Introduction: BOLD fMRI is a powerful noninvasive approach for studying brain function, however, ... more Introduction: BOLD fMRI is a powerful noninvasive approach for studying brain function, however, physiological interpretation of the signal is limited since it reflects a combination of changes in CBV, CBF, blood oxygenation and metabolism (1-4). While methods exist for non-invasive absolute CBF quantification, CBV measurement has been invasive and difficult especially in human subjects. Most calibrated fMRI studies typically assume CBV=0.88CBF based on the absolute CBV-CBF relationship obtained in monkey brains under hypo/hypercapnia using PET (5). This practice is controversial since this relation may vary across brain regions, functional challenges, and species, as demonstrated by published power parameters ranging from 0.18-0.64 in steady-state across species, stimuli, modalities, relative and absolute contributions of whole/partial blood compartments (6-16). This study presents MRI validation of the absolute CBV-CBF relationship in humans.
EFFECTS OF PROPOFOL ANESTHESIA AND SEX ON CEREBRAL BLOOD FLOW Pelin Aksit Ciris, Maolin Qiu, and ... more EFFECTS OF PROPOFOL ANESTHESIA AND SEX ON CEREBRAL BLOOD FLOW Pelin Aksit Ciris, Maolin Qiu, and R. Todd Constable Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, United States, Diagnostic Radiology, Yale University, School of Medicine, New Haven, CT, United States, Diagnostic Radiology, Neurosurgery, and Biomedical Engineering, Yale University, School of Medicine, New Haven, CT, United States
Purpose: To improve the geometric fidelity and spatial-resolution of multi-b diffusion-weighted M... more Purpose: To improve the geometric fidelity and spatial-resolution of multi-b diffusion-weighted MRI of the prostate. Materials and Methods: An accelerated segmented diffusion imaging sequence was developed and evaluated in 25 patients undergoing multi-parametric MRI exams of the prostate. A reduced field-of-view was acquired using an endo-rectal coil. The number of sampled diffusion weightings, or b-factors, was increased to allow estimation of tissue perfusion based on the intra-voxel incoherent motion (IVIM) model. Apparent diffusion coefficients (ADCs) measured with the proposed segmented method were compared to those obtained with conventional single-shot echoplanar imaging (EPI). Results: Compared to single-shot EPI, the segmented method resulted in faster acquisition with twofold improvement in spatial resolution and a greater than threefold improvement in geometric fidelity. ADC values measured with the novel sequence demonstrated excellent agreement with those obtained from the conventional scan (R 2 =0.91 for b max =500 s/mm 2 and R 2 =0.89 for b max =1400 s/mm 2). IVIM perfusion fraction was 4.0+/−2.7% for normal peripheral zone, 6.6+/ −3.6% for normal transition zone and 4.4+/−2.9% for suspected tumor lesions. Conclusion: The proposed accelerated segmented prostate diffusion imaging sequence achieved improvements in both spatial resolution and geometric fidelity, along with concurrent quantification of IVIM perfusion.
Journal of neuroimaging : official journal of the American Society of Neuroimaging, Jan 10, 2018
Diffusion magnetic resonance imaging (dMRI) provides preoperative maps of neurosurgical patients&... more Diffusion magnetic resonance imaging (dMRI) provides preoperative maps of neurosurgical patients' white matter tracts, but these maps suffer from echo-planar imaging (EPI) distortions caused by magnetic field inhomogeneities. In clinical neurosurgical planning, these distortions are generally not corrected and thus contribute to the uncertainty of fiber tracking. Multiple image processing pipelines have been proposed for image-registration-based EPI distortion correction in healthy subjects. In this article, we perform the first comparison of such pipelines in neurosurgical patient data. Five pipelines were tested in a retrospective clinical dMRI dataset of 9 patients with brain tumors. Pipelines differed in the choice of fixed and moving images and the similarity metric for image registration. Distortions were measured in two important tracts for neurosurgery, the arcuate fasciculus and corticospinal tracts. Significant differences in distortion estimates were found across proc...
We propose a method for the automated identification of key white matter fiber tracts for neurosu... more We propose a method for the automated identification of key white matter fiber tracts for neurosurgical planning, and we apply the method in a retrospective study of 18 consecutive neurosurgical patients with brain tumors. Our method is designed to be relatively robust to challenges in neurosurgical tractography, which include peritumoral edema, displacement, and mass effect caused by mass lesions. The proposed method has two parts. First, we learn a data-driven white matter parcellation or fiber cluster atlas using groupwise registration and spectral clustering of multi-fiber tractography from healthy controls. Key fiber tract clusters are identified in the atlas. Next, patient-specific fiber tracts are automatically identified using tractography-based registration to the atlas and spectral embedding of patient tractography. Results indicate good generalization of the data-driven atlas to patients: 80% of the 800 fiber clusters were identified in all 18 patients, and 94% of the 800...
