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Mammography is the radiography examination of the breast tissue (soft tissue radiography). To visualize normal structures and pathology within the breast, it is essential that sharpness, contrast and resolution are maximized. The optimizes, in the image, the relatively small differences in the absorption characteristics of the structures comprising the breast. A low kVp value, typically 28 kVp, is used. Radiation dose must be minimized due to the radio-sensitivity of breast tissue.
Mammography is carried out on both symptomatic women with known history or suspected abnormality of the breast and as screening procedure in well, asymptomatic woman. Consistency of radiography technique and image quality is essential, particularly in screening mammography, where comparison with formal films is often essential. Whilst other technique such as magnetic resonance imaging (MRI) and ultrasound have a role in breast imaging, mammography is undertaken to image the breast most commonly is hence in this chapter.
ANATOMY OF THE BREAST .
In the adult female, each of the mammary glands or breast 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 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.
The surface anatomy includes the nipple, a small projection containing a collection of duct openings from the secretory glands within the breast tissue. The pigmented area surrounding the nipple is termed the areola, a circular area of different colour surrounding a central point. The junction of the interior part of the breast with the anterior chest wall is called inframammary fold (IMF). The axillary tail is a band of tissue that wraps around the pectoral muscle laterally
The width of the breast, called the mediolateral diameter, on most patients is greater than the vertical measurement, from top to bottom. The vertical measurement, which may be described as the craniocaudad diameter, averages from 12 to 15 centimeters at the chest wall. The mammographer must realize that more breast tissue exits than the obvious tissue that extends from the chest. Mammary tissue is overlying the costocartilages near the sternum, and breast tissue is extending well up into the axilla. This breast tissue extending into the axilla called the tail of the breast of the axillary prolongation of the breast.
Craniocaudad diameter (12 to 15cm)
Tail of breast
METHODS OF LOCALIZATION.
Two methods are commonly used to subdivide the breast into smaller areas for localization proposes. The quadrant system, is easiest to use. Four quadrant can be described by using the nipple as the center. These quadrant are the UOQ (upper outer quadrant), the UIQ (upper inner quadrant) LOQ (lower outer quadrant), and the LIQ (lower inner quadrant.
A second method, called the clock system, compares the surface of the breast to the face of clock. A problem with the clock method arises when a medial or lateral portion of either breast is described. What is described at 3 o'clock in the right breast has to describe at 9 o'clock in the left breast.
If either the referring physician or the patient has to felt a mass of any suspicious area in either breast, one of these methods is used to describe the area of special interest to radiology personnel.
FRONTAL VIEW ANATOMY
The glandular tissue of the breast is divided into 15 or 20 lobes arranged like the spokes of a wheel surrounding the nipple.
The glandular lobes, made up of a number of individual lobules, are not clearly separated but are grouped in a radial arrangement. Distally smallest lobules consist of clusters of rounded alveoli. Upon glandular stimulation, peripheral cells of the alveoli from oil globules in their interior, which when ejected into the lumen of the alveoli constitute milk globules. The clusters of alveoli that make up the lobules are interconnected and drain by individual ducts. Each duct enlarges into a small ampulla that serves as a reservoir for milk just before terminating in a tiny opening on the surface of the nipple.
The various subdivisions of these ducts and associated ampullae are activated during pregnancy to prepare for lactation and, following birth to produce milk for the newborn.
Breast carcinoma (cancer) - carcinoma of the breast is divided into two categories, non-invasive and invasive. Non-invasive carcinoma is a distinct lesion of the breast that has the potential to become invasive cancer. These lesions are restricted to the glandular lumen and do not have access to the lymphatic system or blood vessels. Non-invasive cancer may also be termed in situ. Ductal carcinoma in situ (DCSI) is located within the breast duct and has not spread to other areas of the breast. Lobular carcinoma in situ (LCIS) as abnormal cells that have been detected in one or more of the breast lobes. Non-invasive cancers (DCIS and LCIS) comprise approximately 15% to 20% of all breast cancer diagnoses. The common form of breast cancer is invasive or infiltration ductal carcinoma. This type comprises approximately 80% of all breast cancer diagnoses. Invasive cancer is believed to arise in the terminal duct lobular unit. The majority of these cancers cannot be specified without histological evaluation. Invasive cancer of the cancer of the breast carries the worst overall prognosis of the cancers.
Cysts - cysts fluid-filled sacs that 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 a cyst, ultrasonography and needle biopsies are required.
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 non-invasive.
X-RAY EQUIPMENT FEATURES.
A mammography system comprises:
ƒ¨ A high-voltage generator - the generator must be able to maintain a very accurate kVp in the range of 25-35 kVp. High tube currents of about 100 mA are used for mammography and about 25 mA for magnification mammography using fine focus. The generators are commonly constant potential with a low kVp ripple.
