Instrumentation Exam 3 Review

What is the name of the noise reduction technique with iterative reconstruction and what advantage does it provide the MLEM algorithm?

Bayesian Interference Method. MLEM algorithm converges quicker.

Why do truncation artifacts occur?

Durring the transmission scan parts of the body fall outside of the field of view. (parts not seen in all projections)

What happens to the location of the information if the scattered photons fit within your acquisition window?

Information is mislocated giving less image contrast and lesion detection.

Are low or high energy gamma rays more attenuated? What is beam hardening?

Low energy. The energy of the photon beam changing as it is attenuated.

What to low pass and high pass filters do?

Low-pass filter allows low-frequency components to pass. (Removes high-frequency components) Reduces noise, but blurs edges. High-pass filter allows high-frequency components to pass, while attenuating low frequencies. Sharpens organ boundaries, but increases noise.

What is partial volume effect?

The within voxel averaging that occurs in any 3D imaging technique, due to limited finite size of voxels. Objects less than twice the size of the camera's FWHM will be influenced by surrounding tissue.

What is the Nyquist frequency, and can it be extended?

The maximum frequency possible. One pixel contains low frequency background, the next high frequency noise, = 0.5. (32 peaks divided by 64 pixels on a 64 x 64 matrix.) No, it cannot be extended, some of the information will be lost.

How is pixel sizing done, and in what directions?

Two point sources separated by a measured distance, divided by the number of pixels between the two point sources; measured in the X & Y directions and the results should be within a few % of each other or X & Y gains need to be adjusted.

Must SPECT and CT scans be precisely aligned for proper attenuation correction? How about patient motion and heavy breathing?

Yes, all may create artifacts.

Can background appear to be more significant than it is in SPECT?

Yes, because an area of increased background may be viewed from many different angles. Ramp filter may try to limit in FBP and iterative reconstruction may try to reduce or eliminate.

Can resolution recovery be modeled into an iterative reconstruction method, and where?

Yes, forward projection step.

May SPECT uniformity be measured using a 1 gallon bleach bottle and following the AAPM protocol? What is an expected measurement?

Yes, integral uniformity should be between 10 - 29%.

Is motion correction software available?

Yes, manual and automatic

Is resolution lost during the SPECT acquisition concerning the distance to what is being imaged, and what may be done to help?

Yes. Body contour, body orbits, long-bore collimators, input during the forward projection step of the iterative reconstruction process.

Is noise significant in SPECT imaging? Why, and what does it decrease?

Yes. Low counts produce noise. Decreases contrast.

Compare the axis of rotation with the center of rotation.

- Axis of Rotation: the imaginary line that runs through the middle of where the detectors rotate around. - Center of Rotation: center of the 3D computer matrix.

What are the two methods of SPECT reconstruction?

- Filtered backprojection - Iterative reconstruction - used most often today.

What are the limitations associated with SPECT imaging?

- Limited number of emissions - Noise due to low count statistics - Attenuation of the radiopharmaceutical by patient's body

What is NEMA, and what do they do, and not do?

- National Electrical Manufacturers Association A trade association that sets up the methods to test nuclear medicine cameras. - Doesn't set the standards for the camera's performance. - Also does this for many other pieces of electrical equipment.

What are the two artifacts created by simple backprojection?

- Star Artifacts - Streak Artifacts

What are the 2D slices presented for the physician to read? Orthogonal slices?

- Transverse, Coronal, Sagittal - Short axis, Horizontal long axis, Vertical long axis

What are the recommended quality control tests for SPECT tomographs, and how often should they be done?

- Uniformity - Daily - Collimator visual inspection - Daily - Center of rotation - Weekly, biweekly - Spatial resolution /linearity(bars) - Weekly, biweekly - High-count flood/uniformity correction map - Weekly, biweekly - Tomographic spatial resolution - Quarterly, annually - Jaszczak phantom - Quarterly, annually - Tilt-angle check - Quarterly - Collimator-hole angulation - Annually

How will a COR line source appear when reconstructed correctly? Incorrectly?

- Within limits: it appears as a point. - Incorrect: it appears as a donut with a cold hole or if 180 degree collection was made, then as a tuning fork.

How is the spacial resolution procedure performed for SPECT?

