Design of a logarithmic k-space spiral trajectory.

Spiral acquisitions are used in fast cardiac imaging because they traverse k-space efficiently and minimize flow artifacts. A variable pitch logarithmic spiral trajectory is designed to critically sample the low-frequency region in k-space and gradually undersample the high-frequency region. An approximate analytical expression for the trajectory provides a fast means to calculate the gradient waveforms and the sampled data points. A numerical method is introduced based on the trajectory curvature and the rate of change in the gradient magnitude with time for the composite Archimedean-logarithmic trajectory. The pulse sequence is implemented and images are acquired on phantoms and human hearts. The images show improved image resolution and some improvement in image quality as a result of increased extent in k-space and reduction in aliasing artifacts, respectively.

Combined connectivity and a gray-level morphological filter in magnetic resonance coronary angiography.

A connectivity algorithm combined with a new gray-level morphological filter dramatically improves the segmentation of tortuous coronary arteries from 3D MRI. Small coronary arteries are segmented from the larger ventricles with a new filter. These blood vessels are segmented from the noise background with connectivity. Coronary angiograms were computed in nine datasets acquired on volunteers with 3D stack of spirals and contrast-enhanced navigator sequences by both a maximum intensity projection and surface rendering. Surface images provided depth information needed to distinguish branching arteries from crossing veins. Magn Reson Med 43:892-895, 2000.

3D MR coronary artery segmentation.

Coronary arteries are segmented from the blood pool using mathematical morphology operations from a 3D magnetic resonance spiral acquisition on a continuously breathing healthy volunteer. The segmented volume is maximal intensity projected at different views to yield coronary angiograms showing the left anterior descending artery (LAD), right coronary artery (RCA), and left circumflex artery (LCX). Magnetic resonance coronary angiography provides a retrospective rotating view of the coronary artery tree that complements oblique reformatted sections.

Optimization of spoiled gradient-echo phase imaging for in vivo localization of a focused ultrasound beam.

The parameters of a spoiled gradient-echo (SPGR) pulse sequence have been optimized for in vivo localization of a focused ultrasound beam. Temperature elevation was measured by using the proton resonance frequency shift technique, and the phase difference signal-to-noise ratio (SNR delta phi) was estimated in skeletal muscle and kidney cortex in 10 rabbits. Optimized parameters included the echo time equivalent to T2* of the tissue, the longest repetition time possible with a 20-s sonication, and the flip angle equivalent to the Ernst angle. Optimal SPGR phase imaging can detect a sonication beam with a peak phase difference of 0.55 radian, which corresponds to a temperature elevation of 7.3 degrees C. The sonication beam can be localized within one voxel (0.6 x 0.6 x 5 mm3) at power levels that are below the threshold for thermal damage of the tissue.

Computer-assisted interactive three-dimensional planning for neurosurgical procedures.

We have used three-dimensional reconstruction magnetic resonance imaging techniques to understand the anatomic complexity of operative brain lesions and to improve preoperative surgical planning. We report our experience with 14 cases, including intra- and extra-axial tumors and a vascular malformation. In each case, preoperative planning was performed using magnetic resonance imaging-based three-dimensional renderings of surgically critical structures, such as eloquent cortices, gray matter nuclei, white matter tracts, and blood vessels. Simulations, using the interactive manipulation of three-dimensional data, provided an efficient and comprehensive way to appreciate the anatomic relationships. Interactive three-dimensional computer-assisted preoperative simulations provided otherwise inaccessible information that was useful for the surgical removal of brain lesions.

Simultaneous magnetic resonance phase and magnitude temperature maps in muscle.

Noninvasive magnetic resonance temperature maps that are used to monitor thermal ablation of tissue are described. In magnetic resonance images, thermally induced proton nuclear magnetic resonance frequency shifts, and changes in the longitudinal relaxation time produce both phase and magnitude changes in the MR signal. Temperature maps with improved sensitivity are derived from the complex-difference nuclear magnetic resonance signal. Bovine muscle specimens were heated with focused ultrasound to model thermal surgery and create a known thermal distribution to test the method. Resulting MR images acquired in 2 s produce temperature maps with 1 min resolution and 2 degrees C temperature sensitivity. The temperature sensitivity was increased by extending the acquisition to 5 s, by decreasing the receiver bandwidth, and increasing the echo time.

