PET: Quality Control

QC purpose

assess scanner performance over time

Verify scanner is operating properly

Identify problems prior to scanning patients

Provide validation of system corrections

What QC PET scans are done daily?

Blank (air) scan, Daily quality assurance, PMT gain adjustment

Blank scan:

short acquisition with Ge68 rod source or liquid phantom. Asses sinogram, attenuation correction functional? Like a daily flood.

Common Daily QC Report (GE PET): Coincidence; Singles; Dead time; Timing; Energy shift

Defective block shows up as __ on Daily Quality Assurance data

black

Daily Quality Assurance

Start of the day for “hybrid” (PET/CT) systems

Potential Daily Quailty assurace problems

• Scanner instability

• Detector module / PMT/ preamp failure

• Loose cables, connectors, boards

• Inoperable gantry motors, source loader

Daily Quality Assurance includes

PMT Gain adjustment, assessment of detector function

What QC PET scans are done weekly?

Coincidence timing check/calibration

Energy window calibration

What QC PET scans are done Quarterly?

Normalization, Activity/scanner calibration, tomographic uniformity, crystal map.

What QC PET scans are done annually?

PET/CT alignment

Normalization scan process

5-10+ hours

Ge68 rod source, solid phantom. F18 liquid phantom.

Assess sinogram

like gamma camera uniformity

Normalization scan purpose

Corrects for the variations in efficiency in lines of response in the PET sinograms

Activity Calibration:

External well counter or dose calibrator. F18 liquid phantom.

1. Phantom concentration

2. ROI count intensity (counts/voxel)

3. Calibration factor

Calibration factor:

Units of counts (or counts/second) → activity concentration (kBq/ml or uCi/ml) (SUV)* SUV accuracy? SUVs using NAC?

Linear diagonal defect from detector block failure

multiple ring artifacts from detector elements

PET SUV

1.0 ± 10% (or 15%)

Attenuation correction artifact: Overcorrected (iodine contrast), False positive

What QC corrects for variation in efficiency in LOR in each slice of the sinogram?

Normalization corrections

Normalization scan

applied to sinograms to remove variations in event detection efficiencies along the LOR. Caused by detector electronic variations, geometrical variations, individual detector efficiencies. May lead to ring artifacts.

Normalization correction

multiplicative correction map is applied to acquired sinograms. Saved and applied to all patient studies. Needs to be 2D (directly with rod source or phantom) and 3D (component based).

T/F: An axial CT scan is a step-and-shoot type of CT scan.

True

The pixel values/CT numbers for compact bone is _____ HU

250-1000

The following image depicts _____ and could be remedied by _____.

noise/photon starvation; increasing the mA

Pitch =

Table Movement per 360 degree rotation (mm) / (# of slices) (slice width in mm)

How can partial volume averaging be reduced?

Scan with thinner slices

Presence of the non-collinearity of the annihilation photons and the finite positron range are inherent properties of positron emission tomography resulting in:

Positional innaccuracy

In PET scanning raw data is acquired and identified as coincidence events along their LOR are stored in the raw data format called:

sinograms

Property of PET detectors that allows them faster timing signals for coincidence detection and to work at high count rates is called

decay constant

Truncation artifacts in PET/CT imaging are produced by

Difference in size of FOV between PET and the CT

The process of aligning images so that corresponding features can easily be related is called:

image registration

The use of a low-dose CT scan in place of a conventional PET transmission scan

improves accuracy of the scan interpretation

The property of the scintillation detector described as the number of scintilla- tions produced by each incident photon is called:

light output

Which scintillators has the highest light output?

NaI (Tl) (thallium-doped sodium iodide)

A piece of equipment that sorts out photons of different radionuclides with different photon energies and to separate scattered photons from the useful ones is called

Pulse height analyzer

Which of the following scintillators commonly used in PET imaging has the highest stopping power?

BGO (bismuth germinate)

Radiochemical purity tests

HPLC and TLC

What does radio nuclidic purity measure

how much of a sample is the target isotope

Chemical purity QC tests for…

Raw materials

Solvents

Starting materials

By-products

Degradation products

F18 FDG radiochemical purity Limit

95% pure

Mo Breakthrough must be less than

<0.15 µCi Mo/mCi Tc

Mo/Tc99m chemical purity limit

10 ug / mL of Al3+ Aluminum ion

Sr-82/Rb-82 breakthrough limit

Sr-82 content must not exceed 0.02 uCi/mCi of Rb-82

Sr-85 content must not exceed 0.2 uCi/mCi of Rb-82

Generator System Diagram

How is a Sr-82/Rb-82 generator used?

Eluted straight into patients for cardiac perfusion exams

How is a Ge-68/Ga-68 generator used

attached to a localizer molecule via kit

Transient equilibrium generator

Parent Nuclide 10 -100 times half-life of Daughter

Secular equilibrium generator

Parent Nuclide 1000 times or more half-life of Daughter

What are the Secular equilibrium generators

Ge68/Ga68 Generator

Sr82/Ru82 Generator

Molybdenum Column

Tech 99m Radionuclidic purity

Mo99

Tech 99m chemical purity

Aluminum

Magnets

bending momentum to accelerate particles, strength determines ion energy. Stronger magnet – more kinetic energy.

Dees

two flat electrons. In presence of magnetic field, negative ion particles are pushed and pulled, accelerating in circles. As kinetic energy increases, diameter increases. Strength of magnetic field determines the final energy of the ions.

Ion source

negative particles are hydrogen ions produced by ionizing hydrogen gas with tungsten filament.

Extraction foil

neg charge ions reach max diameter, carbon foil is used to strip off orbital electrons, creating a proton or deuteron (+).

Targets

Positive ions are ejected, striking a specific target causing a nuc reaction and creating a short-lived radionuclide. Desired radionuclide determines what ionizing particle and target material are used. Can be solid liquid or gas.

Diffusion pump

generates vacuum to accelerate negative ions.