**1. Introduction**

Breast cancer is one of the most common cancers in Saudi Arabia [1] and, thus, is an important health problem [2]. In the Western world, it is the second most frequent cause of cancer death in women (after lung cancer) [3]. Statistics show that a large number of women in Europe, North America, Australia and many Latin-American countries suffer from this life-threatening disease [4]. Worldwide, in the year 2005, the number of new cases exceeded 1.2 million [3]. Breast cancer is rare in women below the age of 20 years and less common below the age of 30 years, but it is more aggressive and thus has a lower survival rate. The incidence rate, however, rises dramatically over the age of 50 years. This could be due to several risk factors such as family history, genetics, early menstruation, late menopause and other factors that have not yet been identified. Breast cancer can also occur in males and often fatal, but it is extremely rare. The above problems have prompted global governments to put constant efforts to increase patient's recovery level against this disease. Early and accurate detection with mass screening programmes helps improve a woman's chances for successful treatment. It also minimizes pain, suffering and anxiety that surround patients and their families.

The current and the most cost-effective technique used for screening and diagnosis of breast cancer is X-ray mammography. It is the state of the art for earlier detection to improve both prognosis and survival rate [5]. This may be due to its good availability, high sensitivity and relatively low cost/patient. Despite the above efforts, the mortality rate of breast cancer still remains high and in the UK, for example, accounts for ~17% of all female deaths [6, 7]. This is due to some limitations of the current mammographic procedures. As a result, a large number of cases with positive mammography results undergo invasive surgical breast biopsies. Breast biopsy is still widely used and thus is the only fail-safe method to determine whether a lesion is malignant. Of all biopsy cases, only about 25% prove to be malignant. Moreover, a majority of the diagnosed women below the age of 50 have a dense breast tissue. This is a problem as it obscures lesions and results in false-negative mammography.

In addition, the size, shape and appearance of the female breast are not constant but undergo a number of changes during the lifetime of women. For instance, changes occur during the menstrual cycle and more pre-/postmenopause. In addition, the age of the subject not only influences the shape but also parenchymal density of the breast. Thus younger women tend to have denser breasts (more fibro-glandular tissue), whilst postmenopausal women have breasts containing a larger adipose component. This makes the X-ray mammogram far more effective in older women as the fat content is more radio-translucent (appears darker) than glandular tissue (appears underexposed) in younger women [8].

The above discussion suggests that both the shape and parenchymal density of the breast impose particular constraints on the choice of imaging modality. The imaging technique should be powerful for initial detection and subsequent follow-up of the diseases. At present, no single technique can be used for all cases of breast cancer detection without showing certain clinical or technical limitations. This implies necessity to address the specific needs that can help for breast tumour imaging to overcome these limitations. For instance, breast compression is often needed as it holds the breast still and enhances the spatial resolution. It also evens out the breast thickness and reduces scatter in X-ray or gamma-ray imaging in case of scintimammography (SM) [9], thus increasing image sharpness. Moreover, it spreads out the tissue so that small abnormalities will not be obscured by the overlying breast tissue. Since the breast is an external organ and extends to the chest wall, it requires appropriate views to be taken. For instance, in X-ray mammography a lateral (from the side) view of the breast allows separation of the chest wall from lesions deep within the breast.

Furthermore, mammography involves the radiological examination of the breast using equipment specifically designed for, and dedicated to, imaging breast tissue. This equipment is primarily used for the detection of breast cancer at an early stage. It is widely used in screening programme involving healthy populations of women. Early detection of breast cancer in a healthy population places particular demands on radiological equipment as high-quality images are required at a low dose. Symptomatic patients may also benefit from the development of mammography equipment that produces high-quality images for breast screening. Perhaps because of the exacting demands of mammography, acceptability criteria and suspension levels are well developed [10, 11]. It has been an accepted practice that mammography should be performed on X-ray equipment designed and dedicated specifically for imaging breast tissue, due to the clinical imaging requirements for high-quality image. In practice, either film/screen or digital detectors may be used. Both qualitative and quantitative acceptability criteria have been published for X-ray mammography by considering the image quality needed clinically in screening programmed.
