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Module 2 Transcript

©2012 ASRT. All rights reserved. Breast Imaging Basics: Module 2 Transcript

Breast Imaging Basics: Module 2 – Equipment and Instrumentation

1. Breast Imaging Basics – Equipment and Instrumentation

Welcome to Module 2 of Breast Imaging Basics — Equipment and Instrumentation. This module was written by James N. Johnston, Ph.D., R.T. (R)(CV), MRT.

2. License Agreement and Disclaimer

3. Module Objectives

After completing this module, you will be able to: • Explain the basic function and components of mammography equipment. • Distinguish between analog and digital mammography equipment. • Identify the methods used to store mammography images.

4. Introduction

Mammography is the only radiographic examination that is fully regulated by the federal government through the Mammography Quality Standards Act (MQSA). MQSA outlines specific performance and operation requirements for mammography equipment in addition to qualifications for personnel involved. This module discusses the basic physics of the mammography unit and image receptors, along with display and storage.

5. Imaging Challenges

Breast anatomy and tissue composition present unique diagnostic imaging challenges. The fibrous and glandular tissues that compose the breast are very similar in density and atomic number, making them homogeneous radiographically. The quantity of fatty tissue in the breast increases as a woman matures, however, providing some subject contrast and subsequent radiographic contrast. Mammography equipment and imaging techniques have evolved over time to meet the unique challenges of imaging breast tissues and to deliver the highest quality images with minimum radiation dose and discomfort to the patient.

6. Generator

Today’s mammography systems use single-phase input, high-frequency generators. These generators are small and inexpensive compared with older 3-phase systems. The more compact size of the generators reduces the mammography department’s space requirements. The design of the current generation of mammography units is self-contained and freestanding, which simplifies installation and reduces installation costs. The drawing on this slide shows the steps used to generate current for the x-ray tube. The high-frequency generator uses capacitor banks, rectifiers and inverter circuits to deliver a smooth 5 to 10 kHz power source with less than 1% ripple. To accomplish this, the single-phase alternating current (AC ) supply is first rectified, in effect making it direct current, or DC. The current then passes through a capacitor bank where the voltage waveform is smoothed, and next through an inverter circuit where it is changed to high frequency (5 to 10 kHz). At this point, the power moves through a step-up transformer, where it becomes high voltage in addition to high


frequency. The power must again be rectified after the transformer phase, so it passes through another rectifier bank and then through another capacitor bank where it again is smoothed before traveling to the x-ray tube.

7. Generators

The high-frequency generator is an efficient design that produces a near constant potential voltage waveform. When used in x-ray production, such a waveform translates to a higher effective energy x-ray beam. The net result for the mammographer is precise control of kilovoltage (kV) and milliamperage per second (mAs), a higher average x-ray output for a given set of exposure techniques and improved image quality. The net result for the patient is lower radiation dose per image. Precise control of kV and mA are very important in mammography because the modality uses much lower kV and mA ranges. The typical kV output range for a mammography unit is 20 to 40 kV, adjustable in 1-kV increments. The typical mA output range is 20 to 100 mA with the selection options based on which focal spot is being used. The exposure linearity and reproducibility also are very good for these generators. This is important in mammography because linearity and reproducibility allow for more accurate image comparisons over time, a critical factor in screening and follow-up care.

8. Mammography Tube

One of the mammography components that is modified most significantly from conventional radiography design is the x-ray tube. The special design adaptations of the mammography tube were developed because of the tissue density and composition of the breast. One of these adaptations is lower kV settings to enhance the subtle differences in tissue densities. The anode represents another unique mammography adaptation. To understand this modification, let’s briefly review atomic structure and the atomic interactions in the tube target and patient. This drawing shows the commonly used Bohr model illustrating the basic structure of the atom. Recall that the atom is composed of 3 fundamental particles: protons, neutrons and electrons. The positively charged protons and neutrons, which have no charge, compose the nucleus of the atom. The negatively charged electrons orbit the nucleus in discrete energy levels or shells; the closest shell to the nucleus is lettered “K” and the subsequent shells are labeled alphabetically following it (L, M, N, O and so on).

9. Binding Energy

The force of attraction between an atom’s electrons and nucleus is called binding energy and

the closer the electron’s orbit or shell is to the nucleus, the stronger the binding energy. As a

result, the K-shell electrons of a given atom are the most tightly bound and possess the greatest

binding energy. To overcome this binding energy and ionize the atom (or remove the electron

from its orbit) the filament electron from the cathode must possess an energy equal to or

slightly greater than the binding energy of the shell.

The K-shell interaction is generally of greatest use in producing radiographs. When a K-shell electron is removed from its orbit and an outer-shell electron drops to fill that vacancy, a K-


characteristic photon is produced. The energy of the characteristic photon is equal to the difference in the binding energies of the shells involved.

10. K-shell Interaction

In tissue, the K-shell interaction creates photoelectric absorption. This interaction, along with photon transmission, produces the shades of gray that represent the radiographic image or mammogram. Characteristic photons also are generated in this tissue interaction but are called secondary radiation when produced in the body. For any given anatomic part, the mammographer selects the kV to be in the K-edge range of the tissues of interest. K-edge refers to the K-shell binding energy and photon energy necessary to remove a K-shell electron.

11. Knowledge Check

Answer the following question.

12. Knowledge Check


13. Tungsten Target

Tungsten is the target material for conventional x-ray tubes. It has a K-shell binding energy of 69.5 kiloelectron volts (keV). To create K-characteristic photons in a tungsten target, the filament electron must have at least 69.5 keV of energy. However, because mammography typically uses settings in the 25 to 28 kV range, no K-characteristic radiation is produced and the beam is composed entirely of bremsstrahlung photons. The energy range of bremsstrahlung photons creates a continuous emission spectrum, or a bell-shaped curve, that peaks at about one-third of the kV used. If used for mammography, a relatively high number of photons would be above the K-edge for breast tissue and result in scatter, which would degrade image quality. Those below the K-edge for breast tissue would be absorbed by the tissue, increasing patient dose. Because higher atomic number metals such as tungsten increase the likelihood of bremsstrahlung production, they are not the favored targets for breast imaging.

