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Involuntary celibates, or incels, are part of a growing online subculture. Incels are men who are unable to engage in a sexual relationship with a woman and who experience significant distress and anger as a result. In recent years, high-profile incidents of violence perpetrated by incels or those who share incel ideology have increased research attention. Incels communicate online and share several characteristics with other online extremist groups. While only a fraction of incels engage in such violence, a broader spectrum of violence should be considered, including online harassment or general violence against women. This study sought to examine how ongoing engagement on an online incel forum affects changes in incel comments in terms of expressed anger and sadness and use of incel violent extremist language. We collected comments made on an incel forum over a 3-month period. We then identified prolific users and included their comments in our analysis. To assess how their language changed, we used a text-processing program (LIWC: Linguistic Inquiry and Word Count) to assess the extent to which anger, sadness, and incel violent extremist language were expressed in the comments. Our findings indicated that incels express more anger in their comments than users on other platforms such as Reddit, Facebook, and Twitter. However, they did not express greater sadness. Further, we found that incels are already quite angry and sad when they join the forum, and they already use a fair amount of incel vocabulary. Initially, these aspects of their language increase, but they flatten over time. This pattern suggests that introduction to and embracing of incel ideology occurs elsewhere on the Internet, and prior to people joining an incel forum. Implications in terms of prevention of online radicalization and future directions are discussed., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interests with respect to the authorship and/or publication of this article.

Violence is a harmful, complex and gendered act that impacts individuals and communities financially, physically, socially and psychologically. Many studies have investigated how and why gender impacts perceptions of intimate partner violence (IPV), but little research has investigated its effect on stranger violence, despite being publicly perceived as causing greater fear and harm. Th is study examines the effects of gender attitudes and stereotypes on perceptions of stranger violence, specifically attributions of blame, affective response, alleged motivations and attitudes regarding the acceptability of violence (in general). Data was collected from 265 United Kingdom (UK) adults using an online survey posted on Amazon's Mechanical Turk (MTurk). Results indicate that like IPV, gender affects perceptions of stranger violence. Male and female perpetrators of stranger violence are perceived differently, and gender differences are evident between male and female participants. Practical implications of the findings are discussed., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Maladaptive personality traits, such as 'dark personalities' are found to result in a diverse set of negative outcomes, including paraphilic interests and associated (illegal) behaviors. It is however unclear how these are exactly related, and if related, if then only those individuals higher on dark personality traits and higher impulsivity engage in paraphilic behaviors. In the current study, 50 participants were recruited to investigate the relationship between Dark Tetrad personality traits (i.e., narcissism, psychopathy, Machiavellianism and everyday sadism), paraphilic interests (arousal and behavior) and the moderating role of impulsivity. Personality and paraphilic interests were investigated through self-report questionnaires. Impulsivity was measured both through self-reported dysfunctional impulsivity and the P3 event related potential using electroencephalography during the Go/No-Go task (i.e. response inhibition). The results showed that there was a positive association between psychopathy, sadism and paraphilic interests. Whereas everyday sadism was associated with paraphilic (self-reported) arousal, psychopathy was associated with paraphilic behavior. Although P3 amplitude was not associated with paraphilic interests, self-reported dysfunctional impulsivity was associated with paraphilic behavior specifically. However, there was no moderating role of dysfunctional impulsivity and response inhibition (P3) in the relationship between psychopathy and paraphilic behavior. Findings indicate that the relation between specific dark personalities and paraphilic interests may be more complex than initially thought. Nevertheless, risk assessment and intervention approaches for paraphilia and related behavior both may benefit from incorporating Dark Tetrad and impulsivity measurements., (© 2024. The Author(s).)

