Maintenance and manipulation in spatial working memory: dissociations in the prefrontal cortex.

Two experiments were conducted to compare thec ries of the functional organization of spatial working memory within the human prefrontal cortex. In Experiment I, memory set size for locations was parametrically varied, allowing for the assessment of BOLD signal across maintenance requirements. In the sec ond experiment, manipulation of spatial information held in working memory was contrasted with simple maintenance of that information. Both experiment evoked significant activity in a distributed spatia working memory network. Although dorsolateral prefrontal activation increased monotonically with memory set size, this region was differentially engaged in task conditions involving explicit manipulation of in ternal representations. Activation in the superior frontal sulcal region was associated with maintenance of spatial information, increasing with memory se size. In contrast, ventrolateral prefrontal activation was present only at the highest memory set size, possibly due to the differential use of organizational strategies with more complex stimuli. These results sup port claims that the dorsolateral prefrontal cortex is involved in the manipulation of internal representa tions and that the superior frontal sulcal region is involved in the maintenance of spatial information but they suggest a complex role for the ventrolatera prefrontal region.

Genetic influences on brain structure.

Here we report on detailed three-dimensional maps revealing how brain structure is influenced by individual genetic differences. A genetic continuum was detected in which brain structure was increasingly similar in subjects with increasing genetic affinity. Genetic factors significantly influenced cortical structure in Broca's and Wernicke's language areas, as well as frontal brain regions (r2(MZ) > 0.8, p < 0.05). Preliminary correlations were performed suggesting that frontal gray matter differences may be linked to Spearman's g, which measures successful test performance across multiple cognitive domains (p < 0.05). These genetic brain maps reveal how genes determine individual differences, and may shed light on the heritability of cognitive and linguistic skills, as well as genetic liability for diseases that affect the human cortex.

The efficacy of a continuous quality improvement (CQI) method in a radiological department. Comparison with non-CQI control material.

OBJECTIVE: To study the efficacy of continuous quality improvement (CQI) compared to ordinary management in an on-duty radiology department. MATERIAL AND METHODS: Because of complaints regarding delivery of on-duty radiological services, an improvement was initiated simultaneously at two hospitals, at the HUCH (Helsinki University Central Hospital) utilising the CQI-method, and at the OUH (Oulu University Hospital) with a traditional management process. For the CQI project, a team was formed to evaluate the process with flow-charts, cause and effect diagrams, Pareto analysis and control charts. Interventions to improve the process were based on the results of these analyses. RESULTS: The team at the HUCH implemented the following changes: A radiologist was added to the evening shift between 15:00-22:00 and a radiographer was moved from the morning shift to 15:00-22:00. A clear improvement was achieved in the turn-around time, but in the follow-up some of the gains were lost. Only minimal changes were achieved at the OUH, where the intervention was based on traditional management processes. CONCLUSION: CQI was an effective method for improving the quality of performance of a radiology department compared with ordinary management methods, but some of this improvement may be subsequently lost without a continuous measurement system.

Quantitative functional lung imaging with synchrotron radiation using inhaled xenon as contrast agent.

Small airways play a key role in the distribution of ventilation and in the matching of ventilation to perfusion. The purpose of this study was to introduce an imaging method that allows measurement of regional lung ventilation and evaluation of the function of airways with a small diameter. The experiments were performed at the Medical Beamline of the European Synchrotron Radiation Facility. Monochromatic synchrotron radiation beams were used to obtain quantitative respiration-gated images of lungs and airways in two anaesthetized and mechanically ventilated rabbits using inhaled stable xenon (Xe) gas as a contrast agent. Two simultaneous images were acquired at two different energies, above and below the K-edge of Xe. Logarithmic subtraction of the two images yields absolute Xe concentrations. This technique is known as K-edge subtraction (KES) radiography. Two-dimensional planar and CT images were obtained showing spatial distribution of Xe concentrations within the airspaces, as well as the dynamics of filling with Xe. Bronchi down to 1 mm in diameter were visible both in the subtraction radiographs and in tomographic images. Absolute concentrations of Xe gas were calculated within the tube carrying the inhaled gas mixture, small and large bronchi, and lung tissue. Local time constants of ventilation with Xe were obtained by following the evolution of gas concentration in sequential computed tomography images. The results of this first animal study indicate that KES imaging of lungs with Xe gas as a contrast agent has great potential in studies of the distribution of ventilation within the lungs and of airway function, including airways with a small diameter.