14. Molybdenum Target

Molybdenum is the target material of choice for mammography tubes. Molybdenum has a K-shell binding energy of 20 keV and an atomic number of 43 vs tungsten’s atomic number of 74. When molybdenum is used with selections in the 25 to 28 kV range, there is a significant spike in K-characteristic photons with very little resulting bremsstrahlung photons. Filtration of the beam addresses the production of the bremsstrahlung photons. A molybdenum filter removes many of the high-energy bremsstrahlung photons through photoelectric events within the filter according to the K-edge principle. This is referred to as the molybdenum/molybdenum combination. Filtration minimizes the scatter that high-energy photons produce and improves image quality. The lower-energy bremsstrahlung photons still contribute to patient dose, but the contribution is acceptably low.

15. Rhodium Target


Rhodium is a better target material for imaging larger, denser breasts. The material has a slightly higher atomic number of 45 and produces a slightly higher energy beam. It also generates a spike in K-characteristic photons but with an energy level 2 to 3 kV higher than molybdenum. This slightly higher energy beam results in better penetration of denser or larger breasts. Rhodium also produces more bremsstrahlung photons, however. When matched with a rhodium filter— again taking advantage of the K-edge principle — many of the higher-energy bremsstrahlung photons are removed, reducing scatter production. This is the rhodium/rhodium combination. Rhodium has more low-energy bremsstrahlung photons compared with molybdenum, making rhodium less than ideal for routine mammography.

16. Biangular Anode

A mammography tube is available with a biangular anode that offers both a molybdenum anode track and a rhodium anode track. The mammographer can select the molybdenum target for average breast size and composition or the rhodium target for larger or denser breasts.

17. Filtration

Recall that filtration is used to adjust the energy of the beam. As with conventional radiography, filtration removes low-energy photons that would otherwise be absorbed by the tissues and increase patient dose. In mammography, a filter of the same element as the target (ie, Mo/Mo or Rh/Rh) is used to remove high-energy photons that would result in scatter and decrease image contrast. The idea is to limit the beam to the 17 to 20 kV range, creating the high contrast necessary to distinguish low subject-contrast tissues and microcalcifications; this is essential to mammography interpretation. For this purpose, filters of 0.03 mm of molybdenum or 0.025 mm of rhodium are used. The mammographer usually has the option of selecting either a molybdenum or rhodium filter, with or without the additional option of the rhodium target.

18. Filtration

When imaging larger or denser breasts, the mammographer can use the rhodium filter with the molybdenum target to yield a higher energy beam better suited for that breast type. As with conventional radiography, this filtration is in addition to the inherent filtration provided by the target window and compression paddle. The total filtration must be 0.30 mm aluminum equivalent at 30 kV, or 0.25 mm aluminum equivalent at 25 kV.

19. Beryllium Target Window

The target window of the mammography tube is made of beryllium. This material, with a very low atomic number of 4, minimizes absorption of the low-energy beam as it exits the tube. Beryllium’s filtration contribution is about 0.1 mm aluminum equivalent; it is included with the total filtration calculation, but is intended to be minimal.

20. Knowledge Check


21. Knowledge Check


22. Cathode and Filaments


The cathode end of the mammography tube is a bit more conventional. It is made up of 2 filaments and a negatively charged focusing cup. Some tubes might use only one filament and limit the size of the space charge by increasing the negative charge on the focusing cup. This is called a negative bias. Recall that the space charge is created by the electrons that are ejected from the cathode due to the extreme heat created within the filament. Filaments are necessarily small because of the high-resolution requirements of mammography, the short source-to-image distance of the mammography unit, and physical and geometric restrictions of the filament. A filament cannot resolve anything smaller than itself. Generally, filaments measure 0.1 mm for small focal spots and 0.3 mm for large focal spots. The smaller focal spot is used with magnification images, and the large focal spot is used for routine imaging.

23. System Geometry

The mammography system geometry makes use of some fundamental imaging principles and incorporates other innovative ones. Let’s review some definitions:

The actual focal spot is the actual area of the target struck by electrons.

The effective focal spot is the focal spot area as projected toward the patient and image

receptor.

The line focus principle describes angling the face of the anode target to maintain a

large focal spot for heat dissipation, while creating a smaller, effective focal spot to

maintain or improve image resolution.

The anode heel effect describes the loss of beam intensity on the anode side of the

beam; the effect is caused when the heel of the anode target, which is partially in the

beam path, absorbs some photons.

24. Short Source-to-Image Distance

The mammography unit uses a source-to-image distance of 24 to 30 inches, which is relatively short compared with the 40 to 45 inches used in general radiography. Mammographers must compensate for this shorter source-to-image distance to avoid image quality problems. In general, a short source-to-image distance results in magnification of the area of interest and geometric unsharpness. To regain the needed high resolution in the mammography system, a mammographer uses a fractional focal spot of 0.3 mm for routine mammography and 0.1 mm for magnification mammography. A short source-to-image distance can cause some tissue at the chest wall to be missed. Mammographers can make adjustments to compensate for this problem. First, they can orient the mammography tube so that the cathode is over the chest wall. Second, they can shift the tube toward the patient so that the focal spot (true beam center) is over the chest wall. And third, the mammographer can use a greater target angle to tilt the entire tube with the cathode side down. This puts the true vertical center of the beam at the chest wall, ensuring that all of the critical structures are imaged.

25. Focal Spot and Target Angle

Adjustments also help with several other potential problems. The cathode positioned over the chest wall takes advantage of the anode heel effect by placing the thickest part of the breast


under the cathode side. This placement evens out the exposure relative to tissue thickness and density. The typical effective focal spot is keystone shaped, with the larger portion on the cathode end. This means that the anode end of the focal spot as projected on the image receptor is smallest, or closest to true size, and the cathode end is largest. To minimize the normally occurring focal spot blur, the central ray is moved over the chest wall, and the entire cathode side of the beam is eliminated.

