Biomedical subjects

Peter M Jakob

Publications and source records attributed to Peter M Jakob.

At least 19 recordsLinked to original sources

PubMed2006

Structure-specific magnetic field inhomogeneities and its effect on the correlation time.

We describe the relationship between the correlation time and microscopic spatial inhomogeneities in the static magnetic field. The theory takes into account diffusion of nuclear spins in the inhomogeneous field created by magnetized objects. A simple general expression for the correlation time is obtained. It is shown that the correlation time is dependent on a characteristic length, the diffusion coefficient of surrounding medium, the permeability of the surface and the volume fraction of the magnetized objects. For specific geometries (spheres and cylinders), exact analytical expressions for the correlation time are given. The theory can be applied to contrast agents (magnetically labeled cells), capillary network, BOLD effect and so forth.

Magnetic Resonance Imaging↗

PubMed2006

Diffusion-weighted imaging with a rapid repeated hole-burning sequence.

Diffusion-weighted imaging in the presence of extremely short T2-relaxation time is generally not feasible with a standard PGSE experiment due to the superposed signal decays caused primarily by T2-relaxation and secondarily by diffusion. Here, we present a new method for diffusion-weighted imaging achieved by a nearly T2-independent pre-experiment where a DANTE-pulse train is repeated rapidly.

Journal Article↗

PubMed2006

Multimodal functional cardiac MRI in creatine kinase-deficient mice reveals subtle abnormalities in myocardial perfusion and mechanics.

A decrease in the supply of ATP from the creatine kinase (CK) system is thought to contribute to the evolution of heart failure. However, previous studies on mice with a combined knockout of the mitochondrial and cytosolic CK (CK(-/-)) have not revealed overt left ventricular dysfunction. The aim of this study was to employ novel MRI techniques to measure maximal myocardial velocity (V(max)) and myocardial perfusion and thus determine whether abnormalities in the myocardial phenotype existed in CK(-/-) mice, both at baseline and 4 wk after myocardial infarction (MI). As a result, myocardial hypertrophy was seen in all CK(-/-) mice, but ejection fraction (EF) remained normal. V(max), however, was significantly reduced in the CK(-/-) mice [wild-type, 2.32 +/- 0.09 vs. CK(-/-), 1.43 +/- 0.16 cm/s, P < 0.05; and wild-type MI, 1.53 +/- 0.11 vs. CK(-/-) MI, 1.26 +/- 0.11 cm/s, P = not significant (NS), P < 0.05 vs. baseline]. Myocardial perfusion was also lower in the CK(-/-) mice (wild-type, 6.68 +/- 0.27 vs. CK(-/-), 4.12 +/- 0.63 ml/g.min, P < 0.05; and wild-type MI, 3.97 +/- 0.65 vs. CK(-/-) MI, 3.71 +/- 0.57 ml/g.min, P = NS, P < 0.05 vs. baseline), paralleled by a significantly reduced capillary density (histology). In conclusion, myocardial function in transgenic mice may appear normal when only gross indexes of performance such as EF are assessed. However, the use of a combination of novel MRI techniques to measure myocardial perfusion and mechanics allowed the abnormalities in the CK(-/-) phenotype to be detected. The myocardium in CK-deficient mice is characterized by reduced perfusion and reduced maximal contraction velocity, suggesting that the myocardial hypertrophy seen in these mice cannot fully compensate for the absence of the CK system.

Animals↗

PubMed2006

Accelerated volumetric MRI with a SENSE/GRAPPA combination.

PURPOSE: To combine the specific advantages of the generalized autocalibrating partially parallel acquisitions (GRAPPA) technique and sensitivity encoding (SENSE) with two-dimensional (2D) undersampling. MATERIALS AND METHODS: By splitting the 2D reconstruction process into multiple one-dimensional (1D) reconstructions, the normal 1D GRAPPA method can be used for image reconstruction. Due to this data-handling process, a GRAPPA reconstruction is performed along the phase-encoding (PE) direction and effectively a SENSE reconstruction is performed along the partition-encoding (PAE) direction. RESULTS: In vivo experiments demonstrate the successful implementation of the SENSE/GRAPPA combination. Experimental results with up to 9.6-fold acceleration using a prototype 32-channel receiver head coil array are presented. CONCLUSION: The proposed SENSE/GRAPPA combination for 3D imaging allows the GRAPPA method to be applied in combination with 2D undersampling. Because the SENSE/GRAPPA combination is not based on knowledge of spatial coil sensitivities, it should be the method of choice whenever it is difficult to extract the sensitivity information.

