Biomedical subjects
Publications and source records attributed to N J Pelc.
We describe a technique for three-dimensional cine MR imaging. By using short repetition times (TR) and interleaved slice encoding, volumetric cine data can be acquired throughout the cardiac cycle with a temporal resolution of approximately 80 msec. A T1-shortening agent is used to produce contrast between blood and myocardium. A comparison between the acquisition times of this and several other two-dimensional techniques is presented.
In several applications, MRI is used to monitor the time behavior of the signal in an organ of interest; e.g., signal evolution because of physiological motion, activation, or contrast-agent accumulation. Dynamic applications involve acquiring data in a k-t space, which contains both temporal and spatial information. It is shown here that in some dynamic applications, the t axis of k-t space is not densely filled with information. A method is introduced that can transfer information from the k axes to the t axis, allowing a denser, smaller k-t space to be acquired, and leading to significant reductions in the acquisition time of the temporal frames. Results are presented for cardiac-triggered imaging and functional MRI (fMRI), and are compared with data obtained in a conventional way. The temporal resolution was increased by nearly a factor of two in the cardiac-triggered study, and by as much as a factor of eight in the fMRI study. This increase allowed the acquisition of fMRI activation maps, even when the acquisition time for a single full time frame was actually longer than the paradigm cycle period itself. The new method can be used to significantly reduce the acquisition time of the individual temporal frames in certain dynamic studies. This can be used, for example, to increase the temporal or spatial resolution, increase the spatial coverage, decrease the total imaging time, or alter sequence parameters e.g., repetition time (TR) and echo time (TE) and thereby alter contrast. Magn Reson Med 42:813-828, 1999.
We present a method (DMESH) for nonrigid cyclic motion analysis using a series of velocity images covering the cycle acquired, for example, from phase-contrast magnetic resonance imaging. The method is based on fitting a dynamic finite-element mesh model to velocity samples of an extended region, at all time frames. The model offers a flexible tradeoff between accuracy and reproducibility with controllable built-in spatiotemporal smoothing, which is determined by the fineness of the initially defined mesh and the richness of included Fourier harmonics. The method can further provide a prediction of the analysis reproducibility, along with the estimated motion and deformation quantities. Experiments have been conducted to validate the method and to verify the reproducibility prediction. Use of the method for motion analysis using displacement information (e.g., from magnetic resonance tagging) has also been explored.
Whenever a linear gradient is activated, concomitant magnetic fields with non-linear spatial dependence result. This is a consequence of Maxwell's equations, i.e., within the imaging volume the magnetic field must have zero divergence, and has negligible curl. The concomitant, or Maxwell field has been described in the MRI literature for over 10 years. In this paper, we theoretically and experimentally show the existence of two additional lowest-order terms in the concomitant field, which we call cross-terms. The concomitant gradient cross-terms only arise when the longitudinal gradient Gz is simultaneously active with a transverse gradient (Gx or Gy). The effect of all of the concomitant gradient terms on phase contrast imaging is examined in detail. Several methods for reducing or eliminating phase errors arising from the concomitant magnetic field are described. The feasibility of a joint pulse sequence-reconstruction method, which requires no increase in minimum TE, is demonstrated. Since the lowest-order terms of the concomitant field are proportional to G2/B0, the importance of concomitant gradient terms is expected to increase given the current interest in systems with stronger gradients and/or weaker main magnetic fields.
This paper describes a technique for characterizing the gradient subsystem of a magnetic resonance (MR) system. The technique uses a Fourier-transform analysis to directly measure the k-space trajectory produced by an arbitrary gradient waveform. In addition, the method can be easily extended to multiple dimensions and can be adapted to measuring residual gradient effects such as eddy currents. Several examples of gradient waveform and eddy-current measurements are presented. Also, it is demonstrated how the eddy-current measurements can be parameterized with an impulse-response formalism for later use in system tuning. When compared to a peak-fitting analysis, this technique provides a more direct extraction of the k-space measurements, which reduces the possibility of analysis error. This approach also has several advantages as compared to the conventional eddy-current measurement technique, including the ability to measure very short time constant effects.
The ability to track motion from cine phase-contrast (PC) magnetic resonance (MR) velocity measurements was investigated using an in vitro model. A computer-controlled deformable phantom was used for the characterization of the accuracy and precision of the forward-backward and the compensated Fourier integration techniques. Trajectory accuracy is limited by temporal resolution when the forward-backward technique is used. With this technique the extent of the calculated trajectories is underestimated by an amount related to the motion period and the sequence repetition time, because of the band-limiting caused in the cine interpolation step. When the compensated Fourier integration technique is used, trajectory accuracy is independent of temporal resolution and is better than 1 mm for excursions of less than 15 mm, which are comparable to those observed in the myocardium. Measurement precision is dominated by the artifact level in the phase-contrast images. If no artifacts are present precision is limited by the inherent signal-to-noise ratio of the images. In the presence of artifacts, similar in magnitude to those observed in vivo, the reproducibility of tracking a 2.2 x 2.2 mm2 region of interest is better than 0.5 mm. When the Fourier integration technique is used, the improved accuracy is accompanied by a reduction in precision. We verified that tracking three-dimensional (3D) motion from velocity measurements of a single slice can lead to underestimations of the trajectory if there is a through-plane component of the motion that is not truly represented by the measured velocities. This underestimation can be overcome if volumetric cine phase-contrast velocity data are acquired and full three-dimensional analysis is performed.
