**2.1 Mammography**

Mammography is a gold standard imaging method for breast cancer screening, detection and diagnosis in women under forty with a relatively high sensitivity in the range of 85-90% (Berg et al., 2004). This method is a national screening program in the United Kingdom (Glasspool & Evans., 2000). Although mammography is an effective imaging tool, it is not without limitations like many other diagnostic modalities. First, the sensitivity of mammography decreases dramatically in young patients due to their dense breast tissues. On the other hand, its ability to detect malignant lesions in young female patients decreases to 68% (Nystrom et al., 2002, Kopans, 1992). As a result, some patients with breast cancer are missed and some others without a malignant tumor undergo unnecessary biopsies due to incorrect mammography findings. Secondly, mammography in patients evaluated following breast surgery or radiotherapy, is unreliable with a false negative rate of 25-45%, because it cannot always differentiate benign from malignant diseases (Fass, 2008). Many efforts have ever been taken to establish new tests to enable us to collect more complete information by non-invasive methods. These efforts minimize the use of breast biopsy in women who do not have breast cancer. Therefore, many other imaging modalities such as ultrasonography (US), magnetic resonance imaging (MRI), diffuse optical tomography (DOT), computed tomography (CT) and measurement of tumor markers in blood serum have been initiated to increase the diagnostic accuracy of mammography.

#### **2.2 Ultrasonography**

Ultrasonography is an important adjunct to mammography for both diagnosis and characterization of breast cancer and is routinely used in this role (Fass, 2009). It is noninvasive, easily available, relatively cheap and also recommended for pregnant or lactating women when ionizing radiation may not be recommended (Schueller et al., 2008). It has been found that ultrasonography, when combined with mammography, can prevent up to 22% of unnecessary biopsies (Zonderland, 2000). Although ultrasonography can reliably differentiate a cystic lesion from a solid lesion, it does not provide a high specificity to distinguish benign from malignant lesions (Balleyguier et al., 2008; Sauer et al., 2005). Another disadvantage of this method as a screening tool, when applied to general population, is low sensitivity and specificity and furthermore it is operator-dependent (Liberman, 2000).

#### **2.3 Magnetic Resonance Imaging (MRI)**

Breast cancer was one of the first that was examined using MRI (Ross et al., 1982). This method is very useful for further evaluation when mammography and ultrasonography are indeterminate to study the presence or location of a suspect abnormality (Glasspool et al., 2000). However, due to cost reasons, low access and high false positives, MRI is not yet considered as a screening exam for breast cancer except for special cases. MRI of breast cancer is recommended in the repeated screening of patients who have the increased risk of radiation induced DNA mutations (Fass, 2008). MRI is used to screen women with a family history of breast cancer, women with very dense breast tissues or women with silicone implants that can obscure pathology in mammography (Fass, 2008).