A biomarker of cancer aggressiveness, such as hypoxia, could substantially impact treatment decis... more A biomarker of cancer aggressiveness, such as hypoxia, could substantially impact treatment decisions in the prostate, especially radiation therapy, by balancing treatment morbidity (urinary incontinence, erectile dysfunction, etc.) against mortality. R2 (*) mapping with Mono-Exponential (ME) decay modeling has shown potential for identifying areas of prostate cancer hypoxia at 1.5T. However, Gaussian deviations from ME decay have been observed in other tissues at 3T. The purpose of this study is to assess whether gradient-echo signal decays are better characterized by a standard ME decay model, or a Gaussian Augmentation of the Mono-Exponential (GAME) decay model, in the prostate at 3T. Multi-gradient-echo signals were acquired on 20 consecutive patients with a clinical suspicion of prostate cancer undergoing MR-guided prostate biopsies. Data were fitted with both ME and GAME models. The information contents of these models were compared using Akaike's information criterion (se...
Dual-pathway sequences have been proposed to help improve the temperature-to-noise ratio (TNR) in... more Dual-pathway sequences have been proposed to help improve the temperature-to-noise ratio (TNR) in MR thermometry. The present work establishes how much of an improvement these so-called "PSIF-FISP" sequences may bring in various organs and tissues. Simulations and TNR calculations were validated against analytical equations, phantom, abdomen, and brain scans. Relative TNRs for PSIF-FISP, as compared to a dual-FISP reference standard, were calculated for flip angle (FA) = 1 to 85 º and repetition time (TR) = 6 to 60 ms, for gray matter, white matter, cervix, endometrium, myometrium, prostate, kidney medulla and cortex, bone marrow, pancreas, spleen, muscle, and liver tissues. PSIF-FISP was TNR superior in the kidney, pelvis, spleen, or gray matter at most tested TR and FA settings, and benefits increased at shorter TRs. PSIF-FISP was TNR superior in other tissues, e.g., liver, muscle, pancreas, for only short TR settings (20 ms or less). The TNR benefits of PSIF-FISP increa...
Journal of magnetic resonance imaging : JMRI, Jan 12, 2016
To assess whether R2* mapping with a standard Monoexponential (ME) or a Gaussian Augmentation of ... more To assess whether R2* mapping with a standard Monoexponential (ME) or a Gaussian Augmentation of the Monoexponential (GAME) decay model better characterizes gradient-echo signal decays in gynecological cancers after external beam radiation therapy at 3T, and evaluate implications of modeling for noninvasive identification of intratumoral hypoxia. Multi-gradient-echo signals were acquired on 25 consecutive patients with gynecologic cancers and three healthy participants during inhalation of different oxygen concentrations at 3T. Data were fitted with both ME and GAME models. Models were compared using F-tests in tumors and muscles in patients, muscles, cervix, and uterus in healthy participants, and across oxygenation levels. GAME significantly improved fitting over ME (P < 0.05): Improvements with GAME covered 34% of tumor regions-of-interest on average, ranging from 6% (of a vaginal tumor) to 68% (of a cervical tumor) in individual tumors. Improvements with GAME were more promin...
ABSTRACT Introduction: The use of MRI for cardiac stress testing has been hampered by the need fo... more ABSTRACT Introduction: The use of MRI for cardiac stress testing has been hampered by the need for ECG gating, which does not perform well under conditions of changing heart rate and/or irregular heart motion. Real-time MRI avoids this problem, but until recently has not enjoyed high enough frame rates to properly visualize heart motion under stress conditions. Techniques incorporating training data sets [1] can be used to increase frame rates, but these also can break down when heart motion becomes irregular. We have developed a 32-channel real-time cardiac MRI system and used it to image the heart at up to 48 effective frames per second during exercise stress testing, without the inclusion of training sets. Methods: A 32-channel prototype GE MRI system was employed [2] to perform real-time imaging together with a 32-element cardiac receiver array [3]. The cardiac array comprised twenty-one 75 mm diameter circular elements mounted in a hexagonal array over the left-to-center chest wall and eleven 107 mm elements centered under the back. Custom imaging software was utilized to collect raw data from all 32 channels across multiple computers, reconstruct the images, and display them on the scanner monitor in real time. A main program directed the overall real-time application and provided the user interface, with controls to interactively change the scan plane [2]. A Beowulf cluster comprising eight nodes, each with dual 2.2 GHz AMD Opteron™ processors, has now been incorporated into this system as a real-time reconstruction engine, connected via private Gigabit Ethernet. Real-time imaging was performed using a FIESTA (balanced steady-state free precession) pulse sequence with a field of view of 33 cm, slice thickness of 8 mm, and TR of 2.8 ms. 128 readout points were collected, and 72 phase encodes with a half-Fourier acquisition scheme, using over-scans of 4 to 16 lines. ASSET acceleration factors between 2 and 4 were employed, along with a sliding reconstruction (slide factor of 2) to achieve effective imaging speeds of up to 48 frames per second. For accelerated imaging, low-resolution scans were first used to interactively move to the desired scan plane while collecting B1 field maps to calibrate the subsequent ASSET reconstruction. Switchover to accelerated imaging was achieved by a single button push on the user interface. Real-time scanning was performed on four normal volunteers, with exercise stress testing on a single volunteer. Results: Figure 1 shows still frames from a movie acquired at 22 frames per second in a four-chamber view of the heart. This used a x2 sliding reconstruction and an ASSET factor of two. Reducing the half-Fourier over-scans from 16 to 4 lines increased the effective scan rate to 31 frames per second, with some loss of SNR. Raising the ASSET factor to 4 with 8 over-scan lines resulted in frame rates of 44 frames per second, as exemplified by the coronal data set of Fig. 2. While the SNR is reduced relative to Fig. 1, the image quality is still sufficiently high to enable quantification of cardiac wall thickening. Figure 3 shows a plot of wall thickening versus time, measured on a 48 frame-per-second data set acquired immediately after exercise had increased the heart rate to an average of 85 BPM from a resting rate of 52 BPM in a normal volunteer.
