Breast imaging Modalities
Breast imaging has developed mainly during the last three decades (Fig. 1). In the fifties and sixties breast imaging was not part of the routine diagnostic armamentarium of most x-ray departments possibly with
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Figure 1.
a) Radiographic equipment used by one of the pioneers of mammography, Dr Raul Leborgne. Montevideo, in the 1940s.
b) Modern mammography unit featuring high frequency generator, bi-metal anode, magnification capability, power assisted compression device, automatic exposure contro, and Bucky. The X-ray tube and examination table are mounted on an arm which can be rotated around a horizontal axis. This makes it easy to obtain images in different projections without moving the patient who can be seated or standing during the examination.
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b
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one exception - galactography (ductography). Today,
mammography is among the most frequent
x-ray examinations performed. This development has paralleled a continuous increase in the incidence of breast cancer, especially in the industrialized countries. Nationwide mammographic screening programs for breast cancer have been introduced in several countries.
Although mammography is the dominating breast imaging modality, ultrasound is rather extensively used as a complimentary modality. Magnetic resonance imaging has shown promising results, although its role remains to be defined. Transillumination of the breast (light scanning) has been claimed by some authors to be as accurate as mammography in the diagnosis of breast cancer. In a Swedish multi centre study, however, light scanning turned out to be far less sensitive and specific than mammography, especially in the diagnosis of non-palpable carcinomas.
Thermography of the breast was used in many centres in the sixties and seventies, but the results were disappointing, and its use has largely been abandoned.
Soft tissue radiography like mammography implies imaging of tissues with relatively small differences in absorption characteristics, mainly fat and fibroglandular tissue. To achieve a high subject contrast, appropriate radiation quality is critical. Factors influencing radiation quality are tube target material, added filtration and kV setting.
In the late sixties the molybdenum tube was introduced by Charles Gros together with a dedicated mammography unit. Molybdenum target tubes with beryllium windows and molybdenum filters operated at kV settings of about 25 to 28 produce a relatively monochromatic radiation providing a high subject contrast. Tungsten tubes with special filtration as well as bi-metal tubes made of molybdenum and rhodium are also currently being used by some manufacturers of mammography units.
A major step forward in the technology of soft tissue radiology was taken in the seventies by introducing intensifying screens, usually made of rare earth phosphors, such as gadolinium or lanthanum. Combining such a screen with high contrast film of relatively high speed the radiation dose could be reduced by a factor of 50 compared with direct exposure films.
 | Figure 2.Characteristic curves illustrating films with high and low contrast. |
Film contrast is defined as the slope of the characteristic curve (Fig. 2); the steeper the curve, the higher the contrast. In addition to the inherent properties of the film, the processing conditions are critical. Ideally, a processor dedicated to
mammography should be used, although this is not always possible. The processing should be monitored with daily sensitometry (see below). Most
mammography units today come with a reciprocating grid which reduces scattered radiation and thus improves contrast. The trade-off is a doubling of the radiation dose which is considered acceptable.
Another technique to improve image quality is magnification. By using a small focal spot (about 0,1 mm focal spot size) and an air gap several effects are obtained. The scatter is reduced by the air gap, the effective noise in the recording system is reduced, and the magnified image is easier to view. A magnification of 1,7 to 2,0 is usually used. Again, the trade-off is a higher dose.
Ultrasound examination of the breast helps to clarify problematic lesions that have been detected on physical examination or mammography. For this type of focused examination a hand-held 7,5 MHz transducer is generally used. The operator should have a thorough knowledge of US as well as mammography and breast pathology. Ultrasound is of value in the following situations:
 a | Figure 3.
35-year old woman with a 2 cm palpable mass at the 12 o'clock position in the right breast.
a) On mammography very dense breast parenchyma was demonstrated without clearly definable tumor corresponding to the physical
finding.
b) On ultrasound examination a 1,9 by 1,4 cm hypoechoic lesion with irregular border is seen. |
 b |
- In the evaluation of a well circumscribed nodule on the mammogram. The differentiation between a
cystic and a solid
lesion is usually easy and accurate. However, some cysts may be moderately echogenic and therefore difficult to identify as cysts. Some circumscribed carcinomas may be of very low
echogenicity, and sometimes even show some echo enhancement and thus simulate cysts. When a
lesion has been demonstrated to be solid, the differentiation between
benign and
malignant is often impossible.
- US is also valuable in the evaluation of any discrepancy between physical findings and mammography. Thus, in the presence of a physical finding with a negative mammogram, especially in the dense breast, US can often demonstrate the presence or absence of a focal lesion (Fig. 3). Similarly, in the presence of an unclear density on the mammogram US can often confirm or dispel a suspicion of a focal lesion.
- US is useful when analysing lesions which cannot be demonstrated by mammography for anatomic reasons. One example is a juxtathoracic tumor, another is lymph nodes high in the axilla.
- US guided fine-needle aspiration biopsy (FNAB) is often a quick and accurate procedure. One advantage over x-ray guidance is that the puncture can be performed under direct visual control. Either a centrally perforated transducer or a freehand technique may be used. The procedure is facilitated if an assistant operates the syringe which is connected to the needle by a flexible tube. The examiner thus operates the transducer with one hand and the needle with the other. US guide d FNAB has been shown to be as accurate as x-ray guided FNAB.
- US can also be used to perform preoperative wire localization of nonpalpable breast lesions.
- US is superior to mammography in the evaluation of silicone breast implants, especially regarding rupture and leakage, and also in the guidance of FNAB of palpable and non-palpable lesions adjacent to an implant. Breast Doppler US has shown some promising results but should still be considered experimental.
There are several limitations to the US examination of the breast. It is quite operator dependent. The sonographic contrast between tumors and fatty tissue is poor. Furthermore, lesions presenting as small calcifications without a mass cannot be demonstrated. Thus, e.g. pure intraductal carcinoma without focal tumor can usually not be seen.
Magnetic resonance imaging (MRI)
MRI of the breast has been demonstrated to be of value in certain situations, provided the correct technique and dedicated surface coils are used. Several investigations have shown that dynamic studies of the enhancement pattern after intravenous administration of contrast medium provide important differential diagnostic information. Malignant tumors almost always enhance faster and more intensely than benign tumors (Fig. 4). Cellular fibroadenomas, some types of proliferative mastopathy and inflammatory lesions as well as postoperative scar tissue during the first six months after operation may show an enhancement pattern similar to carcinoma. On the other hand, MR has been shown to be of value in the postoperative breast after about six months, especially in patients with silicone implants which are often difficult to evaluate mammographically
 a | Figure 4.48-year old woman who had felt a mass in her left breast.
a) MR examination with a T1 weighted 3 D gradient echo sequence (FLASH, Siemens) shows a low signal area centrally in the breast.
b) After injection of intra
venous contrast medium (Gadolinium-DTDA, 0,1 mmol/kg body weight) a
substantial signal enhancement is seen. On histopathologic examination an invasive carcinoma was found (Courtesy of Dr Beata Boné, Huddinge Hospital, Stockholm). |
 b |
MR of the breast is limited by its complexity and cost. This may change in the future with small dedicated breast MR machines.
Experience is accumulating regarding the use of storage phosphor based digital mammography. Although the spatial resolution is somewhat inferior to conventional screen film mammography this is compensated for by a higher contrast. Important factors are image algorithms and postprocessing of the images. Although there are still limitations in the characterization of small calcifications, digital techniques will be important in the future. Actually, there are already commercially available equipments for stereotactic biopsy, using digital technology (CCD camera) providing near real time image display.
Ingvar Andersson and Baldur F. Sigfússon