Stalking perpetrators may suffer from maladaptive personality traits, particularly if they stalk in the context of an (ex-)intimate relationship. To date, no study has examined how different personality attributions may relate to stalker motivation, or the behaviors they engage in, and how this differs across victim-perpetrator relationships. Further, the perspective of the victim is often not taken into consideration, even though most stalking victims know their stalker intimately and a majority are stalked by a former or current partner. The present study employed a correlational design to assess the relationship between stalking behaviors, motivation to stalk, and personality attributions, as perceived by the victim across an ex-intimate or other victim-perpetrator relationship. The study sample consisted of 100 victims of stalking (63% ex-intimate; 85% female) who were recruited through a National Stalking Helpline. Results align with and extend the results of previous researchers, most notably the high proportion of reported Cluster B-aligned personality attributions among stalkers, as well as the proportion of more under-researched personality attributions, and their associated risks. Victims of an ex-intimate partner were more likely to report their stalker was motivated by intimacy, and personality attributions aligned with both borderline and paranoid PD were more often reported than in other relationship contexts. Results and clinical implications are discussed., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Paraphilia is defined as a condition in which sexual excitement relies on fantasizing about and/or participating in unusual sexual behavior. Although recent studies have assessed the concordance between paraphilic interests and paraphilic behaviors, few studies have studied which individual traits and demographics predict engaging in paraphilic behaviors, or the level of concordance between arousal and behavior. The current study replicated and expanded Joyal and Carpentier's 2022 study. We assessed concordance between paraphilic arousal and behavior. Further, we assessed the impact of Dark Tetrad traits, impulsivity, social desirability and demographic variables on engaging in paraphilic behaviors using self-report questionnaires in a sample from the general population and FetLife. Finally, we were interested in whether these individual differences moderated the concordance between arousal and behavior. Results indicated high concordance between paraphilic arousal and behavior for all paraphilias except pedophilia and hebephilia. Younger, male participants were more likely to engage in various paraphilias than women and older participants. Machiavellianism was linked with lower paraphilic behavior, particularly impulsive or risky ones. Psychopathy predicted engaging in more deviant or illegal paraphilias, whereas sadism only showed an association for engaging in frotteurism and narcissism was not a predictor for engaging in any paraphilia. For several paraphilias, individual traits moderated the effect of arousal on behavior. Implications of these findings and future directions are discussed.

There are clinical practice and operational reasons why it may be appropriate to primarily focus on general risk factors when supervising people convicted of sexual crime in the community. General risk domains may be particularly relevant when supervision officers engage in frequent reassessment of acute dynamic risk factors. We tested the ability of a case management tool, the Dynamic Risk Assessment for Offender Re-entry, to discriminate community based, short-term general (all outcome) recidivism versus nonrecidivism among people convicted of sexual crime ( n = 562). We tested the predictive discrimination validity of each DRAOR item and then subscale scores in univariate and multivariate models (also controlling for general static risk). DRAOR scores were associated with general recidivism outcomes and effect sizes were generally similar or stronger compared to models with people convicted of nonsexual crime ( n = 2854). DRAOR Acute scores were consistently and incrementally related to general recidivism outcomes beyond other scores. In practice, case managers should remain aware that people convicted of sexual crime are at risk for nonsexual recidivism outcomes and assess problematic functioning broadly alongside problems in sexual domains. Clinically, interconnection among domains potentially provides multiple avenues for effective intervention., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Sexual abuse of children remains a widespread problem with well-documented, adverse consequences. Often, abuse ending is contingent on a disclosure made by the victim, but victims delay disclosure if they tell someone at all. The factors associated with (non)disclosure are complex and interrelated. In this paper, we propose a new theoretical framework (Mimicry Deception Theory; MDT) to explore various aspects of the grooming process, using a qualitative content analysis of US court appeal cases (N = 25). Specifically, we focus on how MDT components contribute to the likelihood of a CSA disclosure. MDT is made up of five components: Victim Selection, Community Integration, Complexity of Deception, Resource Extraction, and Detectability. These five components allow us to look at several characteristics of abuse in tandem and examine how they interact to impact various outcomes, such as (non)disclosure. We provide a detailed codebook for this framework, that can be used to systematically extract relevant information from large amounts of data. Through the application of this framework, we were able to identify several factors that may play a role in delayed or non-disclosure. Further, we found repeat offenders were likely to use the exact same methods of access, grooming, and remaining undetected across victims. Implications for prevention, as well as clinical interventions with perpetrators as well as victims are discussed., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The rise of the #MeToo movement highlights the prevalence of sexual victimization and gives a voice to victims who may have been silent before. Nevertheless, survivors or victims of sexual violence who come forward may be blamed or not believed. These reactions are evident both with adult and child victims. Further, fears about false accusations of sexual misconduct may negatively impact responses to disclosures. This study aimed to examine gender differences in perceptions toward the #MeToo movement, and the extent to which these translate into a skeptical response to disclosure. Further, we wanted to explore whether proximity to false allegations of sexual violence was linked with more negative responses and whether use of self-affirmations may decrease the likelihood of such a response. Through an online survey ( N = 235) on Amazon's Mechanical Turk, we assessed participants' exposure to and perceptions of the #MeToo movement. Further, we asked them about their proximity to sexual violence (victimization or perpetration) and to false allegations. Using a threat manipulation (news article about false accusation) and a self-affirmation exercise, we studied the effects of both variables on responses to disclosure. Results indicated that after reading an article about a false accusation, male participants were more likely to blame a victim of childhood sexual abuse and to perceive the abuse as less harmful, compared with female participants. Further, we found that self-affirmation was linked with more supportive responses to a disclosure. These findings highlight the threatening nature of false accusations of sexual violence for men, and how this threat may shape the narrative regarding sexual violence. Opportunities to use self-affirmation to change this narrative to a more supportive one are discussed.

