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Biomedical subjects

S Brockstedt

Publications and source records attributed to S Brockstedt.

12 recordsLinked to original sources
PubMed2000

Clinical lacunar syndromes as predictors of lacunar infarcts. A comparison of acute clinical lacunar syndromes and findings on diffusion-weighted MRI.

OBJECTIVES: To evaluate if patients with acute lacunar syndromes have acute lacunar infarcts or other types of cerebral lesions on diffusion-weighted MRI. METHODS: Patients with acute lacunar syndromes underwent echo-planar diffusion MRI of the brain within 3 days after stroke onset. Localization and size of lesions with hyperintense signal were determined, compared with clinical characteristics and with findings on follow-up T2-weighted MRI. RESULTS: Twenty-three patients participated in the study. Thirteen patients had pure motor stroke, 1 pure sensory stroke, 8 sensorimotor stroke, and 1 ataxic hemiparesis. Twenty-two patients had at least one lesion with increased signal on diffusion-weighted MR images. These acute lesions were in the internal capsule/ basal ganglia/thalamus in 13 patients, subcortical white matter in 5 patients, brainstem in 2 patients, cortex (multiple small lesions) in 1 patient, and cortex + basal ganglia in 1 patient. The median volume of the lesions was 0.6 ml on the initial examination and on follow-up, of 17 patients after 1 to 5 months, 0.5 ml. CONCLUSIONS: Almost all patients with acute ischemic lacunar syndromes have acute lesions on echo-planar diffusion-weighted MRI within 3 days after stroke onset. These lesions are mostly small and subcortical, compatible with lacunar infarcts caused by single penetrating artery occlusion, but in a minor proportion of patients (2 of 23 in our study) a cortical involvement is found.

Acute Disease↗
PubMed1999

Radiological diagnosis of acute stroke. Comparison of conventional MR imaging, echo-planar diffusion-weighted imaging, and spin-echo diffusion-weighted imaging.

PURPOSE: To compare conventional MR imaging, echo-planar diffusion-weighted imaging (EP-DWI) and spin-echo diffusion-weighted imaging (SE)-DWI at radiological diagnosis of acute stroke. MATERIAL AND METHODS: Twenty-seven patients (30-85 years old) were examined. Clinical examination was performed before MR imaging. All MR examinations were assessed by an experienced neuroradiologist blinded to clinical findings. RESULTS: In EP-DWI, every patient had a lesion corresponding to the clinical findings. EP-DWI was used as the gold standard. In conventional PD+T2 imaging, 23/59 focal lesions were interpreted as acute, which was false in 11 lesions, and 36/59 lesions were considered to be old, 6 were in fact acute. Nine acute lesions were only detected retrospectively and 12 acute lesions were not detected at all on PD+T2. SE-DWI including the apparent diffusion coefficient correlated fairly well with EP-DWI but the procedure was impractical. CONCLUSION: EP-DWI is reliable for diagnosis of early ischemic stroke, while SE-DWI performs reasonably well. Conventional PD+T2 imaging is not reliable for diagnosis of early ischemia.

Acute Disease↗
PubMed1999

Triggering in quantitative diffusion imaging with single-shot EPI.

PURPOSE: The aim of this study was to evaluate any possible effects of brain motion, CSF pulsations and other possible sources of physiological motion in electrocardiographic (ECG) triggered and non-triggered single-shot echo-planar imaging (EPI) measurements of diffusion. MATERIAL AND METHODS: Three different triggering protocols were evaluated in 6 healthy volunteers: 1) ECG triggering with time delay (TD) 100 ms; 2) ECG triggering with TD 400 ms; and 3) no triggering at all. RESULTS: The results obtained showed that white matter mean apparent diffusion coefficient (ADC) values were similar for all triggering protocols and that the reproducibility in ADC measurements using diffusion-weighted (DW) EPI was good. The average standard deviation of the ADC values was, however, higher for ADC maps obtained without ECG triggering. CONCLUSION: The use of prospective diastolic ECG triggering significantly improves the accuracy of quantitative diffusion measurements but for routine clinical diffusion imaging, where quantitative data is of less importance, the accuracy obtained without ECG triggering can be considered adequate.