26. Reference Axis

With the true center of the beam at the chest wall, the center of the beam relative to the region of interest actually is the center of the anode half of the beam; in mammography, this is called the reference axis. The relatively small target angle of mammography tubes is expressed relative to this reference axis and is called the reference axis target angle. The effective target angle, measured between the true center and anode face, is 22° to 24°. However, the reference angle, measured between the reference axis and target face, is 7° to 12°, which is similar to radiographic tubes. Tilting the entire tube about 6° fully exposes the image receptor at a short source-to-image distance, while maintaining the smaller effective focal spot. This also allows the tube manufacturer to use a larger actual target angle for greater heat load capacity, a more complex version of the line focus principle.

27. Compression Devices

Compression is perhaps the most dreaded and certainly the most uncomfortable part of a mammography examination for the patient. However, compression is an essential step in obtaining a quality image. The mammographer must balance use of adequate compression to achieve appropriate image quality with concern for patient comfort. If too little compression is used, an inferior mammogram will result and compromise diagnostic quality. However, if too much compression is used, the patient might find the exam unbearable and fail to return for future screenings, defeating the goal of regular screening mammography.


The purpose of compression is to reduce the breast diameter to a smaller, more uniform thickness. Doing so has several advantages. Reducing the thickness also reduces the mAs needed to obtain the image, which lowers patient dose. Scatter production also is decreased because less kV is needed to penetrate tissues for quality contrast resolution. Reduced breast thickness also minimizes the distance between the anatomic structures of interest and the image receptor, known as object-to-image distance, which improves image detail and resolution. Compression immobilizes the breast, reducing the possibility of motion blur. Compressing the breast forces the structures into side-to-side positions, which reduces superimposition and improves the visibility of the areas of interest. Finally, compression evens out the thickness of the breast from chest wall to nipple, presenting a more uniform exposure to the image receptor and uniform optical density.



The general design features of the compression device are dictated by MQSA. The compression device consists of a compression paddle made of plexiglass, a motor drive with a foot-operated switch and a manual adjustment knob. The paddle should be radiolucent and be the same size and shape as the image receptor. The paddle also should have a 90° edge on the chest-wall side to uniformly compress the breast tissue at the chest wall and maximize the quantity of tissue included in the image. The paddle must not deflect or bow during compression more than 1 cm from parallel to the receptor in any direction. Bowing any more than 1 cm causes disproportionate compression across the breast and compromises image quality.


The mammography compression drive mechanism must provide 25 to 45 lbs, or 111 to 200 newtons, of force. The power-drive mode is foot operated to allow the mammographer to have both hands free to position the breast. The power drive should not exceed 45 lbs of force, and MQSA requires periodic testing of the safety switch to prevent application of more force. A mechanical adjustment of the compression is available for “fine” compression adjustments.

31. Spot Compression

A spot compression paddle also is available for localized compression to further separate superimposed structures and increase compression of a small area to improve “focused” imaging. The spot compression paddle is generally about 5 cm in diameter and made of radiolucent plastic.

32. Grids

As with general radiography, a grid is used in mammography to improve image contrast by reducing the amount of scatter radiation that reaches the image receptor. Because a much lower kVp range is used in mammography, however, special consideration is given to the materials used and the design of the mammography grid. Mammography grids have a ratio of 3:1 to 5:1 and a frequency of 30 to 50 lines per cm; a 4:1 ratio and 40 lines per cm are most common. Most mammography grids are linear and focused to the source-to-image distance.

33. Grids

General radiography grids use lead strips as the absorptive material and aluminum as the interspace material. Mammography grids also use lead for absorption, but the interspace material is typically a carbon fiber, fine wood or similar low atomic number material because aluminum is too absorptive for use with mammography exposure techniques. The grid can be stationary, but most are moving grids to blur the grid lines and reduce grid artifacts. Unlike general radiography grids that reciprocate, mammography grids only move in one direction. Mammography grids must be available in the image receptor sizes offered by the facility.

34. High Transmission Cellular Grid

A newer design for mammography grids is the high transmission cellular grid. This grid uses a cross-hatch design, that is, grid lines running in both directions, as originally introduced by Gustav Bucky in 1913. This design has greater scatter clean-up because it absorbs scatter photons in both the length and width dimensions of the grid, whereas a linear grid only absorbs photons perpendicular to the direction of the grid lines.


35. High Transmission Cellular Grid

The general disadvantages of the traditional cross-hatch design are increased grid artifact and higher patient dose because mAs must be increased to counter the removal of both scatter and remnant photons. The high-transmission cellular grid design addresses these disadvantages. It uses copper instead of lead as the absorptive material and a unique microprocessor-controlled precision motor to move the grid and eliminate grid artifacts. The combination of copper as the absorber and air as the interspace material yields a significantly better contrast improvement factor, while using approximately the same exposure technique as would be used with a 5:1 linear grid.

36. Grid vs Nongrid Imaging

When any grid is used for an examination, there is an increase in exposure technique compared to imaging without a grid. This increase is approximately twice the exposure for a mammography technique that does not use a grid. However, the contrast improvement is substantial at about 40% when using a grid, and the radiation dose is still quite low. The use of a grid further cleans up the remnant beam by removing scatter that would otherwise reduce contrast and degrade the image. The mammographer’s goal in using a grid is to strike a balance between image quality and patient dose.

37. Knowledge Check

Match the following terms with their definitions.

38. Knowledge Check


39. Magnification

In general, magnification is used in medical imaging to improve the visibility of small structures by increasing the object-to-image distance, or purposefully placing the anatomic part at a specific distance from the image receptor. This also is true in mammography, but with the added challenge of the need to apply compression and use a small enough focal spot to minimize geometric blur. Recall that mammography uses a shorter source-to-image distance, which further challenges the process because a short source-to-image distance increases magnification. Magnification in mammography is performed by first attaching a radiolucent plastic stand to the image receptor platform. The purpose of the stand is to increase the object-to-image distance sufficiently to magnify the area of interest 1.4 to 2 times normal size, depending on the equipment manufacturer. MQSA requires that the system offer at least one magnification value within the 1.4 to 2.0 magnification range. Compression still is used, but the general compression paddle is replaced with one that is modified for magnification images.

