Jonathon A. Nye, Ph.D Emory University School of Medicine Atlanta, GA

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1 Artifacts related to CT Jonathon A. Nye, Ph.D Emory University School of Medicine Atlanta, GA Acknowledgements: Fabio Esteves MD 1, James Galt PhD 1, Perry Sprawls PhD 1, Robert Eisner PhD 2, James Hamill PhD 3, Tinsu Pan PhD 4 1 Emory University Hospital, Atlanta, GA 2 Emory University Midtown Hospital, Atlanta, GA 3 Siemens Medical Solutions, Knoxville, TN 4 MD Anderson Cancer Center Society of Nuclear Medicine, Continuing Education, Saturday, June Financial disclosures: None

2 Artifact systematic discrepancy between true attenuation of an object and that represented in the image Atif Artifact torigins i based on the Physics (attenuation properties of object) Scanner (detector properties) Protocol (helical or cine mode acquisition) Patient (motion corruption) Barrett, JF and Keat N. RadioGraphics 24:

3 Typical CT numbers Air Lung ~-700 Fat ~-80 to120 Water 0 +/-5 Brain ~40 Soft Tissue ~40 to ~100 Bone 200 to > 600 Metal >1000

050 Water 140 kvp 120 kvp 80 kvp Bilinear scaling is most common 0.

4 CT PET µ-map Compton interactions dominate in low-z materials: air, water and soft tissue Photoelectric contributions dominate for mixtures of bone and soft tissue 511 kev µ [1/cm] Water 140 kvp 120 kvp 80 kvp Bilinear scaling is most common method to derive 511keV µ-values CT number [HU] from CT HUs CT HU PET µ (Bi-linear) matrix 6-10mm FWHM Gaussian Filter

5 Beam hardening (Physics-based) Selective removal of soft x-rays Hardened beam become more penetrating Mean energy shifts up

Beam hardening (Physics-based) Projections

attenuation then those passing through the

in the center compared to the edges Beam

Figures on right show improvement in CT number

6 Beam hardening (Physics-based) Projections through the periphery experience less attenuation then those passing through the center Reconstructed CT numbers are depressed in the center compared to the edges Beam hardening can be corrected by adding filtration Figures on right show improvement in CT number with increasing tube potential 80 kvp 13 ma 300 msec 120 kvp 200 ma 300 msec

7 Photon Starvation (Physics-based) Collection of some high attenuating projections resulting in low statistics Reconstruction with FBP magnifies the noise in these projections Can be overcome by Increasing tube current Automated tube current modulation

8 Ring Artifacts (Scanner-based) An inconsistent i detector response compared to its neighbors Caused by variations in detector gain or failed detector element Misinformation is projected in all angles Failed detector Detector projections Rotation direction

9 Ring artifacts (Scanner-based) Resulting ring artifact

10 Streaks (Patient-based) Caused by highly attenuating object in field-of-view Attenuation properties are too high to be correctly digitized The reconstructed object radiates streaks

defibrillators, dental implants, jewelry, surgical staples Work around

11 Metallic implants (Patient-based) Can occur in presence of: Prosthetics, catheterization, pacemakers and leads, implanted defibrillators, dental implants, jewelry, surgical staples Work around includes: Change slice angle, increase kilovoltage Software based dm metal artifact reduction

12 Metal artifact reduction A B C Metal artifact on = 1514nCi/mL Metal artifact t off = 1693nCi/mL 11.8% change

13 1. Thresholding 2. Pixel value reassignment 3. Morphological corrections 4. Smoothing [1/cm] Hamill et al. Phys Med Biol. 2006;51(11): µ 0 Off On Pixel Value

17 Barium Enema

18 MIP

21 Oral Contrast

24 Application of oral contrast media in coregistered positron emission tomography-ct METHODS: 30 patients received oral Gastrografin 45 min before data acquisition. iti The second group (30 pts) received no contrast medium. Two reviewers in consensus compared FDG-uptake in the gastrointestinal tract in PET images of both groups. RESULTS: No significant difference in FDG uptake in PET images in all regions of the gastrointestinal tract except the ascending colon was seen in both groups. No correlation was found in the location of increased FDG uptake and contrast media in the CT images. CONCLUSION: An oral contrast agent can be used for coregistered PET-CT without the introduction of artifacts in PET. Dizendorf EV et al. AJR Am J Roentgenol 2002 Aug;179(2):477-81

