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Biomedical subjects

L R Schad

Publications and source records attributed to L R Schad.

At least 19 recordsLinked to original sources
PubMed1992

Radiotherapy treatment planning of basal meningiomas: improved tumor localization by correlation of CT and MR imaging data.

A localization technique, based on three-dimensional CT and MR imaging data for precision radiotherapy of basal meningiomas, is presented. Indications for radiotherapy included unresected tumors, gross disease remaining despite surgery, and recurrences. The patient's head was fixed in a stereotactic localization system which is usable at the CT, MR and the linear accelerator installations. The geometrical distortion of MR imaging data was evaluated in three dimensions by phantom measurements. The geometrical distortion was "corrected" (reducing displacements to the size of a pixel) by calculations based on modelling the distortion as a fourth order two-dimensional polynomial. The target volume was defined in three-dimensional MR imaging data after application of 0.1 mmol/kg b.w. Gd-DTPA solution and transferred precisely from MR onto CT data to provide a map of the radiation attenuation coefficient for dose calculation. The superior soft tissue contrast of MR showed an excellent tumor delineation especially when the bony base of the skull obscured the target in CT images. Target volume, calculated dose distribution, and critical structures could be transferred between CT and MR imaging data and displayed as three-dimensional shaded structures for better assessment for matching of target volume and dose distribution. With the described planning system a more precise target definition of basal meningiomas was possible by integration of the superior tumor delineation in MR compared with CT.

Computer Graphics↗
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 angiography. Its use in pulmonary and mediastinal space-occupying lesions].

MR angiography (MRA) proved to be promising combined to MR imaging (MRI) in the assessment of intrathoracic masses. Sequential FLASH 2D angiograms were acquired in breath-hold technique using the following parameters: TR = 30 ms, TE = 10 ms, FA = 30 degrees. Section thickness was 5 mm with 1 mm overlap between sequential sections. Individual conditions of the examination were achieved by an automatized control procedure. Targeted MIP postprocessing resulted in 3D reconstructions illustrating vascular anatomy and avoiding superimposition. Presentation should be done by cine-mode for better spatial impression. This method was evaluated in a prospective study of 21 patients with malignant pulmonary and mediastinal masses in addition to spin-echo imaging. The diagnostic contribution concerning the relationship between the mass and the vasculature like displacement, stenosis, and poststenotic perfusion defect were assessed.

Adult↗
PubMed1992

Computer-aided classification of malignancy in astrocytomas. I. The value of nuclear parameters obtained by automated black and white image analysis.

Ninety-three selected cases of astrocytomas including glioblastomas (astrocytomas grades 1-4) were evaluated by means of Feulgen-stained microscopic slides for nuclear parameters obtained by automated black and white image analysis (ABWIA). The goal was to determine to what extent nuclear features evaluated by ABWIA were applicable as classifiers for the computer-aided numerical classification of malignancy in astrocytomas. Before the automated evaluation, all tumours had been subjectively graded according to the Mayo Clinic grading rules as delineated by Ringertz. Twenty-three nuclear parameters were evaluated and tested for their classification impact. With a model of five parameters (number of nuclei per area, mean of the convex form factor, extinction sum, extinction variation, and full-width-half-maximum of the extinction distribution) the highest reclassification rate of 75% correctly reclassified cases was obtained. Although this is a good result for a classification using only nuclear parameters, it is too poor for practical application. Thus, nuclear parameters evaluated by ABWIA alone are insufficient for numerical classification models assessing the malignant expression of astrocytomas.

Astrocytoma↗
PubMed1992

[Measurement of the blood flow velocity in the pulmonary arteries using the magnetic resonance technique].

MR blood velocity measurements were performed by the RACE technique in a plane perpendicular to the flow of the pulmonary arteries. MR findings were correlated with those of perfusion scintigraphy, Doppler US and right heart catheter (thermodilution). The ratio of MR blood flow measurements of right and left pulmonary arteries correlated well with the results of perfusion scintigraphy (RPA to LPA) and Doppler. Poor correlation was found when comparing MR blood flow measurements with right heart catheter since absolute flow measurements can be superimposed by neighboring blood vessels in complex anatomic situations.

Adolescent↗
PubMed1991

Three-dimensional time-of-flight MR-angiography and the surgical indication of brainstem cavernomas.

This study was designed to assess the diagnostic value of 3D time-of-flight MR-angiography in cerebral cavernomas. In seven patients, nine out of ten cavernomas were removed by microsurgery. While MR-angiography demonstrated well branches of brain arteries adjacent to the lesions, no flow signal in the vascular malformations was observed. On the other hand, there was a high intensity signal induced by methaemoglobin in those three patients with brainstem cavernomas who had experienced a significant bleeding attack seven months prior to admission. It had a spotted appearance in MR-angiography with volumes of the largest spots around 1.8 cm3. It is suggested that this spot signal could be used as a path marker for the surgical approach in brainstem cavernomas.

