1. Nuclear Medicine Physics
Single Photon Emission Computed Tomography (SPECT)
Jerry Allison, Ph.D.
Department of Radiology
Medical College of Georgia

2. Medical College of Georgia And Sameer Tipnis, Ph.D.
A note of thanks to
Z. J. Cao, Ph.D.
Medical College of Georgia
And
Sameer Tipnis, Ph.D.
G. Donald Frey, Ph.D.
Medical University of South Carolina
for
Sharing nuclear medicine presentation content

3. Tomographic NM imaging (SPECT)
Single Photon Emission Computed Tomography
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

4. Scintillation Camera
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps, 4th edition, 2012

5. Scintillation Camera PMT Event Positioning Network
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps, 4th edition, 2012

6. Tomographic imaging (SPECT)
Produce tomographic images by acquiring conventional gamma camera projection data at several angles around the patient
Similar to CT

7. SPECT
Provide 3-D images to eliminate overlaying and underlying activity of a slice
better contrast
more accurate lesion localization
more demanding technically and longer data acquisition
more severe image noise

8. SPECT data acquisition
Generally, two detectors mounted at 180 or 90 on a rotation gantry

9. SPECT data acquisition
a sequence of 2-D static images at different angular positions (views)
detector rotation range
180º with 2 perpendicular detectors or
360º with 2 opposite detectors
45º RAO
In principle, rotation of the gamma camera about 180° allows for the
acquisition of sufficient projections for tomographic reconstruction. However,
in practice, opposing views acquired 180° apart differ due to various factors
(photon attenuation, depth dependent collimator response) and SPECT data are
commonly acquired over 360°.
45º LPO

10. SPECT data acquisition
Circular or elliptical orbit, which is better?
closer to the patient  better spatial resolution

11. SPECT Image Acquisition
(Improves spatial resolution)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

12. Data Collection: Configuration
Non Cardiac
Cardiac
Delete??????

13. SPECT Image Acquisition
Typically 2 camera heads, rotating around patient
Projection images every 3 – 6 degrees
~ 30 s / projection, ~ 15 minutes total
Matrix - 64 x 64 or 128 x 128
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

14. Data Collection: Angular Stops
3 to 6 degrees is common
a lesser number causes streaking
a larger number does not improve image quality
Step-shoot or continuous acquisition, which is better??????
Step & Shoot Characteristics
Some loss of time
Less Blur

15. View number for 360º SPECT number of views = matrix size 128 views

16. Each projection has 128 data points
An image with 128 x 128 matrix
Contains 128 projections
Each projection has 128 data points
Equivalent to 128 slice CT
(i.e., 128 tomographic slices per rotation)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

17. Sinogram (for one of many slices)
There are many (e.g. 64 or 128) sinograms per rotation.
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

18. © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

19. Back Projection
Leads to blurring in image (streaks and star-like artifacts)
Deleted Kuhl’s Brain Imaging

20. Filtered Backprojection
Suppress blurring through filtering the projections
A high-pass filter (ramp filter) can be used to suppress blurring
A problem that is immediately apparent is the blurring (star-like artifacts) that occur. One would expect that a high-pass filter could be used to eliminate blurring, and that is the case. The optimal way to eliminate these patterns in the noiseless case is through a ramp filter. The combination of back projection and ramp filtering is known as filtered back projection.

21. Filtered Back Projection (of noiseless data)
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

22. Filter Amplitude Frequency
Ramp filter
Amplitude
Ramp w/ rolloff filter
Frequency
Define K(L) & L
In the absence of noise, a ramp filter works well
For noisy images, a ramp filter with roll-off is required

23. Filter Ramp Ramp with some roll-off filter
Suppresses blurring but enhances noise
Ramp with some roll-off filter
Smoothes the image and suppresses noise
Trade off noise vs resolution
Roll-off filter characteristics are adjustable

24. Filter Types
© Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

25. Filter Types (A) Butterworth: (less noise, more smooth)
(B) Butterworth: (more noise, less smooth)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

26. Filter Applied filter is the product of: Ramp
User selected/characterized filter
Shepp-Logan
Hahn
Butterworth
Weiner
Hamming
Hanning (MCG: Philips, generally turned off)

27. Filter Applied filter is the product of: Procedure for applying filter
Ramp
User selected/characterized filter
Shepp-Logan (cut-off freq)
Hann (cut-off freq)
Butterworth (order (slope), critical freq (0.707 response)
Procedure for applying filter
1D projection of each view is converted to spatial frequency using a Fourier transform
Ramp filter with roll-off is applied in spatial frequency space (k-space)
Filtered projection is recovered with inverse Fourier transform
Back projections performed to reconstruct image

28. Selection of Filters for SPECT
Filters trade noise for resolution
No standard way to optimize filter choice
Patient to patient variation
Physician preferences
Vendor recommendation
Embellish from Cherry.

29. Iterative Reconstruction (IR)
Filtered Back Projection has some limits
Various corrections needed
Attenuation
Compton Scatter
Ordered Subsets Expectation Maximization (OSEM) is a common iterative reconstruction algorithm
Improve comments on limitations of filtered back projection.