25 Focal Tracer Uptake: A Potential Artifact in Contrast-Enhanced Dual-Modality PET/CT Scans METHODS: Thirty patients t underwent whole-body PET/CT in the presence of intravenous and oral iodinated contrast agent. RESULTS: The contrast-induced PET artifact was present in 4 (13%) of 30 patients. CONCLUSION: Contrast-enhanced dual-modality PET/CT examinations may result in a PET artifact that is due to the transient bolus passage of undiluted intravenous contrast agent. Antoch G et al. J Nucl Med 2002 Oct;43(10):

26 Respiratory Motion (Patient-based) projections Bed direction Patient exhales Diaphragm moved up Patient inhales Diaphragm is down Detector ring Patient inhales or exhales a large volume of air The diaphragm is present in one projection but not in the successive projections as the bed moves The bed position eventually reaches the diaphragm a second time Resulting reconstructed images appear to have two diaphragm lobes

27 Mushroom artifact Reconstructed image Bed direction Detector ring

Scan type (Protocol-based) Sequential/Cine/Step-and-shoot

for every 180+fan angle Highly susceptible to motion

scanned Used in heart and lung imaging Spiral/Helical/Volume

Table is in different position for each projection Suppresses

positions Used in whole-body Became standard after 1990 Perry

30 Scan type (Protocol-based) Sequential/Cine/Step-and-shoot Table is stationary during scan Table moves to a new position for every 180+fan angle Highly susceptible to motion artifacts Image data only available for table positions scanned Used in heart and lung imaging Spiral/Helical/Volume scanning Continuous table movement during scan (z-direction) Table is in different position for each projection Suppresses motion artifacts Image data available over continuous slice positions Used in whole-body Became standard after 1990 Perry Sprawls PhD, Sprawls Educational Foundation

31 Stair step/contour (Protocol-based) Observed in sequential/cine scanning as discontinuities iti between slices Common in cardiac imaging employing ECG gating Image interpolated half-way between steps

Slow CT protocols for AC Is filtered back-projection the best algorithm for

Hawkeye Slow rotation ti Low-Output t CT Less expensive than traditional CT Lower

Streak artifacts are caused by motion of the chest during acquisition of CT slice.

Galt, Adaptive Bayesian Iterative Transmission Reconstruction for Attenuation Correction

32 Slow CT protocols for AC Is filtered back-projection the best algorithm for reconstructing CT scans to be used for attenuation correction? Hawkeye Slow rotation ti Low-Output t CT Less expensive than traditional CT Lower radiation dose (much lower ma) Slow rotation may produce artifacts in chest imaging Streak artifacts are caused by motion of the chest during acquisition of CT slice. Ji Chen, Ernest V. Garcia, Russell D. Folks, Aharon Peretz, and James R. Galt, Adaptive Bayesian Iterative Transmission Reconstruction for Attenuation Correction in Myocardial Perfusion Imaging with SPECT/Slow- Rotation Low-Output CT Systems, International Journal of Biomedical Imaging, vol. 2007, Article ID 18709, 7 pages, 2007.

33 Summary Many CT artifacts can be addressed by changes to the CT acquisition protocol Post-image artifact reduction tools are becoming increasingly i available Viewing non-ac PET images is useful in identifying patient-based artifacts Respiratory motion is a confounding factor in most PET/CT datasets CT contrast artifacts currently cannot by addressed by changing acquisition protocol parameters or post- imaging manipulation

34 CT for PET attenuation correction Most dedicated CT protocols are high dose compared to doses adequate for PET AC Calculated Dose by JAN Protoco kvp mas Collimation Pitch Length CTDI vol DLP ED* l [mm] [mm] [mgy] [mgycm] [msv] LD-CT LD-CT HD-CT HD-CT LD low dose; HD high dose Data summarized from Brix et al. JNM 46: *DLP->ED using 0.017mSv/mGy-cm The literature trend is decreasing CT dose for PET AC

