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Power Efficiency Guide

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3.1.1 Introduction

While a variety of x-ray units have been used in mammography since its inception (Bassett et al., 1992; Gold, 1992; Vyborny and Schmidt, 1989), it is now widely recognized that quality mammog-raphy requires a dedicated mammographic x-ray unit (ACR, 1993; DHHS, 1987; Haus, 1990; Yaffe, 1991). In order to meet the stringent imaging needs of mammography such a unit must be equipped with a variety of essential features discussed in this Section. These include a small focal spot coupled with a relatively long source-to-image-receptor distance (SID) to minimize blur; a low energy x-ray beam and a specialized mammographic grid to provide high subject contrast; and specialized equipment for firm, uniform compression. Without these features, it is almost impossible to visualize small nonpalpable masses and very small microcalcifications, often the only indications of early carcinoma. Use of non-dedicated radiographic equipment can result in missing many cancers and can lead to unwarranted biopsies, and is prohibited under MQSA (1992).

A number of authors have described the need for and features of dedicated, specially designed, mammographic equipment (AAPM, 1990; NCRP, 1986). A review of these descriptions indicates that there are a number of features that should be incorporated into a dedicated mammographic unit. Probably, the most comprehensive description of the features of a dedicated mammographic unit is that prepared by ACR (1993) and is frequently cited below. Another summary of these issues appears in Seminars in Breast Disease (Haus, 1999a).

The minimum set of features for an acceptable dedicated unit has been set by MQSA (1992) regulations. These regulatory requirements are outlined in Table 3.1.

In establishing these requirements FDA drew heavily on the ACR document mentioned above (ACR, 1993). The requirements of the final regulations apply to all mammography units under the purview of MQSA (1992), whether they are used for screening or diagnostic ("problem-solving") mammography.

Table 3.1—MQSA equipment requirements.

Adapted from MQSA (1992) Section 900.12(b) Equipment [also published in CFR Title 21, Chapter 1, Part 900-Mammography, Subpart B-Section 900.12 (b) Equipment]

(b) Equipment. Regulations published under Secs. 1020.30, 1020.31, and 900.12(e) of this chapter that are relevant to equipment performance should also be consulted for a more complete understanding of the equipment performance requirements.

(1) Prohibited equipment. Radiographic equipment designed for general purpose or special nonmammography procedures shall not be used for mammography. This prohibition includes systems that have been modified or equipped with special attachments for mammography. This requirement supercedes the implied acceptance of such systems in Sec. 1020.31(f)(3) of this chapter.

(2) General. All radiographic equipment used for mammography shall be specifically designed for mammography and shall be certified pursuant to Sec. 1010.2 of this chapter as meeting the applicable requirements of Secs. 1020.30 and 1020.31 of this chapter in effect at the date of manufacture.

(3) Motion of tube-image receptor assembly.

(i) The assembly shall be capable of being fixed in any position where it is designed to operate. Once fixed in any such position, it shall not undergo unintended motion.

(ii) The mechanism ensuring compliance with paragraph (b)(3)(i) of this section shall not fail in the event of power interruption.

(4) Image receptor sizes.

(i) Systems using screen-film image receptors shall provide, at a minimum, for operation with image receptors of 18 x 24 centimeters (cm) and 24 x 30 cm.

(ii) Systems using screen-film image receptors shall be equipped with moving grids matched to all image receptor sizes provided.

(iii) Systems used for magnification procedures shall be capable of operation with the grid removed from between the source and image receptor.

(5) Light fields. For any mammography system with a light beam that passes through the x-ray beam-limiting device, the light shall provide an average illumination of not less than 160 lux (15 foot candles) at 100 cm or the maximum source-image receptor distance (SID), whichever is less.

(6) Magnification.

(i) Systems used to perform noninterventional problem solving procedures shall have radiographic magnification capability available for use by the operator.

(ii) Systems used for magnification procedures shall provide, at a minimum, at least one magnification value within the range of 1.4 to 2.0.

(7) Focal spot selection.

(i) When more than one focal spot is provided, the system shall indicate, prior to exposure, which focal spot is selected.

(ii) When more than one target material is provided, the system shall indicate, prior to exposure, the preselected target material.

(iii) When the target material and/or focal spot is selected by a system algorithm that is based on the exposure or on a test exposure, the system shall display, after the exposure, the target material and/or focal spot actually used during the exposure.

(8) Compression. All mammography systems shall incorporate a compression device.

(i) Application of compression. Effective October 28, 2002, each system shall provide:

(A) An initial power-driven compression activated by hands-free controls operable from both sides of the patient; and

(B) Fine adjustment compression controls operable from both sides of the patient.

(ii) Compression paddle.

