Biomedical subjects

B Siewert

Publications and source records attributed to B Siewert.

At least 19 recordsLinked to original sources

PubMed1995

Acute human stroke studied by whole brain echo planar diffusion-weighted magnetic resonance imaging.

Our purpose was to use whole brain echo planar magnetic resonance imaging (MRI) to identify and characterize diffusion abnormalities in acute cerebral ischemia. We studied 40 patients as early as 3 hours after onset of signs and symptoms of cerebral ischemia. Diffusion-weighted imaging (DWI) of the entire brain could be completed in 3 seconds or, using seven different diffusion sensitivities (maximum b = 1,271 sec/mm2), in 48 seconds. Measurements and synthetic maps were made of apparent diffusion coefficients (ADC), a physiological parameter that characterizes the self-diffusion of water in tissue. Early ischemic lesions were identified with DWI as hyperintense regions of decreased ADC in all patients who subsequently developed infarction, before changes were evident on conventional MRI in cases studied earlier than 6 hours after onset of ischemic symptoms. Lesions as small as 4 mm in diameter were identified. The extent of lesions within white matter was best defined by controlling for the anisotropic effect of axonal orientation. The mean ADC (+/- SD) for control regions in the 36 patients was 9.15 (+/- 2.91) x 10(-4) mm2/sec. Mean ADC of ischemic regions was 56% of control values at 6 hours or less and stayed significantly reduced for 3 to 4 days after onset of ischemia. The relative ADC increased progressively over time to be pseudonormalized at 5 to 10 days and elevated in the chronic state, making the distinction of acute lesions adjacent to chronic infarcts readily apparent. DWI with echo planar imaging measures a unique physiological parameter that is sensitive to ischemic changes before conventional MRI. Its potential role in the quantitative study of human stroke pathophysiology and therapeutics is the subject of further investigation.

Adult↗

PubMed1995

Brain lesions in patients with multiple sclerosis: detection with echo-planar imaging.

PURPOSE: To evaluate the detection of brain lesions with echo-planar imaging relative to conventional spin-echo (SE) imaging. MATERIALS AND METHODS: In 17 patients (three men, 14 women; mean age, 31 years) with multiple sclerosis, the following were compared: single-shot proton-density- and T2-weighted and thin-section T2-weighted echo-planar, proton-density- and T2-weighted multishot echo-planar, and conventional SE sequences. Quantitative and qualitative criteria as well as lesion detectability were evaluated. The proton-density-weighted SE sequence was used as the standard of reference. RESULTS: Multishot sequences were superior to single-shot sequences in image quality and lesion detectability. With the multishot proton-density-weighted sequence, 53 of 54 large lesions and 23 of 30 small lesions were detected; with the single-shot proton-density-weighted sequence, 38 of 54 large lesions and five of 30 small lesions were detected. CONCLUSION: With multishot echo-planar sequences, detectability of large lesions is similar to that with conventional SE imaging. Susceptibility artifact is diminished in comparison to single-shot echo-planar sequences.

Adult↗

PubMed1995

Diffusion and perfusion imaging techniques.

Diffusion imaging techniques including the Stejskal-Tanner and the stimulated emission of amplitude echoes (STEAM) pulse sequences are discussed. The calculation of apparent diffusion coefficient (ADC) maps is reviewed. The perfusion imaging techniques of blood oxygen level dependent (BOLD) and first pass bolus perfusion techniques as well as the newer technique of echo-planar imaging with signal targeting and altering radiofrequency (EPISTAR) are also discussed. Finally, the theory of intravoxel incoherent motion (IVIM) and its relationship to both diffusion and perfusion phenomena is examined.

Brain Diseases↗

PubMed1995

Stroke and ischemia.

With the introduction of diffusion imaging the diagnosis and acute stroke can be made within minutes of clinical onset. In combination with perfusion imaging, tissue viability can be assessed. The etiology of ischemia can in most cases be investigated by MR angiography. The current applications of these techniques are reviewed, and pitfalls as well as problems in the diagnosis of acute stroke are discussed. With the availability of these techniques, patients can almost instantaneously be approved for pharmacotherapy and monitored and thus clinical outcome potentially can be improved tremendously.

Acute Disease↗

PubMed1994

Signal targeting with alternating radiofrequency (STAR) sequences: application to MR angiography.