Algorithms↗

PubMed2006

Controlled aliasing in volumetric parallel imaging (2D CAIPIRINHA).

The CAIPIRINHA (Controlled Aliasing In Parallel Imaging Results IN Higher Acceleration) concept in parallel imaging has recently been introduced, which modifies the appearance of aliasing artifacts during data acquisition in order to improve the subsequent parallel imaging reconstruction procedure. This concept has been successfully applied to simultaneous multi-slice imaging (MS CAIPIRINHA). In this work, we demonstrate that the concept of CAIPIRINHA can also be transferred to 3D imaging, where data reduction can be performed in two spatial dimensions simultaneously. In MS CAIPIRINHA, aliasing is controlled by providing individual slices with different phase cycles by means of alternating multi-band radio frequency (RF) pulses. In contrast to MS CAIPIRINHA, 2D CAIPIRINHA does not require special RF pulses. Instead, aliasing in 2D parallel imaging can be controlled by modifying the phase encoding sampling strategy. This is done by shifting sampling positions from their normal positions in the under-sampled 2D phase encoding scheme. Using this modified sampling strategy, coil sensitivity variations can be exploited more efficiently in multiple dimensions, resulting in a more robust parallel imaging reconstruction.

Humans↗

PubMed2006

In vivo quantitative three-dimensional motion mapping of the murine myocardium with PC-MRI at 17.6 T.

This work presents a method that allows for the assessment of 3D murine myocardial motion in vivo at microscopic resolution. Phase-contrast (PC) magnetic resonance imaging (MRI) at 17.6 T was applied to map myocardial motion in healthy mice along three gradient directions. High-resolution velocity maps were acquired at three different levels in the murine myocardium with an in-plane resolution of 98 mum, a slice thickness of 0.6 mm, and a temporal resolution of 6 ms. The applied PC-MRI method was validated with phantom experiments that confirmed the correctness of the method with deviations of <1.7%. Myocardial in-plane velocities between 0.5 cm/s and 2.2 cm/s were determined for the healthy murine myocardium. Through-plane velocities of 0.1-0.83 cm/s were measured. Velocity data was also used to calculate the myocardial twist angle during systole at different slices in the short-axis view.

Algorithms↗

PubMed2006

Direct parallel image reconstructions for spiral trajectories using GRAPPA.

The use of spiral trajectories is an efficient way to cover a desired k-space partition in magnetic resonance imaging (MRI). Compared to conventional Cartesian k-space sampling, it allows faster acquisitions and results in a slight reduction of the high gradient demand in fast dynamic scans, such as in functional MRI (fMRI). However, spiral images are more susceptible to off-resonance effects that cause blurring artifacts and distortions of the point-spread function (PSF), and thereby degrade the image quality. Since off-resonance effects scale with the readout duration, the respective artifacts can be reduced by shortening the readout trajectory. Multishot experiments represent one approach to reduce these artifacts in spiral imaging, but result in longer scan times and potentially increased flow and motion artifacts. Parallel imaging methods are another promising approach to improve image quality through an increase in the acquisition speed. However, non-Cartesian parallel image reconstructions are known to be computationally time-consuming, which is prohibitive for clinical applications. In this study a new and fast approach for parallel image reconstructions for spiral imaging based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) methodology is presented. With this approach the computational burden is reduced such that it becomes comparable to that needed in accelerated Cartesian procedures. The respective spiral images with two- to eightfold acceleration clearly benefit from the advantages of parallel imaging, such as enabling parallel MRI single-shot spiral imaging with the off-resonance behavior of multishot acquisitions.

Algorithms↗

PubMed2006

2D-GRAPPA-operator for faster 3D parallel MRI.

When using parallel MRI (pMRI) methods in combination with three-dimensional (3D) imaging, it is beneficial to subsample the k-space along both phase-encoding directions because one can then take advantage of coil sensitivity variations along two spatial dimensions. This results in an improved reconstruction quality and therefore allows greater scan time reductions as compared to subsampling along one dimension. In this work we present a new approach based on the generalized autocalibrating partially parallel acquisitions (GRAPPA) technique that allows Fourier-domain reconstructions of data sets that are subsampled along two dimensions. The method works by splitting the 2D reconstruction process into two separate 1D reconstructions. This approach is compared with an extension of the conventional GRAPPA method that directly regenerates missing data points of a 2D subsampled k-space by performing a linear combination of acquired data points. In this paper we describe the theoretical background and present computer simulations and in vivo experiments.