Magnetic resonance angiography (MRA) was performed by using RF pulses designed to excite a limited spatial extent in two orthogonal directions. The restriction in the second spatial dimension can be used to increase inflow enhancement and to improve small field-of-view imaging. A rectangular excitation was produced with an "echo-planar" k-space trajectory and a sinc-modulated RF waveform. In vivo images have demonstrated that vessels are more clearly delineated with the two-dimensional excitation. Aliasing artifacts in small field-of-view imaging are significantly reduced, although in some cases complete elimination is not possible due to the nature of the gradient trajectory.
Dynamic cardiac imaging in MRI is a very challenging task. To obtain high spatial resolution, temporal resolution, and signal-to-noise ratio (SNR), single-shot imaging is not sufficient. Use of multishot techniques resolves this problem but can cause motion artifacts because of data inconsistencies between views. Motion artifacts can be reduced by signal averaging at some cost in increased scan time. However, for the same increase in scan time, other techniques can be more effective than simple averaging in reducing the artifacts. If most of the energy of the inconsistencies is limited to a certain region of kappa-space, increased sampling density (oversampling) in this region can be especially effective in reducing motion artifacts. In this work, several variable-density spiral trajectories are designed and tested. Their efficiencies for artifact reduction are evaluated in computer simulations and in scans of normal volunteers. The SNR compromise of these trajectories is also investigated. The authors conclude that variable-density spiral trajectories can effectively reduce motion artifacts with a small loss in SNR as compared with a uniform density counterpart.
A new generation of high power gradient systems which allow much faster MR imaging as well as shorter echo times has recently become available. Some of these high-speed gradient systems impose limits on the percentage of time during which the gradient can change in amplitude (slewing duty cycle). While this limitation may be immaterial to many 2DFT and echo planar imaging methods, a traditional circular spiral trajectory is difficult to use on these systems because its gradient waveforms change during the entire course of the trajectory so that the slewing duty cycle during the readout period is 100%. We describe a piecewise-linear spiral trajectory which is composed of linear segments and rounded corners. This trajectory reduces the slewing duty cycle while maintaining the desirable imaging properties of circular spirals including interleaving by simple gradient rotation. For one representative example, the slewing duty cycle is reduced to 46%. A conventional gridding method was used for image reconstruction, but a new numerical algorithm to calculate the density compensation factor was required. Use of piecewise-linear spiral trajectories reduces the impact imposed by limited gradient slewing duty cycle.
Phase contrast magnetic resonance imaging (MRI) can provide in vivo myocardial velocity field measurements. These data allow densely spaced material points to be tracked throughout the whole heart cycle using, for example, the Fourier tracking algorithm. To process the tracking results for myocardial deformation and strain quantification, we developed a method that is based on fitting the tracking results to an appropriate local deformation model. We further analyzed the accuracy and precision of the method and provided performance predictions for several local models. In order to validate the method and the theoretical performance analysis, we conducted controlled computer simulations and a phantom study. The results agreed well with expectations. Human heart data were also acquired and analyzed, and provided encouraging results. At the signal-to-noise ratio (SNR) level and spatial resolution expected in clinical settings, the study predicts strain quantification accuracy and precision that may allow the technique to become a practical and powerful noninvasive approach for the study of cardiac function, although clinically acceptable data acquisition strategies for three-dimensional (3-D) data are still a challenge.
An in vitro study was performed to investigate the effects of B(o) inhomogeneity on magnetic resonance images of flow. Controlled inhomogeneity gradients (Gi) were applied and the magnitude of the artifacts produced was quantified for different echo delay times (TE). Both steady and pulsatile flows were examined. In the presence of an inhomogeneity gradient, signal loss is apparent if the flow is pulsatile and/or if the slice thickness is large. The signal loss increases with increasing TE and Gi. With pulsatile flow, ghosting artifacts are also generated. These increase in intensity with increasing TE and Gi. In vivo, field inhomogeneity due to susceptibility variations is large enough to produce these effects. Representative time-of-flight images obtained of a normal volunteer with two different TEs demonstrate the effect in vivo. Flow-related signal loss and artifacts, therefore, increase with increasing TE independent of the moments of the applied gradients.