We describe a time of flight subtraction method for cine MR angiography that provides nearly perfect suppression of background signal intensity with excellent flow contrast. The method consists of a preparation phase, during which the longitudinal magnetization of the target tissue is inverted on alternate acquisitions and the background tissue is presaturated, followed by a readout phase using a cine segmented turboFLASH sequence with a shared echo modification to improve temporal resolution. With appropriate alternation of the phases of the radiofrequency excitation pulses, there is cancellation of the background signal intensity but flow signal is optimized. By using a thick section (up to 25 mm), substantial portions of the vascular territory are encompassed in a single plane. This permits rapid, dynamic assessment of flow patterns in areas such as the circle of Willis, carotid bifurcation, or renal arteries. Applications of the method for bright and dark blood cine MR angiography are demonstrated.

Aorta, Abdominal↗

PubMed1994

[The value of MR tomography after transsphenoidal hypophyseal resection. A retrospective comparison between endocrinology, operation and MRT].

Postoperative magnetic resonance images of 40 patients after transsphenoidal surgery for pituitary adenoma were evaluated retrospectively. The signal intensities of packing material and residual tumour were analysed. T1-weighted spin-echo sequences were obtained in coronal and sagittal views with and without intravenous injection of contrast media. Residual tumour demonstrated homogeneous or inhomogeneous signal intensity on the native scan with homogeneous or inhomogeneous enhancement. Packing material demonstrated a homogeneous signal intensity on the images obtained without injection of contrast media with a peripheral enhancement or alternate layers of low and intermediate signal intensity with alternate enhancement after injection of contrast media. In 5 of 54 examinations different results were found in MRI, endocrinology and surgery (two false negative and one false positive MRI in correlation with endocrinology; two residual tumours on MRI where surgery was supposed to be complete). Application of intravenous contrast media facilitates the interpretation of postoperative examination of the pituitary gland as well as comparison with preoperative examinations and the knowledge of the intraoperative procedure.

Adenoma↗

PubMed1994

[The role of magnetic resonance angiography prior to the transjugular placement of a portosystemic stent shunt (TIPS)].

The authors employed magnetic resonance angiography (MRA) to guide catheter placement for transjugular intrahepatic portosystemic stent shunt (TIPS) procedures in 14 of 24 patients, and compared the results to the 10 patients who did not have prior planning based on MRA. Two-dimensional time-of-flight venography was performed during breath holding, and projection venograms were formatted in sagittal, coronal and axial planes. MRA defined venous anatomy sufficiently well to shorten the procedure and helped to minimize invasiveness. With MRA guidance, intrahepatic needle punctures were significantly fewer (without MRA guidance: mean 12.1; with MRA guidance: mean 3.5, p < 0.001) and associated complications were absent (without MRA guidance: failed placement, N = 1; bleeding requiring blood transfusions, N = 2; death complicating intraperitoneal haemorrhage with haemobilia, N = 1, and hepatic capsular perforation, N = 1). The average time for the procedure was 2.8 hours without MRA guidance and 1.8 hours with MRA guidance (p < 0.0005). The authors conclude that MR angiography is a useful technique to define portal and hepatic venous anatomy prior to TIPS, and planning based on MRA may decrease the difficulty and length of the procedure.

Adult↗

PubMed1994

[The in-vivo diffusion measurements of the liver, kidneys, spleen and m. erector with an echo-planar imaging system in normal subjects].

A diffusion sensitive pulse sequence using a stimulated echo (STEAM) type excitation and echo-planar (EPI) readout was developed on a 1.5 T echo-planar MR imaging system. Using this sequence the authors measured the apparent diffusion coefficients (ADC) of water in normal human liver, spleen, muscle and kidney in 12 volunteers. ADCs measured in normal volunteers were: liver, 1.39 +/- 0.16 x 10(-3) mm2/s, spleen, 0.95 +/- 0.15 x 10(-3) mm2/s, muscle, 1.99 +/- 0.16 x 10(-3) mm2/s, kidney, 3.54 +/- 0.47 x 10(-3) mm2/s. A large scatter of the ADCs in the kidneys was found in the different degrees of hydration status among the volunteers and also due to inter-subject variability. With further clinical experience, in vivo diffusion measurements of the abdominal organs may prove helpful in the identification and classification of abdominal disease by MRI.

Adult↗

PubMed1994

Qualitative mapping of cerebral blood flow and functional localization with echo-planar MR imaging and signal targeting with alternating radio frequency.

