Biomedical subjects

Peter M Jakob

Publications and source records attributed to Peter M Jakob.

36 records · Page 2Linked to original sources

PubMed2004

Assessment of human pulmonary function using oxygen-enhanced T(1) imaging in patients with cystic fibrosis.

Indirect qualitative MRI of pulmonary function is feasible using the paramagnetic effects of oxygen physically dissolved in blood. In this study, a more quantitative oxygen-enhanced pulmonary function test based on the slope of a plot of R(1) vs. oxygen concentration-the oxygen transfer function (OTF)-was developed and tested in a pool of five healthy volunteers and five patients with cystic fibrosis (CF). The lung T(1) relaxation rate, R(1), under normoxic conditions (room air, 21% O(2)), and the response to various hyperoxic conditions (40%-100% O(2)) were studied. Lung T(1) in healthy volunteers showed a relatively homogeneous distribution while they breathed room air, and a homogeneous decrease under hyperoxic conditions. Lung T(1) in CF patients showed an inhomogeneous distribution while they breathed room air, and the observed lung T(1) decrease under hyperoxia depended on the actual state of the diseased lung tissue. In the selected group of CF patients, areas with reduced OTF also showed reduced perfusion, as confirmed by qualitative contrast-enhanced MR pulmonary perfusion imaging. The results demonstrate that this completely noninvasive oxygen-enhanced pulmonary function test has potential for clinical applications in the serial diagnosis of lung diseases such as CF. .

Adolescent↗

PubMed2004

Inversion recovery TrueFISP: quantification of T(1), T(2), and spin density.

A novel procedure is proposed to extract T(1), T(2), and relative spin density from the signal time course sampled with a series of TrueFISP images after spin inversion. Generally, the recovery of the magnetization during continuous TrueFISP imaging can be described in good approximation by a three parameter monoexponential function S(t) = S(stst)(1-INV exp(-t/T(*) (1)). This apparent relaxation time T(*) (1) <or= T(1) depends on the flip angle as well as on both T(1) and T(2). Here, it is shown that the ratio T(1)/T(2) can be directly extracted from the inversion factor INV, which describes the relation of the signal value extrapolated to t = 0 and the steady-state signal. Analytical expressions are given for the derivation of T(1), T(2), and relative spin density directly from the fit parameters. Phantom results show excellent agreement with single point reference measurements. In human volunteers T(1), T(2), and spin density maps in agreement with literature values were obtained.

Algorithms↗

PubMed2004

Contrast-enhanced dental MRI for visualization of the teeth and jaw.

A technique for contrast-enhanced dental MRI is described that enables 3D visualization of the oral cavity, including the jaw and teeth. Since teeth are MR-invisible, the basic principle of this technique is that the teeth and jaw can be observed indirectly through contrast with a surrounding MR-visible medium. For this purpose, the oral cavity is filled with a nontoxic substance, such as water or MR contrast media, that gives a high MR signal. A 3D data set covering the entire buccal space is acquired, and the image intensities are inverted. Since isosurface reconstructions of the teeth and jaw, as well as panoramic views analogous to orthopantomography, can be extracted from these data, contrast-enhanced dental MRI may be useful as a flexible tool for dentistry and orthodontics. Moreover, contrast-enhanced dental MRI works without radiation exposure, and therefore it is an interesting alternative to X-ray-based imaging modalities such as conventional radiography and dental CT. In this article, some preliminary results obtained with contrast-enhanced dental MRI are shown in order to demonstrate the feasibility and performance of this new approach.

Adult↗

PubMed2004

Field-of-view limitations in parallel imaging.

