Donald B. Twieg

Widespread reward-system activation in obese women in response to pictures of high-calorie foods.

Behavioral studies have suggested that exaggerated reactivity to food cues, especially those associated with high-calorie foods, may be a factor underlying obesity. This increased motivational potency of foods in obese individuals appears to be mediated in part by a hyperactive reward system. We used a Philips 3T magnet and fMRI to investigate activation of reward-system and associated brain structures in response to pictures of high-calorie and low-calorie foods in 12 obese compared to 12 normal-weight women. A regions of interest (ROI) analysis revealed that pictures of high-calorie foods produced significantly greater activation in the obese group compared to controls in medial and lateral orbitofrontal cortex, amygdala, nucleus accumbens/ventral striatum, medial prefrontal cortex, insula, anterior cingulate cortex, ventral pallidum, caudate, putamen, and hippocampus. For the contrast of high-calorie vs. low-calorie foods, the obese group also exhibited a larger difference than the controls did in all of the same regions of interest except for the putamen. Within-group contrasts revealed that pictures of high-calorie foods uniformly stimulated more activation than low-calorie foods did in the obese group. By contrast, in the control group, greater activation by high-calorie foods was seen only in dorsal caudate, whereas low-calorie foods were more effective than high-calorie foods in the lateral orbitofrontal cortex, medial prefrontal cortex, and anterior cingulate cortex. In summary, compared to normal-weight controls, obese women exhibited greater activation in response to pictures of high-calorie foods in a large number of regions hypothesized to mediate motivational effects of food cues.

Differences in auditory processing of words and pseudowords: an fMRI study.

Although there has been great interest in the neuroanatomical basis of reading, little attention has been focused on auditory language processing. The purpose of this study was to examine the differential neuroanatomical response to the auditory processing of real words and pseudowords. Eight healthy right‐handed participants performed two phoneme monitoring tasks (one with real word stimuli and one with pseudowords) during a functional magnetic resonance imaging (fMRI) scan with a 4.1 T system. Both tasks activated the inferior frontal gyrus (IFG), the posterior superior temporal gyrus (pSTG) and the inferior parietal lobe (IPL). Pseudoword processing elicited significantly more activation within the posterior cortical regions compared with real word processing. Previous reading studies have suggested that this increase is due to an increased demand on the lexical access system. The left inferior frontal gyrus, on the other hand, did not reveal a significant difference in the amount of activation as a function of stimulus type. The lack of a differential response in IFG for auditory processing supports its hypothesized involvement in grapheme to phoneme conversion processes. These results are consistent with those from previous neuroimaging reading studies and emphasize the utility of examining both input modalities (e.g., visual or auditory) to compose a more complete picture of the language network. Hum. Brain Mapping 14:39–47, 2001.

Assessment of brain tumor angiogenesis inhibitors using perfusion magnetic resonance imaging: Quality and analysis results of a phase I trial

To determine thresholds of quality for a T2*‐weighted perfusion magnetic resonance imaging (MRI) study and evaluate the effects of an angiogenesis inhibitor on relative blood flow and volume changes in brain tumor patients in a multi‐institution setting.

Baseline conditions and subtractive logic in neuroimaging

Discrepancies in the patterns of cortical activation across studies may be attributable, in part, to differences in baseline tasks, and hence, reflect the limits of the subtractive logic underlying much of neuroimaging. To assess the extent of these effects, three of the most commonly used baseline conditions (rest, tone monitoring, and passive listening) were compared using phoneme discrimination as the experimental task. Eight participants were studied in a fMRI study with a 4.1 T system. The three baseline conditions systematically affected the amount of activation observed in the identical phoneme task with major affects in Brocas area, the left posterior superior temporal gyrus, and the left and right inferior parietal regions. Two central findings were: 1) a differential effect of baseline within each region, with the rest baseline condition producing the greatest amount of activation and the passive listening condition producing the least, and 2) systematic baseline task activation in the inferior parietal regions. These results emphasize the relativity of activation patterns observed in functional neuroimaging, and the necessity to specify the baseline processes in context to the experimental task processes. Hum. Brain Mapping 14:228–235, 2001.

Iterative reconstruction of single-shot spiral MRI with off resonance

A variety of applications and research directions in magnetic resonance imaging which require fast scan times have recently become popular. In order to satisfy many of the requirements of these applications, snapshot imaging methods, which acquire an entire image in one excitation, are often used. These snapshot techniques are relatively insensitive to motion and can allow rapidly occurring processes to be imaged. However, snapshot imaging techniques acquire data over a relatively long period, during which off-resonance phase can accumulate, leading to image degradation. This degradation often limits the usefulness of the images. Presented here is a method to iteratively reconstruct an image acquired by a spiral snapshot technique and to remove image degradation due to off resonance. This iterative method does not assume that the inhomogeneity is slowly varying within the image, allowing better results than with deblurring techniques which do not take abrupt changes into account. Although presented here with a spiral imaging technique, the iterative algorithm is general enough to be applied to a variety of snapshot imaging techniques.

Parsing local signal evolution directly from a single-shot MRI signal: a new approach for fMRI.