PURPOSE: To create qualitative maps of cerebral blood flow (CBF) with the EPISTAR (echo-planar imaging and signal targeting with alternating radio frequency) technique. MATERIALS AND METHODS: The EPISTAR technique was performed in a pig model of hypercapnia and then tested in 26 volunteers by using various paradigms for cortical activation. Echo-planar images were acquired with and without use of a radio-frequency inversion pulse applied to inflowing arterial spins. A qualitative map of CBF was then created by subtracting the image obtained without the radio-frequency pulse from that obtained with the radio-frequency pulse. RESULTS: Progressively more distal portions of the tagged vessels were seen as the inflow time was lengthened until cortical enhancement was seen for inflow times of approximately 1 second or longer. Signal intensity increases from rest to sensorimotor activation ranged from 13% to 193%. CBF changes in the motor strip, primary visual cortex, and the motor area for eye movements were well localized to the cortical gray matter ribbon. CONCLUSION: The EPISTAR technique is a rapid, noninvasive means for creating qualitative maps of CBF.

Animals↗

PubMed1994

Fast MR imaging of the liver: quantitative comparison of techniques.

PURPOSE: To compare several techniques for fast magnetic resonance (MR) imaging of focal liver lesions. MATERIALS AND METHODS: Ninety patients (37 men and 53 women, aged 19-92 years [mean, 54 years]) with 137 focal liver lesions (56 metastases, 13 hepatocellular carcinomas, 52 hemangiomas, 16 cysts) underwent MR imaging with rapid acquisition spin-echo (RASE), T1-weighted fast low-angle shot (FLASH), turboFLASH, segmented turboFLASH, and T2-weighted conventional and turbo spin echo (SE). Images were analyzed for spleen-to-liver and lesion-to-liver signal difference-to-noise ratios (SD/Ns). RESULTS: Turbo SE T2-weighted imaging had the highest SD/N for spleen-to-liver (P < .01) and for lesion-to-liver (P < .02) contrast. Segmented T1-weighted turboFLASH imaging had the second highest SD/N for spleen-to-liver (P < .001) contrast and was better overall than other T1-weighted sequences for depicting liver lesions (P < .01). Results at segmented turboFLASH imaging were second best for hemangiomas (P < .01). For metastases, no significant difference was found for results with the T1-weighted sequences. CONCLUSION: Segmented T1-weighted turboFLASH and turbo SE T2-weighted imaging hae advantages over conventional techniques for liver imaging.

Carcinoma, Hepatocellular↗

PubMed1994

CT of the acute abdomen: findings and impact on diagnosis and treatment.

An acute abdomen is a clinical condition characterized by severe abdominal pain that develops suddenly over several hours or less [1]. Abdominal tenderness and rigidity, either generalized or localized, usually are severe and indicate an urgent need for prompt diagnosis and treatment. The underlying cause of acute abdomen varies, and some cases require immediate surgical treatment, whereas for others, surgery is unnecessary or contraindicated. This need for prompt diagnosis and treatment should not preclude an appropriate investigation to establish the precise diagnosis before undertaking surgery [1, 2].

Abdomen, Acute↗

PubMed1993

Portal magnetic resonance angiography.

MR angiography has shown definite clinical use in the portal venous system. Methods have been developed for noninvasive assessment of portal venous anatomy and blood flow using a variety of techniques. Time-of-flight techniques for portal angiography and both time-of-flight and phase-contrast techniques for flow measurement are reviewed.

Humans↗

PubMed1992

[Cryptogenetic ascites. Attempts at original pathophysiologic explanation of a monomorphic sonographic image pattern].

In 15,645 consecutive ultrasound examinations of the abdomen (1986 to 1988), free fluid in the peritoneal cavity was found in 247 patients by internal trial during 397 sessions (= 2.5%). Most frequent basic diagnosis for the reason of this symptom were tumorous diseases (99 patients corresponding to 40.1%), cirrhosis of the liver (52 patients corresp. to 22.1%) and heart failure (31 patients corresp. to 12.6%, among these complex gayprooft myocardial insufficiency 24, right heart failure 7). Ovarian cysts or cystomas (7), acute/chronic-recurrent pancreatitis (6), Crohn's disease (3), infections (3), rheumatoid disorders (3), nephrotic syndrome (2), and extra-uterine pregnancy (2) were more rarely represented. In 23 patients (corresp. to 9.3%) the cause of an ascites remained obscure. Among these, a high prevalence of the female sex in the premenopausal age was remarkable with a score of 20:3 (statistically significant difference in terms of the other patients of our group). This observation suggests that an ovarian factor plays a role in the development of ascites in the absence of other evident causes. The literature implies that endometriosis is rather prominent, followed by oligosymptomatic infections or inflammatory diseases.

Adolescent↗

PubMed1992

[Spontaneous necrosis of the femoral condyle--new findings in T2-weighted spin-echo sequences and gradient-echo studies].