Parallel imaging is one of the most promising developments in recent years for the acceleration of MR acquisitions. One area of practical importance where different parallel imaging methods perform differently is the manner in which they deal with aliasing in the full-FOV reconstructed image. It has been reported that sensitivity encoding (SENSE) reconstruction fails whenever the reconstructed FOV is smaller than the object being imaged. On the other hand, generalized autocalibrating partially parallel acquisition (GRAPPA) has been used successfully to reconstruct images with aliasing in the reconstructed FOV, as in conventional imaging. The disparate behavior of these methods can be easily demonstrated by a few simple illustrative examples. Additional in vivo examples using GRAPPA and modified SENSE (mSENSE) make this distinction clear. These experiments demonstrate that SENSE fails to reconstruct correct images when coil sensitivity maps are used that do not automatically account for the object size and therefore the aliasing in the reconstructed images. However, with the use of aliased high-resolution coil sensitivity maps, accurate SENSE reconstructions can be generated. On the other hand, GRAPPA produces images with an aliasing appearance that is exactly as would be expected from normal nonaccelerated acquisitions. An understanding of these effects could potentially lead to fewer operator-dependent errors, as well as a better understanding of the differences between the underlying reconstruction processes.

Computer Simulation↗

PubMed2004

Myocardial perfusion measurements by spin-labeling under different vasodynamic states.

In this study absolute myocardial perfusion was determined using a spin-labeling magnetic resonance imaging (MRI) technique at 2 Tesla. The technique was applied to 16 healthy volunteers at resting conditions, adenosine-induced stress, and oxygen breathing. Overall myocardial quantitative perfusion was determined as 2.3 +/- 0.8 mL/g/min (rest), 4.2 +/- 1.0 mL/g/min (adenosine), and 1.6 +/- 0.6 mL/g/min (oxygen), respectively. T1 of left ventricular blood pool decreased from 1709 +/- 101 ms (rest) to 1423 +/- 61 ms (oxygen), whereas T1 of right ventricular blood did not change significantly (1586 +/- 126 ms and 1558 +/- 150 ms). In conclusion, the presented technique for quantification of myocardial perfusion is an alternative to contrast agent-based methods. The spin labeling method is noninvasive and easily repeatable and it could therefore become an important tool to study changes in myocardial perfusion under different vasodynamic states.

Adenosine↗

PubMed2004

Towards a single-sequence neurologic magnetic resonance imaging examination: multiple-contrast images from an IR TrueFISP experiment.

OBJECTIVE: The objective of this study was to reconstruct images bearing multiple contrasts from a single sequence magnetic resonance imaging (MRI) experiment. MATERIALS AND METHODS: Using a segmented IR-TrueFISP imaging sequence, the signal recovery after inversion and alpha/2 preparation was sampled in 6 volunteers. These images were used to generate T1, T2, and spin-density maps, allowing construction of images with multiple contrasts, including T1-, T2-, spin-density-weighted, and also FLAIR contrast. Traditionally acquired images bearing the corresponding contrast were obtained for comparison. Regression analysis was performed to compare the synthetic and traditionally acquired images for the whole brain and a region of interest in the occipital region. RESULTS: The synthetic images closely reproduced the contrast from the "standard" examination. Using regression analysis, the obtained image signal intensities for the calculated images compare favorably (P <0.0001-<<0.000001) with images acquired using multiple sequences. CONCLUSIONS: Perfectly registered images with any desired contrast based on T1, T2, and spin density, along with underlying quantitative maps, can be obtained using a single IR-TrueFISP sequence.

Brain↗

PubMed2004

SMASH, SENSE, PILS, GRAPPA: how to choose the optimal method.

Fast imaging methods and the availability of required hardware for magnetic resonance tomography (MRT) have significantly reduced acquisition times from about an hour down to several minutes or seconds. With this development over the last 20 years, magnetic resonance imaging (MRI) has become one of the most important instruments in clinical diagnosis. In recent years, the greatest progress in further increasing imaging speed has been the development of parallel MRI (pMRI). Within the last 3 years, parallel imaging methods have become commercially available, and therefore are now available for a broad clinical use. The basic feature of pMRI is a scan time reduction, applicable to nearly any available MRI method, while maintaining the contrast behavior without requiring higher gradient system performance. Because of its faster image acquisition, pMRI can in some cases even significantly improve image quality. In the last 10 years of pMRI development, several different pMRI reconstruction methods have been set up which partially differ in their philosophy, in the mode of reconstruction as well in their advantages and drawbacks with regard to a successful image reconstruction. In this review, a brief overview is given on the advantages and disadvantages of present pMRI methods in clinical applications, and examples from different daily clinical applications are shown.

