Image acquisition

Question 1

What is frame mode imaging ?

Answer 1

Image parameters are set before imaging, usually stored as protocols, i.e. mat size, mag, mat depth, e window and termination condition

Question 2

What is list mode imaging?

Answer 2

Data is stored as x,y,z and t 
Can rebin/frame as desired (flexible)
Inc post-processing time
No check Im during imaging
Option not always available

Question 3

What is static imaging?

Answer 3

Images are displayed as a matrix
Little or no post-pro
Take diff angle views to get more info

Question 4

Why is whole body imaging used?

Answer 4

Easier to interpret than multiple statics

Question 5

How are whole body images created?

Answer 5

1)Step and shoot:
Series of static images
Digitally stitched together

2) Single sweep
Scan speed pre-selected
Counts stored in pos mat based on cam position at the time of recording

Question 6

What is dynamic imaging?

Answer 6

A series of sequential static frames
Used to image rapidly changing distribution within the patient
ex. renography

Question 7

What is cardiac gated imaging?

Answer 7

Several frames acquired covering the cardiac cycle
Acquired over many cycles
Can bin counts based on phases

Question 8

What is SPECT?

Answer 8

3D GC projection imaging

Question 9

What are the issues with 2D imaging?

Answer 9

2D view of a 3D object
There is no depth information
Overlying structures are superimposed leading to a loss of contrast compared to 3D

Question 10

How are planar images motion corrected? (4 steps)

Answer 10

1 Produce line images from a dynamic sequence (compress along each row/clm)
2 Stack the line images and est x/y motion from the deviations in pos of max I
3 Translate each frame according to the disp co-ords
4 Sum the translated frames

Question 11

How are SPECT images motion corrected?

Answer 11

Review sinograms and linograms for discontinuities

The shift req to realign the components is then calculated and the projections are translated

Question 12

What are the limitations of motion correction?

Answer 12

Reduces spatial res as req interpolation
Out of plane motion cannot be corrected
Distribution changes can confound the algorithms

Question 13

How are colour scales used?

Answer 13

Help simplify diagnosis by maximising visible contrast

Question 14

Describe Filtered Back Projection

Answer 14

Back Project each planar image onto three dimensional image matrix
More views – better reconstruction
1/r blurring, even with infinite number of views
So must apply a ramped filter to remove the blur

Question 15

How does using a ramp filter effect projection images?

Answer 15

Gives increases weighting to higher spatial frequencies

But, amplifies noise

Question 16

How can ramp filters be improved?

Answer 16

Multiply with another function to rolls off the ramp effect at higher frequencies
For example, mult by a gaussian

Question 17

What are the limitations of back projection?

Answer 17

Requires filtering to correct
Can introduce noise and streaking artefacts
Filtering can reduce issues but reduces resolution

Question 18

What are the 2 types of attenuation correction?

Answer 18

Chang and Sorenson

Question 19

What is Sorenson correction?

Answer 19

Assumes known uniform attenuation, with object size estimate from an orthogonal projection.
Modifies projections before reconstruction

Question 20

What is Chang correction?

Answer 20

1) Assumes known uniform attenuation over the object and requires object boundary is defined a priori.
2) Reconstructs the uncorrected image
3) Calculate correction matrix and applies it as a scaling factor

Question 21

What is second order chang?

Answer 21

1) Uses first order result to create new, corrected projections
2) Subtracts the corrected projections from acquired data to give an ‘error’ projection set,
3) Then the ‘error’ set is reconstructed, subjected to first order correction and added to corrected image

Question 22

What are the limitations of attenuation correction?

Answer 22

Inaccurate corrections can cause artefacts
Usually designed for use without scatter correction u values should be broad beam
Assumes uniform atten, good for soft tissues/bone poor for lung/thorax

Question 23

What is the minimum number of projections required to avoid streaking?

Answer 23

Need sampling ∆𝜃≤𝑑/(1 ⁄ 2 𝐷)

For 360 ∆𝜃=2𝜋/𝑁 so 𝑁≥𝐷/𝑑
For 180 N/2 will do

min is approx same no views as matrix size

Question 24

What is iterative reconstruction?

Answer 24

Iteration is process of successively better “guesses”

Creates an image by refining the expected projections in comparison to those recorded MLEM

Question 25

Minimum N of iterative projections required?

Answer 25

Within a circle of diameter D there will be
𝜋 ((𝐷⁄𝑑))⁄4 pixels D=diam d=pixel sep

Ignoring atten and res loss N/2 proj provide new info (in 360)

Therfore:
𝑁/2 𝐷/𝑑≥𝜋/4 (𝐷/𝑑)^2 𝑜𝑟 𝑁≥𝜋/2 𝐷/𝑑
less than for FBP

Question 26

What are the main Iterative algorithms?

Answer 26

Conjugate Gradient
Maximum Likelihood Expectation Maximisation (MLEM) (and its offshot OSEM)
Maximum A Posteriori Expectation Maximisation (MAPEM)

Question 27

What is MLEM?

Answer 27

Each iteration of the algorithm has two steps:

1) In the expectation step the formula giving the likelihood of any reconstructed image given the measured data is formed
2) In the maximisation step the image that is most likely given the measured data is found

Question 28

What is OSEM ?

Answer 28

Ordered Subsets Expectation Maximisation

OSEM accelerates convergence by updating the image estimate following a comparison of only a sub-set of the projection data to the current estimated projections

Reduces comp demand, inc convergence by factor of the no of sub-sets used

Must carefully select subsets to obtain best convergence

Question 29

Possible refinements to atten corr?

Answer 29

• Integrate a system specific correction into the initial projection
• Can use CT data to provide an atten corr map

Question 30

How is scatter correction used?

Answer 30

Not widely available
Example is monte carlo
Atten corr must account for scatter corr

Question 31

What are the advantages of SPECT/CT?

Answer 31

CT can provide an accurate atten corr map even with low dose/qual CT

Simplifies location of lesions by providing improve co-registration of the modalities - full dose CT not required

Faster diag/staging - one stop imaging

CT atten corr allow for accurate tracer uptake quantisation

Question 32

Issues with image fusion ?

Answer 32

SPECT-CT images are acquired sequentially not concurrently - swaps gantries

But patient motion betw ims can cause alignment issues esp. breathing issues - must check reg with phantom ims