The rise of the #MeToo movement has shed light on the prominence of sexual violence, and its victims who often remain silent. Despite increasing awareness, victims or survivors of sexual violence who disclose may be faced with negative reactions such as disbelief or blame. Such reactions extend to child victims of sexual abuse. This study aimed to shed light on gender differences in responses to sexual violence against a backdrop of #MeToo. Through an online survey ( N  = 253) on Amazon's Mechanical Turk, we assessed participants' exposure to and perception of the #MeToo movement. In addition, we measured proximity to a victim or perpetrator of sexual violence. The effect of these variables on participants' response to a disclosure of childhood sexual abuse was examined. Results indicated that men are more likely to perceive the movement as threatening than women. Furthermore, a discrepancy in proximity to sexual violence emerged, with women more likely to know a victim and men more likely to know a perpetrator. In response to a disclosure of childhood sexual abuse, men were more likely to respond in a skeptical manner than women. Positive perceptions of the #MeToo movement translated into more supportive responses to a disclosure. Proximity to a victim of sexual violence did not impact how people responded to a disclosure, but proximity to a perpetrator was associated with a more negative response. Although the aim of this movement is to give a voice to victims of sexual violence, it may trigger a defensive response from men, which makes them more skeptical toward disclosures of victimization.

Mimicry Deception Theory (MDT) argues that deception varies along a long- to short-term continuum. Long-term deception involves complex deception, community integration, slow resource extraction, and low detectability, whereas short-term deception is the opposite. To date, no self-report scale exists that assesses a dispositional orientation toward long-term deception. Across four studies, we developed and validated a Mimicry Deception Scale (MDS) to assess individuals' dispositional orientation toward long-term deception. Using theoretically driven items, we found a reliable four-factor structure through exploratory and confirmatory factor analyses. Further, MDS components were strongly correlated and had acceptable internal consistency. For convergent validity, long-term MDS positively correlated with Machiavellianism, conscientiousness, and planning. Finally, the test-retest reliability of the MDS was acceptable, and the complexity facet of the MDS predicted successful lying over time. The findings have implications for how to profile, identify, and recognize patterns of deception, especially with respect to long-term patterns using self-report.

Victims of Childhood Sexual Abuse (CSA) may be met with skeptical responses when they disclose their experience. The fear of such a response means that most victims delay telling anyone about the abuse. The aim of this study was to explore how contextual factors of abuse such as victim gender and age, perpetrator gender, and victim-perpetrator relationship affect a response to CSA. Further, we explored what personality traits may predict a skeptical response. An online questionnaire with 357 undergraduate students asked participants to read a vignette describing CSA, and to indicate the extent to which they believed the vignette, whether they blamed the victim or perpetrator, and what punishment they would recommend for the perpetrator. Results indicated that older victims were less likely to be believed and more likely to be blamed than younger victims. Further, we found more blame was placed on a perpetrator of intrafamilial abuse compared with extrafamilial abuse, but only if the perpetrator was male. Female perpetrators were punished less harshly than male perpetrators, but female intrafamilal perpetrators received harsher punishment than female extrafamilial perpetrators. No such effect was found for male perpetrators. Openness to Experience was associated with a more supportive response to CSA, as was Agreeableness. Implications and future directions are discussed., (© Copyright 2021 Springer Publishing Company, LLC.)

