nuclear medicine

p.3 What are the two types of imaging? Describe what they are.

Anatomical (structure: size, shape, and density) and Functional (biochemical or physiological activity).

p.3 What is functional (molecular) imaging?

Visualization, characterization, and measurement of biological processes at the molecular and cellular levels in humans and other living systems.

p.3 What type of imaging is nuclear medicine?

Functional imaging.

p.4 Briefly describe the steps of nuclear medicine.

Radioactive tracer injected in blood stream (intravenously).
Radiotracer accumulates in specific organs.
Distribution of radiotracer in body is imaged.
Speed and location of radiotracer uptake analyzed to determine tissue health.

p.4 What the three nuclear medicine modalities?

Planar Scintigraphy, SPECT, PET

p.4 What does SPECT stand for?

Single Photon Emission Computed Tomography

p.4 What does PET stand for?

Positron Emission Tomography

p.5 What does CT stand for?

Computed Tomography

p.5 What is a radiotracer?

A chemical compound in which one or more atoms have been replaced by a radioisotope.

p.5 What is a radioisotope?

Atoms from the same element whose nuclei have a different number of neutrons with an unstable atomic nucleus (imbalance between neutrons and protons) --> spontaneous change in nucleus composition.

p.5 Can you make any radiotracer?

Yes, but only a few are approved by the FDA.

p.5 What happens why the number of neutrons is larger than the number of protons? Give an example.

Neutron eliminates electron in excess to become a Proton.
Carbon 14 (16 p⁺, 8 n) --> Nitrogen 14 (7 p⁺, 7 n) + β⁻ emission

p.5 What happens why the number of protons is larger than the number of neutrons? Give an example.

Proton becomes Neutron when a Neutron emits positron in excess.
Carbon 11 (6 p⁺, 5 n) --> Boron 11 (5 p⁺, 6 n) + β⁺ emission

p.5 What is a positron?

A particle with an electron mass that is positively charged.

p.5 Generally, is the number of protons or neutrons larger?

Generally, neutrons > protons

p.6 What is a half-life, t1/2?

Time for radioactivity to drop to one-half of its value.

p.6 What is the equation for half-life?

t1/2 = ln 2 / λ
where λ is the decay rate

p.6 How does the effective half-life differ from the regular half-life?

The effective half-life should consider elimination through biological processes (the biological half-life, t1/2, bio)

p.6 What is the equation for the effective half-life? (he prob won't ask this but its good to recognize ig)

t1/2, eff = (t1/2 * t/2, bio) / (t1/2 + t1/2, bio)

p.6. What is the equation for the number of radioactive nuclei at any point in time?

N = N₀ ^(-λt)
where N₀ is the nuclei at t=0

p.7 What is the most widely used radiotracer for scintigraphy? Why?

Technetium, ⁹⁹ᵐTc

• Meta stable state exists with long half-life (

p.7 What are some examples of positron emission radiotracers? Why are they chosen?

¹¹C (t1/2 = 20.4 min), ¹³N (t1/2 = 9.96 min), ¹⁵O (t1/2 = 2.03 min), ¹⁸F (t1/2 = 109.8 min), ¹²⁴I (t1/2 = 4.15 days)

• Chemistry known for substitution of H or OH
• Short half-life (minimize radiation dose)
• Require a cyclotron and fast labelling chemistry

p.8 Describe a technetium generator and how it works.

• Alumina ceramic column with radioactive ⁹⁹Mo absorbed on its surface
• Housed in a lead shield
• Saline drawn through the column, to which ⁹⁹ᵐTc (from ⁹⁹Mo decay) binds
• Radiotracer is collected and purity determined
• Generator is replaced every week.

p.9 Describe a cyclotron and how it works.

• Accelerates hydrogen atoms to 3/4 the speed of light.
• Radioisotopes are then produced in a cyclotron by high energy bombardment of heavy water.
• The radioisotopes are then used to prepare radiotracers.

p.10 Describe a gamma camera and how it works.

• Device used to image gamma radiation emitting radioisotopes.
• ⁹⁹ᵐTc radiotracer is injected and accumulates (sodium pertechnetate NaTcO₄ for GI, other radiotracers with bond to Tc targeting specific organs: Ceretec, Sestamibi)
• It will emit gamma rays from the body.

p.10 What does the production of gamma rays tell us?