Problem MRI-guided cardiac interventions have gained significant interest over the past decade. M... more Problem MRI-guided cardiac interventions have gained significant interest over the past decade. MRI provides the potential advantage of intra-operative anatomical imaging for the purpose of targeting, monitoring and control of the treatment. Several groups are currently working on enabling MRI-guided catheter based cardiac interventions (1-2). MRI can be used to target the therapy in cases where anatomical foci (such as myocardial scars) coincide with the electrophysiological foci. MRI has the potential to monitor the acute effect of tissue ablation as well and post-operative control. MRI can also be used for device tracking, using image based tracking, MR tracking or electro magnetic (EM) tracking. Current approaches for cardiac MRI-guidance are either to use pre-operative datasets or real-time imaging. The purpose of this study was to enable concomitant navigation on 3D cardiac cine loops as well as real-time imaging using ECG triggering for accurate temporal registration and synchronization.
This article reviews state-of-the-art MR acquisition strategies for brain imaging including seque... more This article reviews state-of-the-art MR acquisition strategies for brain imaging including sequences for imaging anatomy and morphometry, microstructure particularly in the white matter, and functional imaging. Recent developments in terms of acquisition strategies are reviewed for each of these topics, and the trade-offs made with some of the different strategies are discussed.
This work describes a real‐time imaging and visualization technique that allows multiple field of... more This work describes a real‐time imaging and visualization technique that allows multiple field of view (FOV) imaging. A stream of images from a single receiver channel can be reconstructed at multiple FOVs within each image frame. Alternately, or in addition, when multiple receiver channels are available, image streams from each channel can be independently reconstructed at multiple FOVs. The implementation described here provides for real‐time visualization of the placement of guidewires and catheters on a dynamic roadmap during interventional procedures. The loopless catheter antenna, an electrically active intravascular probe, was used for MR signal reception. In 2D projection images, the catheter and surrounding structures within its diameter of sensitivity appear as bright signal. The simplicity of the resulting images allows very‐narrow‐FOV imaging to decrease imaging time. Very‐narrow‐FOV images are acquired on MR receiver channels that collect guidewire or catheter data. The...
Purpose-The relationship between cerebral blood volume (CBV) and cerebral blood flow (CBF) underl... more Purpose-The relationship between cerebral blood volume (CBV) and cerebral blood flow (CBF) underlies blood oxygenation level dependent functional MRI signal. This study investigates the potential for improved characterization of the CBV-CBF relationship in humans, and examines gender effects as well as spatial variations in the CBV-CBF relationship. Methods-Healthy subjects were imaged non-invasively at rest and during visual stimulation, constituting the first MRI measurement of the absolute CBV-CBF relationship in humans with complete coverage of the functional areas of interest. Results-CBV and CBF estimates were consistent with literature, and their relationship varied both spatially and with gender. In a region of interest (ROI) with stimulus induced activation in CBV and CBF at the P<0.05 significance level, a power function fit resulted in CBV = 2.1 CBF 0.32 across all subjects, CBV = 0.8 CBF 0.51 in females and CBV = 4.4 CBF 0.15 in males. Exponents decreased in both genders as ROIs were expanded to include less significantly activated regions. Conclusion-Consideration for potential gender related differences, as well as regional variations under a range of physiological states, may reconcile some of the variation across literature and advance our understanding of the underlying cerebrovascular physiology.
