Biomedical subjects

G Brix

Publications and source records attributed to G Brix.

16 recordsLinked to original sources

PubMed1992

Correction of spatial distortion in magnetic resonance angiography for radiosurgical treatment planning of cerebral arteriovenous malformations.

A treatment planning system based on magnetic resonance (MR) angiographic imaging data for the radiosurgery of inoperable cerebral arteriovenous malformations is reported. MR angiography was performed using a three-dimensional (3D) velocity-compensated fast imaging with steady-state precession (FISP) sequence. Depending on the individual MR system, inhomogeneities and nonlinearities induced by eddy currents during the pulse sequence can distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. If necessary, these errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "2D-phantom" displaying "pincushion" distortion in the image, and a "3D-phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 2-3 mm to 1 mm) by calculations based on modeling the distortion as a fourth order 2D polynomial. Displacement, warp, and tilt of the image plane may be corrected by adjustment of the gradient shimming currents. After correction, the accuracy of the geometric information is limited only by the pixel resolution of the image (= 1 mm). Precise definition of the target volume could be performed by the therapist either directly in the MR images or in calculated projection MR angiograms obtained by a maximum intensity projection algorithm. MR angiography provides a sensitive, noninvasive 3D method for defining target volume and critical structures, and for calculating precise dose distributions for radiosurgery of cerebral arteriovenous malformations.

Humans↗

PubMed1992

[MR tomographic studies of the cerebral circulation: the methodological principles and initial clinical experiences with T2*-weighted gradient-echo sequences and CM administration as a bolus].

Paramagnetic contrast agents produce local magnetic field inhomogeneity when they pass through the cerebrovascular system. This effect can be monitored with T2*-weighted gradient echo images, which show transient signal loss, while a bolus of GdDTPA is passing through the brain tissue. This signal loss is correlated to the local cerebral tissue perfusion and the local cerebral blood volume. In a prospective study 10 volunteers and 14 patients with cerebral infarcts and brain tumours were examined. After bolus application of GdDTPA a dynamic series of rapid T2*-weighted gradient echo images were recorded, and the local dynamics of contrast flow in brain tissue was examined. From the series of images, local changes in signal intensity were calculated pixel by pixel and presented as parameter images. By this method, infarcted areas and brain tumours can be distinguished from normal tissue by their contrast flow dynamics. The abnormal contrast dynamics depend on changes in local tissue perfusion and alterations in local blood volume. The separation of the influence of both parameters however is still an unsolved problem.

Adolescent↗

PubMed1992

[MR tomography of the bone marrow in malignant systemic diseases. Diagnosis and therapeutic follow-up control].

This paper presents a short survey of the current status of bone marrow diagnosis in systemic neoplastic disease by means of MR imaging. The different patterns of bone marrow infiltration from lymphomas and leukaemias are presented and the differential diagnoses are discussed. Apart from the primary diagnosis the relevance of conventional MRI and chemical shift imaging for therapy follow up and after-care is discussed.

Bone Marrow↗

PubMed1992

[Bone marrow changes following radiotherapy. Results of MR tomography].

Magnetic resonance imaging (MRI) offers a new approach in the morphologic evaluation of the bone marrow. Physiologic and pathologic changes can be assessed with very high sensitivity. Radiotherapy induces acute depletion of the hematopoietic bone marrow, resulting in fatty degeneration. With MRI it is possible to evaluate the changes during irradiation and it also discloses the long-term fatty degeneration after radiotherapy. The irradiated bone marrow mostly exhibits a homogeneous hyperintense pattern on T1-weighted images. This allows clear recognition of the former target volumes. Our quantitative studies based on chemical shift imaging data reveal a lack of recovery of hematopoiesis after radiotherapy with 30 Gy or more. These results are independent of patients' age and of the interval after radiotherapy.

Adipose Tissue↗

PubMed1992

[MR tomography and MR angiography--a new method for the planning of the irradiation of large abdominal fields].

