Publications by Year: 2008

Junichi Tokuda, Shigehiro Morikawa, Hasnine A Haque, Tetsuji Tsukamoto, Kiyoshi Matsumiya, Hongen Liao, Ken Masamune, and Takeyoshi Dohi. 2008. “Adaptive 4D MR imaging using navigator-based respiratory signal for MRI-guided therapy.” Magn Reson Med, 59, 5, Pp. 1051-61.Abstract
For real-time 3D visualization of respiratory organ motion for MRI-guided therapy, a new adaptive 4D MR imaging method based on navigator echo and multiple gating windows was developed. This method was designed to acquire a time series of volumetric 3D images of a cyclically moving organ, enabling therapy to be guided by synchronizing the 4D image with the actual organ motion in real time. The proposed method was implemented in an open-configuration 0.5T clinical MR scanner. To evaluate the feasibility and determine optimal imaging conditions, studies were conducted with a phantom, volunteers, and a patient. In the phantom study the root mean square (RMS) position error in the 4D image of the cyclically moving phantom was 1.9 mm and the imaging time was approximately 10 min when the 4D image had six frames. In the patient study, 4D images were successfully acquired under clinical conditions and a liver tumor was discriminated in the series of frames. The image quality was affected by the relations among the encoding direction, the slice orientation, and the direction of motion of the target organ. In conclusion, this study has shown that the proposed method is feasible and capable of providing a real-time dynamic 3D atlas for surgical navigation with sufficient accuracy and image quality.
Jong-Hwan Lee, Heather M O'Leary, HyunWook Park, Ferenc A Jolesz, and Seung-Schik Yoo. 2008. “Atlas-based multichannel monitoring of functional MRI signals in real-time: automated approach.” Hum Brain Mapp, 29, 2, Pp. 157-66.Abstract
We report an automated method to simultaneously monitor blood-oxygenation-level-dependent (BOLD) MR signals from multiple cortical areas in real-time. Individual brain anatomy was normalized and registered to a pre-segmented atlas in standardized anatomical space. Subsequently, using real-time fMRI (rtfMRI) data acquisition, localized BOLD signals were measured and displayed from user-selected areas labeled with anatomical and Brodmann's Area (BA) nomenclature. The method was tested on healthy volunteers during the performance of hand motor and internal speech generation tasks employing a trial-based design. Our data normalization and registration algorithm, along with image reconstruction, movement correction and a data display routine were executed with enough processing and communication bandwidth necessary for real-time operation. Task-specific BOLD signals were observed from the hand motor and language areas. One of the study participants was allowed to freely engage in hand clenching tasks, and associated brain activities were detected from the motor-related neural substrates without prior knowledge of the task onset time. The proposed method may be applied to various applications such as neurofeedback, brain-computer-interface, and functional mapping for surgical planning where real-time monitoring of region-specific brain activity is needed.
Stephan E. Maier and Robert V. Mulkern. 2008. “Biexponential Analysis of Diffusion Related Signal Decay in Normal Human Cortical and Deep Gray Matter.” Magn Reson Imaging, 26, 7, Pp. 897-904.Abstract

Diffusion imaging with high-b factors, high spatial resolution and cerebrospinal fluid signal suppression was performed in order to characterize the biexponential nature of the diffusion-related signal decay with b-factor in normal cortical gray and deep gray matter (GM). Integration of inversion pulses with a line scan diffusion imaging sequence resulted in 91% cerebrospinal fluid signal suppression, permitting accurate measurement of the fast diffusion coefficient in cortical GM (1.142+/-0.106 microm2/ms) and revealing a marked similarity with that found in frontal white matter (WM) (1.155+/-0.046 microm2/ms). The reversal of contrast between GM and WM at low vs high b-factors is shown to be due to a significantly faster slow diffusion coefficient in cortical GM (0.338+/-0.027 microm2/ms) than in frontal WM (0.125+/-0.014 microm2/ms). The same characteristic diffusion differences between GM and WM are observed in other brain tissue structures. The relative component size showed nonsignificant differences among all tissues investigated. Cellular architecture in GM and WM are fundamentally different and may explain the two- to threefold higher slow diffusion coefficient in GM.