Artifacts↗

PubMed2003

A brief review of parallel magnetic resonance imaging.

Since the 1980s, the implementation of fast imaging methods and dedicated hardware for MRI scanners has reduced the image acquisition time from nearly an hour down to several seconds and has therefore enabled a widespread use of MRI in clinical diagnosis. Since this development, the greatest incremental gain in imaging speed has been provided by the development of parallel MRI (pMRI) techniques in late 1990s. Within the past 2 years, parallel imaging methods have become commercially available, which means that pMRI is now available for broad clinical use. In the clinical routine, virtually any MRI method can be enhanced by pMRI, allowing faster image acquisitions without any increased gradient system performance. In some cases pMRI can even result in a significant gain in image quality due to this faster acquisition. In this review article, the advantages and the disadvantages of pMRI in clinical applications are discussed and examples from many different daily applications are given.

Brain Neoplasms↗

PubMed2003

Quantitative perfusion mapping of the human lung using 1H spin labeling.

PURPOSE: To evaluate the feasibility and reproducibility of a noninvasive, rapid and quantitative pulmonary perfusion mapping method using a two-compartment tissue model in combination with a (1)H spin labeling technique. MATERIALS AND METHODS: Ten healthy volunteers and three patients with cystic fibrosis (CF) were examined on a 1.5-T whole-body scanner. Global and selective lung T(1) maps based on an inversion recovery Snapshot FLASH technique were acquired from each subject with breath-holds at end-expiration. For comparison, corresponding Gd-DTPA-enhanced (1)H MR perfusion images were also obtained from each CF patient. RESULTS: Quantitative perfusion maps were calculated from the global and selective T(1) maps. The measured perfusion rates of the upper right lung in volunteers ranged from 400 to 600 mL/100 g/minute. The method showed a high intra-study reproducibility and low relative errors. In CF-patients, perfusion defects detected using Gd-DTPA-enhanced MR imaging were also detected using the spin labeling method. The perfusion rates of diseased lung tissues were less than 200 mL/100 g/minute. CONCLUSION: Noninvasive, robust and quantitative (1)H MR mapping of pulmonary perfusion was successfully performed using a rapid lung T(1) mapping in combination with spin labeling within the imaging slice. The proposed method has the potential to provide both important qualitative functional information and quantitative pulmonary perfusion rates in various lung diseases at various stages without the need of contrast agents.

Adult↗

PubMed2003

Quantitative assessment of myocardial perfusion with a spin-labeling technique: preliminary results in patients with coronary artery disease.

PURPOSE: To determine perfusion and coronary reserve in human myocardium without contrast agent using a spin labeling technique. MATERIALS AND METHODS: Assessment of myocardial perfusion is based on T1 measurements after global and slice-selective spin preparation. This magnetic resonance imaging (MRI) technique was applied to 12 healthy volunteers and 16 patients with suspected coronary artery disease under resting conditions and adenosine-induced vasodilatation. RESULTS: In volunteers, quantitative perfusion was calculated as 2.4 +/- 1.2 mL/g/minute (rest) and 3.9 +/- 1.3 mL/g/minute (adenosine), respectively. Perfusion reserve was 2.1 +/- 0.6. In patients, when comparing perfusion reserve in the anterior and posterior myocardium, reduced values according to a stenotic supplying vessel could be seen in seven of 11 patients who underwent stress testing. In these patients, the relative difference of coronary reserve was 44% +/- 18%. Two patients without stenosis of coronary arteries showed no differences in coronary reserve (with a relative change of 2 +/- 2%). CONCLUSION: In patients with single-vessel coronary artery disease, differences in coronary reserve were clearly detectable when comparing anterior and posterior myocardium. The spin labeling method is noninvasive and easily repeatable, and it could therefore become an important tool to study changes in myocardial perfusion.