40. Magnification

An increase in object-to-image distance typically results in a loss of resolution because of geometric blur. In general radiography, the radiographer usually compensates for magnification by increasing the source-to-image distance. The only factor the mammographer can use to reduce the geometric blur associated with magnification is to reduce focal spot size. Typical


mammograms use focal spot sizes of 0.3 mm. For magnification images, a 0.1-mm focal spot is used. This smaller focal spot returns the system resolution to its normal level of approximately 20 line pairs per mm.

41. Magnification

Because the magnification stand creates an air gap, much of the more divergent scatter photons miss the receptor, which improves contrast. An air gap has a similar effect on the image as a grid, and so the grid is removed during magnification exposures. With analog mammography magnification, it’s necessary to increase exposure to the breast because of a failure of the reciprocity law, meaning that a disproportionate increase in exposure is required as exposure time lengthens. This is a photographic property associated with film-screen imaging. Most digital systems are not affected by reciprocity law failure. The skin dose also increases significantly with magnification techniques — by as much as 2 to 3 times — because the breast is much closer to the radiation source. The plus side of this problem is that more photons interact with the breast tissue and contribute to the image, thereby reducing image noise and improving resolution.

42. Magnification

Let’s summarize what we’ve covered so far. Magnification is used in mammography to improve the visibility of very small structures. When magnifying breast tissue, scatter radiation reaching the receptor is reduced because of the air gap, which improves image contrast. Further, image noise is decreased improving resolution because more photons interact to produce the image.

43. Techniques

Compared with general radiography systems, the control of exposure factors for mammography systems is somewhat unique. Depending on the manufacturer and unit design, the mammographer can select kVp, mA, exposure time, anode material, filter material, grid, density setting and radiation detector placement.

44. Automatic Exposure Control

Like the automatic exposure control (AEC) system of general radiographic equipment, the AEC of a mammography unit, is designed to terminate exposure when a predetermined density is reached. However, the mammography AEC uses a complex algorithm to calculate total exposure time. The algorithm takes into account information from a test exposure to determine breast density, the compression paddle for breast thickness, as well as the density setting selected by the mammographer and length of exposure. Conventional radiography AEC devices are calibrated to a particular image receptor speed or speed class. The mammography AEC systems are calibrated for a variety of circumstances such as 2 different image receptor speeds or speed classes, grid and nongrid exposures, and magnification and nonmagnification modes. Typically, the AEC device is designed to deliver an optical density of 1.4 to 2.0 for an American College of Radiology (ACR) plexiglass phantom imaged using standard exposure factors.

45. Diode Detectors


The mammography unit typically uses a single ionization chamber detector or 3 solid-state semiconducting diode detectors located behind the image receptor. The single detector is a D-shaped sensor with a sensor area of about 5 square centimeters (cm2). The 3-detector arrangement uses three detectors that are 1 square centimeter (cm2), with 2 detectors about 4 cm apart. The third detector is located halfway between the other detectors and about 2 cm in front of them. Each detector has from 3 to 10 stop positions, depending on the manufacturer. The detector should be placed beneath the densest part of the breast, usually at the chest wall or below the glandular tissue.

46. Density Settings

The AEC typically has settings to adjust density at least two settings above and two settings below the normal setting through a density compensation circuit. Each of these settings represents a 12% to 15% change in mAs or a 0.15 change in optical density. The system also has a backup timer that is preset to terminate exposure at 600 mAs for grid exposure techniques and 300 mAs for nongrid exposure techniques. The AEC should be used for all routine mammography exams. According to MQSA regulations, the mammographer must develop an AEC technique chart listing all routine settings, particularly kVp settings, based on breast composition. The mammographer also should consult prior mammograms to determine the densest part of the breast for proper AEC detector location.

47. Full Automatic Exposure

Mammography systems typically have an automatic exposure mode, which also may be known as auto filter mode or full auto mode depending on the manufacturer. When operated in full automatic exposure mode, the mammography unit automatically selects and controls all options: kVp, mA, exposure time, density setting, anode material, filter material, grid and detector location. The unit generally is operated in this mode because the breast varies considerably in its composition, thickness and density, and it would be very difficult for the mammographer to determine optimum technique. However, the mammographer can control options in the manual mode.

48. Auto kVp

Auto kVp mode allows mammographers to select filter, density setting and sensor placement. The kVp is automatically determined. With some units the kVp may be automatically adjusted within the first few milliseconds of the exposure.

49. Auto mAs

Auto mAs mode, or auto time mode, allows the selection of kVp, filter, density and sensor placement. The mAs is automatically determined within the first few milliseconds of exposure.

50. Full Manual

Full manual mode allows the mammographer to select all exposure parameters. The use of modes other than full auto are designed to allow the mammographer to compensate for known variations. For example, it might not be desirable to image male breasts in full auto mode because the placement of the sensor at the chest wall would overexpose breast tissue based on the density of the pectoral muscle. Manual mode also is necessary for in-place projections of women with implants to avoid overexposing breast tissue. Other circumstances such as patients


with pacemakers or implantable venous catheters might require the use of a semiautomatic mode.

51. Knowledge Check


52. Knowledge Check


53. Analog-to-Digital Transition

Mammography is nearing the end of the transition from analog to digital imaging in the United States. However, some facilities still use film-screen systems, and the information is part of the curriculum and certification exam for mammography. Even in fully digital departments, it is useful to understand film-screen technique to appreciate the function and differences of digital systems and to understand how mammography has evolved to its current state.