Nathan McDannold, Natalia Vykhodtseva, and Kullervo Hynynen. 2008. “Blood-brain barrier disruption induced by focused ultrasound and circulating preformed microbubbles appears to be characterized by the mechanical index.” Ultrasound Med Biol, 34, 5, Pp. 834-40.Abstract
This work investigated the effect of ultrasonic frequency on the threshold for blood-brain barrier (BBB) disruption induced by ultrasound pulses combined with an ultrasound contrast agent. Experiments were performed in rabbits using pulsed sonications at 2.04 MHz with peak pressure amplitudes ranging from 0.3 to 2.3 MPa. BBB disruption was evaluated using contrast-enhanced magnetic resonance imaging. The threshold for BBB disruption was estimated using probit regression. Representative samples with similar amounts of contrast enhancement were examined in light microscopy. Results from these experiments were compared with data from previous studies that used ultrasound frequencies between 0.26 and 1.63 MHz. We found that the BBB disruption threshold (value where the probability for disruption was estimated to be 50%) expressed in terms of the peak negative pressure amplitude increased as a function of the frequency. It appeared to be constant, however, when the exposures were expressed as a function of the mechanical index (peak negative pressure amplitude estimated in situ divided by square root of frequency). Regression of data from all frequencies resulted in an estimated mechanical index threshold of 0.46 (95% confidence intervals: 0.42 to 0.50). Histologic examination of representative samples with similar amounts of blood-brain barrier disruption found that the number of regions containing extravasated red blood cells per unit area was substantially lower on average for lower ultrasound frequencies. This data suggests that the mechanical index is a meaningful metric for ultrasound-induced blood-brain barrier disruption, at least for when other parameters that are not taken into account by the mechanical index are not varied. It also suggests that lower frequency sonication produces less red blood cell extravasation per unit area.
Zhikui Xiao, W. Scott Hoge, RV Mulkern, Lei Zhao, Guangshu Hu, and Walid E. Kyriakos. 2008. “Comparison of Parallel MRI Reconstruction Methods for Accelerated 3D Fast Spin-echo Imaging.” Magn Reson Med, 60, 3, Pp. 650-60.Abstract

Parallel MRI (pMRI) achieves imaging acceleration by partially substituting gradient-encoding steps with spatial information contained in the component coils of the acquisition array. Variable-density subsampling in pMRI was previously shown to yield improved two-dimensional (2D) imaging in comparison to uniform subsampling, but has yet to be used routinely in clinical practice. In an effort to reduce acquisition time for 3D fast spin-echo (3D-FSE) sequences, this work explores a specific nonuniform sampling scheme for 3D imaging, subsampling along two phase-encoding (PE) directions on a rectilinear grid. We use two reconstruction methods-2D-GRAPPA-Operator and 2D-SPACE RIP-and present a comparison between them. We show that high-quality images can be reconstructed using both techniques. To evaluate the proposed sampling method and reconstruction schemes, results via simulation, phantom study, and in vivo 3D human data are shown. We find that fewer artifacts can be seen in the 2D-SPACE RIP reconstructions than in 2D-GRAPPA-Operator reconstructions, with comparable reconstruction times.