(A) Systems shall be equipped with different sized compression paddles that match the sizes of all full-field image receptors provided for the system. Compression paddles for special purposes, including those smaller than the full size of the image receptor (for "spot compression") may be provided. Such compression paddles for special purposes are not subject to the requirements of paragraphs (b)(8)(ii)(D) and (b)(8)(ii)(E) of this section.

(B) Except as provided in paragraph (b)(8)(ii)(C) of this section, the compression paddle shall be flat and parallel to the breast support table and shall not deflect from parallel by more than 1.0 cm at any point on the surface of the compression paddle when compression is applied.

(C) Equipment intended by the manufacturer's design to not be flat and parallel to the breast support table during compression shall meet the manufacturer's design specifications and maintenance requirements.

Table 3.1—continued.

(D) The chest wall edge of the compression paddle shall be straight and parallel to the edge of the image receptor.

(E) The chest wall edge may be bent upward to allow for patient comfort but shall not appear on the image.

(9) Technique factor selection and display.

(i) Manual selection of milliampere seconds (mAs) or at least one of its component parts (mA) and/or (s) shall be available.

(ii) The technique factors [peak tube potential3 in kilovolt (kV) and either tube current in mA and exposure time in seconds, or the product of tube current and exposure time in mAs] to be used during an exposure shall be indicated before the exposure begins, except when automatic exposure controls (AEC) are used, in which case the technique factors that are set prior to the exposure shall be indicated.

(iii) Following AEC mode use, the system shall indicate the actual kilovoltage peak (kVp)a and mAs used during the exposure. The mAs may be displayed as mA and seconds.

(10) Automatic exposure control.

(i) Each screen-film system shall provide an AEC mode that is operable in all combinations of equipment configuration provided, e.g., grid, nongrid; magnification, nonmagnification; and various target-filter combinations.

(ii) The positioning or selection of the detector shall permit flexibility in the placement of the detector under the target issue.

(A) The size and available positions of the detector shall be clearly indicated at the x-ray input surface of the breast compression paddle.

(B) The selected position of the detector shall be clearly indicated.

(iii) The system shall provide means for the operator to vary the selected optical density from the normal (zero) setting.

(11) X-ray film. The facility shall use x-ray film for mammography that has been designated by the film manufacturer as appropriate for mammography.

(12) Intensifying screens. The facility shall use intensifying screens for mammography that have been designated by the screen manufacturer as appropriate for mammography and shall use film that is matched to the screen's spectral output as specified by the manufacturer.

(13) Film processing solutions. For processing mammography films, the facility shall use chemical solutions that are capable of developing the films used by the facility in a manner equivalent to the minimum requirements specified by the film manufacturer.

(14) Lighting. The facility shall make special lights for film illumination, i.e., hot-lights, capable of producing light levels greater than that provided by the view box, available to the interpreting physicians.

(15) Film masking devices. Facilities shall ensure that film masking devices that can limit the illuminated area to a region equal to or smaller than the exposed portion of the film are available to all interpreting physicians interpreting for the facility.

From MQSA (1992) Section 900.12(e)(5)(x)—Radiation Output (x) Radiation output.

(A) The system shall be capable of producing a minimum output of 4.5 mGy air kerma per second [513 milliroentgen (mR) per second] when operating at 28 kVp in the standard mammography (moly/moly) mode at any SID where the system is designed to operate and when measured by a detector with its center located 4.5 cm above the breast support surface with the compression paddle in place between the source and the detector. After October 28, 2002, the system, under the same measuring conditions shall be capable of producing a minimum output of 7.0 mGy air kerma per second (800 mR per second) when operating at 28 kVp in the standard (moly/moly) mammography mode at any SID where the system is designed to operate.

(B) The system shall be capable of maintaining the required minimum radiation output averaged over a 3.0 second period.

Adapted from MQSA (1992) Section 900.12(e)(5)(iii)(A)—System Resolution

(A) System Resolution.

(1) Each x-ray system used for mammography, in combination with the mammography screen-film combination used in the facility, shall provide a minimum resolution of 11 cycles/millimeters (mm) (line-pairs/mm) when a high contrast resolution bar test pattern is oriented with the bars perpendicular to the anode-cathode axis, and a minimum resolution of 13 line-pairs/mm when the bars are parallel to that axis.

(2) The bar pattern shall be placed 4.5 cm above the breast support surface, centered with respect to the chest wall edge of the image receptor, and with the edge of the pattern within 1 cm of the chest wall edge of the image receptor.

(3) When more than one target material is provided, the measurement in paragraph (e)(5)(iii)(A) of this section shall be made using the appropriate focal spot for each target material.

aIn this Report, the name used for this quantity is "operating potential," expressed as "kilovolt peak (kVp)" (see Glossary).

