V Andrew Stenger

University of Hawaiʻi at Hilo, Hilo, Hawaii, United States

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Publications (95)490.76 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: PurposeTo design low peak and integrated power simultaneous multislice excitation radiofrequency pulses with transmit field inhomogeneity compensation in high field MRI.Theory and Methods The “interleaved greedy and local optimization” algorithm for small-tip-angle spokes pulses is extended to design multiband (MB) spokes pulses that simultaneously excite multiple slices, with independent spokes weight optimization for each slice. The peak power of the pulses is controlled using a slice phase optimization technique. Simulations were performed at 7T to compare the peak power of optimized MB spokes pulses to unoptimized pulses, and to compare the proposed slice-independent spokes weight optimization to a joint approach. In vivo experiments were performed at 7T to validate the pulse's function and compare them to conventional MB pulses.ResultsSimulations showed that the peak power-minimized pulses had lower peak power than unregularized and integrated power-regularized pulses, and that the slice-independent spokes weight optimization consistently produced lower flip angle inhomogeneity and lower peak and integrated power pulses. In the brain imaging experiments, the MB spokes pulses showed significant improvement in excitation flip angle and subsequently signal homogeneity compared to conventional MB pulses.Conclusion The proposed MB spokes pulses improve flip angle homogeneity in simultaneous multislice acquisitions at ultrahigh field, with minimal increase in integrated and peak radiofrequency power. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 09/2014; · 3.27 Impact Factor
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    ABSTRACT: PurposeQuantitative susceptibility map (QSM) reconstruction is ill posed due to the zero values on the “magic angle cone” that make the maps prone to streaking artifacts. We propose projection onto convex sets (POCS) in the method of steepest descent (SD) for QSM reconstruction. Methods Two convex projections, an object-support projection in the image domain and a projection in k-space were used. QSM reconstruction using the proposed SD-POCS method was compared with SD and POCS alone as well as with truncated k-space division (TKD) for numerically simulated and 7 Tesla (T) human brain phase data. ResultsThe QSM reconstruction error from noise-free simulated phase data using SD-POCS is at least two orders of magnitude lower than using SD, POCS, or TKD and has reduced streaking artifacts. Using the l1-TV reconstructed susceptibility as a gold standard for 7T in vivo imaging, SD-POCS showed better image quality comparing to SD, POCS, or TKD from visual inspection. ConclusionPOCS is an alternative method for regularization that can be used in an iterative minimization method such as SD for QSM reconstruction. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 03/2014; · 3.27 Impact Factor
  • Robert J Anderson, Benedikt A Poser, V Andrew Stenger
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    ABSTRACT: Simultaneous multislice (SMS) imaging can significantly increase image acquisition rates and improve temporal resolution and contrast in gradient-echo blood oxygen level-dependent (BOLD) functional MRI (fMRI) experiments. Through-plane signal loss due to B0 inhomogeneities at air-tissue interfaces limits fMRI of structures near the nasal cavity and ear canals. This study implemented spectral-spatial (SPSP) radiofrequency pulses for reduced through-plane signal loss across multiple simultaneously excited slices. Multiband (MB) and power independent of number of slices (PINS) methods are combined with SPSP excitation for signal loss compensation in slice-accelerated human brain imaging. Nine simultaneous slices of 5-mm thickness and 20 mm apart were excited using standard MB radiofrequency pulses and the proposed SPSP-SMS pulses, yielding coverage of 36 slices in four shots with 350-ms volume pulse repetition time. The pulses were compared in breath-hold fMRI at 3T. The SPSP-SMS pulses recovered ∼45% of voxels with signal loss in standard SMS images. Activation in areas of signal recovery increased by 26.4% using a 12.6-ms SPSP-MB pulse and 20.3% using a 12.1-ms SPSP-PINS pulse. It is demonstrated that SPSP-SMS pulses can improve BOLD sensitivity in areas of signal loss across simultaneous multiple slices. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 12/2013; · 3.27 Impact Factor
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    ABSTRACT: Background / Purpose: In this study we intended to:create an EPI method that provides signal-loss compensation in a single shotProvide images that are inherently free of Nyquist ghostingExtension to simultaneous multi-slice (“multiband”) EPI Main conclusion: The results indicate that single-shot z-shimming with IDEA EPI is a promising remedy for EPI signal loss. The benefit of “ghost-free” IDEA EPI images remains. A full activation study to assess IDEA EPI z-shimming vs other approaches is currently ongoing.
