Breast cancer is the most prevalent malignant tumor among women worldwide. It has become an important public health issue of global concern, and is one of the major malignant tumors that seriously threaten women’s lives and health and consume a huge amount of health resources, characterized by high prevalence, high mortality, high medical and health expenditures, and low awareness rates, with nearly 60% of new breast cancer patients occurring in developed countries. It is predicted that in the United States in 2013, the number of new breast cancer patients will be about 230,000, accounting for 29% of the total number of female malignant tumors, ranking first; and the death rate will be about 39,000 people, accounting for about 14% of the malignant tumors, ranking second only to lung cancer. Compared with Europe and the United States, China’s breast cancer patients show a trend of rejuvenation. Research shows that the median age of female breast cancer patients in China is 48 years old, which is about 10 years earlier than that in western countries. Therefore, improving the standardized diagnosis and treatment of breast cancer is an important issue for people’s health. At present, the detection methods of breast cancer mainly include clinical examination (palpation), imaging examination and laboratory examination. Clinical examination has high specificity but low sensitivity, and it is usually detected when the lesion reaches a certain volume or when the patient shows obvious clinical symptoms, and by then the tumor has already reached the progressive stage. Due to the lack of specific tumor markers, laboratory tests cannot be used as a diagnostic method for breast cancer, and only play a certain monitoring role in the follow-up process after tumor treatment. With the development of imaging equipment and technology, imaging examination has become an indispensable method to diagnose breast cancer. Imaging examination not only has higher accuracy, but also can detect earlier tumors significantly earlier than clinical examination. Currently, breast imaging methods recognized both at home and abroad include mammography, ultrasonography, MRI and nuclear medicine examination. As a gonadal organ, the mammary gland is regulated by the level of estrogen and progesterone in the body and undergoes changes. Changes in breast fibroglandular tissue occur not only during pregnancy and breastfeeding, but also slightly during the menstrual cycle. In addition, breast tissue undergoes constant changes throughout a woman’s life. As a result, the anatomical presentation of the normal mammary gland is extremely varied. Various imaging methods of the breast have different imaging principles, and each has its own advantages and disadvantages, so how to optimize the selection of imaging methods has become a matter of concern for clinicians. The selection should be based on the purpose of the imaging examination, combined with the anatomical and physiological characteristics of the mammary gland. I. Anatomy and physiological characteristics of the breast Mammary tissue is mainly composed of fibrous glands (parenchyma) and connective tissue (stroma). Breast tissue is fully developed only after the 1st full-term pregnancy, and fat infiltration of breast tissue appears after the 1st breastfeeding in most women. The breast parenchyma consists of 15-20 lobes or segments, each of which is connected to a duct. The terminal ductal lobular unit is the most basic tissue unit, and it consists of the extrafollicular terminal ducts, the intrafollicular terminal ducts, and the terminal alveolar ducts. The innermost cuboidal epithelial cells are arranged along the ducts. Myoepithelial cells form a discontinuous layer between the line-like arrangement of cuboidal cells and the basement membrane. The basement membrane separates the epithelial cells from the connective tissue. If the basement membrane is invaded by tumor cells, the tumor is an invasive carcinoma. Near the nipple, the cuboidal cells of the duct are replaced by squamous cells. The connective tissue within the mammary gland determines the type and density of the mammary gland and consists of interlobular and intralobular components. The interlobular connective tissue surrounds the terminal ductal lobular units, which show edematous changes under the influence of the hormones of the menstrual cycle. Since the edematous phase occurs during the secretory phase of the menstrual cycle (approximately 7 d before menstruation), in order to avoid interference with the detection and diagnosis of lesions due to this sex hormone-related physiologic hyperplasia, it is recommended that mammography be scheduled 7-14 d after menstruation, if the condition permits. The arterial blood supply to the mammary gland is derived from the branches of the subclavian arteries (internal mammary artery), and the axillary arterial branches (lateral thoracic artery and intercostal arteries). The lymphatic drainage pathway of the mammary gland runs from the deeper part of the gland to the local lymph nodes, with at least 75% of the drainage going to the axillary lymph nodes and the rest to the paraspinal lymph nodes (medial thoracic). Sentinel lymph node is the 1st lymph node that receives lymphatic drainage from the tumor area, and this lymph node is the 1st station of tumor lymphatic metastasis. Advantages and limitations of each breast imaging method 1. Mammography: based on the different relative absorption rate of different tissues to X-ray, it can distinguish them on the image. x-ray is sensitive to calcification in the breast, and it can detect calcified foci with a diameter of <2mm, which is considered to be an important sign of many early breast cancers. However, radiography is less sensitive in detecting isodense masses, especially in the denser breast. In addition, this examination has radiation, and high-dose X-ray irradiation is considered to be one of the high-risk factors for breast cancer. 