Biomedical subjects

B Nordell

Publications and source records attributed to B Nordell.

At least 19 recordsLinked to original sources

PubMed2002

Cortical activation during rhythmic hand movements performed under three types of control: an fMRI study.

Echoplanar fMRI was used to measure changes in cortical activation during the performance of a simple hand movement task under three types of voluntary control. Each of three imaging series alternated a task with rest: passive (in which the experimenter moved the hand), voluntary against low resistance, and voluntary against higher resistance. Contralateral activation was observed in the supplementary motor area (SMA), the primary motor cortex (M1), and the somatosensory cortex (S1) in all three tasks in each subject, whereas ipsilateral activation differed in each cortical region for each task. SMA had the widest prevalence of ipsilateral activation in all three tasks. In the M1, ipsilateral activation was observed in all but 1 subject in the two voluntary tasks but in only a few subjects in the S1 in any of the tasks. Quantitative changes in signal intensity and spatial extent of activation differentiated the voluntary tasks from the passive task and were most pronounced in the S1.

Adolescent↗

PubMed2000

Noise considerations in the determination of diffusion tensor anisotropy.

In this study the noise sensitivity of various anisotropy indices has been investigated by Monte-Carlo computer simulations and magnetic resonance imaging (MRI) measurements in a phantom and 5 healthy volunteers. Particularly, we compared the noise performance of indices defined solely in terms of eigenvalues and those based on both the eigenvalues and eigenvectors. It is found that anisotropy indices based on both eigenvalues and eigenvectors are less sensitive to noise, and spatial averaging with neighboring pixels can further reduce the standard deviation. To reduce the partial volume effect caused by the spatial averaging with neighboring voxels, an averaging method in the time domain based on the orientation coherence of eigenvectors in repeated experiments has been proposed.

Adult↗

PubMed2000

Analysis of the Look-Locker T(1) mapping sequence in dynamic contrast uptake studies: simulation and in vivo validation.

An alternative to the pulse sequences at present used in dynamic contrast uptake MRI is the dynamic LL-EPI T(1) mapping method. This method generates T(1) estimates in a few seconds, thereby allowing dynamic studies. A particular advantage of the LL-EPI technique is that it provides the opportunity to generate spatial and temporal information about the paramagnetic contrast agent concentration independently of the inflow rate. This paper illustrates, by computer simulations, the accuracy of the estimated 1/T(1) value when using the LL-EPI technique in situations that are not supported by the model. The simulated situations not supported by the model are those in which the longitudinal and transversal relaxation rates change during the T(1) mapping. The most critical moment occurs during a bolus passage of contrast agent when the concentration gradient is large. The computer simulations of the LL-EPI T(1) mapping method in non-supported situations show that in normal perfused capillary tissue the error in the estimated 1/T(1) value is within the absolute error of 0.1 s(-1) in most simulated situations, although in a typical vessel the simulations do indicate that the stated absolute error tolerance of 0.5 s(-1) is exceeded relatively easily. However, this transgression can be rectified by a non-bolus injection of the contrast agent media.

Brain↗

PubMed1999

Phantom and in vivo study of the Look-Locher T1 mapping method.

This paper describes and tests the LL-EPI method for obtaining quantitative T1 estimates in a few seconds thereby allowing dynamic T1 studies. It is shown that the method works even when there is an inflow into the imaged volume, e.g., in a vessel. No calibration is needed. The method has been tested in a phantom study with several different scan parameter set-ups, with and without inflow. The method shows robustness and individual scan parameters and inflow rates do not influence the ability to calculate the Gd-DTPA concentration. Linearity prevail between the measured 1/T1 and the Gd-DTPA concentration in the range 150 < T1 < 2500 ms. In a dynamic Gd-DTPA phantom study, it was shown that the dynamic LL-EPI T1 mapping technique was three times more sensitive than the signal from a T2*-weighted EPI sequence. In an in vivo study, dynamic T1 mapping of the Gd-DTPA uptake in a meningioma was performed. Inspection of the uptake curves indicates that the method is feasible in clinical perfusion studies.

Brain↗

PubMed1993

A theoretical study of amplitude modulation and time shifting in quantitative MR measurements of motion in brain tissue.

MR imaging pulse sequences can be made sensitive to motion by adding gradients with different strengths at different time intervals. In the well-known phase mapping method, such velocity encoding gradients are used to obtain phase information linear to the velocity of the studied object in the direction of the gradient. When very low velocities are studied, a long duration velocity-encoded gradient is required to obtain sufficient velocity sensitivity. In such cases, variation in the object velocity during the execution of the sequence may hamper the accuracy of the method. In this study, we have made a computer simulation of the performance of a phase mapping method sequence (TE = 46 msec) designed for quantitative studies of motion in brain tissue. Using a Gaussian-shaped velocity input function, the time shifting and the amplitude modulation properties of the sequence was studied for various values of the duration, defined as the full width of tenth of maximum (FWTM), of the input function. The time shift corresponded well to the center of the 180 degrees RF pulse, and the amplitude modulation was seen to decrease with increasing time duration of the velocity input function. Applied on in vivo data, where an approximately gaussian-shaped brain motion velocity pattern was assumed to have a duration of 150 msec, the amplitude modulation of the sequence was estimated to 2%.