Recent improvements in parallel imaging have been driven by the use of greater numbers of indepen... more Recent improvements in parallel imaging have been driven by the use of greater numbers of independent surface coils placed so as to minimize aliasing along the phase‐encode direction(s). However, gains from increasing the number of coils diminish as coil coupling problems begin to dominate and the ratio of acceleration gain to expense for multiple receiver chains becomes prohibitive. In this work, we redesign the spatial‐encoding strategy in order to gain efficiency, achieving a gradient encoding scheme that is complementary to the spatial encoding provided by the receiver coils. This approach leads to “O‐space” imaging, wherein the gradient shapes are tailored to an existing surface coil array, making more efficient use of the spatial information contained in the coil profiles. In its simplest form, for each acquired echo the Z2 spherical harmonic is used to project the object onto sets of concentric rings, while the X and Y gradients are used to offset this projection within the i...
A simple iterative algorithm, termed deconvolution-interpolation gridding (DING), is presented to... more A simple iterative algorithm, termed deconvolution-interpolation gridding (DING), is presented to address the problem of reconstructing images from arbitrarily-sampled k-space. The new algorithm solves a sparse system of linear equations that is equivalent to a deconvolution of the k-space with a small window. The deconvolution operation results in increased reconstruction accuracy without grid subsampling, at some cost to computational load. By avoiding grid oversampling, the new solution saves memory, which is critical for 3D trajectories. The DING algorithm does not require the calculation of a sampling density compensation function, which is often problematic. DING's sparse linear system is inverted efficiently using the conjugate gradient (CG) method. The reconstruction of the gridding system matrix is simple and fast, and no regularization is needed. This feature renders DING suitable for situations where the k-space trajectory is changed often or is not known a priori, such as when patient motion occurs during the scan. DING was compared with conventional gridding and an iterative reconstruction method in computer simulations and in vivo spiral MRI experiments. The results demonstrate a stable performance and reduced root mean square (RMS) error for DING in different k-space trajectories.
Purpose-Cerebral blood volume (CBV) changes in many diverse pathologic conditions, and in respons... more Purpose-Cerebral blood volume (CBV) changes in many diverse pathologic conditions, and in response to functional challenges along with changes in blood flow, blood oxygenation, and the cerebral metabolic rate of oxygen. The feasibility of a new method for non-invasive quantification of absolute cerebral blood volume that can be applicable to the whole human brain was investigated. Methods-Multi-slice data were acquired at 3 T using a novel inversion recovery echo planar imaging (IR-EPI) pulse sequence with varying contrast weightings and an efficient rotating slice acquisition order, at rest and during visual activation. A biophysical model was used to estimate absolute cerebral blood volume at rest and during activation, and oxygenation during activation, on data from 13 normal human subjects. Results-Cerebral blood volume increased by 21.7% from 6.6±0.8 mL/100 mL of brain parenchyma at rest to 8.0±1.3 mL/100 mL of brain parenchyma in the occipital cortex during visual activation, with average blood oxygenation of 84±2.1% during activation, comparing well with literature. Conclusion-The method is feasible, and could foster improved understanding of the fundamental physiological relationship between neuronal activity, hemodynamic changes, and metabolism underlying brain activation; complement existing methods for estimating compartmental changes; and potentially find utility in evaluating vascular health.
Purpose: To determine and track the planar orientation of active interventional devices without u... more Purpose: To determine and track the planar orientation of active interventional devices without using localizing RF microcoils. Materials and Methods: An image-based tracking method that determines a device's orientation using projection images was developed. An automated and a manual detection scheme were implemented. The method was demonstrated in an in vivo mesocaval puncture procedure in swine, which required accurate orientation of an active transvascular needle catheter. Results: The plane of the catheter was determined using two projection images. The scan plane was adjusted automatically to follow the catheter plane, and its orientation with respect to a previously acquired target plane was displayed. The algorithm facilitated navigation for a fast and accurate puncture. Conclusion: Using image-based techniques, with no mechanical design changes, the orientation of an active intravascular probe could be tracked.
The success of x-ray fluoroscopy-guided coronary catheterization depends in part on the ability t... more The success of x-ray fluoroscopy-guided coronary catheterization depends in part on the ability to obtain simultaneous and real-time visualization of the guidewire, guiding catheter, and anatomy of the chest. The hypothesis explored in this paper is that magnetic resonance imaging (MRI) could provide this ability. This hypothesis was tested with loopless antennas used as the guidewire and a guiding catheter and two surface coils, each connected to four different receiver channels of a GE 1.5-T CV/I MRI scanner. Experiments were conducted on six healthy dogs. Intravascular antennas were inserted in the right carotid artery and maneuvered in the aorta while running a fast gradient-echo sequence (TR/TE 5/1.3 msec, flip angle 7°). Real-time projection images of the chest anatomy, together with the guidewire and guiding catheter, were obtained. Positioning of the MRI guiding catheter either in the descending aorta, ascending aorta, or heart was achieved easily. This study represents a step toward MRIguided coronary catheterization.
