Publications by Year: 2011

2011
Maier T, Strauss G, Bauer F, Grasser A, Hata N, Lueth TC. Distance Measurement in Middle Ear Surgery using a Telemanipulator. Med Image Comput Comput Assist Interv. 2011;14 (Pt 1) :41-8.Abstract

In this article, a new tool for the intraoperative measurement of distances within the middle ear by means of a micromanipulator is presented. The purpose of this work was to offer the surgeon a highly accurate tool for measuring the distances between two points in the 3D operational field. The tool can be useful in various operations; this article focuses, however, on measuring the distance between the stapes footplate and the long process of the incus of the middle ear. This distance is important for estimating the proper prosthesis length in stapedotomy for treating otosclerosis. We evaluated the system using a simplified mechanical model. Our results show that the system can measure distances with a maximum error of 0.04 mm.

Sundaram P, Mulkern RV, Wells III WM, Triantafyllou C, Loddenkemper T, Bubrick EJ, Orbach DB. An Empirical Investigation of Motion Effects in eMRI of Interictal Epileptiform Spikes. Magn Reson Imaging. 2011;29 (10) :1401-9.Abstract

We recently developed a functional neuroimaging technique called encephalographic magnetic resonance imaging (eMRI). Our method acquires rapid single-shot gradient-echo echo-planar MRI (repetition time=47 ms); it attempts to measure an MR signal more directly linked to neuronal electromagnetic activity than existing methods. To increase the likelihood of detecting such an MR signal, we recorded concurrent MRI and scalp electroencephalography (EEG) during fast (20-200 ms), localized, high-amplitude (>50 μV on EEG) cortical discharges in a cohort of focal epilepsy patients. Seen on EEG as interictal spikes, these discharges occur in between seizures and induced easily detectable MR magnitude and phase changes concurrent with the spikes with a lag of milliseconds to tens of milliseconds. Due to the time scale of the responses, localized changes in blood flow or hemoglobin oxygenation are unlikely to cause the MR signal changes that we observed. While the precise underlying mechanisms are unclear, in this study, we empirically investigate one potentially important confounding variable - motion. Head motion in the scanner affects both EEG and MR recording. It can produce brief "spike-like" artifacts on EEG and induce large MR signal changes similar to our interictal spike-related signal changes. In order to explore the possibility that interictal spikes were associated with head motions (although such an association had never been reported), we had previously tracked head position in epilepsy patients during interictal spikes and explicitly demonstrated a lack of associated head motion. However, that study was performed outside the MR scanner, and the root-mean-square error in the head position measurement was 0.7 mm. The large inaccuracy in this measurement therefore did not definitively rule out motion as a possible signal generator. In this study, we instructed healthy subjects to make deliberate brief (<500 ms) head motions inside the MR scanner and imaged these head motions with concurrent EEG and MRI. We compared these artifactual MR and EEG data to genuine interictal spikes. While per-voxel MR and per-electrode EEG time courses for the motion case can mimic the corresponding time courses associated with a genuine interictal spike, head motion can be unambiguously differentiated from interictal spikes via scalp EEG potential maps. Motion induces widespread changes in scalp potential, whereas interictal spikes are localized and have a regional fall-off in amplitude. These findings make bulk head motion an unlikely generator of the large spike-related MR signal changes that we had observed. Further work is required to precisely identify the underlying mechanisms.

Risholm P, Balter J, Wells III WM. Estimation of delivered dose in radiotherapy: the influence of registration uncertainty. Med Image Comput Comput Assist Interv. 2011;14 (Pt 1) :548-55.Abstract

We present a probabilistic framework to estimate the accumulated radiation dose and the corresponding dose uncertainty that is delivered to important anatomical structures, e.g. the primary tumor and healthy surrounding organs, during radiotherapy. The dose uncertainty we report is a direct result of uncertainties in the estimates of the deformation which aligns the daily cone-beam CT images with the planning CT. The accumulated radiation dose is an important measure to monitor during treatment, in particular to see if it significantly deviates from the planned dose which might indicate that either the patient was not properly positioned before treatment or that the anatomy has changed due to the treatment. In the case of the latter, the treatment plan should be adaptively changed to align with the current patient anatomy. We estimate the accumulated dose distribution, and its uncertainty, retrospectively on a dataset acquired during treatment of cancer in the neck and show the dose distributions in the form of dose volume histograms.