Adenosine↗

PubMed2003

Visualization of myocardial microstructure using high-resolution T*2 imaging at high magnetic field.

The analysis of myocardial microstructure in vivo is important for the determination of myocardial contractility and function. The purpose of the present study was to demonstrate that high-resolution T*2 imaging has the potential to visualize the microstructure of beating, isolated rat hearts. To perform T*2 imaging, a multiple gradient-echo sequence was implemented on an 11.75 Tesla microscopy system. An in-plane resolution of 78 microm and a slice thickness of 250 microm were achieved in 24 min. In comparison to histological sections, the T*2 maps showed an excellent spatial correspondence to the myocardial fiber structure. To demonstrate the utility of this technique, morphologic alterations in myocardial microstructure were investigated in hearts with chronic myocardial infarction. Scar tissue and the extent of the infarcted region were clearly visualized and quantified using high-resolution T*2 imaging.

Animals↗

PubMed2003

Time-resolved flow measurement in the isolated rat heart: characterization of left coronary artery stenosis.

The investigation of flow behavior in coronary arteries is of great importance for an understanding of heart failure and heart regulation mechanisms. The purpose of the present study was to demonstrate that flow velocity can be quantified in the coronary arteries of the isolated rat heart with high-resolution phase contrast MRI. A phase contrast cine-FLASH imaging sequence was used for flow quantification with an in-plane resolution of 70 microm and a slice thickness of 500 microm. With time-resolved measurements, coronary flow over the heart cycle was analyzed. Furthermore, the flow behavior in coronary stenosis was investigated and the degree of stenosis was quantified with MR phase contrast imaging. To achieve the required spatial resolution and a satisfactory signal-to-noise ratio, the experiments were performed at 11.75 T.

Animals↗

PubMed2003

Investigation of the microstructure of the isolated rat heart: a comparison between T*2- and diffusion-weighted MRI.

Myocardial fiber structure can be determined with diffusion-weighted (DW) MRI as well as with high-resolution T*(2) imaging. The purpose of the present study was twofold: to provide a more quantitative description of T*(2)-based myocardial fiber contrast, and to compare the T*(2)-based fiber structure with high-resolution (78 microm in-plane, 1-mm slice thickness) DW images of the isolated rat heart at 11.75 T. This study demonstrates that the static dephasing regime is responsible for visualization of myocardial microstructure, and that the dynamic dephasing regime can be neglected. In comparison with DW experiments, T*(2) mapping and DW images yield almost equivalent information on myocardial fiber structure.

Animals↗

PubMed2003

NMR-microscopy with TrueFISP at 11.75T.

The purpose of this paper is to demonstrate that a fully balanced gradient echo technique (TrueFISP) can be used for microscopic experiments at high static magnetic field strengths. TrueFISP experiments were successfully performed on homogeneous and inhomogeneous objects at 11.75T. High-resolution TrueFISP images were obtained from phantoms, plants, formalin-fixed samples, and from an isolated beating rat heart with an in-plane resolution of 78 micro m and a slice thickness of 500 micro m. The signal-to-noise ratio (SNR) gain of TrueFISP compared to conventional gradient echo or spin echo sequences will allow faster acquisition times or an improvement in spatial resolution for microscopic experiments.

Animals↗

PubMed2002

Generalized autocalibrating partially parallel acquisitions (GRAPPA).

In this study, a novel partially parallel acquisition (PPA) method is presented which can be used to accelerate image acquisition using an RF coil array for spatial encoding. This technique, GeneRalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) is an extension of both the PILS and VD-AUTO-SMASH reconstruction techniques. As in those previous methods, a detailed, highly accurate RF field map is not needed prior to reconstruction in GRAPPA. This information is obtained from several k-space lines which are acquired in addition to the normal image acquisition. As in PILS, the GRAPPA reconstruction algorithm provides unaliased images from each component coil prior to image combination. This results in even higher SNR and better image quality since the steps of image reconstruction and image combination are performed in separate steps. After introducing the GRAPPA technique, primary focus is given to issues related to the practical implementation of GRAPPA, including the reconstruction algorithm as well as analysis of SNR in the resulting images. Finally, in vivo GRAPPA images are shown which demonstrate the utility of the technique.