30. Calculation Includes: Attenuation Scatter Blur with depth
Assume
Some Image (I)
Calculate
Projections (P’)
Calculation Includes:
Attenuation
Scatter
Blur with depth
Compare to
Measured Projection (P)
Use P’ & P
to form corrections
Image estimate may be a uniform image
Assumed image undergoes forward projection to produce sinogram
Form New
Image (I’)
Is I-I’< *
Done

31. Iterative Reconstruction
Slow compared to filtered back projection
Commonly used for PET
Being used increasingly in SPECT
IR used for ~all Philips SPECT at MCG
Siemens C-Cam??????
Has become the standard for SPECT?????

32. Image recon - Iterative
Common IR recon is the OSEM
For OSEM, # iterations (I) and # subsets (S) affect image quality
 # (I/S)   noise, but sharper images
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

33. Iteration 1
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

34. Iteration 5
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

35. Iteration 10
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

36. Brain Phantom IR OSEM FBP
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

37. Non-filter Noise Factors
Collimator
Matrix
64 x 64
128 x128
Slice thickness
Time per stop/ Number of Stops
Administered Dose
Deleted:
Number of matrix element rows

38. Data Collection Whole Image is collected for each view
64 x 64 or 128 x 128
Each row makes a slice
Multiple slices can be added to reduce noise
Anything higher than 128 x 128?

39. Data Collection: Counts
Determination of Number of Image Counts
Activity in patient
Time per stop
Number of Stops

40. Attenuation Correction
Like all radionuclide imaging there is a problem due to attenuation.
Correction can be important for judging the activity of lesions

41. Attenuation correction
s traveling smaller paths through pt (nearer to camera) have less attenuation compared to those from deeper in pt
 for AC can be assumed or measured
Chang (assumed), Measured - Gd rods (older) or CT (new)
CT can be non-diagnostic (low power, cone-beam) or fully diagnostic depending on the scanner model
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

42. Attenuation in SPECT I0 = I1e+a (attenuation corrected intensity)
Probability of detection / correct intensity I0, dependent
on the depth at which  originates  need to know “a”
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

43. Chang’s AC method Image first reconstructed without AC
Contours of image used to estimate t for each projection,  assumed to be constant
Average ACF determined for each pixel (x,y) from all projections
Reconstructed image corrected pixel-by-pixel
Works well for area with approximately constant attenuation like head, abdomen but not for areas like chest / thorax
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

44. I(x) = I0e-mx (x is thickness of tissue between pixel & detector)
Chang’s Method
Assume uniform attenuation
m linear attenuation coefficient
m is ~0.15/cm for Tc-99m for soft tissue
I(x) = I0e-mx
(x is thickness of tissue
between pixel & detector)
0.15/cm is the narrowbeam attenuation coefficient for 140 keV photons in soft tissue
Use of the broadbeam coefficient, 0.12/cm, helps to account for attenuation and scatter in uniform radioactivity distributions
In Chang’s method, m is often set to ~0.12/cm to better account for Compton scatter

45. Uniform phantom with evenly distributed 99mTc
Chang method
Proper AC
Low counts in center
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

46. SPECT/CT

47. AC in SPECT/CT
Accurate / realistic -map obtained for each projection using CT
 values used in Chang’s algorithm to correct pixel-by-pixel
AC here is more realistic (since  is not assumed to be constant)
Current SPECT/CT systems use this method
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

48. Philips Astonish NM Recon Software
The Astonish: Ordered Subsets Expectation Maximization (OSEM)
Compensation for the blurring effects of the collimator built into the reconstruction
Resolution Recovery allows recovery of some of the original resolution
Astonish uses the distance from the detector to the object of interest recorded as a function of angle by the camera during acquisition and geometric properties of the specific collimator
Has become the standard for SPECT?????

49. Philips Astonish NM Recon Software
Used on essentially all Philips NM SPECT images at MCG
Iterations: 4
Subsets: 16
CT data used for attenuation correction (except brains due to EEG electrode artifacts)
Chang’s AC invoked for brains
Has become the standard for SPECT?????

50. Common SPECT Problems Patient motion
System Alignment (Center of rotation issues)
Collimator issues
Distance issues
Loss of resolution with distance

51. Center of Rotation
SPECT assumes heads always look at a constant central rotation point

52. COR (Spatial Alignment)
Image will have blurring and circular artifacts
COR must be tested periodically for all heads
SPECT of three point sources
Generally done with system QC software
SPECT of 3 point sources
?????

54. COR correction
SPECT image of a uniform phantom on a camera with poor COR correction
Incorrect COR correction introducing a ring artifact, degrading spatial resolution
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

55. COR correction
COR correction is used when reconstructing tomo-graphic data to correct for minor misalignment between the center of the image and the axis of rotation.
COR corrections are stored in a correction table and are applied automatically after a data set has been acquired.
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

56. Collimator Issues Collimators are not completely uniform
A high count flood must be stored to correct for collimator non-uniformities
20 M for 5% for 128 x128
Check or delete “20 M for 5% for 128 x128”

57. Patient Studies Advantages Disadvantages No overlapping structures
3 dimensional lesion locations
Fusion with high resolution images (CT, MRI)
Disadvantages
Time consuming (motion)
Images are noisy