Functional Brain Imaging: Some Parameters for Success
Alkire, Michael T. M.D.
IN 1981, Phelps et al.1
published a positron emission tomography study showing that cerebral glucose utilization increases in the human visual cortex when subjects are scanned with their eyes open versus
when they are scanned with their eyes closed. Not a dramatic finding by today’s standards, but this landmark study helped found modern functional brain imaging because it demonstrated that those brain regions that are relatively more active have a regionally specific increase in metabolism, which can be noninvasively measured in vivo
in the living human brain. Twenty-three years later, we can now image regional changes in human brain activity without the risk of radioactivity exposure that positron emission tomography imaging requires by using the widely available functional magnetic resonance imaging (fMRI) technique with blood oxygenation level–dependent (BOLD) contrast.2,3
This technique exploits the fact that changes in the oxygen state of hemoglobin lead to measurable changes in the local magnetic field susceptibility of the blood and surrounding tissue. By tracking these susceptibility changes over time during a physiologic manipulation (i.e.
, a cognitive or sensorimotor challenge), one can measure localized changes in cerebral blood flow as a correlate of brain activity at finer spatial and temporal resolution than that afforded by existing positron emission tomography techniques. Application of fMRI with BOLD contrast to pain4
and anesthesia research5
are still in their infancy, and a multitude of questions regarding the proper application of this technique remain to be determined. In this issue of Anesthesiology, Ibinson et al.6
move the field forward by providing answers to three parameters that are relevant to the conduct of fMRI studies of pain.
The first parameter addressed by Ibinson et al.
concerns the question of signal attenuation. In an fMRI pain study where a pain stimulus is presented to subjects in an on-and-off manner (a so-called boxcar design), does the fMRI BOLD signal get smaller from one boxcar to the next as the session continues? Some studies suggest that it fades or attenuates from one block to the next,7,8
whereas other studies have not reported this effect.9,10
Ibinson et al.
found the BOLD signal in the fourth “on” block to be significantly lower than that seen in the first “on” block in the three brain regions of the anterior cingulate cortex, the somatosensory cortex, and the cerebellum, with the greatest attenuation (i.e.
, as much as 50%) occurring in the somatosensory cortex.
The second parameter addressed by Ibinson et al.6
concerns the period of time over which the BOLD signal might remain suppressed, given within-session signal attenuation. Ibinson et al.
determined whether the attenuation from one session would continue into another session that starts 4 min after the first. If so, this might be problematic for any study in which the experimental conditions are changed between sessions. If there is an attenuation carryover effect, the second imaging session would start with a reduced BOLD signal and, if an investigator did not counterbalance the active and control scan sessions, a systematic error might be falsely attributed to an experimental manipulation. Fortunately, for future studies, a signal attenuation carryover effect is not evident when a 4-min between-session break is used.
The third parameter addressed by Ibinson et al. concerns the most effective echo time to use in a pain study. These investigators determined whether a 40-ms or a 60-ms TE time would provide the most robust BOLD signal for imaging pain, given a repetition time of 3 s on a 1.5-T scanner. Their results do not reveal much difference between an echo time of 40 or 60 ms, and either seems appropriate.
The source of the within-session BOLD signal attenuation is not known, and many postulates have been offered. Activation of descending pain modulation pathways is one postulate, but this seems unlikely because the psychophysical perception of pain does not fade over a 30-s block of time,7,11
and it may actually increase near the end of that time window.9
Pain-induced global changes in cerebral blood flow have also been postulated,6,8,12
and this idea deserves further study. Another possibility may be stimulus-induced motion that directly varies with the BOLD signal. This is an important additional parameter for future fMRI studies to consider.
The studies showing BOLD signal attenuation have also noted some difficulty with pain-induced subject movement. Indeed, 31% of the scan sessions in the study of Ibinson et al.
were eliminated from analyses after applying an exclusion threshold limiting head motion to less than 1 mm of movement from one scan to the next and excluding sessions showing significant task-correlated motion. In the pain study of Kurata et al.
pain “on” stimulation periods were limited to 15 s, as opposed to 30 s for visual and motor tasks, to reduce pain-induced movement artifact and suggested that “more sophisticated techniques to correct head-motion artifacts . . . might permit longer duration of pain stimulus.” All of these studies applied the motion correction technique of realigning all of the session scans to the first scan in the series, but is this sufficient to correct for stimulus-induced motion artifact in the fMRI time series? As stated by Friston et al.13
in reference to their retrospective motion correction procedure, “This [motion correction] approach is predicated on the observation that movement-related effects are extant even after perfect realignment.”