Diagnosis of meningeoma. A comparison of costs before CT, during CT and after introduction of MR imaging.

OBJECTIVE: To assess whether the capital investment required by advances in radiological technology is offset by savings in the direct costs of diagnostic services. MATERIAL AND METHODS: Meningeoma was used as an indicator case. All meningeoma patients from three study periods were included: Twenty patients in 1976-77 before the introduction of CT, 22 patients in 1984-85 when CT was used and 16 patients in 1992 when MR imaging had replaced CT as the most informative imaging modality. Radiological and other diagnostic investigations, and the hospital stay were identified and cost analyzed. RESULTS: The costs of radiological examinations increased from 293 Euros in 1976-77 to 513 Euros in 1992. The average number of diagnostic examinations per patient decreased from 5.1 in 1976 77 to 2.4 in 1992. The length of hospital stay decreased from 11.5 to 2.7 days and the total costs of the diagnostic work-up decreased to one-third of the original, i.e. from 3423 Euros in 1976-77 to 1282 Euros in 1992. CONCLUSION: The costs of the radiological examinations rose, but the development of radiological technology simplified the diagnostic practice. The hospital stay drastically decreased. The total costs of diagnostic work-up per patient dropped to one-third of the baseline costs.

MRI pontine hyperintensity after supratentorial ischemic stroke relates to poor clinical outcome.

BACKGROUND AND PURPOSE: MRI studies in patients with atherosclerosis often reveal ill-defined hyperintensity in the pons on T2-weighted images. This pontine hyperintensity (PHI) does not fulfill the criteria of a brain infarct, and its clinical relevance is not established. We examined the frequency, as well as the radiological and clinical correlates, of PHI in poststroke patients. METHODS: Three hundred nineteen patients were studied 3 months after supratentorial ischemic stroke with the use of 1.0-T MRI. Brain infarcts, atrophy, white matter hyperintensities, and PHI were registered. The clinical outcome was assessed 3 and 15 months after the stroke. RESULTS: Of the patients, 152 (47.6%) had PHI. The risk factors for stroke did not differ in patients without or with PHI. PHI was related to a higher frequency (P=0.002) and larger volume (P<0.001) of supratentorial brain infarcts, to parietal (P=0.020) and temporal (P=0.002) atrophy, to central atrophy (P< or =0.040), and to white matter hyperintensity grade (P<0.001). Brain infarcts that affected the corpus striatum (putamen, caudate, and pallidum) (P< or =0. 011) or pyramidal tract (P<0.001) were more frequent in patients with PHI. The 3- and 15-month outcomes were worse in patients with PHI (P< or =0.004). The total volume of brain infarcts (OR 1.22), mean atrophy (OR 3.59), and PHI (OR 3.76) were independent correlates of a poor 15-month outcome. CONCLUSIONS: PHI after supratentorial ischemic stroke deserves attention because it relates to poor clinical outcome.

A shielded Overhauser marker for MR tracking of interventional devices.

Improvements to an active MR tracking technique are described. Real-time position monitoring of interventional procedures can be realized by incorporating a small marker that emits an NMR signal into the tip of an interventional device, and the marker's emitted NMR signal is enhanced by use of the Overhauser phenomenon. A significant advance over prior designs has achieved by making the marker have a cylindrical shape and by confining the saturation energy to the marker's interior. The performance of the improved active marker was verified in the laboratory and in vitro. The experiments demonstrated that the marker was visible in MR images when inserted in different excised tissues, and even in air, with positive contrast and with various imaging sequences. The tissue magnetization was minimally perturbed, and the marker emitted a variable but enhanced signal in all orientations in the magnetic field. The marker can potentially be used to mark locations on the body for frameless stereotaxy or to identify inserted devices.