A clinical, noninvasive, MR imaging-monitored ultrasound surgery method.

A noninvasive method of tissue ablation that is guided and monitored with magnetic resonance (MR) imaging has been developed. The method uses sharply focused ultrasound transducers of different focal lengths to induce a localized temperature elevation during a short exposure (1-20 seconds). A hydraulic, computer-controlled positioning device moves the transducer in an MR imager. The positioner is built into a standard cradle in the imager. The system includes cavitation detection and power monitoring circuitry for patient safety. The target volume is outlined with cross-sectional MR images obtained immediately before sonication. By means of the software, the focus is moved to ablate the volume defined with the images. The temperature elevation during the exposure is monitored by means of the proton resonance frequency shift with fast gradient-echo sequences, and the necrosed volume is demonstrated with T2-weighted fast spin-echo images. This method has been extensively tested in in vivo animal experiments and is now undergoing clinical trial.

Estimation of average flow in ungated 3D phase contrast angiograms.

Three dimensional (3D) phase contrast angiograms contain velocity data, which is discarded after the reconstruction of the projections. In extension to earlier work on velocity quantification with ungated 2D phase data, this paper shows that a useful estimate of the average velocity and flow rate can be extracted from ungated 3D phase contrast angiograms. Simulations and experiments in a phantom and in vivo were performed. For pulsatile flow and strong spin saturation, an over-estimation of the flow rate at the net in-flow end of the imaging volume and underestimation at the net out-flow end was observed. Imaging at lower RF tip angles yielded flow rates close to the correct value within the entire imaging volume. In contrast to ungated 2D experiments, the flow rates determined by repeated 3D experiments showed no variation.

Superconducting open-configuration MR imaging system for image-guided therapy.

PURPOSE: To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. MATERIALS AND METHODS: A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, and nonmagnetic displays and with position-monitoring probes and device-tracking instrumentation. RESULTS: The magnet homogeneity was 12.3 ppm, and the gradient field was linear to within 1% over an imaging region 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences, were demonstrated at a rate of one image every 1.5 seconds. CONCLUSION: MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

Focused US system for MR imaging-guided tumor ablation.

PURPOSE: To measure the performance characteristics of a focused ultrasound (US) system for magnetic resonance (MR) imaging-guided tumor ablation. MATERIALS AND METHODS: The authors constructed a focused US system for MR imaging-guided tumor ablation. The location of the heated region and thermal dose were monitored with temperature-sensitive MR images obtained in phantoms and rabbit skeletal muscle after application of each sonic pulse. RESULTS: The region heated by the focused ultrasound beam was within 1 mm of that observed on temperature-sensitive fast gradient-echo MR images of in vivo rabbit skeletal muscle. Analysis of heat flow and the rate of coagulation necrosis provided an estimate of the size of the ablated region that was in agreement with experimental findings. CONCLUSION: MR imaging provides target definition and control for thermal therapy in regions of variable perfusion or in tissues that are not well characterized.

MR temperature mapping of focused ultrasound surgery.

Deep lying soft tissue tumors may be treated by a nonincisional surgical procedure executed inside an MR imaging system using a thermal effect delivered by a focused ultrasound transducer. A prototype system is constructed to assess MRI thermal monitoring and the localization of the heat zone in muscle. The temperature distribution of the focal spot is imaged with MRI while mechanically moving the transducer with an hydraulic 3-axis positioner. Acoustic power is applied with a spherical shell transducer using 1- to 10-s duration pulses at frequencies of 1.5 MHz to selectively coagulate tissue at 60-70 degrees C. The procedure is monitored with a series of fast second gradient echo, T1-weighted, temperature sensitive MR sequences. Acquisitions are optimized for high temperature sensitive images that yield the thermal diffusivity, heat flow time constant and the focal spot size in muscle. MR temperature maps of muscle provide localization and dosimetry both in the focal region and near field.