In this work a new single‐shot MRI method, single‐shot parameter assessment by retrieval from signal encoding (SS‐PARSE), is introduced. This method abandons a fundamental simplifying assumption that is used in conventional MRI methods. Established MRI methods implicitly assume that the local intrinsic signal does not change its amplitude or phase during signal acquisition, even though these changes may be substantial, especially during the relatively long signals used in single‐shot image acquisitions. SS‐PARSE, on the other hand, acknowledges local decay and phase evolution, and models each signal datum as a sample from (k,t)‐space rather than k‐space. Because of this more accurate signal model, SS‐PARSE promises improved performance in terms of accuracy and robustness, but requires more intensive reconstruction computations. The theoretical properties of the method are discussed, and simulation results are presented that demonstrate more robust and accurate measurements of relaxation rate changes associated with brain activation in functional MRI (fMRI), freedom from geometric errors due to off‐resonance frequencies, and better tolerance of the large susceptibility gradients that occur naturally in parts of the brain. In addition, this technique has the potential to assess nonexponential relaxation behavior during a single‐shot signal. Magn Reson Med 50:1043–1052, 2003.

Study of temporal stationarity and spatial consistency of fMRI noise using independent component analysis

Spatial independent component analysis (ICA) was used to study the temporal stationarity and spatial consistency of structured functional MRI (fMRI) noise. Spatial correlations have been used in the past to generate filters for the removal of structured noise for each time-course in an fMRI dataset. It would be beneficial to produce a multivariate filter based on the same principles. ICA is examined to determine if it has properties that are beneficial for this type of filtering. Six fMRI baseline datasets were decomposed via spatial ICA. The time-courses associated with each component were tested for wide-sense stationarity using the wide sense stationarity quotient (WSS). Each dataset was divided into three subsets and each subset was decomposed. The components of first and third subset were matched by the strength of their correlation. The components produced by ICA were found to have largely nonstationary time-courses. Despite the temporal nonstationarity in the data, ICA was found to produce consistent spatial components. The degree of correlation among components differed depending on the amount of dimension reduction performed on the data. It was found that a relatively small number of dimensions produced components that are potentially useful for generating a spatial fMRI filter.

Smaller regional gray matter volume in homeless african american cocaine-dependent men: a preliminary report.

Models of addiction include abnormalities in parts of the brain involving executive function/inhibitory control. Although previous studies have reported evidence of structural abnormalities in cocaine-dependent individuals, none have specifically targeted the homeless. The present preliminary study investigated brain structure in such an understudied group, homeless, crack-cocaine-dependent African American men (n = 9), comparing it to that in healthy controls (n = 8). Structural data were analyzed using voxel based morphometry (VBM) and a regions of interest (ROI) analysis. Homeless cocaine-dependent individuals had smaller gray matter volume in dorsolateral prefrontal cortex, anterior cingulate, the cerebellum, insula, and superior temporal gyrus. Most of these areas subserve executive function or inhibitory control. These results are similar to those found in most previous studies of non-homeless cocaine-dependent individuals. Reduced gray matter in executive function/inhibitory control regions of the brain in cocaine-dependent individuals may be a preexisting risk factor for the development of addiction and/or a consequence of drug abuse.

Basic Properties of SS-PARSE Parameter Estimates

Single shot parameter assessment by retrieval from signal encoding (SS-PARSE) is a recently introduced method to obtain quantitative parameter maps from a single-shot (typically 65 ms) magnetic resonance imaging (MRI) signal. Because it explicitly models local magnetization decay and phase evolution occurring during the signal 1) it can provide quantitative estimates of local transverse magnetization magnitude and phase, frequency, and relaxation rate and 2) it is free of geometric distortion or blurring due to field nonuniformities within the tissues. These properties promise to be advantageous in functional brain MRI (fMRI) and other dynamic imaging applications. In this paper, the basic phenomena underlying the performance of SS-PARSE in practice are discussed. Basic sources of bias errors in the parameter estimates are discussed, and performance of the method is characterized in terms of parameter estimates from simulation, experimental phantoms, and in vivo studies. Characteristics of the sum-of-square-error cost function and the iterative search algorithm are discussed, and their relative roles in determining estimation accuracy are described. Practical guidelines for use of the method are presented and discussed. In vivo parameter maps are also presented.

Improved depiction of small anatomic structures in MR images using Gaussian-weighted spirals and zero-filled interpolation

Partial-volume artifacts reduce the contrast and continuity of small structures in magnetic resonance images. Zero-filled interpolation (ZFI) has been known for some time as a useful technique to reduce partial-volume artifacts and improve the appearance of small structures and edges. However, its use is limited by the fact that ZFI can exacerbate image artifacts. For example, it can exacerbate Gibbs ringing, also known as the truncation artifact, which manifests itself as spurious ringing around sharp edges. Currently, the most common technique to address this problem is post-acquisition filtering, which causes blurring in the image. Using ZFI in conjunction with a variable-density sampling method designed to reduce ringing is proposed as a possible solution to this problem. This approach is demonstrated with a Gaussian-weighted spiral and is compared to conventional spiral sampling both with and without the application of a filter used to reduce ringing. The two spiral sampling techniques are compared using simulations, phantom images, and in vivo brain images. The Gaussian-weighted spiral demonstrates reduced ringing without the loss of spatial resolution commonly associated with post-acquisition filtering. Additionally, this sampling technique is shown to work well in conjunction with ZFI to reduce partial-volume artifacts without the apparent increase in Gibbs ringing usually associated with zero-filled reconstruction. This approach will be most useful for imaging techniques such as MR angiography which are known to be sensitive to partial-volume effects, as well as when imaging anatomic regions associated with more severe Gibbs ringing.