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In the adult female, each of the mammary glands or breasts is a conic or hemispheric eminence located on the anterior and lateral chest walls. Breast size varies from one individual to another and even in the same woman, depending on her age and the influence of various hormones. However, the usual breast extends from the anterior portion of the second rib down to the sixth or seventh rib and from the lateral border of the sternum well into the axilla.
Before the examination begins, the mammographer will explain the procedure and ask the patient to put on a gown, preferably one designed for mammography, which allows exposure of only the breast that is being examined. The patient will be instructed to remove any jewellery, talcum powder, or antiperspirant that may cause artifacts on the radiographic image.
The mammographer will document relevant patient history as per departmental protocol. Generally this patient history will include the following:
Number of pregnancies
Family history of cancer including breast cancer
Medication such as hormone therapy that currently taking
Description of problem, such as screening mammogram, lumps, pain, and discharge
The mammographer should also note location of scars, palpable masses, moles, warts, tattoos, etc.
In mammography, the great variability of the breast, with respect to the proportion of fatty tissue to fibro-glandular tissue, presents certain technical difficulties. In producing a superior quality mammogram, the shape and countour of the normal breast poses additional problems to the mammographer.
The base of the breast is that portion near the chest wall, whereas the area near the nipple is termed the apex. In either the craniocaudad or the mediolateral projection, the base of the breast is much ticker and contains much denser tissues than at the apex. To overcome this anatomic difference, compression is used in combination with a specially designed tube so that the more intense central ray (CR) of the x-ray beam penetrates the ticker base of the breast.
The most distinctive aspect of the mammography machine is that is the unique design of the x-ray tube, which has a molybdenum target with small focal spots of 0.3 and 0.1 mm. Rhodium has recently been introduced as an optimal anode material. The focal spots must be this size because of the size of the cancer calcifications, which are typically less than 1.0 mm in size.
The anode configuration produces a prominent heel effect resulting from the short source-to-image receptor distance (SID) and the use of a narrow reference target angle. Because the x-ray tube is aligned with the cathode placed over the base of the breast (at the chest wall) and the anode outward toward the apex (nipple area), the heel effect fortunately can be used to maximum advantage. Because the cathode side of the x-ray beam has significantly greater intensity of x-rays compared with the anode side, a more uniform-density breast image can be produced, because the more intense x-rays are at the base, where tissue thickness is greater. Most mammograms utilize grids, automatic exposure control (AEC), and the important breast compression device.
The AEC chambers on most mammography systems are adjustable in up to 10 positions from the chest wall to the nipple region. To ensure adequate exposure of the more dense/thick tissues, generally the chamber under the chest wall or to the more dense tissue area should be selected. Exceptions to this include special projection, such as magnification and spot compression views.
All mammography machines contain a compression device that is used to compress the breast. Improvements in breast compression technology in recent years have greatly improved the visibility of detail in breast images. The compression device is made of a plastic that allows transmission of the low energy x-rays. The device should have a straight chest wall edge to allow the compression to grasp the breast tissues close to the chest wall. Compression is controlled by the technologist and is typically applied at 25 to 45 pounds of force.
In addition to the standard compression device, a smaller "spot" device may be used to compress localized areas. The compression device should be checked regularly to ensure that it is working properly and applying the correct amount of pressure. Appropriately applied compression is one of the critical components in the production of a high quality mammogram. Six benefits of using compression are to:
Decrease the thickness of the breast
Bring the breast structure as close to the image receptor as possible
Decrease dose and scattered radiation
Decrease motion and geometric unsharpness
Separate breast structure
These six benefits identify how image quality or resolution is improved by reducing scatter and also by reducing magnification of breast structures. Therefore the overall breast thickness has also been greatly reduced, which reduces the ratio of scatter to primary radiation by one half.
The magnification method is used to enlarge specific areas of interest such as small lesions or microcalcifications. This requires an x-ray tube with a 0.1 mm focal spot to maintain image resolution. Magnification of 1 Â½ to 2 times can be obtained by inserting a magnification platform between the image receptor and the breast, thereby magnifying the part resulting from increased OID. This magnification technique can be used with most mammogram projections.
Patient dose is significant in mammography, as seen by the dose icon boxes included on each positioning page. A skin dose of 800 to 900 mrad and a mean glandular dose (MGD) of 130 to 150 mrad is common for a 4 cm thickness mammogram, which is much higher than for most other body parts. For example, a much thicker 30 cm lateral lumbar spine at 90 kV, 50 mAs, has a skin dose of 1000 to 1300 and a midline dose of 130 to 180 mrad. The reason for the relatively high dose for mammograms is the very low kV (25 to 28) and the high mAs (75 to 85) required.