One-dimensional NMR thermal mapping of focused ultrasound surgery.

OBJECTIVE: The aim of this study was to evaluate a technique for real-time monitoring of tissue temperature and tracking of the heat source during minimally invasive thermal interventions such as focused ultrasound surgery. MATERIALS AND METHODS: A temperature-sensitive NMR line scan pulse sequence was directed interactively from a workstation during the application of focused ultrasound to samples of excised bovine skeletal muscle. The NMR signal along a sensitive line was monitored during and after heating by means of a scrolling display on the workstation. RESULTS: The temperature sensitivity was found to be approximately 2 degrees C with a time resolution of 300 ms along a line intersecting the ultrasonic focal point. Experimental temperature rises determined from the NMR signal showed close agreement with theoretical temperature behavior derived from the heat equation. Temperature quantitation capabilities were lost upon onset of thermal denaturation and coagulation. CONCLUSION: This technique could serve as a noninvasive guide in tracking the heat source and in monitoring thermal dose during focused ultrasound surgery and other minimally invasive thermal interventions.

Somatosensory evoked response source localization using actual cortical surface as the spatial constraint.

We localized right median nerve somatosensory evoked responses in a normal human subject using an equivalent dipole method applied to magnetic field recordings. High resolution, 3-dimensional MRI data were used to confine source locations to the cortical surface. Results localized in Brodmann area 3b corresponding to location of hand somatosensory cortex derived from direct brain stimulation studies. The solution was unique and total computational time for an exhaustive, brute-force search was small and the results realistic due to applied anatomical constraints. This study demonstrates feasibility of accurate, non-invasive, realistic localization of dynamic human cortical function using spatial constraints provided by MRI images.

Magnetic resonance-guided thermal surgery.

A demonstration of MR guided thermal surgery involved experiments with imaging of focused ultrasound in an MRI system, measurements of the thermal transients and a thermal analysis of the resulting images. Both the heat distribution and the creation of focused ultrasound lesions in gel phantoms, in vitro bovine muscle and in vivo rabbit muscle were monitored with magnetic resonance imaging. Thermal surgical procedures were modeled by an elongated gaussian heat source where heat flow is controlled by tissue thermal properties and tissue perfusion. Temperature profiles were measured with thermocouples or calculated from magnetic resonance imaging in agreement with the model. A 2-s T1-weighted gradient-refocused acquisition provided thermal profiles needed to localize the heat distribution produced by a 4-s focused ultrasound pulse. Thermal analysis of the images give an effective thermal diffusion coefficient of 0.0015 cm2/s in gel and 0.0033 cm2/s in muscle. The lesions were detected using a T2-weighted spin-echo or fast spin-echo pulse sequence in agreement with muscle tissue sections. Potential thermal surgery applications are in the prostate, liver, kidney, bladder, breast, eye and brain.

3D phase contrast MRI of cerebral blood flow and surface anatomy.

Noninvasive MR acquisition of blood flow and stationary tissue provides three-dimensional display of flow and of the vascular and brain surfaces. The calculated motion of a simulated bolus injection is derived from the measured velocity vector field and is animated to resemble cine angiography. Simulation of a bolus injection into the basilar artery of a healthy volunteer shows the blood flow into the posterior cerebral arteries.

MR-guided focused ultrasound surgery.

Magnetic resonance guided focused ultrasound surgery provides a minimally invasive controlled method for selectively destroying deep-lying tissue. A thermal analysis of focused ultrasound provides an estimate of the time-dependent temperature distribution and thermal dose required for ultrasound surgery. The temperature distribution is estimated by accumulating heat sources, considering the effects of thermal conductivity, heat content, and perfusion. In this study, both gel phantoms and excised in vitro bovine muscle specimens were imaged in a 1.5 T MR system while heated with a 5 cm diameter, 10 cm focal length, 1.1 MHz transducer. During sonication, the thermal effects were observed with T1-weighted pulse sequences. Below a critical temperature, the heat zone appeared as a dark spot that moved with the focal spot. Above a critical thermal dose, the in vitro tissue was irreversibly altered and the focal lesion was observed on both the MR image and the specimen slice.

