Filters, Collimators and Grids

Introduction

Recall the two mechanisms by which diagnostic x-rays are produced:

• Bremsstrahlung radiation is a continuous spectrum of x-ray photons formed from the energy loss of decelerated electrons as they bombard a dense target.
• Characteristic x-ray production begins with the ejection of inner K-shell electrons from the target atom due to the absorption of kinetic energy from incident electrons which exceed the binding energy for that inner shell electron. An outer shell electron then fills the orbital vacancy and in the process emits a photon with discrete energy level equivalent to the energy difference.

The resultant unfiltered spectrum is a continuous range of photon energies with a maximum determined by kVp (set by the operator/radiographer/technician) and sharp spikes of intensity at energy levels corresponding to the characteristic x-rays. Various kinds of filtration then changes the shape of the spectrum.

1. Filtration

Filtering is the process of removing low energy x-rays from the beam spectrum which has low penetrative power and therefore contributes to patient dose (particularly skin/entrance surface dose) and degrades image quality due to scatter.

Beam hardening refers to the preferential removal of lower-energy photons from a polychromatic beam with filtration.

1.1. Inherent filtration

The materials which comprise the x-ray tube assembly attenuate the beam as photons exit the tube and this is referred to as inherent filtration. This includes;

• target anode (via anode heel effect)
• glass envelope
• insulating oil
• housing exit window

Importantly, this level of filtration can not be controlled, hence it’s inherent effect.

1.2. Anode heel effect

Due to the anode angle, x-rays produced deep within the anode material which are emitted at angles perpendicular to the incident path of the electron beam will be attenuated by more anode material. This anode heel effect results in reduced x-ray beam intensity at the anode end of the x-ray tube.

Factors affecting the anode heel effect include:

• anode angle: the magnitude of the effect depends on the angle. By increasing the angle, the amount of target material perpendicular to the anode is decreased resulting in less attenuation of x-rays produced.
• source-to-image-detector distance: Increasing the SID reduces the heel effect by allowing more divergence of the x-ray beam which consequently produces a more uniform image.
• field size: Decreasing the field size using collimators to absorb peripheral variations will achieve a more uniform beam.

The anode heel effect can be used strategically in practice by positioning the patient such that thicker anatomy is at the cathode side of the x-ray tube (e.g. in mammography, the denser chest wall region will be closer to the cathode side)

1.2. Added filtration

Removable and interchangeable metal sheets which vary in thickness and type of metal (e.g. aluminium 2 – 3 mm) are used to remove low energy Bremmstrahlung radiation via the photoelectric effect.

Source: EPA Radiation Guideline 6: Post 2015 equipment

1.3. Total filtration

Total filtration (inherent + added filtration) must be 3mm Al for a 80kVp generator

Filtration reduces x-ray intensity (quantity) and quality (shape of beam spectrum), but not the maximum energy of the x-ray beam spectrum.

The total filtration can be inferred from a measurement of the Half-Value-Layer (HVL) or IPSM 64 can be used a lookup table.

Half Value Layer

A material’s half-value layer (HVL), or half-value thickness, is the thickness of the material at which the intensity of radiation entering it is reduced by one half.

The HVL is inversely proportional to the linear attenuation coefficient (LAC) of a material. The LAC is the probability that a photon interacts (absorbed or scattered) per unit length it travels in a specific material. Hence, the greater the LAC, the lower the HVL of the material.

1.4. Compensating filters

Used to achieve a more uniform exposure when imaging areas of large tissue density variation.

e.g. wedge filters variations in tissue or tissue density along longitudinal plane, trough filters for chest radiography plane of transverse axis

Limits on filter thickness

Reduced Image Quality for minimal dose reduction

If too much filtration is used then there will be a decrease in radiographic contrast.
– Hardened spectrum: Less Photoelectric interactions

Increased tube loading, longer exposure times and increased risk of motion artifacts.