Brain↗

PubMed1993

On the pulsatile nature of intracranial and spinal CSF-circulation demonstrated by MR imaging.

Cerebrospinal fluid (CSF) flow was studied in 24 healthy volunteers using gated MR phase imaging. The subarachnoid space (SAS) was divided into 5 compartments depending on the magnitude of the pulsatile CSF flows: a high velocity compartment in the area of the brain stem and spinal cord, 2 slow ones at the upper and lower extremes of the SAS, and finally 2 intermediate velocity compartments in between. The main pulsatile spinal flow channel had a meandering pattern. The extraventricular CSF-circulation can be explained by pulsatile CSF flow without the necessity of assuming existence of a net flow. A successive time offset during the cardiac cycle has been found in the fronto-occipital direction of the interplay between the arterial expansion, brain expansion, volume changes of the CSF spaces and of the veins. It is proposed to name this time offset the intracranial "volume wave" (VoW).

Adult↗

PubMed1993

Target delineation in radiosurgery for cerebral arteriovenous malformations. Assessment of the value of stereotaxic MR imaging and MR angiography.

A study of 6 selected arteriovenous malformation (AVM) patients was performed to investigate the feasibility of delineating an AVM on MR images and to compare the AVM volume outlined on different images. Conventional stereotaxic angiograms, stereotaxic MR images and MR angiograms using several different pulse sequences were obtained prior to radiosurgery. Treatment plans were made from the conventional stereotaxic angiograms. These plans were then transferred to a separate dose planning computer which displayed the MR images with the superimposed isodose lines. The radiated volumes of AVM and brain tissue were measured from these MR images. Last, an assessment was made of the radiation volume needed for an appropriate treatment of the AVM if the treatment plan was made from the MR images rather than from the conventional stereotaxic angiogram. It was possible to delineate medium and large size AVM nidi on stereotaxic MR images based on an integration of information obtained from various pulse sequences. The estimated volumes of the AVM nidi were found to be larger on the conventional stereotaxic angiograms than on the stereotaxic MR images. Consequently, a dose plan based on a conventional stereotaxic angiogram would result in a higher integral dose to the brain with the same target dose. By using reliable MR information it is expected that the volume of brain exposed to radiation could be decreased and the adverse effects of stereotactic radiosurgery for AVM thereby minimized.

Adult↗

PubMed1992

Pulsatile brain movement and associated hydrodynamics studied by magnetic resonance phase imaging. The Monro-Kellie doctrine revisited.

Brain tissue movements were studied in axial, sagittal and coronal planes in 15 healthy volunteers, using a gated spin echo MRI sequence. All movements had characteristics different from those of perfusion and diffusion. The highest velocities occurred during systole in the basal ganglia (maximum 1.0 mm/s) and brain stem (maximum 1.5 mm/s). The movements were directed caudally, medially and posteriorly in the basal ganglia, and caudally-anteriorly in the pons. Caudad and anterior motion increased towards the foramen magnum and towards the midline. The resultant movement occurred in a funnel-shaped fashion as if the brain were pulled by the spinal cord. This may be explained by venting of brain and cerebrospinal fluid (CSF) through the tentorial notch and foramen magnum. The intracranial volume is assumed to be always constant by the Monro-Kellie doctrine. The intracranial dynamics can be viewed as an interplay between the spatial requirements of four main components: arterial blood, capillary blood (brain volume), venous blood and CSF. These components could be characterized, and the expansion of the arteries and the brain differentiated, by applying the Monro-Kellie doctrine to every moment of the cardiac cycle. The arterial expansion causes a re-moulding of the brain that enables its piston-like action. The arterial expansion creates the prerequisites for the expansion of the brain by venting CSF to the spinal canal. The expansion of the brain is, in turn, responsible for compression of the ventricular system and hence for the intraventricular flow of CSF.

Adult↗

PubMed1992

Three-dimensional dose mapping from gamma knife treatment using a dosimeter gel and MR-imaging.

