Co

Fig. 3.2. Geometry of a mammography x-ray tube.

Image Receptor

Fig. 3.2. Geometry of a mammography x-ray tube.

dimension of a 24 x 30 cm cassette. For a 60 cm SID, this requires an effective target angle of 22 degrees. In turn, this can be achieved by using an x-ray tube with a 22 degree target or by appropriately tilting an x-ray tube with a smaller target angle (e.g., a six degree tilt of a 16 degree target).

The effective size of the x-ray tube focal spot (feff) is another important consideration in mammography and issues related to feff have been the topic of continuing debate since before the advent of dedicated mammographic systems. Detailed discussions have been provided by a number of authors over the past few decades (Braun, 1979; Gabbay, 1994), including the effect of the intensity distribution of the focal spot on geometric unsharpness (Nickoloff et al., 1990). Along with the sharpness of the image receptor, the feff will determine the limiting resolution of the imaging system given the system geometry (SID and patient support to image-receptor distance). Considering the need to resolve microcalcifications that may be present in the breast, the feff needs to be small enough to minimize blur.

Modern mammographic screen-film combinations are capable of resolving 20 cycles mm1 (ACR, 1993). Their noise properties, however, usually restrict visualization in biological structures to about 10 cycles mm1 and below (Kratzat, 1988). As a consequence, in order to insure that the focal spot will not cause excessive blur, it has been recommended that the feff be able to resolve 12.5 cycles mm1 (AAPM, 1990). Because of the imaging geometry in mammography, the feff is largest at the chest wall and decreases anteriorly (Figure 3.3). Thus, the feff should meet the 12.5 cycles mm1 requirement at the chest wall. Furthermore, the blur due to the focal spot is greatest for objects such as microcalcifications at the top surface of the breast, farthest from the image receptor (Figure 3.4). Therefore, the 12.5 cycles mm1 requirement must be met for an object in the plane located at the breast entrance surface.

It can be directly determined if a mammographic system meets this criterion by imaging a resolution pattern placed near the chest-wall edge of the x-ray field, 4.5 cm above the plane of the breast support (i.e., at the typical location of the breast entrance surface). However, when considering the resolving capabilities of mammographic systems, it has been traditional to write specifications in terms of the feff and to measure performance by measuring feff. For contact mammography, the conditions mentioned above (resolving 12.5 cycles mm1 at the chest wall for an object at the entrance surface of the breast) are met when the effective focal-spot size meets the following criterion (AAPM, 1990):

Cathode

Fig. 3.3. Variation in focal-spot shape and size with position (Barnes, 1999).

Cathode

Fig. 3.3. Variation in focal-spot shape and size with position (Barnes, 1999).

where, the magnification M = SID/(SID - 5) and SID - 5 is the distance from the focal spot to the entrance surface of the breast (assuming contact mammography, a 4.5 cm breast thickness, and a 0.5 cm separation between the patient support and the image-receptor plane). The effective focal-spot sizes required to meet these conditions are listed in Table 3.4 for several SIDs.

This approach involves a number of complications. For example, the feff will generally be larger than the "nominal" focal-spot size listed in the unit's specifications. The National Electrical Manufacturers Association (NEMA, 1992) has set standards for focal-spot size and specified how x-ray tube focal spots may be labeled. As indicated in Table 3.5, the measured dimensions of the focal spot (length and width) are allowed to exceed the "nominal" (labeled) focal-spot size by up to 50 percent in width and length and by 100 percent in length when the nominal size is 0.3 mm or greater.

Fig. 3.4. Limiting resolution as a function of distance from the image-receptor and focal-spot size (Haus, 1999b).

TABLE 3.4—Effective focal-spot size (feff ) required at various SIDs (12.5 cycles mm-1 at 5 cm above the image receptor).

SID (cm) feff (mm)a

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