Subdiaphragmatic radiation therapy in malignant lymphoma requires complete irradiation of retroperitoneal lymph nodes, spleen and splenic pedicle and optimal shielding of radiosensitive structures. The aim of our study was to develop a new method for individual field definition by using MR tomography and MR angiography. In 38 patients with malignant lymphoma coronal MR tomograms and MR angiograms of the abdominal vessels were obtained and superimposed by a specially created computer program. By using a Subtraskop the MRT/MRA superposition was geometrically projected onto the simulation film in correct scale. The target volume could individually be defined and was compared to standard treatment planning according to the definition by Abbatucci. In all patients an exact irradiation of the spleen, the splenic pedicle and prominent lymphatic masses was possible. Furthermore, this resulted in a 32% reduction of treated kidney volume in the average of patients. Noninvasiveness, acceptable costs and high accuracy support the application of this new method in clinical routine.

Abdomen↗

PubMed1990

Fast and precise T1 imaging using a TOMROP sequence.

Proton spin-lattice (T1) relaxation time images were computed from a data set of 32 gradient-echo images acquired with a fast TOMROP (T One by Multiple Read Out Pulses) sequence using a standard whole-body MR imager operating at 64 MHz. The data acquisition and analysis method which permits accurate pixel-by-pixel estimation of T1 relaxation times is described. As an example, the T1 parameter image of a human brain is shown demonstrating an excellent image quality. For white and gray brain matter, the measured longitudinal relaxation processes are adequately described by a single-component least-squares fit, while more than one proton component has to be considered for fatty tissue. A quantitative analysis yielded T1 values of 547 +/- 36 msec and 944 +/- 73 msec for white and gray matter, respectively.

Brain↗

PubMed1990

[MR-tomographic quantification of infiltrative bone marrow changes and therapeutic follow-up of patients with hairy cell leukemia].

The effect of treatment on bone marrow infiltration in hairy cell leukaemia was followed by means of MRT in five patients. A semi-quantitative method has been developed in order to judge the effect of treatment. The results correlate well with the findings on iliac crest biopsies. The method could partly replace the invasive iliac crest biopsies, which are used for therapy monitoring.

Bone Marrow↗

PubMed1990

Evaluation of proton density by magnetic resonance imaging: phantom experiments and analysis of multiple component proton transverse relaxation.

The quantitative evaluation of proton density by magnetic resonance imaging (MRI) is limited as a result of non-uniformities in the intensity distribution of the images and by the fact that only part of the protons of the tissue contribute to the image signal. This study was undertaken to estimate the accuracy of proton density measurements using a standard whole-body MR imager operating at 1.5 T. First, phantom experiments were performed to examine the possibility of an intensity correction. For the test phantom the systematical errors in the computed proton densities were reduced from 5 to 1% after correction. Secondly, proton transverse relaxation curves of biological tissues were measured in vitro on an MR spectrometer. A multi-exponential analysis of the data shows that for spin-echo times TE greater than 10 ms in total between 10 and 30% of the protons of the tissue do not contribute to the image signal. In all tissues a proton component with a free induction decay (FID) time T2* less than 32 microseconds was observed. In the time range TE greater than 10 ms two proton components can be distinguished in muscle and fatty tissue. Finally, it will be shown that a pixel-orientated two-exponential analysis of spin-echo images leads to a much more homogeneous density image than one-exponential computation, since tissue-specific biexponentiality and partial volume effects are taken into account. As a conclusion, the hydrogen density of biological tissues can be evaluated at best with an overall error of 10% from MR images for TE greater than 10 ms. This accuracy is insufficient for a pixel-orientated neutron therapy planning.

Adipose Tissue↗

PubMed1989

[Bone marrow changes in Hodgkin's disease: MR tomography and chemical shift imaging].