A new method has been investigated for the mapping of dose distributions in three dimensions delivered by the Leksell gamma knife. The irradiation unit is used to selectively treat small volumes in the brain with single high doses of ionising radiation--a treatment procedure known as radiosurgery. The dosimetry method investigated utilises a dosimeter gel consisting of ferrous sulphate solution and agarose which is, prior to irradiation, loaded into a cavity in a spherical phantom. Chemical changes induced in the gel by the radiation are measured by means of an MR-scanner. This imaging method permits rapid evaluation of the dose distribution in an irradiated volume. It thus offers a potential verification of individual radiation intracranial target treatment regimes as well as quality assurance measurements, assuming that the precision and accuracy of the dose mapping are adequate. The dose and its distribution registered by the gel dosimeter, in this initial experiment, are in good agreement with corresponding computed data obtained with the KULA treatment planning system of the gamma knife. The gel has thus the potential of being an attractive alternative dose mapping method to those used at present in radiosurgery, i.e. radiographic film and small ionisation chambers. The precision of the dosimeter gel is, however, not yet sufficient high to be used as a basic dosimetry system for the gamma knife.

Gels↗

PubMed1992

MR imaging, flow and motion.

The present work is intended as a nonmathematical review of the role of flow and motion in nuclear magnetic resonance (MR) imaging. A historical review of MR flow measurement techniques is given, followed by a short overview of flow models in vitro and in vivo. The theory behind the influence of motion on the modulus and phase MR signal information is discussed and effects such as washin/washout, flow-induced signal void, phase offset, and phase dispersion are defined. A simple approach to the concept of MR angiography is given, and methods for quantitative flow measurements such as the phase mapping technique, are surveyed. Aspects of the measurement of diffusion and microcirculation are given, and finally, an overview of the role of MR flow imaging in present and future clinical application is given.

Blood Circulation↗

PubMed1991

Improved portal film image quality in radiation therapy with high energy photons.

Various metal screen-film combinations have been investigated in order to determine the best radiographic image. The quality of these different combinations has been evaluated by measuring the scattered to primary film dose ratio S/P. The S/P ratio increases with increasing atomic number of the front screen for 4 MV x-rays but shows no significant difference for 8 MV x-rays. For rear screens the S/P ratio is slightly increased for higher atomic numbers. A metal with an atomic number around 26-29 should be an optimal metal screen regarding quality aspects. A cassette of stainless steel has, in clinical use for portal and/or verification films, given very good images.

Humans↗

PubMed1990

Accuracy and precision of the computerized brain atlas programme for localization and quantification in positron emission tomography.

The computerized brain atlas programme (CBA) provides a powerful tool for the anatomical analysis of functional images obtained with positron emission tomography (PET). With a repertoire of simple transformations, the data base of the CBA is first adapted to the anatomy of the subject's brain represented as a set of magnetic resonance (MR) or computed tomography (CT) images. After this, it is possible to spatially standardize (reformat) any set of tomographic images related to the subject, PET images, as well as CT and MR images, by applying the inverse atlas transformations. From these reformatted images, statistical images, such as average images and associated error images corresponding to different groups of subjects, may be produced. In all these images, anatomical structures can be localized using the atlas data base and the functional values can be evaluated quantitatively. The purpose of this study was to determine the spatial and quantitative accuracy and precision of the calculated regional mean values. Therefore, the CBA was applied to regional CBF (rCBF) measurements with [11C]fluoromethane and PET on 26 healthy male volunteers during rest and during three different physiological stimulation tasks. First, the spatial accuracy and precision of the reformation process were determined by measuring the spread of defined anatomical structures in the reformatted MR images of the subjects. Second, the mean global CBF and the mean rCBF in the average PET images were compared with the global CBF and rCBF in the original PET images. Our results demonstrate that the reformation process accurately transformed the individual brains of the subjects into the standard brain anatomy of the CBA. The precision of the reformation process had an SD of approximately 1 mm for the lateral dislocation of midline structures and approximately 2-3 mm for the dislocation of the inner and outer brain surfaces. The quantitative rCBF values of the original PET images were accurately represented in the reformatted PET images. Moreover, this study shows that the application of the CBA improves the analysis of functional PET images: (a) The average PET images had a low background noise [0.4 ml/100 g/min +/- 0.7 (SD)] compared to the mean rCBF changes specifically induced by physiological stimulation. (b) The reformatted PET images had a voxel volume of 10.9 mm3. Owing to this high sampling resolution, it was possible to differentiate the mean rCBF changes in adjacent activated fields such as the left motor hand area from the sensory hand area and the left premotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1990

Fourier analysis of cerebrospinal fluid flow velocities: MR imaging study. The Scandinavian Flow Group.

An interleaved pseudocinematographic FLASH (fast low-angle shot) sequence with additional pulsed gradients for flow encoding was used to quantify cerebrospinal fluid (CSF) flow velocities and CSF production. Flow-dependent phase information was obtained by subtracting two differently encoded phase images. The phase information in the resultant image was converted to flow velocity with a calibration curve with the slope 26.5 radian.m-1.sec. The velocity versus time function was Fourier transformed and a continuous curve was fitted to the measured data with use of the first three harmonics. The in vivo measurements showed a significant variation in flow velocities in the cerebral aqueduct (range, 6-51 mm/sec). Calculated CSF production was in the range of 0.6-1.2 L/24 h. The present method gives valuable information about CSF hydrodynamics in an imaging time of less than 8 minutes.