    19th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2013; 08/2013
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    ABSTRACT: Simultaneous multislice (SMS) acquisitions have recently received much attention as a means of increasing single-shot imaging speed. SMS acquisitions combine the advantages of single-shot sampling and acceleration along the slice dimension which was previously limited to three-dimensional (3D) volumetric acquisitions. A two-dimensional description of SMS sampling and reconstruction has become established in the literature. Here, we present a more general 3D Fourier encoding and reconstruction formalism for SMS acquisitions that can easily be applied to non-Cartesian SMS acquisitions. An "SMS 3D" k-space is defined in which the field of view along the slice select direction is equal to the number of excited slices times their separation. In this picture, SMS acceleration can be viewed as an undersampling of SMS 3D k-space that can be freely distributed between the in-plane and slice directions as both are effective phase-encoding directions. Use of the SMS 3D k-space picture is demonstrated in phantom and in vivo brain acquisitions including data obtained with blipped-controlled aliasing in parallel imaging sampling. SMS sensitivity encoding reconstruction is demonstrated as well as non-Cartesian SMS imaging using blipped spiral trajectories. The full framework of reconstruction methods can be applied to SMS acquisitions by employing a 3D k-space approach. The blipped-controlled aliasing in parallel imaging method can be viewed as a special case of undersampling an SMS 3D k-space. The extension of SMS methods to non-Cartesian 3D sampling and reconstruction is straightforward. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 07/2013; · 3.27 Impact Factor
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    ABSTRACT: PURPOSE: A technique is described for simultaneous multislice (SMS) excitation using radiofrequency (RF) parallel transmission (pTX). METHODS: Spatially distinct slices are simultaneously excited by applying different RF frequencies on groups of elements of a multichannel transmit array. The localized transmit sensitivities of the coil geometry are thereby exploited to reduce RF power. The method is capable of achieving SMS-excitation using single-slice RF pulses, or multiband pulses. SMS-pTX is demonstrated using eight-channel parallel RF transmission on a dual-ring pTX coil at 3 T. The effect on B1 (+) homogeneity and specific absorption rate (SAR) is evaluated experimentally and by simulations. Slice-GRAPPA reconstruction was used for separation of the collapsed slice signals. RESULTS: Phantom and in vivo brain data acquired with fast low-angle shot (FLASH) and blipped-controlled aliasing results in higher acceleration (CAIPIRINHA) echo-planar imaging are presented at SMS excitation factors of two, four, and six. We also show that with our pTX coil design, slice placement, and binary division of transmitters, SMS-pTX excitations can achieve the same mean flip angles excitations at ∼30% lower RF power than a conventional SMS approach with multiband RF pulses. CONCLUSION: The proposed SMS-pTX allows SMS excitations at reduced RF power by exploiting the local B1 (+) sensitivities of suitable multielement pTX arrays. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 05/2013; · 3.27 Impact Factor
  • Jazmin Camchong, Victor Andrew Stenger, George Fein
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    ABSTRACT: BACKGROUND: We previously reported that when long-term abstinent alcoholics (LTAA; with no drug comorbidity) are compared to controls, they show increased resting state synchrony (RSS) in the executive control network and reduced RSS in the appetitive drive network suggestive of compensatory mechanisms that may facilitate abstinence. The aim of the present study was to investigate whether long-term abstinent alcoholics with comorbid stimulants dependence (LTAAS) show similar RSS mechanisms. METHODS: Resting-state functional MRI data were collected on 36 LTAAS (20 females, age: 47.85±7.30), 23 LTAA (8 females, age: M=47.91±6.76), and 23 non-substance abusing controls (NSAC; 8 females, age: M=47.99±6.70). Using seed-based measures, we examined RSS with the nucleus accumbens (NAcc) and the subgenual anterior cingulate cortex (sgACC). RESULTS: Results showed commonalities in LTAA and LTAAS RSS (similar enhanced executive control RSS and left insula RSS) as well as differences (no attenuation of appetitive drive RSS in LTAAS and no enhancement of RSS in right insula in LTAA). CONCLUSIONS: We believe these differences are adaptive mechanisms that support abstinence. These findings suggest common as well as specific targets for treatment in chronic alcoholics with vs without comorbid stimulant dependence.
