In an external magnetic field, the magnetic moments of spin of hydrogen nuclei align themselves and create macroscopic magnetization in the direction of the field. This magnetization is rotated by 90 degrees into the transverse plane when appropriate electromagnetic radiation is applied. The magnetization then precesses and shrinks in the transverse plane with a time constant T2, because the magnetic moments of spin spread out with time. This process is known as transverse relaxation. After transverse relaxation, the magnetization builds up in the direction of the external magnetic field with a time constant T1, because the magnetic moments of spin begin to realign in the direction of the external magnetic field. This second process is referred to as longitudinal relaxation. In fact, the magnetization decays much faster in the transverse plane due to inhomogeneities of the external magnetic field than it would on the basis of transverse relaxation alone. When another appropriate form of electromagnetic radiation is delivered, fanned-out magnetic moments of spin will refocus and reestablish the magnetization. This technique, which can cancel the influence of inhomogeneities, is called the spin-echo sequence. This magnetization precesses around the axis of the external magnetic field and induces an echo signal in a coil. TE is the time between a 90-degree rotation of the magnetization and echo-signal production, and TR is the time between consecutive 90-degree rotations of the magnetization. T2-weighted images are generated using a long TE and a long TR, and T1-weighted images using a short TE and a short TR.

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The potential role of magnetic resonance (MR) imaging in the diagnosis of breast disorders has been investigated since the late 1970s. However, conventional MR imaging has not been reliable for diagnosis of breast masses, and so contrast MRI has been performed. In particular, the gradient echo technique combined with administration Gd-DTPA (i. e. dynamic MRI) improves lesion detection and tissue characterization of breast masses. In recent studies, breast MR imaging has shown up to : 100% sensitivity in the detection of breast cancer. On the other hand, its specificity varies between 37% and 97% because of the fact that many benign lesions are also enhanced with Gd-DTPA, similar to breast cancer. Recently, the use of MR imaging has been felt to be of value in prebiopsy examination to reduce the number of surgical biopsies for false-positive mammography. To improve the specificity of MR imaging, MR imaging-guided localization and biopsy for breast lesions will be necessary.

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In our hospital, magnetic resonance imaging of the breast (MR-mammography, MRM, 0.5T) was performed using T1-weighted gradient echo with fat suppression and magnetization transfer contrast before and after infusion of Gd-DTPA using key hole imaging which substituted localized dynamic study. In our experience, MRM is useful for diagnosis of difficult cases and surgical planning for breast preservation because of its high tumor-gland contrast and clear three-dimensional demonstration. MRM is also useful for determining the response to preoperative chemotherapy. However, tumor visibility depends mainly on the nature of the background breast glands.

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In order to examine the rate of correct diagnosis by mammography at initial mass screening for breast cancer, we carried out a retrospective study of mammography findings in 267 cases of breast cancer detected at Asahikawa Cancer Screening Center.
The screening was performed by physical examination, and in cases where disease was suspected, mammography, ultrasonography, and needle biopsy were done. Mammographically, 172 cases (64.4%) were cancer-positive, 58 cases (21.7%) were suspicious for cancer, and 37 cases (13.9%) were cancer-negative. Patients below 50 years of age and those with tumors of small diameter (<20 mm) showed a significantly lower rate of cancer positivity than patients aged 50 years or more and those with tumors 20 mm or more in diameter. Mammographic abnormalities were not specific, since these changes were also found in normal subjects and patients with benign diseases.
Therefore, we concluded that mammography without physical examination at initial mass screening has a high risk of missing breast cancer. Mass screening for breast cancer should be performed by physical examination involving inspection and palpation at the first instance. If any suspicious findings are obtained, mammography, ultrasonography, and needle biopsy should be done.

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We performed a study of 448 women with breast cancer who visited the breast clinic of Nagano Cancer Center, and underwent mammography (MMG) and ultrasonography (US) at the last examination, between October 1983 and December 1993. These women subsequently underwent surgery at several hospitals after referral from our center. Definitive diagnoses of breast cancer were made histopathologically.
Both MMG and US findings were graded using five-step criteria (indicating the level of malignancy) from I (normal) to V (definite malignancy) The diagnostic efficiencies of the two examinations were then compared clinicopathologically. Furthermore, on the basis of the results obtained, we considered supplemental diagnostic methods that could be introduced for mass screening of breast cancer.
1) There was a significant difference between the incidence of cases diagnosed as V·IV· III b (malignancy) by US (71.9%) and those diagnosed by MMG (60.9%).
2) Among cases suspected to be breast cancer by palpation, the proportion diagnosed as V · IV·III b (malignancy) by US (83.3%) was significantly higher than that diagnosed by MMG (73.7%). Among cases suspected to be mastopathy by palpation, the proportion diagnosed as malignant by US was higher than that diagnosed by MMG.
3) The false negativity rate for palpation combined with MMG was higher than that for palpation with US.
4) Among cases without palpable masses, the proportion diagnosed as malignant by MMG was higher than that diagnosed by US. However, for palpable tumors less than 5.0 cm in diameter, the proportion of cases diagnosed as malignant by US was higher than that diagnosed by MMG.
5) In terms of the gross appearance of the cut surface, grade of cancer invasion, histological type and grade of chubbiness, the diagnostic efficiency of US was shown to be slightly better than that of MMG.
The above results suggest that US should be considered as a supplemental method for introduction to mass screening of breast cancer.

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