A head coil system with an integrated orbiting transmission point source mechanism for attenuation correction in PET/MRI.

The combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) provides a benefit for diagnostic imaging. Still, attenuation correction (AC) is a challenge in PET/MRI compared to stand-alone PET and PET-computed tomography (PET/CT). In the absence of photonic transmission sources, AC in PET/MRI is usually based on retrospective segmentation of MR images or complex additional MR-sequences. However, most methods available today are still challenged by either the incorporation of cortical bone or substantial anatomical variations of subjects. This leads to a bias in quantification of tracer concentration in PET. Therefore, we have developed a fully integrated transmission source system for PET/MRI of the head to enable direct measurement of attenuation coefficients using external positron emitters, which is the reference standard in AC. Based on a setup called the 'liquid drive' presented by Jones et al (1995) two decades ago, we built a head coil system consisting of an MR-compatible hydraulic system driving a point source on a helical path around a 24-channel MR-receiver coil to perform a transmission scan. Sinogram windowing of the moving source allows for post-injection measurements. The prototype was tested in the Siemens Biograph mMR using a homogeneous water phantom and a phantom with air cavities and a Teflon (PTFE) cylinder. The second phantom was measured both with and without emission activity. For both measurements air, water and Teflon were clearly distinguishable and homogeneous regions of the phantom were successfully reproduced in the AC map. For water the linear attenuation coefficient was measured as (0.096 ± 0.005) cm-1 in accordance with the true physical value. This combined MR head coil and transmission source system is, to our knowledge, the first working example to use an orbiting point source in PET/MRI and may be helpful in providing fully-quantitative PET data in neuro-PET/MRI.

Response distribution as an explanation of the mirror effect.

Response distribution has recently been proposed as an explanation of the mirror effect in recognition memory. According to the proposal, participants presented with distinctive sets of items (e.g., low- and high-frequency words) vary their responses to give an equal number of positive responses (e.g., the sum of hits and false alarms) to each set. Four experiments tested this proposal. Two experiments showed that the mirror effect is present in the absence of distinctive sets of items. Two experiments showed that the mirror effects is present in the absence of response equalization. Wherever the response distribution hypothesis can be tested, it fails.

Encoding, repetition, and the mirror effect in recognition memory: symmetry in motion.

Attention/likelihood theory has been used to explain the mirror effect in recognition memory. The theory also predicts that any manipulation that affects the recognition of old items will also affect recognition of the new items-more specifically, that all the underlying distributions will move and that they will move symmetrically on the decision axis. In five experiments, we tested this prediction. The first two experiments used encoding tasks during study to change recognition performance for high- and low-frequency words. The results show symmetrical dispersion of the underlying distributions. The final three experiments used repetition to increase recognition performance. Repetition produced a symmetrical pattern of movement that was different from that produced by encoding task. This pattern is, however, also covered by attention/likelihood theory. A further extension of the theory was used to predict response times.

Intralist interference in recognition memory.

Three experiments on recognition memory were carried out to define the nature of intralist interference effects. Experiment 1 replicated the findings of an earlier study (A. I. Schulman, 1971) on what appeared to be combined study (input) and test (output) order effects and added information on the presence of speed-accuracy trade-off effects. Experiment 2 demonstrated that only test order was effective and that study order effects did not occur. Experiment 3 demonstrated again that only test order was effective and also showed that the effect remained when response times were controlled. Attention/likelihood theory was fitted to the data of the final, clarified interference effect.

Attention/likelihood theory: reply to Hintzman (1994).

D. L. Hintzman's (1994) criticism of our theory on recognition memory consists of 2 points: An equation of attention/likelihood theory has been incorrectly written and the likelihood ratios of the theory can be replaced by another, preferable transformation. Both of these points are discussed and rebutted.

The regularities of recognition memory.

Three regularities in recognition memory are described with supporting data: the mirror effect, the order of receiver operating characteristic slopes, and the symmetry of movement of underlying distributions. The derivation of these regularities from attention/likelihood theory is demonstrated. The theory's central concept, which distinguishes it from other theories, is the following: Ss make recognition decisions by combining information about new and old items, the combination made in the form of likelihood ratios. The central role of the likelihood ratios extends the implications of signal detection theory for recognition memory. Attention/likelihood theory is fitted to data of 2 series of experiments. One series involves yes-no tests and confidence ratings, the other forced-choice experiments. It is argued that the regularities require a revision of most current theories of recognition memory.

Speed versus accuracy instructions, study time, and the mirror effect.