3 line source placed along the Z-axis of a water filled phantom, 360 degree acquisition, 128 x 128 matrix with 15 cm radius of rotation. Reconstruct with Ramp filter then the FWHM and FWTM are calculated using the transaxial slices in both X & Y directions. AAPM suggests moving a line source in the X direction and measuring the axial FWHM in successive transverse slices.

Planar uniformity, according to the AAPM, should be corrected to what percent, and what artifact may appear if it is not brought to that percent or below?

3% Bullseye artifact - if both detectors. Incomplete ring - if only one detector.

How many counts are collected and analyzed for integral and differential uniformity and how are those parameters calculated?

64 x 64 matrix: 30 - 60 million counts 128 x 128 matrix: 120 million counts [(high count pixel - low count pixel)/(high count pixel + low count pixel)] x 100 Should be 3% or less.

What two matrix sizes are used during SPECT acquisition, and which gives better resolution?

64 x 64, and 128 x 128 128 x 128 gives better resolution, but a 64 x 64 zoomed in may give the equivalent by reducing the area covered by each pixel.

What should be collected week/biweekly for uniformity correction when doing SPECT?

A high count correction flood.

How is a SPECT acquired?

A number of 2D projections are collected around the patient that are backprojected into the 3D matrix.

What is contrast resolution, and is it better in planar or SPECT?

Ability to distinguish between two areas of an image due to differences in count density. Much better in SPECT.

What is attenuation?

Amount of radiation detected reduced by scatter (Compton scatter) or absorption (photoelectric effect).

How small can motion be during SPECT acquisition and create significant artifacts?

As little as 1/2 a pixel.

Does attenuation and scatter affect SPECT images?

Attenuation distorts count density, and scatter mis-positions the information.

What are the 3 categories of an object being represented in spacial frequency domain?

Background, object data, noise

How is COR calibration performed?

Camera rotates around 1 or multiple point sources, 32 stops, 360 degrees rotation, head remains parallel to the AOR. COR analysis program determines an X-offset which is applied to all projections to exactly line up the opposing detector views. Matches the COR with the AOR. X-offset should be within 0.5 pixels or there will be a loss of resolution with the SPECT images. An unacceptable Y-offset error is indicative of camera head tilt.

What cases ray artifacts?

Come from the back projection process when very hot or cold lesions may be insufficiently corrected by the filter used. Artifacts typically show up in the transverse slices.

What would happen with high counts in FBP due to noise in a pixel and how is it treated?

Could cause falsely higher counts in vicinity voxels. A low pass filter rolling off the ramp filter could treat the high-frequency noise.

What is assumed in the 2D projections that is not true?

Counts are always exactly the same in the 2D projections and they are not. They do not take into account simple statistics and noise due to low counts in the projections.

What parameters does the Butterworth filter use?

Critical frequency and power factor. Change critical frequency - the same as changing cut-off frequency. Change power factor - raise value = less noise, less resolution smoother. Lower value = more resolution, more noise, grainier.

What is an iteration? What increases the number of iterations?

Cycle, noise. There needs to be a compromise between the readability of the 3D image and the noise introduced (use a filter to subtract out the noise.)

What does SPECT do with the radiopharmaceutical distribution within the body?

Eliminates the superposition of activity in front of and behind the object being imaged.

What is a surface rendering 3D image? How do areas of decreased pharmaceutical concentration appear?

Exterior pixels are connected together to look like a cage then shaded in. Low counts look like holes.

May the CT attenuation map be easily used in FBP? Where is the information inserted in the iterative reconstruction?

FBP - no Iterative - forward projection step

What methods may be used to identify and correct patient motion in SPECT imaging?

Fiducial markers, sinograms, and motion correction software

Where is scatter correction placed during the iterative reconstruction and how is it done?

Forward projection step. Scatter is measured at different depths in air and in phantoms to get an idea of how many gammas are scattered in tissue, while attenuation maps can be used estimate the effect on different body types in the scatter model.

What filters use cut-off frequencies, and what do they do?

Hann, Hamming Increase value - better resolution, but more noise, grainier. Decrease value - less resolution, less noise, smoother

What kind of filter is a Ramp filter, what does it do?

High-pass, erases the blurring from simple back projection in the transaxial plane. Highest resolution filter, but yields a noisy image.

What are the advantages of CT attenuation correction over TBAC?

Higher photon flux, don't have to replace source, possibly decreased radiation exposure to the technologist, and CT anatomical detail is produced.

