Tolga Çukur

Attention during natural vision warps semantic representation across the human brain

Little is known about how attention changes the cortical representation of sensory information in humans. On the basis of neurophysiological evidence, we hypothesized that attention causes tuning changes to expand the representation of attended stimuli at the cost of unattended stimuli. To investigate this issue, we used functional magnetic resonance imaging to measure how semantic representation changed during visual search for different object categories in natural movies. We found that many voxels across occipito-temporal and fronto-parietal cortex shifted their tuning toward the attended category. These tuning shifts expanded the representation of the attended category and of semantically related, but unattended, categories, and compressed the representation of categories that were semantically dissimilar to the target. Attentional warping of semantic representation occurred even when the attended category was not present in the movie; thus, the effect was not a target-detection artifact. These results suggest that attention dynamically alters visual representation to optimize processing of behaviorally relevant objects during natural vision.

Improving Non-Contrast-Enhanced Steady-State Free Precession Angiography with Compressed Sensing

Flow‐independent angiography offers the ability to produce vessel images without contrast agents. Angiograms are acquired with magnetization‐prepared three‐dimensional balanced steady‐state free precession sequences, where the phase encodes are interleaved and the preparation is repeated before each interleaf. The frequent repetition of the preparation significantly decreases the scan efficiency. The number of excitations can instead be reduced with compressed sensing by exploiting the compressibility of the angiograms. Hence, the phase encodes can be undersampled to save scan time without significantly degrading image quality. These savings can be allotted for preparing the magnetization more often, or alternatively, improving resolution. The enhanced resolution and contrast achieved with the proposed method are demonstrated with lower leg angiograms. Depiction of the vasculature is significantly improved with the increased resolution in the phase‐encode plane and higher blood‐to‐background contrast. Magn Reson Med, 2009.

Non-contrast-enhanced flow-independent peripheral MR angiography with balanced SSFP.

Flow‐independent angiography is a non‐contrast‐enhanced technique that can generate vessel contrast even with reduced blood flow in the lower extremities. A method is presented for producing these angiograms with magnetization‐prepared balanced steady‐state free precession (bSSFP). Because bSSFP yields bright fat signal, robust fat suppression is essential for detailed depiction of the vasculature. Therefore, several strategies have been investigated to improve the reliability of fat suppression within short scan times. Phase‐sensitive SSFP can efficiently suppress fat; however, partial volume effects due to fat and water occupying the same voxel can lead to the loss of blood signal. In contrast, alternating repetition time (ATR) SSFP minimizes this loss; however, the level of suppression is compromised by field inhomogeneity. Finally, a new double‐acquisition ATR‐SSFP technique reduces this sensitivity to off‐resonance. In vivo results indicate that the two ATR‐based techniques provide more reliable contrast when partial volume effects are significant. Magn Reson Med, 2009.

Functional subdomains within human FFA.

The fusiform face area (FFA) is a well-studied human brain region that shows strong activation for faces. In functional MRI studies, FFA is often assumed to be a homogeneous collection of voxels with similar visual tuning. To test this assumption, we used natural movies and a quantitative voxelwise modeling and decoding framework to estimate category tuning profiles for individual voxels within FFA. We find that the responses in most FFA voxels are strongly enhanced by faces, as reported in previous studies. However, we also find that responses of individual voxels are selectively enhanced or suppressed by a wide variety of other categories and that these broader tuning profiles differ across FFA voxels. Cluster analysis of category tuning profiles across voxels reveals three spatially segregated functional subdomains within FFA. These subdomains differ primarily in their responses for nonface categories, such as animals, vehicles, and communication verbs. Furthermore, this segregation does not depend on the statistical threshold used to define FFA from responses to functional localizers. These results suggest that voxels within FFA represent more diverse information about object and action categories than generally assumed.

Positive Contrast with Alternating Repetition Time SSFP (PARTS): A Fast Imaging Technique for SPIO-Labeled Cells

There has been recent interest in positive‐contrast MRI methods for noninvasive tracking of cells labeled with superparamagnetic iron‐oxide nanoparticles. Low‐tip‐angle balanced steady‐state free precession sequences have been used for fast, high‐resolution, and flow‐insensitive positive‐contrast imaging; however, the contrast can be compromised by the limited suppression of the on‐resonant and fat signals. In this work, a new technique that produces positive contrast with alternating repetition time steady‐state free precession is proposed to achieve robust background suppression for a broad range of tissue parameters. In vitro and in vivo experiments demonstrate the reliability of the generated positive contrast. The results indicate that the proposed method can enhance the suppression level by up to 18 dB compared with conventional balanced steady‐state free precession. Magn Reson Med, 2010.