Background: Despite the frequent occurrence and sizeable consequences of merger abandonment in other sectors, there is no thorough understanding of merger abandonment in health care., Purpose: The purpose of this study is to improve the understanding of determinants of health care merger abandonment., Approach: On the basis of the literature on merger abandonment, we formulated a framework on potential determinants of health care merger abandonment. We then constructed a survey that was sent to 70% of all executives of Dutch health care organizations (response rate = 35%, n = 291). We provide descriptive overviews of open, multiple-response, and multiple-choice questions on merger abandonment and use chi-square tests and Fisher's exact tests to test whether abandoned and completed merger processes differ., Findings: About 62% of the respondents were involved in at least one merger process during the period of 2005-2012. Thirty-eight percent of these respondents reported that their last merger case ended prematurely (n = 53). The most frequently mentioned determinants of merger abandonment are changing insights on the desirability and feasibility during merger processes, incompatibilities between executives, and insufficient support for the merger from internal stakeholders. We did not find significant relationships between merger abandonment and executives' previous merger experience, degree of organizational diversification, health care sector, size differences, or other organizational differences., Discussion: Our findings partially confirm results from previous studies, especially with regard to the importance of changing insights and incompatibilities between the involved executives in merger abandonment. We also find that pressure from internal stakeholders, particularly nonexecutive directors, and distrust, fear, and animosity play an important role in merger abandonment., Practice Implications: To minimize the organizational and societal costs of abandoned mergers, we advise executives who engage in mergers to construct backup plans with alternative strategies in case the merger is abandoned and to conduct a thorough analysis of pros and cons before the merger.

A class of fast magnetic spectroscopic imaging methods using continuously oscillating gradients for four-dimensional (three spatial and one spectral) localization is introduced. Sampling may start immediately following the application of an RF excitation pulse, thus enabling measurement of spin density, chemical shift, and relaxation rates of short-T2 species. For spatial localization, steady-state sinusoidal gradient waveforms are used to sample a ball in k space. The two types of trajectories presented include: (1) continuously oscillating gradients with continuously rotating direction used for steady-state free-precession imaging and (2) continuously oscillating gradients followed by a spoiler directed along discrete projections. Design criteria are given and spatial-spectral and spatial-temporal reconstruction methods are developed. Theoretical point-spread functions and signal-to-noise ratios are derived while considering T2*, off-resonance effects, and RF excitation options. Experimental phantom, in vivo, and in vitro 1H and 23Na images collected at 2.35 T are presented. The 1H images were acquired with isotropic spatial resolution ranging from 0.03 to 0.27 cm3 and gradient-oscillation frequencies ranging from 600 to 700 Hz, thus allowing for the separation of water and lipid signals within a voxel. The 23Na images, acquired with 500 and 800 Hz gradient waveforms and 0.70 cm3 isotropic resolution, were resolved in the time domain, yielding spatially localized FIDs.

Cryoablation has recently become a useful procedure for the treatment of prostatic and hepatic tumors, primarily because of advances in the ability to monitor visually the freezing process with ultrasound. Success of the procedure depends in large part on how well the ice front can be positioned to destroy pathologic tissue, while sparing healthy tissue. This study describes a cryogen delivery system that can be used in conjunction with magnetic resonance (MR) image-guided cryoablation, and an automatic control system that uses MR image guidance in a feedback loop to control the ice front trajectory. Edge-detected MR images are used to determine the current ice front location at each time interval, providing feed-back to an automatic control system that adjusts the flow of cryogen to the cryoprobe. Numerical simulations and experimental results demonstrate that an ice front with cylindrical symmetry can be accurately controlled using this MR image-guided feedback control scheme.

A formalism for the calculation of fields generated by current distributions on hyperbolic surfaces of revolution is presented and used to generate designs for shielded gradient coils. One application is the development of small, insertable coils suitable for head imaging while allowing clearance for shoulders. This technique demonstrates that efficient designs that offer substantial advantages over conventional cylindrical designs are possible.

A growing body of evidence suggests that rhesus macaques may be a good model of human brain aging. We used positron emission tomography (PET) and 18F-fluorodeoxyglucose (FDG) to measure regional cerebral metabolic rates for glucose (rCMRglc) in young and aged rhesus macaques to determine if age-related decreases, such as those reported in humans, also occur in macaques. Whereas the aged animals had lower metabolic rates in every brain region studied, the largest differences were in left temporal cortex. The largest differences were also observed in left temporal cortex when relative rCMRglc values were used. Both rCMRglc and relative rCMRglc were marked by substantial individual variation within the aged group. This variation may parallel the variation observed in behavioral studies. Future studies that include both PET and behavioral measures should help determine if there is a relationship between age-related changes in rCMRglc and behavior.