Neculai Archip, Olivier Clatz, Stephen Whalen, Simon P Dimaio, Peter M Black, Ferenc A Jolesz, Alexandra J Golby, and Simon K Warfield. 2008. “Compensation of geometric distortion effects on intraoperative magnetic resonance imaging for enhanced visualization in image-guided neurosurgery.” Neurosurgery, 62, 3 Suppl 1, Pp. 209-15; discussion 215-6.Abstract

OBJECTIVE: Preoperative magnetic resonance imaging (MRI), functional MRI, diffusion tensor MRI, magnetic resonance spectroscopy, and positron-emission tomographic scans may be aligned to intraoperative MRI to enhance visualization and navigation during image-guided neurosurgery. However, several effects (both machine- and patient-induced distortions) lead to significant geometric distortion of intraoperative MRI. Therefore, a precise alignment of these image modalities requires correction of the geometric distortion. We propose and evaluate a novel method to compensate for the geometric distortion of intraoperative 0.5-T MRI in image-guided neurosurgery. METHODS: In this initial pilot study, 11 neurosurgical procedures were prospectively enrolled. The scheme used to correct the geometric distortion is based on a nonrigid registration algorithm introduced by our group. This registration scheme uses image features to establish correspondence between images. It estimates a smooth geometric distortion compensation field by regularizing the displacements estimated at the correspondences. A patient-specific linear elastic material model is used to achieve the regularization. The geometry of intraoperative images (0.5 T) is changed so that the images match the preoperative MRI scans (3 T). RESULTS: We compared the alignment between preoperative and intraoperative imaging using 1) only rigid registration without correction of the geometric distortion, and 2) rigid registration and compensation for the geometric distortion. We evaluated the success of the geometric distortion correction algorithm by measuring the Hausdorff distance between boundaries in the 3-T and 0.5-T MRIs after rigid registration alone and with the addition of geometric distortion correction of the 0.5-T MRI. Overall, the mean magnitude of the geometric distortion measured on the intraoperative images is 10.3 mm with a minimum of 2.91 mm and a maximum of 21.5 mm. The measured accuracy of the geometric distortion compensation algorithm is 1.93 mm. There is a statistically significant difference between the accuracy of the alignment of preoperative and intraoperative images, both with and without the correction of geometric distortion (P < 0.001). CONCLUSION: The major contributions of this study are 1) identification of geometric distortion of intraoperative images relative to preoperative images, 2) measurement of the geometric distortion, 3) application of nonrigid registration to compensate for geometric distortion during neurosurgery, 4) measurement of residual distortion after geometric distortion correction, and 5) phantom study to quantify geometric distortion.

Peter Kazanzides, Tian Xia, Clint Baird, George Jallo, Kathryn Hayes, Nobuyuki Nakajima, and Nobuhiko Hata. 2008. “A cooperatively-controlled image guided robot system for skull base surgery.” Stud Health Technol Inform, 132, Pp. 198-203.Abstract
We created an image-guided robot system to assist with skull base drilling by integrating a robot, a commercial navigation system, and an open source visualization platform. The objective of this procedure is to create a cavity in the skull base to allow access for neurosurgical interventions. The motivation for introducing an image-guided robot is to improve safety by preventing the surgeon from accidentally damaging critical structures during the drilling procedure. Our approach is to attach the cutting tool to the robot end-effector and operate the robot in a cooperative control mode, where robot motion is determined from the forces and torques applied by the surgeon. We employ "virtual fixtures" to constrain the motion of the cutting tool so that it remains in the safe zone that was defined on a preoperative CT scan. This paper presents the system design and the results of phantom and cadaveric experiments. Both experiments have demonstrated the feasibility of the system, with average overcut error at about 1 mm and maximum errors at 2.5 mm.
Ferenc A Jolesz and Nathan McDannold. 2008. “Current status and future potential of MRI-guided focused ultrasound surgery.” J Magn Reson Imaging, 27, 2, Pp. 391-9.Abstract
The combination of the imaging abilities of magnetic resonance imaging (MRI) with the ability to delivery energy to targets deep in the body noninvasively with focused ultrasound presents a disruptive technology with the potential to significantly affect healthcare. MRI offers precise targeting, visualization, and quantification of temperature changes and the ability to immediately evaluate the treatment. By exploiting different mechanisms, focused ultrasound offers a range of therapies, ranging from thermal ablation to targeted drug delivery. This article reviews recent preclinical and tests clinical of this technology.
