298 MAMMOGRAPHY

field hyperpolarized noble gas MRI. J Magn Reson 2005;174: 116–124.

42.Kvistad KA, et al. Breast lesions: Evaluation with dynamic contrast-enhanced T1-weighted MR imaging and with T2 - weighted first-pass perfusion MR imaging. Radiology 2000; 216:545–553.

43.Kuhl CK, Schild HH. Dynamic interpretation of MRI of the breast. JMRI 2000;12:965–974.

MAGNETOCARDIOGRAPHY. See BIOMAGNETISM.

MANOMETRY, ANORECTAL. See ANORECTAL

MANOMETRY.

MANOMETRY, ESOPHAGEAL. See ESOPHAGEAL

MANOMETRY.

MAMMOGRAPHY

PEI-JAN PAUL LIN

Beth Israel Deaconess

Medical Center

Boston, Massachusetts

INTRODUCTION

It has been shown that reduced breast cancer mortality in the past decade can be attributed to the high sensitivity of screening mammography in detecting nonpalpable lesions (1,2). There has been a wide variety of equipment and imaging modalities employed in the breast cancer detection and imaging; ranging from ultrasound imager, X-ray mammography, computed tomography scanner (CT), to magnetic resonance imager (MRI). Additionally, thermography, light diaphanography, electron radiography, and microwave radiometry have also been utilized, experimentally, to detect breast cancer without much success. Brief explanations of these imaging modalities have been described in the first edition of this Encyclopedia, NCRP Report No.85, and in a review article by Jones (3–5). Among those modalities; ultrasound, X-ray mammography, CT, and MRI, X-ray mammography is the most practical and relatively inexpensive, and is the only main stream of equipment available for breast cancer detection. At present, the ultrasound imager is often employed as an adjunct to the X-ray mammography and is not the primary screening imaging device. However, when mammography is mentioned, it is normally meant to say ‘‘X-ray mammography’’. For these reasons, this article will devote all of its effort to X-ray mammography.

THE MAMMOGRAPHY QUALITY STANDARDS ACT OF 1992 (MQSA), (PUBLIC LAW 102–539)

Breast cancer was a major public health issue in the early 1990s; U.S. Congress enacted MQSA ‘‘to ensure that all women have access to quality mammography for the

detection of breast cancer in its earliest, most treatable stages’’. Thus, it is required by law that facilities providing mammography services be properly accredited and be certified by the U.S. Food and Drug Administration (FDA). ‘‘Accreditation and Certification’’ of mammography facilities are beyond the scope of this article, and interested readers are requested to refer to the FDAs WEB Site (6), and the American College of Radiology (ACR) WEB Site (7).

X-RAY MAMMOGRAPHY

Conventional X-ray equipment was initially employed for breast cancer imaging with industrial (thick) emulsion film, or portal imaging film for use in radiation therapy as the image receptor in order to visualize the small microcalcifications, prior to, as late as 1970s. The breast entrance dose of this imaging process exceeded well over 85 mGy per film ( 10 R per film) and the radiographic techniques were typically in the range of 45–55 kVp, and 1000 mAS with a radiation beam quality of half-value layer (HVL) ¼ 1.0– 1.5 mm of aluminum (mmAl) (8). The X-ray beam spectrum produced by a conventional X-ray tube, equipped with tungsten anode, is not necessarily optimized for breast cancer detection. The mammography images obtained in this manner had the desired spatial resolution ( 20 lp mm 1), but had a less than desirable radiographic contrast. This combination of ‘‘high entrance dose’’ with ‘‘low radiographic contrast’’ was not an acceptable approach. The radiology community was searching for a new breast imaging solution. In the 1970s, xeromammography imaging plates provided the much needed improvement in image quality and lowered the breast entrance dose by a factor of two thirds to one half compared to using the thick emulsion industrial type film (9).

Due to its unique patient positioning of breast imaging, the geometrical arrangement of dedicated mammography units should be pointed out. As shown in Fig. 1, with the

X-ray tube

Collimator

Radiaion field

Patient

Examination table

Screen-film cassette

Anti-scatter grid

Figure 1. Geometric arrangement of conventional radiography. The X-ray system, the anatomy of interest, and the screen-film cassette are centered and aligned for exposure.

