Attenuation Correction

Question 1

Photon attenuation

Answer 1

• absorption and scattering interactions
• degree of attenuation depends on: photon energy, thickness of tissue, linear attenuation coefficient

Question 2

Attenuation coefficients

Answer 2

Linear attenuation coefficient
* Symbolized by μ or μl
* Expressed in cm-1
Mass attenuation coefficient
* Symbolized by μ/p or μm = μl/p
* Expressed in cm2/g

Question 3

Narrow-beam geometry

Answer 3

• Narrows acceptance of scattered photons
• Requires narrow aperture
• Absorbed halfway between source and detector

Question 4

Broad-beam geometry

Answer 4

• Allows acceptance of scattered photons

Question 5

Photon depth

Answer 5

• attenuation increases with depth
• apparent decrease in deep vs. superficial activity

Question 6

Attenuation correction

Answer 6

• AC can be applied before, during, or after reconstruction
• methods: arthmetic mean, geometric mean, chang, transmission (ct, rod source)

Question 7

Arithmetic and geometric mean

Answer 7

• correction based on the path length of photons from origin to the body surface
• opposing views are combined using arithmetic mean or geometric mean
• 360 acquisition
• a constant attenuating medium is assumed
• AC factor is applied prior to reconstrution

Question 8

Chang method

Answer 8

• body contour defined by ROI
• AC factors calculated based on distance from ROI, then applied to each pixel
• a constant attenuating medium is assessed
• performed post-reconstruction

Question 9

Transmission methods

Answer 9

• an external source of radiation is used to measure tissue attenuation
• radionuclide and CT options
• transmission scan generates an AC map
• AC factors based on the LAC of the tissue
• AC map is applied to the emission data to correct for attenuation
• applied prior to FBP and incorperated into IR

Question 10

RN Tranmission Scans

Answer 10

• Blank/reference scan performed prior to transmission scan
• Transmission and emission scans are acquired using the same detector system
• Simultaneous vs. sequential scans
• Transmission scan performed with flood source, line source(s), or moving line source
• Typically, long-lived sources (e.g., 153Gd)
• Must take the difference between photon energies into account

Question 11

CT transmission scans

Answer 11

• Best method of AC in current practice
• Transmission and emission scans are
  acquired sequentially
• CT coefficients need to be adjusted to
  reflect the difference in x-ray and photon
  energies

Question 12

Attenuation artifacts

Answer 12

• Without AC, artifacts include:
• Hot rim
• Areas of decreased uptake
• False positives/perfusion defects may result
• With AC, artifacts include:
• Motion
• Truncation
• Contrast/metal

Question 13

Scatter correction

Answer 13

• Scattered photons of up to 50 degrees have up to a 50% chance of acceptance
• Scatter will increase the counts in the ray sum

Question 14

Method 1 scatter correction

Answer 14

• use asymmetric window

Question 15

Method 2

Answer 15

• Compton scattering window subtraction
• Uses two PHA windows
• Subtract the second window from the first, then reconstruct