    Drug and alcohol dependence 04/2013; · 3.60 Impact Factor
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    ABSTRACT: Simultaneous multi-slice (SMS) imaging with receiver arrays allows for greatly accelerated 2D single-shot acquisitions with BOLD or diffusion-weighted EPI, and spirals. The commonly used SMS excitation methods require specially designed multi-band (MB) or PINS RF pulses to excite N slices. In this abstract we explore the use of parallel transmission (pTX) for SMS excitation, which can be achieved by uniquely frequency shifted slice-select pulses on subsets of pTX elements that are closets to the slices they excite. pTX has been shown to be useful for managing B + and B homogeneity and SAR. Using a dual-ring 3D pTX coil design and blipped-CAIPIRINHA EPI, we show that factor-2 SMS excitation with parallel transmission (pTX) can readily be achieved with conventional single-band RF pulses. Factor-4 SMS using dual-band RF pulses applied to each ring are also shown. For pTX coils with inherent TX-sensitivity encoding along the slice direction, the approach can reduce total required RF power (global SAR) compared to MB pulses with N frequency bands used on all coil elements as well as accelerating SMS multi-dimensional excitations.
    21st Annual Meeting of the International Society for Magnetic Resonance in Medicine, Salt Lake City, USA; 04/2013
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    Jazmin Camchong, Victor Andrew Stenger, George Fein
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    ABSTRACT: BACKGROUND: We previously reported that when compared with controls, long-term abstinent alcoholics (LTAA) have increased resting-state synchrony (RSS) of the inhibitory control network and reduced synchrony of the appetitive drive network, and hypothesized that these levels of synchrony are adaptive and support the behavioral changes required to maintain abstinence. In this study, we investigate whether these RSS patterns can be identified in short-term abstinent alcoholics (STAA). METHODS: Resting-state functional magnetic resonance imaging data were collected from 27 STAA, 23 LTAA, and 23 nonsubstance abusing controls (NSAC). We examined baseline RSS using seed-based measures. RESULTS: We found ordered RSS effects from NSAC to STAA and then to LTAA within both the appetitive drive and executive control networks: increasing RSS of the executive control network and decreasing RSS of the reward processing network. Finally, we found significant correlations between strength of RSS in these networks and (i) cognitive flexibility, and (ii) current antisocial behavior. CONCLUSIONS: Findings are consistent with an adaptive progression of RSS from short- to long-term abstinence, so that, compared with normal controls, the synchrony (i) within the reward network progressively decreases, and (ii) within the executive control network progressively increases.
    Alcoholism Clinical and Experimental Research 02/2013; · 3.42 Impact Factor
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    ABSTRACT: Background / Purpose: Echo planar imaging (EPI) is popular in BOLD functional magnetic resonance imaging (fMRI) due to its acquisition speed and sensitivity. Unfortunately it is very prone to artifacts including Nyquist ghosts, as the oscillating readout gradient requires reversal of alternate lines. The reconstruction is hence sensitive to gradient delays, causing violation of the Nyquist criterion that leads to N/2 ghost artifacts. The ghost image is a superimposed copy of the image, and even activation, shifted by 1/2 FoV. Main conclusion: We propose an approach termed “Interleaved Dual-Echo with Acceleration EPI” (IDEA EPI): two inherently ghost-free images are acquired separately under the positive and negative gradients. The echo time difference between the two images also allows extraction of field maps for “dynamic” unwarping of EPI time series.
    18th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2012; 08/2012
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    ABSTRACT: Background / Purpose: Simultaneous multi-slice acquisition (SMS) allows considerable temporal resolution (TR) reduction. SMS has recently attracted much interest for echo-planar imaging (EPI) based BOLD fMRI and diffusion-tensor imaging (DTI). In 2D single-shot acquisitions SMS achieves very effective speed-ups by the nominal multi-slice factor where parallel imaging hardly helps. Main conclusion: We investigated multiplexed single-shot spiral-in sequences with multi-band (MB) and PINS6 excitation pulses:spirals are an efficient way to cover k-spaceinward-spiral provides a “compact” sequence without much dead time: TR~TEFull brain coverage at 3.5x3.5x5mm 3 with a multi-slice factor of 3 yields TR= 310ms and fully samples cardiac pulsations.
