PROjECTION RADIOGRAPHY
6.3.5. Other methods of scatter reduction
g ng
CIF 1 SF 1 SF
= −
− (6.21)
where, sfg and sfng are the scatter fractions with and without the grid.
table 6.3 gives the bucky factor and cif for a range of grids for the commonly used lucal patient equivalent phantoms.
6.3.4.3. Grid selection
from the data in table 6.3, it is clear that for each radiographic projection there is an optimum grid ratio that will provide adequate scatter reduction, with an acceptable increase in dose to the patient. for example, chest and abdomen projections on adults would require ratios of 10:1 or 12:1. Practically, however, the grid is permanently fitted to the cassette holding device in radiographic tables and wall mounted devices, and these generally have a ratio of at least 10:1.
consequently, grid use is generally far from optimized.
6.3.4.4. Grid artefacts and alignment
there are several possible misalignments that will lead to artefacts in projection images. additionally, a damaged grid will generate artefacts and must be replaced. figure 6.11 illustrates the possible misalignments of the grid. in practice, it is possible for a number of these to be present at once. note that the moving grid is laterally decentred during operation, although the degree of offset is small on average.
table6.3. bucky factoR anD cif at the centRe of the iMage RecePtoR foR a selection of RaDiogRaPhic gRiDs, using a 25.4 cm × 25.4 cm X Ray fielD With the lucal chest anD abDoMen Patient eQuiValent PhantoMs, 120 kV foR the chest Data anD 70 kV foR the abDoMen Data
grid ratio sf sPR bucky factora cif
lucal chest phantom
no grid 0.390 0.640 — 1.0
6:1 0.188 0.231 1.53 1.33
8:1 0.150 0.176 1.62 1.39
10:1 0.123 0.141 1.69 1.44
12:1 0.103 0.115 1.75 1.47
16:1 0.076 0.082 1.85 1.51
lucal abdomen phantom, soft tissue region
no grid 0.712 2.472 — 1.0
6:1 0.371 0.588 2.79 2.18
8:1 0.302 0.433 3.20 2.42
10:1 0.256 0.345 3.53 2.58
12:1 0.221 0.283 3.81 2.71
16:1 0.174 0.211 4.27 2.87
lucal abdomen phantom, spine region
no grid 0.837 5.155 — 1.0
6:1 0.472 0.894 — 3.25
8:1 0.388 0.636 — 3.76
10:1 0.327 0.486 — 4.14
12:1 0.276 0.382 — 4.45
16:1 0.203 0.254 — 4.91
a the low bucky factor is due to the scatter level in chest radiography being low, as the lungs act as a large air gap, and the thickness of other tissues is relatively low. the bucky factor for the abdomen is constant for each grid.
FIG. 6.11. Grid misalignments leading to cut-off.
6.3.5.1. Collimation
section 6.3.2.2 describes the effect of X ray field size on scatter fraction.
since smaller field sizes reduce the scatter fraction, it is good practice to collimate the X ray beam to as small an area as possible for each projection, thereby improving image quality and reducing patient dose. this requires radiographers with a good knowledge of anatomy and good judgement of the wide variety of patients to ensure that the region of clinical interest is included in each image, without needlessly irradiating tissues that are not of interest. care is required not to collimate too tightly, which would increase the possibility of the region of clinical interest being cut off, resulting in a repeated exposure and increased dose.
6.3.5.2. Compression
the data in section 6.3.2.3 clearly demonstrate the increase in scatter fraction with patient thickness. therefore, if the patient thickness can be reduced during exposure, by applying a compression band for example, then the amount of scatter will be reduced. this has further benefits, because a shorter exposure
time can be used, reducing movement blur and patient dose. alternatively, the kV setting may be reduced, improving the contrast or snR in the image. for these reasons, among others, compression is routinely used in mammography (see section 9.3.2).
6.3.5.3. Air gap
as described in section 6.2.4, the use of an increased oiD, or air gap, results in magnification and a reduction in scatter fraction. this is because the divergent scattered rays will be increasingly less likely to strike the image receptor as the oiD is increased, and are therefore much less likely than the primary rays to strike the image receptor.
the effect of the oiD on the scatter fraction is illustrated in fig. 6.12.
FIG. 6.12. Effect of air gaps (OID) on scatter fraction for a range of FIDs. Xp is the source–patient exit distance. PMMA phantom of 20 cm thickness, X ray field size of 20 cm × 20 cm, 90 kV.
6.3.5.4. Scanning slit
as explained in section 6.3.2.2, the scatter fraction depends on the X ray field size. slot scanning systems take advantage of this to minimize the scatter fraction by using a very small area X ray field with a slit aperture, which must be scanned across the patient to produce the image. such systems generally
feature a pre-patient collimator to define the fan shaped beam, and a post-patient collimator to intercept any scattered radiation produced.
for digital systems, the area detector may be replaced by silicon or similar strip detectors, in which case the post-patient collimation is not required. these systems may feature a single fan beam scanned across the region of interest, or a number of fan beams. the latter allows for faster scanning and shorter exposure times. the basic principle is illustrated in fig. 6.13. these systems are capable of good scatter rejection without the necessity for a grid, but require smooth and precise movement of the collimator systems and stable X ray generator performance. there is increased probability of artefacts compared with conventional methods, and the longer exposure time required increases the risk of movement blur in the images and reduces the life of the X ray tube.
FIG. 6.13. Principle of operation of a multislit scanning system for scatter reduction. The image receptor and post-patient collimator may be replaced by strip detectors in digital systems.
REFERENCES
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