Lateral View of Mammography Examination Setup

X-ray tube

Collimator

Radiaion field

Chest wall

Compression paddle

Breast holder

Anti-scatter grid

Screen-film cassette

	(a) Correct alignment of X-ray	(b) Improper setup. The "black"
	system with the patient's
		triangular shaped area is not
	chest wall included in the
		in the field-of-view.
	field-of-view.

Figure 2. Geometry of dedicated mammography systems. On the left side of (a), the geometrical arrangement of a dedicated mammography unit is correctly setup where the center of the radiation beam is aligned to the chest wall of the patient with the compression cone pressing down on the breast being imaged. On the right side (b), improper setup for mammography examination is evident. The ‘‘black’’ triangular area is not included in the image captured by the image receptor, potentially missing the suspected cancer site.

conventional radiography system, the X-ray tube is setup in the center of the anatomy of interest. The radiation field is a projection of diverging X rays restricted by the collimator, centered to the area of interest. The geometry of a dedicated mammography system on the other hand is offset so as to maximize inclusion of breast tissue close to the chest wall (see Fig. 2a). If the geometry were setup in the same manner as the conventional radiography as depicted in Fig. 2b, the black triangular area of the breast would not be included in the imaging field possibly missing a suspected cancer site.

THE PHYSICS OF MAMMOGRAPHIC IMAGING

Considering that various tissues in the breast are radiologically similar, but not identical, the task of differentiating the fibrous, ductal, and glandular tissues is extremely difficult. This is due to the fact that (1) water; the main ingredient of human tissue, has a density of r ¼ 1.0 g cm 3

and effective atomic number of Zeff ¼ 7.4–7.6, and (2) fat has a density of ¼ 0.9 g cm 3 and effective atomic number

of Zeff ¼ 5.9–6.5 (10). Thus, various breast tissues with varying degrees of fat and water contents are very similar from radiological point of view.

The development of screen-film mammography was, in reality, paired with the redesigning of dedicated X-ray equipment for breast imaging. Breasts are relatively thin (physical thickness and X-ray attenuation property) compared to other body parts. Radiographs of extremities, for example, are obtained with X-ray tube potential in the

MAMMOGRAPHY 299

range of 50–60 kVp. Breast tissues contain no high attenuation anatomy, such as bones, a lower tube potential (< 35 kVp) would be more suitable for breast imaging (11). Use of lower tube potential has a potential benefit of taking advantage of the photoelectric effect in differentiating the subtle differences of breast tissues.

It should be pointed out that there are five basic ways that X-ray photons interact with matter; they are (1) coherent scattering, (2) photoelectric effect, (3) Compton effect, (4) pair production, and (5) photodisintegration (12). Of these five interactions, photoelectric effect and Compton effect predominantly contribute to the image formation in diagnostic radiology. The probabilities of photoelectric effect and the Compton effect can be expressed as following;

Photoelectric effect Z3=E3	ð1Þ
Compton effect E Z	ð2Þ

In equations (1) and (2), E is the photon energy, and Z is the atomic number. Clearly, from Eq. 1, the lower the X- ray photon energy, and the higher the atomic number the probability of the photoelectric effect will be higher. Since the photoelectric effect is proportional to the ‘‘cube’’ of

(Z/E), the differential of breast tissues in Zeff is ‘‘enhanced’’ in the image formation. Thus, a better approach

to differentiate and image the breast tissues is to employ a lower X-ray tube potential for imaging. While this approach is ‘‘good’’ for imaging, the radiation dose absorbed by the breast would still be a major concern.

