Lecture 16: The Moly-Cow

Question 1

Describe the process in which Technetium builds up for use in hospitals.

Answer 1

1) Molybdenum-99 has a half life of 67 hours and decays to Technetium-99.
2) As the Mo-99 decays, Tc-99m builds up.
3) When the activities are equal, the Tc-99m is removed and used for nuclear medicine studies.

Question 2

What is the half-life of Tc-99m?

Answer 2

~ 6 hours

Question 3

What is the equation for the decay of Mo-99?

Answer 3

N_1 = number of Mo-99 molecules
t = time
λ_1 = decay constant

Question 4

What is the equation for the build up of Tc-99m?

Answer 4

N_1 = number of Mo-99 molecules
N_2 = number of Tc-99m molecules
t = time
λ_1 = Mo-99 decay constant
λ_2 = Tc-99m decay constant

Question 5

Describe the graph that represents the Moly-Cow being ‘milked’

Answer 5

Question 6

Why is Tc-99m a good radioisotope?

Answer 6

1) It is primarily a gamma-emitter, and gamma-photon energy is ideal.
2) The half-life is short (so well notated ton scans).
3) It can be attached to interesting molecules.
4) It is cheap.

Question 7

Define gamma scintigraphy

Answer 7

The use of a gamma-camera to produce projection image of radioisotope distribution.

Question 8

A collimator is used in gamma scintigraphy for _______ ________.

Answer 8

Spatial discrimination.

Question 9

What detects gamma rays in gamma scintigraphy?

Answer 9

A sodium iodide crystal, backed by a photo-multiplier tube.

Question 10

A gamma camera is the ________ equivalent of the planar x-ray.

Answer 10

Functional

Question 11

What does the gamma camera image?

Answer 11

Organ function

Question 12

Describe a collimator

Answer 12

A hexagonal grid of lead with partitions known as septa. It defines the spatial resolution of a gamma camera.

Question 13

What is the equation for the spatial resolution of a gamma camera?

Answer 13

R = resolution
l = collimator thickness
d = diameter of septa
z = distance to sample

Question 14

How is the resolution of a gamma camera increased?

Answer 14

• Smaller diameter of septa
• Larger collimator thickness
• Smaller distance to sample (this is limited by patient size)

Question 15

Define Single Photon Emission Computed Tomography (SPECT)

Answer 15

An adapted for of gamma scintigraphy in which the gamma-camera is rotated 360º around the patient, collecting a projection at each orientation. This allows for 3D functional images to be produced using back projection reconstruction.

Question 16

How are quantitative measurements made using SPECT?

Answer 16

1) The signal is measured at the region of interest.
2) A signal is also measured in a control ROI to correct for background radiation.
3) The uptake of tracer in the ROI is expressed as a % of the administered activity.
4) Corrections for radioactive decay are made.
5) Graphs of activity over time are produced.

Question 17

Give an example of what an activity graph would look like for the absorption and emission of a radiotracer

Answer 17