    18th Annual Meeting of the Organization for Human Brain Mapping (OHBM) 2012; 08/2012
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    ABSTRACT: Echo planar imaging (EPI) is most commonly used for blood oxygen level-dependent fMRI, owing to its sensitivity and acquisition speed. A major problem with EPI is Nyquist (N/2) ghosting, most notably at high field. EPI data are acquired under an oscillating readout gradient and hence vulnerable to gradient imperfections such as eddy current delays and off-resonance effects, as these cause inconsistencies between odd and even k-space lines after time reversal. We propose a straightforward and pragmatic method herein termed "interleaved dual echo with acceleration (IDEA) EPI": two k-spaces (echoes) are acquired under the positive and negative readout lobes, respectively, by performing phase encoding blips only before alternate readout gradients. From these two k-spaces, two almost entirely ghost free images per shot can be constructed, without need for phase correction. The doubled echo train length can be compensated by parallel imaging and/or partial Fourier acquisition. The two k-spaces can either be complex averaged during reconstruction, which results in near-perfect cancellation of residual phase errors, or reconstructed into separate images. We demonstrate the efficacy of IDEA EPI and show phantom and in vivo images at both 3 T and 7 T. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 03/2012; · 3.27 Impact Factor
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    ABSTRACT: Susceptibility induced signal loss is a limitation in gradient echo functional MRI. The through-plane artifact in axial slices is particularly problematic due to the inferior position of air cavities in the brain. Spectral-spatial radiofrequency pulses have recently been shown to reduce signal loss in a single excitation. The pulses were successfully demonstrated assuming a linear relationship between susceptibility gradient and frequency, however, the exact frequency and spatial distribution of the susceptibility gradient in the brain is unknown. We present a spiral spectroscopic imaging sequence with a time-shifted radiofrequency pulse that can spectrally decompose the through-plane susceptibility gradient for spectral-spatial radiofrequency pulse design. Maps of the through-plane susceptibility gradient as a function of frequency were generated for the human brain at 3T. We found that the linear relationship holds well for the whole brain with an optimal slope of -1.0 μT/m/Hz. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 02/2012; · 3.27 Impact Factor
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    Weiran Deng, Cungeng Yang, V Andrew Stenger
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    ABSTRACT: Multidimensional radiofrequency (RF) pulses are of current interest because of their promise for improving high-field imaging and for optimizing parallel transmission methods. One major drawback is that the computation time of numerically designed multidimensional RF pulses increases rapidly with their resolution and number of transmitters. This is critical because the construction of multidimensional RF pulses often needs to be in real time. The use of graphics processing units for computations is a recent approach for accelerating image reconstruction applications. We propose the use of graphics processing units for the design of multidimensional RF pulses including the utilization of parallel transmitters. Using a desktop computer with four NVIDIA Tesla C1060 computing processors, we found acceleration factors on the order of 20 for standard eight-transmitter two-dimensional spiral RF pulses with a 64 × 64 excitation resolution and a 10-μsec dwell time. We also show that even greater acceleration factors can be achieved for more complex RF pulses.
    Magnetic Resonance in Medicine 02/2011; 65(2):363-9. · 3.27 Impact Factor
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    Cungeng Yang, Weiran Deng, V Andrew Stenger
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    ABSTRACT: Susceptibility artifacts and transmission radio frequency (RF) field (B(1) +) inhomogeneity are major limitations in high-field gradient echo MRI. Previously proposed numerical 2D spectral-spatial RF pulses have been shown to be promising for reducing the through-plane signal loss susceptibility artifact by incorporating a frequency-dependent through-plane phase correction. This method has recently been extended to 4D spectral-spatial RF pulse designs for reducing B(1) + inhomogeneity as well as the signal loss. In this manuscript, we present simple analytical pulse designs for constructing 2D and 4D spectral-spatial RF pulses as an alternative to the numerical approaches. The 2D pulse capable of exciting slices with reduced signal loss and is lipid suppressing. The 4D pulse simultaneously corrects signal loss as well as the B(1) + inhomogeneity from a body coil transmitter. The pulses are demonstrated with simulations and with gradient echo phantom and brain images at 3T using a standard RF body coil. The pulses were observed to work well for multiple slices and several volunteers.
    Magnetic Resonance in Medicine 02/2011; 65(2):370-6. · 3.27 Impact Factor
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    ABSTRACT: Head motion during (1)H MR spectroscopy acquisitions may compromise the quality and reliability of in vivo metabolite measurements. Therefore, a three-plane image-based motion-tracking module was integrated into a single-voxel (1)H MR spectroscopy (point-resolved spectroscopy) sequence. A series of three orthogonal spiral navigator images was acquired immediately prior to the MR spectroscopy water suppression module in order to estimate head motion. By applying the appropriate rotations and translations, the MR spectroscopy voxel position can be updated such that it remains stationary with respect to the brain. Frequency and phase corrections were applied during postprocessing to reduce line width and restore coherent averaging. Spectra acquired during intentional head motion in 11 volunteers demonstrate reduced lipid contamination and increased spectral reproducibility when motion correction is applied.
