A novel optically transparent RF shielding for fully integrated PET/MRI systems.

Preclinical imaging benefits from simultaneous acquisition of high-resolution anatomical and molecular data. Additionally, PET/MRI systems can provide functional PET and functional MRI data. To optimize PET sensitivity, we propose a system design that fully integrates the MRI coil into the PET system. This allows positioning the scintillators near the object but requires an optimized design of the MRI coil and PET detector. It further requires a new approach in realizing the radiofrequency (RF) shielding. Thus, we propose the use of an optically transparent RF shielding material between the PET scintillator and the light sensor, suppressing the interference between both systems. We evaluated two conductive foils (ITO, 9900) and a wire mesh. The PET performance was tested on a dual-layer scintillator consisting of 12 × 12 LSO matrices, shifted by half a pitch. The pixel size was 0.9 × 0.9 mm2; the lengths were 10.0 mm and 5.0 mm, respectively. For a light sensor, we used a 4 × 4 SiPM array. The RF attenuation was measured from 320 kHz to 420 MHz using two pick-up coils. MRI-compatibility and shielding effect of the materials were evaluated with an MRI system. The average FWHM energy resolution at 511 keV of all 144 crystals of the layer next to the SiPM was deteriorated from 15.73 ± 0.24% to 16.32 ± 0.13%, 16.60 ± 0.25%, and 19.16 ± 0.21% by the ITO foil, 9900 foil, mesh material, respectively. The average peak-to-valley ratio of the PET detector changed from 5.77 ± 0.29 to 4.50 ± 0.39, 4.78 ± 0.48, 3.62 ± 0.16, respectively. The ITO, 9900, mesh attenuated the scintillation light by 11.3 ± 1.6%, 11.0 ± 1.8%, 54.3 ± 0.4%, respectively. To attenuate the RF from 20 MHz to 200 MHz, mesh performed better than copper. The results show that an RF shielding material that is sufficiently transparent for scintillation light and is MRI compatible can be obtained. This result enables the development of a fully integrated PET detector and MRI coil assembly.

Top-down facilitation of visual recognition.

Cortical analysis related to visual object recognition is traditionally thought to propagate serially along a bottom-up hierarchy of ventral areas. Recent proposals gradually promote the role of top-down processing in recognition, but how such facilitation is triggered remains a puzzle. We tested a specific model, proposing that low spatial frequencies facilitate visual object recognition by initiating top-down processes projected from orbitofrontal to visual cortex. The present study combined magnetoencephalography, which has superior temporal resolution, functional magnetic resonance imaging, and a behavioral task that yields successful recognition with stimulus repetitions. Object recognition elicited differential activity that developed in the left orbitofrontal cortex 50 ms earlier than it did in recognition-related areas in the temporal cortex. This early orbitofrontal activity was directly modulated by the presence of low spatial frequencies in the image. Taken together, the dynamics we revealed provide strong support for the proposal of how top-down facilitation of object recognition is initiated, and our observations are used to derive predictions for future research.

Sequence learning in human ocular smooth pursuit.

It has been established that repeated presentation of a transient target motion stimulus such as a constant-velocity ramp leads to the build up of steady state (SS), anticipatory smooth pursuit eye movements after two or three presentations. Each SS response is then composed of the anticipatory component of nonvisual origin, a visual component associated with the stimulus presentation and another nonvisual component that represents the decay of the response after extinction of the stimulus. Here we investigated the interactions that occur when each motion stimulus was itself a sequence containing more than one ramp component. Ramp components had a velocity of 15 degrees /s or 30 degrees /s to left or right and were separated by gaps of 200 ms duration. In an initial experiment, responses to 2-ramp stimuli were examined and compared with responses to the single-ramp stimuli from which they were constituted. We present evidence that the anticipatory, nonvisual components of the double-ramp response result from the linear summation of the nonvisual components of the responses to the constituent single-ramp components. In a 2nd experiment, we examined responses to a wide variety of 4-ramp sequences and again found evidence that, in the SS, the responses were formed from the linear summation of the constituent single-ramp components. Regression analysis performed on the velocity at onset of each ramp component indicated that this nonvisual part of the response was predictive of the upcoming ramp component. To confirm this, unexpected changes were introduced into single ramp components of the 4-ramp sequence after at least five prior presentations of the sequence had allowed a SS response to be established. Subjects continued to initiate a response to the modified component that was appropriate in velocity and direction for the corresponding part of the previous sequence and inappropriate for the newly modified stimulus. This preprogrammed response persisted unmodified for more than 170 ms after onset of the modified ramp component. In contrast, in the second presentation of the new sequence, the anticipatory component of the response was highly correlated with the SS response of the new sequence, but not with that of the prior sequence, showing that the preprogrammed response had been modified very rapidly. Similar behaviour was observed whichever of the 4-ramp components was modified, indicating that the velocity and direction of the anticipatory response to each component had been preprogrammed. The results suggest that velocity information related to at least four elements of a sequence can be temporarily stored and subsequently released with appropriate temporal order to form an anticipatory response throughout the whole sequence.

The role of expectancy and volition in smooth pursuit eye movements.

The most important factor allowing the generation of pursuit eye movements prior to target onset is confidence in the likelihood of imminent target appearance. We show how these anticipatory pursuit responses are essentially ballistic motor primitives and how the signal that drives them in normally defined by stored information concerning target speed, duration and direction. But we also show how static cues may be used to grade the level of these motor primitives 'on-line'. We further demonstrate that, when concatenated, these graded motor primitives can be rapidly combined to form predictive smooth movement trajectories in response to complex multi-ramp sequences.

Recovery of megakaryocytopoiesis in vitro in children with acute lymphoblastic leukemia during induction of remission.

In children with acute lymphoblastic leukemia (ALL), megakaryocytopoiesis was investigated in vitro by the semisolid agar culture technique. In untreated ALL the median number of committed megakaryocyte progenitor cells (CFU-Mk) in the bone marrow was 2 (range less than 0.1-8) per 10(5) bone marrow cells instead of 30 (range 14-93) in controls, the impairment being dependent on the degree of leukemic bone marrow infiltration. However, if the number of CFU-Mk was related to residual nonleukemic bone marrow cells only, two-thirds of the children investigated had a frequency of CFU-Mk within the normal range. After 2 weeks of induction therapy the majority of the children had a low number of CFU-Mk in the bone marrow (median 6, range 0.5-40), a fact that could no longer be explained by a dilution of CFU-Mk by leukemic cells. After 4 weeks of chemotherapy (day 29) the frequency of CFU-Mk had risen to 18 (range 3-67) per 10(5) bone marrow cells, a value still significantly (p less than 0.01) below the normal range. In contrast to the changes in the number of CFU-Mk the median cell number per megakaryocyte colony remained constant during induction of remission. After cessation of long-term chemotherapy all children investigated had a normal number of CFU-Mk, suggesting that no permanent chemotherapy-related damage to committed megakaryocyte progenitor cells was induced.