Introduction: BOLD fMRI is a powerful noninvasive approach for studying brain function, however, ... more Introduction: BOLD fMRI is a powerful noninvasive approach for studying brain function, however, physiological interpretation of the signal is limited since it reflects a combination of changes in CBV, CBF, blood oxygenation and metabolism (1-4). While methods exist for non-invasive absolute CBF quantification, CBV measurement has been invasive and difficult especially in human subjects. Most calibrated fMRI studies typically assume CBV=0.88CBF based on the absolute CBV-CBF relationship obtained in monkey brains under hypo/hypercapnia using PET (5). This practice is controversial since this relation may vary across brain regions, functional challenges, and species, as demonstrated by published power parameters ranging from 0.18-0.64 in steady-state across species, stimuli, modalities, relative and absolute contributions of whole/partial blood compartments (6-16). This study presents MRI validation of the absolute CBV-CBF relationship in humans.
EFFECTS OF PROPOFOL ANESTHESIA AND SEX ON CEREBRAL BLOOD FLOW Pelin Aksit Ciris, Maolin Qiu, and ... more EFFECTS OF PROPOFOL ANESTHESIA AND SEX ON CEREBRAL BLOOD FLOW Pelin Aksit Ciris, Maolin Qiu, and R. Todd Constable Radiology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, United States, Diagnostic Radiology, Yale University, School of Medicine, New Haven, CT, United States, Diagnostic Radiology, Neurosurgery, and Biomedical Engineering, Yale University, School of Medicine, New Haven, CT, United States
Purpose: To improve the geometric fidelity and spatial-resolution of multi-b diffusion-weighted M... more Purpose: To improve the geometric fidelity and spatial-resolution of multi-b diffusion-weighted MRI of the prostate. Materials and Methods: An accelerated segmented diffusion imaging sequence was developed and evaluated in 25 patients undergoing multi-parametric MRI exams of the prostate. A reduced field-of-view was acquired using an endo-rectal coil. The number of sampled diffusion weightings, or b-factors, was increased to allow estimation of tissue perfusion based on the intra-voxel incoherent motion (IVIM) model. Apparent diffusion coefficients (ADCs) measured with the proposed segmented method were compared to those obtained with conventional single-shot echoplanar imaging (EPI). Results: Compared to single-shot EPI, the segmented method resulted in faster acquisition with twofold improvement in spatial resolution and a greater than threefold improvement in geometric fidelity. ADC values measured with the novel sequence demonstrated excellent agreement with those obtained from the conventional scan (R 2 =0.91 for b max =500 s/mm 2 and R 2 =0.89 for b max =1400 s/mm 2). IVIM perfusion fraction was 4.0+/−2.7% for normal peripheral zone, 6.6+/ −3.6% for normal transition zone and 4.4+/−2.9% for suspected tumor lesions. Conclusion: The proposed accelerated segmented prostate diffusion imaging sequence achieved improvements in both spatial resolution and geometric fidelity, along with concurrent quantification of IVIM perfusion.
Journal of neuroimaging : official journal of the American Society of Neuroimaging, Jan 10, 2018
Diffusion magnetic resonance imaging (dMRI) provides preoperative maps of neurosurgical patients&... more Diffusion magnetic resonance imaging (dMRI) provides preoperative maps of neurosurgical patients' white matter tracts, but these maps suffer from echo-planar imaging (EPI) distortions caused by magnetic field inhomogeneities. In clinical neurosurgical planning, these distortions are generally not corrected and thus contribute to the uncertainty of fiber tracking. Multiple image processing pipelines have been proposed for image-registration-based EPI distortion correction in healthy subjects. In this article, we perform the first comparison of such pipelines in neurosurgical patient data. Five pipelines were tested in a retrospective clinical dMRI dataset of 9 patients with brain tumors. Pipelines differed in the choice of fixed and moving images and the similarity metric for image registration. Distortions were measured in two important tracts for neurosurgery, the arcuate fasciculus and corticospinal tracts. Significant differences in distortion estimates were found across proc...
We propose a method for the automated identification of key white matter fiber tracts for neurosu... more We propose a method for the automated identification of key white matter fiber tracts for neurosurgical planning, and we apply the method in a retrospective study of 18 consecutive neurosurgical patients with brain tumors. Our method is designed to be relatively robust to challenges in neurosurgical tractography, which include peritumoral edema, displacement, and mass effect caused by mass lesions. The proposed method has two parts. First, we learn a data-driven white matter parcellation or fiber cluster atlas using groupwise registration and spectral clustering of multi-fiber tractography from healthy controls. Key fiber tract clusters are identified in the atlas. Next, patient-specific fiber tracts are automatically identified using tractography-based registration to the atlas and spectral embedding of patient tractography. Results indicate good generalization of the data-driven atlas to patients: 80% of the 800 fiber clusters were identified in all 18 patients, and 94% of the 800...