54. Acquisition

With analog systems, film is used for image acquisition, display and archival purposes. Once the film is exposed and processed, the mammogram’s characteristics are fixed. For example, if the image is too light, nothing can be done and that portion of the examination must be repeated. If the image is too dark, it can be illuminated somewhat by a hot light, but might be so dark that the image must be repeated. With digital systems, acquisition, display and image storage involve different systems. In some instances, the different systems can be a detriment, such as the separation of the mammographer’s visual cues for proper exposure and image quality. However, in other cases, separate systems can be an asset, such the ability of postprocessing functions to enhance image characteristics. Independent systems provide the opportunity for departments to maximize and customize each system, thereby improving imaging efficiency and quality, an important advantage in such a heavily regulated modality.

55. Digital vs Analog Characteristics

The general response characteristics of digital and analog systems also are different. Analog response to exposure is nonlinear and creates an H and D curve. The curve is fairly steep and represents a high-contrast image. With mammography, the lower end of the body of the curve represents the useful range of densities. Digital response to exposure is linear. Though the linear line represents a low-contrast image, it also shows the digital system’s ability to respond to a much wider range of exposure values — from very low to very high. Digital systems can then be windowed and leveled, that is, focused on a specific range of densities anywhere along this continuum. Mammography highlights the low end of exposure and provides exquisite high-resolution, detailed mammograms. Look at this illustration. If the horizontal axis is exposure and the vertical axis is latitude the line marked A represents analog response to exposure. Using the same horizontal and vertical axes, the line marked B represents a digital response to exposure.


56. Film

MQSA requires a facility to use a film that has been designated by the manufacturer as appropriate for mammography and at least 2 film sizes: 18 x 24 cm and 24 x 30 cm. The standard film for mammography is a single-emulsion slow film that produces high contrast and high resolution. The single emulsion is somewhat thicker than the 2 emulsions used in conventional radiographic film. The single emulsion also contains a cubic grain (crystal) structure approximately 0.5 to 0.9 µm in size, which delivers a higher contrast in the lower end of the useful range of densities and makes it ideally suited for mammography. The other side of the film is coated with an antihalation backing to prevent the backscattering of light produced by the screen. The film is green-sensitive to spectrally match the commonly used green light-emitted screens. The speed and contrast of this film is well-suited to the extremely low subject contrast of the breast.

57. Film

Mammography film maximizes the subtle differences in breast tissue and its slow speed delivers a high-resolution image. The slow speed also translates to low image noise. This is because more radiation is used to create the image. Though more radiation translates to a higher patient dose, the trade-off between low noise and dose is acceptable.

58. Screens

MQSA also requires a facility to use a screen that is appropriate for mammography and is spectrally matched to the film being used. A single screen is matched with a single-emulsion film. The most commonly used screen phosphor is gadolinium oxysulfide, which is a green light-emitting rare-earth phosphor. The screen is designed to be very slow to minimize noise. Slow speed means that more radiation is required to produce the image compared with a faster screen. The slow screen greatly reduces overall radiation dose compared with direct exposure because it replaces some radiation with light to expose the film emulsion.

59. Screen Positioning

The screen is positioned in the back of the cassette, and the film is placed in the cassette with the emulsion facing toward the screen and away from the tube. This arrangement minimizes light crossover, which reduces resolution loss. The speed of mammography screens does not directly compare to the speed of general radiography screens. For example, a 100-speed mammography screen is 50% to 75% slower than a 100-speed general radiography screen. The difference in numeric expressions of speed stems from the fact that Kodak arbitrarily assigned a value of 100 to its first mammography screen.

60. Cassettes

Mammography cassettes are made of plastic or a low atomic number carbon fiber material to minimize interference with the remnant beam exiting the patient to expose the film. The single intensifying screen is mounted on a foam pad on the back plate of the cassette to create firm film-screen contact. As previously mentioned, the film is placed in the cassette with the emulsion side against the screen to minimize light spread and a loss of resolution. The back plate also is made of plastic or a carbon fiber material to minimize interference with the exit radiation interacting with the AEC detector.

61. Processor


The mammography processor is the single greatest variable in the imaging chain. For this reason, MQSA has strict operation and quality control (QC) guidelines for the processor’s operating parameters. With such rigid guidelines, a dedicated mammography processor, usually a 90-second processor, typically is maintained in the radiology department. Researchers experimented with the use of extended development processors, but it was more difficult to maintain proper temperatures and chemical strengths in those processors, and they slowed patient throughput. MQSA regulations require strict daily monitoring of the dedicated mammography processor. Patients cannot be imaged until QC procedures are performed and the processor is found to be operating within parameters or until an identified problem is corrected.

62. Cassette-based System

Digital mammography may be cassette-based or cassette-less. The cassette-based system, also referred to as computed radiography, or CR, uses a photostimulable phosphor plate housed in a plastic cassette. Cassette-based systems can be used with traditional film-screen mammography units; the CR cassette is placed in the Bucky after AEC recalibration, much like a film-screen cassette would be used. The phosphor atoms of the plate are ionized when the plate is exposed to radiation. The liberated electrons are trapped in a higher energy level just beyond the valence shell of the atom and in a quantity proportional to the radiation exposure received.

63. Cassette-based System

The pattern of trapped electrons represents the latent image; the cassette is placed in a CR reader, where the plate is removed and scanned by a laser. The laser moves back and forth in a precise manner across the plate, using mirrors or an optical lens system. As the laser scans the plate, the electrons are freed and emit their excess energy as a dim blue light. This light is collected through special optics and filtered to prevent light from the laser from adding to or interfering with the light signal. The light is then sent to a photodetector where it is converted to an electrical signal. From this device, the signal travels to a computer where it is processed, digitized and displayed. Once in this form, the digital image can be stored in a picture archiving and communication system (PACS) and transmitted electronically to other locations.

64. Cassette-less System, Indirect Capture

Cassette-less systems use either indirect capture or direct capture; there are 2 types of indirect capture. One indirect capture method uses a charge-coupled device (CCD), scintillator and optics. The CCD is a light-sensitive device that can respond to low light intensities and a wide range of higher light intensities. The scintillator is a cesium iodide phosphor that is coupled to the CCD using fiber-optic bundles or an optical lens system. The scintillator captures the x-rays of the remnant beam and converts them to light. The light is transmitted to a CCD array via a fiber optics or lens system, where it is converted to an electrical signal that is sent to the computer for processing and display.