Obstein KL, Patil VD, Jayender J, San José Estépar R, Spofford IS, Lengyel BI, Vosburgh KG, Thompson CC. Evaluation of colonoscopy technical skill levels by use of an objective kinematic-based system. Gastrointest Endosc. 2011;73 (2) :315-21.Abstract

BACKGROUND: Colonoscopy requires training and experience to ensure accuracy and safety. Currently, no objective, validated process exists to determine when an endoscopist has attained technical competence. Kinematics data describing movements of laparoscopic instruments have been used in surgical skill assessment to define expert surgical technique. We have developed a novel system to record kinematics data during colonoscopy and quantitatively assess colonoscopist performance. OBJECTIVE: To use kinematic analysis of colonoscopy to quantitatively assess endoscopic technical performance. DESIGN: Prospective cohort study. SETTING: Tertiary-care academic medical center. POPULATION: This study involved physicians who perform colonoscopy. INTERVENTION: Application of a kinematics data collection system to colonoscopy evaluation. MAIN OUTCOME MEASUREMENTS: Kinematics data, validated task load assessment instrument, and technical difficulty visual analog scale. RESULTS: All 13 participants completed the colonoscopy to the terminal ileum on the standard colon model. Attending physicians reached the terminal ileum quicker than fellows (median time, 150.19 seconds vs 299.86 seconds; p<.01) with reduced path lengths for all 4 sensors, decreased flex (1.75 m vs 3.14 m; P=.03), smaller tip angulation, reduced absolute roll, and lower curvature of the endoscope. With performance of attending physicians serving as the expert reference standard, the mean kinematic score increased by 19.89 for each decrease in postgraduate year (P<.01). Overall, fellows experienced greater mental, physical, and temporal demand than did attending physicians. LIMITATION: Small cohort size. CONCLUSION: Kinematic data and score calculation appear useful in the evaluation of colonoscopy technical skill levels. The kinematic score appears to consistently vary by year of training. Because this assessment is nonsubjective, it may be an improvement over current methods for determination of competence. Ongoing studies are establishing benchmarks and characteristic profiles of skill groups based on kinematics data.

Yuan J, Mei C-S, Madore B, McDannold NJ, Panych LP. Fast Fat-suppressed Reduced Field-of-view Temperature Mapping using 2DRF Excitation Pulses. J Magn Reson. 2011;210 (1) :38-43.Abstract

The purpose of this study is to develop a fast and accurate temperature mapping method capable of both fat suppression and reduced field-of-view (rFOV) imaging, using a two-dimensional spatially-selective RF (2DRF) pulse. Temperature measurement errors caused by fat signals were assessed, through simulations. An 11×1140μs echo-planar 2DRF pulse was developed and incorporated into a gradient-echo sequence. Temperature measurements were obtained during focused ultrasound (FUS) heating of a fat-water phantom. Experiments both with and without the use of a 2DRF pulse were performed at 3T, and the accuracy of the resulting temperature measurements were compared over a range of TE values. Significant inconsistencies in terms of measured temperature values were observed when using a regular slice-selective RF excitation pulse. In contrast, the proposed 2DRF excitation pulse suppressed fat signals by more than 90%, allowing good temperature consistency regardless of TE settings. Temporal resolution was also improved, from 12 frames per minute (fpm) with the regular pulse to 28 frames per minute with the rFOV excitation. This technique appears promising toward the MR monitoring of temperature in moving adipose organs, during thermal therapies.

