High-Resolution 3D Breath-hold Coronary Artery Imaging at 3T using Wideband SSFP

論文發表人：李緒磊 (南加州大學電機所博士班)

http://www.ismrm.org/07/

在 3T MRI 運用 SSFP 技術可提供高訊噪比及高對比影像，但 off-resonance 效
應限制了此一技術中可使用的 TR 參數，降低空間解析度上限。Wideband SSFP 交
互使用兩個不同的 TR 以產生兩個交替出現的穩定態，可抑制 off-resonance
artifact 並提昇影像解析度。在此報告中我們展示了於 3T MR 上進行的三維
breath-hold GRE、SSFP、Wideband SSFP 冠狀動脈影像，其在頻率編碼方向的解
析度為 0.5 毫米。Wideband SSFP 抑制 off-resonance artifact 的能力使其具
有在 3T MR 上取得高解析度冠狀動脈影像的潛力。


Coronary artery imaging using SSFP at 3 Tesla exhibits high SNR and CNR,
however off-resonance effects limit the TR and hence the achievable
spatial resolution. Wideband SSFP sequences utilize an oscillating
steady state to suppress off-resonance artifacts and therefore improve
spatial resolution. We demonstrate coronary artery imaging at 3T using
three-dimensional breath-hold GRE, SSFP, and wideband SSFP sequences,
with an in-plane resolution of 0.5 mm in the frequency-encoding
direction. Wideband SSFP suppresses off-resonance artifacts and is a
potential solution for generating high-resolution coronary artery images
at 3 Tesla.

Integrity-aware Bandwidth Guarding Approach in P2P Networks

論文發表人：江文惠(南 加州大學電機工程學系)

 

http://www.ifip-networking.org/index.htm 

 

大部份的網路互助系統都面臨一個主要的問題 , 總是有些使用者 ( freediers ) 不願意付出而只想從中得利. 以同儕網路系統 ( P2P Network ) 來說, 就是有些使用者只想下載檔案而不願意上傳檔案給他人. 這樣的使用者對系統來說是多餘的, 會給其它使用者帶來負擔並降低整體系統效能. 因次我們提出一個有效率的演算法來分辨這些不合群的使用者 . 主要的想法為監控可用頻寬與使用者網路流量之間的相對關係. 首先我們會先比較現有可用頻寬量測工具的特色, 選擇最適合來監控同儕網路的工具, 接著, 提出我們評估使用者是否為 freeriders 的演算法以及我們的實驗結果. 實驗結果證實我們的演算法確實能夠分辨出對整體網路有貢獻的使用者以及只想從中得利而不想有所貢獻的使用者. 藉由這些資訊我們能建立更穩固的底層網路架構 . 並以此為基礎來發展許多同儕網路系統 ( P2P system ) 或多點傳播系統 ( multicast applications ) .

 

Most Internet-based collaborative computing systems face the major problem: freeriding. The abundance of freeriders, and load imbalance it creates, punishes those peers who do actively contribute to the network by forcing them to overuse their resources. Hence, the overall system performance becomes to degrade quickly. The goal of this paper is aimed to provide an efficient approach to distinguish the dishonest peers from the honest peers. The key idea of our approach is to make use of the relationship between the perceived throughput and the available bandwidth. First, we do a comprehensive study of available bandwidth estimation tools. Next, we propose integrity-aware bandwidth guarding algorithm, which is designed according to the perceived throughput and the available bandwidth estimation. Finally, the simulation results illustrate that our approach can correctly identify dishonest peers and be of great help in constructing a better overlay structure for many peer-to-peer and multicast applications.

 

Transfer (vs. specificity) following different amounts of perceptual learning in tasks differing in stimulus orientation and position

論文發表人：劉效樺 (加州大學爾灣校區認知科學系博士班)

 

http://www.visionsciences.org/meeting.html

 

「知覺學習」指經由學習或是練習之後，在知覺作業表現的增進。這類知覺學習對作業的刺激屬性具有部分或全部的特定性，如：視覺刺激的旋轉或是視覺位置的改變。本研究中，我們探討不同刺激方向與位置所產生之遷移效果（反之，特定性）與初始練習量的函數關係。從先前的研究(Jeter et al., VSS 2004) ，我們已知中度精確的方向辨別作業，呈現部分遷移效果；因此，操弄不同的初始學習量，所產生的遷移效果得以被衡量。受試者在初始的學習階段，被要求對於在視野周邊的Garbor圖形（也就是 -35°±5°，或 55°±5°），如NW和SE位置，做方向判斷作業（相對於基底角度是順時針或是反時針）；遷移作業則是將視覺刺激轉換至相對的視野周邊NE和SW位置。方向判斷作業是在雜訊交錯或是無雜訊的條件下進行訓練與測驗。使用Staircase方法來測量不同練習量之後的對比閾限(contrast threshold)值。如預期，結果顯示接受4個區段（2天）與8個區段（4天）的初始訓練，比起未接受初始練習(0區段) ，所產生的遷移效果量大。初始訓練與遷移之後的訓練擁有約略連續的power函數。在初始訓練階段關係，多接受的訓練並未產生更大的遷移效果。此結果與預期訓練能產生部分遷移的簡單模式不一致；可能是對於額外練習之飽足效應(saturating benefits) 。這些結果將與先前的知覺學習文獻中的例子一併考慮。