The studies that did not report a within-session pain-induced attenuation of the BOLD response used slightly different techniques that may have minimized motion-related effects. Pain-induced motion artifact may have been reduced in the study by Apkarian et al.9
because they used a surface head coil and they had their subjects move their own hands onto the pain stimulus, which allowed their subjects to anticipate and prepare for the pain. In a recent study by our group, we used the approach of Friston et al.13
for motion correction. With this approach, the within-session time series movement parameters that were used to realign the time series are entered into the design matrix of the study such that the linear translations and rotations of the image volume are treated as confounds within the statistical model. This procedure effectively eliminates the signal covariance that is directly due to subject movement and is particularly effective at minimizing or removing stimulus-induced movement effects. Friston et al.13
claim that, using their motion correction technique, their own “empirical analyses suggest that (in extreme situations) over 90% of fMRI signal can be attributed to movement, and that this artifactual component can be successfully removed.”
Motion-induced artifact in the fMRI time series will continue to pose a problem for pain researchers well into the future. This is especially true for studies that attempt to precisely quantify changes in the evoked hemodynamics caused by various pain stimuli. Perhaps recent developments in prospective motion correction techniques will help to minimize concern related to stimulus-evoked motion.14
The potential for functional imaging to help elucidate mechanisms of pain and elucidate anesthesia is enormous. The work of Ibinson et al. reminds us that that any field of research moves forward sometimes with giant steps and sometimes by simply understanding the basic parameters that produce the experimental signals of interest.
1. Phelps ME, Kuhl DE, Mazziota JC: Metabolic mapping of the brain’s response to visual stimulation: Studies in humans. Science 1981; 211:1445–8
2. Ogawa S, Tank DW, Menon R, Ellermann JM, Kim SG, Merkle H, Ugurbil K: Intrinsic signal changes accompanying sensory stimulation: Functional brain mapping with magnetic resonance imaging. Proc Natl Acad Sci U S A 1992; 89:5951–5
3. Kwong KK, Belliveau JW, Chesler DA, Goldberg IE, Weisskoff RM, Poncelet BP, Kennedy DN, Hoppel BE, Cohen MS, Turner R, Cheng H-M, Brady TJ, Rosen BR: Dynamic magnetic resonance imaging of human brain activity during primary sensory stimulation. Proc Natl Acad Sci U S A 1992; 89:5675–9
4. Davis KD, Wood ML, Crawley AP, Mikulis DJ: fMRI of human somatosensory and cingulate cortex during painful electrical nerve stimulation. Neuroreport 1995; 7:321–5
5. Antognini JF, Buonocore MH, Disbrow EA, Carstens E: Isoflurane anesthesia blunts cerebral responses to noxious and innocuous stimuli: A fMRI study. Life Sci 1997; 61:L349–54
6. Ibinson JW, Small RH, Algaze A, Roberts CJ, Clark DL, Schmalbrock P: Functional magnetic resonance imaging studies of pain: An investigation of signal decay during and across sessions. Anesthesiology 2004; 101:960–9
7. Becerra LR, Breiter HC, Stojanovic M, Fishman S, Edwards A, Comite AR, Gonzalez RG, Borsook D: Human brain activation under controlled thermal stimulation and habituation to noxious heat: An fMRI study. Magn Reson Med 1999; 41:1044–57
8. Kurata J, Thulborn KR, Gyulai FE, Firestone LL: Early decay of pain-related cerebral activation in functional magnetic resonance imaging: Comparison with visual and motor tasks. Anesthesiology 2002; 96:35–44
9. Apkarian AV, Darbar A, Krauss BR, Gelnar PA, Szeverenyi NM: Differentiating cortical areas related to pain perception from stimulus identification: Temporal analysis of fMRI activity. J Neurophysiol 1999; 81:2956–63
10. Alkire MT, White NS, Hsieh R, Haier RJ: Dissociable brain activation responses to 5-Hz electrical pain stimulation: A high-field functional magnetic resonance imaging study. Anesthesiology 2004; 100:939–46
11. Koyama Y, Koyama T, Kroncke AP, Coghill RC: Effects of stimulus duration on heat induced pain: The relationship between real-time and post-stimulus pain ratings. Pain 2004; 107:256–66
12. Coghill RC, Sang CN, Berman KF, Bennett GJ, Iadarola MJ: Global cerebral blood flow decreases during pain. J Cereb Blood Flow Metab 1998; 18:141–7
13. Friston KJ, Williams S, Howard R, Frackowiak RS, Turner R: Movement-related effects in fMRI time-series. Magn Reson Med 1996; 35:346–55
14. Thesen S, Heid O, Mueller E, Schad LR: Prospective acquisition correction for head motion with image-based tracking for real-time fMRI. Magn Reson Med 2000; 44:457–65
This article has been cited 1 time(s).
© 2004 American Society of Anesthesiologists, Inc.
Publication of an advertisement in Anesthesiology Online does not constitute endorsement by the American Society of Anesthesiologists, Inc. or Lippincott Williams & Wilkins, Inc. of the product or service being advertised.