Image Quality Vs. Radiation Dose balance

Absorption edges: K-edge filtering

Recall that the photoelectric effect occurs only when the incident x-ray exceeds the binding energy of the inner shell electron. Just above this level, materials have absorption edges corresponding to a sudden increase in the chance of PE interacting. These edges correspond to the discrete energies at which an electron can be ionised/excited from a certain atomic shell. For example, the K-edge, corresponds to the energy required to eject a K-shell (most inner shell) electron and ionize the atom. However, the probability of PE absorption decreases rapidly as the photon energy (E) further increases above the k-edge, and is proportional to:

PE ∝ Z3/E3

In mammography the K-edge Filter
• Aim to achieve a mono-energetic spectrum:
– Use a filter with a selective k-edge (e.g Mo @ 20 keV)
– Lower Brem. removed which will only contribute to dose
– Higher energy photons removed just above k-edge
– Significant proportion of Mo/Mo spectrum is in 17.4 – 19.6 keV

Contrast Agent K Edge filter

Iodine and Barium can be introduced in the patient to improve contrast
by manipulating their k-edges:
• Increase Z of target tissue and increase PE effect interaction
probability
• As the effective beam kV increases to above the k edge of iodine,
more absorption occurs and better contrast with surrounding soft
tissue
• Match the spectra to k-edge of contrast agent

Scatter

Ideal image contrast:
– Unattenuated photons interact in the detector
– Photons that interact in the patient are either
absorbed via PE or interact via Compton and the
scattered photon is not incident on the detector

Compton scattered photons reach detector

Scatter decreases contrast and increases noise

Factors affecting scatter

Increase patient size/anatomy

Increased field size

Increase kVp

a) ↑ Proportion of photons that
Compton scatter
b) Scattered photons are more
penetrating – escape patient and
reach detector

Effect on image quality
– increasing of blurring
– loss of contrast
– Increased noise
• Effect on patient dose
– Backscatter leads to increasing superficial dose (skin in
fluoroscopy / breast in AP chest)
Possible reduction through :
 Collimating tightly to area of interest
 limitation of the irradiated volume (e.g.: breast
compression in mammography)
 use of a scatter reduction technique e.g. grids

Contrast Degradation Factor (CDF) (ideally ~ 1):
CDF=1/(1+S/P)
where: S/P: ratio of the scattered to primary radiation

Collimator

Light field radiation field congruence test

Exposure of detector with
radio-opaque markers set to
edge of light beam

2. Anti-scatter Grids

X-ray absorbing septa aligned with the focal spot of the beam are separated by a series of radiolucent interspaces (Aluminium (Z = 13, ρ = 2.7 g/cm3) or Carbon Fibre (Z = 6, ρ = 1.8 g/cm3) – higher primary transmission)

Focal spot alignment ensures primary photons contributing to the images traverse unimpeded through the radiolucent interspaces and scattered photons are absorbed by the septa.

• Grid Characterisation Parameters
  • Grid Ratio (most important factor determining
  • Focal Length
  • Grid Frequency
  • Bucky Factor
  • Primary and Scatter Transmission Factors

Grid ratio = height of grid (h)thickness of interspace material (D) 

Grid cut off 

• Undesireable attenuation of primary beam due to malalignment
• Occurs at higher grid ratios

Air Gap Technique

• An air gap technique is where the patient is
  deliberately moved away from the image receptor.
  • The distance used is about 20-30 cm.
  • A smaller proportion of the scattered photons strike
  the detector – ↑ Contrast
  • Image is magnified so therefore a larger FFD has to
  be used : ~ 3 m to 4 m
  • This technique is less effective at the higher
  energies as the scattered radiation is more forward.
• proportional to the air gap size
  • Increase dose to patient
  because of Inverse Square
  Law between patient and
  receptor
  • Image is magnified so need to
  use longer FDD
  • Not used routinely – used to
  be used for some chest
  imaging