65. Cassette-less System, Indirect Capture

The other indirect capture method also uses a scintillator with cesium iodide or gadolinium oxysulfide as the phosphor, photodetectors and thin-film transistors. A glass substrate is divided into a grid of pixels. Each pixel contains a photodetector with an attached thin-film transistor. The substrate matrix is then covered with the scintillator layer.


The scintillator absorbs the x-ray energy of the remnant beam and converts it to light energy. The light energy is absorbed by the photodetectors and converted to an electrical charge. The electrical charge is stored by the thin-film transistor until it is activated and transmits its signal. The signal, which is proportional to the x-ray energy exposure to that area, is sent to a computer for processing and display.

66. Cassette-less System, Direct Capture

The primary disadvantage of the 2 indirect capture methods is that they incorporate an additional step of converting x-rays to light and then to an electrical signal, which can result in a loss of resolution. The direct capture method solves this drawback by not using a scintillator. This method incorporates a biased electrode top layer, an amorphous selenium middle layer and a thin-film transistor array as the bottom layer; all of the layers are on a glass substrate. Before exposure, a bias charge is set up by the electrode top layer across the amorphous selenium layer. X-rays then are absorbed by the amorphous selenium layer, creating electrical charges proportional to the exposure energy. These electrical charges are transmitted to the underlying electronics where they are amplified and converted to a digital signal that is passed to the thin-film transistor array. The array transmits the signal to the computer for processing and display.

67. Analog-to-Digital Converter

No matter the acquisition method, the mammography image is in an analog form that must be passed through an analog-to-digital converter to become an electronic file. The converter digitizes the information for the computer to process. As the signal is transmitted, it is sampled. The sampling process is a time-based event that occurs at specific locations and specific intervals along the extracted signal. The frequency at which the data sample is collected is referred to as the sampling frequency; the closer the samples are to each other, the greater the frequency. Sampling pitch is an expression of sampling frequency and refers to the discrete nature of the sampling process. With cassette-based systems, the sampling pitch represents the distance between the scanning laser beam positions during plate processing. For cassette-less systems, it is the distance between adjacent detector elements.

68. Matrix

The analog signal is divided into a pixel matrix during digitization. The size of the matrix determines the resolution. Larger matrices have smaller pixels and produce greater resolution. The distance between the centers of adjacent pixels is called pixel pitch and is measured in microns. Mammograms have very large matrices to produce the necessary high resolution.

69. Grayscale

When the signal is digitized, the signal location, that is, where the signal originates in the matrix, determines spatial resolution; the intensity of the signal determines the grayscale. During the digitization process, the shades of gray are assigned based on intensity, or the number of photons striking a given area of the detector. The greater the number of photons incident on an


area of the detector — as represented by a pixel — the darker the assigned shade of gray will be.

70. Bit Depth

Bit depth refers to the number of shades of gray that can be displayed within a given pixel. Digital systems generally have bit depths of 214, or 16,384 shades of gray. The computer controls this part of the process and passes the image data through a series of processing steps to create the displayed image. Each manufacturer has developed proprietary algorithms to process the image data.

71. Display

The display of mammograms, whether analog or digital, must meet certain requirements to show the necessary anatomic details. Breast imaging centers must pay attention to ambient light levels in the viewing and reading room, as well as image luminescence provided by the view boxes or display monitors. Adequate image display was one of the greatest challenges in the development of digital mammography.

72. View Box Performance

Because of the high-contrast nature of mammograms and the goal of detecting very small lesions, viewing conditions are critically important in mammography. There are no specific MQSA requirements regarding view-box design, but the ACR recommends the following: The view box used for analog mammogram interpretation should provide a luminance level of at least 3,000 candelas per square meter, twice the luminance of general radiography view boxes. Further, the illumination levels of the reading room should be 50 lux or less. Extraneous sources of light such as light from other rooms, windows or view boxes should be eliminated or reduced. Ambient sources of light decrease image contrast and limit the interpreting physician’s ability to visualize the maximum densities of the film. Finally, MQSA requires that masking devices be available to the interpreting physician. The view box must be masked to limit the viewing area to only the exposed portion of the mammogram.

73. Hot Light

MQSA requires a facility to have a hot light capable of producing light levels greater than the mammography view box, generally more than 3,000 candelas per square meter. A hot light can enhance the darkest parts of the analog image. The anatomic details may be present but might be difficult to see under the luminance conditions of the view box.

74. Knowledge Check


75. Knowledge Check


76. Knowledge Check


77. Computer-aided Detection


Computer-aided detection (CAD) uses complex computer-based algorithms to detect suspicious areas and mark them for further evaluation by the radiologist. CAD may be used with scanned and digitized film-screen mammograms, but usually is used in conjunction with full-field digital mammography. The pattern recognition software scans for variations in breast architecture, density, presence of calcifications and other indications of malignancy based on image density variations. CAD was developed by using thousands of mammograms with known cancers to “train” the system. CAD compares the scanned image against the data and marks suspicious areas. CAD is an inherent part of digital systems. The digital image already exists as a data set that the CAD algorithm can analyze.

78. Computer-aided Detection

CAD is an aid for the radiologist, a sort of second pair of eyes, and is not intended to replace the radiologist’s interpretation. Some physicians have likened it to the “spell check” function of a word processing program. CAD has 2 basic outputs. A CADe system evaluates image data to detect and mark potential lesions. A CADx system evaluates a known or suspicious lesion area, either detected by the radiologist or by a CADe evaluation, to determine the likelihood that the lesion is malignant.

79. Digital Viewing Systems

Digital viewing systems are divided into acquisition workstations and review workstations. In general, a workstation is made up of a monitor, computer and image display software. The primary differences between the workstations are their information access, image manipulation capabilities and monitor resolutions.