Conventional X-ray tubes employ tungsten as the target material and produce a broad X-ray spectrum through the bremsstrahlung process. At X-ray tube potential of 35 kVp and lower, the tube potential used on most dedicated mammography systems, the tungsten target X-ray tubes would produce broad energy spectrum that contribute less to the image formation and more to the radiation dose. Tungsten, rhodium, and molybdenum are ideal for use in X-ray production due to their relatively high melting points than other metallic elements. Typically, dedicated mammography equipment is equipped with molybdenum target X-ray tube with beryllium window (port), and 30-mm thick molybdenum filter. The X rays, generated at 25–30 kVp tube potential, produced by the molybdenum target X-ray tube contain a large fraction of characteristic X rays at energies 17–20 keV (13,14). It is, therefore, quite natural to optimize and utilize these characteristic X rays for image formation and, consequently, patient exposure reduction at the same time. Most dedicated mammography equipment employ a combination of molybdenum target with aluminum, molybdenum, or rhodium filters. The characteristic X rays generated at the molybdenum target have X-ray photon energies of 17.4 and 19.6 keV, just below the K-absorption edge of 20 keV, thus are quite transparent to the molybdenum filter. This is illustrated in Fig. 3. On the left of Fig. 3a, is a schematic drawing of the X-ray spectrum generated at 30 kVp with a molybdenum target and aluminum filter. In the middle of Fig. 3b, is the same system with 30 mm thick molybdenum filter. Notice that the K-absorption edge curve (dashed curve) of molybdenum shows that the X-ray photons with energies just above

300 MAMMOGRAPHY

Relative Intensity

(a) Mo Target/Al Filter

1.0

Kα=17.4

0.8

0.6

Kβ=19.6

0.4

0.2

0

10 20 30 Photon Energy (keV)

Relative Intensity

(b) Mo Target/Mo Filter

1.0

0.8

K-edge=20.0

0.6

0.4

0.2

0

10 20 30 Photon Energy (keV)

Relative Intensity

(c) Rh Target/Rh Filter

1.0

Kα=20.2

0.8

Kβ=22.7 K-edge=23.2

0.6

0.4

0.2

0

10 20 30 Photon Energy (keV)

Figure 3. The spectrum of X rays generated with molybdenum and rhodium. On the far left (a) is the X-ray spectrum generated by a molybdenum target at 30 kVp and filtered with aluminum filter. The characteristic X rays Ka (17.4 keV), and Kb (19.6 keV) appear as the two peaks in the graph. In the middle (b) is the X-ray spectrum generated by the same X-ray system in part a, but is filtered with 30 mm thick molybdenum. The K-absorption edge curve is shown in dashed line. On the far right (c) is an X-ray spectrum generated by a rhodium target at 30 kVp and filtered with 20 mm thick rhodium. The general shapes of middle figure and far right figure are similar with differences in the peak energies of K-characteristic X rays (Ka ¼ 20.2 keV, and Kb ¼ 22.7 keV), and the K-absorption edge energy (23.2 keV).

20 keV would be preferentially absorbed than those just below 20 keV. Similarly, the same can be said for the rhodium target with rhodium filter as shown in Fig. 3c. Figure 3b and c are graphically speaking quite similar. It is noteworthy to point out that a careful study of Fig. 3b, and c will reveal that X-ray beams generated from rhodium target X-ray tube ‘‘must not’’ be filtered with molybdenum! The intensity of rhodium K-characteristic X rays (Ka, and Kb) would be in the energy range where the molybdenum K absorption is high. Taking advantage of the spectral information, in 1991,GE introduced the Senographe DMR unit equipped with a dual track and dual filter (molybdenum and rhodium) X-ray tube for mammography applications. Some of the physical characteristics and the spectral energy data of molybdenum, rhodium, and tungsten are summarized in Table 1.

For illustration purposes and descriptions of imaging components, following this paragraph, the screen-film mammography (SFM) system manufactured by GE, the Senogaphe DMR mammography unit, is shown in Fig. 4. The photograph in Fig. 4 represents the overall external

Table 1. K-Characteristic X Rays and K-Absorption Edge of Molybdenum, Rhodium, and Tungsten

	Molybdenum	Rhodium	Tungsten
Atomic Number	42	45	74
Kaa	17.4	20.2	59.3
Kba	19.6	22.7	69.1
K-edgea	20.0	23.7	69.5

aEnergy in keV.

and mechanical design of a typical ‘‘dedicated’’ X-ray mammography unit. It represents ‘‘the overall’’ design with respect to the tilting gantry with the X-ray tube housing at the top and the image receptor at the bottom. And, the gantry is attached to a column (or stand), which houses the elevation mechanism of entire gantry.

