Jonathan Kirsch

Simultaneous in vivo spectral editing and water suppression

Water suppression is typically performed in vivo by exciting the longitudinal magnetization in combination with dephasing, or by using frequency‐selective coherence generation. MEGA, a frequency‐selective refocusing technique, can be placed into any pulse sequence element designed to generate a Hahn spin‐echo or stimulated echo, to dephase transverse water coherences with minimal spectral distortions. Water suppression performance was verified in vivo using stimulated echo acquisition mode (STEAM) localization, which provided water suppression comparable with that achieved with four selective pulses in 3,1‐DRYSTEAM. The advantage of the proposed method was exploited for editing J‐coupled resonances. Using a double‐banded pulse that selectively inverts a J‐coupling partner and simultaneously suppresses water, efficient metabolite editing was achieved in the point resolved spectroscopy (PRESS) and STEAM sequences in which MEGA was incorporated. To illustrate the efficiency of the method, the detection of γ‐aminobutyric acid (GABA) was demonstrated, with minimal contributions from macromolecules and overlying singlet peaks at 4 T. The estimated occipital GABA concentration was consistent with previous reports, suggesting that editing for GABA is efficient when based on MEGA at high field strengths.

Load balancing and locality in range-queriable data structures

We describe a load-balancing mechanism for assigning elements to servers in a distributed data structure that supports range queries. The mechanism ensures both load-balancing with respect to an arbitrary load measure specified by the user and geographical locality, assigning elements with similar keys to the same server. Though our mechanism is specifically designed to improve the performance of skip graphs, it can be adapted to provide deterministic, locality-preserving load-balancing to any distributed data structure that orders machines in a ring or line.

Byzantine replication under attack

Existing Byzantine-resilient replication protocols satisfy two standard correctness criteria, safety and liveness, in the presence of Byzantine faults. In practice, however, faulty processors can, in some protocols, significantly degrade performance by causing the system to make progress at an extremely slow rate. While ldquocorrectrdquo in the traditional sense, systems vulnerable to such performance degradation are of limited practical use in adversarial environments. This paper argues that techniques for mitigating such performance attacks are needed to bridge this ldquopracticality gaprdquo for intrusion-tolerant replication systems. We propose a new performance-oriented correctness criterion, and we show how failure to meet this criterion can lead to performance degradation. We present a new Byzantine replication protocol that achieves the criterion and evaluate its performance in fault-free configurations and when under attack.

Parathyroid Localization and Implications for Clinical Management

CLINICAL CONTEXT The prevalence of hyperparathyroidism, especially primary hyperparathyroidism, has increased in recent decades due to improvements in diagnostic techniques with a corresponding surge in parathyroid surgery, leading to the development of focused, minimally invasive surgical approaches. Focused parathyroidectomy is predicated on preoperative localization of suspected parathyroid pathology. As a result, there has been a proliferation of parathyroid imaging modalities and protocols, resulting in confusion about their indications and applications. EVIDENCE ACQUISITION Bibliographies from clinical trials and review articles published since 2000 were reviewed and supplemented with targeted searches using biomedical databases. We also employed our extensive clinical experience. EVIDENCE SYNTHESIS The best-studied modalities for parathyroid localization are nuclear scintigraphy and sonography and are widely applied as initial studies. Multiple variations exist, and several additional noninvasive imaging techniques, such as computed tomography and magnetic resonance, are described. The exquisite anatomical detail of 4-dimensional computed tomography must be balanced with significant radiation exposure to the thyroid gland. Invasive venous PTH sampling and parathyroid arteriography have important roles in remedial cases. Due to considerable heterogeneity in imaging, multidisciplinary collaboration between endocrinologists, surgeons, and radiologists is beneficial. CONCLUSIONS Parathyroid localization is indicated in surgical candidates. Crucial considerations when selecting an imaging study include availability, cost, radiation exposure, local expertise, and accuracy. Additional factors include the patients anticipated pathology and whether it is de novo or refractory disease. An approach to imaging for patients with primary hyperparathyroidism is presented.