Adult↗

PubMed2006

Single-shot quantitative perfusion imaging of the human lung.

The major drawback to quantitative perfusion imaging using arterial spin labeling (ASL) techniques is the need to acquire two images (tag and control), which must be subtracted in order to obtain a perfusion-weighted image. This can potentially result in misregistration artifacts, especially in lung imaging, due to varying lung inflation levels in different breath-holds. In this work a double inversion recovery (DIR) imaging technique that yields perfusion-weighted images of the human lung in a single shot is presented. This technique ensures the complete suppression of background tissue while it preserves signal from the blood. Furthermore, the perfusion-weighted images and an additional (independent) acquired reference scan can be used to obtain quantitative perfusion information from the lungs.

Adult↗

PubMed2006

Autocalibrated coil sensitivity estimation for parallel imaging.

Parallel imaging has proven to be a robust solution to the problem of acquisition speed in MRI. These methods are based on extracting spatial information from an array of multiple surface coils in order to speed up image acquisition. One of the most essential elements of any parallel imaging method is the information describing the coil sensitivity distribution throughout the sample. This paper covers some of the advanced methods to obtain coil sensitivity-related information, focusing particularly on the class of methods referred to as autocalibrating. These methods all acquire the data for coil sensitivity estimation directly before, during or directly after the reduced data acquisition. After a review of standard methods for coil sensitivity estimation, some of the basic and advanced autocalibrating methods are reviewed, and some example applications shown.

Algorithms↗

PubMed2006

Perspectives and limitations of parallel MR imaging at high field strengths.

In medical magnetic resonance imaging (MRI) imaging, it is standard practice to use MR scanners with a field strength of 1.5 Tesla. Recently, an ongoing trend towards higher field strengths can be observed, with a new potential clinical standard of 3.0 Tesla. High-field MR imaging, with its intrinsic higher signal-to-noise ratio (SNR), can enable new applications for MRI in medical diagnosis, or can serve to improve existing methods. The use of high field MRI is not without its limitations, however. Besides SNR, other unwanted effects increase with a higher field strength. Without correction, these high field problems can cause a serious loss in image quality. An elegant way to address these problems is the use of parallel imaging. In many clinical applications, parallel MRI (pMRI) is part of the standard protocol, as pMRI can enhance virtually every MRI application without necessarily affecting the contrast behavior of the underlying imaging sequence. In addition to the speed advantages offered by pMRI, the capability of parallel imaging to reduce significant high field-specific problems, thereby improving image quality, will be of major importance.

Brain↗

PubMed2005

Dynamics of 23Na during completely balanced steady-state free precession.

The dynamics of 23Na during completely balanced steady-state free precession (SSFP) have been studied in numerical simulations and experiments. Results from both agree well. It is shown that during SSFP multiple quantum coherences are excited and that their excitation affects the observable signal. The signal response to the sequence parameters (flip angle, TR, and RF pulse phase cycle) shows a structure which can not be described by the Bloch equations. Due to excitation of T31 (s,a), the amplitude ratio of the fast and slowly decaying components deviates from 3:2 and is a function of the sequence parameters. The results shown here represent a basis for the implementation and optimization of 23Na-SSFP imaging sequences.

Area Under Curve↗

PubMed2005

T1 mapping of the entire lung parenchyma: Influence of the respiratory phase in healthy individuals.

PURPOSE: To determine the effect of respiratory phase on the T1 values of the entire lung. MATERIALS AND METHODS: We calculated T1 relaxation time on a pixel-by-pixel analysis of a series of Snapshot FLASH tomograms acquired after a single inversion pulse. Two sets of coronal T1 maps were acquired, one in full suspended inspiration and one in suspended expiration. RESULTS: The average of the mean T1 values over the entire lung in inspiration was 1199+/-117 msec, the average of the mean T1 values in expiration was 1333+/-167 msec. This difference was statistically significant, at P=0.005. In inspiration the average coefficient of variation was 182+/-56 msec, in expiration it was 153+/-41 msec. This difference was not statistically significant. CONCLUSION: T1 measurements of the entire lung are feasible. Our findings underline the need for a close monitoring of the inspirational status during MR examinations of the thorax, notably when T1 measurements of the lung are performed.

Adult↗

PubMed2005

Controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) for multi-slice imaging.