 

Perceptual learning refers to the improvement in performance in perceptual tasks through learning or practice. Often, this learning is partially or wholly specific to stimulus features of the task, such as rotation of the stimulus or change of visual position. In this study, we investigated transfer (or conversely specificity) as a function of the extent of initial practice for tasks that differed in stimulus orientation and position. We chose to test a moderate-precision orientation discrimination task known to exhibit partial transfer (Jeter et al., VSS, 2004) so that the extent of transfer after different amounts of initial learning could be assessed. Subjects discriminated the orientation (clockwise or counter-clockwise of a base angle) of a peripheral Gabor in initial training (i.e., -35°±5° or 55°±5°) with locations such as the NW and SE positions and in a 90° rotated transfer task in the NE and SW positions. Discrimination was trained and tested in the presence and absence of white external noise. Staircase methods measured contrast thresholds as a function of practice. As expected, the results show improvements in initial performance for the transfer task following 4 blocks (2 days) and 8 blocks (4 days) of initial training compared to 0 (no) initial training. Initial training and training after transfer had approximately equal power function rates. Additional blocks of initial training yielded little improvement in the transfer task. These results were not consistent with a simple model in which each initial training block yielded a (fractional) benefit at transfer (p < 0.001), but may suggest saturating benefits with additional practice. These results are considered in the context of previous examples of perceptual learning in the literature.

Common and differential brain regions associated with retrieving spatial and non-spatial information from episodic and semantic memory

論文發表人：林君昱 (亞歷桑納大學心理系認知神經科學研究所博士班)

 

http://www.cnsmeeting.org/

 

本研究利用功能性核磁共振大腦影像掃描（fMRI）檢驗與情節及語意記憶中空間及非空間訊息提取歷程有關之大腦區域。在學習階段，受試者觀看六組在螢幕上呈現之圖片。每組圖片包含四個位於螢幕四個角落之不同物品。受試者必須記起各物品圖片的細節及其位置。在測驗階段，受試者須回答一系列之相關比較性問題。每個問題中，有二個物品的名稱以文字型式呈現以作為比較。測驗問題共包含五大類：情節記憶-空間訊息、情節記憶-非空間訊息、語意記憶-空間訊息、語意記憶-非空間訊息，以及控制組。在語意記憶問題中之物品有可能是已學習過的或是新呈現的。研究結果發現，當人類回憶提取不同類之訊息時，許多相似的大腦區域都有參與，但同時也各自利用了一些獨特的大腦區域來完成任務。例如，顳葉內側後緣與情節記憶及空間訊息提取較為相關，但顳葉內側前緣則與語意記憶及非空間訊息提取較相關。內側頂葉（Precuneus）與「熟悉感」較相關，但及外側頂葉則與空間訊息處理較相關。本研究亦討論了這些結果及各變項間之交互作用。本研究之結果提供了人類記憶神經機制更深一層的認識。

The present study examined the brain regions involved in the retrieval of spatial and non-spatial information from episodic or semantic memory using event-related fMRI. In the study phase, participants viewed six object arrays (each had pictures of objects in four quadrants), and were instructed to remember the objects, their locations, and the details of their appearance. At test, participants were presented with verbal names of two objects at a time during scanning and made relational judgments on questions from the following conditions: EPISODIC SPATIAL, EPISODIC NON-SPATIAL, SEMANTIC SPATIAL, SEMANTIC NON-SPATIAL, and CONTROL. Objects in the semantic conditions were either from the study phase or new. Results indicated similar brain activation patterns across all the memory conditions. However, the various conditions yielded differential activations in terms of magnitude, cluster size, and location in specific sub-regions. For example, posterior medial temporal lobe (MTL) was more active for episodic and spatial conditions than semantic and non-spatial conditions, while anterior MTL showed the opposite pattern. Precuneus and lateral parietal regions also showed differential activations that might be related to "perceived familiarity" and spatial processing, respectively. For instance, even when using the same semantic questions that did not require episodic retrieval (e.g. which object is normally softer), precuneus was more active when the objects in the questions had been studied in the study phase than when they were new, perhaps reflecting involuntary feelings of familiarity. Interactions between conditions and the implications of the results will be discussed.