Susceptibility Induced Loss of Signal: Comparing PET and f MRI on a Semantic Task

1. Susceptibility Induced Loss of Signal:Comparing PET and fMRIon a Semantic Task Devlin et al. (in press)

2. Introduction • fMRI and PET are both popular techniques for investigating neural correlates of cognitive processes • Several advantages to fMRI: • greater temporal and spatial resolution allowing for event-based and trial-based experiments • doesn’t require exposure to radioactive isotopes

3. Introduction • PET findings, however, are not always able to be replicated with fMRI paradigms • Imaging tissue near air-filled sinuses can result in geometric distortion or worse, loss of BOLD signal (i.e., susceptibility artifacts)

4. Courtesy of Dr. James Danckert

5. Introduction • Given the close proximity of major language areas to air-filled sinuses, fMRI investigation of language processing has been particularly problematic • Similar investigations of lexical-semantic processing show activation of these temporal language regions with PET but not with fMRI (Perani et al., 1999; Kiehl et al., 1999)

6. How are these susceptibility artifacts overcome? • define regions of interest (ROI) a priori to increase statistical power • use Worsley et al.’s (1996) small volume statistical correction

7. Study Objective • Investigate the usefulness of the Worsley et al.‘s (1996) statistical correction by examining the activation patterns present during a semantic categorizing task using both PET and fMRI

8. Methods. Semantic Categorization Task • Semantic Categorization Task. Subjects read 3 cued words consecutively presented, then made a decision as to whether the fourth (target) word belonged to that category • Example: dolphin, seal, walrus, OTTER • Stimuli: cued and target words displayed for 200ms at 400ms intervals • Responses: “Same” and “Different” were indicated by right- or left-mouse clicks; 1750ms provided after target word for response

9. Methods. Letter Categorization Task 2) Letter Categorization Task. Same stimulus and response characteristics but with no lexical or semantic component. Example: fffffff, ffff, ffffff, FFFFF

10. PET Experiment

11. Methods. PET Participants. 8 healthy, English speaking males aged 21 – 47 (mean 28) Stimuli presentation. • Blocked design • Twelve 90 sec. scans (8 semantic categorization, 4 letter categorization) were presented. 45 sec. of stimuli presentation, 45 sec. of blank screen • Each subject saw the conditions in a different order

12. Methods. PET Functional Imaging Details • GE Advance PET Scanner • 35 image planes, each 4.25mm thick. • Axial field-of-view (FOV) = 15.3cm • Voxel size = 2.34mm x 2.34mm x 4.25mm

13. Results: PET Broca’s area (BA44/45) R. cerebellum Inferior temp. gyrus (BA 20) Ant-medial temp. pole (BA 38) Figure 1. Areas of activation in the Semantic Minus Letter categorization comparison

14. Results: PET • Areas of Activations: R. cerebellum, L. inferior temporal gyrus, L. anterior medial temporal pole, and Broca’s area • All activations were significant at the cluster level • All activations except that observed in the temporal pole was significant at the voxel level

15. Conclusions. PET • Activation areas are consistent with previous studies looking at lexico-semantic activation • Both activated regions of the temporal lobe are areas affected by susceptibility artifacts

16. fMRI Experiment

17. Methods. fMRI • Methodology was the same as PET study but with 192 semantic trials (8 trials/block with 12 semantic semantic blocks/session with two sessions)

18. Methods. fMRI • Biophysical Parameters: • Varian-Siemens 3T MR scanner used • A head gradient coil used along with a birdcage head radio-frequency coil • A gradient-echo EPI sequence was used for image collection (TR 3s, TE 30ms, 64 x 64 resolution, 256mm x 256mm FOV) • 21 slices with 6mm slice thickness and in-plane resolution of 4mm.

19. Results. fMRI Broca’s area (BA 44/45) BA44/45 R. cerebellum BA 8 R-Hem “Broca’s” Figure 1. Areas of activation in the Semantic Minus Letter categorization comparison

20. Results. fMRI • Areas of activation: L. frontal lobe extending from the inferior frontal gyrus (BA47) into Broca’s area (BA44/45), R. frontal (BA44/45), and L. medial surface of the superior frontal cortex (BA8) • No reliable activations in the temporal lobe

21. Applying Statistical Correction to fMRI data • Worsley et al.’s (1996) small volume correction calculation was applied • The correction showed a reliable region of activation in the L. inferior temporal lobe, a region that was shown to be activated in the PET experiment • The correction, however, did not reveal activation in the L. anterior-medial temporal cortex, another area that was activated in the PET experiment

22. Results. fMRI and PET activation overlap Figure 3. A comparison between the semantic activation in the PET and fMRI experiments. Overlapping activation is shown in yellow. Red = PET; Green = fMRI

23. Author’s Discussion • Same task yielded differences in activation in PET and fMRI paradigms • The statistical correction compensated for the susceptibility artifact in affected temporal regions only when signal loss was relatively small • Other compensatory measures such as tailored RF pulse sequences may be more successful in recovering signal loss in susceptible temporal regions

24. Were the fMRI and PET Experiments Exactly the Same? • Different number of trials (96 vs. 192) • Length of stimulus blocks (45s vs. 30s) • Normalization of the images (PET template vs. EPI template) • Specification of the GLM (diff. in deg. of freedom) • Different participants

25. Appropriate Control Condition? • Letter and Semantic Categorization tasks differed by more than one factor… • differed in required semantic processing • differed in whether or not the participant was required to read

26. Areas of Activation Observed only in fMRI • Medial frontal (premotor area)