Biomedical subjects

Clemens Bos

Publications and source records attributed to Clemens Bos.

9 recordsLinked to original sources

PubMed2006

Three-dimensional radial ultrashort echo-time imaging with T2 adapted sampling.

The application of 3D radial sampling of the free-induction decay to proton ultrashort echo-time (UTE) imaging is reported. The effects of T2 decay during signal acquisition on the 3D radial point-spread function are analyzed and compared to 2D radial and 1D sampling. It is found that in addition to the use of ultrashort TE, the proper choice of the acquisition-window duration TAQ is essential for imaging short-T2 components. For 3D radial sampling, a maximal signal-to-noise ratio (SNR) with negligible decay-induced loss in spatial resolution is obtained for an acquisition-window duration of TAQ approximately 0.69 T2. For 2D and 1D sampling, corresponding values are derived as well. Phantom measurements confirm the theoretical findings and demonstrate the impact of different acquisition-window durations on SNR and spatial resolution for a given T2 component. In vivo scans show the potential of 3D UTE imaging with T2-adapted sampling for musculoskeletal imaging using standard MR equipment. The visualization of complex anatomy is demonstrated by extracting curved slices from the isotropically resolved 3D UTE image data.

Algorithms↗

PubMed2005

Analysis and correction of gradient nonlinearity and B0 inhomogeneity related scaling errors in two-dimensional phase contrast flow measurements.

Phase contrast flow measurements will be increasingly biased at eccentric positions, where nonlinearity of gradients and inhomogeneity of the main field become important. In theory, they scale the result of phase contrast flow values in two ways: incorrect velocity encoding of moving spins and geometric distortion of the vessel cross-sectional area. A flow phantom, consisting of a 3D grid of interconnected tubes, was used to determine the spatial dependence of the associated scaling factors, which demonstrate that scaling errors in flow can be as large as 20% within the examined volume of 336 x 336 x 336 mm(3). The same phantom was also used to determine and minimize concomitant gradient effects. Correction of the off-center flow values with the local scaling factors and the concomitant gradient phase improves the measurement accuracy substantially, both in the flow phantom and in a volunteer study.

Adult↗

PubMed2005

Simultaneous monitoring of temperature and T1: methods and preliminary results of application to drug delivery using thermosensitive liposomes.

A method is presented to obtain temperature and longitudinal relaxivity measurements simultaneously and in near real-time. Quantitative relaxivity values are obtained from the signal magnitude from fast Look-Locker EPI data, whereas phase information from all signal samples on the recovery curve is combined to provide temperature values using the proton resonance frequency method. The utility of this technique is illustrated in an in vitro experiment with thermosensitive liposomes, which are studied as potential micro vehicles for local drug delivery. The method allowed measuring the evolution of relaxivity during RF-heating of liposomes containing a paramagnetic contrast agent, demonstrating increase of liposome permeability near the phase transition temperature. Potential applications are monitoring of local drug delivery using thermosensitive liposomes, and confirmation of reaching the liposomes' threshold temperature during thermal therapy.

Diffusion↗

PubMed2005

Real-time interactive viewing of 4D kinematic MR joint studies.

Assessment of soft tissue in normal and abnormal joint motion today gets feasible by acquiring time series of 3D MRI images. However, slice-by-slice viewing of such 4D kinematic images is cumbersome, and does not allow appreciating the movement in a convenient way. Simply presenting slice data in a cine-loop will be compromised by through-plane displacements of anatomy and "jerks" between frames, both of which hamper visual analysis of the movement. To overcome these limitations, we have implemented a demonstrator for viewing 4D kinematic MRI datasets. It allows to view any user defined anatomical structure from any viewing perspective in real-time. Smoothly displaying the movement in a cine-loop is realized by image post processing, fixing any user defined anatomical structure after image acquisition.

Algorithms↗

PubMed2004

In vivo MR imaging of intravascularly injected magnetically labeled mesenchymal stem cells in rat kidney and liver.

PURPOSE: To evaluate in vivo magnetic resonance (MR) imaging with a conventional 1.5-T system for depiction and tracking of intravascularly injected superparamagnetic iron oxide (SPIO)-labeled mesenchymal stem cells (MSCs). MATERIALS AND METHODS: This study was conducted in accordance with French law governing animal research and met guidelines for animal care and use. Rat MSCs were labeled with SPIO and transfection agent. Relaxation rates at 1.5 T, cell viability, proliferation, differentiation capacity, and labeling stability were assessed in vitro as a function of SPIO concentration. MSCs were injected into renal arteries of healthy rats (labeled cells in four, unlabeled cells in two) and portal veins of rats treated with carbon tetrachloride to induce centrolobular liver necrosis (labeled cells and unlabeled cells in two each). Follow-up serial T2*-weighted gradient-echo MR imaging and R2* mapping were performed. MR imaging findings were compared histologically. RESULTS: SPIO labeling caused a strong R2* effect that increased linearly with iron dose; R2* increase for cells labeled for 48 hours with 50 microg of iron per milliliter was 50 sec(-1) per million cells per milliliter. R2* was proportional to iron load of cells. SPIO labeling did not affect cell viability (P > .27). Labeled cells were able to differentiate into adipocytes and osteocytes. Proliferation was substantially limited for MSCs labeled with 100 microg Fe/mL or greater. Label half-life was longer than 11 days. In normal kidneys, labeled MSCs caused signal intensity loss in renal cortex. After labeled MSC injection, diseased liver had diffuse granular appearance. Cells were detected for up to 7 days in kidney and 12 days in liver. Signal intensity loss and fading over time were confirmed with serial R2* mapping. At histologic analysis, signal intensity loss correlated with iron-loaded cells, primarily in renal glomeruli and hepatic sinusoids; immunohistochemical analysis results confirmed these cells were MSCs. CONCLUSION: MR imaging can aid in monitoring of intravascularly administered SPIO-labeled MSCs in vivo in kidney and liver.