Enhanced spectral shaping in steady-state free precession imaging

Balanced steady‐state free precession (SSFP) is hindered by the inherent off‐resonance sensitivity and unwanted bright fat signal. Multiple‐acquisition SSFP combination methods, where multiple datasets with different fixed RF phase increments are acquired, have been used for shaping the SSFP spectrum to solve both problems. We present a new combination method (weighted‐combination SSFP or WC‐SSFP) that preserves SSFP contrast and enables banding‐reduction and fat‐water separation. Methods addressing the banding artifact have focused on either getting robust banding‐reduction (complex‐sum SSFP) or improved SNR efficiency (sum‐of‐squares SSFP). The proposed method achieves both robust banding‐reduction and an SNR efficiency close to that of the sum‐of‐squares method. A drawback of fat suppression methods that create a broad stop‐band around the fat resonance is the wedge shape of the stop‐band leading to imperfect suppression. WC‐SSFP improves the suppression of the stop‐band without affecting the pass‐band performance, and prevents fat signal from obscuring the tissues of interest in the presence of considerable resonant frequency variations. The method further facilitates the use of SSFP imaging by providing a control parameter to adjust the level of banding‐reduction or fat suppression to application‐specific needs. Magn Reson Med, 2007.

Multiple repetition time balanced steady-state free precession imaging.

Although balanced steady‐state free precession (bSSFP) imaging yields high signal‐to‐noise ratio (SNR) efficiency, the bright lipid signal is often undesirable. The bSSFP spectrum can be shaped to suppress the fat signal with scan‐efficient alternating repetition time (ATR) bSSFP. However, the level of suppression is limited, and the pass‐band is narrow due to its nonuniform shape. A multiple repetition time (TR) bSSFP scheme is proposed that creates a broad stop‐band with a scan efficiency comparable with ATR‐SSFP. Furthermore, the pass‐band signal uniformity is improved, resulting in fewer shading/banding artifacts. When data acquisition occurs in more than a single TR within the multiple‐TR period, the echoes can be combined to significantly improve the level of suppression. The signal characteristics of the proposed technique were compared with bSSFP and ATR‐SSFP. The multiple‐TR method generates identical contrast to bSSFP, and achieves up to an order of magnitude higher stop‐band suppression than ATR‐SSFP. In vivo studies at 1.5 T and 3 T demonstrate the superior fat‐suppression performance of multiple‐TR bSSFP. Magn Reson Med 62:193–204, 2009.

Multiple-profile homogeneous image combination: application to phase-cycled SSFP and multicoil imaging.

Signal inhomogeneities in MRI often appear as multiplicative weightings due to various factors such as field‐inhomogeneity dependencies for steady‐state free precession (SSFP) imaging or receiver sensitivities for coil arrays. These signal inhomogeneities can be reduced by combining multiple data sets with different weights. A sum‐of‐squares combination is typically used due to its simplicity and near‐optimal signal‐to‐noise ratio (SNR). However, this combination may lead to residual signal inhomogeneity. Alternatively, an optimal linear combination of the data can be performed if the weightings for individual data sets are estimated accurately. We propose a nonlinear combination to improve image‐based estimates of the individual weightings. The signal homogeneity can be significantly increased without compromising SNR. The improved performance of the method is demonstrated for SSFP banding artifact reduction and multicoil (phased‐array and parallel) image reconstructions. Magn Reson Med 60:732–738, 2008.

Fat-Water Separation with Alternating Repetition Time Balanced SSFP

Balanced SSFP achieves high SNR efficiency, but suffers from bright fat signal. In this work, a multiple‐acquisition fat‐water separation technique using alternating repetition time (ATR) balanced SSFP is proposed. The SSFP profile can be modified using alternating repetition times and appropriate phase cycling to yield two spectra where fat and water are in‐phase and out‐of‐phase, respectively. The signal homogeneity and the broad width of the created in‐phase and out‐of‐phase profiles lead to signal cancellation over a broad stop‐band. The stop‐band suppression is achieved for a wide range of flip angles and tissue parameters. This property, coupled with the inherent flexibility of ATR SSFP in repetition time selection, makes the method a good candidate for fat‐suppressed SSFP imaging. The proposed method can be tailored to achieve a smaller residual stop‐band signal or a decreased sensitivity to field inhomogeneity depending on application‐specific needs. Magn Reson Med 60:479–484, 2008.

Hadamard slice encoding for reduced-FOV diffusion-weighted imaging.

To improve the clinical utility of diffusion‐weighted imaging (DWI) by extending the slice coverage of a high‐resolution reduced field‐of‐view technique.