2.Breast ultrasound examination: Breast ultrasound examination is painless, radiation-free, can be repeated for a short period of time, and is suitable for women of any age and any physiological period (including pregnancy and lactation). Ultrasound is more accurate in determining the nature of lumpy lesions (cystic or solid), and is helpful in detecting dense breast lumps that are difficult to visualize on X-ray. However, ultrasound is not clear for small calcified foci; it is difficult to diagnose tiny lumps in hyperplastic glands and special type of breast cancer; the image features of some benign and malignant diseases overlap; it is highly dependent on the operator's skills and difficult to analyze retrospectively. 3, Breast MRI: MR is widely used in the diagnosis of breast diseases in recent decades because of its high soft tissue resolution and no radiation. Dynamic enhancement MRI can not only show the morphological information of lesions, but also provide functional information such as lesion blood supply, water molecule diffusion, cell membrane choline metabolism, etc., which is a more sensitive means of breast imaging. However, due to the slightly lower diagnostic specificity of breast MRI, the U.S. National Comprehensive Cancer Network (NCCN) guideline has made more stringent provisions for breast MRI imaging, requiring the use of special breast coils, the need to have MR-guided puncture biopsy capability and professional breast MR diagnostic team and other conditions. Conditions. 4. Breast nuclear medicine examination: PET-CT makes use of the metabolic differences between normal tissues and tumor tissues to diagnose tumors, which has high diagnostic sensitivity and specificity, and can evaluate the metastasis of tumor lymph nodes and systemic metastasis (N-stage and M-stage), providing basis for the selection of treatment plan. However, specific imaging agents for breast cancer have not yet been applied in the clinic. Breast screening is a regular breast examination for healthy people, which is aimed at detecting early breast cancer. Through breast cancer screening, patients can get early diagnosis and treatment, which can effectively reduce the death rate of breast cancer by 20%-30%. 2013 NCCN guideline in the United States suggests that for non-high-risk healthy people, breast surgical checkups should be carried out every 1-3 years between the ages of 25-40 years old, and breast surgical checkups and mammography should be carried out annually after the age of 40 years old. In China, mammography and breast ultrasound are the main methods of breast screening due to differences in breast parenchyma (smaller and denser) and economic constraints. Moreover, the median age of breast cancer in China is about 48 years old, which is earlier than in Europe and the United States and other countries, so it is recommended to advance the screening age to 40 years old. For high-risk groups (previous history of breast cancer or precancerous lesions, family history of breast cancer, BRCA1/2 gene mutation, adolescents who have had chest radiation therapy, etc.), it is recommended that from the person. The purpose of diagnostic examination is to clarify the diagnosis. When it is difficult to achieve a clear diagnosis with a single imaging examination, it is often necessary to combine multiple imaging methods for comprehensive analysis. Commonly used examination methods include mammography, ultrasound and MRI. 1, mammography and ultrasound: 2 methods complement each other, joint application is the "golden combination" of clinical diagnosis of breast lesions. With the exception of ultrasound, which is preferred for breast lesions in young adults (under 30-35 years of age), pregnant and breastfeeding women, mammography and ultrasound are recommended for the evaluation of all diagnostic imaging tests, and the combination of the two tests can improve the sensitivity and negative predictive value of breast cancer diagnosis. Age and type of glandular density affect the sensitivity of diagnostic mammography in women and have little effect on the sensitivity of ultrasound. The results of the study showed that the sensitivity of mammographic lesion detection and diagnosis was higher in the >50-year-old group of screeners than in the ≤50-year-old group, and the sensitivity of the 2 tests for breast cancer detection was higher in the menopausal group than in the nonmenopausal group, with statistically significant differences in both cases. For patients with dense glands, the diagnostic sensitivity of ultrasound was higher than that of X-ray photography; for lesions mainly characterized by calcification, the diagnostic sensitivity of X-ray photography was higher than that of ultrasound; for lesions that were positive by clinical palpation, the diagnostic sensitivity of ultrasound was better than that of X-ray photography; for lesions that were negative by clinical palpation, the sensitivity difference between the two examination methods was not statistically significant. (1) Clinically suspected breast lesions: the following imaging protocols are recommended for those with abnormal clinical breast exams: breast ultrasound is preferred for those <35 years of age, and the need for mammography is determined according to the degree of suspicion of the lesion, and the treatment plan is determined according to the classification of the breast imaging report and data system assessment; for those ≥35 years of age, mammography combined with breast ultrasound is performed. (2) For those who have no abnormality in clinical breast examination: when mammography finds abnormalities such as lump, calcification, structural distortion, asymmetric densification, etc., it is necessary to firstly compare the results with previous mammography results to make clear whether it is a new abnormality or not; for the tiny calcified lesions, localized compression magnification can be added to further observe the morphology of the lesion; if it is manifested as a lump, then ultrasonography can be used to further observe it. 2, Breast MRI: Due to the high sensitivity but low specificity of breast MRI compared with conventional imaging methods, it is usually not used as an advantage for breast cancer diagnosis. Its maximum spatial resolution is 1.9-3.3mm, and it has a greater