THE SCREEN-FILM MAMMOGRAPHY

The screen-film mammography employs the same basic image receptor system as conventional radiographic imaging. While conventional radiography employs a doubleemulsion film sandwiched between two intensifying \ screens yielding a spatial resolution of (up to) 8 lp mm 1 for a detailed screen (15), a typical SFM image receptor consists of a single-emulsion film and a single thin rareearth phosphor intensifying screen yielding a spatial resolution of 20 lp mm 1 (16).

In order to optimize the efficiency of the SFM, manufacturers including Agfa, Kodak, Konica, and Fuji, have produced matching pairs of the intensifying screen and film specifically for use with mammography. And, to further improve the sensitometric characteristics of the screen-film, the processing chemistry, particularly the developer, have also been carefully prescribed along with its development conditions. Note that the sensitometric characteristics of screen-film system refers to the ‘‘photographic effect or blackening effect’’ of the screen-film system in response to the X-ray absorption (17). An example of this matching pair of intensifying screen and film is depicted in Fig. 5, the emission and absorption

Figure 4. A typical dedicated mammography unit; GE Senographe DMR. (Courtesy of GE Healthcare.) The overall mechanical design of a typical dedicated X-ray mammography showing the tilting gantry with the X-ray tube housing at the top and the image receptor at the bottom. The gantry (or the elongated C arm) is normally attached to a column or a stand in which the elevation mechanism of entire gantry is housed.

characteristics of intensifying screen and film employed in SFM. Note that the emission of 550 nm wavelength light from AD Mammo Screens is matched by the absorption spectra of the AD-M Film.

					MAMMOGRAPHY				301
			Spectral characteristics
	1.0							20	transmittanceFilter
sensitivityRelative			AD-M film
	0.5						SLG-8U	10
		AD mammo screens
	0	400	450	500	550	600	650	0
	350							700

Wavelength (nm)

Figure 5. Spectral characteristics of screen-film mammography system. The AD Mammo screens are green-emitting and the AD-M film is orthochromatic. The figure shows the light-emitting spectrum of the AD Mammo Screens, the spectral sensitivity curve of the AD-M film, and the transmittance spectrum of SLG-8U safety light filter. (Courtesy of Fujifilm Medical Systems USA, Inc.)

In order to accommodate the varying size of breasts, two imaging cassette sizes are available with dedicated mammography equipment, they are 18 24 cm (8 10 in.), and 24 30 cm (10 12 in.). The film is loaded on the topside of the cassette with the emulsion side facing the intensifying screen. Figure 6 is a schematic drawing showing the crosssection of a typical screen-film mammography cassette (the single emulsion film, single intensifying screen) with enlarged views of the film (top, right), and the screen (bottom, right). The ‘‘sponge’’, in the cassette, is employed to assure good screen-film contact.

THE ANTISCATTER GRID

The antiscatter grid is placed between the breast holder, and the cassette slot, refer to Fig. 4. Scattered radiation is one of the main causes of degrading the image quality in X-ray imaging. The antiscatter grid is employed to minimize the scattered radiation from reaching the screen-film system while allowing the primary radiation to pass through. Although, mammography examinations are typically conducted with X-ray potentials <30 kVp, would still require a moving (reciprocating) antiscatter grid to cleanup the scattered radiation. Typically, the antiscatter grid used in mammography has a grid ratio of 5 : 1, or 4 : 1. While the exposure time in X-ray mammography imaging is relatively long ( 1 s), the speed of the moving grid must be

Protective coating

Antihalation layer

Cassette front

Film base

Film base	Lower emulsion
Emulsion	Upper emulsion
Intensifying screen	Protective coating
Screen base	Phosphor layer
Sponge	(Gd2O2:Tb)
	Fine grain
(for film-screen contact)
	Highly transparent binder
Cassette back	High packing density	Figure 6. The cross-sectional view of screen-
		film cassette. (Courtesy of Fujifilm Medical
	Screen base
		Systems USA, Inc.)

302 MAMMOGRAPHY

carefully adjusted to avoid artifact associated with grids. For the antiscatter grids, the grid line rate must be sufficiently high, or the attenuation material structure must be so designed to avoid being imaged as grid artifacts on the mammograms. The honeycomb design antiscatter grid; Cellular (HTC) Grid utilized in LoRad mammography systems and marketed by Hologic is well known for its high transmission of useful primary radiation with increased absorption of scattered radiation (18).