Nickolai Sheikov, Nathan McDannold, Shipra Sharma, and Kullervo Hynynen. 2008. “Effect of focused ultrasound applied with an ultrasound contrast agent on the tight junctional integrity of the brain microvascular endothelium.” Ultrasound Med Biol, 34, 7, Pp. 1093-104.Abstract
Previous studies have investigated a potential method for targeted drug delivery in the central nervous system that uses focused ultrasound bursts combined with an ultrasound contrast agent to temporarily disrupt the blood-brain barrier (BBB). The purpose of this work was to investigate the integrity of the tight junctions (TJs) in rat brain microvessels after this BBB disruption. Ultrasound bursts (1.5-MHz) in combination with a gas contrast agent (Optison) was applied at two locations in the brain in 25 rats to induce BBB disruption. Using immunoelectron microscopy, the distributions of the TJ-specific transmembrane proteins occludin, claudin-1, claudin-5, and of submembranous ZO-1 were examined at 1, 2, 4, 6 and 24 h after sonication. A quantitative evaluation of the protein expression was made by counting the number of immunosignals per micrometer in the junctional clefts. BBB disruption at the sonicated locations was confirmed by the leakage of i.v. administered horseradish peroxidase (HRP, m.w. 40,000 Da) and lanthanum chloride (La(3+), m.w. approximately 139 Da). Leakage of these agents was observed at 1 and 2 h and, in a few vessels, at 4 h after ultrasound application. These changes were paralleled by the apparent disintegration of the TJ complexes, as evidenced by the redistribution and loss of the immunosignals for occludin, claudin-5 and ZO-1. Claudin-1 seemed less involved. At 6 and 24 h after sonication, no HRP or lanthanum leakage was observed and the barrier function of the TJs, as indicated by the localization and density of immunosignals, appeared to be completely restored. This study provides the first direct evidence that ultrasound bursts combined with a gas contrast agent cause disassembling of the TJ molecular structure, leading to loss of the junctional barrier functions in brain microvessels. The BBB disruption appears to last up to 4 h after sonication and permits the paracellular passage of agents with molecular weights up to at least 40 kDa. These promising features can be exploited in the future development of this method that could enable the delivery of drugs, antibodies or genes to targeted locations in the brain.
Nathan McDannold, Natalia Vykhodtseva, and Kullervo Hynynen. 2008. “Effects of acoustic parameters and ultrasound contrast agent dose on focused-ultrasound induced blood-brain barrier disruption.” Ultrasound Med Biol, 34, 6, Pp. 930-7.Abstract
Previously, it was shown that low-intensity focused ultrasound pulses applied along with an ultrasound contrast agent results in temporary blood-brain barrier (BBB) disruption. This effect could be used for targeted drug delivery in the central nervous system. This study examined the effects of burst length, pulse repetition frequency (PRF), and ultrasound contrast agent dose on the resulting BBB disruption. One hundred nonoverlapping brain locations were sonicated through a craniotomy in experiments in 26 rabbits (ultrasound frequency: 0.69 MHz, burst: 0.1, 1, 10 ms, PRF: 0.5, 1, 2, 5 Hz, duration: 20 s, peak negative pressure amplitude: 0.1 to 1.5 MPa, Optison dosage 50, 100, 250 microl/kg). For each sonication, BBB disruption was evaluated using contrast-enhanced magnetic resonance imaging. The BBB disruption threshold (the pressure amplitude yielding a 50% probability for BBB disruption) was determined using probit regression for the three burst lengths tested. Tissue effects were examined in light microscopy for representative locations with similar amounts of contrast enhancement from each group. While changing the PRF or the Optison dosage did not result in a significant difference in the magnitude of the BBB disruption (p > 0.05), reducing the burst length resulted in significantly less contrast enhancement (p < 0.01). The BBB disruption thresholds were estimated to be 0.69, 0.47 and 0.36 MPa for 0.1, 1 and 10 ms bursts, respectively. No difference was detected in histology between any experimental groups. This data suggests that over the range of parameters tested, BBB disruption is not affected by PRF or ultrasound contrast agent dose. However, both the BBB disruption magnitude and its threshold depend on the burst length.
Nathan McDannold, Clare M Tempany, Ferenc A Jolesz, and Kullervo Hynynen. 2008. “Evaluation of Referenceless Thermometry in MRI-guided Focused Ultrasound Surgery of Uterine Fibroids.” J Magn Reson Imaging, 28, 4, Pp. 1026-32.Abstract

PURPOSE: To clinically assess a previously described method (Rieke, Magn Reson Med 2004) to produce more motion-robust MRI-based temperature images using data acquired during MRI-guided focused ultrasound surgery (MRgFUS) of uterine fibroids. MATERIALS AND METHODS: The method ("referenceless thermometry") uses surface fitting in nonheated regions of individual phase images to extrapolate and then remove background phase variations that are unrelated to temperature changes. We tested this method using images from 100 sonications selected from 33 patient MRgFUS treatments. Temperature measurements and thermal dose contours estimated with the referenceless method were compared with those produced with the standard phase-difference technique. Fitting accuracy and noise level were also measured. RESULTS: In 92/100 sonications, the difference between the two measurements was less than 3 degrees C. The average difference in the measurements was 1.5 +/- 1.4 degrees C. Small motion artifacts were observed in the phase-difference imaging when the difference was greater than 3 degrees C. The method failed in two cases. The mean absolute error in the surface fit in baseline images corresponded to a temperature error of 0.8 +/- 1.4 degrees C. The noise level was approximately 40% lower than the phase-difference method. Thermal dose contours calculated from the two methods agreed well on average. CONCLUSION: Based on the small error when compared with the standard technique, this method appears to be adequate for temperature monitoring of MRgFUS in uterine fibroids and may prove useful for monitoring temperature changes in moving organs.