Tempany CM, McDannold NJ, Hynynen K, Jolesz FA. Focused Ultrasound Surgery in Oncology: Overview and Principles. Radiology. 2011;259 (1) :39-56.Abstract

Focused ultrasound surgery (FUS) is a noninvasive image-guided therapy and an alternative to surgical interventions. It presents an opportunity to revolutionize cancer therapy and to affect or change drug delivery of therapeutic agents in new focally targeted ways. In this article the background, principles, technical devices, and clinical cancer applications of image-guided FUS are reviewed.

Obstein KL, San José Estépar R, Jagadeesan J, Patil VD, Spofford IS, Ryan MB, Lengyel BI, Shams R, Vosburgh KG, Thompson CC. Image Registered Gastroscopic Ultrasound (IRGUS) in human subjects: a pilot study to assess feasibility. Endoscopy. 2011;43 (5) :394-9.Abstract

BACKGROUND AND STUDY AIMS: Endoscopic ultrasound (EUS) is a complex procedure due to the subtleties of ultrasound interpretation, the small field of observation, and the uncertainty of probe position and orientation. Animal studies demonstrated that Image Registered Gastroscopic Ultrasound (IRGUS) is feasible and may be superior to conventional EUS in efficiency and image interpretation. This study explores whether these attributes of IRGUS will be evident in human subjects, with the aim of assessing the feasibility, effectiveness, and efficiency of IRGUS in patients with suspected pancreatic lesions. PATIENTS AND METHODS: This was a prospective feasibility study at a tertiary care academic medical center in human patients with pancreatic lesions on computed tomography (CT) scan. Patients who were scheduled to undergo conventional EUS were randomly chosen to undergo their procedure with IRGUS. Main outcome measures included feasibility, ease of use, system function, validated task load (TLX) assessment instrument, and IRGUS experience questionnaire. RESULTS: Five patients underwent IRGUS without complication. Localization of pancreatic lesions was accomplished efficiently and accurately (TLX temporal demand 3.7 %; TLX effort 8.6 %). Image synchronization and registration was accomplished in real time without procedure delay. The mean assessment score for endoscopist experience with IRGUS was positive (66.6 ± 29.4). Real-time display of CT images in the EUS plane and echoendoscope orientation were the most beneficial characteristics. CONCLUSIONS: IRGUS appears feasible and safe in human subjects, and efficient and accurate at identification of probe position and image interpretation. IRGUS has the potential to broaden the adoption of EUS techniques and shorten EUS learning curves. Clinical studies comparing IRGUS with conventional EUS are ongoing.

Tokuda J, Mamata H, Gill RR, Hata N, Kikinis R, Padera RF, Lenkinski RE, Sugarbaker DJ, Hatabu H. Impact of Nonrigid Motion Correction Technique on Pixel-wise Pharmacokinetic Analysis of Free-breathing Pulmonary Dynamic Contrast-enhanced MR Imaging. J Magn Reson Imaging. 2011;33 (4) :968-73.Abstract

PURPOSE: To investigates the impact of nonrigid motion correction on pixel-wise pharmacokinetic analysis of free-breathing DCE-MRI in patients with solitary pulmonary nodules (SPNs). Misalignment of focal lesions due to respiratory motion in free-breathing dynamic contrast-enhanced MRI (DCE-MRI) precludes obtaining reliable time-intensity curves, which are crucial for pharmacokinetic analysis for tissue characterization. MATERIALS AND METHODS: Single-slice 2D DCE-MRI was obtained in 15 patients. Misalignments of SPNs were corrected using nonrigid B-spline image registration. Pixel-wise pharmacokinetic parameters K(trans) , v(e) , and k(ep) were estimated from both original and motion-corrected DCE-MRI by fitting the two-compartment pharmacokinetic model to the time-intensity curve obtained in each pixel. The "goodness-of-fit" was tested with χ(2) -test in pixel-by-pixel basis to evaluate the reliability of the parameters. The percentages of reliable pixels within the SPNs were compared between the original and motion-corrected DCE-MRI. In addition, the parameters obtained from benign and malignant SPNs were compared. RESULTS: The percentage of reliable pixels in the motion-corrected DCE-MRI was significantly larger than the original DCE-MRI (P = 4 × 10(-7) ). Both K(trans) and k(ep) derived from the motion-corrected DCE-MRI showed significant differences between benign and malignant SPNs (P = 0.024, 0.015). CONCLUSION: The study demonstrated the impact of nonrigid motion correction technique on pixel-wise pharmacokinetic analysis of free-breathing DCE-MRI in SPNs.