THE COMPUTED RADIOGRAPHY FOR MAMMOGRAPHY

Computed radiography was introduced to radiology in 1981 and the ‘‘digitization’’ of routine radiographic examinations had started in earnest. The CR image plate housed in cassette replaced, directly, the screen-film cassette in routine radiography applications. This direct replacement design required no mechanical modifications on the existing radiographic equipment. With the introduction of CR, a whole new set of technology including the optical CR readers, image processing software programs, image display subsystems, and so on, made the filmless radiology department within an achievable reality (19).

The difference between the routine CR and the CR for mammography (CR-M) is largely due to the optical response of the CR phosphor; thus, the radiation dose required to reduce the image noise, and the spatial resolution capability for mammography applications. In 2001, Fuji introduced the FCR 5000MA, which was used in the America College of Radiology Imaging Network (ACRIN); Digital versus Screen-Film Mammography (DMIST) study (20,21). While the study had already been concluded, the official results are in the final preparation stage, and have not been released yet. The FCR 5000MA differentiated itself from the previous generation of CR products by utilizing a 50 mm pixel spot size for the laser and introduced dual side IP reading to the market.

In January 2004, Fuji introduced the Profect CS, or ‘‘ClearView-CS’’ in the United States. Clear View CS is pending FDA approval and not yet commercially available in the United States. Fuji recently finished the Premarketing Approval (PMA) application to FDA, and the approval is anticipated sometime during the second half of 2005 (22). Note, however, that the 100 mm pixel spot size digital mammography system has been in use for the past few years in Europe, Australia, and Japan. The mechanism in which ‘‘how’’ the IP and the CR reader work together to produce an image is beyond the scope of this article and readers are suggested to turn to publications available (23,24), or corresponding articles in this Encyclopedia.

The obvious advantage of CR-M, and the full field digital mammography (FFDM) systems, is its wide dynamic range and its linear response to radiation. In screen-film mammography, over-or underexposure was one of the main causes of ‘‘repeat’’ examinations. Such exposure related repeats can be minimized due to the wide latitude in exposure acceptable for imaging. Furthermore, the images are no longer ‘‘fixed’’ or ‘‘limited’’; the subject contrast and the overall image brightness can be adjusted for image interpretation by adjusting the display window ‘‘level’’, and

			Comparison of dynamic range
	105		Imaging plate (IP)										3	Optical
	103
relativeIP's luminescence	104													density
	101												2
	102
													1
	100					Screen-film
			10–2		10–1 100			101		102
					Exposure (mR)

Figure 7. Comparison of dynamic range of CR and screen-film image receptors. (Courtesy of Fuzifilm Medical Systems, USA, Inc.) The image plate (IP) has wider linear response to radiation extends over four decades of dose and eliminates the inherent limitations of the ‘‘toe’’ and ‘‘shoulder’’ portions of the film’s H&D Curve. (Courtesy of Fujifilm Medical Systems USA, Inc.)

‘‘width’’. The display ‘‘window level’’ corresponds to the film optical density in SFM, and the ‘‘widow width’’ corresponds to the contrast. In digital imaging systems, both window level and width may be adjusted to optimize the image rendered on the monitor. Figure 7 shows the response differences of these two image receptor systems to the radiation exposure. The exposure range for the CRs ‘‘Image Plate’’ is wider than that of a typical screen-film combination by an order of 2 to 3 in magnitude.

THE STEREOTACTIC BREAST BIOPSY MAMMOGRAPHY

Breast biopsy is often required when an area of suspicious malignancy site is revealed after a mammography is obtained. In order to accurately extract the specimen, a stereo mammogram may be obtained so that the biopsy needle can be accurately inserted to the site where it is suspected of malignancy; core biopsy (25). Most dedicated mammography equipment are designed to perform routine (screen-film, or FFDM) mammography, and with an attachment to perform stereotactic mammography and core biopsy in an ‘‘up right’’ posture; either the patient is standing or sitting on a chair.

From a pair of stereo images separated by 308 ( 158 from the centerline), using the triangulation technique, the three-dimensional (3D) coordinates of the suspicious site can be calculated within an accuracy of 1 mm (25), see the schematic diagram of stereotactic imaging geometry in Fig. 8. It is essential that the patient remain still before and after the acquisition of the stereo images. The core biopsy can be localized accurately only if the patient positioning remain the same.