Michael Wang, Robert Rohling, Cheryl Duzenli, Brenda Clark, and Neculai Archip. 2008. “Evaluation of targeting errors in ultrasound-assisted radiotherapy.” Ultrasound Med Biol, 34, 12, Pp. 1944-56.Abstract
A method for validating the start-to-end accuracy of a 3-D ultrasound (US)-based patient positioning system for radiotherapy is described. A radiosensitive polymer gel is used to record the actual dose delivered to a rigid phantom after being positioned using 3-D US guidance. Comparison of the delivered dose with the treatment plan allows accuracy of the entire radiotherapy treatment process, from simulation to 3-D US guidance, and finally delivery of radiation, to be evaluated. The 3-D US patient positioning system has a number of features for achieving high accuracy and reducing operator dependence. These include using tracked 3-D US scans of the target anatomy acquired using a dedicated 3-D ultrasound probe during both the simulation and treatment sessions, automatic 3-D US-to-US registration and use of infrared LED (IRED) markers of the optical position-sensing system for registering simulation computed tomography to US data. The mean target localization accuracy of this system was 2.5 mm for four target locations inside the phantom, compared with 1.6 mm obtained using the conventional patient positioning method of laser alignment. Because the phantom is rigid, this represents the best possible set-up accuracy of the system. Thus, these results suggest that 3-D US-based target localization is practically feasible and potentially capable of increasing the accuracy of patient positioning for radiotherapy in sites where day-to-day organ shifts are greater than 1 mm in magnitude.
Serena H Wong, Ronald D Watkins, Mario Kupnik, Kim Butts Pauly, and Butrus T Khuri-Yakub. 2008. “Feasibility of MR-temperature mapping of ultrasonic heating from a CMUT.” IEEE Trans Ultrason Ferroelectr Freq Control, 55, 4, Pp. 811-8.Abstract
In the last decade, high intensity focused ultrasound (HIFU) has gained popularity as a minimally invasive and noninvasive therapeutic tool for treatment of cancers, arrhythmias, and other medical conditions. HIFU therapy is often guided by magnetic resonance imaging (MRI), which provides anatomical images for therapeutic device placement, temperature maps for treatment guidance, and postoperative evaluation of the region of interest. While piezoelectric transducers are dominantly used for MR-guided HIFU, capacitive micromachined ultrasonic transducers (CMUTs) show competitive advantages, such as ease of fabrication, integration with electronics, improved efficiency, and reduction of self-heating. In this paper, we will show our first results of an unfocused CMUT transducer monitored by MR-temperature maps. This 2.51 mm by 2.32 mm, unfocused CMUT heated a HIFU phantom by 14 degrees C in 2.5 min. This temperature rise was successfully monitored by MR thermometry in a 3.0 T General Electric scanner.
Caleb H Farny and Greg T Clement. 2008. “Feasibility of ultrasound phase contrast for heating localization.” J Acoust Soc Am, 123, 3, Pp. 1773-83.Abstract
Ultrasound-based methods for temperature monitoring could greatly assist focused ultrasound visualization and treatment planning based on sound speed-induced change in phase as a function of temperature. A method is presented that uses reflex transmission integration, planar projection, and tomographic reconstruction techniques to visualize phase contrast by measuring the sound field before and after heat deposition. Results from experiments and numerical simulations employing a through-transmission setup are presented to demonstrate feasibility of using phase contrast methods for identifying temperature change. A 1.088-MHz focused transducer was used to interrogate a medium with a phase contrast feature, following measurement of the baseline reference field with a hydrophone. A thermal plume in water and a tissue phantom with multiple water columns was used in separate experiments to produce a phase contrast. The reference and phase contrast field scans were numerically backprojected and the phase difference correctly identified the position and orientation of the features. The peak temperature reconstructed from the phase shift was within 0.2 degrees C of the measured temperature in the plume. Simulated results were in good agreement with experimental results. Finally, employment of reflex transmission imaging techniques for adopting a pulse-echo arrangement was simulated, and its future experimental application is discussed.