A closed-form integration method is derived and analyzed for computing motion trajectories from velocity field data, particularly as measured by phase contrast (PC) cine MR imaging. By modeling periodic motion as composed of Fourier harmonics and integrating the material velocity of the tracked point in the frequency domain, this method gives an unbiased trajectory estimate in the presence of white measurement noise and eddy current effects. When applied to cine PC data, the method can incorporate compensation for the frequency response of the cine interpolation, offering a further improvement on the tracking accuracy. In simulation and phantom studies, the estimated trajectories were in excellent agreement with the true trajectories. Encouraging results have also been obtained on data from volunteers.
In some dynamic imaging applications, only a fraction, 1/n, of the field of view (FOV) may show considerable change during the motion cycle. A method is presented that improves the temporal resolution for a dynamic region by a factor, n, while maintaining spatial resolution at a cost of square root of n in signal-to-noise ratio (SNR). Temporal resolution is improved, or alternatively, total imaging time is reduced by reducing the number of phase encodes acquired for each temporal frame by 1/n. To eliminate aliasing, a representation of the signal from the static outer portion of the FOV is constructed using all the raw data. The k-space data derived from this representation is subtracted from the original data sets, and the differences correspond to the dynamic portion of the FOV. Improved resolution results are presented in phantom studies, and in vivo phase contrast quantitative flow imaging.
We describe a technique to increase the velocity-to-noise ratio (VNR) of phase-contrast magnetic-resonance velocity images based on making three measurements/flow encoding axis rather than the usual two. A phase-aliased high first moment data set and a nonaliased low first moment data set are acquired, and the high-moment data are phase unwrapped using the low-moment data. The VNR of the resulting measurement is given by that of the high-moment measurement and increases linearly with the first moment. A factor of 4 gain in VNR was observed with only a 50% increase in scan time. Thus, this method is a much more efficient way to increase VNR than simple averaging.
Interleaved spiral scanning of k-space is an efficient and fast method for imaging dynamic processes. In this article, a cine version of interleaved spiral imaging is presented. The method is shown to overcome the "lightning-flash" artifacts of the conventional triggered (gated) method. Compared with the segmented k-space 2DFT method, it achieves better temporal resolution in a comparable or shorter scan time. Preliminary human studies show that the method is a promising tool for imaging dynamic processes.
PURPOSE: To evaluate a magnetic resonance (MR) imaging method that allows for simultaneous resolution of both the cardiac and respiratory cycles. MATERIALS AND METHODS: Conventional and phase-contrast cine sequences were modified to provide additional resolution of the respiratory cycle. Data were collected in 11 healthy volunteers during MR imaging of the heart and portal vein. The imaging time was increased over that of a conventional cine acquisition by a factor equal to the number of frames in the respiratory cycle. Data were compared with those from comparable sequences in which only one motion cycle was resolved. RESULTS: In the heart, motion due to cardiac dynamics was separated from respiration-induced excursions. The extent of motion could be measured, and artifacts were minimized. Changes in flow rate as a function of both motion cycles were resolved and quantified in both the portal vein and superior vena cava. CONCLUSION: This method allows for simultaneous resolution of cardiac and respiratory motion cycles and helps provide a more physiologic view of the effects of cardiac and respiratory variations.
The sensitivity of nuclear magnetic resonance to motion can be harnessed to produce quantitative measurements of low flow velocity and volume flow rate. MR flow quantification is noninvasive, can be performed in arbitrary locations and in any image orientation, and is fairly rapid. This article discusses the basic techniques with particular attention to phase contrast quantitation of volume flow rate, especially the determinants of accuracy and reproducibility in these measurements.
BACKGROUND: After the occlusion of an internal carotid artery the principal source of collateral flow is through the arteries of the circle of Willis, but the size and patency of these arteries are quite variable. Study of the anatomy of the collateral pathways in patients with internal-carotid-artery occlusion with or without infarction in the watershed area of the deep white matter may identify patterns that afford protection from ischemic infarction. METHODS: Using conventional magnetic resonance imaging and three-dimensional phase-contrast magnetic resonance angiography, we evaluated 29 consecutive patients (32 hemispheres at risk) with angiographically proved occlusion of the internal carotid artery. Four collateral pathways to the occluded vessel were evaluated: the proximal segment of the anterior cerebral artery, the posterior communicating artery, the ophthalmic artery, and leptomeningeal collateral vessels from the posterior cerebral artery. RESULTS: Only features of the ipsilateral posterior communicating artery were related to the risk of watershed infarction. The presence of posterior communicating arteries measuring at least 1 mm in diameter was associated with the absence of watershed infarction (13 hemispheres, no infarcts; P < 0.001). Conversely, there were 4 watershed infarcts in the 6 hemispheres with posterior communicating arteries measuring less than 1 mm in diameter and 10 infarcts in the 13 hemispheres with no detectable flow in the ipsilateral posterior communicating artery. CONCLUSIONS: A small (< 1 mm in diameter) or absent ipsilateral posterior communicating artery is a risk factor for ischemic cerebral infarction in patients with internal-carotid-artery occlusion.