Stereotactic mammography requires that the breast being examined is compressed (just as in routine mammography), but the patient should remain standing still while the mammogram is being processed. The processing time alone takes at least 90 s with typical automatic film processor. Thereafter, the best location of the biopsy needle entrant site, and the coordinates of the sampling must be determined. The prolonged time represents substantial

+/– 15°

Z

Y

X

Compressed breast

Breast holder

Imaging plane/film

Right image Left image Target

Figure 8. Schematic drawing of geometrical setup for stereotactic mammography. From the stereo images, with known geometry and the X–Y coordinates, the depth in Z direction is calculated via triangulation.

discomfort on the part of the patient not to mention the anxiety of fear for the possibility of being diagnosed and confirmed as having breast cancer.

DIGITAL SPOT MAMMOGRAPHY

Before the introduction of FFDM, in order to ease the discomfort of undergoing the biopsy process, a small detector with imaging area of 5 5 cm had been developed by LoRad, for example, so that the stereo images of the breast can be displayed immediately after exposures are made. This is one of the successful biomedical applications that have resulted from collaborative technology transfer programs between the National Aeronautics and Space Administration (NASA), the National Cancer Institute (NCI), and the U. S. Department of Health and Human Services Office on Women’s Health (OWH) (26). Since the imaging area is only 5 5 cm, it is referred to as Digital Spot Mammography. The biopsy coordinate is then calculated via the built-in software program with a shorter overall examination time.

In addition, dedicated breast biopsy systems have been developed where the patient would be lying on her stomach (recumbent). The breast under examination is positioned through a hole in the examination table and aligned with the X-ray equipment and the biopsy needle gun. The recumbent core biopsy mammography unit manufactured by LoRad is depicted in Figs. 9 and 10. LoRad is now one of the brand names sold under Hologic. A similar recumbent core biopsy unit, called Mammo Test Biopsy System, is available from Fisher Imaging.

FULL FIELD DIGITAL MAMMOGRAPHY

The CR-M is a direct replacement of the screen-film cassette. In other words, the IP cassette replaced the screen-

MAMMOGRAPHY 303

Figure 9. The recumbent stereotactic core biopsy system. A recumbent design provides a stable patient positioning to acquire a pair of stereo images for accurate localization of biopsy site while the patient lays flat on her stomach with comfort during the entire examination. (Courtesy of Hologic.)

film cassette. Therefore, no major mammography equipment modification would be necessary. In DR on the other hand, just as in conventional radiography and fluoroscopy applications, the image receptor system may be built into the image receptor compartment as an integral part of the imaging assembly (27,28). Manufacturers had attempted to physically fit the DR detector assembly, or more accurately; the Flat Panel (FP) detector assembly, into the space occupied by the SFM cassette. To date, no commercial product of FP detector assembly has been fitted as a direct physical replacement of the SFM cassette is available (29). In other words, due to technical difficulties, there is no FP detector assembly that is sufficiently compact to fit into the space designed to accommodate the SFM cassette. The image acquired with FP detector is transmitted to and processed by the image processing chain thereafter.

While there are a hand full of FFDM systems either under development or manufactured for clinical applications, three FFDM units are currently available in the United States, and are described here (27). The fact that

Figure 10. The zoomed view of the recumbent stereotactic core biopsy system. In the zoomed view, the breast under the examination is positioned through the opening in the tabletop. (Courtesy of Hologic.)

304 MAMMOGRAPHY

Figure 11. Schematic drawing of an amorphous silicon digital detector. (Courtesy of GE Healthcare.)

these three systems are designed to cover an imaging field of at least ( ) 18 24 cm, the smaller cassette size of SFM, the name; FFDM was assigned. This is to signify the full size imaging filed coverage as opposed to the small field detector of LoRad’s Digital Spot Mammography, or Fischer Imaging’s Mammo Test Biopsy System.

There is a trend that mammography equipment is being transformed to ‘‘digital’’ format. One reason of the slow pace of this transformation is the large initial capital expenditure of FFDM. Both, analog and digital formats are expected to coexist for many more years to come. The impact of Fuji’s CR-M cannot be ignored due to the fact that it is possible to convert and replace the screen-film cassette directly. Any of the existing SFM units will be able to join the ‘‘digital era’’.