Magnetic resonance imaging white matter hyperintensities and mechanism of ischemic stroke.

BACKGROUND AND PURPOSE: We sought to determine the relations between infarct subtype and white matter hyperintensities (WMHIs) on MRI. MATERIALS AND METHODS: We studied 395 ischemic stroke patients with 1. 0-T MRI. The number of lacunar, border-zone, and cortical infarcts was registered. WMHIs were analyzed in 6 areas. Univariate and multivariate statistical analyses were used to find the risk factors for different infarct subtypes and to study the connections between WMHIs and brain infarcts. RESULTS: Lacunar infarcts were associated with hypertension (odds ratio [OR], 1.79; 95% CI, 1.17 to 2.73), alcohol consumption (OR, 1.96; 95% CI, 1.17 to 3.28), and age (OR, 1. 03; 95% CI, 1.00 to 1.06). Border-zone infarcts were associated with carotid atherosclerosis (OR, 2.20; 95% CI, 1.15 to 4.19). Atrial fibrillation (OR, 3.02; 95% CI, 1.66 to 5.50) and carotid atherosclerosis (OR, 1.94; 95% CI, 1.12 to 3.36) were independent positive predictors, and history of hyperlipidemia (OR, 0.44; 95% CI, 0.26 to 0.75) and migraine (OR, 0.48; 95% CI, 0.25 to 0.93) were negative predictors for cortical infarcts. Patients with lacunar infarcts had more severe WMHIs than patients with nonlacunar infarcts in all WM areas (P

High-accuracy MR tracking of interventional devices: the Overhauser marker enhancement (OMEN) technique.

A new technique for visualization of interventional devices in magnetic resonance imaging is presented. Determination of the position of an invasive device is made possible by incorporating into the device a small marker that emits the NMR signal. This signal is enhanced by the use of the Overhauser phenomenon. This technique differs from the earlier reported techniques for marking interventional instruments in the sense that the contrast between the marker and tissue is not based on different relaxation rates, but on NMR signal enhancement. A prototype marker was constructed and inserted into an inductively fed loop-gap resonator that couples saturation energy with the marker. Circuit analogies are presented that model the Overhauser phenomenon and the coupling circuit. In vitro experiments demonstrated that the marker is visible in MR images up to a slice thickness of 50 mm when inserted in excised animal liver and fat tissues.

Regional gray matter, white matter, and cerebrospinal fluid distributions in schizophrenic patients, their siblings, and controls.

BACKGROUND: Cortical gray matter volume reductions and cerebrospinal fluid (CSF) volume increases are robust correlates of schizophrenia, but their sources have not been established conclusively. METHODS: Structured diagnostic interviews and magnetic resonance imaging scans of the brain were obtained on 75 psychotic probands (63 with schizophrenia and 12 with schizoaffective disorder), ascertained so as to be representative of all such probands in a Helsinki, Finland, birth cohort; 60 of their nonpsychotic full siblings; and 56 demographically similar control subjects without a personal or family history of treated psychiatric morbidity. RESULTS: Patients with schizophrenia and their siblings exhibited significant reductions in cortical gray matter volume and significant increases in sulcal CSF volume compared with controls. The patients, but not their siblings, also exhibited significant reductions in white matter volume and significant increases in ventricular CSF volume. Regional effects were most robust when component volumes were expressed as percentages of overall regional volumes; in this case, for patient and sibling groups, gray matter volume reductions and sulcal CSF volume increases were significantly more pronounced in the frontal and temporal lobes than in the remainder of the brain. None of the group differences varied significantly by sex or hemisphere. CONCLUSIONS: Structural alterations of the cerebral cortex, particularly in the frontal and temporal lobes, are present in patients with schizophrenia and in some of their siblings without schizophrenia; such changes are thus likely to reflect genetic (or shared environmental) effects. Ventricular enlargement is unique to the clinical phenotype and is thus likely to be affected primarily by nonshared causative factors.