Cryosurgery of the skin is a common treatment for both benign and malignant skin cancers. Monitoring the depth of the frozen lesion during cryosurgery, either by estimation based on the lateral spread of freeze at the skin surface or via thermocouples, may be inaccurate because of the heterogeneous nature of tissue. We describe an integrated cryosurgical probe and magnitude resonance imaging probe which we use to obtain high resolution MR images of skin, subcutaneous muscle and the frozen lesion during cryosurgery.

Conventional constant angular velocity sampling of the surface of a sphere results in a higher sampling density near the two poles relative to the equatorial region. More samples, and hence longer sampling time, are required to achieve a given sampling density in the equatorial region when compared with uniform sampling. This paper presents a simple expression for a continuous sample path through a nearly uniform distribution of points on the surface of a sphere. Sampling of concentric spherical shells in k-space with the new strategy is used to design 3D selective inversion and spin-echo pulses. These new 3D selective pulses have been implemented and verified experimentally.

The involvement of the corpus callosum in aging and Alzheimer's disease (AD) is not clear. We measured cross sectional areas of the entire corpus callosum (CC), as well as the front 20% (genu), middle 60% (body), and posterior 20% (splenium) of the structure from a midsagittal MRI slice in AD patients (N = 20), and young (N = 16) and old (N = 13) control subjects. We found that mean CC area in young controls was 570 +/- 107 mm2. Aging did not significantly affect the mean area of the CC (562 +/- 98 mm2). A small, significant reduction was seen in AD in comparison to the young control group (480 +/- 133 mm2). However, AD is accompanied by a large and statistically significant reduction in the genu area in comparison to both young and old control subjects. A trend toward an age-dependent reduction in the body area is also accentuated in AD patients who showed significantly smaller callosal bodies than young controls. We conclude that selective changes within corpus callosum accompany aging and AD pathology.

The gradient-selected triple-quantum-filtered (GS3Q) experiment, developed to improve the contrast in NMR imaging of sodium-23 (23Na) in the human brain, is limited by low signal-to-noise ratio (SNR). We have analyzed the GS3Q experiment and show here that the improvement in GS3Q-filtered 23Na SNR as the repetition time (TR) decreases is accompanied by the appearance of spurious one-quantum (1Q) 23Na signals. An improved filter with better suppression of spurious 1Q 23Na signals is obtained by adding a preparatory crusher gradient and two-step phase cycling to a conventional GS3Q filter. The relative contributions of 3Q coherence and spurious 1Q coherences to the conventional and modified-GS3Q-filtered signals are calculated, providing a measure of the effectiveness of each GS3Q filter. The filters were implemented on a 2.35 T medium-bore spectrometer and their predicted properties verified. SNR measurements from GS3Q-filtered three-dimensional images of an agarose gel phantom indicate that 0.012 M 23Na images in the human brain can be acquired with 8 cm3 voxels and SNR of 10 in 30 minutes at 2.35 T, assuming similar relaxation times.

Radiation injury, a major hazard of central nervous system (CNS) radiotherapy, was investigated using sequential studies with positron emission tomography (PET) and magnetic resonance imaging (MRI) in beagle dogs with both helium and neon-ion hemibrain irradiation. All dogs receiving 7.5-11 Gy of neon showed no signs of radiation injury to 3 years after irradiation. Dogs receiving > or = 13 Gy neon or helium succumbed to radiation necrosis and died 21-32 weeks after irradiation. The findings of imaging studies for all dogs who succumbed to radiation necrosis were normal until 3-6 weeks before death. Sequential studies were performed using 0.5 T MRI spin-echo and inversion recovery imaging sequences, and high-resolution (2-3 mm) PET with 18F deoxyglucose and 82Rb. The same axial slices (within 1-2 mm) were imaged repeatedly (weekly) after irradiation until death. The earliest CNS changes were seen as decreased metabolic activity in the cortex of the irradiated hemisphere with PET or an increase in signal intensity in the periventricular white matter on T2-weighted spin-echo imaging on MRI. From the time this increase in signal intensity was first observed, T1 and T2 values increased steadily in both the gray and white matter until death. The changes in white matter were consistently greater than those in gray matter. The results of PET, MRI, and histopathological examinations support the theory that both cellular and vascular mechanisms are involved in radiation necrosis.