Attention/likelihood theory is a model of recognition memory designed to explain the mirror effect (Glanzer & Adams, 1985, 1990). The theory and the effect were studied using speed versus accuracy instructions and short versus long exposure of stimuli. Speed versus accuracy instructions during test and short versus long exposure of stimuli during study were used to vary the number of features sampled from stimuli. When the number of features sampled was reduced either by speed instructions or by shorter exposure, recognition performance was impaired. The theory predicts that in such cases, all distances between underlying distributions will contract. That means, moreover, that when recognition accuracy is decreased for old stimuli, it is also decreased for new stimuli. These predictions were supported by the data from three experiments.

Forgetting and the mirror effect in recognition memory: concentering of underlying distributions.

The mirror effect is a strong regularity in recognition memory: If there are two conditions, A and B, with A giving higher recognition accuracy, then old items in A are recognized as old better than old items in B, and also new items in A are recognized as new better than new items in B. The mirror effect is explained by attention/likelihood theory, which also makes several new, counterintuitive predictions. One is that any variable, such as forgetting, that affects recognition changes the responses to new as well as old stimuli. In terms of underlying distributions, forgetting produces concentering, the bilateral movement of distributions, both new (noise) and old (signal), toward a midpoint. Data from two forced-choice experiments are reported that support the prediction of concentering and other predictions drawn from the theory. It is argued that current theories of memory, which are strength theories, cannot handle these regularities.

An examination of the continuous distractor task and the "long-term recency effect".

The continuous distractor task has yielded a so-called "long-term recency effect" that appears to call into question the dual-storage explanation of serial position effects in free recall. In this study, we show that the "long-term recency effect" is really a short-term storage effect, resulting from adaptation to the repeated presentation of a particular type of distractor throughout the list. This adaptation, a time-sharing process, permits short-term storage to carry out its normal functions. Experiment 1 shows that an appropriate postlist distractor task does in fact eliminate the "long-term recency effect." This finding supports the assertion that the effect is a product of short-term storage. Experiment 2 demonstrates the benefits and costs of the time-sharing process, relative to standard free recall, for both long-term and short-term storage. The findings support the time-sharing hypothesis. Experiment 3 replicates Experiment 2, with a change in procedure that rules out output interference as a mechanism responsible for the results of Experiment 2. Data are also presented on the development of the adaptation over trials. It is concluded that the adaptation and time-sharing processes need to be included in the dual-storage model of short-term storage.

The mirror effect in recognition memory: data and theory.

The mirror effect is a regularity in recognition memory that requires reexamination of current views of memory. Five experiments that further support and extended the generality of the mirror effect are reported. The first two experiments vary word frequency. The third and fourth vary both word frequency and concreteness. The fifth experiment varies word frequency, concreteness, and the subject's operations on the words. The experiments furnish data on the stability of the effect, its relation to response times, its extension to multiple mirror effects, and its extension beyond stimulus variables to operation variables. A theory of the effect and predictions that derive from the theory are presented.

Free recall and word finding ability in normal aging and senile dementia of the Alzheimer's type: the effect of item concreteness.

This study examined the effect of item concreteness on free recall and word finding ability in three groups: young adults, normal old adults, and individuals with senile dementia of the Alzheimer's type (SDAT). The results of Experiment 1 showed, in addition to an overall decline in recall across the three groups, an attenuation with normal aging of the memory advantage of concrete over abstract words. The SDAT group, however, did not show this attenuation. Experiment 2 compared word finding ability for concrete and abstract items. Word finding was significantly impaired in SDAT but not in normal aging. Furthermore, the SDAT group did much worse on the abstract items. This difficulty with the retrieval of abstract words can explain the unexpected concreteness effect in the SDAT group in Experiment 1. The attenuation of the concreteness effect was predicted on the basis of the communication hypothesis of age-related cognitive decline, which attributes age deficits to a breakdown of the memory network. The results are consistent with the reduced activation of information in this network.

Picture superiority in free recall: the effects of normal aging and primary degenerative dementia.

A key factor in the decline of memory with age may be a breakdown of communication in the information network involved in memory and cognitive processing. A special case of this communication is assumed to underlie the picture superiority effect in recall. From this hypothesis it follows that the picture superiority effect should lessen with age. In Experiment 1, three groups of adults (young, old normal, and old memory-impaired) were tested in free recall of pictures and word lists. As predicted, the picture superiority effect declined with age. Experiment 2 replicated these findings and showed, moreover, that the picture superiority effect can be reestablished in normal old adults by instructing them to verbalize overtly during item presentation.

The pattern of memory loss resulting from intravenously administered diazepam.

A word recognition task was designed to determine the stage in memory affected by a single 10-mg intravenous injection of diazepam and the duration of the effect. Injection in three experimental subjects produced an anterograde amnesia for the 14 to 24-minute period immediately after injection. Memory loss resulted from impaired storage, the stage during which information is entered into memory. Retention and retrieval stages of memory were unaffected. This temporary amnesia may result from increased inhibition in the hippocampal system produced by diazepam, which shares many properties with the inhibitory neurotransmitter gamma-aminobutyric acid.