Nuclear medicine (only RT relevant applications) and PET

Question 1

What are the indications which lead to a bone scan?

Answer 1

Staging of a cancer known to metastasis
Investigate bone pain in cancer
Investigate treatment response
Loosening/infection of a joint replacement

Question 2

What is a typical protocol for a bone scan?

Answer 2

Inject with 600MBq of Tc-99m HDP
Image 2-3 hours later
Image the whole body with LEHR collimators

Question 3

What is a typical protocol for a parathyroid SPECT scan?

Answer 3

Inject 800MBq of Tc-99m MIBI
Static image at 10mins and 2hrs
SPECT at 2hrs
Perform low dose CT

Question 4

In bone scans what uptake would suggest tumourous disease?

Answer 4

Single sided uptake, away from joints and other known areas of degenerative uptake

Question 5

What are the two PET tracers used for oncology purposes?

Answer 5

F-18 FDG

Fluorothymidine (FLT) - used for assessing tumour response

Question 6

Why is exact attenuation correction possible for PET?

Answer 6

The attenuation along a known line of response is independent of the position of the emission along that line

Question 7

Why use quantification?

Answer 7

Lesion characterisation
Response assessment
Data reduction in trials and statistical analysis
Dose optimisation
Testing drug targetting
RT target identification - gives functional target volumes

Question 8

What is the equation for the standard uptake value?

Answer 8

SUV = Activity concentration/(Injected dose/body weight)

Question 9

What are the three measures of body weight and why would each be used?

Answer 9

Body mass - most common
Lean body mass - more consistent across range of body habitus for FDG but more complex to measure
Body surface area - tallies with some dosing regimes

Question 10

What are the three measures of activity concentration?

Answer 10

SUVmax
SUVmean
SUVpeak - the hottest mean value in a small given volume

Question 11

What is the correction to the SUV equation for the existing blood glucose level?

Answer 11

Gluc(mmol/l)/5(mmol/l)

Question 12

What are the common errors when calculating the SUV and what are their effects?

Answer 12

Incorrect cross-calibration between dose calibrator and scanner - systematic error equal to the relative cross calibration
Residual activities in administration system unaccounted for - lower net administration than used to calculate SUV so get lower SUV
Incorrect decay correction - incorrect SUV
Tissued injection - incorrect SUV

Question 13

What are the technical considerations for the SUV calculation and what effect do they have?

Answer 13

Aquisition parameters - affect SNR, get upward bias with low SNR
Image reconstruction - affect convergance, partial voluming effects are worse when convergance is insufficient
ROI strategy - type and size of ROI can change SUV
Normalisation factor and glucose factor used - BSA, BM, and LBM are not comparable, need to use glucose factor for accuracy
Contrast agents - produces attenuation map errors so can effect SUV values

Question 14

What are the patient factors that need to be considered for SUV calculations and what effect do they have?

Answer 14

Blood glucose level - can reduce uptake if high
Uptake period - different point on uptake curve therefore different SUV
Patient comfort - FDG uptake in muscle and brown fat higher due to discomfort
Inflammation - false positively increased SUV
Patient motion/breathing - causes mismatch of attenuation correction, may also smear out counts so get lower SUV

Question 15

What SUV calculation did the PERCIST trial use and why?

Answer 15

SUV peak and lean body mass

Gave hottest SUL for the baseline and follow-up lesions

Question 16

What were the four PERCIST classifications?

Answer 16

Total metabolic response - can’t distinguish lesion at baseline from bloodpool at follow-up
Partial metabolic response - drop in SUL >30% and at least 0.8 in absolute terms
Stable metabolic disease - no change
Progressive metabolic disease - increase in SUL>30% and of at least 0.8 in absolute terms

Question 17

Why use compartmental models?

Answer 17

Derived kinetic parameters may more closely describe pathophysiology

Question 18

What corrections are needed to calculate the activity concentration?

Answer 18

Randoms correction
Normalisation
Dead time correction
Scatter correction
Attenuation correction
PET scanner calibration

Question 19

What is the cause of ‘randoms’ and what is the equation to estimate the rate of randoms per second?

Answer 19

Two uncorrelated single detection events happen within sufficient temporal activity to fall within trues detection window
Cij = 2.tau.ri.rj

Question 20

What are causes of the variable sensitivities which normalisation accounts for?

Answer 20

Axial data summing and 'mashing'
Detector efficiency
Geometric and solid angle effects
Time window alignment
Structural alignment
Septa

Question 21

How is direct normalisation achieved?

Answer 21

Illuminate all possible lines of response with a positron source
Analytical correction is made for non-uniform radial illumination
Normalisation coefficients including all effects are taken to be inversely proportional to the counts acquired in each LOR

Question 22

What are the issues with direct normalisation?

Answer 22

Long acquisition times needed for good quality normalisation matrix
Needs excellent uniformity
Quantity and distribution of scatter is rarely comparable to that of a patient