This brief communication reports the results of preliminary studies performed to evaluate the feasibility of using NMR imaging to monitor the freezing of tissue during cryosurgery. Two tissues were studied, rabbit brain and dog prostate. NMR imaging of these tissues provided a clear distinction between frozen and unfrozen tissue and an accurate assessment of the extent of freezing in real time.

The inability to observe the transient, irregular shape of the frozen region that develops during cryosurgery has inhibited the application of this surgical technique to the treatment of tumors in the brain and deep in visceral organs. We used proton NMR spin-echo and spoiled gradient-echo imaging to monitor the development of frozen lesions during cryosurgery in the rabbit brain and the resulting postcryosurgical changes up to 4 hr after freezing. Spoiled gradient-echo images (TE = 14 ms; TR = 50 ms) were acquired during freezing and thawing at a rate of 15 s/slice. Although the frozen region itself is invisible in MR images, its presence is distinguished easily from the surrounding unfrozen soft tissue because of the large contrast difference between frozen and unfrozen regions. T2-weighted spin-echo images (TE = 100 ms, TR = 2 s) obtained after thawing suggest that edema forms first at the margin of the region that was frozen (cryolesion) and then moves into the region's core. Histological examination showed complete necrosis in the cryolesion and a sharp transition to undamaged tissue at the margin of the lesion and its image. Blood-brain barrier (BBB) damage was investigated using gadolinium-DTPA. The region of edema in the T2-weighted spin-echo images was coincident with the area of BBB damage in the Gd-DTPA-enhanced T1-weighted spin-echo images (TE = 33 ms, TR = 400 ms) and both were distinguishable as areas of high signal relative to the surrounding normal tissue. The results of these experiments indicate that MR can both effectively monitor the cryosurgical freezing and thawing cycle and characterize the postcryosurgical changes in tissue during follow-up.

A high frequency solution of the electromagnetic field produced by a circular surface coil adjacent to a homogeneous conducting, dielectric sphere is used to predict the attainable signal to noise ratio (S/N) and specific absorption rate (SAR) for in vivo 1H NMR spectroscopy experiments from 200 to 430 MHz (4.7-10 T). Above 200 MHz the S/N increases more rapidly with frequency and the SAR increases less rapidly compared with the respective S/N and SAR frequency dependence below 200 MHz. The difference in frequency dependence is due to dielectric resonances of the magnetic field inside the sphere at frequencies above 200 MHz. It is predicted that surface coil 1H NMR experiments may be performed on a head-sized sphere, having conductivity and relative dielectric constant of brain, at frequencies up to 430 MHz without exceeding 8 W/kg local SAR and 3.2 W/kg SAR. The calculations of the S/N and SAR are used to determine optimum surface coil geometries for NMR experiments. The power radiated by the surface coil in the absence of shielding and asymmetries in the received signal with respect to the plane defined by the surface coil axis and the direction of the static magnetic field are significant at high frequency. Experimental measurements of the magnetic field inside a head-sized sphere verify the presence of dielectric resonances at frequencies above 200 MHz.

The changes in hydrostatic pressure and electrical potentials across vessels in the human vasculature in the presence of a large static magnetic field are estimated to determine the feasibility of in vivo NMR spectroscopy at fields as high as 10 T.A 10-T magnetic field changes the vascular pressure in a model of the human vasculature by less than 0.2%. An exact solution to the magnetohydrodynamic equations describing a conducting fluid flowing transverse to a static magnetic field in a nonconducting, straight, circular tube is used. This solution is compared to an approximate solution that assumes that no magnetic fields are induced in the fluid and that has led previous investigators to predict significant biological effects from static magnetic fields. Experimental results show that the exact solution accurately predicts the magnetohydrodynamic slowing of 15% NaCl flowing transverse to 2.3- and 4.7-T magnetic fields for fluxes below 0.5 liter/min while the approximate solution predicts a much more retarded flow.