The principal way patient dose is controlled in mammography is by careful and accurate positioning, which minimizes the need for repeats. The ACR recommends a repeat rate of less than 5% for mammography. The only shielding possible is a waist apron fir shielding the gonadal region. Mean glandular dose is the average breast tissue dose, rather than a specific midline dose as for the lumbar spine and other body parts.
Film-screen mammography continues to be standard in current breast radiography. The greatest benefit of the film-screen system is an excellent image with a low radiation dose, allowing women to hve this examination regularly. The ability to see fine detail, edge sharpness, and soft tissues is a hallmark of a good film-screen mammogram. However, digital mammography (computed radiography or digital radiography) is developing rapidly and has certain distinct advantages over film-screen mammography.
Computed Radiography Mammography
Computed radiography can be used for mammography similar to the way it is used in general radiography with its imaging plate (IP) and image processor. CR cassettes containing imaging plates can be used in existing mammographic systems. CR mammography has certain advantages over film-screen systems, such as operating costs is one advantage of CR over film-screen systems is that the imaging plates can be exposed many times before they need to be replaced. Therefore, considering the cost of film and associated expenses, the use of CR becomes more economical. In addition, the need for chemical processing is eliminated, which is more ecologically sound. Teleradiology options a second important advantage of CR mammography is its ability to retrieve and transfer the images to remote locations for interpretation or consultation. This is referred to as teleradiology, more specifically for transmitting mammogram images electronically, the term telemammography is sometimes used.
Archiving and PACS options, after the images have been interpreted they can be stored electronically at any desired location through the PACS. This a third advantage of digital imaging compared with film-screen systems, because the need for physical storage space for hardcopy films is eliminated as mammogram images are incorporated into existing PACS.
Direct Digital Radiography
Direct digital radiography is a second form of digital imaging that continues to be refined and developed but is not yet in common use. These mammographic systems contain a flat detector that is permanently mounted on the x-ray unit. Comparison studies has shown the newer DR mammographic systems have improved contrast resolution with reductions in patient dose compared with film-screen imaging. There is no imaging plate required with CR. The flat detector captures the remnant x-rays and produces a digital image. The digital imageis then projected on a monitor at the mammographer's workstation for direct viewing and post-processing as needed.
Computer-Aided Detection Systems
Today, the computer is being used as the "second opinion" in mammographic interpretation. Computer-aided detection (CAD) is a technology that has the potential to impact dramatically the diagnosis of breast cancer. CAD systems use a computerized detection algorithm to analyze digital images for various suspicious lesions and provide the radiologist with an estimate of the probability of malignancy. Certain studies have shown that a using a second reader to interpret screening improves the cancer detection rate by as much 10%.
Using computers is advantageous because they do not get fatigue or distracted, nor do they demonstrate intraobserver variation. Clusters of microcalcifications are a good example of objects appropriate for CAD viewing because they differ from normal anatomic structures in density, shape, and size. Detection and classification of micracalcifications and borders of lesions are possible with CAD. Some studies show improvement in microcalcification detection rates. The use of CAD systems is increasing, but they are not expected to replace the radiologist.
ALTERNATIVE MODALITIES AND PROCEDURES
Sonography has been used to image the breast since the mid-1970s. It provides valuable adjunct information for the radiologist, along with the film-screen mammogram and physical examination. Today sonography is an integral part of the mammography department and the mammogram examination. Its major value is its ability to distinguish between a cyst and a solid lesion. It is also used extensively to determine fluid, abscess, hematoma, and silicone gel. Ultrasound has the ability to find cancers in women with dense breasts. Mammographers may also be trained to perform sonography of the breast in addition to film-screen mammography. Image quality is heavily dependent on sonographer expertise. Conventional scanner and hand-held transducer when a high-resolution conventional scanner is used, the patient is positioned supine or rolled slightly onto one side. The hand held transducer is placed on a palpable mass or an area noted on a mammogram.
Mammoscintigraphy may be helpful in confirming breast cancer diagnosis. Technetium-99m-sestamibi is injected as a tracer into the arm opposite the affected breast, breast images are obtained 10 minutes later. This procedure has fallen slightly out of favour because of the high number of false positive result. Sentinal node studies are useful for patients with melanoma, and they are also becoming increasingly useful for breast cancer. This procedure involves injecting sulphur colloid around the lesion subcutaneously. The flow is then observed through the lymph vessels to assess which nodes are affected by the cancer.
Positron Emission Tomography
Positron emission tomography is being used to detect early signs of cancerous growth within the breast. Using the tracer fluorodeoxyglucose (FDG), early cancerous cells san be detected by their increased metabolism. This increase metabolism uses sugar and the FDG tracer molecules at a greater rate as compared with normal breast tissue, making the cancer's location visible with PET. PET also is used following surgery or treatment for breast cancer to determine whether recurrent disease is present in the breast or other parts of the body.