Rapid NMR cardiography with a half-echo M-mode method.

A real-time NMR cardiac profiling pulse sequence has been developed that incorporates two-dimensional (2D) selective excitation and a half-echo readout. The time resolution has been improved by a factor of two relative to the previous flow-compensated, full-echo version. The technique produces a 2D plot of "beam"-axis position versus time, analogous to M-mode echocardiography. In human subjects, details of valve leaflet motion, intracardiac flow, wall motion, and wall thickening may be observed along optimal lines of sight selected interactively. The pulse sequence uses a low-tip-angle 2D selective-excitation pulse derived from a spiral k-space trajectory to excite a narrow cylinder of magnetization, followed by a half-echo readout gradient oriented along the axis of the cylinder. One-dimensional Fourier transformation of the acquired signal results in a magnetization profile along the length of the cylinder, or beam. The pulse sequence is effectively flow compensated without any additional gradient lobes, because the rapid oscillation in the gradient wave forms of the 2D excitation pulse produces relatively small net gradient moments, and the shortened readout gradient has minimal first-order moment relative to center echo. The signal from moving blood can alternatively be velocity encoded by the addition of bipolar gradients along any of the three axes, producing Doppler-like traces of intracardiac blood flow.

Volume rendering and connectivity algorithms for MR angiography.

Several display algorithms for three-dimensional angiographic data are evaluated. The mathematical analysis assumes additive Gaussian noise to predict the background distribution function for maximum intensity projection, sum projection, and connectivity display methods. In the maximum intensity projection method the mean noise level increases with the number of voxels in the ray, while in the sum projection the noise distribution width increases with the projection thickness, but the mean level remains constant. Comparisons of maximum intensity projection, sum projection, and connectivity algorithms applied to an MR angiogram of the circle of Willis are made. Measurements of the noise distribution are in agreement with the analysis. Algorithms combining connectivity with maximum intensity and sum projection are also evaluated. In these methods, a projection image is created using only the voxels marked by connectivity, typically with a 6% threshold of the data. Fine vessels are resolved and background noise is reduced in agreement with the analysis.

3D surface rendered MR images of the brain and its vasculature.

Both time-of-flight and phase contrast magnetic resonance angiography images are combined with stationary tissue images to provide data depicting two contrast relationships yielding intrinsic discrimination of brain matter and flowing blood. A computer analysis is based on nearest neighbor segmentation and the connection between anatomical structures to partition the images into different tissue categories: from which, high resolution brain parenchymal and vascular surfaces are constructed and rendered in juxtaposition, aiding in surgical planning.

Fast MR cardiac profiling with two-dimensional selective pulses.

A rapid-profiling NMR pulse sequence has been designed to provide an interactive, real-time cardiac probe analogous to M-mode ultrasound. The pulse sequence employs a two-dimensional (2D) selective NMR pulse to excite a narrow (nominally 1-cm-diameter) cylinder of magnetization intersecting the heart. This procedure is followed by a readout gradient applied along the length of the cylinder, or "beam," to yield an M-mode type profile with a one-dimensional Fourier transform reconstruction. k-space techniques were used to design 2D pulses which excite cylinders characterized by either Gaussian or square radial excitation profiles. Images of phantoms acquired at 1.5 T confirm the predictions of the k-space analysis. The cylinder can be displaced interactively by modulating the rf excitation and the beam axis can be reoriented to any oblique direction by changing the relative mixing of the gradient waveforms. Flow compensation using bipolar gradient waveforms inverts the contrast of flowing blood and suppresses flow artifacts. A gated cardiac image is acquired as a reference to locate the excitation axis. A series of cardiac experiments was performed on several healthy volunteers. As the beam is moved and rotated to probe the myocardium, the profile plots resemble an M-mode echocardiogram. Unlike in M-mode echocardiography, however, the axis of interrogation is not limited to specific windows, and there is distinct flexibility of contrast. However, the temporal resolution is currently less than that achieved by ultrasound. NMR M-mode profiling provides a direct, fast method of measuring heart motion to assess cardiac function as part of an MR cardiac exam.