Research progresses of breast background parenchymal enhancement of MRI

, , ,

(Department of Radiology, the Third Affiliated Hospital Kunming Medical University,Yunnan Tumor Hospital, Kunming 650118, China)

Research progresses of breast background parenchymal enhancement of MRI

LIQinqing,YANGJun,DINGYingying,LIZhuolin*

(DepartmentofRadiology,theThirdAffiliatedHospitalKunmingMedicalUniversity,YunnanTumorHospital,Kunming650118,China)

Breast background parenchymal enhancement (BPE) is a special form of breast dynamic contrast enhanced MRI. The diagnosis and research of breast diseases which shows BPE is becoming more and more important. The relationship between BPE and the risk factors of breast cancer is one of the focuses in breast MRI. In this review, the definition and typical manifestations of BPE, the relationship between BPE and fibroglandular tissue, and the influence factors of BPE in the breast MRI were reviewed.

Background parenchymal enhancement; Magnetic resonance imaging; Breast neoplasms

动态增强MRI(dynamic contrast enhanced MRI, DCE-MRI)在乳腺疾病的诊断运用广泛[1]，且近年来越来越多的学者开始关注DCE-MRI中发生强化的乳腺纤维腺体组织(fibroglandular tissue, FGT)。目前对DCE-MRI时乳腺背景实质强化(background parenchymal enhancement, BPE)在乳腺疾病诊断中的作用和影响因素、BPE与乳腺癌患病风险及预后评价的相关性等的分析逐渐增多。本文对BPE的定义及典型表现、BPE的影响因素、BPE对MRI诊断的影响、BPE与乳腺FGT含量的关系以及BPE与乳腺癌发病风险的相关性等进行综述。

1 BPE的定义及典型表现

乳腺BPE是指正常的乳腺FGT在DCE-MRI中所表现的不同程度和范围的强化，包括乳腺间质组织和上皮组织的强化[2]。根据强化的腺体占总腺体含量的百分比，将BPE描述为4类：极少量(<25%的腺体组织显示有强化)、少量(25%～50%)、中等量(51%～75%)以及大量(>75%)。Gram等[3-4]对BPE的解释不仅包括强化腺体体积，还包括强化程度，但2014年1月美国放射学院(American College of Radiology, ACR)公布的2013年新版磁共振乳腺影像报告和数据系统(breast imaging-reporting and data system, BI-RADS)[5]中，并未出现类似描述。新版MRI BI-RADS中提到BPE的评估应在增强扫描的第一组图像中进行，即在注射对比剂后约90 s。

典型的BPE常表现为双乳对称性、弥漫性分布的强化[6]，可呈片状或多发点状，最先由乳腺FGT的边缘开始强化，后逐渐向乳腺中央区蔓延，乳晕后方最后强化，该强化模式由乳腺供血动脉的分布所致，可能存在个体差异，甚至同一个人在接受治疗后强化模式也可能发生改变。此外，BPE的典型血流动力学特征与良性病变类似，即早期缓慢上升，随着时间的延迟，范围和强度逐渐扩大、增加。但也可出现早期快速、显著的强化，甚至延迟期呈平台型或廓清型[6]的类似恶性病变的表现。

2 BPE的影响因素

BPE是一个动态变化的过程，不同受检者的BPE不同，甚至同一患者在不同时期的BPE亦有所不同。BPE受组织学、生理学等因素的影响，其程度及范围取决于乳腺组织的血管分布以及对比剂进入组织的渗透性[4,6]。Kuhl等[7]提出影响增强MRI组织强化范围及强化程度的常见因素有：对比增强T1WI脉冲序列的参数、不同乳腺组织的T1WI基线时间、对比剂浓度和剂量、扩散系数及其固有的T1WI弛豫率。此外，对于乳腺组织，还有一些特殊的影响因素，如乳腺血管系统的解剖结构以及激素水平等。

目前，关于BPE产生和调控的确切机制还不清楚[8-9]。已有研究[10-12]证实，激素尤其是雌激素是最重要的影响因素，雌激素的作用可是内源性、外源性，或是内分泌治疗后的结果。乳腺是激素敏感器官[8]，雌激素可刺激乳腺小叶组织和导管腺泡上皮细胞的增生，促进血管形成，还可产生组胺样物质，使乳腺实质内的血管扩张、血管通透性增加，导致乳腺充血，乳房内的水含量发生变化，从而影响T1值[13]，最终导致BPE的改变。

Price等[14]学者研究双侧输卵管、卵巢切除术对乳腺MRI中BPE的影响，发现卵巢切除术后患者的BPE明显降低。Kang等[13,15]研究BPE与绝经状态的关系，发现绝经前女性的BEP明显高于绝经后女性。King等[16]研究报道接受他莫昔芬(一种雌激素受体调节剂)治疗后，BPE及FGT含量出现明显下降，而用药结束后，BPE又可能逐渐恢复。Pike等[17]的研究认为，BPE可能是评价他莫昔芬疗效较好的一个指标。

内源性雌激素的生理作用，可较合理地解释BPE伴月经周期发生变化的原因[8]。月经周期的前半段(月经期及卵泡期或称增殖期，1～2周)，雌激素主要刺激乳腺上皮细胞增殖、腺泡细胞分化以及基底膜增厚，而在月经周期的后半段(黄体期或称分泌期，3～4周)，雌激素的组胺样作用明显，促进乳腺腺泡管腔扩大并分泌物增多，使毛细血管扩张，增加血管通透率，从血管内渗出到乳腺基质的液体量逐渐增加，使乳腺基质和小叶间纤维组织发生水肿，且在分泌阶段(月经周期后半段)最为明显[10]。