Risholm P, Golby AJ, Wells III WM. Multimodal image registration for preoperative planning and image-guided neurosurgical procedures. Neurosurg Clin N Am. 2011;22 (2) :197-206.Abstract

Image registration is the process of transforming images acquired at different time points, or with different imaging modalities, into the same coordinate system. It is an essential part of any neurosurgical planning and navigation system because it facilitates combining images with important complementary, structural, and functional information to improve the information based on which a surgeon makes critical decisions. Brigham and Women's Hospital (BWH) has been one of the pioneers in developing intraoperative registration methods for aligning preoperative and intraoperative images of the brain. This article presents an overview of intraoperative registration and highlights some recent developments at BWH.

Madore B, Panych LP, Mei C-S, Yuan J, Chu R. Multipathway Sequences for MR Thermometry. Magn Reson Med. 2011;66 (3) :658-68.Abstract

MR-based thermometry is a valuable adjunct to thermal ablation therapies as it helps to determine when lethal doses are reached at the target and whether surrounding tissues are safe from damage. When the targeted lesion is mobile, MR data can further be used for motion-tracking purposes. The present work introduces pulse sequence modifications that enable significant improvements in terms of both temperature-to-noise-ratio properties and target-tracking abilities. Instead of sampling a single magnetization pathway as in typical MR thermometry sequences, the pulse-sequence design introduced here involves sampling at least one additional pathway. Image reconstruction changes associated with the proposed sampling scheme are also described. The method was implemented on two commonly used MR thermometry sequences: the gradient-echo and the interleaved echo-planar imaging sequences. Data from the extra pathway enabled temperature-to-noise-ratio improvements by up to 35%, without increasing scan time. Potentially of greater significance is that the sampled pathways featured very different contrast for blood vessels, facilitating their detection and use as internal landmarks for tracking purposes. Through improved temperature-to-noise-ratio and lesion-tracking abilities, the proposed pulse-sequence design may facilitate the use of MR-monitored thermal ablations as an effective treatment option even in mobile organs such as the liver and kidneys.

Aljallad MH, Yuan J, Pilatou MC, McDannold NJ, Panych LP. Multi-resolution MRI Temperature Monitoring in a Reduced Field of View. Magn Reson Imaging. 2011;29 (9) :1205-14.Abstract

PURPOSE: The purpose was to develop a new magnetic resonance imaging technique for fast temperature monitoring with extended volume coverage. MATERIALS AND METHODS: The Multiple Resolutions Along Phase-Encode and Slice-Select Dimensions (MURPS) method was implemented in both a two-dimensional (2D) spoiled gradient echo (SPGR) sequence and a multishot echo-planar imaging (EPI) sequence. Both modified sequences were used to acquire image data from three slices with variable phase-encode resolution and slice thickness. In the SPGR sequence, a 2D resonant frequency pulse was also implemented to enable imaging within a reduced field of view, and this was used to monitor (at 1.5 T) the temperature changes in a live rabbit and in gel phantoms heated by focused ultrasound. A modified EPI sequence was tested during heating of a phantom undergoing motion. RESULTS: The in vivo experiments demonstrated that temperature changes in unexpected locations away from the focal plane, such as near bone structures, could be detected due to the extra volume coverage afforded by the MURPS method. Temperature changes in a moving phantom were resolved using the MURPS EPI sequence with an acquisition rate of three slices every 300 ms. CONCLUSION: The MURPS method enables temperature monitoring over multiple slices without loss of temporal resolution compared with single-slice imaging and, if combined with multishot EPI, enables volume temperature monitoring in moving organs.