There are numerous features recommended by experts that go beyond the minimum set of features required by MQSA (1992). All of the features discussed below are highly desirable for dedicated mammographic units, particularly if the unit is used for both screening and diagnostic or problem-solving mammography. This is especially true if the unit is the only dedicated mammography unit available in a facility. All of these features are summarized in the tables at the end of each subsection.

Before purchasing a dedicated mammographic unit, there are several steps that should be undertaken. The unit's specifications should be reviewed in comparison with the critical features and specifications described below. Current owners of the unit(s) (make and model) under consideration should also be questioned with respect to the adequacy of its performance in each of these critical areas. The radiologist should also review both grid and magnification images of dense or difficult to compress breasts that have been imaged on the unit(s) under consideration. For this purpose, images should be obtained from competent radiologic colleagues rather than through the unit's manufacturer.

3.1.2 Mechanical Assembly and General Considerations

The mammographic unit should rigidly support the x-ray tube housing and image-receptor support device at opposite ends of a C-arm or similar assembly. The C-arm should be designed to allow continuous rotation to permit views to be obtained in various projections with the patient either erect or seated. The system should allow the technologist to rotate the C-arm 180 degrees relative to the vertical axis in one direction and at least 120 degrees and preferably 50 degrees in the other direction (ACR, 1993). This range of angulation allows for both routine and specialized projections, including the reverse CC view in which the breast must be compressed from below. It also insures that the technologist will always be able to compress the breast perpendicular to the long axis of the pectoralis major muscle in the MLO view and will therefore be able to include the posterior portion of the breast on the image despite differences in patient body build.

While it should be possible to position the C-arm of the mammo-graphic unit to achieve any degree of obliquity (continuously variable angulation), detents at the common positioning angles, such as 0, 30, 45, 60 and 90 degrees, on either side of vertical should also be provided to help the technologist achieve reproducible positioning. The degree of angulation of the C-arm should be indicated on the unit and should be easy to read from any position on either side of the image-receptor support.

The C-arm should be designed so that the technologist can move it high enough to accommodate a tall patient and low enough for a patient in a wheelchair. There should be room enough under the image-receptor support and counterweight for a patient's legs if they are in a wheelchair or need to be seated for the examination. In general, this will require a range of vertical motion such that the center of the image-receptor support can be positioned from 66 to 140 cm above the floor for both CC and lateral views (ACR, 1993). If only standing patients need to be accommodated, the range of vertical motion can be from 97 to 140 cm. In addition, if a patient is in a wheelchair, if they must remain seated, or if they can stand but cannot move their feet easily for different views, it is more convenient for the technologist to move the C-arm side-to-side and in-and-out in a longitudinal or transverse motion from the main body of the unit. The unit should allow this flexibility. The unit should permit the technologist to perform more than one function at a time. For example, the technologist should be able to raise the C-arm vertically at the same time that they are lifting the compression paddle.

Controls for adjusting the position and height of the C-arm and for rotating it should be readily accessible to the technologist who must use these frequently throughout the mammographic examination. The unit should be equipped with mechanical, motorized or electromagnetic locks to fix the C-arm in any required position or orientation (ACR, 1993). These locks should be strong enough to prevent motion of the C-arm when the patient leans on the unit. The locks should be released by hand or foot controls and should not release in the event of a power failure.

Motorized controls for compression should be accessible on both sides of the C-arm, as well as being foot controlled, to allow for easy positioning on standard and specialized views. The control for releasing compression should also be on the C-arm, to permit quick release if the patient is feeling faint or suddenly feels that she can no longer tolerate the compression. In the event of a power failure, the compression should be released automatically. The switch for the light field should also be readily accessible, located on both sides of the C-arm or else positioned centrally where it is easy to reach.

A bar support should be available on each side of the C-arm for the patient to grip. Such a support is especially useful after the technologist has raised the patient's arm for the oblique views. The bar should extend to the height of the tube head and below the image-receptor support, so that the patient can reach it easily during positioning for any view. This bar is a necessity for assisting the patient in supporting themselves, thereby, minimizing motion. When the bar is used for CC positioning, the patient should be able to reach it without stretching. The control buttons should not be on the bar support where the patient might accidentally grasp them.

The smaller the tube housing the better. If the tube housing projects toward the patient's head, the technologist will have difficulty including the patient's chest-wall tissue on the image in the CC view. A large tube head will also interfere with positioning for magnification, especially on the CC view. Moreover, on a superola-terial-to-inferomedical oblique view, the patient's shoulder will bump into the tube head. In addition, the tube housing should be equipped with a plastic face shield. This device should be designed to prevent the patient's face or hair from projecting between the x-ray tube and the breast and should not overlap the imaging field.