Adult↗

PubMed1989

31P-NMR-spectroscopy measurements of energy metabolism of in vivo growing ascites tumours following addition of glucose.

The cellular ATP content and the phosphorylation potential, defined as the ATP, ADP and inorganic phosphate (Pi) ratios, of exponentially growing Ehrlich ascites tumour cells were compared with cells at the plateau phase of growth. These phosphorus compounds were measured using 31P-NMR-spectroscopy immediately after removal of the cell material from the host and in their ascites fluid reflecting in vivo growth conditions. Reaching the plateau phase of growth, the ATP content and the phosphorylation potential decreased. Upon addition of glucose, the phosphorylation potential immediately increased. We concluded that the reduced phosphorylation potential was due to a limited availability of glucose in spite of the nearly normal blood glucose concentration found. An increasing diffusion distance from the host to all parts of the tumor is a possible reason for that.

Adenosine Triphosphate↗

PubMed1986

Method for quantification of low flow velocities by magnetic resonance phase imaging.

The aim of this study was to compare the influence of flow in the velocity range 0 to 25 mm/s on modulus, phase, real and imaginary images obtained with a standard magnetic resonance scanner (Siemens Magnetom, 0.5 T), and to develop a simple method for determination of flow velocities in vivo from this information. Using a flow phantom, the flow dependent magnetic resonance imaging (MRI) signal has been studied as a function of flow perpendicular to the image slice with non-doped water (simulating moving cerebrospinal fluid) as well as with water doped with Mn2+ (simulating moving blood) for each of the four mentioned image types. The results show a marked flow dependence on all types of images studied. The variation of the signal with flow in the modulus images is relaxation-time dependent in the studied velocity range and it is non-monotone for non-doped water. In the phase images, however, the variations are monotone and not dependent on relaxation times. In modulus images the curve shape is relatively independent on flow direction, while phase images are clearly dependent on flow direction in the studied velocity range. The signal versus velocity curves for the real and imaginary images show resemblance to those for the modulus and the phase images, respectively. It is concluded that the phase information can be used to generate a signal versus velocity calibration curve, which can be used to quantify low flow velocities in vivo.

Magnetic Resonance Imaging↗

PubMed1986

Cerebrospinal fluid flow studied with gated magnetic resonance imaging during the various parts of the cardiac cycle.

The pulsatile movement of cerebrospinal fluid (CSF) through the Sylvian aqueduct has been studied in two normal volunteers. A standard magnetic resonance scanner was used as well as a routine spin echo sequence. Series of ECG-gated axial images were obtained perpendicular to the long axis of the Sylvian aqueduct. Previously it has been demonstrated by flow phantom experiments that the phase information can be used to obtain a linear relation between phase and flow velocity. By multiplying the CSF flow velocity by the cross-sectional area of the aqueduct of Sylvius obtained in each image, the CSF flow variation during the cardiac cycle could be demonstrated and measured.

Cerebrospinal Fluid↗

PubMed1984

Computer assisted dosimetry of scanned electron and photon beams for radiation therapy.

A computer controlled beam forming system for energies up to 50 MeV has been developed in order to produce high quality electron and photon beams for radiation therapy. The desired radiation field shape and dose distribution are achieved by programming the scanning pattern of a narrow and unfiltered electron or photon beam. The computer that controls the scanning pattern also performs dosimetric analyses in the resultant radiation beams. The system allows real time display of the measured dose distributions at a rate of up to five discrete dose values per second for a 15 cm square field. Measurements in scanned as well as in stationary electron and photon beams at energies of 10, 20 and 50 MeV are presented. Finally, the consequences of photon generated electrons in the very broad high energy photon beams that can be produced by a scanning system are illustrated and discussed.

Computers↗

PubMed1980

Investigation of the possibility of using photoneutron beams for radiation therapy.

The possibility has been investigated of using electrons accelerated by a 50 MeV racetrack microtron for generation of photoneutron beams for radiation therapy. Central axis depth-dose curves have been measured in an A-150 tissue-equivalent phantom. Neutron half-value depths between 4.4 and 5.2 g cm-2 were obtained at an SSD of 100 cm for different converter materials and target geometries. At an absorbed dose ratio of 1:1 for neutrons and photons at the dose maximum, the total absorbed dose rates are estimated to be 0.1 Gy min-1 at 100 micronA electron current and a SSD of 100 cm. At a depth of 5 cm the neutron to photon absorbed dose ratio is typically 1:2 and the OER is expected to be about 1.8. Some dose distributional and radiobiological advantages of a physically mixed beam of neutrons and photons for external beam radiation therapy are discussed.

Electrons↗