Adult↗
PubMed1998

Quantitative diffusion coefficient maps using fast spin-echo MRI.

In this work, we have evaluated the performance of a diffusion-sensitive fast spin-echo (FSE) pulse sequence. The proposed pulse sequence utilises velocity-compensating diffusion-encoding gradients and includes the collection of navigator echoes. Spoiler gradients were inserted in the slice-selecting direction to minimise effects from stimulated echoes. Calculations of the b values showed that cross-terms between imaging gradients and diffusion gradients only led to a marginal increase of b values. Pixel-wise calculation of apparent diffusion coefficient (ADC) maps was performed numerically, considering cross-terms between diffusion-encoding and imaging gradients. The sequences investigated used echo train lengths of 16, 8 and 4 echoes and were encoded in either the slice-, frequency- or phase-encoding direction. In order to allow for higher b values a pulse-sequence version using non-motion compensating diffusion-encoding gradients was written. Phantom measurements were performed and the diffusion coefficients of water and acetone were reasonable. Seven healthy volunteers (age 28-50 years) were examined and apparent diffusion coefficient values agreed well with expected values. Diffusion-weighted images, apparent diffusion coefficient maps and images corresponding to the trace of the diffusion tensor of good quality were retrieved in vivo.

Adult↗
PubMed1998

MRI thermometry in phantoms by use of the proton resonance frequency shift method: application to interstitial laser thermotherapy.

In this work the temperature dependence of the proton resonance frequency was assessed in agarose gel with a high melting temperature (95 degrees C) and in porcine liver in vitro at temperatures relevant to thermotherapy (25-80 degrees C). Furthermore, an optically tissue-like agarose gel phantom was developed and evaluated for use in MRI. The phantom was used to visualize temperature distributions from a diffusing laser fibre by means of the proton resonance frequency shift method. An approximately linear relationship (0.0085 ppm degrees C(-1)) between proton resonance frequency shift and temperature change was found for agarose gel, whereas deviations from a linear relationship were observed for porcine liver. The optically tissue-like agarose gel allowed reliable MRI temperature monitoring, and the MR relaxation times (T1 and T2) and the optical properties were found to be independently alterable. Temperature distributions around a diffusing laser fibre, during irradiation and subsequent cooling, were assessed with high spatial resolution (voxel size = 4.3 mm3) and with random uncertainties ranging from 0.3 degrees C to 1.4 degrees C (1 SD) with a 40 s scan time.

Animals↗
PubMed1997

Quantification of low-velocity motion using a navigator-echo supported MR velocity-mapping technique: application to intracranial dynamics in volunteers and patients with brain tumours.

Gradient-echo pulse sequences with velocity-encoding gradients of 22.5-25 mT/m, were used for brain-motion and CSF-flow studies. To reduce motion artifacts, a phase-correction technique based on navigator echoes was evaluated. Three patients with right-sided parietal tumours were investigated; one astrocytoma grade III-IV, one astrocytoma grade I-II and one benign meningioma. In healthy volunteers, a maximal brain-tissue velocity of (0.94 +/- 0.26) mm/s (mean +/- 1SD) was observed, which is consistent with previously presented results. The phase correction was proven useful for reduction of artifacts due to external head movements in modulus and phase images, without loss of phase information related to internal motion. The tissue velocity within the astrocytomas was low during the entire cardiac cycle. An abnormally high rostral velocity component was, however, observed in the brain tissue frontal to the astrocytomas. In all patients, an abnormal CSF flow pattern was observed. The study of brain motion may provide further understanding of the effects of tumours and other pathological conditions in the brain. When considering intracranial motion as a source of error in diffusion/perfusion MRI, the present study suggests that a pathology can alter the properties of brain motion and CSF flow considerably, leading to a more complex impact on diffusion/perfusion images.

Adult↗
PubMed1997

The perfusion fraction in volunteers and in patients with ischaemic stroke.

The fractional volume of capillary blood, i.e. the perfusion fraction f, was measured with the aid of an echo-planar imaging protocol originally designed for the measurement of water diffusion. In healthy volunteers, reasonable f values were obtained. In patients with cerebral ischaemic stroke, a marked decrease in the f value was seen in the infarcted region as compared with corresponding values in the contralateral hemisphere. We suggest that perfusion-fraction measurements may add to the diagnostic value of water-mobility examinations in patients with ischaemic disease.