Clare Poynton, Mark Jenkinson, Stephen Whalen, Alexandra J Golby, and William Wells. 2008. “Fieldmap-free retrospective registration and distortion correction for EPI-based functional imaging.” Med Image Comput Comput Assist Interv, 11, Pt 2, Pp. 271-9.Abstract
We describe a method for correcting the distortions present in echo planar images (EPI) and registering the EPI to structural MRI. A fieldmap is predicted from an air / tissue segmentation of the MRI using a perturbation method and subsequently used to unwarp the EPI data. Shim and other missing parameters are estimated by registration. We obtain results that are similar to those obtained using fieldmaps, however neither fieldmaps, nor knowledge of shim coefficients is required.
Yanmei Tie, Stephen Whalen, Ralph O Suarez, and Alexandra J Golby. 2008. “Group Independent Component Analysis of Language fMRI from Word Generation Tasks.” Neuroimage, 42, 3, Pp. 1214-25.Abstract

Language fMRI has been used to study brain regions involved in language processing and has been applied to pre-surgical language mapping. However, in order to provide clinicians with optimal information, the sensitivity and specificity of language fMRI needs to be improved. Type II error of failing to reach statistical significance when the language activations are genuinely present may be particularly relevant to pre-surgical planning, by falsely indicating low surgical risk in areas where no activations are shown. Furthermore, since the execution of language paradigms involves cognitive processes other than language function per se, the conventional general linear model (GLM) method may identify non-language-specific activations. In this study, we assessed an exploratory approach, independent component analysis (ICA), as a potential complementary method to the inferential GLM method in language mapping applications. We specifically investigated whether this approach might reduce type II error as well as generate more language-specific maps. Fourteen right-handed healthy subjects were studied with fMRI during two word generation tasks. A similarity analysis across tasks was proposed to select components of interest. Union analysis was performed on the language-specific components to increase sensitivity, and conjunction analysis was performed to identify language areas more likely to be essential. Compared with GLM, ICA identified more activated voxels in the putative language areas, and signals from other sources were isolated into different components. Encouraging results from one brain tumor patient are also presented. ICA may be used as a complementary tool to GLM in improving pre-surgical language mapping.

Sai Chun Tang and Gregory T Clement. 2008. “A harmonic cancellation technique for an ultrasound transducer excited by a switched-mode power converter.” IEEE Trans Ultrason Ferroelectr Freq Control, 55, 2, Pp. 359-67.Abstract
The aim of this study is to evaluate the feasibility of using harmonic cancellation for a therapeutic ultrasound transducer excited by a switched-mode power converter without an additional output filter. A switching waveform without the third harmonic was created by cascading two switched-mode power inverter modules at which their output waveforms were pi/3 phase shifted from each other. A PSPICE simulation model for the power converter output stage was developed. The simulated results were in good agreement with the measurement. The waveform and harmonic contents of the acoustic pressure generated by a 1-MHz, self-focused piezoelectric transducer with and without harmonic cancellation have been evaluated. Measured results indicated that the acoustic third harmonicto- fundamental ratio at the focus was small (-48 dB) with harmonic cancellation, compared to that without harmonic cancellation (-20 dB). The measured acoustic levels of the fifth harmonic for both cases with and without harmonic cancellation also were small (-46 dB) compared to the fundamental. This study shows that it is viable to drive a piezoelectric ultrasound transducer using a switched-mode power converter without the requirement of an additional output filter in many high-intensity focused ultrasound (HIFU) applications.
