1. Nuclear Medicine Physics
SPECT/CT, PET/CT, PET/MR
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. SPECT vs PET PET SPECT (Simultaneous acquisition)
(Step-and-shoot acquisition)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

4. SPECT vs PET imaging Attribute SPECT PET Detection Single s
Coincident s
Radionuclides
99mTc, 67Ga, 111In
18F, 82Rb, 13N, E
70 – 300 keV
511 keV
Spatial res.
 10 – 12 mm
 mm
Atten.Correction
No / Yes*
Yes
* Possible with SPECT/CT or transmission source systems
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

5. SPECT & PET SPECT – 2 views from opposite sides
Res.  collimator res., which degrades rapidly with increasing distance from collimator face
PET – Simultaneous acquisition
Res.  detector width; is max in center of ring
SPECT sensitivity ~ 0.02%
Huge losses due to absorptive collimators
PET sensitivity- 2D ~ 0.2%; 3D ~ 2% or higher
High sensitivity due to ACD (electronic collimation)
Allows higher frequency filters / higher spatial resolution
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

6. SPECT/CT, PET/CT, PET/MR
Combine a PET scanner
(or gamma camera) with a CT
(or MR) scanner on the same
gantry to share the same
patient table
Acquire CT (or MR) and PET
(or SPECT) images sequentially
(or simultaneously) with minimum patient movement and then use software to fuse the images

7. Why PET/CT, PET/MR, and SPECT/CT?
Fusion of PET (or SPECT) and CT (or MR) images provides a clear background for tumors and hence better tumor localization.
The CT (not MR) image provides a patient-specific attenuation map for PET (or SPECT) attenuation compensation.

8. PET/CT scanner First PET/CT scanner: 1998
Currently all PET scanners have CT.

9. SPECT/CT scanner
Introduced in early 2000’s

10. PET/CT image fusion

11. SPECT/CT images
Oncology

12. From CT image to attenuation map
Attenuation coefficient m depends on photon energy.
The average energy of CT x-rays is ~70 keV while the photons are 511 keV in PET or 140 keV in SPECT.
CT image is segmented to bone, muscle, and lungs.
Apply different scaling factors (e.g. m511/m70 for PET) to tissues, e.g to muscle and to bone, to convert the CT image to m map.

13. Artifacts in PET (or SPECT) caused by CT
Since CT image is used for PET (or SPECT) attenuation compensation, CT image artifacts may cause artifacts in PET (or SPECT) reconstructed image.
CT artifacts may be caused by
metal, patient movement, or image truncation,
contrast
misalignment between CT and PET (or SPECT) due to respiration and other patient motions.

14. Respiration misalignment in PET/CT
PET comp

15. Misalignment in SPECT/CT

16. Misalignment in SPECT/CT

17. Head motion in PET
Corrected

18. PET/MR
Functional and anatomic image fusion, MR provides better soft tissue contrast than CT  better lesion localization particularly in brain imaging
No ionizing radiation to the patient from MR so radiation dose is cut down to about half
It is difficult to obtain patient-specific MR-based attenuation map for PET reconstruction.

19. Simultaneous PET/MR
PET inserted in the MR ring so that the data acquisition is truly simultaneous, e.g. Siemens Biograph mMR (PET + 3T MR)

20. Sequential PET/MR
PET separated from MR but sharing the same patient table. Data acquisition is sequential, e.g. Philips TF Ingenuity (PET + 3T MR)

21. Main technical challenges of PET/MR
Reduce the interference between PET and MR components.
PMT replaced by avalanche photodiodes, or
PMT installed far away from the MR ring using long optical fibers
PET electronics shielded from RF and gradient fields
MR coils shielded from the metal PET insert
Patient-specific, MR-based attenuation map for PET attenuation compensation?!?!

22. PET/MR human brain
PET MRI fused

23. PET/MR human brain PET/CT PET/MR PET/MR PET/MR
w.o. m comp trans-based MR-based

24. PET/MRI breast cancer
MRI fused PET

25. Attenuation Correction: A Review
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

26. Attenuation correction
s traveling smaller paths through pt (nearer to camera) have less attenuation compared to those from deeper in pt
Inaccurate representation for the same amount of radioactivity
AC allows correct representation of the distributed radioactivity
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

27. Attenuation correction
SPECT:  for AC can be assumed or measured
Chang (assumed), Measured - Gd rods (older) or CT (new)
CT can be non-diagnostic (low power) or fully diagnostic
PET:  for AC is measured
CT (fully diagnostic)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

28. Attenuation in SPECT I0 = I1e+aD1 a
I1 = I0e-a t
I0 = I1e+a
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

29. Chang’s AC method Image first reconstructed without AC
Contours of image used to estimate t for each projection,  assumed to be constant
ACF determined for every projection
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

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

31. SPECT-CT

32. 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

33. Attenuation in PET
P2 = e-b a b
P1 = e-a t
P = P1 × P2 = e-a × e-b
= e-(a+b)
= e-t D1 D2
Probability of detection dependent only on the total thickness

34. Attenuation Correction in PET/CT
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

35. Attenuation Correction
Without AC
With AC
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

36. Attenuation in PET
For each LOR, probability of detection dependent only on the total thickness
Every point along LOR has SAME ACF (No depth dependance)
Attenuation more severe than in SPECT but much easier to correct
By far the most important data correction in PET
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

37. PET/CT co-registration
After acquisition, check PET / CT registration
Scanners have manual QC tool for checking co-reg
If unsatisfactory, adjust co-reg using QC tool and repeat PET reconstruction
Proper co-registration of PET and CT data is critical for proper diagnosis
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

38. Attenuation Correction
Misregistration
Lateral walls of myocardium in the PET data, corrected with the lower attenuation of lung tissue
Proper registration
Lateral walls of myocardium corrected with the higher attenuation of heart tissue
(Ref: Attenuation correction of PET cardiac data with low-dose average CT in PET/CT, Tinsu Pan et al, Med. Phys. 33, October 2006)
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

39. Typical PET / CT imaging
(1)
(2)
512 x512 128 x128
120 kVp  511 keV
Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

40. October 7, Researchers at the University of California, Davis (UC Davis) have received a five-year, $15.5 million grant to develop what they are calling the world's first total-body PET scanner.
National Cancer Institute and will fund the Explorer project, led by Simon Cherry, PhD, distinguished professor of biomedical engineering and Ramsey Badawi, PhD, a professor of radiology.
The total-body PET scanner would image an entire body all at once, and it would acquire images much faster or at a much lower radiation dose by capturing almost all of the available signal from radiopharmaceuticals. … the design would line the entire inside of the PET camera bore with multiple rings of PET detectors.
… such a total-body PET design could reduce radiation dose by a factor of 40 or decrease scanning time from 20 minutes to 30 seconds