In all current parallel imaging techniques, aliasing artifacts resulting from an undersampled acquisition are removed by means of a specialized image reconstruction algorithm. In this study a new approach termed "controlled aliasing in parallel imaging results in higher acceleration" (CAIPIRINHA) is presented. This technique modifies the appearance of aliasing artifacts during the acquisition to improve the subsequent parallel image reconstruction procedure. This new parallel multi-slice technique is more efficient compared to other multi-slice parallel imaging concepts that use only a pure postprocessing approach. In this new approach, multiple slices of arbitrary thickness and distance are excited simultaneously with the use of multi-band radiofrequency (RF) pulses similar to Hadamard pulses. These data are then undersampled, yielding superimposed slices that appear shifted with respect to each other. The shift of the aliased slices is controlled by modulating the phase of the individual slices in the multi-band excitation pulse from echo to echo. We show that the reconstruction quality of the aliased slices is better using this shift. This may potentially allow one to use higher acceleration factors than are used in techniques without this excitation scheme. Additionally, slices that have essentially the same coil sensitivity profiles can be separated with this technique.

Abdomen↗

PubMed2005

Dynamic autocalibrated parallel imaging using temporal GRAPPA (TGRAPPA).

Current parallel imaging techniques for accelerated imaging require a fully encoded reference data set to estimate the spatial coil sensitivity information needed for reconstruction. In dynamic parallel imaging a time-interleaved acquisition scheme can be used, which eliminates the need for separately acquiring additional reference data, since the signal from directly adjacent time frames can be merged to build a set of fully encoded full-resolution reference data for coil calibration. In this work, we demonstrate that a time-interleaved sampling scheme, in combination with autocalibrated GRAPPA (referred to as TGRAPPA), allows one to easily update the coil weights for the GRAPPA algorithm dynamically, thereby improving the acquisition efficiency. This method may update coil sensitivity estimates frame by frame, thereby tracking changes in relative coil sensitivities that may occur during the data acquisition.

Algorithms↗

PubMed2005

Oxygen-enhanced proton imaging of the human lung using T2.

Magnetic susceptibility gradients caused by tissue/air interfaces lead to very short T(2)* times in the human lung. These susceptibility gradients are dependent on the magnetic susceptibility of the respiratory gas and therefore should influence T(2)* relaxation. In this work, a technique for quantitative T(2)* mapping of the human lung during one breath hold is presented. Using this method, the lung T(2)* relaxation time was measured under normoxic (room air, 21% O(2)) and hyperoxic (100% O(2)) conditions to verify this assumption. The mean T(2)* difference between room air and 100% O(2) is about 10% and contains ventilation information, since only ventilated regions contribute to signal change due to different susceptibility gradients.

Adult↗

PubMed2005

Parallel magnetic resonance imaging using the GRAPPA operator formalism.

In this article it is shown that GRAPPA reconstruction can be reformulated as a matrix operator, similar to ladder or propagator operators used in quantum mechanics, that shifts data in k-space. Using this formalism, it is shown that there exists an infinitesimal GRAPPA operator that shifts data in k-space by arbitrarily small amounts. Other desired k-space shifts can then be accomplished through repeated applications of this infinitesimal GRAPPA operator. Implications of these ideas are described.

Algorithms↗

PubMed2004

In vivo detection limits of magnetically labeled embryonic stem cells in the rat brain using high-field (17.6 T) magnetic resonance imaging.

Stem cell transplantation is a promising therapeutic approach for several neurological disorders. However, it has yet to fulfill its high expectations, partially due to the lack of a reliable noninvasive method for monitoring the biodistribution of the grafted stem cells in vivo. We have used high-resolution magnetic resonance imaging (MRI) at 17.6 T, combined with efficient magnetic labeling of the stem cells with iron oxide nanoparticles, in order to assess the in vivo detection limit in small animal models. Injection of different concentrations of magnetically labeled stem cells in gel phantoms led to significant reductions in image intensity from small cellular clusters of less than 10 cells. To determine the detection limit in vivo, various numbers of both labeled and unlabeled cells were injected stereotactically into the striatum of rats. Significant hypointense signal changes were observed for 100 labeled cells. After injection of approximately 20 labeled cells, signal reduction at the injection site was observed but could not be assigned unambiguously to the cells. Our results show that high-field MRI allows tracking of a minimal number of cells in vivo, well below the number used in previous studies, opening the possibility of gaining new insights into cell migration and differentiation.

Animals↗