A patient with Hodgkin's disease, who had received various forms of treatment, was examined by MR tomography and scintigraphy; the results are discussed together with the findings of the bone marrow biopsy. A spectroscopic imaging technique (modified Dixon method) and quantitative evaluation of the fat and water components of the RF signal provide new diagnostic information that may improve the differential diagnosis. Serial examinations following supra-lethal therapy and bone marrow transplantation have been able to show changes in the bone marrow and these could be evaluated quantitatively.

Adult↗

PubMedDate not supplied

1H-spectroscopic imaging using a modified Dixon method.

Inhomogeneities of the static magnetic field and the different susceptibilities of the various types of tissue are a serious problem for all imaging methods of spectral separation of fat and water. In the Dixon method this problem is solved by using the absolute values of the image signals for the separation. In image regions where the fat signal is greater than the water signal, however, this results in an incorrect assignment of the computed solutions. A modified Dixon method was developed to easily carry out the spectral separation completely over the entire image by interactively building up a phase correction matrix after the data acquisition. The spectral delineation of the fat tissue finds an interesting application in the treatment planning with fast neutrons in accounting for the increase in dose.

Adipose Tissue↗

PubMedDate not supplied

Two-exponential analysis of spin-spin proton relaxation times in MR imaging using surface coils.

Proton relaxation time measurements were performed on a standard whole body MR imager operating at 1.5 T using a conventional surface coil of the manufacturer. A combined CP/CPMG multiecho, multislice sequence was used for the T1 and T2 relaxation time measurements. Two repetition times of 2000 ms (30 echoes) and 600 ms (2 echoes) with 180 degrees-pulse intervals of 2 tau = 22 ms were interleaved in this sequence. A two-exponential T2 analysis of each pixel of the spin-echo images was computed in a case of an acoustic neurinoma. The two-exponential images show a "short" component (T2S) due to white and gray matter and a "long" component (T2S) due to the cerebrospinal fluid. In the fatty tissue two components with T2S = 35 +/- 3 ms and T2L = 164 +/- 7 ms were measured. Comparing with Gd-DTPA imaging the relaxation time images show a clear differentiation of vital tumor tissue and cerebrospinal fluid.

Equipment Design↗

PubMedDate not supplied

Multiexponential proton spin-spin relaxation in MR imaging of human brain tumors.

In vivo measurements of proton relaxation processes in human brain tumors have been performed by magnetic resonance (MR) imaging using a whole-body superconductive MR scanner, operating at 1.5 T. The T1 and T2 relaxation time measurements were based on a combined Carr-Purcell/Carr-Purcell-Meiboom-Gill sequence with two interleaved repetition times and 32 echoes. First, comparative measurements in the imager and with the spectrometer of relaxation times were performed on phantoms containing fluids of different T1 and T2 to evaluate accuracy. A maximum deviation of approximately 10% was found. Multislicing with a gap width of one slice thickness influenced the accuracy of T1 relaxation measurement. A gap width of at least two times the slice thickness was necessary for reliable determination of T1. No influence on T2 values was observed by multislicing. Second, in human head imaging the multiexponential behavior of the T2 decay curves has been analyzed in each pixel, where the mean square deviation has been used as a criterion to discriminate between mono- and biexponential behavior. Mean values of monoexponential T1 and multiexponential T2 relaxation data for white matter, gray matter, CSF, edema, and tumor were sampled in 12 patients with brain tumors. T2 showed monoexponential behavior in white and gray matter, whereas CSF, edema, and tumor showed distinct biexponentiality. The biexponential analysis generally yields "fast" and "slow" components with T2f = 80 +/- 17 ms and T2s = 2,030 +/- 210 ms for CSF (partial volume effect), T2f = 104 +/- 25 ms and T2s = 677 +/- 152 ms for edematous tissues, T2f = 97 +/- 19 ms and T2s = 756 +/- 99 ms for tumor tissues, respectively. Using a stepwise discriminant analysis by forward selection, the two best discriminating parameters of the multiexponential relaxation analysis for each pair of classification groups have been selected. For the discrimination of edematous and tumor tissues a retrospective overall accuracy of 94% has been found.