Zada G, Agarwalla PK, Mukundan S, Dunn IF, Golby AJ, Laws ER. The neurosurgical anatomy of the sphenoid sinus and sellar floor in endoscopic transsphenoidal surgery. J Neurosurg. 2011;114 (5) :1319-30.Abstract

OBJECT: A considerable degree of variability exists in the anatomy of the sphenoid sinus, sella turcica, and surrounding skull base structures. The authors aimed to characterize neuroimaging and intraoperative variations in the sagittal and coronal surgical anatomy of healthy controls and patients with sellar lesions. METHODS: Magnetic resonance imaging studies obtained in 100 healthy adults and 78 patients with sellar lesions were reviewed. The following measurements were made on midline sagittal images: sellar face, sellar prominence, sellar angle, tuberculum sellae angle, sellar-clival angle, length of planum sphenoidale, and length of clivus. The septal configuration of the sphenoid sinus was classified as either simple or complex, according to the number of septa, their symmetry, and their morphological features. The following measurements were made on coronal images: maximum width of the sphenoid sinus and sellar face, and the distance between the parasellar and midclivus internal carotid arteries. Neuroimaging results were correlated with intraoperative findings during endoscopic transsphenoidal surgery. RESULTS: Three sellar floor morphologies were defined in normal adults: prominent (sellar angle of < 90°) in 25%, curved (sellar angle 90-150°) in 63%, flat (sellar angle > 150°) in 11%, and no floor (conchal sphenoid) in 1%. In healthy adults, the following mean measurements were obtained: sellar face, 13.4 mm; sellar prominence, 3.0 mm; sellar angle, 112°; angle of tuberculum sellae, 112°; and sellar-clival angle, 117°. Compared with healthy adults, patients with sellar lesions were more likely to have prominent sellar types (43% vs 25%, p = 0.01), a more acute sellar angle (102° vs 112°, p = 0.03), a more prominent sellar floor (3.8 vs 3.0 mm, p < 0.005), and more acute tuberculum (105° vs 112°, p < 0.01) and sellar-clival (105° vs 117°, p < 0.003) angles. A flat sellar floor was more difficult to identify intraoperatively and more likely to require the use of a chisel or drill to expose (75% vs 25%, p = 0.01). A simple sphenoid sinus configuration (no septa, 1 vertical septum, or 2 symmetric vertical septa) was noted in 71% of studies, and the other 29% showed a complex configuration (2 or more asymmetrical septa, 3 or more septa of any kind, or the presence of a horizontal septum). Intraoperative correlation was more challenging in cases with complex sinus anatomy; the most reliable intraoperative midline markers were the vomer, superior sphenoid rostrum, and bilateral parasellar and clival carotid protuberances. CONCLUSIONS: Preoperative assessment of neuroimaging studies is critical for characterizing the morphological characteristics of the sphenoid sinus, sellar floor, tuberculum sellae, and clivus. The flat sellar type identified in 11% of people) or a complex sphenoid sinus configuration (in 29% of people) may make intraoperative correlation substantially more challenging. An understanding of the regional anatomy and its variability can improve the safety and accuracy of transsphenoidal and extended endoscopic skull base approaches.

Pohl KM, Konukoglu E, Novellas S, Ayache N, Fedorov A, Talos I-F, Golby AJ, Wells III WM, Kikinis R, Black PM. A new metric for detecting change in slowly evolving brain tumors: validation in meningioma patients. Neurosurgery. 2011;68 (1 Suppl Operative) :225-33.Abstract

