Geometrical rejection

This section describes the techniques which reduce the amount of scattered radiation that reach the detector using physical geometry.

Anti-scatter grids

60 Chapter 3. Scatter and its effects on mammograpghy

(a) (b)

(c)

Figure 3.15: The geometry used to study the source of scatter in a realistic mammography scenario is shown in (a), where black arrows have been drawn to illustrate possible scattering paths from the compression paddle and breast support into the image receptor. (b) represents a 2D surface plot of the total SPR across the image receptor. Profiles of the SPR from the various layers included in the simulations along the vertical white line are shown in (c).

3.5. Scatter reduction techniques 61

film-screen and FFDM. They were introduced in section 2.4.5 in conjunction with the typical parameters used to study their performance (BF , CIF , Tp and Ts).

The two anti-scatter grid designs available in mammography are linear and cellular. The linear grid is the preferred geometry employed by most of the manufacturers (e.g. Siemens, GE, etc.). However Hologic mammography systems use a cellular geome- try [59].

Cellular anti-scatter grids, in general, have been found to outperform linear designs as they provide a higher CIF and lower BF than linear ones [47, 58, 61]. This increase in CIF is achieved by the larger scatter absorption from the cellular geometry of the grid. On the other hand, the reduction in BF can be due to the higher Tp associated

to a less dense septa material, copper, and low grid ratios (r) of 3.8, which are lower in comparison with the usual ratio of 5 observed in linear anti-scatter grids. Furthermore, interspace material can play an important role, as the attenuation of primary photons is lower in air than in the materials employed by linear anti-scatter grids.

Table 3.3 illustrates data published by Boone et al. [61] to show the performance of two anti-scatter grids (linear and cellular). They have been calculated using a 60mm thick breast phantom representing 50% adipose 50% glandular tissue and an X-ray beams set at 28kVp Mo/Mo target/filter combination.

Table 3.3: Sample anti-scatter grid parameters calculated by Boone et al. [61] for a 60mm thick breast phantom of 50% glandularity using an X- ray energy spectrum of 28kVp Mo/Mo. The linear and cellular anti-scatter grids have been previously described by Rezentes et al. [58].

Anti-scatter design Grid ratio r Septa material SPR CIF BF Tp(%)

No Grid — — 0.69 — — 100

Linear Grid 5 Lead 0.21 1.40 1.83 77

Cellular Grid 3.8 Cooper 0.10 1.54 1.70 91

It is observed that the cellular anti-scatter grid provides a better scatter rejection (i.e. lower SP R), which is translated into a higher CIF . The decrease of the BF is due to the higher Tp observed. Note that the Tp value is always less than 100% as the septa

material, interspace material and grid covers absorb a small amount of the primary photons as they pass through the grid.

Air gap

It has been seen that the anti-scatter grid is the most common geometric scatter re- jection technique. However, an alternative scatter rejection method is to resort to using a large air gap [60, 102]. These are found mainly in paediatric, magnification mammography and sometimes in chest radiography [102].

In section 3.4.4 it was described that the scattered radiation was slowly reduced by

air gap distances up to 30mm. However, large air gaps are required to achieve a

considerable scatter reduction. With large air gaps, the scatter intensity is reduced following the inverse square law such that much of the scattered X-ray photon flux leaves the system without interacting within the detector.

62 Chapter 3. Scatter and its effects on mammograpghy

However, the use of large air gaps has drawbacks. The magnified image obtained

will suffer from larger focal spot geometric blurring, reduction of the FOV and the dose transfered to the breast is also increased [61, 131]. Large air gaps are used in mammography when the magnification of a specific region within the breast is required, and in this case a fine focal spot is used to reduce geometric unsharpness.

Other techniques

Several alternatives to anti-scatter grids and air gaps have been investigated in the literature.

˚

Aslund and Cederstr¨om [114] studied scatter reduction when using multi slit digital mammography. A multi slit is comprised of two collimators. One is placed above the breast (pre-collimator), while the second one is located below the breast (post- collimator). If a perfect alignment of both collimators is achieved, the Tp is 100%.

Moreover, results have suggested that this geometry produces the lowest SP R values, where the largest source of scattered X-ray photons is the image receptor [114]. In a slot-scan geometry, a collimator is placed near the X-ray tube to match the area of a narrow image receptor. Then the X-ray tube rotates, together with the narrow image receptor, thus the entire breast is scanned. Slot-scanning has a similar CIF than anti-scatter grids, however, the breast dose is lower [132] as Tp approaches to 100%.

This was confirmed by Boone et al. [61], who found that slot-scan produces lower BF than cellular anti-scatter grids while maintaining the same CIF .