ƒ¨ X-ray tube - the x-ray tube has certain special design features, including a beryllium window, which attenuates less of the low-energy beam than glass, a small focus (â‰¤0.06mm) and a molybdenum target.
ƒ¨ Tube filtration - the tube filter is 0.03mm of molybdenum or 0.5mm aluminium equivalent to absorb below the optimum kVp range. Some equipment has dual filtration capabilities, e.g 0.03mm molybdenum for routine use and 0.025mm rhodium to provide dose reduction and good image quality in denser breasts.
ƒ¨ Compression device - breast compression should be applied only to a level that the woman can tolerate and not above 200Newtons. Some equipment has automatic compression available, which optimizes the compression based on the characteristics of the individual breast, to achieve uniform tautness.
ƒ¨ Reciprocating anti-scatter grid - the grid a line space of 35 lines/cm and a grid factor of 5:1. A specially designed moving a grid usually needed, despite its dose implications. The standard grid is 5:1 linear, moving grid. The necessity for a grid depends on the choice of anode material, the total filtration, the tube voltage, and the density and the thickness of breast. An image of satisfactory quality at an acceptable level of average entrance surface dose is essential. For more dense and thicker breasts (>6cm compressed), a tungsten anode, aluminium or other special filtration, e.g rhodium, higher tube voltages and an anti-scatter moving grid are preferable. For thinner breasts (<4cm), the use of an anti-scatter grid may not always be necessary.
ƒ¨ Image-recording system - the image receptor system is described in the image acquisition section.
ƒ¨ AEC system - 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.
POSITIONING AND TECHNICAL CONSIDERATIONS.
Before the examination begins, the radiographer will explain the procedure and the patient to put on gown, preferably one designed for mammography, which allows exposure of only the breast that being examined. The patient will be instructed to remove any jewellery, talcum powder, or antiperspirant that may cause artifacts on the radiographic image.
The radiographer 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.
Medications (e.g hormone therapy) currently taking
Previous mammograms, when and where performed.
Description of problems, such as screening mammogram, lumps, pain, and discharge
The radiographer should also note location of scars, palpable masses, moles, warts, and tattoos.
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 contour of the normal breast poses additional problems to the radiographer.
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 craniocauded or the mediolateral projection, the base of the breast is much thicker 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 thicker base of the breast.
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 image. 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 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 structures as close to the IR as possible.
Decrease dose and scattered radiation.
Decrease motion and geometric unsharpness
Separate breast structure.
These six factors identify how image quality or resolution is improved by reducing scatter and also by reducing magnification of breast structures.
The magnification method is used to enlarge specific areas of interest such as small lesion or microcalcification. This requires an x-ray tube with a 1.0mm focal spot to maintain image resolution. Magnification of 1 ½ 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 IOD. This magnification technique can be used with most mammograms projection.
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 4cm thickness mammogram, which is much higher than for most other body parts. For example, a much thicker 30cm 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 relatively high dose for mammograms is the very low kV (25 to 28) and the high mAs (70 to 85) required.
The principle way patient dose is controlled in mammography is by careful and accurate positioning, which minimizes the need for repeats. The ACR recommends a repeats rate of less than 5% mammography. The only shielding possible is a waist apron for shielding the gonadal region.
FILM SCREEN MAMMOGRAPHY
Film-screen mammography continues to be the standards in current breast radiography. The greatest benefit of the film-screen system is an excellent iamge with a low radiation dose, allowing women to have the examination regularly. The ability to see fine detail, edge sharpness and soft tissue 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-mammography.
COMPUTED RADIOGRAPHY MAMMOGRAPHY
Computed radiology (CR) can be used for mammography similar to the way it is used in general radiography with its imaging plate (IP) and image processor. CR cassette containing imaging plates can be used in existing mammographic systems, such as the following:
Operating costs - one advantages of CR over film-screen system 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.
Archiving and PACS options - after the image have been interpreted, they can be stored electronically at any desired location through the PACS. This is a second advantage of digital imaging compared 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 (DR) is a second form of digital imaging that continued 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 have shown that newer DR mammography systems have improved contrast resolution with reductions in patient dose compared with film-screen imaging. There is no imaging plate as required with CR. The flat detector captures the remnant x-rays and produces a digital image. The digital image is then projected on a monitor at the mammographer's workstation for direct viewing and post-processing as needed.
MAGNETIC RESONANCE IMAGING
MRI has received approval from the FDA as an adjunct screening tool for breast imaging. The number of breast MRI scans performed in the U.S is increasing annually. Although its cost makes it prohibitive for general clinical use, MRI has been clinically proven to be effective for certain special applications.