80. Acquisition Workstation

The mammographer uses the acquisition workstation. This workstation offers a list of exam options and is the mammographer’s primary interface with the imaging chain. It provides a preview of the images as the mammographer acquires them. It also has presets or options to direct where the images go next, for example, the radiologist’s review station or the PACS. The resolution of acquisition monitors is lower than that of the review workstation, and the image characteristics on acquisition workstations might not look the same as those on review workstations.

81. Review Workstations

Review workstations, which also are called diagnostic workstations or reading workstations, are used specifically for mammography interpretation. These workstations must have high-resolution monitors. Although a 3-megapixel monitor is adequate for primary diagnosis, a 5-megapixel matrix of at least 2048 x 2560 is preferred. Workstations also should have enhanced image viewing and manipulation, or postprocessing, features, such as contrast adjustment, brightness adjustment and magnification. The keyboard and mouse can be specially designed to facilitate the radiologist’s interpretation of the images.

82. Review Workstations

Today’s digital systems can produce images up to 15 megapixels each in size. In such cases, the monitor scales the image to display it in a single view and in the process reduces the resolution from the original image. However, some systems offer the option to display a portion of the


image in full resolution, along with the ability to pan the image, so that the radiologist can view the image in its original format.

83. Clinical Viewing Stations

Clinical viewing stations also are available. These workstations generally have limited information, access and image-viewing capabilities. They are intended primarily to allow referring physicians or other clinicians to view the images but do not allow the data set that represents the image to be altered. The monitors have lower resolutions of approximately 2 megapixels and cannot be used for official image interpretation.

84. Hard-copy Device

Mammographers use hard-copy devices to print digital images. A hard-copy device is a laser printer that uses either wet or dry processing methods to create a film copy of a digital image. The use of such devices is becoming less common as digital imaging becomes standard. When a mammogram is printed using such a device, all postprocessing capabilities are lost and the image’s grayscale is limited. A dry laser film might display 50 micrometers spatial resolution. The dry laser system also generally has a higher density and fewer artifacts than wet-processed film.

85. Laser Printer

In general, laser printers have a laser light source, a beam modulator and a series of optics and mirrors to shape and control the beam. The system focuses and modulates the beam in a raster pattern to re-create the digital image line by line on the film. The film then is processed using either a wet or dry processing method. The wet processing method uses a silver halide-based emulsion film that is passed through a developer, fixer, washed and dried, just like conventional film.

86. Dry Processing

The dry processing method, as its name implies, does not require chemicals, but uses a thermal-based technique. Like the wet processing method, the film emulsion is silver based, and the laser creates a latent image that must be processed. However, in addition to the light-sensitive components, dry processing film contains thermally sensitive components that form the visible image when exposed to the thermal head of the printer. The image becomes stable and permanent once the visible image is formed and the film cools.

87. Knowledge Check


88. Storage Challenges

MQSA requires that mammography images be kept for not less than 5 years if a patient is seen regularly and 10 years if no additional images are obtained. Storage of both film and digital images presents unique challenges. In the transition to digital mammography, facilities still require physical space to store analog mammograms and virtual space in the form of a PACS or mini-PACS for digital images. The transition introduces several process issues, such as how to compare a previous analog mammogram to a current digital image.

89. Mammogram Storage


Image storage has long been a challenge in radiology. Because of mammography’s screening and follow-up role, it’s critically important for a facility to properly store and keep track of mammograms and reports. Film file rooms remain in place for a time even after a facility converts to a digital system. Some facilities have considered scanning films into their PACS using a film digitizer, but most have found it too time consuming and cost prohibitive.

90. Mammogram Storage

In mammography, current images must be compared to previous studies. So during a transition from analog to digital systems, analog mammograms must be compared to a patient’s first digital images. Because of the need for comparison, analog mammograms must be stored on site for easy retrieval. Patients or physicians might need prior studies along with new digital images, and a process should be in place for providing access to both analog and digital images. Breast imaging managers should establish time limits for the storage of analog mammograms, eventually phasing out physical storage space as the need declines.

91. PACS

A PACS is a network that facilitates storage, retrieval, transmission and display of images. This network provides for communication between computers and other electronic systems using the DICOM standard, a common electronic language for radiology systems. The digital mammography unit acquires the image, displays it at the workstations and then stores the image electronically in the PACS. PACS archive servers store the images. Storage is a continuous challenge in radiology because of the large size of image files, and it can be a particular challenge in mammography. A single digital mammogram can be 14 to 50 MB in size and a single study as large as 200 MB. Initially, mammography was not supported by most PACS, but since more mammography departments have converted to digital imaging, PACS manufacturers have incorporated mammography storage, retrieval and display capabilities into their products.

92. PACS Archiving

PACS archiving consists of 2 parts: the image manager and image storage. The image manager allows images to be moved back and forth among various workstations and storage. The storage part of the system records the data on a storage medium such as an optical disk or hard drive. Data may be stored online, nearline or offline. Data stored online is immediately accessible to the user.

93. Nearline and Offline Storage

Images sent from the acquisition workstation to the review workstation are stored online for immediate access by the interpreting radiologist. If a patient might return for short-term follow-up, biopsy or surgery, the mammogram might be stored nearline. Nearline storage uses a CD or jukebox with robotic arms to retrieve the data more quickly than from archival storage. It is not as immediately accessible as online storage, but can be retrieved quickly when needed. Examinations that probably will not be accessed for a year or more are stored offline. Offline storage can use CDs, other recording media or offsite servers, such as cloud computing, for archival storage. Images stored offline have to be retrieved and loaded back into the workstation for access.


94. Accessibility Levels

In a perfect world, a mammography department would maintain all examination data online for immediate use whenever needed, but there are 2 limiting factors: cost and physical storage space. Storing data online would tie up workstations and local servers, slowing down all data movement. In addition, online storage would require huge, robust computer systems that would be cost and space prohibitive. Nearline and offline storage offer less expensive options for mid-term and long-term storage.

95. Physical Storage

The physical storage of digital data is a growing problem for many breast imaging facilities. The advantages of digital acquisition and storage still outweigh a return to analog imaging, but digital data storage has presented its own set of challenges, most notably changing storage technology and increasingly large storage requirements.