Algorithms↗

PubMed2002

Magnetization transfer short inversion time inversion recovery enhanced 1H MRI of the human lung.

The unique characteristics of the human lung arising from low proton density and multiple air-tissue interfaces of the alveoli cause difficulty in 1H lung magnetic resonance imaging. In addition, the dominating signal from sources such as the thoracic muscle and subcutaneous fat hampers the visualization of the lung parenchyma. In this contribution, an efficient tissue suppression technique is presented which allows one to significantly enhance lung parenchyma visibility. A short inversion time inversion recovery (STIR) experiment combined with a magnetization transfer (MT) experiment was used for magnetization preparation in order to suppress the signal from muscle. A half-Fourier single-shot turbo spin-echo sequence was used as acquisition module. This approach was used to perform lung anatomical imaging in eight healthy human subjects and five patients with cystic fibrosis. The results obtained demonstrate that with MT-STIR approach high quality human lung images can be obtained and that this approach has the potential for the evaluation of lung pathologies.

Adult↗

PubMed2002

Microscopic spin tagging (MiST) for flow imaging.

In this study, a new strategy for slow flow imaging is proposed. The basic idea is to generate flow contrast on a microscopic level below the spatial resolution of an imaging experiment. Since a microscopic spin tagging scheme is used, this concept is called MiST (Microscopic Spin Tagging). MiST is not a single specific measurement sequence, but rather a new flow sensitive preparation concept which is highly flexible and can be carried out in many ways. The common principle in all possible realizations of MiST is a periodic tagging of magnetization in thin planes (100-200 microm) within the imaging voxels by means of spatially selective RF-pulses. Therefore, flow sensitivity occurs via inflow of fresh spins on a microscopic scale. With this approach, short evolution times are sufficient to introduce inflow contrast and a spatial dependence of inflow times is avoided. The flow sensitive preparation and image orientation are also not connected as they are in conventional time-of-flight techniques. Another powerful feature of MiST is that it can be designed as a non-subtraction method, which results in no signal from stationary spins. Here we present a first realization of the MiST concept and its validation in quantitative flow measurements to demonstrate the feasibility of the proposed preparation concept.

Arteries↗

PubMedDate not supplied

Assessment of cardiovascular apoptosis in the isolated rat heart by magnetic resonance molecular imaging.

Apoptosis, an active process of cell self-destruction, is associated with myocardial ischemia. The redistribution of phosphatidylserine (PS) from the inner to the outer leaflet of the cell membrane is an early event in apoptosis. Annexin V, a protein with high specificity and tight binding to PS, was used to identify and localize apoptosis in the ischemic heart.Fluorescein-labeled annexin V has been used routinely for the assessment of apoptosis in vitro. For the detection of apoptosis in vivo, positron emission tomography and single-photon emission computed tomography have been shown to be suitable tools. In view of the relatively low spatial resolution of nuclear imaging techniques, we developed a high-resolution contrast-enhanced magnetic resonance imaging (MRI) method that allows rapid and noninvasive monitoring of apoptosis in intact organs. Instead of employing superparamagnetic iron oxide particles linked to annexin V, a new T1 contrast agent was used. To this effect, annexin V was linked to gadolinium diethylenetriamine pentaacetate (Gd-DTPA)-coated liposomes. The left coronary artery of perfused isolated rat hearts was ligated for 30 min followed by reperfusion. T(1) and T(2)* images were acquired by using an 11.7-T magnet before and after intracoronary injection of Gd-DTP-labeled annexin V to visualize apoptotic cells. A significant increase in signal intensity was visible in those regions containing cardiomyocytes in the early stage of apoptosis. Because labeling of early apoptotic cell death in intact organs by histological and immunohistochemical methods remains challenging, the use of Gd-DTPA-labeled annexin V in MRI is clearly an improvement in rapid targeting of apoptotic cells in the ischemic and reperfused myocardium.

Animals↗