What type of collimators are favored in SPECT imaging?

Higher resolution. Maybe slant hole to reduce distance, and possibly focused collimators that magnify the image over the crystal.

What does MLEM take into account that FBP does not, and how does it work?

It assumes that the number of counts in each pixel of the 2D projections are affected by Poisson statistics. This allows for it to account for noise in the 2D projection and it attempts to find, not just a single correct solution, but rather a 3D image matrix that is likely to produce the measured 2D projections.

What is a Jasczak phantom and what can it be used to measure?

It is a 20 cm diameter Plexiglas phantom with both rod and sphere inserts of various sizes to simulate lesions. The top of the cylinder just holds water to measure uniformity. Phantom is filled with water and 10 - 20 mCi of Tc-99m . Lay phantom on its side on the imaging table and acquire study. Transaxial slices analyzed for uniformity, resolution, and may calculate lesion contrast by a specific formula.

How does the Chang method of attenuation work and what faulty assumption does it use?

It's mathematical correction assumes all areas to be corrected are homogeneous. Works OK for brain and abdomen, but not anything else.

What are the requirements for the sheet sources for measuring uniformity when doing SPECT?

Less than 1% variance from point to point.

What needs to be done prior to SPECT imaging to reduce motion?

Make the patient comfortable.

What is the most popular method of iterative reconstruction?

Maximum-Likelihood Expectation Maximization (MLEM)

Are the scans created simultaneously in SPECT-CT attenuation correction?

No, they are done sequentially.

Are all filter unit labels the same?

No, there are: cycles/pixel, cycles/cm, and percent Nyquist frequency.

Is SPECT resolution dependent on the depth within the body?

No

Can you change filter setting values for quantitative studies?

No, based on normal populations using set filter values.

What other iterative reconstruction technique is used that allows the reconstruction of a 3D matrix with fewer iterations?

Ordered-Subsets Expectation Maximization (OSEM): Breaks down the whole set of projections into a large number of subsets. This reduces the number of iterations from 50 - 100 to 2 - 10.

Where do artifacts in SPECT images come from?

Photon absorption, Compton scatter, noise, and loss of resolution with distance from the collimator.

When may filtering be done, and what is it best to do?

Prior, during, and after back-projection. Filter in all three dimensions.

What filter eliminates star artifacts?

Ramp filter - eliminates low frequency background activity and is applied prior to each backprojection.

How does iterative reconstruction work?

Starts with an initial guess of the radiopharmaceutical distribution to create an estimated 3D matrix. The guess might assume uniform distribution or a backprojection of the measured projections. The computer creates estimated 2D projections using the 1st guess, plus information related to the physics of imaging, and a patient specific attenuation map. This estimated 2D projection is then compared with the measured 2D projections and the discrepancies are used to modify the estimated 3D matrix. The modified 3D image matrix becomes the starting point for iteration. Iterations are repeated until it converges, where the estimated projections are very similar to the measured projections. `

Define noise.

Statistical variation in the number of counts registered.

What is the most common acquisition format of the SPECT system?

Step and shoot

What are three software techniques used to detect patient motion?

Summed image, sinogram, and cine display

How is a rotational stability test performed and why?

Tape a sheet source to the collimator and image at 0, 90, 180, and 270 degrees to see if system is stable (yields the same uniform image) in all 4 positions. Subtract individual images from 0 degree image, and operator should only see noise.

How does filtered backprojection work?

The 3D image is reconstructed from 2D images that have been backprojected then filtered. Each 2D image shows all the counts as a line of transverse pixels. The many angles allow the 3D image to be reconstructed with filters to block out the overlapping counts. Individual line projections from each angle are stacked on top of each other. (Forms a sinogram type image.) The sinogram has all the projection data need to reconstruct a single slice of the original activity distribution.

Explain how Step and Shoot works.

The camera is stationary, acquiring the image, for a specified length of time or counts, then moves to the next position.

What is the gantry of the SPECT system?

The structure the gamma camera is connected to that allows it to rotate around a patient (frame that supports the detectors). Heaviest part of the instrument.

How does the MLEM iterative reconstruction technique handle noise?

Treats the count density as a Poisson function of the actual radioactive distribution - deals more effectively with the noise inherent in the 2D projection - however, there is increasing noise with increasing number of iterations.