Multiphase segmented k-space velocity mapping in pulsatile flow waveforms.

The aim of the present study was to obtain the precision of flow measurement in breath-hold segmented k-space flow sequences. The results are based on studies of pulsatile flow in a phantom tube. The ultimate purpose is to use these sequences to measure coronary flow. In abdominal and cardiothoracic magnetic resonance imaging the image quality is degraded due to respiratory motion. In the segmented k-space acquisition method, one obtains many phase-encoding steps or views per cardiac phase. This shortens imaging time in the order of phase-encoding lines and makes it possible to image in a single breath-hold, thereby eliminating respiratory artefacts and improving edge detection. With breath-hold multiframe cine flow images it is possible to evaluate flow in all abdominal and cardiothoracic areas, including the coronary arteries. Our study shows that velocity curves shift in time when the number of k-space ky-lines per segment (LPS) are varied; this shift is linear as a function of LPS. The mean velocity Vmean in the center of mass of the pulsatile peak is constant (Vmean = 40.1 +/- 2.9 cm/s) and time t = -10.1 x LPS + 268 (r = 0.993, p < 0.0001). Correlation between theoretical and experimental flow curves is also linear as a function of LPS: C = -0.977 * LPS (r = 0.987, p < 0.0001). It is concluded that velocity curves move with LPS and are smoothed when the breath-hold velocity mapping is used. The more LPS is gathered the more inaccurate results are. LPS 7 or more cannot be considered clinically relevant.

Costs of plain-film radiography in a partially digitized radiology department. An activity-based cost analysis.

PURPOSE: The aim of the study was to analyse the costs of computed radiography (CR) as part of a small picture archiving and communication system (mini-PACS), and to compare these costs with those of conventional analogue radiography using activity-based accounting (ABC). MATERIAL AND METHODS: The study was conducted at the Central Hospital of Vaasa where in 1993 the Radiology Department acquired a mini-PACS with a CR reader, a chest CR unit, and a CT unit as digital image processing modalities. Of altogether 34140 plain-film examinations, 3/4 were made with CR and stored mostly on film, and 1/4 were made with conventional analogue radiography. The costs and activities of these two modes were analysed by means of the ABC method which identifies and allocates indirect costs in radiological procedures. RESULTS: The costs of CR imaging were 9% higher than those of conventional radiography. The costs of the chest CR unit were equal to those of conventional radiography. The difference was due to higher investment costs in digital image processing. The time gained from a reduction in the number of retakes did not shorten the time spent by patients in the examination room, and its effect on film costs was minimal. CONCLUSION: In planning the step-by-step transition of conventional film-based analogue radiography to fully digitized radiography, it should be noted that films are still used in the transition period and that this is associated with higher costs than in the previous system of conventional analogue plain-film imaging.

Variable agreement between visual rating scales for white matter hyperintensities on MRI. Comparison of 13 rating scales in a poststroke cohort.