96. Traditional Storage

Magnetic tape is the oldest medium for data storage and is now all but obsolete. For this method, a ferromagnetic material and fluctuating magnetic field magnetize the tape. The tape is read using a principle similar to the law of electromagnetic induction familiar to radiologic technologists. Digital mammography data are too dense to be recorded in this way. The tape would have to be unmanageably long and moved past the electromagnet extremely quickly. This is why VHS tapes cannot be used to record high-definition digital signals. Optical discs such as CD-ROMs are another familiar and still used storage medium. During recording, a laser burns the photosensitive layer on the disc from the center of the disc outward. The laser is modulated by the data signal, creating a series of light and dark spots along the spiral. At their basic level, digital data are a series of ones and zeros. An optical reader reconstructs the ones and zeros as the digital signal. The laser can start at any point along the spiral and read only segments of the data file or start at the center of the disc and read the disc in its entirety. This is an inexpensive and easy method of storage but is limited by how much data the disc can hold. Even when arranged in redundant arrays, the number of CD-ROMs eventually becomes unmanageably large.

97. Flash Drives

Another storage technology is the high-capacity flash drive. These drives can store hundreds of gigabytes of data. They consist of printed circuit boards arranged in a grid. There are 2 transistors at each intersect of the grid. One is a floating gate and the other is a control gate. A higher charge or lower charge is created by applying electrical charges to the floating gates. The higher charges represent ones and the lower charges represent zeros. When arranged in arrays, flash drives can provide considerable storage in a rather small space.

98. Holographic Storage

Holographic storage also is being explored as a viable archival method. A holographic system uses laser technology and a special holographic medium. A signal beam converts the ones and zeros into an optical checkerboard that is then stored in the medium through a chemical reaction. The checkerboards are stored like pages throughout the depth of the storage medium


where the signal and reference beams intersect. This storage method has the potential to provide 1 terabyte (TB) of storage per cubic centimeter (cm3) of recording medium.

99. Cloud Computing

Cloud computing offers access via the Internet to files stored at remote servers. Several providers are working to offer this solution for PACS storage of mammograms and other medical images. Cloud computing solves problems related to on-site storage and the cost and time spent developing site-based solutions. This archiving method presents security concerns, however. Cloud-based storage also might be called hosted PACS or off-site PACS.

100. Storage Requirements

From the previous descriptions of storage technologies, it’s clear that the challenge is to store increasingly large quantities of data in smaller spaces in a cost-effective manner. Mammography is contributing massive quantities of data to be stored and archived by PACS.

101. Other Storage Considerations

There are other considerations regarding mammography storage. For example, a patient or physician who does not have access to the PACS might request a copy of an examination, which then must be burned to a CD-ROM. Space per disc can be an issue. Further, imaging facilities must have disaster plans that include the backup and off-site storage of all imaging data. The need for the film file room might be eliminated over time, but will be replaced by virtual storage challenges.

102. Conclusion

This concludes Breast Imaging Basics Module 2 — Equipment and Instrumentation. You should now be able to:

Explain the basic function and components of mammography equipment.

Distinguish between analog and digital mammography equipment.

Identify the methods used to store mammography images.

103. Bibliography

104. Bibliography

105. Bibliography

106. Acknowledgements

107. Development Team

108. Module Completion


Bibliography

American College of Radiology. 1999 Mammography Quality Control Manual. Reston, VA: American College of Radiology; 1999. Andolina VF, Lille SL. Mammography Imaging: A Practical Guide. Philadelphia, PA: Lippincott Williams & Wilkins; 2010. Bassett LW, Hendrick RE. Quality Determinants of Mammography. Rockville, MD: U.S. Department of Health and Human Services, Public Health Service Agency for Health Care Policy and Research; 1994. Bick U, Diekmann F. Digital Mammography. Berlin, Germany: Springer International; 2010. Bushong SC. Radiologic Science for Technologists. 9th ed. St Louis, MO: Elsevier Mosby; 2008. Bushberg JT. The Essential Physics of Medical Imaging. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2002. Carlton RR, Adler AM. Principles of Radiographic Imaging: An Art and a Science. 4th ed. Clifton Park, NY: Thomson Delmar Learning; 2006. Colang JE, Johnston JN. PACS storage technology update: holographic storage. Radiol Manage. 2006; 28(3):38-49. Gray JE, Princehorn JA. HTC grids improve mammography contrast [white paper]. Bedford, MA: Hologic Inc. www.hologic.com/data/File/pdf/W-BI-HTC_HTC%20GRID_09-04.pdf. Accessed January 31, 2012. Hayat MA. Cancer Imaging: Instrumentation and Applications. San Diego, CA: Elsevier Academic Press; 2008. Hashimoto BE. Practical Digital Mammography. New York, NY: Thieme Medical Publishers; 2008. Johnston JH, Fauber TL. Essentials of Radiographic Physics and Imaging. St Louis, MO: Elsevier Mosby; 2011. Jong RA, Yaffe MJ. Digital mammography. Can Assoc Radiol J. 2005;56:319-323. Peart O. Mammography and Breast Imaging: Just the Facts. New York, NY: McGraw-Hill Professional; 2005. Rezentes PS, de Almeida A, Barnes GT. Mammography grid performance. Radiology. 1999;210(1):227-232. Wysocki ML, DeVos D. Magnification mammography. Radiol Technol. 1997;68(3):241-241.


Acknowledgements

Subject Matter Experts

Lisa Deans, R.T.(R)(MR)

Stephanie Johnston, M.S.R.S., R.T.(R)(M)(BS)

Myke Kudlas, M.Ed., R.T.(R)(QM)

Kevin Powers Ed.D., R.T.(R)(M)

Special Thanks to:

Hologic, Inc.

Project Management and Instructional Design

Charlotte Hendrix, Ph.D., SPHR

Narration

Vince Ascoli

Sharon Krein

Lead Designer

Nicholas Price

Module Production

Sharon Krein

Jake Rumanek

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