Adult↗
PubMed1991

[MR tomography and MR angiography in cerebral arteriovenous malformations].

Av malformations are cerebral abnormalities with a high risk of bleeding. The role of MRI and MR angiography (MRA) for demonstrating these congenital vascular malformations has been studied. It has been shown that MRI/MRA can provide a rapid and certain diagnosis. MRA can demonstrate arteries to the second set of branches beyond the main cerebral vessels. It has the advantage of being able to produce rapidly and noninvasively 3-D images and, unlike conventional angiography, it can be repeated at will. The disadvantage is the limited special resolution and the difficulty in distinguishing between arteries and veins.

Adolescent↗
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

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↗
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

MRA-guided stereotactic radiation treatment planning for cerebral angiomas.

Stereotactic neurosurgical interventions are characterized by a high risk of rupturing intracranial vessels or damaging vital brain structures. By MRI and MRA, the anatomic information necessary for stereotactic treatment planning may be assessed with a single modality as a high-resolution digital image volume. The adequacy of MR as a modality for stereotactic guidance is demonstrated by the example of the radiosurgical treatment of cerebral angiomas. An optimized acquisition protocol, a stereotactic apparatus, a distortion correction and minimization method, and a 3D treatment planning workstation are elements of a proposed preoperative approach which in a clinical study met with good acceptance.

Algorithms↗
PubMedDate not supplied

Three dimensional image correlation of CT, MR, and PET studies in radiotherapy treatment planning of brain tumors.

A treatment planning system for stereotactic convergent beam irradiation of deeply localized brain tumors is reported. The treatment technique consists of several moving field irradiations in noncoplanar planes at a linear accelerator facility. Using collimated narrow beams, a high concentration of dose within small volumes with a dose gradient of 10-15%/mm was obtained. The dose calculation was based on geometrical information of multiplanar CT or magnetic resonance (MR) imaging data. The patient's head was fixed in a stereotactic localization system, which is usable at CT, MR, and positron emission tomography (PET) installations. Special computer programs for correction of the geometrical MR distortions allowed a precise correlation of the different imaging modalities. The therapist can use combinations of CT, MR, and PET data for defining target volume. For instance, the superior soft tissue contrast of MR coupled with the metabolic features of PET may be a useful addition in the radiation treatment planning process. Furthermore, other features such as calculated dose distribution to critical structures can also be transferred from one set of imaging data to another and can be displayed as three-dimensional shaded structures.

Brain↗
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

Clinical applications of MR angiography in intrathoracic masses.

This is a prospective evaluation of the use of MR angiography (MRA) at 1.5 T in the assessment of intrathoracic masses. Two-dimensional (2D) MRA was obtained sequentially by means of a fast low angle shot (FLASH) technique (repetition time 30 ms, echo time 10 ms, flip angle 30 degrees) one slice per breath-holding. An automated control procedure and instantaneous image reconstruction permitted constant monitoring of the image quality and tailoring of the timing of the scans to each patient's breathing capacity; MRA was successfully completed in all patients. Two-dimensional FLASH angiography was postprocessed into three-dimensional (3D) MR angiography (projections) by a maximum-intensity-projection algorithm; a 3D spatial impression of the MRA was achieved by obtaining 3D MRAs from different viewing angles and by viewing these in a cine-loop. Superimposition of vessels was avoided by creating angiograms of interest of a specific anatomic region. Fifteen patients with malignant or benign intrathoracic tumor were evaluated; their MR findings were correlated with chest radiography, conventional angiography, bolus enhanced CT, and/or perfusion scintigraphy. Magnetic resonance angiography revealed stenosis, distortion, and displacement of vessels by tumors as well as distal perfusion defects caused by proximal tumors. The MRA findings were readily accepted by our clinical colleagues and incorporated into their surgical planning. We believe MRA to be a promising complement to MR imaging in the assessment of intrathoracic masses.

Adult↗
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

Use of MR angiography for stereotactic planning.

With the introduction of MR angiography (MRA) into clinical routine MR protocols, it has become possible now to image flowing as well as stationary tissue with excellent contrast using a single modality. This has opened up new perspectives for planning stereotactic approaches, which are characterized by high risks for damaging intracerebral vessels or vital brain structures. In this article we present an MRA based planning method for the treatment of arteriovenous malformations by stereotactic radiosurgery. It includes flow compensated gradient echo pulse sequences for the acquisition of angiographic MR datasets, a stereotactic MR marker system, an algorithm for the correction of geometric distortion of MR image data, and a three-dimensional workstation system for the creation and evaluation of treatment plans. The latter is based on the concept of simultaneously displaying both MR slice and angiographic projection images. This allows the evaluation of intracerebral vasculature together with brain anatomy. The MRA guided planning approach was tested and compared to a conventional X-ray angiographic technique in a clinical study. Our satisfactory results suggest that MRA is a technique that can be used advantageously for stereotactic planning.

Brain↗