Two disadvantages of using PET for breast imaging are higher cost and radiation exposure. Therefore even though PET has certain valuable applications for early detection of breast disease, the cost of the equipment required and use of short half-life radioactive tracers make the use of PET impractical as a screening tool. Radiation exposure from FDG tracer is approximately 6 times greater than that from a technetium-99m-sestamibi study as used in nuclear medicine.
Magnetic Resonance Imaging
One advantage of MRI is that it can show the whole breast maximally with greater comfort for the patient. Also, recent work with contrast indicates the MRI can show evidence of vascularisation of lesions. Furthermore, it provides better sensitivity and specificity than ultrasound and x-ray mammography. Additional advantages of MRI involve its superior ability to visualize pathology on patients with dense breast tissue and with breast implants. Dense breast tissue as an adjunct to mammographic studies, MRI has shown to be useful for classifying suspicious lesions and microcalcifications that have been identified on mammograms. MRI is especially helpful in evaluating small breasts and very dense breast tissue.
Nowadays, breast implants is more than 1 million women in the U.S and Canada have undergone breast augmentation (surgical implants). Silicone and saline implants are radiopaque, requiring implant displaced (ID) views (Eklund method). Compression is more difficult with implants, and extra care must be taken by the mammographer not to rupture the implant. Automatic exposure control (AEC) also cannot be used with augmented breast, all of which makes imaging of breast tissue with implants a challenge using conventional screening mammograms or ultrasound techniques.MRI has been clinically proven to be most effective in diagnosis problems related to breast implant imaging. For example, with MRI it is possible to evaluate potential intracapsular and extracapsular rupture, including the area posterior to the implant, which is very problematic with either mammography or sonography studies.
In addition to diagnosing implant rupture, it is also important to demonstrate the breast tissue surrounding and the posterior to the implants for the possible malignant growth. Physical examination is more difficult with implants, which also increases the risk for cancer growth without detection. MRI, unlike mammography or sonography, is not hindered by the presence of an implant. Clinical testing is being done with a new kind of radiolucent implant that will allow more effective use of film-screen mammography, including the use of automatic exposure controls. However, the more than 1 million women with radiopaque implants, many of whom are nearing the life expectancy limits of the implants, will require more and more evaluations of the implants for possible rupture or other related problems. This in turn increases the potential role of MRI in breast implant imaging.
Two primary disadvantages of MRI are its high false-positive rate and the high cost, both of which limit its use as a breast screening procedure. However, research and clinical use continue as MRI evolves into playing a larger role in the diagnostic workup for breast lesions.
Screening mammography is important for the early detection of pathologic changes in the breast. These changes can be either benign (noncancerous), or malignant (cancerous). The most common pathologic indications for mammography include the following:
Breast Carcinoma (cancer): Carcinoma of the breast is divided inti two categories, noninvasive and invasive. Noninvasive carcinoma is a distinct lesion of the breast that has potential to become invasive cancer. These lesions are restricted to the glandular lumen and do not hve access to the lymphatic system or blood vessels. Noninvasive cancer may also be termed in situ. Ductal carcinoma in situ (DCIS) is isolated within the breast duct and has not spread to other areas of the breast. Lobular carcinoma in situ (LCIS) is abnormal cells that have been detected in one or more of the breast lobes. Noninvasive cancers (DCIS and LCIS) comprise approximately 15% to 20% of all breast cancer diagnoses.
The most common form of breast cancer is invasive or infiltrating ductal carcinoma. This type comprises approximately 80% of all breast cancer diagnoses. Invasive cancer of the breast carries the worst overall prognosis of the invasive cancers.
Cysts: Cysts are fluid-filled sacs that are benign and appear as well-circumscribed masses. Their density is usually that of the surrounding tissue; however they may also appear denser. To positively diagnose s cyst, ultrasonography and needle biopsies are required.
Fibroedema: Fibroedemas are the most common benign, solid lumps or tumours composed of fibrous and glandular tissue. They are well-circumscribed lesions with clearly defined edges that can be felt during palpation. They typically have the same density as the surrounding tissue. The mass is an overgrowth of the fibrous tissue of the breast lobule.
Gynecomastia: Gynecomastia as from Greek term meaning "woman-like breasts". This is a benign condition of the male breast in which there is a benign glandular enlargement of the breast. Gynecomastia may be unilateral or bilateral but seems to be more pronounced in one breast. It typically presents as a palpable mass by the nipple.
Paget's disease of the nipple: This condition first appears as a crust or scaly nipple sore or as a discharge from the nipple. Slightly more than half of the persons having this cancer also have a lump in the breast. Paget's disease may be invasive or noninvasive.