Scaling Byzantine Fault-Tolerant Replication toWide Area Networks

This paper presents the first hierarchical Byzantine fault-tolerant replication architecture suitable to systems that span multiple wide area sites. The architecture confines the effects of any malicious replica to its local site, reduces message complexity of wide area communication, and allows read-only queries to be performed locally within a site for the price of additional hardware. A prototype implementation is evaluated over several network topologies and is compared with a flat Byzantine fault-tolerant approach

Prime: Byzantine Replication under Attack

Existing Byzantine-resilient replication protocols satisfy two standard correctness criteria, safety and liveness, even in the presence of Byzantine faults. The runtime performance of these protocols is most commonly assessed in the absence of processor faults and is usually good in that case. However, faulty processors can significantly degrade the performance of some protocols, limiting their practical utility in adversarial environments. This paper demonstrates the extent of performance degradation possible in some existing protocols that do satisfy liveness and that do perform well absent Byzantine faults. We propose a new performance-oriented correctness criterion that requires a consistent level of performance, even with Byzantine faults. We present a new Byzantine fault-tolerant replication protocol that meets the new correctness criterion and evaluate its performance in fault-free executions and when under attack.

Steward: Scaling Byzantine Fault-Tolerant Replication to Wide Area Networks

This paper presents the first hierarchical byzantine fault-tolerant replication architecture suitable to systems that span multiple wide-area sites. The architecture confines the effects of any malicious replica to its local site, reduces message complexity of wide-area communication, and allows read-only queries to be performed locally within a site for the price of additional standard hardware. We present proofs that our algorithm provides safety and liveness properties. A prototype implementation is evaluated over several network topologies and is compared with a flat byzantine fault-tolerant approach. The experimental results show considerable improvement over flat byzantine replication algorithms, bringing the performance of byzantine replication closer to existing benign fault-tolerant replication techniques over wide area networks.

Paxos for System Builders: an overview

This paper presents an overview of Paxos for System Builders, a complete specification of the Paxos replication protocol such that system builders can understand it and implement it. We evaluate the performance of a prototype implementation and detail the safety and liveness properties guaranteed by our specification of Paxos.

Customizable Fault Tolerance forWide-Area Replication

Constructing logical machines out of collections of physical machines is a well-known technique for improving the robustness and fault tolerance of distributed systems. We present a new, scalable replication architecture, built upon logical machines specifically designed to perform well in wide-area systems spanning multiple sites. The physical machines in each site implement a logical machine by running a local state machine replication protocol, and a wide-area replication protocol runs among the logical machines. Implementing logical machines via the state machine approach affords free substitution of the fault tolerance method used in each site and in the wide-area replication protocol, allowing one to balance performance and fault tolerance based on perceived risk. We present a new byzantine fault-tolerant protocol that establishes a reliable virtual communication link between logical machines. Our communication protocol is efficient (a necessity in wide-area environments), avoiding the need for redundant message sending during normal-case operation and allowing a logical machine to consume approximately the same wide-area bandwidth as a single physical machine. This dramatically improves the wide-area performance of our system compared to existing logical machine based approaches. We implemented a prototype system and compare its performance and fault tolerance to existing solutions.

Survivable SCADA Via Intrusion-Tolerant Replication

Providers of critical infrastructure services strive to maintain the high availability of their SCADA systems. This paper reports on our experience designing, architecting, and evaluating the first survivable SCADA system-one that is able to ensure correct behavior with minimal performance degradation even during cyber attacks that compromise part of the system. We describe the challenges we faced when integrating modern intrusion-tolerant protocols with a conventional SCADA architecture and present the techniques we developed to overcome these challenges. The results illustrate that our survivable SCADA system not only functions correctly in the face of a cyber attack, but that it also processes in excess of 20 000 messages per second with a latency of less than 30 ms, making it suitable for even large-scale deployments managing thousands of remote terminal units.