The image-receptor support device should be designed to hold the cassette firmly in place with the front edge of the cassette at the chest wall and a minimum of space between the cassette edge and the chest wall (ACR, 1993). There should be <2 mm movement side-to-side and the device should be tight enough to prevent movement of the cassette during an exposure. In addition, the image-receptor support device must be designed to limit the x-ray transmission through the support to no >0.876 pGy air kerma (0.1 mR) for any exposure (CDRH, 2002b).

A radiation shield should be provided to minimize operator exposure. The operator exposure should not exceed 5 mSv y1 (ACR, 1993). Given reasonable assumptions concerning workload and technique [6.58 mGy air kerma (750 mR)] per exposure, four exposures per patient, 40 patients per day, 5 d week1, scattered radiation at the entrance of the shield equal to 0.001 times the exposure at the breast entrance surface, a shield with an attenuation equivalent to 0.08 mm of lead at 35 kVp is appropriate to meet this standard (ACR, 1993). The shield should extend from <15 cm above the floor to a height of 1.85 m. The width of the shield should be sufficient (at least 0.6 m) to provide reasonable assurance that the technologist will not be exposed during the conduct of an examination. If the shield is movable, there should be interlocks to prevent exposure when the shield is not in place. The exposure controls should be designed so that the operator cannot make an exposure when outside the shielded area.

The unit should also provide a means for recording on the patient's images, identifying information concerning the patient, the facility where the film was taken, and the technologist who took the film. Information concerning patient position (view, angulation, etc.) and the appropriate technique variables (target-filter combination, operating potential, milliampere seconds, compressed breast thickness, compression force, etc.) should also be included. See Table 3.2 for a summary of desirable characteristics of the mechanical assembly.

Table 3.2—Summary of the desirable characteristics of the mechanical assembly (ACR, 1993).

C-arm

• continuous rotation (+180 degrees - 120 degrees (preferably -150 degrees)

• accurate angulation indicator available

• controls readily accessible

• small tube housing with plastic facial shield

Locks

• strong enough to prevent C-arm motion

• should not release in the event of a power failure

Compression

• released by hand or foot controls plus controls on C-arm

• release automatically with power failure

Image-receptor support device

• able to be positioned from 66 to 140 cm (97 to 140 cm standing patients only)

• holds the cassette firmly in place

• minimum "dead space" at chest wall

• <2 mm cassette movement side-to-side

• limit the x-ray transmission to <0.876 pGy (0.1 mR) for any exposure Radiation shield

• attenuation equivalent to 0.08 mm of lead at 35 kVp extending from <15 cm above the floor to a height of 1.85 m, width sufficient to insure technologist not exposed

Recording system

• means of recording patient and technique information directly on film

3.1.3 X-Ray Source Assembly

A dedicated screen-film mammographic unit should have an x-ray tube with a molybdenum target and a thin beryllium window (1.5 mm thickness or less) together with an added molybdenum filter [sufficiently thick to meet the minimum half-value layer (HVL) requirements of CDRH (2002b) (ACR, 1993; AHCPR, 1994)]. This combination of target, window and filter materials has been shown to provide excellent contrast for the detection tasks present in mammography when the appropriate operating potential (<28 kVp) is employed (Beaman and Lillicrap, 1982; Feig, 1987; Jennings et al., 1981). The x-ray beam from such a system has the low-energy characteristics required to achieve high subject contrast for breasts of average density and thickness. This is due to the 17.5 and 19.7 keV characteristic x rays from the molybdenum target and the strong suppression of the spectrum at energies >20 keV because of the k-shell absorption edge of the molybdenum filter (Figure 3.1a). Inordinate amounts of filtration in the x-ray beam from a glass window or excess filtration or otherwise inappropriate added filtration would have significant negative consequences (AAPM, 1990; Yaffe, 1991). Not only would beam quality be increased resulting in a loss of subject contrast, but also tube output would be reduced, resulting in increased exposure time. Longer exposure times could lead to problems with patient motion and higher patient doses due to film reciprocity law failure. Adjustments could be made to reduce exposure time (e.g., increasing operating potential, using a higher milliampere and consequently, a larger focal spot, using a faster image receptor, etc.) but, each would have its own negative consequences for image quality (reduced contrast, increased blur, increased noise, respectively).

Alternative target and filter combinations may be employed, if they provide equivalent contrast-detail perceptibility at equal or reduced patient dose. For example, tubes with molybdenum targets and rhodium filters (Figure 3.1b), as well as those with rhodium targets and rhodium filters (Figure 3.1c), and tubes with tungsten targets and rhodium added filration (Figure 3.1d) have been used successfully in imaging patients (Beaman and Lillicrap, 1982). Such combinations are most effective in patients with larger or denser breasts. In such patients, these units can produce both better image quality and lower patient dose. In systems where the filter can be varied (for example, a molybdenum target with both molybdenum and rhodium filters), the type of filter in use should be displayed on the unit.

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