目前，关于BPE与月经周期的关系以及乳腺MR检查的最佳时间仍存在争议。Müller-Schimpfl等[18]发现BPE在月经周期第21～28天(第4周)及第1～6天(第1周)最高，而在第7～20天(第2周)最低。Ellis等[19]推荐的MR检查时间是月经周期的第7～14天。Kang等[13,20]研究结果也显示，月经周期后两周的BPE较前两周高，不利于MR检查，该结果与月经周期中雌激素的生理变化基本吻合。目前认为，在月经周期的第2周BPE水平最低，最适合进行乳腺MR检查。但Kang等[13]发现，即使在月经周期第2周，还会存在BPE较强而影响对MRI结果判读的情况，该研究根据乳腺的FGT含量的不同，将绝经前妇女乳腺BPE的程度进行分类研究，发现脂肪型乳腺BPE在月经周期的第2周较低，第4周最高；而致密型乳腺第4周BPE相对均匀且较低，是最佳检查时间段，而第3周BPE程度最高，应避免检查。Ellis[19]提议对于月经不规律或稀发的女性，可通过检测血清中的孕激素水平，判断患者是否处于最佳的MR检查时间。

3 BPE对MRI的影响

乳腺是低灌注器官，强化的FGT常小于总腺体含量的50%，也可见大于50%、甚至超过75%者。DCE-MRI对于检测乳腺癌具有极高的敏感性，尤其对于呈弥漫性生长的浸润性癌或以良性形态出现的髓样癌，以及乳腺X线摄影呈致密腺体型的乳腺癌患者，乳腺MRI具有更高的诊断价值[21-24]。但BPE对MRI诊断的准确性也会产生影响。在DCE-MRI中，BPE会降低病灶与周围正常乳腺组织之间的对比度，模糊病灶的边缘，甚至掩盖病灶，给病灶的检出带来一定的困难，尤其对于小的肿块或较大范围的非肿块样强化，可能增加病灶的假阴性率[23,25-26]。Uematsu等[26]的研究结果显示，呈中度或显著背景实质强化组的MRI准确性低于极少量或少量强化组；此外，当BPE呈不对称局灶性或区域性表现时，易被误诊为异常征象，从而增加MRI诊断的假阳性率，甚至可能增加不必要的穿刺活检率[4,27-28]。因此，识别不典型的BPE尤为重要，但对于可疑的非肿块样强化[29]有一定困难，此时可通过短期随访、临床触诊、乳腺X线摄影以及超声等的联合运用提高诊断准确率。

4 BPE与乳腺FGT含量的关系

乳腺MRI中FGT含量的定性评估也被纳入ACR最新版(第5版)的BI-RADS[5]，该乳腺MRI特征与传统乳腺X线摄影腺体密度有高度一致性，对乳腺癌患病风险的判断较准确[30-32]。BPE与乳腺FGT含量的关系目前还存在争议。Klifa等[28,33]研究认为BPE与FGT呈正相关，腺体越致密，BPE程度越高。而有研究[13,15,32,34]结果显示FGT与BPE间无明显相关性，极度致密类的乳腺可呈现极少量的强化，而散在纤维腺体类乳腺也可能出现明显、大量的背景实质强化。Kang等[13]研究发现，对于FGT含量丰富的致密型乳腺，丰富的FGT受激素变化的影响较脂肪类更为显著。虽然相较于乳腺FGT含量，还有很多影响因素可对BPE造成影响，但不同FGT含量的乳腺可表现出从极少量到大量强化的BPE。

5 BPE与乳腺癌发病风险的相关性

雌激素的增多与肿瘤发生有关联性[35]，雌激素须与组织中的雌激素受体相结合才能产生作用，而乳腺是激素敏感器官，雌激素可对BPE产生影响[36]。研究[37-39]发现，正常乳腺组织的强化在乳腺癌发病风险的预测以及治疗反应和预后评估方面有重要的作用。King等[37]在DUI 乳腺癌患者和正常受检者乳腺MRI中BPE的研究中发现，BPE呈中度和显著强化的女性较极少量和少量强化者患乳腺癌的可能性更大。Dontchos等[36]亦发现BPE有助于预测乳腺癌患病风险。King等[37]认为BPE可能与乳腺X线摄影腺体密度同样重要、彼此独立，且与乳腺癌患病风险有关。因此，BPE可作为乳腺组织可能发生恶性转变的标志物[40]。

6 小结

BPE不仅可能影响乳腺MRI的诊断，还与乳腺癌的患病风险存在关联。临床需熟悉BPE的不同表现形式、激素及其他因素对BPE的影响，必要时需联合超声、乳腺X线摄影以及临床病史及触诊进行综合判断，以做出最客观的诊断。

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李勤勍(1987—)，女，云南昆明人，硕士，主治医师。研究方向：肿瘤影像研究。E-mail: qinqing_81@139.com

李卓琳，昆明医科大学第三附属医院 云南省肿瘤医院放射科，650118。E-mail: lizhuolin0327@163.com

2016-11-12

2017-06-20

乳腺背景实质强化的MRI研究进展

李勤勍，杨 军，丁莹莹，李卓琳*

(昆明医科大学第三附属医院 云南省肿瘤医院放射科，云南 昆明 650118)

乳腺背景实质强化(BPE)是乳腺动态增强MRI所特有的一种表现形式，在乳腺疾病诊断中的作用以及BPE与乳腺癌患病风险、预后等的关系已成为研究的热点。本文对乳腺背景实质强化的定义及典型表现、BPE对MRI诊断的影响、BPE的影响因素、BPE与乳腺纤维腺体组织含量的关系以及BPE与乳腺癌发病风险的相关性等进行综述。

乳腺背景实质强化；磁共振成像；乳腺肿瘤

R737.9; R445.2

A

1003-3289(2017)08-1273-04

10.13929/j.1003-3289.201611069