BACKGROUND: Change detection is a critical component in the diagnosis and monitoring of many slowly evolving pathologies. OBJECTIVE: This article describes a semiautomatic monitoring approach using longitudinal medical images. We test the method on brain scans of patients with meningioma, which experts have found difficult to monitor because the tumor evolution is very slow and may be obscured by artifacts related to image acquisition. METHODS: We describe a semiautomatic procedure targeted toward identifying difficult-to-detect changes in brain tumor imaging. The tool combines input from a medical expert with state-of-the-art technology. The software is easy to calibrate and, in less than 5 minutes, returns the total volume of tumor change in mm. We test the method on postgadolinium, T1-weighted magnetic resonance images of 10 patients with meningioma and compare our results with experts' findings. We also perform benchmark testing with synthetic data. RESULTS: Our experiments indicated that experts' visual inspections are not sensitive enough to detect subtle growth. Measurements based on experts' manual segmentations were highly accurate but also labor intensive. The accuracy of our approach was comparable to the experts' results. However, our approach required far less user input and generated more consistent measurements. CONCLUSION: The sensitivity of experts' visual inspection is often too low to detect subtle growth of meningiomas from longitudinal scans. Measurements based on experts' segmentation are highly accurate but generally too labor intensive for standard clinical settings. We described an alternative metric that provides accurate and robust measurements of subtle tumor changes while requiring a minimal amount of user input.

Risholm P, Ross J, Washko GR, Wells III WM. Probabilistic elastography: estimating lung elasticity. Inf Process Med Imaging. 2011;22 :699-710.Abstract

We formulate registration-based elastography in a probabilistic framework and apply it to study lung elasticity in the presence of emphysematous and fibrotic tissue. The elasticity calculations are based on a Finite Element discretization of a linear elastic biomechanical model. We marginalize over the boundary conditions (deformation) of the biomechanical model to determine the posterior distribution over elasticity parameters. Image similarity is included in the likelihood, an elastic prior is included to constrain the boundary conditions, while a Markov model is used to spatially smooth the inhomogeneous elasticity. We use a Markov Chain Monte Carlo (MCMC) technique to characterize the posterior distribution over elasticity from which we extract the most probable elasticity as well as the uncertainty of this estimate. Even though registration-based lung elastography with inhomogeneous elasticity is challenging due the problem's highly underdetermined nature and the sparse image information available in lung CT, we show promising preliminary results on estimating lung elasticity contrast in the presence of emphysematous and fibrotic tissue.

Risholm P, Fedorov A, Pursley J, Tuncali K, Cormack R, Wells III WM. Probabilistic Non-rigid Registration of Prostate Images: Modeling and Quantifying Uncertainty. Proc IEEE Int Symp Biomed Imaging. 2011;2011 :553-556.Abstract

Registration of pre- to intra-procedural prostate images needs to handle the large changes in position and shape of the prostate caused by varying rectal filling and patient positioning. We describe a probabilistic method for non-rigid registration of prostate images which can quantify the most probable deformation as well as the uncertainty of the estimated deformation. The method is based on a biomechanical Finite Element model which treats the prostate as an elastic material. We use a Markov Chain Monte Carlo sampler to draw deformation configurations from the posterior distribution. In practice, we simultaneously estimate the boundary conditions (surface displacements) and the internal deformations of our biomechanical model. The proposed method was validated on a clinical MRI dataset with registration results comparable to previously published methods, but with the added benefit of also providing uncertainty estimates which may be important to take into account during prostate biopsy and brachytherapy procedures.

Madore B, Hoge WS, Chao T-C, Zientara GP, Chu R. Retrospectively Gated Cardiac Cine Imaging with Temporal and Spatial Acceleration. Magn Reson Imaging. 2011;29 (4) :457-69.Abstract

Parallel imaging methods are routinely used to accelerate the image acquisition process in cardiac cine imaging. The addition of a temporal acceleration method, whereby k-space is sampled differently for different time frames, has been shown in prior work to improve image quality as compared to parallel imaging by itself. However, such temporal acceleration strategies prove difficult to combine with retrospectively gated cine imaging. The only currently published method to feature such combination, by Hansen et al. [Magn Reson Med 55 (2006) 85-91] tends to be associated with prohibitively long reconstruction times. The goal of the present work was to develop a retrospectively gated cardiac cine method that features both parallel imaging and temporal acceleration, capable of achieving significant acceleration factors on commonly available hardware and associated with reconstruction times short enough for practical use in a clinical context. Seven cardiac patients and a healthy volunteer were recruited and imaged, with acceleration factors of 3.5 or 4.5, using an eight-channel product cardiac array on a 1.5-T system. The prescribed FOV value proved slightly too small in three patients, and one of the patients had a bigemini condition. Despite these additional challenges, good-quality results were obtained for all slices and all patients, with a reconstruction time of 0.98±0.07 s per frame, or about 20 s for a 20-frame slice, using a single processor on a single PC. As compared to using parallel imaging by itself, the addition of a temporal acceleration strategy provided much resistance to artifacts.