The spontaneous necrosis of the femur condyles might cause diagnostic problems because clinical symptoms precede the radiologic changes on conventional x-ray. MRI offers an early detection of the necrosis of the femur condyle like bone necrosis of other localisation. Nine patients were examined by T1- and T2-weighted spin echo- and by T2*-weighted gradient echo sequences. All patients had reduced signal intensity in the subchondral bone marrow on T1-w SE sequences. In three patients new findings were observed on T2-w SE and gradient echoes respectively. These findings allow better understanding in the underlying pathologic-anatomical changes.

Aged↗

PubMed1992

[MR venography in deep venous thromboses of the leg and pelvis. A comparison of 2D single layer images and 3D MIP reconstructions with phlebography].

A total of 22 MR venograms were performed in 7 volunteers and 15 patients suspected of deep vein thrombosis of the lower limb and pelvis. MR findings were compared to conventional venography in all patients. MR venography is a reliable method for the exclusion of thrombosis proximal to the popliteal vein. In the calf veins, diagnosis of thrombosis is not yet reliable. For MR venography 2D-time-of-flight-inflow-technique and secondary 3D-MIP reconstructions were used and compared to each other. With both methods there were no false negative results in comparison to conventional venography. 2D single slice MR venography showed two false positive results in iliac and one in popliteal vein. MIP 3D reconstructions led to seven false positive results (three iliac, two femoral, two popliteal). The exclusive interpretation of MIP-3D reconstruction is not reliable for decision-making in deep venous thrombosis.

Adult↗

PubMed1991

[Chronic hemorrhagic pleural effusion in mediastinal pancreatic pseudocyst].

A 48-year-old patient is presented with chronic hemorrhagic pleural effusion due to a pancreatitis-induced pseudocyst. High levels of pleural fluid amylase and demonstration of the pseudocyst by endoscopic retrograde cholangio-pancreatography (ERCP) provided the diagnosis. After chest tube draining for about two weeks curative surgery was undertaken.

Cholangiopancreatography, Endoscopic Retrograde↗

PubMed1991

[Spontaneous regression of liver metastases in neuroendocrine tumors of the abdomen].

In six patients with hepatically metastasised gastrointestinal tumours of neuro-endocrinal origin, including one each islet cell carcinoma and gastrinoma of the pancreas, one cloacogenic carcinoma (a rare type of anal cancer), and three carcinoid tumours of the small bowel, spontaneous liquefaction inside echogenic liver metastases was demonstrated by ultrasound. The hepatic lesions were classified as being of "bull's-eye-" shape without "badge" pattern. Chemotherapy had no effect on their ultrasonographic imaging. With regard to our observations, liquefying processes were far more common in these well-differentiated tumour metastases than in poorly differentiated fast-proliferating malignancies (about 1:40). Assessment of liquefaction, however, has obviously no influence on the prognosis of progress speed. Ultrasonographic differential diagnosis includes focal inflammatory disease of the liver, if there is no evidence of a primary tumour.

Carcinoid Tumor↗

PubMed1991

[Effects of the PAF antagonist WEB 2086 on hypoxia and angiotensin II-induced pulmonary vasoconstriction in the isolated perfused rat lung].

Using isolated blood-perfused lung preparations of rats, we tested the influence of the PAF antagonist WEB 2086 on vasoconstriction triggered by hypoxia or angiotensin II (A II). If a constant flow was pre-set, changes in the prepulmonarily measured pressure were directly related to the changes of resistance in the pulmonary flow. WEB 2086 reduced the hypoxically conditioned vasoconstriction (HPV) when using blood as perfusion medium, the effect being dependent on the dose (ED50 = 127.3 +/- 21.1 mg/l). HPV was lowered on the average by 82% if the full pharmacologic dose of 800 mg/l WEB 2086 was added to the perfusate. The A II response was weakened to a lesser degree (by 45%). If plasma was used as perfusate, the pressure increase in response to hypoxic stimulation or A II was less marked. However, the relative effect of the PAF antagonist was analogous (attenuation by 83% or 53%, respectively). In chronically hypoxic animals (3 weeks at 10% O2) the relative pressure drop in the lesser circulation after application of WEB 2086 (400 mg/l; HPV; blood as perfusate) was definitely more pronounced (p less than 0.001). The fact that WEB partly antagonises the pulmonary vasoconstriction triggered both by alveolar hypoxy and by angiotensin II, seems to indicate that in both constrictive stimuli PAF participates in the complex mediator mechanism or that WEB 2086 exercises a non-specific vasodilatory effect on the pulmonary flow.

Angiotensin II↗