instrumentation final - slides

What is required during SPECT acceptance testing?

All planar gamma camera tests plus SPECT-specific acceptance tests.

Why must multihead camera calibration factors match closely?

Because differences between heads can create artifacts and non-uniform reconstructions.

What is the maximum allowed difference between calibration factors on multihead cameras?

0.5 percent.

What is the maximum allowed difference between acceptance test results on multihead cameras?

10 percent.

Why is QC more important in SPECT than in planar imaging?

Because any projection error becomes amplified during reconstruction.

What happens when camera rotation is inaccurate in SPECT?

Errors appear in the reconstructed slices and degrade image quality.

What should corrected uniformity be for SPECT?

3 percent or better.

How much variation should be present in a flood source used for SPECT QC?

Less than 1 percent variation in activity.

Why is pixel size accuracy important in SPECT?

Incorrect pixel dimensions distort reconstruction and attenuation correction.

How often must pixel size calibration be performed?

Quarterly.

What does the axis of rotation represent?

The physical line around which the gamma camera rotates.

What does the center of rotation represent?

The center point of the 3D reconstruction matrix.

What indicates correct rotational alignment during COR testing?

Projection midpoints crossing at the same point.

What does the COR analysis program calculate?

The difference between the AOR and COR and the necessary offset.

What source is used for COR testing?

A 1 mCi Tc-99m point source.

Why are 32 stops used in a COR test?

To fully evaluate the camera over a 360-degree rotation.

How many counts per projection are required for COR?

Between 5000 and 20000.

What matrix size is typically used for COR?

128 by 128.

What must be done if the COR X-offset exceeds 0.5 pixel?

Erase the stored correction and generate a new one.

What artifact appears in 360-degree SPECT when COR is incorrect?

A doughnut artifact.

What artifact appears in 180-degree SPECT when COR is incorrect?

A pitchfork artifact.

How does incorrect COR appear in clinical images?

As subtle blurring or distortion due to misplaced activity.

How often should COR be checked?

Weekly until stable, then at regular intervals.

What does the reference scan measure?

Attenuation of gamma rays in air for systems using attenuation correction.

How long does a reference scan require?

Two minutes.

How many counts are needed for a reference scan?

At least 1.8 million counts.

How do you test stability of detector heads?

Acquire images at 0, 90, 180, and 270 degrees using a sheet source.

What should detector head stability images look like when subtracted?

They should contain only noise.

What is collimator hole angulation error?

A collimator hole has a different angle than the rest, causing mispositioning.

How does a 1 percent hole angulation relate to offset error?

It corresponds to approximately 3.5 mm of positioning error.

When should collimator hole angulation be evaluated?

During acceptance testing or when collimator damage is suspected.

What does the Jaszczak phantom test?

Resolution, contrast, and uniformity in SPECT.

What do rods in the Jaszczak phantom measure?

Spatial resolution.

What do spheres in the Jaszczak phantom measure?

Contrast resolution.

Why are SPECT artifacts more severe than planar artifacts?

Reconstruction amplifies small projection errors.

What causes ray artifacts in SPECT?

Filtered backprojection, especially in the presence of hot areas.

What causes ring artifacts in SPECT?

Flood nonuniformity, COR errors, or collimator defects.

How do ring artifacts appear?

As circular alternating bright and dark rings.

What causes motion artifacts in SPECT?

Patient movement between projections.

How do motion artifacts appear?

As blurring or double structures.

What causes truncation artifacts in attenuation-corrected SPECT?

Part of the patient is outside the field of view during transmission imaging.

What does the energy window select?

It selects the range of photon energies around the photopeak that the detector will accept.

Why is a narrow energy window used for calibration?

To precisely locate the exact photopeak.

Why is a wide energy window used for imaging?

To increase counts and improve statistics during patient studies.

What is peak broadening?

The spreading of the energy peak that prevents two close photon energies from being separated.

What does energy resolution measure?

How well a detector can distinguish between gamma rays of different energies.

How is percent energy resolution calculated?

FWHM divided by the photopeak energy times 100.

Why is Cs-137 used to measure energy resolution?

Because it produces a stable 662 keV photopeak.

What does the height of the photopeak represent?

The maximum number of detected events at the true photon energy.

What does FWHM represent?

The width of the peak at half its maximum height.

What does poor energy resolution indicate?