Brain↗

PubMedDate not supplied

Systemic bone marrow disorders: characterization with proton chemical shift imaging.

In a prospective clinical study, 26 patients (22 with malignant lymphoma and 4 with myelofibrosis) and 9 healthy volunteers were examined by conventional magnetic resonance and proton chemical shift imaging (CSI; modified Dixon method). On the basis of the CSI data, a quantitative evaluation of the relative fat and water signal fractions in regions of interest of the femur, pelvis, and spine was performed. In 16 of 17 patients with biopsy-proven bone marrow disorders, CSI revealed a significant reduction in the fat fraction of the bone marrow relative to that of normal volunteers. The visual assessment could detect only 14 of the 17 pathological cases.

Bone Marrow↗

PubMedDate not supplied

Proton chemical shift imaging of bone marrow for monitoring therapy in leukemia.

In three patients with different forms of leukemia, follow-up examinations before, during, and after chemotherapy and bone marrow transplantation were performed by proton chemical shift imaging (1H-CSI). The relative fat and water fractions were computed in representative regions of the marrow in the femur, pelvis, and lumbar spine. On serial examinations the fat fractions increased over time, in agreement with the responses to therapy proven by bone marrow biopsies from the iliac crest. These preliminary results suggest a role for magnetic resonance and CSI in the monitoring of therapy in leukemia and systemic neoplastic diseases.

Acute Disease↗

PubMedDate not supplied

Pharmacokinetic parameters in CNS Gd-DTPA enhanced MR imaging.

Dynamic MR imaging can be used to study tissue perfusion and vascular permeability. In the present article a procedure for dynamic MR is presented, which (a) accurately resolves the fast kinetics of tissue response during and after intravenous infusion of the paramagnetic contrast medium Gd-DTPA and (b) yields a linear relationship between the measured MR signal and the Gd-DTPA concentration in the tissue. According to these features, the measured signal-time curves can be analyzed within the framework of pharmacokinetic modeling. Tissue response has been parameterized using a linear two-compartment open model, with only negligible effects of the peripheral compartment on the central compartment. The three model parameters were fitted to the signal-time data pixel by pixel, based on a set of 64 rapid SE images (SE 100/10 ms, image scan time 13 s, interscan intervals 11 s). This makes it possible to construct parameter images, whereby structures become visible that cannot be distinguished in conventional Gd-DTPA enhanced MR. As a clinical example, the approach is discussed in a case of glioblastoma.

Brain Neoplasms↗

PubMedDate not supplied

MRI and MRA in treatment planning of subdiaphragmatic radiation therapy.

Radiotherapy treatment planning needs optimum definition of target volume in its relative position to normal tissue. The aim of our study was to achieve individual field definition in subdiaphragmatic radiotherapy by visualization of the target volume using fast, breath-held MRI and MR angiography. A modified rapid acquisition SE technique (SE 150/10) was used to obtain a coronal image within a 14 s breath-holding period, displaying kidneys, spleen, and lumbar spine on one slice. Coronal MR angiography acquisition in breath-hold technique was performed using a sequential FLASH-2D sequence (FLASH-2D 30/10/30 degrees). For reconstruction of the MR angiogram in coronal view, we used a maximum intensity projection algorithm. A computer program superimposed the MR angiogram onto the MR image. Correct magnification of the superposition image allowed direct projection onto the simulation film. Problems of distortion and different projection techniques were taken into account and quantified by phantom measurements. The localization error measured in a reference plane was less than 5 mm within a radius of 140 mm. Fourteen cases of Hodgkin disease and non-Hodgkin lymphomas were treated employing the novel technique. By superposition of the MR image and the MR angiogram, demarcation of vascular architecture from parenchymatous organs was achieved. Projection of the MR superposition onto the simulation film yielded accurate and convenient field definition using noninvasive imaging techniques.

Abdomen↗