application prospect in the early diagnosis and screening of breast cancer. For patients with dense or complex breast tissues, as well as patients whose diagnosis cannot be confirmed by anatomical structure imaging techniques (e.g. mammography and ultrasound), it can provide greater help. Image-guided puncture biopsy: When the lesion is still difficult to be diagnosed clearly through comprehensive imaging, pathologic examination of the lesion is needed, and image-guided puncture biopsy is the final method to confirm the diagnosis. Puncture biopsy is usually recommended when the BI-RADS classification is 4. Because of the high false-negative rate of fine-needle aspiration cytology, hollow-needle aspiration histology is currently recommended. Image guidance includes ultrasound-guided, X-ray-guided, and MR-guided puncture or localization, and preference should be given to imaging methods that are easy to perform. Therefore, ultrasound-guided puncture or localization should be performed for lesions that can be detected by ultrasound, followed by X-ray-guided biopsy, and MR-guided puncture or localization is recommended only when both ultrasound and X-ray fail to show the lesion. Follow-up imaging examination Follow-up imaging examination refers to the imaging examination required for breast cancer patients confirmed by pathology, including preoperative clinical staging, determination of efficacy after treatment and evaluation of tumor residue after breast-conserving surgery. 1. Preoperative clinical staging: usually refers to T-staging of the tumor, which determines the size, location and infiltration range of the tumor, detects multicentric foci as well as possible malignant foci in the contralateral breast, so as to provide effective information for the establishment of the clinical treatment plan.MR has a high resolution of the soft tissue, and the sensitivity of detecting breast foci is much higher than that of the other imaging methods of the breast. Therefore, MRI is recommended for preoperative staging, especially for patients undergoing breast-conserving surgery.18F-Fluoro-Deoxyglucose (18F-FDG) PET-CT mammography is valuable for the diagnosis of poorly staged, locally advanced or metastatic lesions. Performing 18F-FDGPET-CT on the basis of standard staging helps to further characterize focal nodules and/or distant metastases of locally advanced breast cancer (staging). 2. Judgement of efficacy after treatment: With the continuous development of medical technology, the treatment of breast cancer has changed dramatically, and it has been developed from pure surgical treatment in the past to the situation of multidisciplinary comprehensive treatment. Comprehensive treatment of breast cancer mainly includes neoadjuvant chemotherapy, surgery, postoperative adjuvant chemotherapy, adjuvant radiotherapy, endocrine therapy and molecular targeted therapy. The evaluation of the efficacy of breast cancer treatment, especially the evaluation of the effect of neoadjuvant chemotherapy, will directly affect the determination of the next treatment plan for the tumor. At present, the more widely used evaluation standard is still the responseevaluation criteria insolidtumors (RECIST). That is, the criteria for evaluating the effectiveness of treatment in terms of changes in the tumor's pre- and post-chemotherapy trajectory. The same methods and techniques should be applied to the review and efficacy evaluation as those used for the baseline examination. Mammography and ultrasonography are both difficult to differentiate between tumor residue after chemotherapy and chemotherapy-induced fibrosis, while breast MRI has become the method of choice for evaluating the efficacy of breast cancer treatment because of its high soft tissue resolution and tomographic imaging. When evaluating the efficacy of treatment using imaging methods, baseline examination must be carried out before treatment.18F-FDGPET-CT can be used for post-treatment monitoring and follow-up, detection of local recurrence and distant metastasis of breast cancer (restaging), especially for the evaluation of the efficacy of treatment for patients with obvious bone metastatic lesions. Tumor residue after breast-conserving surgery: the wide application of breast-conserving treatment has obviously improved the survival quality of breast cancer patients. In recent years, the proportion of breast-conserving surgery in China has been increasing year by year, and tumor residue after surgery has become the biggest problem after breast-conserving surgery. One of the hallmarks of tumor remnants is positive margins in surgical specimens, and the use of imaging means to evaluate tumor remnants is gradually being recognized by clinicians. Because of the obvious postoperative hematoma and fibrous reaction, it is easy to hide the residual tumor tissue. Therefore, MR is a better imaging method for evaluating tumor remnants. Significant malignant signs on the image (such as irregular thickening of hematoma wall and nodular enhancement) suggest that there is residual tumor, and for signs that cannot be clearly diagnosed, patients are recommended to be rechecked after 6 months. As the death rate of breast cancer is increasing year by year in China, imaging examination is of great clinical significance for the detection and diagnosis of breast cancer. Mammography, ultrasound, MR and PET-CT have been widely used in clinical practice. The imaging principles of various imaging methods are different, and each of them has its own advantages and disadvantages in displaying the normal structure and lesions of the breast, and at present, there is no one imaging method that can replace the others. Therefore, various imaging methods are complementary, and the integration of multiple imaging methods has become an important guarantee for accurate diagnosis and precise treatment. Knowing the clinical indications of each examination and choosing the best imaging program can truly improve the detection rate and diagnostic sensitivity of the disease. This will not only benefit patients, but also has important clinical and socio-health economic significance.