That nuclear interaction has happened.

p.10 What is the difference between x-rays and gamma rays in terms of interaction?

X-rays = electron interaction, gamma rays = nuclear interaction.

p.11 What allows for specific gamma rays to be detected?

Collimator

p.11 Approximately how many gamma rays are not detected by a collimator?

99.9%

p.11 What is the geometry of a collimator used for breast and cardiac imaging?

Parallel and tilted in the same direction.

p.11 What is the geometry of a collimator used for magnification?

Converging towards the body.

p.11 What is the geometry of a collimator used for imaging the whole body?

Diverging towards the body.

p.11 What is the geometry of a collimator used for small organs?

Pinhole.

p.12 What does the resolution depend on?

The collimator geometry and the array of photomultiplier tubes.

p.12 What are the two ways to make a larger field of view?

Make the collimator broader, or make it narrower (it would broaden further out).

p.12 How does a broader collimator affect resolution?

Broader collimator = larger field of view = lower resolution.

p.12 How does a scintillator work?

Converts gamma rays to light --> the scintillation crystal hit by rays will emit visible light (thallium-activated sodium iodide).

p.12 What converts gamma rays to light?

Scintillator.

p.12 What converts light to electric signals?

Photomultiplier tube (PMT).

p.13 Whenever a scintillation even occurs, the PMT closest produces the largest current. True or false? Explain.

True. Spatial resolution = dimensions of the PMT

p.13 How do we fix the problem of large currents due to scintillation close to PMTs?

We use the fact that adjacent PMTs produce smaller output currents. Comparing the magnitudes of the currents, we get better location of the scintillation.

p.14 What is an anger logic circuit?

Resistors connected to the output of each PMT.

p.14 What is a PHA?

Pulse Height Analyser. It determines which events correspond to primary radiation.

p.14 How does digitizing voltage give us an energy spectrum?

Since the amplitude of the voltage pulse from PMT is proportional to the energy of the gamma ray.

p.15 What happens when radioactivity is increased?

Larger counts detected.

If radioactivity is extremely high, it can overwhelm the detector systems, leading to saturation or "paralysis," where the system cannot accurately count or measure radiation.

p.15 There is very little background radiation. What does this tell us?

There is no noise, so very high contrast (for the gamma camera). There would be large amounts of scattering -- so we need collimators (but it reduces resolution).

p.15 What is energy resolution of the system usually defined as?

Full-width-half-maximum (FWHM) of the photopeak.

p.15 Why should we consider the dead time of the system?

If an injected dose is large, the rays striking the scintillation crystal can exceed the recording capabilities of the system.

p.15 Why is there a dead time of a system?

Due to the finite recovery and reset times for electronic circuits.

p.15 What are the two behaviours exhibited?

Paralysable and non-paralysable.

p.16 Describe a paralysable behaviour.

System won't respond to a new event for a fixed time due to radioactivity.

• The gamma ray strikes the scintillation crystal that takes 230 ns to release photons.
• Another strike will take further 230 ns to decay - but only one set of photons is produced.
• The number of recorded events can actually decrease.

p.16 Describe a non-paralysable behaviour.

System won't respond to a new event independently of radioactivity.

• Logic circuits or PHA may take time to process signals and any further events are not recorded.
• It dominates the dead time. [t = (N - n) / nN] where n is measured and N is true.

p.18 When the only emission is gamma rays from the radiotracer, what does this mean/tell us?

There is no background signal = high intrinsic contrast.

p.18 Large scattering is corrected by collimation, but signal is reduced. True or false?

True. It also equals lower SNR.

p.18 What is the relationship between low-pass filters and spatial resolution?

Low pass filter --> low spatial resolution images.

p.18 Resolution does not depend on the depth of the emission. True or false.

False.

p.19 What are some factors that affect SNR in scintigraphy?

Amount of radiotracer administered.
Timing of acquisition.
Amount accumulated in organ.
Depth of organ.
Sensitivity of the gamma camera.
Crystal thickness.
Collimator properties and geometry.
Low-pass filter.

p.19 What are some factors that affect resolution in scintigraphy?