Currently, there are three FFDM systems that have received FDA approval and are sold in the United States. The GE Senogaphe 2000D was approved by FDA on January 28, 2000 and ‘‘accredited’’ by ACR in February, 2003. Fischer Senoscan received its FDA approval on September 25, 2001 with subsequent ACR accreditation in August, 2003. Hologic/LoRad’s Selenia received FDA approval and ACR accreditation, respectively, on October 2, 2002, and in September of 2003 (27).

GEs FFDM: SENOGRAPHE 2000D, AND SENOGRAPHE DS

Figure 11 depicts the FP detector developed by General Electric Medical Systems, and the photo inset (top left) shows the Cesium–Iodide (CsI) scintillator crystals. The incident X rays are absorbed by the CsI crystals, which in turn, convert the X-ray photon energy to light. The CsI crystals are deposited in columnar shape so as to minimize the scatter, and optical diffusion of scintillation lights from one column to the other. The underlying photodiode– transistor amorphous Silicon (a-Si) arrays is connected to control data lines, then, converts the light to electrical signals for further processing (30).

GEs FP detector is an indirect-conversion digital detector system. The detective quantum efficiency (DQE) of this FP detector system has been shown to be 60% at Zero Frequency (31,32). Essentially, being the first FFDM system introduced to the commercial market, GEs Senographe

2000D ushered in the digital mammography era to the radiology community. Depicted in Fig. 12 is the secondgeneration FFDM unit, Senographe DS, with the X-ray gantry set to þ 158 with the stereotactic biopsy needle assembly attached. Both Senographe systems (2000D and DS) are equipped with CsI scintillator on an a-Si photodiode–transistor arrays with pixel size of 100 mm and an image matrix size of 1920 2304, covering a field of view 19 23 cm.

FISCHER IMAGING’S SENOSCAN FFDM

On the other hand, Fischer Imaging Corporation’s answer to the FFDM is the Senoscan FFDM unit. The unique feature of Senoscan unit is that it employs a slot mechanism for the X-ray filed collimation that is synchronized to the detector as the slot and detector assembly sweeps across the imaging field, see Fig. 13. The detector of

Figure 12. Photograph of GE FFDM unit; Senographe DS with stereotactic attachment installed. The gantry is tilted to þ158 and prepped for Stereotactic imaging. Note, this is the FFDM version of similar unit shown in Fig. 1, but with zoomed up view for the stereotactic attachment. (Courtesy of GE Healthcare.)

MAMMOGRAPHY 305

X-ray tube

Swing arm pivot point

Filter/collimator

assembly

.88 in							84 in
	14.5 cm					Detector	10.5 cm
	2977cmi

121.69cmin

Figure 13. A schematic drawing of the scanning mechanism of Fischer Senoscan FFDM. (Adapted from Fisher Imaging Sanoscan user’s manual with permission. Courtesy of Fischer Imaging.)

Senoscan unit consists of a layer of thallium activated CsI scintillator connected to charge-coupled-device (CCD) chips via fiber optics (33). Hence, this is also an indirectconversion digital detector system.

The use of slot mechanism also means ‘‘restricting’’ the X-ray beams for exposure. It offers an advantage of less scattered radiation for improved image contrast (34). Therefore, no antiscatter grid is necessary with Senoscan unit. The absence of the antiscatter grid compensate for a less efficient use of the available X rays. In addition, a tungsten anode X-ray tube (35) is employed with this unit, since the tungsten anode X-ray tube would produce X rays more efficiently than a molybdenum or rhodium target. Senoscan is equipped with CCD chips having pixel size of

(1) 27 mm, covering a field of view 11 15 cm under the high resolution mode, and (2) pixel size of 54 mm, covering a field of view 19 29 cm for the normal mode. The image matrix size of this system is 4096 5625.