A decline in detector performance that may require servicing.

Why is tracking energy resolution over time important?

It detects gradual degradation in detector function.

How often is energy resolution typically measured?

Quarterly or annually depending on manufacturer guidelines.

What is background in SPECT imaging?

Nonspecific activity seen from all angles that appears more prominent due to multiple projections.

What improves image quality regarding background?

A cleaner target-to-background ratio.

What is structured background in FBP?

Counts from high-uptake areas spread across the matrix during filtered backprojection.

How is structured background reduced?

By using a ramp filter or iterative reconstruction.

Why is noise a larger issue in SPECT?

Because each projection has fewer counts due to short acquisition times.

Why must noise be evaluated per projection?

Because noise varies from angle to angle, not just overall.

How does FBP affect noise?

It amplifies noise and spreads it into nearby voxels.

What happens when projections are noisy?

Artificial hot or cold spots may appear in reconstructed slices.

What part of the frequency spectrum contains noise?

The high-frequency region.

Why must noise filtering be balanced?

Filtering removes noise but also removes fine detail.

How do iterative reconstruction algorithms treat counts?

They treat them as Poisson data and estimate the most likely distribution.

What is the downside of increasing iterations in iterative reconstruction?

It increases noise.

Why does increasing camera-to-patient distance worsen resolution?

Because gamma rays diverge and the collimator blurs more with distance.

How is resolution loss from distance reduced?

Using body-contour orbits or long-bore collimators.

How does attenuation affect SPECT images?

It reduces detected photons, increases noise, and distorts activity distribution.

Why does Compton scatter reduce image quality?

It adds photons to the wrong locations, decreasing contrast.

Why is patient motion critical in SPECT?

Because motion projects the same structure into different locations.

How is motion detected?

By reviewing projection cine loops or sinograms.

What do discontinuities in a sinogram indicate?

Motion between projections.

What is the partial volume effect?

Loss of accurate activity representation due to voxel averaging.

When does PVE cause major problems?

When the object is smaller than twice the system FWHM.

What is spillover in PVE?

Activity from one region contributing to neighboring voxels.

What does spillover do to hot or cold areas?

It enlarges them and decreases contrast.

What determines contrast in SPECT?

The visibility of differences in uptake between normal and abnormal tissue.

Why are cold lesions hard to detect?

They have low contrast in hot backgrounds.

How is uniformity measured in SPECT?

From transaxial slices of a cylindrical phantom.

Why is SPECT uniformity worse than planar?

Reconstruction amplifies small nonuniformities.

What factors determine spatial resolution in SPECT?

Intrinsic resolution, collimator resolution, matrix size, and angular sampling.

How does noise affect resolution?

More noise degrades resolution.

How can iterative reconstruction improve resolution?

By modeling system geometry and resolution recovery.

Why is collimator choice more important in SPECT?

Because SPECT suffers greater loss of sensitivity and resolution than planar imaging.

Why do longer collimator bores improve resolution?

They restrict accepted angles more effectively.

How do fan-beam and cone-beam collimators improve sensitivity?

They magnify small organs and use the detector area efficiently.

What energy window width is commonly used in SPECT?

A 20 percent window centered on the photopeak.

Why might multiple energy peaks be included in SPECT?

To increase total counts when radionuclides emit several usable energies.

What tradeoff occurs when increasing matrix size?

Resolution improves, but counts per pixel decrease, increasing noise.

How does zoom affect pixel size?

It reduces pixel size and improves effective resolution.

What is continuous rotation?

The camera rotates without stopping for each projection.

What is the downside of continuous rotation?

Motion blur within each projection.

What is step-and-shoot acquisition?

The camera stops for each projection before acquiring counts.

Why is step-and-shoot preferred?

There is less motion blur and better image quality.

What is a circular orbit?

An orbit where the radius remains constant.

Why is an elliptical orbit better than circular?

It fits the patient more closely and improves resolution.

What is a body-contour orbit?

An orbit that follows the patient’s outline for the smallest possible radius.

What happens when too few stops are used in SPECT?

Streak artifacts and reconstruction errors occur.

What rule links matrix size to number of stops?

Use as many stops as pixels per row: 64 stops for 64×64, 128 stops for 128×128.

What is the maximum practical imaging time for SPECT?

About 30 to 45 minutes.