现,极大地抑制了运动伪影问题。此外,SSMD通过滤波技术也显著提高了成像的信噪比。
2.2 便携式烧伤定量成像装置
自主研制的便携式烧伤定量成像装置如
图 2. 便携式烧伤定量成像装置。(a)装置实物图;(b)光路示意图
Fig. 2. Portable quantitative burn imager. (a) Device physical map; (b) optical path schematic
如
2.3 实验方案与数据处理流程
本研究的动物烧伤模型和实验方案已通过温州医科大学实验动物中心动物实验伦理审核(批准号:wydw2022-0116)。选用4只SD大鼠(体重为200~300 g,9~11周龄),以可控恒温电熨斗作为热源,接触烫面是尺寸为85 mm×26 mm且略呈弧面的长方形金属平面。鼠烧伤模型的具体实验过程如下:1)先用电动刮毛机对每只大鼠的背侧区域剃毛,并用刀片刮刀进一步脱毛,保证每只大鼠身上有4处大小与金属接触烫面相匹配的实验区域。2)经腹腔向SD大鼠注射三溴乙醇进行麻醉,先对所有背部实验区域进行测量并作为空白对照组,然后将恒温电熨斗加热到100 ℃,在没有额外压力(仅重力)的情况下作用于每只大鼠背侧的4处实验区域,接触时间分别设置为4、12、24 s(根据文献[16-17,28-29]确定接触时间),如
实验数据按照
图 4. 数据处理流程及数据可视化界面。(a)数据处理流程图;(b)数据可视化界面
Fig. 4. Data processing flowchart and data visualization interface. (a) Data processing flowchart; (b) data visualization interface
1) 对于三种烧伤组(4 s、12 s、24 s),分别在烧伤后的6个时间节点(第5、第10、第30、第60、第90、第120分钟)进行拍摄,获取图片,然后采用SSMD解调法,由式(3)~(4)分别解调出每张图片中三个波长的交流分量和直流分量的二维分布;
2) 由式(5)分别计算出交流分量与直流分量的调制传递函数MTFAC和MTFDC,并基于
3) 从每张光学参数图谱中心附近随机筛选出6个大小为1.04 mm×1.04 mm的感兴趣区域(ROI区域),则同一烧伤组(5个烧伤区域)在相同烧伤节点可以产生30个ROI区域;
4) 对不同烧伤组在每一阶段下的30个ROI区域进行均值与标准差分析,并用于后续的聚类分析。
3 结果与讨论
3.1 鼠烧伤模型的光学参数
本研究共产生了15组烧伤创面,三个不同烧伤程度的样本均有5组。
图 5. 大鼠烧伤皮肤原图。(a)烧伤前(空白组);(b)4 s组;(c)12 s组;(d)24 s组
Fig. 5. Pictures of rat burn skin. (a) Before burn (blank group); (b) 4 s group; (c) 12 s group; (d) 24 s group
图 6. 三个波长下三组不同烧伤区光学参数随时间的变化(每个时间节点对应的参数值为ROI区域的平均值,并给出了其标准差。利用Mann-Whitney U-test比较数据的差异性,显著性差异水平用“*”表示,*表示P<0.05)。(a)~(c) R、G、B波长下各时段烧伤区吸收系数相对于烧伤前的变化;(d)~(f)R、G、B波长下各时段烧伤区约化散射系数相对于烧伤前的变化
Fig. 6. Variation of optical parameters of three groups of different burn areas with time under three wavelengths (the parameter value corresponding to each time period is the average value of ROI area, and its standard deviation is also given. Mann-Whitney U-test is used to compare the differences of data, and the level of significant difference is represented by“*”and * represents P<0.05).(a)-(c) Relative change of absorption coefficient of burn area to the area before burn at each time period under R, G, B wavelengths; (d)-(f) relative change of reduced scattering coefficient of burn area to the area before burn at each time period under R, G, B wavelengths
另外,在烧伤初期难以通过光学参数对三种程度的烧伤进行很好的区分,但随着观测时间的推移,可以发现4 s烧伤组的光学参数变化逐渐平缓或逐渐向未烧伤状态靠近,而12 s组与24 s组的光学参数则均逐渐偏离正常状态。以
Kistler等[33]在探究、建立实验动物可复制的全烧伤模型时发现,在大鼠背部利用电热烫伤仪持续20 s热压可造成Ⅲ度烧伤;杨军等[34]在利用90 ℃烧瓶热压制作大鼠烫烧创面的实验中发现,烧瓶与SD大鼠背部接触5 s可造成Ⅰ度烧伤,接触15 s可造成深Ⅱ度烧伤,接触15~20 s可造成Ⅲ度烧伤。综合本文实验现象与文献结果,本课题组认为轻度烧伤的4 s组属于Ⅰ度烧伤,重度烧伤的12 s组和24 s组分别属于深Ⅱ度烧伤和Ⅲ度烧伤。