A fast imaging technique, missing pulse steady-state free precession (MP-SSFP), is described. MP-SSFP is one of a class of steady-state free precession techniques in which every nth RF pulse is missing. MP-SSFP has been implemented for the case where every third RF pulse is omitted: [-theta 1-r-theta 2-r-echo-r-]. A RF-refocused echo forms at the time of the missing pulse. This echo is less sensitive to field inhomogeneities than the gradient-recalled echoes used in most fast imaging methods. An analytical expression is obtained for the signal strength as a function of the flip angles theta 1 and theta 2, the interpulse interval tau, and the amount of interpulse dephase. The expression shows that theta 1 and theta 2 provide two degrees of freedom to optimize the signal-to-noise ratio and improve tissue contrast. Two different cases, theta 1 = theta 2 and theta 1 = -theta 2, are described to demonstrate the difference in contrast. The first case gives a strong signal from spins with short T1 and short T2 while the second case has contrast very similar to a conventional SSFP technique. The theoretical expression predicts that the signal consists of multiple components which may be observed experimentally by adjusting the gradient pulses.

Phase-sensitive imaging was used to correlate signal distribution with phase shift and velocity distribution in spin-echo magnetic resonance imaging (MRI). Flow-dependent, changing intensity patterns that were seen in a constant-flow phantom study were explained by the simultaneous effects of inflow signal enhancement, first-echo dephasing, and outflow signal loss occurring during laminar flow. In clinical studies, first-echo dephasing was shown during laminar flow in the inferior vena cava. Turbulent flow was demonstrated in the descending thoracic aorta during late systolic flow, and turbulent dephasing-rephasing was shown in the abdominal aorta.

A technique for observing acoustic scattering from individual particles with volumes on the order of 100 micron3 is described. The apparatus consists of an array of focused acoustic transducers, a fluid jet to position the particles, and electronics incorporating quadrature detection. A maximum likelihood estimator is developed for the scattering amplitude that combines the data received from a series of pulses as a particle is carried through the focal zone of the transducers. A calibration method and performance of the apparatus are discussed.

A correction for loss of events due to dead time in dynamic positron emission tomography (PET) is presented. The model employs a paralysing dead time to describe the behaviour of a tomograph over the range of event rates normally encountered in patient studies (up to 200 000 events/s per detector layer). The Donner 280-crystal positron tomograph has a dead time of 1.8 microseconds/event for observed count rates less than 200 000 events/s. The dead time correction factor is 1.8 at 180 000 events/s. The correction is applied to projection data and region of interest analysis of dynamic PET studies, and formulae for the covariances between corrected projection data and between counts in regions of interest in different images from the same dynamic study are established. At 180 000 events/s, the variance of the actual (corrected) number of events in a region containing 3.34 X 10(5) actual events is predicted from the model to be 3.86 X 10(6) (events)2, more than 10 times the variance that would be expected from a naive assumption of Poisson statistics. These statistical results are verified experimentally. An error of 25% is observed in myocardial flow if dead time compensation is not applied, showing the necessity for this correction.

Nuclear magnetic resonance (NMR) imaging was employed to study 10 patients with Alzheimer's disease (AD) and seven healthy elderly control subjects. Coronal sections were used to make volumetric measurements of the hippocampus, ventricles, subarachnoid space, and brain parenchyma. The hippocampal volume (normalized relative to the size of the lenticular nucleus) was reduced by 40% in the AD group compared to the controls, with no overlap between the two groups. Overall measures of brain atrophy and ventricular and sulcal enlargement also showed significantly different group means, although with overlap between the two groups. Hippocampal atrophy did not correlate with either overall brain atrophy or dementia severity, although the degree of brain atrophy was correlated with dementia severity. These results show that NMR is capable of providing in vivo quantification of diminished hippocampal size in AD which is not correlated with overall brain atrophy and which may differentiate AD from normal aging.

A technique for simultaneously measuring the scattering amplitude of individual particles at two angles is applied to human red blood cells. Using a Rayleigh scattering model, the density and compressibility of the cells may be determined given a priori knowledge of their volume. A calibration method relying on measurements of the bulk properties of particle suspensions is described. Red cell properties in hypotonic and hypertonic hosts are compared with a homogeneous mixture model, and a linear relation between hemoglobin content and scattering amplitude at a 90 deg scattering angle is established.

This study examines slice selection and diffusion as sources of systematic error in T2 estimates obtained from imaging experiments. The selective refocusing pulses in a multislice, two-echo, spin-warp experiment cause slice profiles of sequential echoes to change in shape. This results in bias in T2 images calculated from the two-echo data set. A method to correct the bias is presented. The effect of diffusion on T2 images obtained with typical two-echo imaging sequences was found to be negligible.