Adult↗
PubMed1995

Postoperative pulmonary vascular supply in congenital heart disease evaluated with MR imaging at 0.3 T.

The aim of this study was to investigate the usefulness of static spin-echo MR imaging at low field strength in evaluating postoperative pulmonary arterial supply. Twenty-seven patients operated on for complex congenital heart disease underwent cardiac MR imaging: 27 stenoses in 20 patients and 4 aneurysms were present in the material. Five of the overlooked stenoses were located in the ventricular outflow tract or in the valvular region. All four aneurysms were well depicted. The final diagnosis were based on two-dimensional echocardiography, Doppler and invasive studies. MR correctly evaluated the pulmonary arterial supply in 21 patients. In two cases the evaluation was incomplete and in the remaining four patients MR failed to show the stenoses. MR imaging at 0.3 T is an effective non-invasive tool for postoperative evaluation of pulmonary arteries.

Adolescent↗
PubMed1995

Use of an enhanced gradient system for diffusion MR imaging with motion-artifact reduction.

PURPOSE: A spin-echo diffusion-sensitized pulse sequence using high gradients (23 mT/m) is introduced. MATERIAL AND METHODS: In order to minimize motion artefacts, velocity-compensating gradients, ECG-triggering and post-processing with phase correction and raw data averaging using navigator echoes was performed. The in vitro ratio of diffusion coefficients for water and acetone was determined and the water self-diffusion coefficient at different temperatures was evaluated. The pulse sequence was tested in 7 healthy volunteers and in 2 tumour patients with astrocytomas of grades I-II and III-IV. Both single-slice and multi-slice techniques were used. RESULTS: The incorporation of phase correction clearly improved the quality of both diffusion-encoded images and the calculated diffusion maps. Mean values of the diffusion coefficients in vivo were for CSF 2.66 x 10(-9) m2/s and for white and grey matter 0.69 x 10(-9) m2/s and 0.87 x 10(-9) m2/s, respectively. CONCLUSION: Velocity-compensating gradients in combination with a high gradient strength were shown to be useful for in vivo diffusion MR imaging.

Adult↗
PubMed1993

Vertical field MR imaging of upper thorax and spine in small children. Evaluation of a new surface coil.

To improve image quality in a vertical field MR imaging unit, operating at low field strength (0.3 T), we have designed a half-elliptical coil for use in the upper thoracic region of small children. Our intention was also to shorten the examination time, which until now has been long, because several scans with different coils have been necessary to cover the thoracic region. The experimental coil is designed so that a child's shoulders fit into the central region. The coil consists of 2 serially connected cable-loops, mounted on a foam rubber vest. The coil performance was tested in a phantom and improvements relative to standard coils were demonstrated in in vivo studies. The results indicate that by using the half-elliptical coil, the signal-to-noise (S/N) ratio can be improved by a factor of 2 to 3 in the thoracic region of a child.

Child↗
PubMedDate not supplied

Theoretical and experimental evaluation of phase-dispersion effects caused by brain motion in diffusion and perfusion MR imaging.

We investigated intravoxel phase dispersion caused by pulsatile brain motion in diffusion spin-echo pulse sequences. Mathematical models were used to describe the spatial and temporal velocity distributions of human brain motion. The spatial distribution of brain-tissue velocity introduces a phase spread over one voxel, leading to signal loss. This signal loss was estimated theoretically, and effects on observed diffusion coefficient and perfused capillary fraction were assessed. When parameters from a diffusion pulse sequence without motion compensation were used, and ECG triggering with inappropriate delay times was assumed, the maximal signal loss caused by brain-motion-induced phase dispersion was predicted to be 21%. This corresponds to a 95% overestimation of the diffusion coefficient, and the perfusion-fraction error was small. Corresponding calculations for motion-compensated pulse sequences predicted a 1% to 1.5% signal loss due to undesired phase dispersion, whereas experimental results indicated a signal loss related to brain motion of 4%.

Adult↗