35 CT for PET attenuation correction Many groups have worked on CT AC based on patient weight decreasing CT dose for PET Patient ma kvp AC, while maintaining Weight sufficient quality for [lbs] anatomical localizationli > Lowering CT dose for PET AC can lead to artifacts such as Poor photon penetration in high density areas from low kvp protocols (photon < starvation, beam hardening) High statistical noise unveiling inconsistencies in detector response (rings) Data summarized from Table 2 in Ki Krishnasetty et al. JACR 5: patients total, Mean DLP = 152.1, ED = 2.6mSv

36 One Patient, 3 CT protocols A B C Inspiration breath-hold Free-breathing Free-breathing Pitch = 1.5 Scan time = 2 sec Pitch = 0.5 Scan time = 14 sec Cine/Sequential Average CT Time/bed = 6.1 sec Scan time = 40 sec

37 PET/CT + average CT protocol CT scout 80 kv 10 ma CT (helical) 16 x 1.25 mm collimation, 120 kv, 300 ma, 0.5 sec, pitch 1.375:1 Patients are asked to hold breath at mid-expiration Dose ~ 5.5 to 11 mgy PET (2D) 3 mins/bed and 6 to 7 beds (15 cm/bed) Average CT from cine 5 mgy (1.2 msv for 14 cm coverage) Pan et al, Med. Phys. 2006

38 Average CT only over the thorax ACT Helical CT w/ ACT Helical CT w/o thorax Helical CT Pan et al, Med. Phys. 2006

39 Clinical Studies Mismatch 1: CT diaphgram position lower than PET +57% Mismatch 2: CT diaphgram position higher than PET Pan et al, JNM 05

40 Clinical Studies Mismatch 3: CT diaphgram position lower than PET +56% Mismatch 4: CT diaphgram position lower than PET +100% Pan et al, JNM 05

41 Average CT in cardiac PET HCT- misregistration ACT Pan et al, Med. Phys. 06

42 Cardiac PET/CT Dynamic CT averaging tests dynamic average* rest spiral stress spiral Courtesy of James Hamill, Siemens Medical Solutions and Bob Eisner, Emory University Midtown Hospital * typical patient, no problems with CT image quality 42

43 Cardiac PET/CT (cont.) Dynamic CT averaging (cont.) dynamic average* SLOW rest spiral SLOW stress spiral Courtesy of James Hamill, Siemens Medical Solutions and Bob Eisner, Emory University Midtown Hospital 43

Cardiac PET/CT (cont.) Dyna amic CT (cont.) 44 In this case the ACT image shows motion problems. You see these in the dome of the liver and in sagittal and coronal sections through the heart.

44 Cardiac PET/CT (cont.) Dyna amic CT (cont.) 44 In this case the ACT image shows motion problems. You see these in the dome of the liver and in sagittal and coronal sections through the heart. A user interface lets the operator select a more continuous subset of the dynamic data. This can be overlaid with the PET image a work in progress. Courtesy of James Hamill, Siemens Medical Solutions and Bob Eisner, Emory University Midtown Hospital

45 Helical CT, 120 kvp 76 ma Average CT, 80 kvp 13 ma 1.08 Rb-82 PET/CT 17 sector ratios PET T(Hi Dose CT) / PET(Low Dose CT) PET Data Nye et al Med. Phys. 36:1618

46 Helical CT Rb-82 PET/CT 17 sector ratios PET Data 1.4 Average CT PET(Hi Dose CT) / PET(Low Dose CT) Nye et al Med. Phys. 36:1618

47 Rest Stress Nye et al Med. Phys. 36:1618

48 Conclusions It is reasonable to expect a CT protocol to deliver 1-4mSv whole-body dose for adequate PET AC and CT localization HU to 511 kev µ-map conversion can smooth mild CT artifacts observed in low dose protocols Matching the temporal resolution of PET and CT in thoracic cavity continues to be an interesting challenge

The Latest in Radiation Dose Reduction Techniques in CT

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