A biomarker of cancer aggressiveness, such as hypoxia, could substantially impact treatment decis... more A biomarker of cancer aggressiveness, such as hypoxia, could substantially impact treatment decisions in the prostate, especially radiation therapy, by balancing treatment morbidity (urinary incontinence, erectile dysfunction, etc.) against mortality. R2 (*) mapping with Mono-Exponential (ME) decay modeling has shown potential for identifying areas of prostate cancer hypoxia at 1.5T. However, Gaussian deviations from ME decay have been observed in other tissues at 3T. The purpose of this study is to assess whether gradient-echo signal decays are better characterized by a standard ME decay model, or a Gaussian Augmentation of the Mono-Exponential (GAME) decay model, in the prostate at 3T. Multi-gradient-echo signals were acquired on 20 consecutive patients with a clinical suspicion of prostate cancer undergoing MR-guided prostate biopsies. Data were fitted with both ME and GAME models. The information contents of these models were compared using Akaike's information criterion (se...
Dual-pathway sequences have been proposed to help improve the temperature-to-noise ratio (TNR) in... more Dual-pathway sequences have been proposed to help improve the temperature-to-noise ratio (TNR) in MR thermometry. The present work establishes how much of an improvement these so-called "PSIF-FISP" sequences may bring in various organs and tissues. Simulations and TNR calculations were validated against analytical equations, phantom, abdomen, and brain scans. Relative TNRs for PSIF-FISP, as compared to a dual-FISP reference standard, were calculated for flip angle (FA) = 1 to 85 º and repetition time (TR) = 6 to 60 ms, for gray matter, white matter, cervix, endometrium, myometrium, prostate, kidney medulla and cortex, bone marrow, pancreas, spleen, muscle, and liver tissues. PSIF-FISP was TNR superior in the kidney, pelvis, spleen, or gray matter at most tested TR and FA settings, and benefits increased at shorter TRs. PSIF-FISP was TNR superior in other tissues, e.g., liver, muscle, pancreas, for only short TR settings (20 ms or less). The TNR benefits of PSIF-FISP increa...
Journal of magnetic resonance imaging : JMRI, Jan 12, 2016
To assess whether R2* mapping with a standard Monoexponential (ME) or a Gaussian Augmentation of ... more To assess whether R2* mapping with a standard Monoexponential (ME) or a Gaussian Augmentation of the Monoexponential (GAME) decay model better characterizes gradient-echo signal decays in gynecological cancers after external beam radiation therapy at 3T, and evaluate implications of modeling for noninvasive identification of intratumoral hypoxia. Multi-gradient-echo signals were acquired on 25 consecutive patients with gynecologic cancers and three healthy participants during inhalation of different oxygen concentrations at 3T. Data were fitted with both ME and GAME models. Models were compared using F-tests in tumors and muscles in patients, muscles, cervix, and uterus in healthy participants, and across oxygenation levels. GAME significantly improved fitting over ME (P < 0.05): Improvements with GAME covered 34% of tumor regions-of-interest on average, ranging from 6% (of a vaginal tumor) to 68% (of a cervical tumor) in individual tumors. Improvements with GAME were more promin...
ABSTRACT Introduction: The use of MRI for cardiac stress testing has been hampered by the need fo... more ABSTRACT Introduction: The use of MRI for cardiac stress testing has been hampered by the need for ECG gating, which does not perform well under conditions of changing heart rate and/or irregular heart motion. Real-time MRI avoids this problem, but until recently has not enjoyed high enough frame rates to properly visualize heart motion under stress conditions. Techniques incorporating training data sets [1] can be used to increase frame rates, but these also can break down when heart motion becomes irregular. We have developed a 32-channel real-time cardiac MRI system and used it to image the heart at up to 48 effective frames per second during exercise stress testing, without the inclusion of training sets. Methods: A 32-channel prototype GE MRI system was employed [2] to perform real-time imaging together with a 32-element cardiac receiver array [3]. The cardiac array comprised twenty-one 75 mm diameter circular elements mounted in a hexagonal array over the left-to-center chest wall and eleven 107 mm elements centered under the back. Custom imaging software was utilized to collect raw data from all 32 channels across multiple computers, reconstruct the images, and display them on the scanner monitor in real time. A main program directed the overall real-time application and provided the user interface, with controls to interactively change the scan plane [2]. A Beowulf cluster comprising eight nodes, each with dual 2.2 GHz AMD Opteron™ processors, has now been incorporated into this system as a real-time reconstruction engine, connected via private Gigabit Ethernet. Real-time imaging was performed using a FIESTA (balanced steady-state free precession) pulse sequence with a field of view of 33 cm, slice thickness of 8 mm, and TR of 2.8 ms. 128 readout points were collected, and 72 phase encodes with a half-Fourier acquisition scheme, using over-scans of 4 to 16 lines. ASSET acceleration factors between 2 and 4 were employed, along with a sliding reconstruction (slide factor of 2) to achieve effective imaging speeds of up to 48 frames per second. For accelerated imaging, low-resolution scans were first used to interactively move to the desired scan plane while collecting B1 field maps to calibrate the subsequent ASSET reconstruction. Switchover to accelerated imaging was achieved by a single button push on the user interface. Real-time scanning was performed on four normal volunteers, with exercise stress testing on a single volunteer. Results: Figure 1 shows still frames from a movie acquired at 22 frames per second in a four-chamber view of the heart. This used a x2 sliding reconstruction and an ASSET factor of two. Reducing the half-Fourier over-scans from 16 to 4 lines increased the effective scan rate to 31 frames per second, with some loss of SNR. Raising the ASSET factor to 4 with 8 over-scan lines resulted in frame rates of 44 frames per second, as exemplified by the coronal data set of Fig. 2. While the SNR is reduced relative to Fig. 1, the image quality is still sufficiently high to enable quantification of cardiac wall thickening. Figure 3 shows a plot of wall thickening versus time, measured on a 48 frame-per-second data set acquired immediately after exercise had increased the heart rate to an average of 85 BPM from a resting rate of 52 BPM in a normal volunteer.
