Nuclear Medicine Physics SPECT/CT, PET/CT, PET/MR Jerry Allison, Ph.D. Department of Radiology Medical College of Georgia

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

SPECT vs PET PET SPECT (Simultaneous acquisition) (Step-and-shoot acquisition) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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  5 - 6 mm Atten.Correction No / Yes* Yes * Possible with SPECT/CT or transmission source systems 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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

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.

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

SPECT/CT scanner Introduced in early 2000’s

PET/CT image fusion

SPECT/CT images Oncology

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. 0.495 to muscle and 0.406 to bone, to convert the CT image to m map.

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.

Respiration misalignment in PET/CT PET comp

Misalignment in SPECT/CT

Misalignment in SPECT/CT

Head motion in PET Corrected

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.

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)

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)

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?!?!

PET/MR human brain PET MRI fused

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

PET/MRI breast cancer MRI fused PET

Attenuation Correction: A Review 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

Attenuation in SPECT I0 = I1e+a D1 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” 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

Uniform phantom with evenly distributed 99mTc Low counts in center Chang method Proper AC Chang method Overcorrection 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

SPECT-CT

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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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

Attenuation Correction in PET/CT 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

Attenuation Correction Without AC With AC 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

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) 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

Typical PET / CT imaging (1) (2) 512 x512 128 x128 120 kVp  511 keV 2015 Nuclear Medicine Physics for Radiology Residents Sameer Tipnis, PhD, DABR

October 7, 2015 -- 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 http://www.auntminnie.com/index.aspx?sec=sup&sub=mol&pag=dis&ItemID=112051