An electret dosimeter charged by radiation-induced ionizations in air /

An electret radiation dosimeter for long-term personnel monitoring is described. The design of this prototype (a modified parallel-plate ionization chamber) and the associated isothermal electret charging technique are presented. In the charging process, an external voltage causes ions created in air by the passage of radiation to move towards, and become trapped on, a dielectric (e.g., Mylar, Teflon) that covers the measuring electrode, forming an electret. Once the external voltage is removed, the field across the sensitive volume is produced by the electret charge, such that during subsequent irradiation, ions opposite in sign to those on the electret surface are attracted to the electret thus depleting the charge layer in an amount proportional to the exposure. Further irradiation releases the remaining charge on the electret which is measured with an electrometer. This technique allows the electret to be charged, used in the field, and discharged in situ, without dismantling the dosimeter as is required with other electret dosimeters relying on corona charging or other forming methods. Calibration, energy dependence, exposure range, and guard-ring effects of the dosimeter are discussed. This electret dosimeter may prove to be a viable alternative to film dosimeters and TLDs, and is inherently superior because the measuring medium is air

Prolonged survival and serial magnetic resonance imaging/magnetic resonance spectroscopy changes in infantile Krabbe disease

Krabbe disease may present during infancy, late infancy, or adulthood. Earlier-onset disease is associated with shorter survival times. We present a case of infantile onset Krabbe disease with prolonged survival, initial intracranial optic nerves and optic chiasm hypertrophy, and serial changes on cranial magnetic resonance imaging and magnetic resonance spectroscopy.Peer reviewed: YesNRC publication: Ye

Neuroimaging Assessment of Cerebrovascular Reactivity in Concussion: Current Concepts, Methodological Considerations, and Review of the Literature

Concussion is a form of traumatic brain injury (TBI) that presents with a wide spectrum of subjective symptoms and few objective clinical findings. Emerging research suggests that one of the processes that may contribute to concussion pathophysiology is dysregulation of cerebral blood flow (CBF) leading to a mismatch between CBF delivery and the metabolic needs of the injured brain. Cerebrovascular reactivity (CVR) is defined as the change in CBF in response to a measured vasoactive stimulus. Several magnetic resonance imaging (MRI) techniques can be used as a surrogate measure of CBF in clinical and laboratory studies. In order to provide an accurate assessment of CVR, these sequences must be combined with a reliable, reproducible vasoactive stimulus that can manipulate CBF. Although CVR imaging currently plays a crucial role in the diagnosis and management of many cerebrovascular diseases, only recently have studies begun to apply this assessment tool in patients with concussion. In order to evaluate the quality, reliability, and relevance of CVR studies in concussion, it is important that clinicians and researchers have a strong foundational understanding of the role of CBF regulation in health, concussion, and more severe forms of TBI, and an awareness of the advantages and limitations of currently available CVR measurement techniques. Accordingly, in this review, we (1) discuss the role of CVR in TBI and concussion, (2) examine methodological considerations for MRI-based measurement of CVR, and (3) provide an overview of published CVR studies in concussion patients

A prospective comparison study of MRI versus small bowel follow through in recurrent Crohn's disease

Crohn's disease has its peak age of onset in the third decade (1). Since there is no known cure, and it typically runs a relapsing course, patients are recurrently reinvestigated. In North America, a standard approach to reassessing small bowel disease is to use oral barium and a small bowel follow-through (SBFT). The SBFT has long been a standard radiological approach to the investigation for small bowel Crohn's disease. It is thought to be a reliable and acceptable approach to imaging the small bowel by experts provided that it is a dedicated study, incorporating fluoroscopy with manual palpation (2). Some prefer a small bowel enteroclysis (SBE) (3,4) using a nasoenteric tube; however, our group has previously shown that SBFT can be at least as sensitive as SBE and is preferred by patients as no nasal intubation is required (5).NRC publication: Ye

The fan effect in fMRI: left hemisphere specialization in verbal working memory

We studied the fan effect of verbal working memory using functional magnetic resonance imaging (fMRI). Participants were presented with a sentence-pair matching task that described semantic relationships (e.g. classroom-school). Working memory load and semantic processing were manipulated by increasing the number of sentences to be remembered and varying whether they matched expectation. Increased working memory load elicited activation in left dorsal frontal and left inferior parietal regions, and also delayed the hemodynamic responses. Convergent results from semantic matches occurred in the left parietal lobe, whereas, left ventral frontal activation from mismatches diverged from working memory results. The findings were consistent with behavioural and electrophysiological evidence, with the fan effect in fMRI providing novel insight into the spatiotemporal nature of verbal working memory in the left hemisphere.NRC publication: Ye

Detection versus location judgments in a hidden pattern task: functional MRI and behavioral correlates

We used functional magnetic resonance imaging (fMRI) to assess cortical involvement in a hidden pattern task. The experimental and control conditions involved judgment of the presence/absence versus the position of a complex pattern. Activation specific to hidden pattern identification was concentrated on frontal, dorsal parietal, and mesolimbic cortex. This was consistent not only across individual subjects, but with hidden figures tasks used in previous fMRI investigations. Results suggest that pattern identification relies on a relatively stable neural network controlling selective attention. In combination with fMRI, hidden pattern tasks may be useful in neuropsychological assessment of visual search and object identification.Peer reviewed: YesNRC publication: Ye

A site directed fMRI approach for evaluating functional status in the anterolateral temporal lobes

NRC publication: Ye