BACKGROUND AND PURPOSE: Previous reports on the frequency, extent, and clinical correlates of white matter hyperintensities (WMHIs) have been contradictory. The purpose of this study was to test whether part of this variation could be explained by the different properties of the visual WMHI rating scales used. METHODS: The periventricular (PVHIs) and deep white matter (DWMHIs) hyperintensities of 395 poststroke patients were systematically analyzed and transformed to correspond to 13 different rating scales. The scales were compared with the use of Goodman-Kruskal measures of association. The relative frequencies, means, and medians of PVHI and DWMHI grades as well as Spearman rank correlations between WMHI grade and hypertension were calculated. RESULTS: At best more than 80% of the patients received an equivalent WMHI grade by different scales, but at worst the corresponding values were only 0.4% for PVHI and 18% for DWMHI. At best different scales categorized patients similarly in regard to WMHI grade, but at worst the corresponding values were 8% for PVHI and 57% for DWMHI ratings. The distribution of WMHI grades also varied, and when the effect of age on WMHI was assessed, some of the scales had a ceiling effect and some had a floor effect. Only 1 of the 7 PVHI, 5 of the 9 DWMHI, and 1 of the 3 combined rating scales showed a significant correlation with arterial hypertension, a putative risk factor for WMHIs. CONCLUSIONS: Some of the inconsistencies in previous studies of WMHIs are due to differences in visual rating scales. Our findings may warrant international debate regarding harmonization of WMHI ratings.

Dynamic fast-gradient echo MR imaging of pancreatic tumours.

The purpose of this study was to determine the diagnostic utility of dynamic magnetic resonance imaging (MRI) of the pancreas. Twenty-eight adult patients with known or suspected pancreatic tumours were examined. Pre- and post-gadolinium (GdDTPA-BMA) scans were obtained in combination with an oral negative contrast medium (ferristene) to mark the gastrointestinal tract. In 6 cases a more precise diagnosis could be made by dynamic MRI compared to unenhanced MRI. Surgery could confirm the MR diagnosis based on contrast enhancement in 83% compared to 78% for CT. The results of signal intensity (SI) measurements show that a combination of differences in baseline values before enhancement and the slope of enhancement within the first 20 s is a reliable criterion to distinguish between normal pancreas and hypovascular tumours. These tumours already show lower SI values before as well as lower slopes after early enhancement. Mainly two effects facilitate the final MRI diagnosis: (1) the delineation of the pancreas from the duodenum by the negative contrast medium, and (2) the enhancement pattern of pancreatic tumours by gadolinium-enhanced dynamic MRI compared to normal tissue within the early enhancement after contrast injection.

Electrophysiological evidence for cross-modal plasticity in humans with early- and late-onset blindness.

It is commonly believed that sensory deprivation can lead to cross-modal reorganization in an immature but not in a mature brain. The results of the present study suggest, however, that plasticity between sensory modalities is possible even in adults: activity indicating involvement of parietal or occipital brain areas in pitch-change discrimination was found in individuals blinded after childhood. Event-related brain potentials of early blinded (before the age of 2 years), late-blinded (12-28 years of age), and sighted adults were recorded to stimulus sequences consisting of standard tones occasionally replaced by deviant tones. Even when participants were not attending to auditory stimuli, the deviant tones elicited the mismatch negativity (MMN) in each group. There were no significant MMN front-back scalp distribution differences among the groups. However, when participants were detecting deviant stimuli, these stimuli elicited N2 and P3 waves that were posterior in distribution in both groups of blind participants relative to those of the sighted participants. These results suggest that cross-modal reorganization may occur even in the mature human brain.

Interventional MR imaging. Demonstration of the feasibility of the Overhauser marker enhancement (OMEN) technique.

PURPOSE: The poor localization facility of interventional instruments in MR imaging has been one of the major obstacles to the popularization of interventional MR imaging. It has been suggested that the Overhauser enhancement be used to generate markers of small size and high visibility. This article studies the feasibility of a localization marker based on this method. MATERIAL AND METHODS: A small Overhauser marker was constructed on the tip of a coaxial cable and comparative images were taken by a 0.23 T imager with and without electron spin irradiation. RESULTS: During irradiation an enhanced signal intensity from the marker was observed. The signal from the marker also exceeded the signal from a 0.25 mmol MnCl2 reference phantom. CONCLUSION: Its small size and high signal-to-noise ratio, together with immunity to most system nonlinearities and imaging errors, makes the Overhauser marker a promising localization method for the accurate positioning of interventional devices. The method may be applied at any field strength, and markers are visible in images obtained with any practical imaging sequence.