Adipocytes↗

PubMed2004

Liver tumors: MR imaging of radioactive holmium microspheres--phantom and rabbit study.

PURPOSE: To investigate the use of magnetic resonance (MR) imaging in the administration and biodistribution of holmium-loaded poly(L-lactic acid) microspheres (Ho-PLLA-MS) in liver tumors. MATERIALS AND METHODS: MR imaging measurements were obtained in phantoms, three ex vivo rabbit livers, and four livers in living rabbits. When applicable, measurements were compared with those on scintigraphic images. The transverse relaxivity R2* of the Ho-PLLA-MS was determined in a phantom study. The in vivo animal experiments were performed by using rabbits with an implanted VX2 tumor. Detection of passing Ho-PLLA-MS to estimate lung shunting was performed in a scaled model of the vena cava. RESULTS: In the ex vivo liver experiments, the feasibility of real-time MR imaging during administration of microspheres was demonstrated. The in vivo rabbit experiments demonstrated that MR imaging can depict radioactive, nonradioactive, and decayed Ho-PLLA-MS after treatment for as long as they remain in the body. Furthermore, this study showed the ability of dynamic MR imaging to detect single doses of passing Ho-PLLA-MS. CONCLUSION: Ho-PLLA-MS used for internal radionuclide therapy can be imaged clearly in vivo with MR imaging.

Animals↗

PubMed2003

On the artifact of a subvoxel susceptibility deviation in spoiled gradient-echo imaging.

In MRI, susceptibility-based negative contrast amplifies the effect of objects that are too small to be detected by water displacement or intrinsic contrast properties. In this work, a simplified description of the susceptibility artifact of a subvoxel object in spoiled gradient-echo imaging is presented that focuses on the elimination of signal in its vicinity: the dephased-volume. The size and position of the dephased-volume are investigated using 3D time-domain simulations and in vitro experiments in which scan parameters and object magnetic moment are systematically varied. Overall signal loss is found to be linearly related to a dephasing parameter that contains the susceptibility difference with tissue, object volume, and echo time (TE), and thus allows the magnetic moment of the object to be assessed. Gradient strength, in-plane resolution, fractional echo, and slice orientation have limited influence. For the settings used, the center of mass of the artifact was always within 0.5 mm of the object's in-plane position.

Artifacts↗

PubMed2002

Underestimation of access flow by ultrasound dilution flow measurements.

For hemodialysis access surveillance, flow measurements are increasingly considered important because they identify accesses at risk of thrombosis. Usually these flow measurements are performed with the ultrasound dilution technique. In a previous patient study it was observed that the resulting flow values were systematically low as compared to magnetic resonance flow measurements, but a satisfactory explanation was lacking. In the present study, we will demonstrate by hemodynamic calculations and in vitro experiments that this discrepancy can be explained by a temporary reduction of the access flow rate, caused by the reversed needle configuration during ultrasound dilution flow measurements. In this configuration. blood is injected retrogressively at one needle and flow between the needles is increased, causing an increased dissipation of energy. The proposed explanation is subsequently confirmed in a patient with a loop graft, by measuring the blood velocity by Doppler ultrasound as a function of reversed dialyzer flow rate. Apart from the ultrasound dilution technique, these findings are applicable to other recently proposed methods for measuring access flow that employ the reversed needle configuration.

Biophysics↗

PubMed2002

Hemodialysis access imaging: comparison of flow-interrupted contrast-enhanced MR angiography and digital subtraction angiography.

PURPOSE: To compare flow-interrupted contrast material-enhanced magnetic resonance (MR) angiography with conventional digital subtraction angiography (DSA) for hemodialysis access imaging. MATERIALS AND METHODS: Twenty-two accesses (14 arteriovenous grafts [AVGs], eight arteriovenous fistulas [AVFs]) in 18 consecutive patients were imaged with flow-interrupted contrast-enhanced MR angiography and subsequent conventional DSA. MR image quality was assessed as excellent, good, or nondiagnostic. Anastomotic diameters in AVGs and postanastomotic diameters in AVFs were measured in consideration of an adjacent normal segment. Reductions in the diameter of the lumen and interobserver differences were analyzed with method comparison as described by Bland and Altman and expressed as the mean difference with its 95% confidence limits (CLs) (mean +/- 2 SDs). RESULTS: Image quality obtained with flow-interrupted contrast-enhanced MR angiography was considered excellent in 73% (16 of 22) and good in 23% (5 of 22). Method comparison analysis between MR angiography and DSA indicated a mean difference of 3.2% (95% CLs: -26.7%, 33.1%) for observer 1 and 4.1% (95% CLs: -23.8%, 32.1%) for observer 2. Interobserver analysis at MR angiography indicated a mean difference of 3.2% (95% CLs: -28.8%, 35.2%), and that at DSA indicated a mean difference of 3.6% (95% CLs: -9.4%, 16.7%). CONCLUSION: Image quality and anatomic depiction with flow-interrupted contrast-enhanced MR angiography and with DSA were comparable.

Angiography, Digital Subtraction↗