The Filter

·         Filtration is the process of shaping the X-ray beam to increase the ratio of photons useful for imaging to those photons that increase patient dose or decrease image contrast.

·         Overall effect is an increase in the mean energy of the XR beam

·         Increase contrast and decrease patient dose

·         In a radiological examination the x-ray beam is filtered at three different levels:

1.      The x-ray tube and its housing (inherent filtration)

·         A Beryllium window may be used to decrease filtration

2.      Sheets of metal placed in the path of the beam (added filtration)

·         Ideally, absorb low-energy photons, transmit high-energy photons

·         Utilize the photoelectric effect (attenuation) to do this – by selecting a material with appropriate Z#.

·         Single filter or compound (consisting of 2 or more layers of different materials with higher Z# material faces XR tube)

·         Filtration expressed using the equivalent of aluminum of a certain thickness

·         Filtration varies with thickness (increase thickness – increase filtration, but over a certain thickness for certain kVp increases in thickness may not have any significant advantage over lower thickness)

o   <50 kVp: 0.5 mm Al

o   50 - 70 kVp: 1.5 mm Al

o   >70 kVp: 2.5 mm Al

·         See fig 6-1 C: pg 89

·         Increase filtration (increase mean energy): Decrease exposure dose

·         Major disadvantage of filtration is reduction in the intensity of the XR beam (see fig 6-1 C: pg 89)

·         Compensation filters (e.g Wedge filters; Trough filters; Bow-tie filters in CT) obtain films of more uniform density when the object part thickness changes from one side to the other

·         K-edge filters (heavy metal filters) make use of the K-absorption edge of elements with Z# greater than 60 which offers advantages when imaging barium and iodine

o   Barium and iodine offer greatest contrast when they absorb the XR efficiently

o   K-edge filters produce an XR beam which has a high number of photons in the specific energy range of the barium or iodine k-absorption edges.

o   Fig 6-3 and 6-4 C: 91

o   Molybdenum filters are use specifically in mammography

3.      The patient

·         The polychromatic x-ray beam from the tube contains a high number of low energy photons

·         Low-energy photons have a negligible chance of getting through the patient, thereby contributing to patient dose, but adding nothing to the image

·         Some of the very low energy photons are stopped as they exit the tube by the glass window (inherent filtration)

·         Beryllium provides very little filtration and is used as a window in mammography (where low-energy photons are needed)

·         Filters may also be added to increase the filtration effect – these are designed to preferentially absorb the low energy photons

·         Added filtration does not affect the maximum energy of the x-ray beam spectrum

·         Added filtration will always reduce the x-ray tube output