Kekhia H, Rigolo L, Norton I, Golby AJ. Special surgical considerations for functional brain mapping. Neurosurg Clin N Am. 2011;22 (2) :111-32.Abstract

The development of functional mapping techniques gives neurosurgeons many options for preoperative planning. Integrating functional and anatomic data can inform patient selection and surgical planning and makes functional mapping more accessible than when only invasive studies were available. However, the applications of functional mapping to neurosurgical patients are still evolving. Functional imaging remains complex and requires an understanding of the underlying physiologic and imaging characteristics. Neurosurgeons must be accustomed to interpreting highly processed data. Successful implementation of functional image-guided procedures requires efficient interactions between neurosurgeon, neurologist, radiologist, neuropsychologist, and others, but promises to enhance the care of patients.

Aja-Fernández S, Tristán-Vega A, Hoge WS. Statistical Noise Analysis in GRAPPA using a Parametrized Noncentral Chi Approximation Model. Magn Reson Med. 2011;65 (4) :1195-206.Abstract

The characterization of the distribution of noise in the magnitude MR image is a very important problem within image processing algorithms. The Rician noise assumed in single-coil acquisitions has been the keystone for signal-to-noise ratio estimation, image filtering, or diffusion tensor estimation for years. With the advent of parallel protocols such as sensitivity encoding or Generalized Autocalibrated Partially Parallel Acquisitions that allow accelerated acquisitions, this noise model no longer holds. Since Generalized Autocalibrated Partially Parallel Acquisitions reconstructions yield the combination of the squared signals recovered at each receiving coil, noncentral Chi statistics have been previously proposed to model the distribution of noise. However, we prove in this article that this is a weak model due to several artifacts in the acquisition scheme, mainly the correlation existing between the signals obtained at each coil. Alternatively, we propose to model such correlations with a reduction in the number of degrees of freedom of the signal, which translates in an equivalent nonaccelerated system with a minor number of independent receiving coils and, consequently, a lower signal-to-noise ratio. With this model, a noncentral Chi distribution can be assumed for all pixels in the image, whose effective number of coils and effective variance of noise can be explicitly computed in a closed form from the Generalized Autocalibrated Partially Parallel Acquisitions interpolation coefficients. Extensive experiments over both synthetic and in vivo data sets have been performed to show the goodness of fit of out model.

Jing Y, Clement GT. On the Use of Gegenbauer Reconstructions for Shock Wave Propagation Modeling. J Acoust Soc Am. 2011;130 (3) :1115-24.Abstract

In therapeutic ultrasound, the presence of shock waves can be significant due to the use of high intensity beams, as well as due to shock formation during inertial cavitation. Although modeling of such strongly nonlinear waves can be carried out using spectral methods, such calculations are typically considered impractical, since accurate calculations often require hundreds or even thousands of harmonics to be considered, leading to prohibitive computational times. Instead, time-domain algorithms which generally utilize Godunov-type finite-difference schemes are commonly used. Although these time domain methods can accurately model steep shock wave fronts, unlike spectral methods they are inherently unsuitable for modeling realistic tissue dispersion relations. Motivated by the need for a more general model, the use of Gegenbauer reconstructions as a postprocess tool to resolve the band-limitations of the spectral methods are investigated. The present work focuses on eliminating the Gibbs phenomenon when representing a steep wave front using a limited number of harmonics. Both plane wave and axisymmetric 2D transducer problems will be presented to characterize the proposed method.

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