Typical values: 5-8 mm at low depth, 1-2 cm at high depth.
Thickness of scintillation crystal.
Resolution of position encoder.
Depth at which radiotracer accumulates.

p.19 What are some factors that affect CNR in scintigraphy?

Low resolution.
Filter to increase SNR affects CNR.

p.20 Briefly describe SPECT.

Injected radioactive tracer.
Moving gamma camera.
Tomographic reconstruction of image.
Often combined with CT.

p.21 What emits positrons?

The radiotracer that is injected into the patient.

p.21 What happens when a positron encounters an electron?

The particles annihilate each other and produce two photons.

p.21 Is PET's emission dose high or low?

Pretty low.

p.21 What are the values of the variables in the equation: E = mc²?

m = 9.1 x 10⁻³¹ kg
c = 3 x 10⁸ m/s
E = 8.2 x 10⁻¹⁴ J = 511 keV

p.21 What is the relationship between energy and attenuation?

High energy = low attenuation.

p.22

p.24 What is the most common tracer?

F¹⁸ - fluorodeoxiglucose: glucose analogue

p.24 Where do F¹⁸ tracers accumulate?

Where high glucose metabolism occurs = cancer.

p.24 Explain the movement of positrons.

Positrons will travel a random path until encountering an electron.

p.24 Where are the photons emitted?

The photons are not emitted where the atom decays.

p.25 What is the function of scintillator material in PET detection?

It emits light when hit by radiation.

p.25 What device converts the light from scintillator material into an electrical signal?

Photomultiplier tube.

p.25 What introduces errors in the location of radioactive decay in PET detection?

Physical limitations on the size of the detectors.

p.25 What is the energy level that materials in PET detection are tuned for?

511 eV.

p.25 What does Annihilation Coincidence Detection involve?

Time-stamping each event.

p.25 What is the coincidence resolving time in PET detection?

The time-window for the second ray to be assigned to the same annihilation event.

p.25 Under what condition is a pair of gamma rays accepted as a true coincidence?

If the second ray strikes a detector that is in line of response with the first detector.

p.26 How is the image formed in PET detection?

Using tomography principles.

p.26 What type of corrections are required intrinsic to PET?

Attenuation correction and accidental coincidences.

p.26 What is attenuation correction in PET?

Using CT-segmented images to apply attenuation values at 511 keV.

p.27 What are accidental coincidences in PET detection?

When the line of response is incorrectly assigned due to two annihilations occurring closely in time.

p.27 What can cause an incorrect line of response in PET detection?

Scattering of a gamma ray.

p.28 What provides contrast in nuclear medicine images?

The emission of gamma rays from tissues.

p.28 What percentage of events does PET capture compared to SPECT?

PET captures 2-10% of events, while SPECT captures 0.01-0.03%.

p.28 What are some factors that affect SNR in PET?

Amount of radiotracer administered and biological targeting
Timing of events
Gamma ray attenuation in the patient
System sensitivity
Image post-processing
Registration with CT (for attenuation correction)

p.29 What are some factors that affect resolution in PET?

Effective positron range in tissue before it annihilates
Non-colinearity of gamma rays
Dimension of detector crystal (surface and thickness)
Typical values 3-6 mm

p.29 What are some factors that affect CNR in PET?

Low resolution and SNR
Non-specific uptake by healthy tissue

p.30 What is an example of a purpose of whole body scans in nuclear medicine?

To localize metastases.

p.30 What is an example of how brain PET is used in clinical settings?

For diagnosing Alzheimer's disease.

p.31 What is an example of a clinical application of cardiac imaging in nuclear medicine?

Assessing myocardial viability and perfusion.

What is scintigraphy?

Detection with gamma camera of emission from radiotracer.

What is SPECT?

Tomographic image from gamma camera detection at multiple angles.

Together, what do PMT and anger logic unit do?

Converts and localizes light.

What does dead time cause?

Registered events < real events.
(paralysable and non-paralysable)

Describe the process of positron emission.

1. Radiotracer emits positron.
2. Positron encounters electron and annihilates.
3. Two photons are produced and travel in opposite directions.

Describe the process of positron emission detection.

1. Detector accounts for signals from coincident gamma rays.
2. The more events, the higher the signal detected.
3. Detector = scintillator + photomultiplier.