Problem MRI-guided cardiac interventions have gained significant interest over the past decade. M... more Problem MRI-guided cardiac interventions have gained significant interest over the past decade. MRI provides the potential advantage of intra-operative anatomical imaging for the purpose of targeting, monitoring and control of the treatment. Several groups are currently working on enabling MRI-guided catheter based cardiac interventions (1-2). MRI can be used to target the therapy in cases where anatomical foci (such as myocardial scars) coincide with the electrophysiological foci. MRI has the potential to monitor the acute effect of tissue ablation as well and post-operative control. MRI can also be used for device tracking, using image based tracking, MR tracking or electro magnetic (EM) tracking. Current approaches for cardiac MRI-guidance are either to use pre-operative datasets or real-time imaging. The purpose of this study was to enable concomitant navigation on 3D cardiac cine loops as well as real-time imaging using ECG triggering for accurate temporal registration and synchronization.
This article reviews state-of-the-art MR acquisition strategies for brain imaging including seque... more This article reviews state-of-the-art MR acquisition strategies for brain imaging including sequences for imaging anatomy and morphometry, microstructure particularly in the white matter, and functional imaging. Recent developments in terms of acquisition strategies are reviewed for each of these topics, and the trade-offs made with some of the different strategies are discussed.
This work describes a real‐time imaging and visualization technique that allows multiple field of... more This work describes a real‐time imaging and visualization technique that allows multiple field of view (FOV) imaging. A stream of images from a single receiver channel can be reconstructed at multiple FOVs within each image frame. Alternately, or in addition, when multiple receiver channels are available, image streams from each channel can be independently reconstructed at multiple FOVs. The implementation described here provides for real‐time visualization of the placement of guidewires and catheters on a dynamic roadmap during interventional procedures. The loopless catheter antenna, an electrically active intravascular probe, was used for MR signal reception. In 2D projection images, the catheter and surrounding structures within its diameter of sensitivity appear as bright signal. The simplicity of the resulting images allows very‐narrow‐FOV imaging to decrease imaging time. Very‐narrow‐FOV images are acquired on MR receiver channels that collect guidewire or catheter data. The...
Purpose-The relationship between cerebral blood volume (CBV) and cerebral blood flow (CBF) underl... more Purpose-The relationship between cerebral blood volume (CBV) and cerebral blood flow (CBF) underlies blood oxygenation level dependent functional MRI signal. This study investigates the potential for improved characterization of the CBV-CBF relationship in humans, and examines gender effects as well as spatial variations in the CBV-CBF relationship. Methods-Healthy subjects were imaged non-invasively at rest and during visual stimulation, constituting the first MRI measurement of the absolute CBV-CBF relationship in humans with complete coverage of the functional areas of interest. Results-CBV and CBF estimates were consistent with literature, and their relationship varied both spatially and with gender. In a region of interest (ROI) with stimulus induced activation in CBV and CBF at the P<0.05 significance level, a power function fit resulted in CBV = 2.1 CBF 0.32 across all subjects, CBV = 0.8 CBF 0.51 in females and CBV = 4.4 CBF 0.15 in males. Exponents decreased in both genders as ROIs were expanded to include less significantly activated regions. Conclusion-Consideration for potential gender related differences, as well as regional variations under a range of physiological states, may reconcile some of the variation across literature and advance our understanding of the underlying cerebrovascular physiology.
