Topic 15: Attenuation and scatter

Question 1

Survival probability

Answer 1

Probability that a photon is not scattered (“attenuated”).

Question 2

What is the survival probability for a thin layer of material.

Answer 2

a1 ~ 1-mu*d=exp(-mu*d)

Question 3

For 2 thin layers what is the survival probability?

Answer 3

a1*a2 = exp(-mu1*d1)exp*(-mu2*d2)

Question 4

what is the survival probability for a thick layer of mixed materials?

Answer 4

a = exp

Question 5

Attenuation modelling in SPECT why can the measured data not be precorrected for attenuation?

Answer 5

because the different points on a LOR have different attenuation, which is a problem for FBP but not for other types likes MLEM AND OSEM.

Question 6

How do we approximate the attenuation correction in SPECT using the “Chang Method”

Answer 6

1. Calculate the average attenuation factor for each point in the object/ patient
2. Calculate the correction factor for each point as correction = 1/ mean attenuation.

Question 7

Attenuation correction in PET can it be precorrected?

Answer 7

It can be precorrected because all points on the LOR have the same attenuation.

Better for FBP - still not a problem for MLEM/OSEM/etc.)

Question 8

How do we measure the amounts of attenuation?

Answer 8

SPECT transmission techniques. Measure alpha to find mu.

Question 9

Uses of CT with SPECT or PET

Answer 9

Anatomical localisation

Attenuation correction

Quantification

Complementary diagnostic information