不同烧伤程度的组织在早期通过肉眼观测难以区分,但随着时间的推移,不同烧伤程度的组织的光学参数变化趋势却显著不同,可作为一种有效的烧伤程度评估标准,用于实现对不同烧伤深度的区分。
3.2 基于光学参数的烧伤程度系统聚类分析
考虑到连续监测尽管可以分类但耗时较长,所以本课题组进一步基于光学参数对烧伤程度展开聚类分析,旨在缩短分类时间。
由于K-means算法中聚类个数K的选择过于主观,因此本课题组选用系统聚类算法对烧伤后相同阶段下不同烧伤程度的ROI区域(每阶段下有90个ROI区域)进行二分类,以验证假设。烧伤后的每个时间阶段均存在90个样本,基于三个波长下的吸收系数与约化散射系数较烧伤前的相对变化值(共6个参数),利用欧氏几何计算样本间的距离,采用最短距离法计算样本与小类、小类与小类间的距离,直至所有小类合并为一个大类。具体流程如
图 7. 系统聚类。(a)系统聚类流程图;(b)聚合系数折线图
Fig. 7. Systematic clustering. (a) Systematic clustering flowchart; (b) aggregation coefficient line chart
依据肘部法则绘制各烧伤阶段下的聚合系数折线图,从而估计出最优的聚类数量。本文展示了基于烧伤后120 min测得的90个样本数据绘制出的聚合系数折线图,如
系统聚类倾向于将4 s组归为一类,将12 s组和24 s组归为一类,本文称之为轻度烧伤类与重度烧伤类,与聚类前的假设相吻合。为实现分类结果准确性的可视化,可将聚类结果作为实际分类结果与原先假设相结合绘制出烧伤后每个阶段的类混淆矩阵。绘制出的两个具有代表性的类混淆矩阵如
图 8. 系统聚类结果。(a)烧伤后10 min;(b)烧伤后120 min
Fig. 8. Systematic clustering results. (a) 10 min after burn; (b) 120 min after burn
引入分类准确率(A)表征每个阶段下烧伤分类的准确程度。分类准确率的计算公式为
由
表 1. 系统聚类分类准确率
Table 1. Systematic clustering classification accuracy
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3.3 基于主成分分析的光学参数降维
考虑到系统聚类削弱了各光学参数的差异性,同时为了探究更加精细的分类方法,本课题组利用主成分分析对6个光学参数进行降维。先对6个参数进行巴特利特球形检验,P值<0.05表示适合对其进行主成分分析,得到两个主成分的累计贡献率为93.293%(>85%),即利用两个主成分已经可以反映原变量约93%的信息。因此,将6个光学参数降维,最终获得2个主成分。
表 2. 光学参数的因子载荷
Table 2. Factor loading for optical parameters
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图 9. 不同烧伤阶段的因子箱线图。(a)烧伤后5 min;(b)烧伤后10 min;(c)烧伤后30 min;(d)烧伤后60 min;(e)烧伤后90 min;(f)烧伤后120 min
Fig. 9. Factor boxplot at different burn stages. (a) 5 min after burn; (b) 10 min after burn; (c) 30 min after burn; (d) 60 min after burn; (e) 90 min after burn; (f) 120 min after burn
4 结论
本文基于实时SFDI技术研制出的烧伤定量成像装置具有便携、体积小、精度高、快速定量化检测等诸多优点,相对于目前已有的诊断装置具有较大优势。通过动态监测光学参数的变化,结合聚类分析与参数降维,可以实现烧伤程度与烧伤范围的无创定量评估,为烧伤的精准治疗提供了可靠保证。在后续的工作中,本课题组将增加临床的分类病理验证,并从光学参数中表征出更多的生理参数(例血红蛋白含量、血氧饱和度、黑色素浓度等),探究不同程度的烧伤对生理参数的影响,同时将此技术推广到医院,以极大地缩减患者的治疗周期与成本。
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Article Outline
钟晓雪, 黄国武, 缪弘波, 胡城豪, 刘威, 孙春容, 陈志华, 李港宁, 曹自立, 金鑫, 林维豪. 基于空间频域成像的烧伤程度无创定量评估[J]. 中国激光, 2022, 49(24): 2407205. Xiaoxue Zhong, Guowu Huang, Hongbo Miu, Chenghao Hu, Wei Liu, Chunrong Sun, Zhihua Chen, Gangning Li, Zili Cao, Xin Jin, Weihao Lin. Noninvasive Quantitative Assessment of Burn Degree Based on Spatial Frequency-Domain Imaging[J]. Chinese Journal of Lasers, 2022, 49(24): 2407205.