Recent improvements in parallel imaging have been driven by the use of greater numbers of indepen... more Recent improvements in parallel imaging have been driven by the use of greater numbers of independent surface coils placed so as to minimize aliasing along the phase‐encode direction(s). However, gains from increasing the number of coils diminish as coil coupling problems begin to dominate and the ratio of acceleration gain to expense for multiple receiver chains becomes prohibitive. In this work, we redesign the spatial‐encoding strategy in order to gain efficiency, achieving a gradient encoding scheme that is complementary to the spatial encoding provided by the receiver coils. This approach leads to “O‐space” imaging, wherein the gradient shapes are tailored to an existing surface coil array, making more efficient use of the spatial information contained in the coil profiles. In its simplest form, for each acquired echo the Z2 spherical harmonic is used to project the object onto sets of concentric rings, while the X and Y gradients are used to offset this projection within the i...
A simple iterative algorithm, termed deconvolution-interpolation gridding (DING), is presented to... more A simple iterative algorithm, termed deconvolution-interpolation gridding (DING), is presented to address the problem of reconstructing images from arbitrarily-sampled k-space. The new algorithm solves a sparse system of linear equations that is equivalent to a deconvolution of the k-space with a small window. The deconvolution operation results in increased reconstruction accuracy without grid subsampling, at some cost to computational load. By avoiding grid oversampling, the new solution saves memory, which is critical for 3D trajectories. The DING algorithm does not require the calculation of a sampling density compensation function, which is often problematic. DING's sparse linear system is inverted efficiently using the conjugate gradient (CG) method. The reconstruction of the gridding system matrix is simple and fast, and no regularization is needed. This feature renders DING suitable for situations where the k-space trajectory is changed often or is not known a priori, such as when patient motion occurs during the scan. DING was compared with conventional gridding and an iterative reconstruction method in computer simulations and in vivo spiral MRI experiments. The results demonstrate a stable performance and reduced root mean square (RMS) error for DING in different k-space trajectories.
Purpose-Cerebral blood volume (CBV) changes in many diverse pathologic conditions, and in respons... more Purpose-Cerebral blood volume (CBV) changes in many diverse pathologic conditions, and in response to functional challenges along with changes in blood flow, blood oxygenation, and the cerebral metabolic rate of oxygen. The feasibility of a new method for non-invasive quantification of absolute cerebral blood volume that can be applicable to the whole human brain was investigated. Methods-Multi-slice data were acquired at 3 T using a novel inversion recovery echo planar imaging (IR-EPI) pulse sequence with varying contrast weightings and an efficient rotating slice acquisition order, at rest and during visual activation. A biophysical model was used to estimate absolute cerebral blood volume at rest and during activation, and oxygenation during activation, on data from 13 normal human subjects. Results-Cerebral blood volume increased by 21.7% from 6.6±0.8 mL/100 mL of brain parenchyma at rest to 8.0±1.3 mL/100 mL of brain parenchyma in the occipital cortex during visual activation, with average blood oxygenation of 84±2.1% during activation, comparing well with literature. Conclusion-The method is feasible, and could foster improved understanding of the fundamental physiological relationship between neuronal activity, hemodynamic changes, and metabolism underlying brain activation; complement existing methods for estimating compartmental changes; and potentially find utility in evaluating vascular health.
Purpose: To determine and track the planar orientation of active interventional devices without u... more Purpose: To determine and track the planar orientation of active interventional devices without using localizing RF microcoils. Materials and Methods: An image-based tracking method that determines a device's orientation using projection images was developed. An automated and a manual detection scheme were implemented. The method was demonstrated in an in vivo mesocaval puncture procedure in swine, which required accurate orientation of an active transvascular needle catheter. Results: The plane of the catheter was determined using two projection images. The scan plane was adjusted automatically to follow the catheter plane, and its orientation with respect to a previously acquired target plane was displayed. The algorithm facilitated navigation for a fast and accurate puncture. Conclusion: Using image-based techniques, with no mechanical design changes, the orientation of an active intravascular probe could be tracked.
The success of x-ray fluoroscopy-guided coronary catheterization depends in part on the ability t... more The success of x-ray fluoroscopy-guided coronary catheterization depends in part on the ability to obtain simultaneous and real-time visualization of the guidewire, guiding catheter, and anatomy of the chest. The hypothesis explored in this paper is that magnetic resonance imaging (MRI) could provide this ability. This hypothesis was tested with loopless antennas used as the guidewire and a guiding catheter and two surface coils, each connected to four different receiver channels of a GE 1.5-T CV/I MRI scanner. Experiments were conducted on six healthy dogs. Intravascular antennas were inserted in the right carotid artery and maneuvered in the aorta while running a fast gradient-echo sequence (TR/TE 5/1.3 msec, flip angle 7°). Real-time projection images of the chest anatomy, together with the guidewire and guiding catheter, were obtained. Positioning of the MRI guiding catheter either in the descending aorta, ascending aorta, or heart was achieved easily. This study represents a step toward MRIguided coronary catheterization.
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