    Magnetic Resonance in Medicine 09/2010; 64(3):672-9. · 3.27 Impact Factor
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    ABSTRACT: Susceptibility artifacts and excitation radiofrequency field B(1)+ inhomogeneity are major limitations in high-field MRI. Parallel transmission methods are promising for reducing artifacts in high-field applications. In particular, three-dimensional RF pulses have been shown to be useful for reducing B(1)+ inhomogeneity using multiple transmitters due to their ability to spatially shape the slice profile. Recently, two-dimensional spectral-spatial pulses have been demonstrated to be effective for reducing the signal loss susceptibility artifact by incorporating a frequency-dependent through-plane phase correction. We present the use of four-dimensional spectral-spatial RF pulses for simultaneous B(1)+ and through-plane signal loss susceptibility artifact compensation. The method is demonstrated with simulations and in T(2)*-weighted human brain images at 3 T, using a four-channel parallel transmission system. Parallel transmission was used to reduce the in-plane excitation resolution to improve the slice-selection resolution between two different pulse designs. Both pulses were observed to improve B(1)+ homogeneity and reduce the signal loss artifact in multiple slice locations and several human volunteers.
    Magnetic Resonance in Medicine 07/2010; 64(1):1-8. · 3.27 Impact Factor
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    Weiran Deng, V Andrew Stenger
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    ABSTRACT: Three-dimensional spatial-spectral radiofrequency pulses using a stack-of-spirals trajectory can achieve two-dimensional spatial localization and one-dimensional spectral selection simultaneously. These pulses are useful, for example, in reduced field-of-view applications that also require frequency specificity such as lipid imaging. A limitation of the pulse design is that the length of the spiral trajectory is fixed by the frequency separation of lipid and water. This restricts the highest possible excitation resolution of the spatial profile over a given field of excitation. In this work, we examine the use of periodically rotated variable-density spirals to increase the spatial excitation resolution without changing the frequency selectivity. Variable-density spirals are used to undersample the high spatial frequencies such that higher excitation resolutions can be obtained with a small expense in increased aliasing of the slice profile. The periodic rotation of the spiral trajectories reduces the impact of the undersampling by distributing the aliasing in the frequency domain. The technique is demonstrated with simulations, phantom studies, and imaging human leg muscle at 3 T. It was found in the human study that the spatial excitation resolution could be improved from 6 x 6 to 8 x 8 (matrix size over a fixed field of view) while decreasing aliasing by approximately 40-60%.
    Magnetic Resonance in Medicine 02/2010; 63(3):828-34. · 3.27 Impact Factor
  • W Deng, L Chang, H Nakama, G Fein, VA Stenger
    NeuroImage 07/2009; 47. · 6.25 Impact Factor
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    ABSTRACT: The signal loss susceptibility artifact is a major limitation in gradient-echo MRI applications. Various methods, including z-shim techniques and multidimensional tailored radio frequency (RF) pulses, have been proposed to mitigate the through-plane signal loss artifact, which is dominant in axial slices above the sinus region. Unfortunately, z-shim techniques require multiple steps and multidimensional RF methods are complex, with long pulse lengths. Parallel transmission methods were recently shown to be promising for improving B1 inhomogeneity and reducing the specific absorption rate. In this work, a novel method using time-shifted slice-select RF pulses is presented for reducing the through-plane signal loss artifact in parallel transmission applications. A simultaneous z-shim is obtained by concurrently applying unique time-shifted pulses on each transmitter. The method is shown to reduce the signal loss susceptibility artifact in gradient-echo images using a four-channel parallel transmission system at 3T.
    Magnetic Resonance in Medicine 02/2009; 61(2):255-9. · 3.27 Impact Factor

Publication Stats

8k Citations
490.76 Total Impact Points


  • 2013–2014
    • University of Hawaiʻi at Hilo
      Hilo, Hawaii, United States
  • 2008–2013
    • Honolulu University
      Honolulu, Hawaii, United States
  • 2006–2013
    • University of Hawaiʻi at Mānoa
      • Department of Medicine
      Honolulu, HI, United States
    • University of Michigan
      • Department of Biomedical Engineering
      Ann Arbor, MI, United States
  • 2012
    • Radboud University Nijmegen
      Nymegen, Gelderland, Netherlands
  • 2005–2008
    • University of California, Davis
      Davis, California, United States
  • 1998–2008
    • University of Pittsburgh
      • • Department of Psychiatry
      • • Bioengineering
      • • Psychology
      • • Department of Radiology
      Pittsburgh, PA, United States
  • 2000–2005
    • Carnegie Mellon University
      • Department of Psychology
      Pittsburgh, PA, United States
    • Western Psychiatric Institute and Clinic
      Pittsburgh, Pennsylvania, United States
  • 2004
    • University of Missouri
      • Department of Psychological Sciences
      Columbia, MO, United States
  • 2002
    • University of Florida
      Gainesville, Florida, United States