HOLOGIC/LORAD’S FFDM; SELENIA

Selenia’s image receptor is a 250 mm thick amorphous Selenium (a-Se) photoconductor. Underneath a layer of a-Se photoconductor is a thin-film transistor (TFT) arrays that serves as an active readout mechanism. The TFT arrays are typically deposited onto a glass substrate, which provides a physical support for the entire detector components. Selenia uses a 250 mm thick a-Se photoconductor to capture the X-ray photons impinging on the detector sur-

face without the aid of a scintillator. It is said that a thickness of 250 mm a-Se photoconductor is adequate to stop 95% of the X-ray photons in the mammographic energy range (36). The photon energy is converted to a pair of electron, which is negatively charged, and a positively charged ‘‘hole’’. With bias voltage applied, the signal is read off by the TFT arrays. Thus, this is a directconversion digital detector system. The detector is an a-Se photoconductor and TFT arrays with pixel size of 70 mm, covering a field of view 24 29 cm, resulting in an image matrix size of 3328 4096. A summary of the detector characteristics for these three FFDM systems is given in Table 2, (37).

READING MAMMOGRAPHY IMAGES

Initially, the FFDM images were printed on dry laser printers and read on high luminance view boxes (ACR accreditation required of a luminance of > 3000 cd m 2) (38). All three FFDM systems are equipped with workstations where images are soft-copy read. The workstations are commonly equipped with two 5-megapixel high resolution monitors. With the soft-copy reading, an assortment of image manipulation are possible including, but not limited to, basic window width and window level adjustments, zooming, image reversal, and so on for viewing the details of the pathology. More importantly, the impact of image processing in transforming the acquired raw data set to the image displayed for soft-copy reading should be recognized (39). For example, as can be seen in Fig. 14, substantial differences not only in the brightness and contrast of the image but also in the impression of pathology in the images can be noticed. While all three images are acceptable and diagnostic, the two enhanced images are easier to recognize various details.

SUMMARY

A brief history of mammography was described as the introduction to the X-ray mammography. The SFM continues to play its major role in breast cancer detection while FFDM is gaining its installed base. Upon the anticipated FDA approval, the CR-M is poised for introduction for clinical applications in the second-half of 2005. However, all three modalities employed in breast cancer detection, namely; the SFM, FFDM, and the CR-M are expected to

Table 2. Summary of Imaging Characteristics of FFDM Units

Manufacturer	Model Name	Scintillator		Detector	Pixel Size, mm Image Matrix Size				Imaging Size, L W cm
GE	Senographe 2000D, DS	CsI (Tl)a		TFT	100	1920	2304		19		23
Fisher Imaging	Senoscan	(Tl)		CCD	24/48	4096		5625	22		30
Hologic/LoRad	Selenia	CsI b		TFT	70	3328			25		29
		a-Se					4096
Fuji	DSM	CsI (Tl)	c	CCD	48	1025	1024		5		5
	CR-M	BaFBr(Eu)		Computed	50	1770	2370		18		24
				radiography		2364	2964		24		30

aCsI(Tl) ¼ Talium Activated Cesium Iodide. ba-Se ¼ Amorphous Selenium.

cBaFBr(Eu) ¼ Barium Fluorobromide with Europium.

306 MAMMOGRAPHY

Figure 14. Comparison of ‘‘processed’’ images. The unprocessed image (a) can be dramatically improve its appearance with ‘‘Thickness Equalzation’’ processing (b), or the ‘‘Premium View’’ processing (c). The impact of image processing is quite evident. (Courtesy of GE Healthcare.)

play their roles in contributing to reduction of breast cancer deaths through the early detection of mammography screening.

Display of FFDM images and its workstations are important components of the total picture of FFDM (40,41). However, monitors and workstations associated with diagnostic radiology imaging including mammography are in a domain of their own and no attempt is made to include these subjects in this article. Such subject matters belong to digital image processing and display. Finally, archiving of the acquired digital images is also a very important aspect of digitized diagnostic images in the overall operation of radiology. However, this subject is better handled in Picture Archiving and Communication Systems (PACS), and readers are refered to articles under PACS for additional information.

ACKNOWLEDGMENT

The information and materials presented here had been provided by numerous numbers of representatives and scientists from respective manufacturers mentioned in the main text. The author would like to express his thanks and sincere appreciation by listing the manufacturers here, (in alphabetical order) in lieu of listing the individuals who had provided their supports: General Electric Healthcare, Fischer Imaging Corporation, Fujifilm Medical Systems USA, Inc., and Hologic Inc. (LoRad).

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