Nan-kuei Chen, Koichi Oshio, and Lawrence P Panych. 2008. “Improved image reconstruction for partial Fourier gradient-echo echo-planar imaging (EPI).” Magn Reson Med, 59, 4, Pp. 916-24.Abstract
The partial Fourier gradient-echo echo planar imaging (EPI) technique makes it possible to acquire high-resolution functional MRI (fMRI) data at an optimal echo time. This technique is especially important for fMRI studies at high magnetic fields, where the optimal echo time is short and may not be achieved with a full Fourier acquisition scheme. In addition, it has been shown that partial Fourier EPI provides better anatomic resolvability than full Fourier EPI. However, the partial Fourier gradient-echo EPI may be degraded by artifacts that are not usually seen in other types of imaging. Those unique artifacts in partial Fourier gradient-echo EPI, to our knowledge, have not yet been systematically evaluated. Here we use the k-space energy spectrum analysis method to understand and characterize two types of partial Fourier EPI artifacts. Our studies show that Type 1 artifact, originating from k-space energy loss, cannot be corrected with pure postprocessing, and Type 2 artifact can be eliminated with an improved reconstruction method. We propose a novel algorithm, that combines images obtained from two or more reconstruction schemes guided by k-space energy spectrum analysis, to generate partial Fourier EPI with greatly reduced Type 2 artifact. Quality control procedures for avoiding Type 1 artifact in partial Fourier EPI are also discussed.
Jong-Hwan Lee, Te-Won Lee, Ferenc A Jolesz, and Seung-Schik Yoo. 2008. “Independent vector analysis (IVA): multivariate approach for fMRI group study.” Neuroimage, 40, 1, Pp. 86-109.Abstract
Independent component analysis (ICA) of fMRI data generates session/individual specific brain activation maps without a priori assumptions regarding the timing or pattern of the blood-oxygenation-level-dependent (BOLD) signal responses. However, because of a random permutation among output components, ICA does not offer a straightforward solution for the inference of group-level activation. In this study, we present an independent vector analysis (IVA) method to address the permutation problem during fMRI group data analysis. In comparison to ICA, IVA offers an analysis of additional dependent components, which were assigned for use in the automated grouping of dependent activation patterns across subjects. Upon testing using simulated trial-based fMRI data, our proposed method was applied to real fMRI data employing both a single-trial task-paradigm (right hand motor clenching and internal speech generation tasks) and a three-trial task-paradigm (right hand motor imagery task). A generalized linear model (GLM) and the group ICA of the fMRI toolbox (GIFT) were also applied to the same data set for comparison to IVA. Compared to GLM, IVA successfully captured activation patterns even when the functional areas showed variable hemodynamic responses that deviated from a hypothesized response. We also showed that IVA effectively inferred group-activation patterns of unknown origins without the requirement for a pre-processing stage (such as data concatenation in ICA-based GIFT). IVA can be used as a potential alternative or an adjunct to current ICA-based fMRI group processing methods.
Tian Xia, Clint Baird, George Jallo, Kathryn Hayes, Nobuyuki Nakajima, Nobuhiko Hata, and Peter Kazanzides. 2008. “An integrated system for planning, navigation and robotic assistance for skull base surgery.” Int J Med Robot, 4, 4, Pp. 321-30.Abstract
BACKGROUND: We developed an image-guided robot system to provide mechanical assistance for skull base drilling, which is performed to gain access for some neurosurgical interventions, such as tumour resection. The motivation for introducing this robot was to improve safety by preventing the surgeon from accidentally damaging critical neurovascular structures during the drilling procedure. METHODS: We integrated a Stealthstation navigation system, a NeuroMate robotic arm with a six-degree-of-freedom force sensor, and the 3D Slicer visualization software to allow the robotic arm to be used in a navigated, cooperatively-controlled fashion by the surgeon. We employed virtual fixtures to constrain the motion of the robot-held cutting tool, so that it remained in the safe zone that was defined on a preoperative CT scan. RESULTS: We performed experiments on both foam skull and cadaver heads. The results for foam blocks cut using different registrations yielded an average placement error of 0.6 mm and an average dimensional error of 0.6 mm. We drilled the posterior porus acusticus in three cadaver heads and concluded that the robot-assisted procedure is clinically feasible and provides some ergonomic benefits, such as stabilizing the drill. We obtained postoperative CT scans of the cadaver heads to assess the accuracy and found that some bone outside the virtual fixture boundary was cut. The typical overcut was 1-2 mm, with a maximum overcut of about 3 mm. CONCLUSIONS: The image-guided cooperatively-controlled robot system can improve the safety and ergonomics of skull base drilling by stabilizing the drill and enforcing virtual fixtures to protect critical neurovascular structures. The next step is to improve the accuracy so that the overcut can be reduced to a more clinically acceptable value of about 1 mm.